Nanoarchitectures in dye-sensitized solar cells: metal oxides, oxide perovskites and carbon-based materials

Jasmin S. Shaikh a, Navajsharif S. Shaikh b, Sawanta S. Mali c, Jyoti V. Patil a, Krishna K. Pawar a, Pongsakorn Kanjanaboos b, Chang Kook Hong c, J. H. Kim d and Pramod S. Patil *ae
aThin film materials laboratory, Department of Physics, Shivaji University, Kolhapur 416004, India. E-mail: psp_phy@unishivaji.ac.in; Jasminsshaikh@gmail.com
bMaterials Science and Engineering Program, Faculty of Science, Mahidol University, Bangkok, Thailand
cPolymer Energy Materials Laboratory (Room No. 5B404), School of Advanced Chemical Engineering, Chonnam National University, Gwangju 61186, South Korea
dDepartment of Materials Science and Engineering, Chonnam National University, Gwangju, South Korea
eSchool of Nanoscience and Technology, Shivaji University, Kolhapur 416004, India

Received 9th November 2017 , Accepted 12th February 2018

First published on 12th February 2018


Dye-sensitized solar cells (DSSCs) have aroused great interest and been regarded as a potential renewable energy resource among the third-generation solar cell technologies to fulfill the 21st century global energy demand. DSSCs have notable advantages such as low cost, easy fabrication process and being eco-friendly in nature. The progress of DSSCs over the last 20 years has been nearly constant due to some limitations, like poor long-term stability, narrow absorption spectrum, charge carrier transportation and collection losses and poor charge transfer mechanism for regeneration of dye molecules. The main challenge for the scientific community is to improve the performance of DSSCs by using different approaches, like finding new electrode materials with suitable nanoarchitectures, dyes in composition with promising semiconductors and metal quantum dot fluorescent dyes, and cost-effective hole transporting materials (HTMs). This review focuses on DSSC photo-physics, which includes charge separation, effective transportation, collection and recombination processes. Different nanostructured materials, including metal oxides, oxide perovskites and carbon-based composites, have been studied for photoanodes, and counter electrodes, which are crucial to achieve DSSC devices with higher efficiency and better stability.


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Jasmin S. Shaikh

Dr Jasmin S. Shaikh is currently working as a DST-Women Scientist-A at Shivaji University, Kolhapur, India. She is an alumna of 64th Nobel Laureate Meeting-2016, Lindau, Germany. She worked as a post-doctoral fellow under the World Class University Program (2011–12) at Hanyang University, South Korea and was awarded a Ph.D. in 2011 from Shivaji University, Kolhapur under the guidance of Prof. Pramod S. Patil. Her Ph.D. thesis title is “Deposition of Nanocrystalline CuO based thin films by spin coating technique and their characterizations for supercapacitor application”. After returning from South Korea, she joined the Department of Physics, Savitribai Phule Pune University, India as a Research Assistant under the supervision of Prof. S. I. Patil through the PURSE Program (2012–2013). From 2015, she joined the Department of Physics, Shivaji University. Kolhapur, India under the DST-Women Scientist-A Program. She was the recipient of a Best Young Scientist Award (Oral Presentation), H.S. Gour University, Sagar, India. Her research interests remain in the fields of solar cells, photo-catalysis and supercapacitors.

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Navajsharif S. Shaikh

Mr Navajsharif S. Shaikh is currently doing his Ph.D. at the Materials Science and Engineering Program, Mahidol University in Thailand under the guidance of Dr Pongsakorn Kanjanaboos. He has an M. Tech. post graduate degree from VJTI college of engineering, Mumbai and has completed a B.E. (Mechanical) from SVPM's college of engineering, Malegaon. He has passed the nationwide GATE exam two times and has also served as an assistant professor in the reputed degree engineering college (MHSS) for two years. He is keenly interested in nanomaterials, solar cells and energy storage applications.

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Sawanta S. Mali

Dr Sawanta S. Mali is currently working as an outstanding overseas young researcher and Research Professor at Polymer Energy Materials Laboratory, School of Advanced Chemical Engineering, Chonnam National University, Gwangju, South Korea. He was the recipient of the prestigious “Best Ph. D. thesis Award” by International Solvothermal and Hydrothermal Association (ISHA)-2014. Dr Sawanta received his Ph.D. degree from Shivaji University, Kolhapur, M. S. India in 2013 in polymer solar cells based on nanostructured metal oxides under Prof. Pramod S. Patil, Shivaji University, Kolhapur. After submission of his Ph.D. thesis, he worked at Savitribai Phule Pune University in 2012 as a D. S. Kothari Post-Doctoral Fellow under Prof. Sandesh R. Jadkar, Department of Physics. Then, in 2012 he joined the School of Advanced Chemical Engineering (ACE) at the Polymer Energy Materials Laboratory under Prof. Chang Kook Hong's guidance. In 2015, he joined as an editorial board member of “Nature Scientific Reports” and handles the perovskite solar cell area. Moreover, he was the recipient of an Outstanding Young Researcher Award from the Korean Industrial Engineering Chemistry Society in 2015. He was recognized as an outstanding overseas young researcher by the Korean Research Foundation (KRF) in 2016. Since 2013, he has been working on different types of low-cost photovoltaics, including perovskite solar cells. His current research interest is mainly focused on the synthesis of 2D/3D perovskites and their application in hybrid and flexible perovskite solar cells towards stability.

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Jyoti V. Patil

Miss Jyoti V. Patil, UGC BSR junior research fellow, is currently working as a research student under the guidance of Prof. Pramod S. Patil, Thin Film Materials Laboratory, Department of Physics, Shivaji University, Kolhapur. Miss. Jyoti received her B.Sc. degree from Shivaji University, Kolhapur in 2011, and M.Sc. degree from Shivaji University, Kolhapur in 2013. For her Ph.D. degree, her research title is Studies on Sensitized Solar Cells Based on Electrospun Titanium Oxide. Her research interest is in hydrothermal and electrospinning of 1D/3D nanomaterials and their applications in sensitized solar cells.

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Krishna K. Pawar

Mr Krishna K Pawar is currently working as a research student under the guidance of Prof. Pramod S. Patil, Thin Film Materials Laboratory, Department of Physics, Shivaji University, Kolhapur. Mr Krishna received his B.Sc. degree from D. B. F Dayanand College, Solapur University, Solapur in 2013 and M.Sc. degree from Shivaji University, Kolhapur in 2015. His research interest is in chemical sensors and optoelectronic devices.

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Pongsakorn Kanjanaboos

Dr Pongsakorn Kanjanaboos received his B.A. in physics and economics from Washington University in Saint Louis in 2008. He earned his Ph.D. in physics at the University of Chicago in 2013. In collaboration with Argonne National Laboratory, Pongsakorn investigated the self-assembly, nanomechanics, and application of solution-processed nanoparticle thin films. Because of his interests in both the academic and business sides of R&D, he supplemented his business knowledge at the University of Chicago Booth School of Business. In 2013, Pongsakorn joined the Electrical and Computer Engineering department at the University of Toronto as a postdoctoral fellow to work on solar cells, light emitting diodes, and other optoelectronic devices from solution-processed semiconductors.

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Chang Kook Hong

Prof. Dr Chang Kook Hong completed his Ph.D. in Polymer Engineering from The University of Akron (USA) in 2001. Afterwards he joined the University of Delaware and The University of Southern Mississippi as a Postdoctoral Fellow during 2001–2004. He worked for Samsung Electronics until 2007. Currently he is a Professor at the School of Chemical Engineering. He is also working as a Vice-Dean of the Faculty of Science at Chonnam National University. His main research focuses on energy devices, such as perovskite solar cells and secondary batteries, using polymeric materials and polymer nanocomposites for various applications.

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J. H. Kim

Prof. Jin Hyeok Kim received his Ph.D. in 1996. He is currently a professor in the Department of Materials Science and Engineering, Chonnam National University, South Korea. He has been continuously engaged in this research field for more than 20 years. His research interests include the fabrication and characterization of compound thin film solar cells, especially Cu2ZnSn(S,Se)4 thin films, using various methods, such as sputtering, electro-deposition, sol–gel, and nanoparticles. He is also trying to study the synthesis of thin films of metal-chalcogenides and metal-oxides by physical, chemical, and electrochemical methods and their applications in TCO, gas sensors, energy storage devices, etc.

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Pramod S. Patil

Prof. (Dr) Pramod Shankararao Patil is currently working as a Professor in the Department of Physics, and is the Coordinator of the School of Nanoscience and Technology, Shivaji University, Kolhapur. Also, he is the former Coordinator of the Department of Technology and Energy Technology, Shivaji University, Kolhapur. He is working as a Faculty Coordinator of Science. He was the recipient of the best teacher award in 2015. Prof. Patil received his Ph.D. degree from Shivaji University, Kolhapur, India in 1990. His research interest is in solar cells, gas sensors, electrochromism and supercapacitors.


1. Introduction

Solar energy is an abundant renewable energy resource to meet the global energy demand. The solar cell is the most proficient way to convert photon energy into electrical energy. In first generation solar cells, single crystalline Si materials are often used to fabricate p–n junction-based photovoltaic devices.1 Nowadays, single crystalline Si solar cells have reached an efficiency of 24.7%.2 However, expensive techniques are required for the fabrication of single crystalline Si-based solar cells. Also, Si has a low absorption coefficient (100 cm−1), which means that several hundred microns of thickness are required to absorb a sufficient amount of incident light.3 Currently, single crystalline Si solar cells are the most dominant solar cell due to their high efficiency, but this technology also needs expensive equipment and installation technology, which hampers the idea of low cost production.4,5 However, the efficient establishment of solar cell technology on a global scale requires development of both materials and devices, not only to reduce the fabrication cost, but also to increase efficiency. As an alternative to the silicon solar cell, second generation thin film solar cells fabricated from heterojunctions of two different semiconductors have drawn significant attention from the scientific community to achieve better efficiency/low cost ratio because of their facile adaptability to large scale manufacturing processes. They require less film thickness due to their high optical absorption coefficient (>104 cm−1). Thin film solar cells are photovoltaic cells with p–n heterojunctions (or p–i–n or n–i–p) of two materials with dissimilar electron properties.6,7 For the fabrication of a thin film solar cell, a semiconductor thin absorbing layer is deposited to fulfill the aim of low cost fabrication. Typical examples of thin film solar cells are single p–n junction solar cells, which have been based on GaAs,8,9 CdTe,10,11 CIGS,12,13 or CZTS,14,15 and p–i–n junctions based on a-Si[thin space (1/6-em)]:[thin space (1/6-em)]H,16,17 or C–Si.18,19 The main challenges in thin film solar cells are the complex deposition process, the difficulty in controlling the stoichiometry and the presence of structural defects.6,7 The emergence of third generation solar cell technologies, including dye-sensitized solar cells (DSSCs),20,21 quantum dot based solar cells (QDSSCs),22,23 organic photovoltaics (OPVs)24,25 and perovskite solar cells (PSCs), has fulfilled the requirements of low-cost simple fabrication.26,27 The main challenge for the scientific community is to enhance the performance and stability with low-cost fabrication and installation technology.6 Quantum dot solar cells have reached a highest efficiency of only up to 13.43% and they suffer from poor stability issues.28 In addition, OPVs also face the same problems.9 In respect of this, DSSCs have attracted much attention as they offer simple device design and low cost fabrication processes, and enable large-scale production. The progress of DSSCs has been very slow, but is experiencing positive growth in terms of cost, eco-friendliness with long-term stability and a record efficiency of 14.3% reported in early 2015.29 Overall, the main objective of DSSC research is to find cost-effective and eco-friendly technologies for achieving high efficiency and stability of the DSSC device. A number of prominent research groups have been established around the world and nearly 16[thin space (1/6-em)]000 articles have been published in academic journals (Fig. 1) in the last 10 years.30 According to the IDTechEX company projection report, the market for DSSCs will grow to over $130 million by 2023 for use in automotives, portable electronic devices, wireless sensors and mobile devices.31
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Fig. 1 Research articles published based on different aspects of DSSC research.30 Data source: ISI Web of Knowledge.

The main purpose of this review is to give detailed information on the state-of-the-art and photo-physics to understand the reasons behind the improved performance and new developments in the field. Also, illustration of recent advances in key parts of DSSCs, e.g. the photoanode, counter electrode and dye, has been done by classifying them into three categories, namely metal oxides, oxide perovskites and carbon-based materials. This study will provide a roadmap for researchers towards optimization of DSSCs using advanced material engineering.

1.2 Literature survey and state-of-the-art of DSSCs

After some initial research and development, Grätzel et al. developed a DSSC device based on dye-sensitized nanocrystalline titania in 1991.32 During the last three decades, scientists have devoted numerous efforts to the development of DSSCs in terms of four important components: the sensitizer dye, photoanode, electrolyte and counter electrode. In 1991, the first successful DSSC device was fabricated by using the trimeric ruthenium complex RuL2((μ-CN)Ru(CN)L2)2 (L = 2,2′-bipyridine-4,4′-dicarboxylic acid; L′ = 2,2′-bipyridine) as a dye, 15 nm nanoparticulate TiO2 (3D interconnected nanoparticles deposited as a 10 μm thick film on FTO) as a photoanode, the I/I2 redox couple in an electrolyte with acetonitrile solvent, and bare FTO as a CE, and the device showed a power conversion efficiency (PCE) of 7.12% under the AM1.5 spectral distribution.32 The initial development of the device was done by the use of different ruthenium-based dyes (Ru-dyes). In 1993, a device with a PCE of 10% was fabricated using an N3 dye [RuL2(NCS)2·2H2O (L = 2,2′-bipyridyl-4,4′-dicarboxylic acid)], a nanocrystalline TiO2 film-based photoanode, an iodine-based electrolyte (I/I2 redox couple) and 2 μm-thick platinum (Pt) coated on FTO as the CE. The TiO2 films (8–12 μm thickness) with a high surface area were prepared by the sintering of colloidal titania particles (size of particles ∼15–30 nm) on conducting glass substrates. The N3 dye was loaded onto the TiO2 surface. Furthermore, 4-tert-butylpyridine was coated onto the dye-loaded TiO2 surface to suppress the dark current.33 The electrolytes have been classified into three categories: (i) liquid, (ii) solid and (iii) quasi-solid state.34–36 Quasi-solid state electrolytes are generally synthesized from a polymer and an ionic liquid.37 This quasi-solid state overcomes the problems of liquid electrolytes, such as leakage and volatilization of the electrolyte, corrosion of the CE, and photodegradation of the dye, which can cause a serious decrease in the performance of the device.37–39 The low ionic conductivity, high production cost and less mass diffusion ability of solid state electrolytes have encouraged researchers to find suitable quasi-solid state electrolytes.37–39 In contrast to solid state electrolytes, quasi-solid state electrolytes have the ability to diffuse within nanoporous structures, long-term stability, simple processing techniques and a cost-effective nature.37–39 In 1995, the first quasi-solid state electrolyte-based DSSC device was reported in an attempt to achieve improved stability of the device compared to liquid-based electrolyte devices.40 The cell configuration was ITO/TiO2 (Degussa P-25)/4-4′(dcb)2Ru(SCN)2/polymer electrolyte (mixture of polyacrylonitrile, ethylene carbonate, acetonitrile, propylene carbonate and NaI)/ITO. This device showed improved performance and stability compared to liquid electrolyte-based devices. Since 1997, attempts have been made to fabricate photoanodes other than nanocrystalline TiO2 materials in Ru-based DSSCs. In respect of this, Rensmo et al. reported ZnO (15 nm nanostructures) and Al-doped ZnO crystallite (150 nm) films as photoanodes for the fabrication of cis-bis(4,4′-dicarboxy-2,2′-bipyridine)-bis-(isothiocyanato)-ruthenium(II)-based DSSCs and showed an efficiency of 2 and 0.5%, respectively.41 In 1998, Gratzel et al. introduced 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenyl-amine)9,9′-spirobifluorene (OMeTAD) as a hole conducting material for solid state device fabrication. The device configuration was FTO/4.2 μm thick mesoporous TiO2/Ru(II)L2(SCN)2 (where L is 4,4′-dicarboxy-2,2′-bipyridyl)/OMeTAD/Au and showed a PCE of 0.74%.42 In 2001, a new series of panchromatic ruthenium(II) sensitizers, named as black dyes, was developed from carboxylated terpyridyl complexes of tris-thiocyanato Ru(II) and successfully used for efficient DSSC fabrication, and a PCE of 10.4% was achieved with an iodide-based liquid electrolyte.43 In 2008, Grätzel et al. reported two new polypyridyl ruthenium complexes (coded C101 and C102) with high molar extinction coefficients to enhance the optical absorptivity of mesoporous TiO2 films and showed a PCE of 11.0–11.3% by using cheno as a co-adsorbent and an iodine-based electrolyte.44 Yu et al. reported a record PCE of 12.1% for an Ru-based dye by using C106 dye in conjunction with a Li ion-based electrolyte (EL02).45 Here, a double-layer TiO2 film was used as a photoanode. The first transparent layer of 15 μm of TiO2 particles was deposited on FTO, and subsequently a 6 μm thick second layer of TiO2 particles (the scattering layer) was deposited. This device showed an ∼90% IPCE in the 450 to 700 nm wavelength range. Organic dyes emerged to replace toxic Ru-based dyes and enable low-cost DSSC fabrication in 2003. Uchida et al. introduced a novel indoline dye as an organic and environmentally friendly dye for DSSCs achieving a PCE of 6.1%.46 The photoanode was fabricated from TiO2 nanoparticles and Pt-coated titanium foil was used as a CE. In 2006, a novel organic dye comprising electron donor, electron-conducting, and anchoring groups (JK-1 and JK-2) was reported to suppress the aggregation of organic-based dyes and improve their stability. Here, a 2 μm TiO2 nanoparticle film was used as a photoanode, a Pt electrode was used as a CE and an IPCE of 91% with a PCE of 8.01% was achieved under standard AM 1.5 solar radiation.47 Ito et al. reported a thickness effect of TiO2 films on organic D149-based DSSCs, showing an efficiency of 6.67 and 9.03% for a 6.3 and 12.6 μm film thickness, respectively.20 In 1993, the first report on a porphyrin-based dye for DSSCs was reported with a PCE of 2.6% with a 12 μm TiO2 film sensitized by copper mesoporphyrin.48 The efficiency of porphyrin was further enhanced from 3–13% with respect to an increase of IPCE over period 2011–2014.49,50 In 2014, a record efficiency of 13% for a DSSC device was achieved through the molecular engineering of porphyrin sensitizers, coated SM315, which have the properties of a donor-π-bridge-acceptor, and excellent light harvesting properties with better electrolyte compatibility.49 Here, a 7 μm mesoporous TiO2 film as a photoanode, and an electrolyte containing the [Co(bpy)3]2+/3+ redox couple were employed in device fabrication. In 2010, silyl anchor-based dyes emerged and showed high performance in terms of stability and PCE.51 In 2014, DSSCs fabricated by using a novel alkoxysilyl carbazole (metal free dye) as a dye and a Co3+/2+-complex redox-based electrolyte exhibited PCEs of over 12% with a high Voc (higher than 1 V) by the use of a hierarchical multi-capping treated TiO2 photoanode.52 In 2015, Hanaya et al. reported a record PCE for a DSSC device of 14.3% by using co-photosensitization with an alkoxysilyl anchor dye (ADEK-1) and a LEG4 organic dye, a TiO2 photoanode, an electrolyte containing the [Co(phen)3]3+/2+ redox couple and FTO/Au/graphene as a CE. This device achieved a high PCE since its maximum IPCE value reached 91% with a Voc above 1 V [14% record].29 In addition to the dyes, the photoanode material is another key component of DSSCs. There are different nanoarchitectures of TiO2, such as nanorods,53,54 nanowires,55,56 mesoporous structures,57,58 nanosheets59,60etc., that have been reported for the fabrication of efficient DSSC devices. A photoanode with a high specific surface area and a nanoarchitecture with pronounced light-scattering capacity plays a vital role in achieving a high-efficiency DSSC, since it is required for better carrier transportation,56,57 a high density of dye loading,59,61 and the scattering of light to enhance the absorption of the sensitizer.59,61 Still, TiO2 is the dominant semiconductor in DSSC devices, although different semiconductors have emerged, such as SnO2,62,63 ZnO,64–70 Nb2O5,71–73 WO3,71,74,75 copper oxide76–83etc. A DSSC based on a hierarchical anatase TiO2 nanoarchitecture array of 18 μm in length showed a PCE of 7.34% due to its high surface area and superior light scattering capacity.84 The DSSC device fabricated by layer-by-layer assembly of hierarchical TiO2 nanowires (hyperbranched tree-like morphology-under layer),85 TiO2 hollow submicrometer-sized spheres (hierarchical rambutan-like morphology-intermediate layer) and TiO2 micrometre-sized spheres (hyperbranched urchin-like morphology-top layer) exhibited a boosted PCE of up to 11.01%.85 In a DSSC, the counter electrode has an important role in the performance since it acts as a catalyst for the redox couple reaction (regeneration) and accepts an electron from the external circuit.86,87 The CE should possess two properties: high catalytic activity88 and electronic conductivity.89–91 In the solar cell field, each component of DSSCs has achieved great progress and is under development55,86,92,93 There are various catalysts, such as Pt,88,94 carbon materials,95–97 composites,98–100 and inorganic materials64,101–103etc., that have been used as a CE. Graphene has been extensively utilized in DSSCs owing to its excellent optoelectronic104 and electrocatalytic characteristics;97,105 therefore, it is applicable to fabricate photoanodes as well as CEs.106,107 In addition, its mechanical flexibility makes it applicable to build flexible devices by use in both types of electrodes.106,107Graphene-TiO2 nanocomposite photoelectrode-based DSSC devices show that graphene can effectively enhance the open circuit voltage, short-circuit current density, and PCE (7.02 ± 0.06%) as compared to the pure TiO2 DSSC device.108 The DSSC device fabricated with the use of graphene in both electrodes (photoanode and CE) having the configuration 0.08%GR-TiO2/N3/GR showed a PCE of 7.70%, which is higher than that of the TiO2/N3/Pt-based device (7.28%).109 In 2016, Hu et al. reported a novel method to synthesize a graphene electrode by utilizing a greenhouse gas (CO2) for efficient DSSC device fabrication. In this approach, CO2 was directly converted into 3D crapes myrtle flower-like graphene by the reaction of CO2 gas and Na. The DSSC device with this 3D graphene as a CE exhibited a high PCE of 10.1%, which is much higher than that of the DSSC with a Pt-based CE.110 A survey of the IPCE spectra of different types of dye-based DSSCs and the year-wise progress in DSSCs are illustrated in Fig. 2.29,33,45,47–50,52,111–131
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Fig. 2 Survey of the IPCE spectra of different types of dye-based DSSCs and year-wise progress of DSSCs. (A) Ru-based dye: Reprinted with permission from ref. 33, 44 and 45. Copyright from American Chemical Society. (B) Silyl anchor-based dye: Reprinted with permission from ref. 52, 116 and 29. Copyright from Royal Chemical Society. (C) Porphyrin dye: Reprinted with permission from ref. 118, 129 and 130 and Copyright from American Chemical Society. Reprinted with permission from ref. 49. Copyright 2014 Nature Publishing Group. (D) Organic dye: Reprinted with permission from ref. 126, 124 and 119 and Copyright from American Chemical Society. Reprinted with permission from ref 20. Copyright 2018 Royal Chemical Society. [Note: all above IPCE spectra are extracted from the above-mentioned references with permission from the respective publishers.].

2. Dye sensitized solar cells (DSSCs)

Conventional DSSCs consist of a layer of nano-crystalline (mesoporous) TiO2 particles on a transparent conducting oxide (TCO) substrate with an adsorbed Ru-based dye as a sensitizer, a counter electrode and an electrolyte redox couple.92 The operation steps in DSSCs are: (i) excitation; dye molecules in the DSSC absorb light and undergo an electronic state change from the ground state to the excited state. (ii) The excited dye molecule gains the ability to transfer electrons to the conduction band (CB) of the semiconductor. (iii) Diffusion of electrons in the semiconductor via trapping and de-trapping. (iv) Oxidation–reduction reactions of electrolyte redox couples via electrons travelling through the outer circuit. (v) A dye regeneration step occurs due to reduction of the oxidized dye by the redox couple resulting in the conversion of the dye molecules from the excited state back to the original form and making the dye ready for photoelectron generation again. A schematic representation of the operating principal of DSSCs is shown in Fig. 3.132,133
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Fig. 3 Schematic representation of the operating principal of DSSCs showing the components used in DSSCs.

2.1 DSSC architecture

The choice of the contact materials, working electrode (photoanode), dye and electrolyte should be done according to the following points, since it is necessary to suppress the backward transfer process (recombination reaction) and increase the performance of the DSSC.134

I. Substantial charge transfer to the semiconductor must be carried out at a higher rate than the rate of decay of electrons from the excited state of the dye, that is the lowest unoccupied molecular orbital (LUMO), to the ground state, meaning the highest occupied molecular orbital (HOMO).132,133

II. The LUMO of the dye must be more negative (higher) than the CB of the semiconductor. However, the CB of the semiconductor is required to be more negative (higher) than that of the conducting glass (FTO/ITO/MO).132,133

III. The HOMO of the dye should be more positive than the redox potential of the electrolyte.132,133

2.2 Operational principle of DSSCs

A schematic diagram of a DSSC is shown in Fig. 3. The dye is used as a light absorber in DSSCs. The dye (D) absorbs visible light and is converted into an excited state (D*). During this process, excitation of electrons takes place from the HOMO in the ground state to the LUMO in the excited state of the dye.132,135–137 Hence, oxidization of the dye takes place (D+). The excited electron (photoelectron) is injected from the LUMO state of the dye into the CB of mesoporous TiO2 (eqn (2)). The oxidized dye is regenerated by accepting electrons from the electrolyte redox couple (I/I3−) (eqn (3)).132,135–137 The oxidized redox couple in the electrolyte accepts electrons from the counter electrode to gain the normal state of the redox couple of the electrolyte (eqn (4)). However, during the dye regeneration process, some of the photoelectrons in the CB of TiO2 may recombine with the oxidized dye molecules and electrolyte redox couple, resulting in the loss of photoelectrons, as represented in eqn (5) and (6), respectively. Hence, the rates of the regeneration of the dye and the recombination reaction are important factors in DSSCs, which we will further discuss in the photo-physics section. Successfully, some injected photoelectrons in the CB of TiO2 transfer to the FTO (eqn (7)), which results in the collection of photoelectrons in the FTO (current collector) yielding a photocurrent. However, some loss of photoelectrons takes place due to recombination of photoelectrons in FTO with oxidized dye molecules and the electrolyte redox couple, as represented in eqn (8) and (9), respectively. Also, loss of photoelectrons takes place during oxidization and reduction of the electrolyte redox couple (eqn (10)).

2.3 Photo-physics of DSSCs

To achieve high efficiency in DSSCs, an efficient electron transportation process is essential. Understanding of the electron transfer rates in DSSCs is crucial because the rivalry between the rates of excited-state decay, electron injection via an interface, and recombination governs the quantum yield of electrons transferred into the metal oxide. Researchers have used different techniques to determine these rates, such as photoluminescence and transient visible absorbance measurements.
2.3.1 Charge separation. In conventional p–n junction and photo-electrochemical cell (PEC) solar cells, photogenerated charges are separated by the electric field in the space charge region, which is developed across the junction. In DSSCs, the space charge region is absent due to the particle size of the semiconductor being less than the Debye length LD (lp < LD); the small size of the particles does not support space charge region formation. In DSSCs, charge separation is induced by an electric field, which has developed from the formation of a Helmholtz double layer at the interface between the dye and semiconductor by a diffusion process.138 The Helmholtz layer is formed by cations of the dye and electrolyte, and protons (dye cations and protons formed by dissociation of the acidic dye) (Fig. 4a). These separated charges (electron-holes) are injected and transported in the ETM, electrolyte and CE, respectively. Determination of the kinetics, such as the diffusion coefficient and the lifetime of the different carriers before they reach their respective contacts, is crucial since it strongly influences the performance of solar cells.139 In TiO2 (semiconductor), charge transportation can take place by migration and diffusion. However, due to the nanosize of TiO2, there is an absence of a significant potential gradient, which results in only diffusion electron transportation characteristics. The electrolyte component I3 migrates towards the CE and I migrates towards the TiO2 during redox reactions (Fig. 4b).140Fig. 4c–e show schematic level diagrams of the FTO/TiO2 interface and electrolyte/CE under different conditions: dark equilibrium, under illumination and an applied bias lower than the Voc, and under illumination under open-circuit conditions, respectively. Under dark equilibrium, the Fermi level is equal to the redox energy of the electrolyte: EeqFERedox. The Fermi level inside the TiO2 nanoarchitecture shifts from the equilibrium value with a respective applied external load; ΔVin = (EFERedox)/q. This potential difference reaches its maximum value (ΔVin = Voc.) when there is no overvoltage at the CE (no external load).141
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Fig. 4 (a) Schematic representation of the charge distribution near the photoanode surface before illumination and after illumination. Reprinted with permission from ref. 138. Copyright 2000 American Chemical Society. (b) Schematic of carrier transportation at the photoanode/electrolyte interface. Reprinted with permission from ref. 140. Copyright 2003 American Chemical Society. Schematic representations of the energy levels of TCO/TiO2 and electrolyte/platinized TCO interfaces under different operating conditions: DSSC device (c) in dark conditions, (d) under illumination but at lower applied bias (Vin < Voc) and (e) under open-circuit conditions while under illumination. (c–e) Reprinted with permission from ref. 141. Copyright 2011 Elsevier.
2.3.2 Details of the energy level of the semiconductor, electrolyte and dye. The energy level diagram of the semiconductor, redox couple, dye and CE in a DSSC device is shown in Fig. 5a.21,40,142 This figure represents a scheme of electron exchange among the band gap of TiO2, the dye and the redox couple into the electrolyte. Below the CB, a number of trap states are present. A schematic of the charge distribution at the FTO/TiO2 interface is shown in Fig. 5b.143 Band bending at the FTO/TiO2 interface in contact with the electrolyte is represented in Fig. 5c.143 Some unwanted recombination reactions occur at the TiO2 electrolyte interface due to the presence of trap states in TiO2. Fig. 5d shows the mechanism of electron–hole recombination and a representation of the dye–TiO2 interface.144 The electrochemical potential of a semiconductor is nothing but the Fermi level (EF) and in an electrolyte, it is represented as the redox potential Uredox. EF in non-degenerate semiconductors, which depends on the energy of the CB edge (Ec) and density of CB electrons, is given by:142,145
 
image file: c7nr08350e-t1.tif(11)
where KBT is the thermal energy and Nc is the effective density of CB states. Under equilibrium conditions, the electrochemical potential or Fermi energy of the semiconductor (EF) and electrolyte will be equal. The relation between the energy (Eredox) and the potential (Uredox) of any redox couple can be estimated as:142
 
EF,redox [eV] = −(4.6 ± 0.1) − eUredox [V](12)
where e is the elementary charge and V is volts (unit).

image file: c7nr08350e-f5.tif
Fig. 5 (a) Energy level diagram representing the electron transfer kinetics in DSSCs.21,40,142 Schematics of (b) charge distribution at the FTO/TiO2 interface and (c) band bending at the FTO/TiO2 interface in contact with the electrolyte. (b and c) Reprinted with permission from ref. 143. Copyright 2003 American Chemical Society. (d) Mechanism of electron–hole recombination and representation of the dye–TiO2 interface. Reprinted with permission from ref. 144. Copyright 2002 American Chemical Society.

The oxidized state of the dye (excited state) has a higher negative energy state than the CB of TiO2. Hence, these trap states may trap the electrons from the CB of TiO2 or from the injection of photoelectrons from the excited dye.138

2.3.3 Rate of electron injection. The LUMO level of the dye molecules is more negative than that of MO (ETM). The difference between ELUMO (dye)ECB (MO) provides an enthalpic driving force for electron injection into the CB of MO.142,146 Similarly, the HOMO of the dye is present below the energy level of the electrolyte. This difference in energy will provide a driving force for hole transportation and injection into the electrolyte. In short, upon light illumination, the dye gets into an excited state by transporting an electron from the HOMO to the LUMO level of the dye by lifting a hole in the HOMO level.142,146 Electrons in the excited state of the dye are transported to the CB within ∼10−12 s. However, there is also the possibility of a few electrons in the LUMO of the dye further recombining with holes in the HOMO level of the dye within ∼10−8 s.145 This process is known as recombination. A high photocurrent occurs only when electron transportation is faster than the recombination process. Electron injection within 100 fs has been observed for Ru-based complexes, and ∼100 ps for organic dyes. This indicates that the electron injection process depends on the nature of the dye.147 A minimum injection quantum yield depends on competition between electron injection and sensitizer excited-state decay, which can be deduced by the following relation:142,146,148
 
image file: c7nr08350e-t2.tif(13)

The injection of electrons occurs in picoseconds (ps) and the yield depends on the TiO2 surface conditions, the electrolyte species and the concentration of cations in the electrolyte. Then, the injected electrons are transferred to the TCO. However, some electrons recombine with the oxidized dye (dye cation) or I3− in the electrolyte.

2.3.4 Electron transportation (diffusion of electrons). Under light irradiation, charge transportation occurs at the dye/TiO2 interface through electrons in the dye being injected into the CB of the TiO2 and holes in the dye are occupied by the redox couple. The electrons in the TiO2 electrode are surrounded by cations of dye which results into very low electric field gradient for electron transportation. Hence, transportation of the electrons in the TiO2 occurs by diffusion and these phenomena are deduced by illustrating the diffusion coefficient and diffusion length.138,149 The TiO2 (ETM) nanostructures significantly affect the efficiency, since they provide a high surface area for dye loading, an electron transportation path and high absorption due to the scattering of light within the nanostructures (Mie-scattering). The efficiency of a DSSC depends on the diffusion rate;45,150–153 the recombination rate139,154,155 and the mobility of electrons96,156,157 in the ETM. Therefore, there is a need to investigate these parameters and their influences on the electronic properties. Intensity modulated photocurrent spectroscopy (IMPS),158–161 intensity modulated photovoltage spectroscopy (IMVS),158–161 electrochemical impedance spectroscopy (EIS),100,162,163 small amplitude time transients, current–voltage measurements etc.36,159,164 are usually employed for the determination of the above parameters.

For photoelectrons to collect at the TCO (herein FTO or ITO), they must travel a certain distance in the ETM before charge recombination occurs and this length is known as the diffusion length. The diffusion length depends on the charge collection efficiency, and it can be estimated from the diffusion coefficient and electron lifetime τn using the following relation:36,142

 
image file: c7nr08350e-t3.tif(14)

The diffusion coefficient can be measured using the IMPS technique. This technique is useful for the determination of the time constant for the photocurrent response tpc, which depends on the electron transport time (ttr) and the electron lifetime (tn) according to the following relation:36,142,146

 
(τpc)−1 = (τtr)−1 + (τn)−1.(15)

The electron diffusion coefficient can be estimated by fitting a decay of the current transient via the following relation:36,142,146

 
D = w2/(2.35ttr).(16)

2.3.5 Recombination rate. Recombination is the kinetics that adversely affects the photocurrent and photovoltage through loss of photoelectrons by recombination or trapping with holes.139,154,156,165 In semiconductors, a number of trapping levels are present below the CB.166,167 Some injected electrons from the dye are trapped by these energy levels, which results in the loss of photoelectrons.154,168 It is also possible that a few injected electrons recombine with holes into the HOMO level, meaning that the dye species are oxidized before the dye can be regenerated.169,170 To achieve a high photocurrent, the re-reduction of the oxidized dye by redox species in the electrolyte must be faster than the recombination reactions.169,170
2.3.6 Recombination through trap sites of the semiconductor. Nanocrystalline wide-band gap n-type semiconductors (TiO2,143,171 ZnO,172–174 SnO262,175,176etc.) are used in DSSCs as an ETM. These contain a high density of intra-band defect/surface trap states below the CB edge.132,166,167 The trapping/de-trapping of CB electrons from these trapping states strongly influences the charge transport kinetics, such as the electron collection efficiency,177 recombination kinetics178,179 and dye regeneration process.180 Recombination of electron/holes occurs at the electrode–electrolyte interface through surface trapping sites.181,182 Accumulation of electrons takes place at both the CB and trap states due the negative potential applied to the cell.183 Subsequently, optical excitation of the dye initiates electron injection (kinj) into the CB of the TiO2. The injected electrons are further thermalized into trapping states and subsequently they transfer to the CB of TiO2 through a de-trapping process for the collection of the electrons at the current collector or to the oxidized dye (recombination process Kcr) depending on the external applied potential. Hence the rate of reduction of the dye through the electrolyte redox couple must be faster than Kcr to achieve a high photocurrent quantum yield.183
2.3.7 Role of electrolytes in charge recombination. The recombination of injected electrons can take place by transfer to I3 in the electrolyte. This type of recombination depends on the concentration of the electrolyte and the nature of the cation species. An increased concentration of the electrolyte and short-circuit conditions lead to predominant recombination. The rate of recombination with I3 is represented by the following relation:184
 
Rate = kR1[e]α[I3]β.(17)

Here, α and β are constants that are nearly equal to 1. Under the same concentration of electrons, the influence of the electrolyte affects either kR1 or [I3].

Nakade et al. reported the current–voltage characteristics, electron lifetime and electron diffusion coefficient measurements in order to understand the effects of the constituents in the electrolyte on the charge recombination kinetics.184 They performed a comparative study of electrolytes containing different cations, such as Li+, tetra-n-butylammonium (TBA+) and, 1,2-dimethyl-3-propy;omidazolium (DMPIm+), with and without 4-tert-butylpyridine (tBP), and with various concentrations of the I/I3 redox couple. From the experimental results, it is noted that the Voc decreased in the order of TBA+ > DMPIm+ > Li+ due to the positive shift of the TiO2 CB potential due to the surface adsorption of the cations. The composition and concentration of cations influences phenomena such as the electron injection yield, Voc, the electron diffusion coefficient (Dn) and the timescale of dye regeneration.34 Furthermore, the addition of tBP additives into the electrolyte leads to an increase in the distance between the TiO2 and I3, resulting in a longer electron lifetime.155,184 The recombination reaction between electrons and the electrolyte is retarded by tBP additives and the bulk part of the dye. A schematic diagram of the retardation of the recombination reaction is shown in Fig. 6a.155 Injected electrons in the MO can transfer to the back contact FTO or transfer to the oxidized dye (recombination). Such recombination can be overcome by fast neutralization of the oxidized dye by the redox electrolyte (e.g. I).34,155 Compact high band ceramic materials such as HfO2,178,185,186 Al2O3,185,187 SiO2,185,188,189 MgO190,191 and ZrO2[thin space (1/6-em)]192 have been employed to passivate the surface to control the recombination reactions, by acting as a blocking layer. The use of TiCl4 treatment is also an efficient method to passivate the TiO2 surface.170,192 Li et al. reported an improved efficiency of greater than 7% by the use of a HfO2 blocking layer. They showed the deposition effect of the blocking layer through the atomic layer deposition (ALD) technique on the ITO and on the TiO2 surface.170 According to this report, the recombination of electrons by the dye and the electrolyte redox reaction can be successfully suppressed by using an HfO2 blocking layer. Fig. 6b–d show a schematic representation of three types of fabricated photoanode: (b) TiO2 nanoparticles on ITO; (c) a TCO blocking layer consisting of HfO2 deposited on ITO; and (d) a TiO2 blocking layer consisting of HfO2 deposited on TiO2 nanoparticles. The bottom panel shows the band diagram for each photoanode, indicating possible electron recombination processes, such as (1) TiO2/electrolyte; (2) TiO2/dye; and (3) TCO/electrolyte.170


image file: c7nr08350e-f6.tif
Fig. 6 (a) Schematic diagrams of the retardation of the recombination process in DSSCs by adding bulky groups. Reprinted with permission from ref. 155. Copyright 2015 American Chemical Society. Schematic of the three types of fabricated photoanodes (top panel) for control of the recombination reaction: (b) control consisting of TiO2 nanoparticles on ITO, (c) a TCO blocking layer consisting of HfO2 deposited on ITO, and (d) a TiO2 blocking layer consisting of HfO2 deposited on TiO2 nanoparticles. Bottom panel: The band diagram for each photoanode indicating possible electron recombination processes, such as (1) TiO2/electrolyte, (2) TiO2/dye, and (3) ITO/electrolyte. (b–d) Reprinted with permission from ref. 170. Copyright 2014 American Chemical Society.
2.3.8 Dye regeneration processes. The device performance also depends on the rate of dye regeneration and the conversion of the absorption of incident light to a photocurrent.169,193 The dye regeneration process takes place through reducing the oxidized dye by accepting electrons from the redox electrolyte (I/I3).169,180 A slow dye regeneration process leads to recombination processes, causing adverse effects on the efficiency and stability of the device.194,195 This leads to an increased lifetime of the unstable excited or oxidized state.195 The dye regeneration mechanism and kinetic dynamics are usually investigated by using flash photophysics techniques coupled to transient absorption (TA) spectroscopic measurements.196,197 Recently, femtosecond to nanosecond TA spectroscopic measurements were used to study the dye regeneration kinetics.198,199 The dye degeneration process can occur through two possible mechanisms: (i) a single iodide process (SIP) and (ii) a double iodide process (TIP).142,169,199

Single iodide process:

 
[D+ + I] + I → [D0 + I˙] + I (18)
 
[D0 + I˙] + I → D0 + I2˙ (19)
 
2I2˙ → I + I3 (20)

Double iodide process:

 
[D+ + I] + I → [D+⋯I⋯I] (21)
 
[D+⋯I⋯I] → D0 + I2˙ (22)
 
2I2˙ → I + I3 (23)
where D+ is the oxidized dye and I˙ is a radical.

The regeneration process of the dye is seriously affected by the undesirable recombination of e with D+, termed as electron dye recombination (EDR).

 
D+ + eD(24)

The overall efficiency of the dye regeneration process can be expressed by the following relation:

 
D+ + eD(25)
 
image file: c7nr08350e-t4.tif(26)
 
kobs,D+ = krg[I] + kedrntotχ(27)
where kobs,D+ is the observed pseudo-first order rate constant for the decay of D+, krg is the rate constant of dye regeneration by I, and kedr is the rate constant for electron dye recombination. The dye regeneration reaction takes place via an electron transfer between an oxidized dye molecule and an iodide species in the electrolyte. Therefore, the rate constant of regeneration, krg, depends on the interaction between the dye component and the species in the electrolyte. Hence, the dye must be engineered accordingly to get a large extent of interaction at the dye–electrolyte interface. In 2017, Berlinguette et al. reported X-ray absorption spectroscopy studies that confirmed the halogen bonding at the dye–electrolyte interface.200 Recent nanosecond spectroscopic studies revealed that krg increases with increasing halogen constituent in the dye and eventually it increases with the size of the halogen, for example, Dye-F*+ < Dye-Br*+ < Dye-I*+. The results showed that the values of krg and Voc increased with respect to the size of the halogen atom in the dye–halogen couple. Fig. 7a shows a summary of the dye–halogen series, relevant redox couples and rate constants for electrolyte charge recombination (KCR), dye regeneration (krg) and back electron transfer.200


image file: c7nr08350e-f7.tif
Fig. 7 (a) A schematic of the energy level diagram attributed to the dye-halogen series, and the relevant redox couples and rate constants for electrolyte charge recombination, dye regeneration and back electron transfer in the respective processes. Reprinted with permission from ref. 200. Copyright 2017 Nature Publishing group. (b) Electron dynamics and lifetime of the electron transportation process in DSSCs. Reprinted with permission from ref. 92. Copyright 2009 American Chemical Society. (c) TA for transient absorption measurement of a conventional cell at open circuit voltage. (d) JV characteristics of a regular cell compared with the JV characteristics predicted from the sum of the short-circuit current and dark current. (c and d) Reprinted with permission from ref. 153. Copyright 2013 American Chemical Society.
2.3.9 Electron dynamics and lifetime in ETMs. In the era of DSSCs, electron kinetics is very important to get good performance; for example, the electron injection time is ≈10−11 to 10−13 s, the dye regeneration time is ≈10−11 to 10−9 s and the electron transportation time is ≈10−4 to 10−2 s, whereas the interfacial recombination time is ≈10−4 to 10−2 s.92 The electron lifetime in the conduction band of TiO2 after electron injection from the dye and electron diffusion through the TiO2 film is an important factor to get good performance DSSCs.147,148 The kinetics and time of reaction are given in Table 192,154 and a schematic representation is shown in Fig. 7b.92
Table 1 Charge transportation reactions and their time scales92,154
No Photo-physics parameters Reactions Time scale (s)
1 Photon excitation S/TiO2 + → S*/TiO2 TiO2
2 Electron injection S*/TiO2 + → S+/TiO2 + e/TiO2 10−12 s
3 Dye regeneration S*/TiO2 + RE → S/TiO2 + OX 10−4 s
4 Electrolyte regeneration OX + e/CE → RE 10−6 s
5 Recombination by S* dark reaction e/TiO2 + S+/TiO2 → S/TiO2 10−4 s
6 Recombination by OX, dark Reaction e/TiO2 + OX → RE 10−2 s
7 Electron transportation e/TiO2 + FTO → e/FTO + TiO2 10−3 s
8 Back reaction from FTO to dye e/FTO + S+/DFTO → S/FTO
9 Back reaction from FTO to electrolyte e/FTO + OX → RE
10 Electrolyte reduction reaction OX + e → RE 10−5 s


2.3.10 Determination of the dye regeneration efficiency. Transition absorption (TA) measurements have been used to determine the rate of the electron dye recombination process in a redox-inactive electrolyte solution.136,201 Li et al. reported TA measurements at the maximum power point (MPP) of the current–voltage (JV) characteristics.153 They used a combination of measurements such as TA, differential incident photon-to-current conversion efficiency measurements, and impedance spectroscopy to calculate the electron-dye recombination rate and the overall sensitizer regeneration efficiency. Fig. 7c shows the measured TA for a conventional cell (solution containing redox I) at the open-circuit photovoltage.153 The TA decay of an inert cell (without I redox in the solution) measured without background illumination is shown for comparison. The TA decay for an inert cell (without I redox) is usually used to calculate the EDR rate constant and the dye regeneration efficiency using eqn (1) and (2). However, this method is not strictly applicable to regular cells for a number of reasons. The D+ decay is clearly much faster in the regular cells containing I, which suggests efficient dye regeneration. Fig. 7d shows the measured JV characteristics for a regular cell and compares them with the JV characteristic predicted from the sum of the short-circuit photocurrent and with ideal diode characteristics (dark current). This predicted JV curve represents possible JV curves when the dye regeneration process is 100% efficient and no additional recombination occurs under light illumination. All drops in the current and dye regeneration efficiency are attributed to dye regeneration losses.153
2.3.11 Role of nanoarchitectures in the photoanode. In DSSCs, mesoporous structures and nanoarchitectures of transparent oxide semiconductors (MOs) are used as the photoanode.160,202 MOs provide a large surface area for the adsorption of the dye. Also, they accept electrons in the conduction band from the excited dye and transport them in the external circuit to produce an electric current. An ideal photoanode should: (i) possess a high surface area, (ii) be transparent to visible light, (iii) possess a CB of the MO below the LUMO of the dye, (iv) have a high electron mobility, (v) be inert to the electrolyte and (vi) have the ability to chemisorb the dye on the surface via hydroxyl group defects which are attached to the MO.36 Usually, n-type wide band gap semiconductors such as TiO2,37 ZnO,38 and SnO2[thin space (1/6-em)]39 are used as the photoanode. A number of reports are present on the effect of different nanostructures of MO, such as mesoporous structures,55,77,92 nanorods,54,95,203,204 nanowires,84,205,206 nanotubes,207–210 and different hierarchical nanostructures,207,211–213 on the performance of DSSCs. Electrodeposition,214 hydrothermal,214 spray,214 chemical bath deposition,215,216 laser pulse deposition,217,218 sol–gel,219–221etc. deposition techniques have been used to tune the morphology. An improvement in the performance of DSSCs can be achieved by greater dye loading,222 higher light scattering ability,58,223–225 a faster charge transportation process,226 and a longer electron life time.227 The enhancement of dye loading can be achieved by increasing the thickness of the films and the surface area of the MO. However, the maximum allowed thickness of the film is restricted by the electron diffusion length due to the existence of charge recombination. Hence, it is necessary to increase the absorption of the film without increasing the photoelectrode thickness.228–230 Light scattering effects boost the absorption of the photoanode. Scattering of light is a phenomenon related to the propagation of light in the presence of an object.230 Nanostructures with dimensions equal to the wavelength of light lead to maximum scattering.231,232 Large size particles are applicable due to their large scattering effect. Large scattering of light leads to high absorption in the IR wavelength range and this has also been proved by the Mie theory of scattering.231,232 However, a large particle size leads to a low surface area, which leads to less dye loading.233 Hence, mixtures of large size particles and nanosize particles have been used to get high efficiency DSSCs.234–236 However, the mixing of large size particles and nanosize particles results in a decrease in the total surface area of the film. On the other hand, nowadays, a large number of reports are available on the utilization of 1D nanostructures such as nanorods and nanowires of MO to get a high surface area, fast electron transportation and better light scattering capability.55,235 Hierarchical nanostructures formed by nano building blocks of nanostructures (3D) of MO have been developed to get the abovementioned properties more effectively than with 1D nanostructures.237,238 Surface modification or passivation of the MO is usually carried out by deposition of an insulating layer on the MO surface to reduce charge recombination.167,192 For TiO2, mostly TiCl4 treatment is applied.238–240Fig. 8 shows a schematic representation of electron transportation (black arrow) and scattering of light (red arrow) in (a) mesoporous, (b) 1D, (c) 3D flower-like and (d) hierarchical nanostructures of MO.
image file: c7nr08350e-f8.tif
Fig. 8 Schematic representation of electron transportation (black line) and scattering of light (red line) in (a) mesoporous, (b) 1D, (c) flower-like and (d) hierarchical nanostructures of metal oxides.
2.3.12 Mie scattering theory for application of light absorption. The light scattering phenomenon is applicable to the enhancement of light absorption in DSSCs.154,202,208 In conventional DSSCs, large size particles with a size comparable to the wavelength of the incident light are mixed with nanosize particles.232 The large size particles act as a centre for the scattering of light, whereas the nanosize particles provide a high surface area for appropriate dye loading.228,229,232 Nowadays, hierarchical nanostructures are generally used to enhance absorption through scattering with an increased surface area.85,211,241 Also, these structures provide good electron transport properties, for example nanorods242 and nanoflowers.243 Many theories are used to describe the scattering properties. Usually, Mie scattering and Rayleigh scattering are typically used to describe the scattering properties, and hence the enhancement of absorption of photoanodes with different nanostructured morphologies. For Rayleigh scattering, the particle size is less than the wavelength of light, whereas for Mie scattering, the particle size is comparable with the wavelength of light. In the Rayleigh scattering region, the intensity of scattered light (I) from a single particle is calculated using the following equation:231,232
 
image file: c7nr08350e-t5.tif(28)
where I0 is the incident intensity, r is the scatter radius (particle size), and m is the refractive index of the scattered radius, defined as m = nik (where n is the refraction of light and k is a complex term related to absorption). The criteria for Rayleigh scattering are α ≪ 1 and ≪ 1, where α = 2πr/λ and λ is the wavelength of incident light. θ is the scattering angle. The Rayleigh scattering cross section (σscat,Rayleigh) is calculated by integrating over the sphere surrounding the particle, and can be given as:231,232
 
image file: c7nr08350e-t6.tif(29)

Hence, the cross-section in Rayleigh scattering is proportional to the sixth power of the radius, whereas it is inversely proportional to fourth power of the wavelength.232 The cross-section of Mie scattering is given by the following relation:58,231,232

 
image file: c7nr08350e-t7.tif(30)
where the parameters an and bn are calculated using the Riccati-Bessel functions ψ and ξ.
 
image file: c7nr08350e-t8.tif(31)
 
image file: c7nr08350e-t9.tif(32)

According to Mie scattering theory, QScat,Mie can be given by the expression:

 
image file: c7nr08350e-t10.tif(33)

3. DSSC device fabrication

3.1 Photoanode

In DSSCs, under light expose the dye is separated into free carriers, which results in exciton generation at the dye–electrode interface.142,244 Furthermore, this is followed by charge separation via two mechanisms: (i) by transportation of photoelectrons because the CB of the metal oxide is at a lower energy than the LUMO of the dye; (ii) due to the electron density variation between the metal oxide and the dye. There are different types of materials for photoanodes that have been used in DSSCs, such as metal oxides, organic compounds, perovskite materials and carbon–metal oxide composite materials (Fig. 9).142,244
image file: c7nr08350e-f9.tif
Fig. 9 Schematic representation of different types of materials for photoanodes used in DSSCs.
3.1.1 Metal oxides. Mostly binary metal oxides have been reported as a photoanode in DSSCs.142,244 In 1972, Tributsch first demonstrated a DSSC solar cell by fabricating a chlorophyll-sensitized ZnO electrode.245 A breakthrough PCE was reported by Gratzel's group in 1991 by the use of mesoporous TiO2 nanoparticles as a photoanode.32 The first DSSC was fabricated by using a ZnO photoanode, but later on TiO2 became the most popular metal oxide for DSSCs, mainly due to its better photostability and its performance, which is better than that of other metal oxides such as ZnO and SnO2.246,247 Even though ZnO and SnO2 have a high electronic mobility, which is over 2 orders of magnitude, and ZnO shows a comparable band gap to that of TiO2, still TiO2 is a promising candidate due to its chemical stability.248 SnO2 has a high band gap energy (3.8 eV) and the CB is more positive by about 500 mV than that of TiO2. Hence, SnO2 shows a higher rate of charge injection than ZnO and TiO2.248 Despite the advantages of ZnO and SnO2, they show low PCE. A number of publications are available on the reasoning behind the differences in solar cell performance between ZnO, TiO2 and SnO2.232,246–248
3.1.2 TiO2vs. ZnO photoanode performance. TiO2 photoanodes are widely used in DSSCs due to TiO2 being a relatively cost-effective, abundant, biocompatible and nontoxic, stable n-type semiconductor material.126,146,249 TiO2 exists naturally in three crystalline polymorphs: (i) rutile, (ii) anatase and (iii) brookite. Rutile is the most common and stable polymorph of TiO2. Both rutile and anatase have a tetragonal crystal structure with a = 0.46 nm and c = 0.29 nm for rutile, and a = 0.3782 nm and c = 0.9502 nm for anatase. Brookite exhibits an orthorhombic structure with a = 0.5456 nm, b = 0.9182, and c = 0.5143 nm (Fig. 10a).250 The brookite phase is extremely difficult to obtain as compared to the anatase and rutile phases due to its metastable crystal structure (orthorhombic structure).250,251 The electronic structure of TiO2 is shown in Fig. 10b.252 The Ti ions are in a distorted octahedral environment with the Ti4+ (3d0) electronic configuration. The VB of TiO2 is formed primarily by hybridized oxygen 2p and Ti 3d orbitals, while the CB is made up from pure 3d orbital of titanium. Hence, the electrons in the VB and CB are at a different parity (correspondence); and therefore the transition probability for electrons to return back to the VB is decreased, which ultimately lessens the recombination probability. ZnO has dissimilar parity because the VB is composed of completely filled 3d orbitals, while the CB consists of hybridized s–p orbitals. Hence, ZnO also leads to a decrease in the probability of recombination.253,254 This feature of the electronic structures of TiO2 and ZnO has made them promising photoanodes as compared to other metal oxides. Study of the electron transport properties of ZnO and TiO2 photoanodes for DSSCs revealed that the same optimal thickness of ZnO and TiO2 of 5 nm showed an equal efficiency of 4.4%.167 The maximum efficiency for ZnO is attributed to the high mobility of electrons in ZnO, whereas for TiO2 it is attributed to the low recombination rate, higher dye loading, and fast electron injection. However, for both materials, increasing the thickness of the films results in losses in PCE due to a decrease in surface area and increased optical loss. The decrease in efficiency in ZnO is higher than that in TiO2 due to the low adsorption of dye on ZnO.255 In respect of this, Tiwana et al. reported a comparative study of the electron mobility and injection dynamics of mesoporous ZnO, SnO2 and TiO2 thin films sensitized by Z907 ruthenium dye.248 Using time-resolved terahertz photoconductivity measurements, they showed that TiO2 has faster electron injection from the sensitizer (a few picoseconds) than ZnO and SnO2 (10 to 100 picoseconds). The injection dynamics depend on the dye binding models and density of states available for injection, which vary from site to site. The effective mass of an electron in TiO2 (5–10 me) is higher than that in ZnO (∼0.3me) and SnO2 (∼0.3me). Therefore, the available density of states (DOS) is almost 2 orders of magnitude higher in TiO2 than ZnO and SnO2. Hence, faster electron injection takes place in TiO2 than both ZnO and SnO2. The bulk conductivities of ZnO, TiO2 and SnO2 are in the order TiO2 < ZnO < SnO2.256 In contrast to their bulk counterpart, they found that TiO2 mesoporous nanostructures have higher hall mobilities of electrons than ZnO and SnO2. Another essential factor for efficient electron injection is the requirement of significant extension of the dye wave function into the metal oxide.255,256 Němec et al. proposed that the slow injection of ZnO was attributed to the lower static dielectric constant (∼10) than TiO2 (∼100).257
image file: c7nr08350e-f10.tif
Fig. 10 (a) Crystal structures of TiO2: anatase, rutile and brookite. Reprinted with permission from ref. 250. Copyright 2010 American Chemical Society. (b) Electronic structure of TiO2. Reprinted with permission from ref. 252. Copyright 2000 American Physical Society. (c) Band gap measurement of the three crystalline forms of TiO2: anatase, rutile and brookite. Reprinted with permission from ref. 258. Copyright 2009 American Chemical Society.

3.2 TiO2 photoanodes

Mainly three crystalline forms of TiO2 are studied for DSSCS: anatase having a bandgap of 3.2 eV, rutile having a bandgap of 3.0 eV and brookite having a bandgap of 3.4 eV, as shown in Fig. 10c.258 Due to the wider bandgap of anatase TiO2, it is more chemically stable than the rutile phase. Also, it shows a higher PCE than the rutile phase because of its low packing density, which is attributed to its high surface area.259,260 The performance of a TiO2 solar cell can be improved by controlling the shapes and crystal facets of TiO2.261 In rutile TiO2, the (011) surface has a higher reactivity in catalytic reactions than other surfaces,262 whereas in anatase the (001) facet is more reactive than the (101) facet, which is more thermodynamically stable.262 Numerous morphologies of TiO2 having 0D, 1D, 2D and 3D structures have been developed for DSSCs.263,264 Highly crystalline TiO2 nanoparticles are beneficial because they provide a high surface area for better dye loading.264,265 However, the electron mobility is poorly affected by the grain boundaries between the TiO2 nanocrystals.266,267 In contrast, 1D TiO2 can show good electron transport properties, but possesses a lower surface area.268 Hence, a mixture of 0D and 1D TiO2 nanostructures is beneficial to get good electron transport properties with a high surface area.268 In respect of this, 3D hierarchical TiO2 nanostructures are beneficial to overcome the limitations of 0D and 1D nanocrystals, since they have a high surface area, high optical absorption through the scattering of light and good electron transport properties.262,263,268,269Fig. 11 shows an overview of the different nanoarchitectures of the photoanode (TiO2) and their modification by performing sensitization, doping, grafting etc. with metal nanoparticles, metal atoms, carbon materials and metal oxides. These modified photoanodes can exhibit a high surface area, and good electron transportation characteristics and scattering effects.235,262,263,268,269
image file: c7nr08350e-f11.tif
Fig. 11 An overview of the different nanoarchitectures of photoanodes (TiO2) and their modification by sensitization, doping, grafting etc. by metal nanoparticles, metal atoms, carbon materials and metal oxides for advancement in DSSC devices.
3.2.1 Nanoarchitectures of TiO2. The electron transportation of any materials depends on its band structure and therefore also depends on the bonding characteristics.139,156,270 The bulk counterparts of TiO2 consist of electron transportation through electron–phonon coupling, known as polarons due to their ionic structure.271,272 Here, a small polaron leads to the formation of localized and self-trapped electrons, which contribute to electron transportation via thermally activated hopping of electrons from one electronic state to the next.272,273 However, the electron transportation of nanostructured TiO2 depends on the characteristics of the individual nanoparticles, the modified electronic structure,273 the extent of particle connectivity,274,275 and the geometric structure263,268,276 (shape of the nanoassembly; such as nanorod, nano-flower etc.) of the assembly. Furthermore, quantum size effects are observed in TiO2 when splitting of the electronic energy band into discrete electronic energy levels occurs below the size of 3.5 nm nanoparticles (since the exciton Bohr diameter of TiO2 is 3.5 nm).277 In DSSCs, the TiO2 nanoparticle size has always been higher than the exciton Bohr radius, which is typically in the of 10–25 nm.278 Therefore, a nanocrystalline film of TiO2 consists of a network of bulk crystals with extended energy states arising from the high surface area of the nanocrystals and the presence of surface states from unsaturated bonds, the presence of impurities and deviations in the bond length/bond angle compared to their bulk counterparts.279–281
3.2.2 TiO2 nanoparticles. The efficiency of 13% for a DSSC has been obtained by the use of a porphyrin sensitizer and a TiO2 nanoparticle film as a photoanode.49 The TiO2 nanoparticle film provided a large surface area for dye loading, which resulted in the high efficiency. Recent studies have suggested that anatase TiO2 single crystals with {001} facets have good potential for dye adsorption and charge transportation, since the {001} facets of anatase TiO2 single crystals are extensively reactive and the surface energy is 0.90 J m−2, which is higher than that of the {101} facets having a 0.44 J m−2 surface energy.202Fig. 12a shows a TEM image of anatase TiO2 single crystals with octahedron-like morphology and Fig. 12b shows an HRTEM image, which reveals that this morphology consists of a 3D octahedral structure with the (101) and (001) planes. TiO2 nanoparticles with appropriately controlled {001} facets can provide the advantages of mesoporous TiO2 and surface activity for charge transportation. Anatase TiO2 nanoparticles with 34% exposed {001} facets and TiO2 nanoparticles have been utilized as photoanodes for DSSCs, yielding PCEs of 7.06% and 3.47% respectively. The results indicate that anatase TiO2 nanoparticles with 34% exposed {001} facets possess the characteristics of high surface activity and large special surface area for efficient dye loading.202 But the TiO2 nanoparticle-based photoelectrode has the limitations of low electron transport characteristics and reduced electron lifetime because of the misaligned crystallites,84 random networks of crystallographic morphology,282 and lattice mismatches at the grain boundaries,283 encouraging researchers to find other nanoarchitectures.
image file: c7nr08350e-f12.tif
Fig. 12 (a) TEM images of anatase TiO2 single crystals with octahedron-like morphology. (b) The respective HRTEM image of one specific anatase TiO2 single crystal with octahedron-like morphology that can be made into a 3D octahedral structure with the (101) and (001) planes as indicated; the inset shows the lattice with a distance of 0.35 nm between the (101) facets in the TiO2 anatase phase. Reprinted with permission from ref. 202. Copyright 2009 American Chemical Society. (c) SEM image of TiO2/PVAc nanocomposite fibers fabricated via electrospinning from a DMF solution. (d) SEM image of a TiO2 fiber after an annealing process showing an islands-in-a-sea morphology with nanofibrils. (e–g) HRTEM images of TiO2 nanorods fabricated by mechanical cleavage of nanofibers at 120 °C. (c–f) Reprinted with permission from ref. 297. Copyright 2005 American Institute of Physics.
3.2.3 1D nanoarchitectures of TiO2. The drawbacks of TiO2 nanoparticle-based networks include the random transportation of carriers resulting in the scattering of carriers, which contributes to recombination at the grain boundaries.202,283,284 Also, low electron diffusion coefficients arise from surface states, defects and grain boundaries etc. acting as electron trapping sites, which results in recombination of photocarriers and hence decreases the electron collection efficiency at the back contact.284,285 In this respect, various one-dimensional (1D) TiO2 nanoarchitectures have been reported to obtain excellent electron transport and enhanced light harvesting via the light scattering effect.56,286 So far, nanotubes,208,210 nanowires,56,205 and nanorods54,95,287 of TiO2 have been employed to get electron transportation and decrease the recombination process in DSSCs. To increase the electron transportation (mobility), TiO2 1D polycrystalline nanotube arrays207,208,239,288,289 and single crystalline nanowire arrays290,291 have been reported as a photoanode. TiO2 nanotubes have been obtained by various techniques such as anodization,292 template-based methods292 and sol–gel techniques.292 C. A. Grimes first reported the synthesis of uniform TiO2 nanotube arrays via anodic oxidation of titanium foil in hydrofluoric electrolyte (HF acid).293 The dimensions of the nanotubes can be easily controlled by tuning the electrochemical conditions, such as the concentration of the electrolyte, the applied current density and the pH of the electrolyte. Controlling these parameters can result in uniform TiO2 nanotubes with a controlled pore size (22–110 nm), wall thickness (7–34 nm), and length (200 nm to 1000 μm).294,295 Macák et al. were the first to report the application of anodized TiO2 nanowire arrays on titanium foil as a photoanode in a DSSC with an efficiency of 0.036%.295 Highly ordered TiO2 nanotubes with 15 mm length deposited by a nanoporous alumina template method showed an efficiency of 3.5%.288 A comparative study of the electrodynamics between nanotubes and nanoparticle films showed that the electron transportation times were the same, whereas recombination was significantly slower by ∼10 times in the nanotube films, leading to a significantly improved photocarrier collection efficiency for the nanotube photoelectrodes.296 Electrospun TiO2 fibers have been used to fabricate TiO2 single-crystalline nanorods having (001) directional growth.297 Furthermore, these TiO2 single-crystalline nanorods have been transformed into nanofibers via a mechanical cleavage method (Fig. 12c–f). The efficiency of the TiO2 nanorods is 6.2% for a quasi-solid-state DSSC device due to the high penetration ability of the highly viscous gel electrolyte attributed to the large pores, high surface area and well aligned 1D TiO2 nanorods, contributing to better electron transportation. However, 1D nano-structures of TiO2 provide a low specific surface area compared to fabricated nanoparticles stemming from low dye loading.298 To overcome this drawback and combine the characteristics of high surface area and unidirectional electron transport, different hierarchical TiO2 nanostructures have been reported. An advanced approach has been reported that involves the fabrication of hierarchical TiO2 nanotube-based structures without destroying the tube nanoarchitecture via selective removal of the lower quality inner oxide shell of the nanotubes using a chemical etching process.241 This process made single-walled TiO2 nanotubes and they were further decorated with TiO2 nanoparticles by a simple TiCl4 treatment. The hierarchical TiO2 nanotubes formed with a doubly open-ended structure with an average diameter of 120 nm at the top of layer and 40 nm at the bottom of the layer are shown in Fig. 13a.241Fig. 13b shows a TEM image of hierarchical nanotubes of TiO2 synthesized by dipping a TiO2 nanotube film in Ti(OH)4 solution for 80 min and reveals the TiO2 nanoparticles covering the surface of the nanotubes. The top view and cross-section of the bare nanotubes and hierarchical nanotubes of TiO2 prepared via immersion of the doubly open-ended nanotubes in Ti(OH)4 solution for 0, 40, 60 and 80 min, respectively, are shown in Fig. 13c. With these respective immersion times, the diameter of the nanotubes decreases and the layer of nanoparticles increases. Fig. 13d shows a schematic representation of the formation of nanotube hierarchical electrodes via the formation of TiO2 nanoparticles and the number of layers with respect to the immersion time. These hierarchical nanostructures showed an improved efficiency of up to 8%. The high efficiency of this architecture can be ascribed to (i) improved electron transport properties via the single wall structure, (ii) the TiCl4 treatment resulting in passivation of surface defects, and (iii) the high dye loading that becomes possible due to the decorated TiO2 nanoparticles.
image file: c7nr08350e-f13.tif
Fig. 13 Hierarchical nanostructures of TiO2 photoanodes: (a) schematic diagram and (b) TEM image of the hierarchical nanotubes. (c) HR-SEM image of the top and cross-section of bare nanotubes and hierarchical nanotubes prepared from doubly open-ended nanotubes immersed in Ti(OH)4 solution. (d) Schematic representation of hierarchical nanotube formation with isolated TiO2 nanoparticles. (a–d) Reprinted with permission from ref. 241. Copyright 2015 American Chemical Society. (e) Schematic representation of the synthesis process of vertically aligned TiO2 TNA and HNT films. (f) Anatase single crystalline TiO2 TNAs shown in a TEM image and (g) a HRTEM image of TiO2 HNTs. (e–g) Reprinted with permission from ref. 212. Copyright 2013 Nature Publishing group.
3.2.4 Faceted TiO2 nanostructures. It has been widely accepted that a desirable shape and crystallinity of the photoanode results in a good performance of the DSSC.143,237,238,291 Nowadays, TiO2 with tuned crystalline facets has drawn interest since the reactive {001} facets of anatase can effectively reduce charge recombination, while the {101} facets are thermodynamically more stable.210 TiO2-based DSSCs fabricated using various morphologies, like nanosheets (PCE up to 4.56%),299 microspheres (7.91%),300 nanosheet-based hierarchical spheres (7.51%),301etc. having different percentages of {101} and {001} facets have demonstrated impressive efficiency. To take advantage of 1D and 3D TiO2 nanostructures, it is desirable to fabricate TiO2 nanorods,302 nanowires,291 and nanotubes with walls of small nanocrystals of TiO2.241 The first report on the fabrication of photoanodes for DSSCs by vertically aligning anatase single-crystalline TiO2 nanostructure arrays (TNAs) decorated with octahedrons with exposed {001} facets and hierarchical TiO2 nanotubes (HNTs) on Ti-foil by a two-step hydrothermal technique showed a significant PCE of 4.66% for TNAs and 5.84% for HNTs.241 Here, the bare H-titanate acts as a template for shape conversion (hierarchical structure formation) and 1D orientation. In this process, in NH4F, the TNAs contain TiO2 nanocrystals with exposed {001} facets, while pure water results in the formation of HNTs with walls made up of numerous octahedrons (Fig. 13e).212 As shown in Fig. 13f, the TNAs consist of TiO2 truncated octahedral nanocrystals (∼50 nm) which are randomly oriented along a certain growth axis and the inset figure shows that the interfacial angle between the condensed facets of the TNAs is 68.3°, which matches well with the {001} and {101} facets of anatase. The HNTs consist of small TiO2 octahedra that are 20 nm in width and 50 nm in length, and are dominated by {101} facets attached together in the {100} direction (Fig. 13g).

3.3 TiO2 modified with plasmonic materials and metal oxides

3.3.1 TiO2 surface modification with metal oxides. Several important processes happen at the TiO2/dye/electrolyte interface, such as electron injection into the CB of TiO2 from the dye and recombination via back reaction of the injected electrons with the oxidized dye.244,303 In order to decrease the charge recombination and enhance the electron transportation process, several research groups have reported the surface modification of TiO2 with a thin insulating layer.92,155,304 The other promising wide band gap metal oxides such as MgO,191 NiO181,198 and Nb2O5[thin space (1/6-em)]73,305,306 have been reported as a surface passivation layer on the TiO2 surface to deter the recombination reaction of photogenerated carriers. Implementation of p-type NiO with a wide band gap (∼3.55 eV) as a passivation layer can lead to efficient charge separation across the interface of p-NiO and n-TiO2, and can be applied to suppress the recombination process efficiently.92,181,198 The DSSC device assembled with TiO2 surface modified with Eu3+, Tb3+ co-doped NiO showed a PCE of 8.8%, which was higher than that of TiO2/NiO and pure TiO2.181 The various roles played by the Eu3+, Tb3+ co-doped NiO in enhancing the performance include: (i) decreasing the recombination process, (ii) improving charge separation at the n–p junction, and (iii) increasing the carrier concentration and improving the electron transportation process. Arakawa et al. reported that the enhancement of the performance of a black dye-based DSSC has been achieved by MgO or Al2O3 surface modification of the TiO2 photoelectrode. The efficiency was improved from 10.4% to 10.8%.187
3.3.2 Doped TiO2 photoanodes. Similar to altering the morphology, the electronic and optical properties of TiO2 can be tuned by metal and non-metal doping, such as with V,307 Fe,79 N,308 C,308 S,308 and B.308 Introducing these dopants provides shallow trap states for conduction band electrons.278 On the other hand, In and Co doping results in improvement of the visible light response,278 and vanadium(V) doping increases the electronic conductivity attributed to the large number of oxidation states of vanadium. In TiO2, doping with the above-mentioned metal cations take places at Ti sites,309 whereas doping with non-metal anions such as N, C, B and S occurs at O sites.278 Boron doping produces smaller bandgap n-type TiO2 while N doping produces n or p-type depending on the oxygen atom occupancy state.309 Furthermore, to enhance the visible light efficiency and reduce carrier recombination induced by the dopant, modification of TiO2 by co-doping has been studied by many research groups.310–313 A variety of co-doping materials such as (C, N),314 (N, S)314 and (Cr, N),315 have been doped into TiO2 nanomaterials and have proved to be beneficial for visible light absorption and improving the electronic conductivity of TiO2. In addition, Ti3+ self-doping can be used to modify TiO2 by creating oxygen vacancies. These defects exist intrinsically or can be produced by heating under vacuum and reducing conditions. This reduced TiO2 exhibits visible light absorption and hence is beneficial for simple and cost-effective doping to produce narrow band gap TiO2.278 For example, N-doped TiO2 shows new absorption in the visible region as compared to bare TiO2. N-TiO2 exhibits a significant increase in the IPCE and PCE (overall 8% efficiency) due to increased visible absorption in the region from 370 to 530 nm.316 Recently, the effect of graphene on the performance of DSSCs has been investigated by mixing graphene into N-TiO2 photoelectrodes. The highest PCE of DSSCs with N-TiO2/graphene was 9.32% and this increase was approximately 22%compared to that of N-doped TiO2. This study showed that addition of graphene is beneficial to get high dye loading, lower internal resistance and fast carrier transportation for N-TiO2.171
3.3.3 TiO2-metal oxide nanocomposites. ZnO is an n-type semiconductor with a 3.37 eV wide band gap and a high exciton binding energy of 60 meV.317,318 The high electron mobility of ZnO leads to a high electron transport rate, which decreases recombination.319 However, the efficiency of ZnO photoanode-based solar cells is still lower than that of TiO2 because ZnO is unstable, and easily forms insulating complex structures like Zn2+/dye agglomerates upon dye-loading.174,320 Therefore, electron injection from the dye molecules to the CB of the semiconductor may be hindered. Researchers are used to resolving this issue by combining two different materials like ZnO/TiO2 core–shell nanostructures,321,322 and mixed ZnO/TiO2 nanoarchitectures.209,298,323 The PCE of combined ZnO-TiO2 photoanodes is better than that of bare ZnO photoanodes.
3.3.4 Plasmonic nanoparticles embedded in TiO2. Metallic nanostructures have the ability to control and manipulate light at the nanoscale level based on localized surface plasmon resonance (LSPR) of nanostructures (nanoparticles).324 In LSPR, the electric field of incident photons couples to the oscillating CB of the metal nanostructures, giving rise to absorption in the visible range. This phenomenon is directly proportional to the particle (metal nanostructure) volume and localized oscillating electric fields of the metal nanostructures. The absorption and scattering properties of the metal nanostructures depend on the size and shape of the nanostructures.325–329 Isotropic and anisotropic nanostructures generate intense fields localized at the edges and corners of nanostructures and act as “nanosized light concentrators”.330,331 In DSSC devices, such shape-controlled metal nanostructures coupled to dyes give rise to three different phenomena via scattering and absorption of the incident photons: (i) enhancement in dye absorption: the incident photons are scattered by the nanostructures and further get absorbed by the dye molecules. (ii) Increased total absorption cross-section: the nanostructures act as antennas that couple the plasmonic near field to the dye molecules. (iii) Enhancement in photocurrent: the metal nanostructures convert incident photons to “hot” electrons, which are further rapidly injected into the CB of TiO2 leading to an increase in the photocurrent.332–337 Shape-controlled Au@SiO2 nanocubes (silica-coated Au nanocubes) embedded in a TiO2 photoanode showed a PCE of 7.8%, higher than that of the reference device (a bare TiO2 photoanode) (Fig. 14a).337 An optical absorption study revealed the significant enhancement of optical absorption in the 400–800 nm range due to the incorporation of Au@SiO2 nanocubes into the N719 dye and that in the 400–600 nm range is attributed to the dipole–dipole coupling between the nanocubes and dye molecules (Fig. 14b). Under light exposure, the Au nanocubes absorb incident light with the generation of a confined electromagnetic field at the surface (plasmonic near field) (Fig. 14c). The dye molecules couple to this plasmonic near field, which acts as a secondary source of light resulting in high light harvesting by the dye molecules. This enhanced light absorption results in more photocarriers getting into the sensitizer. Furthermore, photoelectrons from the dye molecules get injected into the CB of TiO2. This leads to enhancement in the overall photocurrent density of the DSSC device. Jang et al. reported Au@Ag core/shell nanoparticles decorated on TiO2 hollow nanoparticles (Au@Ag/TiO2 HNPs) by sol–gel reaction and chemical deposition.338 Here, Au and Ag nanocrystals are formed at the surface of the TiO2 hollow nanoparticles, with a diameter of 15 ± 5 nm and 25 ± 5 nm for the Au and Ag nanocrystals, respectively, resulting in the formation of an Ag shell with a thickness of 7.05 nm around the Au/TiO2 nanoparticles, that is the formation of Au@Ag/TiO2 HNPs (Fig. 14d). The final component Au@Ag/TiO2 HNPs exhibited advantages from both sub-components, the Au/Ag core–shell structures and the TiO2 hollow nanoparticles. Electron-loss spectroscopy has been used to find the elemental distribution of the Au@Ag core–shell nanoparticles (Fig. 14e). The device based on Au@Ag/TiO2 HNPs showed a PCE of 9.7%. A schematic illustration of the Au@Ag/TiO2-based photoanode in a DSSC device with demonstration of the LSPR, high surface area and scattering effect is shown in Fig. 14f. The achieved high PCE of the device is attributed to LSPR with a high surface area (129 m2 g−1) and light scattering effect, which leads to overall enhancement of the light absorption.
image file: c7nr08350e-f14.tif
Fig. 14 (a) A low-magnification TEM image of Au@SiO2 nanocubes with a high-magnification TEM image in the inset. (b) Optical absorption spectra of (i) mesoporous TiO2, (ii) TiO2 + Au@SiO2, (iii) TiO2 + N719 and (iv) TiO2 + Au@SiO2 + N719. (c) Energy level diagram, which illustrates the mechanism of enhancement of absorption of N719 dye due to its coupling with plasmonic nanostructures (Au nanocubes) and the enhancement of the electron transportation characteristics due to the generation of more electrons at the dye by the Au nanoparticles, which are further transported to the CB of TiO2. (a–c) Reprinted with permission from ref. 337. Copyright 2014 American Chemical Society. (d) HR-TEM image of Au@Ag core/shell nanoparticles decorated on TiO2 hollow nanoparticles with an Ag shell thickness of 7.05 nm. (e) The elemental distribution of the Au@Ag core/shell nanoparticles obtained by electron-loss spectroscopy (EELS) dot mapping. (f) Schematic representation of the Au@Ag/TiO2 hollow nanoparticle-incorporated photoanode-based DSSC with a magnified schematic structure of an Au@Ag/TiO2 hollow nanoparticle. (d–f) Reprinted with permission from ref. 338. Copyright 2015 American Chemical Society.

The insertion of plasmonic nanostructures into a photoanode leads to an increase in photocurrent and hence results in the enhancement of the PCE. However, to enhance the Voc the device needs to be modified with a photoanode material that has a high band gap, like inorganic perovskites.

3.4 Metal oxide perovskites

Although binary metal oxides like TiO2,61,228 SnO2,63,339 and ZnO255,340 are widely used as photoanodes, recently ternary metal oxides have been studied. Nowadays, instead of TiO2, perovskite-double oxide materials like SrTiO3,341,342 CaTiO3[thin space (1/6-em)]343 and BaTiO3[thin space (1/6-em)]343 have been reported to obtain high Voc, since these are n-type materials with a comparable bandgap to that of TiO2. In addition, their ionization potential, electronic structure, band gap, and electron affinity can be easily modified by altering their atomic composition.240 Okamoto et al. reported SrTiO3, CaTiO3 and BaTiO3 perovskites as a photoanode for DSSCs with higher Voc values of up to 0.636 and 0.650 V, respectively, compared to that of conventional TiO2 (0.509 V).343 Jayabal et al. achieved a high Voc of 0.73 V for a SrTiO3-based photoanode.341 Although it possesses a high Voc, it has a limited low Jsc, which a hampers its practical applications. Recently, BaSnO3 (BSO)-based perovskite materials have been studied as photoanodes in DSSCs344–346 (Fig. 15a). The potential use of BSO as a photoanode in DSSCs has been suggested by Zhang and co-workers.344 Ternary BSO has been investigated as an ETM of a highly efficient DSSC. It is an important photoanode material due to its similar electronic properties to that of TiO2. BSO is an n-type semiconductor with a band gap of 3.1 eV, and its electronic structure, and optical and electrical properties can be tuned easily by atomic substitution, doping into the Ba or Sn site. Shin et al. reported BSO perovskite nanoparticles for photoanodes in DSSCs with 6.2% efficiency (Fig. 15b).347 This report provided detailed studies of the effects of the thickness of the BSO film and TiCl4 treatment on the physical, chemical and photovoltaic properties. The authors concluded that the formation of the ultrathin layer of TiO2 on the BSO surface led to increased charge collection efficiency by improving the charge transport properties and suppressing the recombination reaction. Rajamanickam et al. reported cubic BSO-based DSSC devices fabricated with three different strategies: (i) a BSO/TiCl4 treatment photoanode, (ii) a BSO photoanode made with a TiO2 scattering layer and (iii) a BSO/TiCl4 treatment/TiO2 scattering layer photoanode showing an efficiency of 3.88%, 1.14% and 5.68%, respectively.240Fig. 15c shows a schematic diagram of BSO-based photoanode fabrication for BSO, BSO/TiO2 scattering layer and BSO/ZnO scattering layer. Surface modification by TiCl4 and a scattering layer of TiO2 or ZnO supports dye adsorption, increases light absorption and controls the recombination reaction resulting in high efficiency and stability. Cubic shaped BSO of 132 nm size was used for device fabrication (Fig. 15d).240 The above-mentioned semiconductors and their modifications are applicable to enhance the photocurrent and photovoltage of a DSSC device. One important factor in DSSC devices is dye loading capability, and hence the fabrication of hierarchical nanostructures of the photoanode can lead to better device performance.
image file: c7nr08350e-f15.tif
Fig. 15 (a) Perovskite crystal structure of BSO. (b) Schematic representation of a highly efficient perovskite BaSnO3-based DSSC. (c) A schematic diagram of bare BSO-, BSO/TiCl4 treatment- and BSO/TiCl4 treatment/scattering layer-based DSSC photoanodes and their maximum PCEs. (d) A TEM image of BSO showing a nanocuboid with a length of ∼123–250 nm and ∼118–236 nm, respectively. (a, c and d) Reprinted with permission from ref. 240. Copyright 2016 Royal Chemical Society. (b) Reprinted with permission from ref. 347. Copyright 2013 American Chemical Society.

3.5 Hierarchical nanostructures of metal oxides

In DSSCs, a monolayer of dye molecules adsorbed on the high surface area of a metal oxide leads to efficient charge separation at the electrode–electrolyte interface. Hierarchical nanostructures have advantages over 1D nanostructures; for example, they possess high dye loading due to their large surface area and better absorption due to scattering of light.84,348,349 In respect of this, hierarchical spherical aggregates have been developed to enhance the light harvesting capability without sacrificing the electron transportation characteristics.350,351 Also, the performance of these particles depends on their shape and size, since this alters the light harvesting and electron transportation characteristics.350–352 Many researchers have used the hydrothermal technique for the synthesis of TiO2 nanoparticles.353,354 In our group, a DSSC device fabricated by the use of rutile TiO2 hierarchical microspheres showed an efficiency 3.81%, which is much higher than that of the reference device (0.67%). The fabricated TiO2 microspheres are made up of rutile TiO2 semispherical nanoparticles. The rutile TiO2 microspheres were prepared by the hydrothermal technique at 160 °C using titanium butoxide as a Ti source and a mixture of toluene (non-polar solvent)-HCl (polar solvent) in the desired amount of H2O. The possible growth mechanism is demonstrated in Fig. 16a.355 In step A, nucleation centers are produced at the water–toluene interface. Subsequently, the respective increase in temperature leads to the formation of TiO2 nanospheres, which further turn into hierarchical microspheres due to the agglomeration process under the pressure and temperature of the system. Cao et al. reported that ZnO aggregates have a higher light capacity, which leads to an enhanced PCE of up to 3.51% compared to that of ZnO nanoparticles (0.6%).247 Polydispersed ZnO aggregates deposited on a substrate by spray pyrolysis showed a substantial enhancement in PCE of up to 7.5%.356 TiO2 spherical aggregates have also been reported, which showed an improved efficiency of up to 10% with respect to bare TiO2 nanoparticles, possibly due to their high light scattering capability and the good interaction between the nanoarchitectures.357 Chen et al. reported dye loading measurements, which have shown higher dye loading capability (∼8.69 × 10−5 mol g−1) for TiO2 aggregate powders as compared to P25 TiO2 nanoparticles (4.25 × 10−5 mol g−1).357 Our group member, Mali et al., reported the surfactant-free growth of TiO2 hierarchical nanostructures, including aggregate nanoparticles, 1D tetragonal nanorods, 3D structures having dendrites containing nanorods and 3D hollow urchin-like structures using a simple hydrothermal technique (Fig. 16b).358 This report showed improved PCE of devices from 2.34 to 7.16% with respect to the above-mentioned nanostructures due to an enhanced surface area and light harvesting capability. Also, electrospun 1D TiO2 nanofibers with different diameters (296 nm to 635 nm) have been prepared by varying the feeding rate (1.0 ml h−1 to 2.5 ml h−1) (Fig. 16c).359 The nanofibers with a diameter of 421 nm (1.5 ml h−1) showed the highest PCE of up to 5.39% due to their high surface area (147.8 m2 g−1), higher diffusion coefficient (Dn = 9.48 × 10−5 cm2 s−1 as determined by an IMPS/IMVS study) and a high dye loading capability (105 μmol cm−2). The tetrapod shape of ZnO has attracted much attention due to its hierarchical nature with an interconnected network. This architecture provides low surface defects and a long diffusion length, which leads to long carrier lifetime, and scattering facets that lead to enhancement of the light harvesting capability. The ZnO tetrapod has four 3D extended arms which endow it with not only a good network but also high porosity and mechanical strength.360,361 The electron mobility of ZnO tetrapods is comparable to that of single-crystal ZnO nanowires at ∼17 cm2 V−1 s−1.359 The PCEs of ZnO tetrapod-based DSSCs are largely dependent on the arm length-to-diameter ratio (packing density), since this determines the interface carrier separation and transportation in the device. A number of reports have suggested that ZnO tetrapod-based DSSCs could achieve a higher efficiency than DSSC devices fabricated from ZnO nanowire arrays and spherical particles. An SnO2 nanoparticle/ZnO tetrapod composite-based DSSC device showed an efficiency of up to 6.31% for a 2[thin space (1/6-em)]:[thin space (1/6-em)]1 ratio of ZnO tetrapods to SnO2 particles.362 Some reported hierarchical nanostructures of ZnO and TiO2 photoanodes are shown in Fig. 17a–i.358,363–370
image file: c7nr08350e-f16.tif
Fig. 16 (a) Schematic of the formation of microspheres of TiO2 from closely packed nanospheres via a hydrothermal route and a demonstration of multiple scattering of light in those nanoarchitectures. Reprinted with permission from ref. 355. Copyright 2012 Royal Chemical Society. (b) FE-SEM images of the TiO2 film deposited via a hydrothermal route at different temperature (100–190 °C) and the respective growth model: (i) nanoparticle clusters, (ii) 1D nanorods, (iii) 3D dendrites containing nanorods (30 nm) and (iv) 3D hollow urchin-like morphology. Reprinted with permission from ref. 358. Copyright 2013 Nature Publishing Group. (c) Schematic of the electrospinning technique and illustration of the formation of various diameter TiO2 nanofibres obtained at different electrospinning feeding rates (1.0 to 2.5 ml h−1). Reprinted with permission from ref. 359. Copyright 2015 Elsevier. (d) Schematic of the formation of tetrapod nanostructures of semiconductors like ZnO and their electron transportation mechanism.

image file: c7nr08350e-f17.tif
Fig. 17 Some reported hierarchical nanostructures of photoanode materials like TiO2 and ZnO, which can be applied to efficient DSSC device fabrication.358,364–370 Reprinted with permission from ref. 363. Copyright 2008 AIP Publishing LLC. Reprinted with permission from ref. 358. Copyright 2013 Nature Publishing group. Reprinted with permission from ref. 370. Copyright 2016 Royal Chemical Society. Reprinted with permission from ref. 364, 367 and 369 from American Chemical Society. Reprinted with permission from ref. 365 and 366. Copyright from Elsevier. Reprinted with permission from ref. 368. IOP Publishing Ltd.

The above-mentioned surface modifications and nanostructures of photoanodes are beneficial in the advancement of DSSC devices. Also, further insertion of carbon materials like carbon nanotubes (CNTs) and graphene into the photoanode can lead to dispersion of the semiconductor matrix (TiO2) and improved electron transportation characteristics.

3.6 Role of carbon-based nanomaterials in metal oxide based ETMs

The main challenge within the photoanode of DSSCs is low photo-electron transportation efficiency across the TiO2 matrix due to the presence of grain boundaries.171,371,372 Low photo-induced electron transfer efficiency is the main limitation of such nanostructured photoanodes. The inefficient charge transfer paths within the photoanodes are the main reason for recombination of photo-induced electrons with oxidizing species and/or tri-iodide ions in the electrolyte, consequentially resulting in a decrease in the photocurrent and efficiency of the DSSC.96,97,373 Several reports have shown that the incorporation of carbon materials in the TiO2 matrix leads to good dispersion of the TiO2 nanoparticles, yielding a highly porous morphology of the resultant nanocomposite.371,374,375 This provides the advantage of high yield dye loading and extra new electron transportation paths, which lead to improvement in the photocurrent.376 In respect of this, researchers have reported that graphene and CNT insertion into the photoanode improves the charge transportation properties in DSSCs.228 Electrospun TiO2 nanorod-CNT nanocomposites for efficient electron transportation processes have been reported. Fig. 18 shows the electron transportation process across the photoanode and the corresponding JV curves:95 (a) CNTs totally enclosed/anchored by TiO2 nanoparticles, (b) CNTs totally enclosed/anchored by TiO2 nanorods, and (c) CNTs within TiO2 nanorods. Fig. 18(d) shows an HRTEM image of TiO2 nanorods that incorporate MWCNTs. Fig. 18(e) shows the JV curves of different DSSCs; the hollow-symbol curve represents a photoanode with a thickness of about 6.6 ± 0.7μm, and the solid-symbol curve represents a photoanode with a thickness of about 14.3 ± 0.3 μm. Specifically, multiwall carbon nanotubes (MWCNTs) are inserted into the TiO2 nanorods by electrospinning with a simple one-step approach. Incorporating the MWCNTs into the TiO2 nanorods resulted in a high efficiency of 10.24% with a high fill factor (FF) of 74%.
image file: c7nr08350e-f18.tif
Fig. 18 Electron transportation process across a photoanode: (a) CNTs totally enclosed/anchored by TiO2 nanoparticles, (b) CNTs totally enclosed/anchored by TiO2 nanorods, and (c) CNTs within TiO2 nanorods. (d) HRTEM image of TiO2 nanorods that incorporate MWCNTs. (e) JV curves of different DSSCs; the hollow-symbol curve represents a photoanode with a thickness of about 6.6 ± 0.7 μ m, and the solid-symbol curve represents a photoanode with a thickness of about 14.3 ± 0.3 μm. Reprinted with permission from ref. 95. Copyright 2013 John Wiley and Sons.
3.6.1 Graphene-based photoanodes. Graphene – a two-dimensional (2D) sp2-bonded carbon sheet – has attracted a great deal of interest due to its unique structure,377 and physical properties, including a high electrical conductivity,108 mechanical flexibility,378 surface area, carrier mobility, chemical stability, and optical transparency.377,379–382 Researchers have exerted great efforts to take advantage of graphene in solar cells. Incorporation of graphene into a TiO2 matrix gives the advantages of low recombination rates, enhanced light scattering and good electron transportation, contributing to improved performance.96,109,373 Also, it improves the dye adsorption and electron lifetime of the TiO2 photoanode leading to an increase in the photocurrent.96,109,373 Hence, regarding the rapidly booming interest in this cutting-edge research area, it is necessary to summarize the achievements and discoveries related to graphene-based photoanodes in DSSCs. Insertion of graphene as a 2D bridge into the TiO2 nanocrystalline electrode of a DSSC results in faster photogenerated electron transportation, lower recombination, and higher light scattering.96,109,373 Graphene incorporation into a TiO2 electrode resulted in an increase in the short-circuit current by 45% without sacrificing the Voc, and improved the total efficiency to 6.97%, which was a more than 39% increase compared to a nanocrystalline TiO2 photoanode, and was also much higher than that of a 1D CNT-TiO2 nanomaterial composite electrode (Fig. 19).383 The CB of CNTs (4.5 eV) is more negative and the Fermi level is between the CB and VB as compared to TiO2, where the CB of TiO2 is 4.0 eV vs. vacuum and the Femi level is near to CB. Incorporation of 1D CNTs into a TiO2 electrode resulted in a limited improvement in efficiency, despite the spatial charge transport characteristics in the 1D material. The reason is that the point contact between the TiO2 nanospheres and columnar form structure of the 1D CNTs is unable to allow proper anchoring. Also, the CNT's more negative potential compared to that of TiO2 and the appearance of the Fermi level in the CNTs at the center of the CB and VB leads to a decrease in the Voc of the cell. In contrast to 1D CNTs, 2D graphene has the benefits of good charge separation due to its excellent conductivity with the ability to make good contact with TiO2 nanoparticles. Graphene is a soft single molecular layered structure, and the advantage of intermolecular forces like physisorption and electrostatic binding leads to excellent charge transfer interactions between graphene and TiO2. These features of graphene enable proper anchoring of nanocrystalline TiO2 on graphene flakes and the formation of graphene bridges, which helps to improve the TiO2 interface contacts. The excellent conduction of photogenerated carriers via the graphene bridges results in reduced recombination of photogenerated electrons and hence improves the overall efficiency of the device. Graphene-TiO2 nanocomposites not only benefit from good electron transportation paths but also have great light-harvesting efficiencies. Tang and Hu showed that TiO2-graphene nanocomposites have high light-harvesting efficiencies. Cheng et al. showed an improved efficiency of a device from 4.78% to 7.68% following graphene incorporation into the device.372 A summary of the photovoltaic performances of DSSCs with different nanostructured ETMs and dyes is given in Table 2.5,49,135,145,159,212,247,384–391
image file: c7nr08350e-f19.tif
Fig. 19 (a and c) 1D and (b and d) 2D nanocomposite electrodes. In graphene-based (2D) nanomaterial composite electrodes, graphene anchors the TiO2 particles, which improves the electron transportation. In CNT-based nanocomposites (1D) there is less intermolecular force and attachment between the TiO2 and CNTs. Therefore, the electron transfer resistance is higher and it is much easier for the recombination process to occur. Reprinted with permission from ref. 383. Copyright 2010 American Chemical Society.
Table 2 Photovoltaic performance of various nanostructures
Material and Physical parameter Dye I sc (mA cm−2) V oc (V) FF PCE (%) Ref.
Mesoporous TiO2 SM371 15.9 0.96 0.79 12.0 49
Mesoporous TiO2 SM315 18.1 0.91 0.78 13.0 49
ZnO aggregate film N3 18.7 0.6.35 45.1 5.4 247
Mesoporous TiO2 beads N719 15.47 0.719 0.72 8.84 392
3D TiO2nanotubes N719 7.67 0.78 60 3.57 387
1D TiO2 nanorods/3D TiO2 nanotubes N719 13.40 0.76 63 6.43 387
YF3:Eu3+ (5 wt%) TiO2 14.894 0.787 66.1 7.741 389
NW arrays of TiO2 N719 7.46 0.839 75 4.66 212
PT/NP TiO2 N719 12.00 0.75 70 6.30 5
TiO2/ZrO2 particles Red-dye 16.5 0.715 0.69 8.1 135
N,S-TiO2@Ag N719 29.05 0.77 37 8.22 393
Hierarchically structured Zn2SnO4 nanobeads SJ-ET1 12.2 0.71 72 6.3 386
Al2O3-coated TiO2 films RuL2(NCS)2 12.1 0.760 0.61 5.6 385
Om TiO2 IF/nc-TiO2 BS Heterojunction N719 16.5 0.83 55 7.5 384
5 μm thick scattering layer of 400 nm TiO2 particles +7 μm thick transparent layer (20 nm particles) D102 9.71 0.674 0.742 4.86 145
D149 12.52 0.707 0.720 6.38
D205 13.73 0.728 0.719 7.18
4 μm TiO2 transparent layer +4.5 μm TiO2 scattering layer/PProDOT counter electrode Y123 13.06 0.998 77.4 10.08 388
3 μm TiO2 +post treatment with (0.05 M TiCl4)/Pt counter electrode (solid state DSSC) N719 dye and DMEII-SN electrolyte 12.72 0.745 71.4 6.8 391
12.67 0.808 69.9 7.2
12.40 0.804 69.3 6.9
12.31 0.806 68.2 6.7


4. Counter electrodes

The counter electrode is an important component in DSSCs. The main role of the counter electrode is (i) back transfer of electrons arriving from the external circuit to the redox system91,154 and (ii) to act as a catalyst for the reduction of the oxidized charge mediator (I/I3). Mostly, Pt is used as a counter electrode in DSSCs due to its low charge transfer resistance (Rct, that is the electrode–electrolyte interface resistance), high exchange current density, good catalytic properties and large surface area.94,146,394,395 However, Pt counter electrodes are expensive and also tend to degrade over time in liquid electrolytes, which causes a decrease in the PCE of the DSSC.94,146,394,395 In this context, cost-effective and stable carbon-based counter electrodes have been reported by several groups.218,396,397 These electrodes exhibit a large surface area, excellent charge transport properties with good catalytic behavior and chemically stability in all types of electrolyte.218,396,397 Carbon having 0D, 1D, 2D and 3D nanostructures shows good performance as a counter electrode.218,396,397 Carbon materials such as carbon quantum dots (CQDs),398,399 carbon nanotubes (CNTs),400 graphene,105,107 fullerene401etc. have been successfully employed as a counter electrode in DSSCs. Also, some groups have reported the use of carbon nanostructure-polymer composite films as a counter electrode.402–406 Some other materials such as Ni,407 CoS,402,408 nickel nitride,409,410 CoNi2S4[thin space (1/6-em)]411etc. have also been reported as a good counter electrode in order to develop cost-effective efficient DSSCs.

4.1 Graphene-based counter electrodes

The properties of carbon materials can be controllably tuned by using different dopants, such as nitrogen,411 sulphur,412 phosphorous413 and metal ions like Co and Ta.411 These dopants in the carbon structure provide different electronic structures and surface characteristics, and also give an excellent catalytic activity according to the doped material, oxygen functional groups and concentration.412–415 A number of research groups have reported the use of a reduced graphene oxide-derived graphene/poly(styrenesulfonate) (PEDOT:PSS) composite as a counter electrode in DSSCs with efficiencies comparable to platinum-based cells.416,417 These reports suggested that oxygen functional groups were responsible for the observed catalytic performance of carbonaceous electrode materials, like graphene. Roy-Mayhew et al. made the first report on the impact of the degree of material functionalization on electrocatalytic performance.418 They showed that oxygen functional groups attached on the carbon framework were responsible for the catalytic activity, which can be further improved by increasing the amount of oxygen functional groups. Also, they demonstrated that functionalized graphene sheet (FGS)-based inks deposited on a nonconductive flexible plastic substrate can be effectively applied as a flexible counter electrode, which would be applicable for flexible DSSC devices.418Fig. 20a presents a schematic representation of functionalized graphene with lattice defects. Functional groups like epoxides and hydroxyls are attached on both sides of the graphene plane, while carbonyl and hydroxyl groups are attached at the edges. Fig. 20b represents the side view highlighting the topography of the graphene sheet. Fig. 20c shows IV curves of DSSCs fabricated with functionalized graphene and Pt CEs, respectively. Nitrogen-doped CNTs and graphene have been extensively studied due to their excellent electrocatalytic performance for triiodide reduction in DSSCs.90,412,414 Nowadays, different nanostructures of graphene, including 1D (nanoribbons90,419) and 0D (graphene quantum dots420) structures, are also effectively used as a counter electrode due to their unique properties, like high active surface edges. Graphene nanoribbons having a 1D structure are elongated strips of graphene with a unique structure and optical properties.420 Xue et al. reported nitrogen-doped graphene nanoribbons (N-GNRs), which have a high content of nitrogen of up to 6.5 atom%, for the counter electrode in DSSCs.90 Here, graphene oxide nanoribbons have been synthesized by the unzipping of CNTs via a chemical process. Furthermore, the nanoribbons were annealed in an ammonia/argon gas mixture at 800 °C for 1 h to get N-GNRs (Fig. 21a). The device was fabricated by the use of N-GNRs as a CE, TiO2 as a photoanode, disulfide/thiolate redox as an electrolyte and N719 dye (Fig. 21b). The SEM image of the N-GNRs revealed that the CNTs are unzipped into strip-like graphene and this was further confirmed by the AFM image (Fig. 21c and d). The N-GNR counter electrode gave a higher efficiency of 5.07% for the device as compared to Pt (3.09%) and graphene (3.89%) based counter electrodes. This result suggests that N-GNR counter electrodes have excellent catalytic activity, which comes from the large number of edge/doping-induced defects and low charge transfer resistance, making them an excellent candidate in different counter electrodes. A 3D honeycomb-like graphene sheet (HSG) has been reported by using a simple chemical reaction between Li2O and CO.394 This 3D HSG structure was formed by connecting curved graphene sheets with a thickness of about 2 nm and the size of the honeycomb structures was about 50–500 nm (Fig. 21e). IPCE spectra of the HSG-based device showed an IPCE of above 50% over a range of 500–650 nm and above 60% at 550 nm (Fig. 21f). The CVs of three HSGs show two pairs of oxidation and reduction peaks. These peaks have a low peak-to-peak separation between the oxidation and reduction peaks, with a higher current density suggesting better electrocatalytic activity for the reduction of I3− to I in DSSCs (Fig. 21g). The optimized nanostructures showed excellent catalytic performance as a counter electrode for DSSCs with an efficiency as high as 7.8% due to the synergetic effect of high conductivity, surface edges and the presence of defects.
image file: c7nr08350e-f20.tif
Fig. 20 (a) Schematic representation of functionalized graphene with lattice defects. Functional groups like epoxides and hydroxyls are attached on both sides of the graphene plane, while carbonyl and hydroxyl groups are attached at the edges. (b) Side view highlighting the topography of the graphene sheet. (c) IV curves of DSSCs fabricated from functionalized graphene and Pt CEs, respectively. Reprinted with permission from ref. 418. Copyright 2010 American Chemical Society.

image file: c7nr08350e-f21.tif
Fig. 21 Schematic representation of (a) the technique for N-doped graphene nanoribbon (NGNR) synthesis, and (b) the fabrication of a disulfide/thiolate redox DSSC with N-GNRs as a counter electrode. (c and d) SEM images of N-GNRs taken under different magnifications. (e) An AFM image of the N-GNRs. (e) High-angle annular dark field (HAADF) image of honeycomb-structured graphene (HSG). (f) IPCE of DSSCs with HSG counter electrodes. (g) Cyclic voltammetry curves of an HSG electrode. (a–d) Reprinted with permission from ref. 90. Copyright 2015 Royal Chemical Society. (e–g) Reprinted with permission from ref. 394. Copyright 2013 John Wiley and Sons.

4.2 Transition metal-based counter electrodes

Transition metal-based materials like WO3,421,422 V2O5,423 Nb2O3,423 NiO424and TiP2O7[thin space (1/6-em)]98 have been demonstrated to have excellent catalytic performance in different redox reactions because of their multiple oxidation states. Of the metal oxides, nickel oxide shows good catalytic performance due to its high chemical stability, good charge capacity and well-matched valence band energy (∼−4.96 eV) favorable for charge transfer processes. Despite the presence of the above-mentioned properties of nickel oxide, its actual use in electrocatalysts is restricted due to its low conductivity behavior.425–428 In respect of this, S -doped nickel oxide has been used to increase the conductivity, resulting in an efficiency of 5.04%.424 Titanium nitride-polystyrene sulfonate-doped poly(3,4-ethylene-dioxythiophene) (PEDOT:PSS) nanocomposites have been prepared by mechanical mixing of TiN and PEDOT:PSS under ultra-sonication treatment, and possess high electrical conductivity and excellent electrocatalytic activity.429 These nanocomposites overcome the aggregation problem, which leads to fewer catalytic active sites, poor electronic conductivity and weak bonding between the particles and FTO substrate, resulting in good performance with an efficiency of 7.06%, which is higher than that of the device with a Pt-based counter electrode (6.57%).429Fig. 22 shows typical SEM images of the TiN-PEDOT:PSS composite films: (a) TiN-PEDOT:PSS particles, (b) TiN-PEDOT:PSS rods, (c) TiN-PEDOT:PSS spheres, and (d) IV curves of the above-mentioned electrodes.429 A TiOPC nanocomposite was prepared by a simple chemical approach via an annealing process in a nitrogen atmosphere for carbon thermal transformation of TiP2O7 into TiOPC.98 The following reactions took place during thermolysis of TiP2O7 and vaporization of the P species in carbon, and then the material was further reduced by using carbon resulting in the formation of TiP2O7−xCx. These steps are illustrated in a schematic diagram (Fig. 23a). The SEM image of the oxidized TiOPC revealed a sheet-like morphology (Fig. 23b). The TiOPC nanocomposite with a porous structure showed enhanced electrocatalytic activity, resulting in a high PCE of 8.65% (Fig. 23c).98
 
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image file: c7nr08350e-t12.tif(35)

image file: c7nr08350e-f22.tif
Fig. 22 Typical SEM images of TiN-PEDOT:PSS composite films. (a) TiN-PEDOT:PSS particles, (b) TiN-PEDOT:PSS rods, and (c) TiN-PEDOT:PSS spheres. (d) IV curves of the above-mentioned samples. Reprinted with permission from ref. 429. Copyright 2012 American Chemical Society.

image file: c7nr08350e-f23.tif
Fig. 23 (a) Illustration of the synthetic process for a highly active nanocomposite material, TiOPC, which contains titanium, phosphorous, oxygen, and carbon, as an efficient counter electrode in an I3−/I electrolyte. (b) SEM image of oxidized TiOPC. (c) IV curves of DSSCs with Pt, C, PC3, TiO2/C3, TiOP, and TiOPC CEs. Reprinted with permission from ref. 98. Copyright 2016 American Chemical Society.

4.3 Transition metal-non metal-based CEs

In the above section, we survey the transition metal-based CEs and their respective nanocomposites with carbon, nitrogen and conducting polymers. In relation to this, researchers have been very interested in the performance of nanomaterials of metal-non metal composite materials as compared to high-cost Pt CEs. MxPy represents important metal/non-metal catalytic materials with compositions usually ranging from (y/x < 1) to (y/x > 1), that is from metal-rich to phosphorous-rich polyphosphide structures, where phosphorous atoms are present in the form of larger polyphosphides or isolated anions via direct P–P bonding.430–432 These phosphides have the advantages of good conductivity, abundance in nature, low cost and chemical inertness, resulting in diverse applications in catalyst systems such as those for water splitting, the oxygen reduction reaction and counter electrodes.430–432 Moreover, several reports have suggested that composition and nanostructure govern the excellent catalytic properties of Ni-rich phosphides for catalyzing I3 reduction in DSSCs.433,434 Hydrothermally grown Ni2P/C10 shows a high PCE of 9.57% attributed to Niδ+ and Pδ active sites and the metal-like conductivity accounts for the catalytic performance.197 On the basis of catalytic results of unique hexagonal structures of Ni2P/C10 by CV measurements, Ni–P bonds produce a “ligand effect” and induce positive charges on Ni compared to that of P, which results in metallic conductivity (Fig. 24a).435 Also, the positive charges on Ni are responsible for high adsorption of triiodide ions compared to non-metallic atoms. Therefore, the synergetic effects of fast adsorption of triiodide onto the Ni site, the ability to break I–I bonds, and excellent electron transportation can be attributed to the metallic nature of Ni2P and the catalytic activity of the carbon structure. The SEM image with corresponding EDX mapping and energy-dispersive X-ray spectroscopy (EDS) of the Ni2P/C10 composite revealed 3–5 μm sized granular Ni2P particles with good consistency for Ni and P with the absence of O in the same region (Fig. 24b). In order to examine the role of carbon in controlling the Ni2P particle size and enhancing the extent of phosphidation, the TEM and SAED pattern of Ni2P/C10 have been examined (Fig. 24c). This study revealed the formation of a lattice fringe of Ni2P with an interplanar distance of 0.49 nm, which was attributed to the (100) plane and also confirmed the formation of carbon-anchored Ni2P particles. The SAED pattern of the same revealed the (111), (201), (210), (211) and (300) planes of the hexagonal crystal structure and the polycrystalline nature of the Ni2P/C10 nanocomposite.
image file: c7nr08350e-f24.tif
Fig. 24 (a) Schematic representation of the catalytic mechanism for a Ni2P/C counter electrode. (b) SEM and EDX mapping of the Ni2P/C10 composite for identification of the Ni, P and O content. (c) Corresponding selected area electron diffraction (SAED) patterns for the identification of the carbon structure. Reprinted with permission from ref. 435. Copyright 2017 American Chemical Society.

4.4 Quaternary and pentanary semiconductors

Quaternary and pentanary semiconductors like Cu2ZnSnS4 (CZTS) and Cu2ZnSnSSe4 (CZTSSe) are applicable to the fabrication of low-cost CE electrodes.436–438 These materials have attracted considerable interest due to their biocompatibility and eco-friendliness.436–438 In CZTS, the kesterite and stannite crystal structures have a similar unit cell, but different occupations of Zn, Cu and Sn cations. Researchers are interested in determining the effect of the concentration of Cu, Zn and Sn cations, and S anions on the performance of devices due to the tuning of properties like conductivity and catalytic activity with respect to the concentration of cations and anions, respectively. Mali et al. reported the synthesis of kesterite (CZTS) nanofibers via electrospinning using polyvinylpyrrolidone (PVP) and cellulose acetate (CA) polymer at different concentrations of PVP and CA (Fig. 25a).439 The phase confirmation of CZTS was done by XRD studies (Fig. 25b). Fig. 25c and d show TEM images of CZTS materials that were synthesized with different concentrations of PVP and it was found that the nanofibers have a size between 100–150 nm in diameter. The SEM image revealed that the nanofibers are quite long, that is in the micrometer range (Fig. 25e). The DSSC devices fabricated by the use of PVP-CZTS (Fig. 25f) and CA-CZTS showed a PCE of 3.10% and 3.90%, respectively (Fig. 25g). The synthesis of Cu3InSnSe5 nanoparticles and their modification with Pt and Au have been reported. The IV curves revealed that pt-Cu3InSnSe4 and Au-Cu3InSnSe5 give comparative efficiencies of 5.8% and 7.6% compared to that of a Pt-based CE, respectively.
image file: c7nr08350e-f25.tif
Fig. 25 (a) Schematic illustration of the experimental set-up for electrospun CZTS nanofibers as a counter electrode for DSSCs. (b) XRD pattern and (c and d) HRTEM analysis of CZTS nanofibers prepared from different precursors. (e) surface morphology of CZTS nanofibers. (f) Device architecture and (g) JV curves for the respective counter electrodes. Reprinted with permission from ref. 439. Copyright 2014 American Chemical Society.

4.5 Perovskite materials

Perovskites, having the ABO3 type crystal structure, e.g. SrRuO3, where A is an alkaline earth metal or a lanthanide and B is a transition metal element, are widely used as catalysts for hydrogen evolution reactions and electro-oxidation in fuel cells to replace costly Pt catalysts.197–199 Recently, in 2016, Guo and co-worker reported for the first time a sputtered SrRuO3 film as a counter electrode, which acts as an electrocatalyst towards I3 reduction in DSSCs with an efficiency 6.48%.440 In this device, the electrocatalytic activity depends on the lattice mismatch between the sputtered SrRuO3 (SRO) film and the substrate of MgAl2O4 (MAO) single crystals through epitaxial strain. The same group reported an increase in the efficiency of SRO-based counter electrodes by insertion of graphene quantum dots (GQDs) into the perovskite matrix with an efficiency of 8.05%, which is higher than that of conventional Pt CEs (7.44%).441 Here, SrRuO3 nanoparticles were synthesized by a facile hydrothermal technique. The SEM image revealed the formation of nanoparticles with an approximate size of 40 nm (Fig. 26A). The XRD study confirmed the formation of the orthorhombic crystal structure of SrRuO3 (Fig. 26B). In order to synthesize the SRO-GQD nanocomposite, water-soluble GQDs were synthesized via a hydrothermal route. Photographic images showing the dispersed GQDs and freeze-dried GQDs resulting in yellow colored aggregates are respectively shown in Fig. 26C and D. The TEM and HR-TEM images of the GQDs show the formation of isolated GQDs having a size in the range of 2–5 nm (Fig. 26E) and a lattice spacing distance of 0.34 nm, which corresponds to the graphite (002) facet (Fig. 26F). The Raman study revealed the formation of a highly crystalline and graphitic structure of GQDs, since the ratio of the intensities of the disordered band (D band at 1375 cm−1) and crystalline band (G band at 1572 cm−1), (ID/IG), was equal to 0.74 (Fig. 26G). The TEM image of the SRO-GQD nanocomposite revealed that the GQDs are decorated on the surface of the SRO nanoparticles (Fig. 26H). A schematic representation of the electrolyte regeneration process for the SRO and SRO–GQD hybrid CEs is shown in Fig. 26I. The IV characteristics of the DSSCs using the different CEs measured under AM 1.5 simulated solar illumination (100 mW cm−2), (a) SRO–GQD CE, (b) SrRuO3 CE and (c) Pt CE, are shown in Fig. 26J.
image file: c7nr08350e-f26.tif
Fig. 26 (A) Top view FESEM image of a SrRuO3(SRO) CE and (B) powder XRD pattern of the as-synthesized SRO nanoparticles. Photograph of the (C) as-prepared GQD aqueous solution and (D) freeze-dried GQDs. (E) TEM image, (F) HRTEM image and (G) Raman spectrum of the synthesized GQDs. (H) TEM image of the GQD-decorated SRO nanoparticles. (I) A schematic representation of the electrolyte regeneration process for the SRO and SRO–GQD hybrid CE. (J) IV characteristics of the DSSCs using different CEs measured under AM 1.5 simulated solar illumination (100 mW cm−2). (a) SRO–GQD CE, (b) SrRuO3 CE and (c) Pt CE. Reprinted with permission from ref. 441. Copyright 2017 Royal Chemical Society.

5. Dyes as light absorber materials in DSSCs

In DSSCs, to produce a high photocurrent, it is necessary that the LUMO orbital of the dye be at a higher energy level than that of the semiconductor.126,142 The dye must have a high molecular optical absorption coefficient and the optical gap between the HOMO and LUMO should absorb light from the blue end of the visible spectrum to the near infrared.93,142 Also, the dye should have high adsorption or strong coupling properties on the semiconductor. This coupling phenomenon occurs due to the typical complex structure of the dye.20,44,137,442 In conventional dyes, the metal complexes consist of a central metal ion like Ru with ancillary ligands (L), typically bipyridines (bpy), terpyridines,443 or Os(II),443 which have the ability to link the dye to the semiconductor. These metal ion dyes show light absorption in the visible and near-IR region of the solar spectrum, which comes from a metal to ligand charge transfer transition. The most efficient Ru-polypyridyl sensitizers are cis-di(thiocyanato)-bis(4,4′-dicarboxy-2,2′-bipyridine)-Ru(II), coded as N3,166,444 the doubly deprotonated form of N3 dye which is coded as N71945,344 and (tri-thiocyanato-4,4′,4′′-tricarboxy-2,2′:6′,2′′-terpyridine)-Ru(II), coded as N749146,244 or black dye. However, Ru-based dyes are very expensive due to the use of Ru metal.146,244 In order to replace these expensive dyes, synthetic organic445,446 and vegetable-based dyes442 have been used. These dyes show properties of (i) efficient light absorption capabilities in the near infrared range, (ii) higher ε and (iii) possible tailorability of the molecular structure and photo-physical characteristics.442,443,445–447 Examples of organic synthetic dyes are porphyrin,137 phthalocyanine,448,449 chlorin,450,451 perylene452,453etc. dyes. Usually, vegetable dyes are synthesized from eco-friendly vegetables, and examples include betalains,454,455 carotenoids,442 anthocyanins442 and chlorophylls.456 A liquid electrolyte containing the I/I3 redox couple is usually used for hole transportation.457–459 Due to the leakage problem in a liquid electrolyte-based DSSC, more attention has been paid to the fabrication of solid-state DSSCs using solid-state electrolytes as a hole transporting materials.457,458,460 Solid-state DSSCs have an optimum thickness limitation (1–2 μm) and absorb only 20% of incident light.457,460,461 The low allowed thickness of the films leads to difficulty in achieving effective pore filling and inefficient charge collection.457,460,461 It is necessary to select a dye based on the trade-off between the spectral absorption range and absorption coefficient.457,460,461 Liquid electrolyte-based DSSCs show good efficiency when using a dye with a broad absorption spectrum due to the large allowed thickness of the films, which is useful for high loading of the dye and hence results in efficient DSSCs.462,463 Counterpart solid-state DSSCs do not show good efficiency with dyes that have a broad absorption spectrum because of the lower availability of dye loading due to thickness limitations.464–466 Some researchers have resolved these issues by using a mixture of fluorescent and absorbing dyes to get a broadening of the spectral response without reducing the absorption coefficient of the dye. This type of dye is called an energy relay dye (ERLD).467,468 In ERLDs, the fluorescent dye transfers energy via Forster resonant energy transfer (FRET) to the lower energy gap (near infrared) sensitizing dye. FRET may provide high efficiencies in SS-DSSCs by allowing the combination of high-extinction materials to absorb the whole solar spectrum.20,469 Mostly, GQDs, metal nanoparticles, semiconductor nanoparticles and fluorescence dyes have been used in connection with regular metal-based dyes.468,470,471 This concept was introduced by Shankar and co-workers.151 FRET-based devices have been fabricated using black dye, N719 (as an acceptor dye) and zinc phthalocyanine (as a donor dye). These devices consist of the donor dye dissolved in electrolyte and the acceptor dye anchored on a TiO2 nanowire array surface. Fig. 27a shows a schematic diagram of the concept of the FRET photovoltaic device, which enables FRET-enhanced nanowire DSSCs.151 Close-packed nanowires are beneficial to keep the distance between the donors and acceptors within the Forster radius. Energy transfer from the donor to acceptors in solution was illustrated by the emission spectra as shown in Fig. 27b. A four-fold increase in the quantum yield of absorption was observed for red photons in the TiO2 nanowire-based DSSCs. Zaban and co-workers reported the use of core/shell quantum dots of a low bandgap semiconductor material (core) incorporated into a TiO2 shell. Small band gap materials are unable to donate electrons to TiO2 and hence they relax to the ground state through fluorescence with high efficiency.469 By tuning the size of the quantum dots, it is easy to match the fluorescence of the donor and acceptor absorbance. FRET-based materials must fulfil the following requirements to get a high PCE: (i) good fluorescence properties of the donor materials, (ii) good match of the absorbance range of the acceptor material with the fluorescence range of the donor material, and (iii) both materials must be within the Forster radius, that is several nanometers. Energy transfer of resonance is independent of the matching of the HOMO–LUMO levels of the dye pair. The Forster radius (R0) is defined as the separation distance between the donor and acceptor materials at which the transfer efficiency is 50%. The Forster radius is related to the orientation factor between the donor–acceptor dipoles (k) and the donor fluorescence quantum yield (ΦD). ΦD must be high to achieve a useful value of R0.472
 
image file: c7nr08350e-t13.tif(36)
where NA is the Avogadro constant (6.022 × 1023), and n is the index of refraction (medium surrounding the donor and acceptor). The rate of energy transfer from a single donor to a single acceptor can be calculated by using the Forster radius.151
 
image file: c7nr08350e-t14.tif(37)
where r is the distance between the donor and acceptor, and τD is the lifetime of the donor excited state in the absence of the acceptor. The critical distance is given by the following relation:151
 
image file: c7nr08350e-t15.tif(38)
where λs is the wavelength of the luminescence maximum and As is absorption, given by:
 
image file: c7nr08350e-t16.tif(39)

image file: c7nr08350e-f27.tif
Fig. 27 (a) Schematic diagram of a FRET-based DSSC device and (b) emission spectra. Reprinted with permission from ref. 151. Copyright 2009 American Chemical Society.

Energy transfer can be calculated using the following relation:151

 
image file: c7nr08350e-t17.tif(40)

However, this system is also limited to absorbing a broad region of visible light. The efficiency of DSSCs can be improved by using mobile quantum dots (QDs) functionalized with thiol ligands in an electrolyte.473 QDs serve as mediators to receive and retransmit energy to the sensitizing dyes, therefore amplifying photon collection by the sensitizing dyes in the visible range and enabling up-conversion of low energy photons to high energy photons for dye absorption.473 However, researchers are still searching for appropriate material combinations for fabrication of efficient FRET-based DSSCs. There are a number of acceptor-donor pairs, such as Z907 (acceptor)//NaYF4:Er3+/Yb3+ nanoparticles (donor),474 squaraine dye (acceptor)//CdS QDs (donor),475 5-carboxy-2-(((3-(1,3 dihydro-3,3-dimethyl-1-ethyl-2H-indol-2-ylidene)methyl)-2-hydroxy-4 oxo-2-cyclobuten-lylidene)methyl)3-3-dimethyl-1-octyl-3H-indolium (SQ02) (acceptor)//CdSe/CdS/ZnS quantum dots (QDs) (donor),476 that have been reported. Graphene quantum dots (GQDs) as energy relay antennas and Ru-based N719 acceptor dyes have been reported.476Fig. 28a is a TEM image of the GQDs. Fig. 28b shows the radiative energy transfer (RET), and interaction between the GQDs and N719 dye resulted in an efficiency (η) of 7.96%, which is 30% more than that of an N719-based DSSC. Fig. 28c shows a schematic representation of the energy level diagram and the electron and energy transportation process between the GQDs and N719 dyes. Fig. 28d shows a schematic representation of the overall energy transfer, electron transportation and interaction in the GQD-based DSSC device. This high efficiency is attributed to the inherent properties of the GQDs, like tunable bandgap and fluorescence, wide absorption spectra, high molecular extinction coefficient, up-conversion emission, high photostability, hot electron injection and chemical inertness.469


image file: c7nr08350e-f28.tif
Fig. 28 (a) TEM image of the GQDs. (b) JV curves of N719 and N719/GQD. (c) Schematic representation of the energy level diagram and energy transfer process between the GQDs and N719 dyes. (d) Schematic representation of the overall energy transfer, electron transportation and interaction in a GQD-based DSSC device. Reprinted with permission from ref. 476. Copyright 2011 Elsevier.

6. Hole transporting materials

The electrolyte has a dynamic role in hole transportation in DSSCs. On photoexcitation, the dye injects electrons into the CB of the semiconductor, while holes are captured by the electrolyte and further transported to the counter electrode. Hole transportation though the electrolyte is carried out by redox shuttles in the electrolyte, such as triiodide/iodide (I/I3). Mostly, the (I/I3) redox shuttle is used as a hole transporter. However, its main drawback is a large mismatch between the oxidation potential of the electrolyte (I/I3−) and the dye, because (I/I3−) electrolytes have an oxidation potential of 0.35 V vs. NHE and the dye oxidation potential is ∼−1.0 V vs. NHE,98 which limits the Voc to between 0.7 and 0.8 V. Also, this couple is corrosive towards metal electrodes, such as Ag, Au and Cu, which leads to less stability. In contrast to iodine/iodide-based electrolytes, cobalt polypyridine complexes have drawn attention due to their higher redox potential and lower corrosiveness. Ionic liquids have also received attention due to their chemical and thermal stability. Also, they have a high viscosity, which is applicable to solid-state DSSCs. However, the low ionic mobility of ionic liquid leads to low-efficiency DSSC devices. Mostly, [Bmin][Tf2N] and [C4mim][PF6] are used as ionic liquid-based solvents. For solid-state DSSCs, mostly spiro-OMeTAD polyethylene oxide is used as a hole transporting material.217,218

7. Summary and outlook

In the near future, DSSCs will become a competitive technology due to their eco-friendly and cost-effective nature, the simple fabrication technology required, and the ongoing progress that is being achieved by developing different nanostructures, dyes, and counter electrodes. Recently, a new strategy was used to improve the efficiency of DSSCs by using a combination of dyes, like a TiO2 photoanode sensitized with a porphyrin dye and an organic dye. Fluorescent dyes have provided high efficiencies in SS-DSSCs via FRET processes in combination with high-extinction coefficient materials to absorb the whole solar spectrum. The discovery of different new dyes, and the manipulation of different nanoarchitectures of the ETM and counter electrode will open new horizons in eco-friendly solar cell device fabrication. The engineering of the photoanode needs to increase the dye loading capability, electron transportation characteristics and scattering phenomenon. The above-mentioned characteristics of photoanodes have been obtained by designing semiconductor materials with mesoporous, nanorod, nanowire and hierarchical etc. nanostructures. Also, doping, sensitization, alloy formation and surface modification etc. of basic semiconductors like TiO2 have been employed to get prominent photoanodes. The CE plays an important role in the dye regeneration reaction and circuit completion of the DSSC device. In current DSSC development, prominent CEs have been fabricated by the use of different carbon materials, metal oxides, metal nitrides and inorganic perovskite materials due to their high conductivity and better electrocatalytic activity.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

One of the authors, Dr Jasmin S. Shaikh, Women Scientist-A, acknowledges the Women Scientist project of Department of Science (DST), Delhi, India for providing funds through grant No. SR/WOS-A/PM-1037/2014(G). This research work was supported by the Basics Science Research Program through the National Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2014R1A2054051). This work was also supported by the Korea Research Fellowship Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (2016H1D3A1909289) through an outstanding overseas young researcher fellowship to Dr Sawanta S. Mali. Dr Pongsakorn Kanjanaboos acknowledges support from EGAT & NSTDA (funding number FDA-CO-2560-5449-TH) and the Faculty of Science, Mahidol University.

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