Rare earth ion doped phosphors for dye-sensitized solar cells applications

Abstract

Dye-sensitized solar cells (DSSCs) have attracted extensive attention as one of the promising alternatives to silicon solar cells. However, DSSCs have a maximum absorption in the visible light of solar spectrum, which confines their power conversion efficiency. Lots of research efforts have been focused on extending light absorption to enhance the conversion efficiency. Rare earth ion doped up/down conversion materials is an available approach to compensate for the non-absorbable wavelength region of DSSCs via converting ultraviolet and near-infrared radiation to visible emission. In addition to the light-harvesting enhancement, light-scattering effect and recombination loss can also be achieved in DSSCs by utilizing upconversion (UC) or downconversion (DC) materials. Moreover, the introduction of UC or DC facilitates to improve the stability of solar cells. In this review paper, the performance of dye-sensitized solar cells based on up or down conversion materials will be introduced.

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Nannan Yao

Nannan Yao received a BS degree in the School of Physics and Technology from University of Jinan in 2014. She is currently a MSc student majoring in optoelectronic materials and devices at the University of Jinan. Her current research is focused on improving the performance of dye sensitized solar cells.

Jinzhao Huang

Jinzhao Huang is an associated Professor of the School of Physics and Technology at University of Jinan. He graduated from Linyi University with a BSc in 2003. He received a PhD from Beijing Jiaotong University in 2008. Dr Huang's research interests focus on solar cells.

Ke Fu

Ke Fu received his BS in Physics from the University of Jinan in 2014. He is currently a Master degree candidate majoring in optoelectronic materials and devices at the University of Jinan, China. His work focuses on the design and synthesis of novel hybrid nanomaterials for photocatalysis.

Xiaolong Deng

Dr Xiaolong Deng is an assistant Professor of the School of Physics and Technology at the University of Jinan. He graduated from Northwestern Polytechnical University with a BSc in 2006, and received an MEng from Shanghai University in 2009. He received a PhD from Konkuk University in 2013. Dr Deng's research interests focus on the synthesis and characterization of advanced sub-micro/nanomaterials for photocatalysis, conversion and energy storage applications.

Meng Ding

Dr Meng Ding is an assistant Professor of the School of Physics and Technology University of Jinan. She graduated from Hebei Normal University with a BSc in 2006, and received an MSc from Jilin University in 2009. She received a PhD from Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences in 2012. Dr Ding's research interests focus on the design, synthesis and characterization of advanced micro/nanomaterials for LED and photoelectric detection applications.

Xijin Xu

Xijin Xu, received his PhD degree in 2007 from the Institute of Solid State Physics, Chinese Academy of Sciences. During the following years, He worked in Nanyang Technological University in Singapore, National Institute for Materials Science (NIMS), and Australia. He started his research career at the University of Jinan as a full professor from 2011, and his research mainly focuses on functional micro/nano-materials and devices, including the constructions and applications of functional catalytic material, optoelectronic materials, especially on their applications in solar cells, supercapacitors, lithium-ion batteries and gas sensors. Up to 2015, He has published above 70 SCI papers in international journals including ACS Nano, Adv. Funct. Mater., Small and Scientific Reports, etc.

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1. Introduction

Recently, clean renewable energy utilization has been proposed as an effective solution for environmental pollution and global warming.^1,2 Solar energy acting as the most abundant renewable energy source can directly convert solar radiation to electricity by the application of photovoltaic devices.³ Silicon-based solar cells, the first generation of photovoltaic systems, are the majority of the solar electric market. The second generation solar cells are based on thin-film technology, including CdTe, CIGS and so on. However, the issues of high cost and environmentally harmful waste in the processing technologies of these devices lead to a constant progress in the development of new photovoltaic systems. To avoid these problems, the third generation systems start to emerge in the photovoltaic market, which commonly introduced double junctions, triple junctions and nanotechnology into solar cell fabrication methods. Dye sensitized solar cells (DSSCs) as one of the third generation photovoltaic cells have attracted much attention due to their low cost, easy fabrication procedures, environmentally friendly components and relatively high conversion efficiency.^4–6 Since Grätzel and his co-workers first designed dye sensitized solar cells in 1991, considerable efforts have been devoted to improve the performance of DSSCs over the past decade.⁷ To date, a high conversion efficiency of 14.5% has been achieved for liquid-based DSSCs.⁸ The most important functions of DSSCs in converting solar energy to electricity are the conversion of the photons to carriers and the transport of photogenerated carriers through the device. In general, the most conventionally used Ru dyes (e.g., N719, N749) with a band-gap of 1.8 eV can only absorb visible light in the wavelength range between 300 and 800 nm,^9,10 thus 50% of solar irradiation in the ultraviolet and infrared regions is not utilized. To overcome this drawback, co-sensitization of photoanode has been used to increase light absorption of DSSCs.^11,12 What's more, some researches have demonstrated that highly luminescent energy-relay dyes in the electrolyte can absorb higher energy photons and then undergo Förster transfer to sensitizing dye, broadening the spectral response of solar cells.^13,14 Actually, the conversion efficiency of DSSCs is not only limited by light harvesting efficiency, but also the electron injection efficiency as well as the rate of charge recombination.^15–17

Generally, downconversion (DC) and upconversion (UC) are consist of rare earth (RE) doped host material, and the transitions between the 5d orbital and 4f orbital are shielded from the influence of the host lattice because of the filled outer shell in these ions. RE doped near infrared (NIR)-to-visible upconvertor or ultraviolet (UV)-to-visible downconvertor serve as an excellent luminescent conversion material arising from their intra 4f transitions, which have been widely applied in photovoltaic devices.^18–20 The usage of DC or UC is an effective way to improve the performance of DSSCs. For one reason, the capability of light harvesting has a primary impact on the performance of DSSCs. However, DSSCs with a dye-sensitized TiO₂ photoanode have a maximum absorption in the visible region of the total incident solar irradiation.²¹ To reduce energy loss in DSSC, UC and DC phosphors were used, in which infrared and ultraviolet radiation energy is converted to visible light that can be better match the absorbed spectrum of the dye so as to enhance conversion efficiency of the solar cell. Therefore, the performance of DSSCs will be effectively improved by modifying the solar energy spectrum.²² For another, the light scattering is considered as another approach that can make an impact on the light-harvesting capability of the photoelectrode by utilizing optical enhancement effects. There have been many studies on enhancing the light-harvesting efficiency of photo-electrodes by adding submicrometer-scale particles as light scatterers, resulting in a significant advance in DSSCs. However, the large-sized particles in photoanode films may lower the internal surface area of the photoelectrode film and increase the distance of light. Some groups have been reported that RE luminescence materials embedded in TiO₂ can not only scatter light within the photoanodes but also achieve luminescence conversion. In addition, the recombination losses of DSSCs can be reduced by using RE ion doped materials.

This present paper aims to review the recent progresses in the development of RE ions doped luminescent materials for improving the performance of DSSCs. Especially, this work focuses mainly on the light-harvesting enhancement, light-scattering effect, recombination loss and stability improvement in DSSCs via utilizing UC or DC materials.

2. UC phosphors

Photon UC is a non-linear optical process in which the sequential absorption of two or more low energy photons leads to the emission of a high energy photon.²³ It is anti-Stokes law in its basic statement, that is to say the output photon energy is stronger than input photon energy. In 1959, Bloembergen first made a proposal for an infrared quantum counter (IRQC), which used super excitation as a detector to detect and count infrared (IR) photons.²⁴ And then Auzel suggested that energy transfers between RE ions could occur between two excited-state ions in upconversion processes in 1966.²⁵ After that, many detail researches have been carried out and a large amount of reviews on UC have been presented by Auzel,^26,27 Mita and Nagasawa,²⁸ and Garlick.²⁹ Three basic UC mechanisms have been achieved: energy transfer upconversion (ETU), excited-state absorption (ESA) and photon avalanche (PA). Various two-photon UC mechanisms are presented in Fig. 1.³⁰ The simplest mechanism is ground state absorption/excited state absorption (GSA/ESA). And the very efficient process of upconversion by sequential energy transfers which referred to as ETU. The ETU step involves a sequential absorption of two photons that transfer nonradiative energy from an excited ion (sensitizer) to another neighboring ion (activator) with the sensitizer concomitantly relaxing back to the ground state by nonradiative process.³¹

Fig. 1 Various two-photon UC mechanisms. Reproduced by the permission of ACS (ref. 30: Chem. Rev., 2004, 104, 139–174).

UC phosphors are comprised of Ln³⁺ doping an inorganic crystalline host lattice with low concentrations. Generally, the host materials are required for low lattice phonon energies, close lattice matches to dopant ions, and good chemical stability.^32–34 The most commonly used host materials are NaYF₄,³⁵ LaF₃,³⁶ Y₂O₃ (ref. 37) and ZnO³⁸ and so on. Lanthanide ions of Er³⁺, Ho³⁺, Tm³⁺, Pr³ have attracted much attentions for the investigation of upconverter, and Yb³⁺ ion is usually co-doped as sensitizer to increase NIR absorption intensity of the upconverter.^39,40 Among these UC phosphors, the Er³⁺/Yb³⁺ couple was particularly famous pair until now,⁹ the proposed energy transfer mechanism between Yb³⁺ and Er³⁺ is depicted in Fig. 2.³⁵ Also, typical photoluminescence (PL) spectra of common Ln³⁺ doped different host materials under NIR excitation are summarized in Table 1.

Fig. 2 The energy transfer mechanism between Yb³⁺/Er³⁺. Reproduced by the permission of Elsevier (ref. 35: Appl. Surf. Sci., 2015, 333, 23–33).

Table 1 The PL spectral of rare earth ions doped various host materials under an excitation around 980 nm

UC	Excitation (nm)	Emission (nm)	Reference
NaYF4:Yb^3+,Er^3+	980	521; 540; 653	35
NaYF4:Yb^3+,Pr^3+,Er^3+	976	470; 483; 524; 540; 585; 608; 642; 654; 692; 722	41
NaYF4:Yb^3+,Tm^3+,Co^2+	980	475; 800	42
CaF2:Yb^3+,Er^3+	980	520; 540; 658	43
LiYF4:Tb^3+,Yb^3+	960	541; 550; 654; 669	44
TiO2:Er^3+,Yb^3+	980	526; 547; 659	45
ZnO/TiO2:Er^3+,Yb^3+	980	675; 657; 557; 545	38
Y2O3:Er^3+	980	555; 590	37
BiVO4:Nd^3+	980	635; 720; 770	46
SrIn2O4:Tm^3+,Yb^3+	980	486; 657	47
SrIn2O4:Ho^3+,Yb^3+	980	546; 673	47

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3. DC phosphors

DC followed the well-known principle of the Stokes law, is a luminescence process where a high energy photon is split into two low energy photons.⁴⁸ The first theoretical possibility of downconversion was proposed in 1957 by Dexter, and the quantum efficiency of downconversion exceeds one because more than one photon is emitted for each incoming photon.⁴⁹ Experimentally, it was not until 1974 that Piper et al.⁵⁰ and Sommerdijk et al.⁵¹ independently reported the observation of quantum cutting in YF₃:Pr³⁺. It was reported that Pr³⁺ excited under 185 nm released a blue photon of 407 nm and a red photon of 620 nm due to the ¹S₀-¹I₆ and ³P₀-³F_J, ³H_J transition respectively. DC has been investigated for decades in photovoltaic device after a theoretical study carried out by Trupke et al.⁴⁸ in 2002, in which a theoretical efficiency of 38.6% was achieved by using DC materials. While many researchers have been focused on mechanism of DC, and several DC mechanisms of energy level schemes for two ions are illustrated in Fig. 3.⁵² In Fig. 3(a), the mechanism of quantum cutting for single ion is shown, which emitted unfavorable spectrum: ultraviolet and infrared part. Fig. 3(b)–(d) presents the DC mechanisms that involve energy transfer (➁) and cross-relaxation (➀) between two different RE ions. After excitation of ion A, energy transfer and cross-relaxation occur from A to B. Fig. 3(c) and (d) illustrates the cross-relaxation process followed emission from ion A to ion B.

Fig. 3 (a) Two photon emission from a single ion; (b) down-conversion with ion pairs by cross-relaxation from ion A to ion B and energy transfer from ion A to ion B with emission from ion B, (c and d) cross-relaxation followed emission from ion A to ion B. Reproduced by the permission of Elsevier (ref. 52: J. Lumin., 2000, 87, 1017–1019).

A typical DC consists of a host lattice doped with a few mol% of an activator ion, such as the Eu³⁺ ion. To avoid the IR and UV losses encountered with a single ion mentioned above, it was attempted to use energy transfer between ion pairs.⁵³ An example of the DC process for ZnO:Eu³⁺,Dy³⁺ is presented in Fig. 4,⁵⁴ where after excitating ZnO by incident light, the photo-induced electrons are generated, followed by trapping via the defect states. And then the energy transfer occurs between ZnO and Eu³⁺,Dy³⁺ ions, resulting in the characteristic emissions of Eu³⁺,Dy³⁺. Besides, typical emission spectra of common Ln³⁺ doped different host materials under UV excitation are summarized in Table 2.

Fig. 4 Schematic diagrams of energy transfer process between ZnO and Eu³⁺,Dy³⁺ ions. Reproduced by the permission of Springer (ref. 54: Optoelectronics Letters, 2014, 10, 161–163).

Table 2 The PL spectral of DC doped with rare earth ions under an UV excitation

DC	Excitation (nm)	Emission (nm)	Reference
ZnO:Eu^3+	395	591; 616; 696	55
ZnO:Ce^3+	354	399; 566	56
TiO2:Tm^3+	364	800	57
ZnO:Eu^3+,Tb^3+	320	491; 549; 581; 611	58
ZrO2:Eu^3+,Yb^3+,Y^3+	393	592; 613; 651; 704; 969; 1035	59
SrTiO3:Nd	325	904; 1090	60
NaYF4:Ho^3+,Yb^3+	480	540; 650; 750; 965; 1015	61
Y2O3:Pr^3+,Yb^3+	482	511; 1030; 1075	62
BaF2:Eu^3+,Gd^3+	202	527; 556; 594; 620; 656; 702	63

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4. UC phosphors for DSSCs applications

Enhanced light-harvesting efficiency in UV and NIR region is an attractive issue in the field of solar cells.^22,64 The utilization of DC or UC phosphors is an alternative way to reduce the photon loss.^65,66 In recent years, research on UC phosphors is extended from conventional solar cells to more advanced solar cells like organic solar cell and DSSCs.^67–69 The possible mechanisms for enhancing performance of DSSCs is that UC phosphors have the ability to convert two low energy infrared photons to one high energy photon that can be absorbed by dyes. And efficient UC PL facilitates to generate more electron–hole pairs. Another approach to improve efficiency of solar cell is light management by using light scatters or surface plasmon resonances. The scattering component could modify the photon paths and extend the traveling distance of the incident light in the photoelectrodes, thereby enhancing the probability of photons being captured by the sensitizers. Many studies have suggested that the incorporation of the UC materials into the photoanodes could further improve the light harvesting efficiency as light scatters as well as reduce the electron–hole recombination.^20,70,71 Therefore, the UC phosphors have tremendous application potential as spectrum modifier to extend the response to NIR radiation for improve the efficiency of DSSCs. Besides, typical UC phosphors with different host materials for DSSCs applications are summarized in Table 3.

Table 3 The UC phosphors for DSSCs applications

Sample	JSC (mA cm^−2)	VOC (V)	FF	η (%)	Reference
NaYF4:Yb^3+,Er^3+	8.32	0.79	0.66	4.32	74
NaYF4:Yb^3+,Er^3+@SiO2	14.19	0.76	0.66	7.28	9
NaGdF4:Yb^3+,Er^3+,Fe^3+/Ag	12.62	0.78	0.71	7.05	71
TiO2:Er^3+	13.38	0.78	0.63	6.63	20
Lu2O3:Tm^3+,Yb^3+	13.41	0.76	0.65	6.63	80
TiO2:Er^3+,Yb^3+	18.9	0.77	0.62	8.98	83
TiO2:Y^3+	15.9	0.74	0.77	9.0	84
YOF:(Yb^3+,Er^3+)	13.2	0.78	0.64	6.57	82
Y0.78Yb0.20Er0.02F3/TiO2	15.58	0.80	0.65	7.9	85

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4.1 Fluoride-based UC for DSSCs application

Lanthanide doped UC phosphors were applied in DSSC for the first time by Shan and Demopoulos in 2010.¹⁶ In their study, the Er³⁺–Yb³⁺ co-doped LaF₃ and then combined with TiO₂ (UC-TiO₂) was used as an UC layer to fabricate a triple-layer working electrode for DSSCs. In Fig. 5, two up-conversion emission bands centered at green (543 nm) and red emission (655 nm) are observed under 980 nm irradiation, which are corresponding to the transition from excited state ⁴F_7/2 and ⁴F_9/2 to ⁴I_15/2 of Er³⁺ respectively. However, the ratio of emission intensity for green and red radiation is lower in N719 sensitized UC-TiO₂ film than that in UC-TiO₂ film. This decrease can be attributed to the efficient absorption of green emission at around 543 nm by the N719 dye. Therefore, the NIR light can be utilized by DSSCs with the presence of UC-TiO₂ film. However, this composite film was proved to be ineffective to enhance the photocurrent and efficiency due to the apparent charge recombination at the interfaces between the UC-TiO₂ and electrolyte.

Fig. 5 (a) Upconversion fluorescence spectra of UC-TiO₂ film and (b) the N719-sensitized UC-TiO₂ film. Reproduced by the permission of Wiley (ref. 16: Adv. Mater., 2010, 22, 4373–4377).

In order to allow full utilization of the upconverted light and simultaneously avoid the charge recombination mentioned above, other composite layers like core–shell structure were investigated for application in DSSCs. Among the UC phosphors, the hexagonal phase sodium yttrium fluoride (β-NaYF₄) as one of the excellent host materials has been widely studied in practical application due to the low phonon energy of lattices, especially for the systems co-doped with Er³⁺ activator and Yb³⁺ sensitizer.^72,73 Recently, Zhang et al.⁷³ used NaYF₄:Yb³⁺,Er³⁺/TiO₂ core–shell nanoparticles as photoanode of DSSCs. This special core–shell structure showed excellent capacity to convert infrared to two visible emission bands in the green and red spectral regions, at the same time, the semiconductor character was also retained. The DSSCs with NaYF₄:Yb³⁺,Er³⁺/TiO₂ core–shell structure show a higher efficiency than that of pure TiO₂ and a mixed photoelectrode due to the increased spectral response of DSSCs and its semiconductor character. Furthermore, in order to achieve light reflecting and NIR light harvesting as well as overcome the problem of electron trapping caused by surface defects and ligands of β-NaYF₄:Yb³⁺,Er³⁺ crystals, Zhao et al.⁷⁴ have prepared a highly uniform core/double–shell structure β-NaYF₄:Yb³⁺,Er³⁺@SiO₂@TiO₂, in which amorphous SiO₂ as inner shell creates an electrical isolation for UC core and the outer TiO₂ shell can prevent the decrease of dye loading. As a result, an enhanced efficiency of 10.9% was achieved by employing this core/double–shell structure in DSSC.

Apart from introducing the UC materials directly into photoanodes of DSSCs, using external NIR light harvesting and light reflecting bifunctional layers is an alternative strategy yet. An upconversion phosphor β-NaYF₄:Yb³⁺ (18%), Er³⁺ (2%) was applied at the back of Si solar cells, which leads to a longer path length of the incoming light.⁷⁵ Similarly, Shan et al.⁷⁶ applied β-NaYF₄:Er³⁺,Yb³⁺ nanoplatelets on the external side of the counter electrode in DSSCs as shown in Fig. 6, which exhibits two functions of light-reflection and near-infrared light harvesting. And a 10% enhancement of photocurrent and efficiency of DSSCs was obtained. They also directly applied the UC nanoplatelets on the top of TiO₂ layer in the interior of the cell to eliminate the charge recombination with electrolyte. Similar results were reported by Guo et al.⁷⁷ when the bifunctional β-NaYF₄:Yb³⁺,Er³⁺@SiO₂ was incorporated into TiO₂ nanocrystalline porous film.

Fig. 6 The novel DSSCs consisting of one Internal TiO₂ transparent layer and external rear layer of β-NaYF₄b:Er³⁺,Yb³⁺ nanoplatelets. Reproduced by the permission of ACS (ref. 76: Appl. Mater. Inter., 2011, 3, 3239–3243).

For further improving the light absorption and photocurrent of DSSCs, many groups have combined localized surface plasmon resonance (LSPR) and upconversion-effect. Ramasamy et al.⁷¹ reported 21.3% enhancement in efficiency by using a rear reflector structure consisting of β-NaGdF₄:Yb,Er,Fe upconversion nanoparticles and silver particles in DSSCs. The UC nanoparticles can absorb NIR light and emit visible photons, while silver particles can enhance the upconversion luminescence because of the surface plasmonic effect and large scattering efficiency. Moreover, UC materials with metal nanoparticles (β-NaYF₄:Yb³⁺,Er³⁺@SiO₂@Au) have been successfully prepared as a multifunctional layer on the top of transparent TiO₂ film in the internal DSSCs.⁷⁸ In this composite structure (Fig. 7(a)), SiO₂ coated on NaYF₄:Yb³⁺,Er³⁺ acted as interface layer for Au attachment and an isolating layer to avoid the electron trapping and capture by UC material. When under illumination, spectral conversion from NIR light to visible radiation occur in NaYF₄:Yb³⁺,Er³⁺@SiO₂ and randomly scatter the light to TiO₂ film (Fig. 7(b)). After decorating Au on core–shell structure, the light can be effectively concentrated in the interior cell due to the presence of LSPR. Meanwhile, from Fig. 7(c) the emission intensity of upconversion could be increased because the plasmonic resonance frequency of Au overlaps well with the green and red emission of the upconversion emission. Hence, the outstanding light scattering, LSPR and enhanced UC emission from NaYF₄:Yb³⁺,Er³⁺@SiO₂@Au composites layer will result in an enhanced J_SC (15.84 mA cm⁻²) and η (8.23%) for DSSCs in this work.

Fig. 7 (a) The DSSCs device structure with β-NaYF₄:Yb,Er@SiO₂@Au nanocomposites; (b) the possible mechanism of light harvesting in device; (c) upconversion fluorescence spectra of different composites under 980 nm laser; (d) J–V curves of DSSCs with different photoanode. Reproduced by the permission of RSC (ref. 78: J. Mater. Chem. A, 2014, 2, 16523–16530).

4.2 Oxide-based UC for DSSCs application

In recent years, up-conversion oxide phosphors such as TiO₂:Er³⁺, TiO₂:Er³⁺,Yb³⁺, Lu₂O₃:Tm³⁺, Yb³⁺, ZnO:Er³⁺,Yb³⁺, Y₂O₃:Yb³⁺,Tm³⁺ and so on have been widely investigated in photovoltaic devices. Recently, TiO₂ with Er activator ions as an photoanode material of DSSCs was provided by Li et al.²⁰ An enhancement of 62.9% in conversion efficiency was obtained compared to the reference cell due to the presence of TiO₂:Er³⁺. The luminescence spectrum of UC-TiO₂ excited under 980 nm shown in Fig. 8(a) exhibits a blue peak at 439 nm, a green peak around 540 nm and a red region from 650 nm to 700 nm, which overlap with the absorption of N719. Therefore, an optimal performance of DSSCs was achieved with UC-TiO₂/TiO₂ at a ratio of 7 : 5. In addition, when RE ions Er³⁺ are substituted for Ti⁴⁺ ion lattice sites in the TiO₂ film, a p-type doping effect occurs, similarly to that trivalent positive ion doped in Si semiconductors, resulting in the elevation of the energy level of oxide film and the increase of V_OC. And Er modified TiO₂ mixed with raw TiO₂ also serve as a potential driver for excited electrons to inject into the inner raw TiO₂ layer, which can suppress the recombination of electrons and holes, consisting with the results of electrochemical impedance spectroscopy (EIS) measurement. Besides, a hybrid photoanode consisting of three-dimensional shuttle like up-converting Y₂O₃:Er³⁺ and ZnO nanocrystalline was incorporated in ZnO-based DSSCs.⁷⁹ Such a composite constructor film can improve the effective harvesting of the solar light in NIR region by incorporation of UC phosphors, as well as generate significant light scattering due to the presence of large particle size. Furthermore, several other groups reported Lu₂O₃:Tm³⁺,Yb³⁺,⁸⁰ Er³⁺–Yb³⁺ co-doped TiO₂ (ref. 58) and so on, as luminescence medium, were also introduced into the photoanode, which improve incident light harvest via a conversion luminescence process and increases photocurrent and overall efficiency of DSSCs.

Fig. 8 (a) The PL spectrum of UC powders excited at 980 nm; (b) J–V curves of DSSCs based on TiO₂:Er/TiO₂ with different ratios. Reproduced by the permission of Elsevier (ref. 20: J. Solid State Chem., 2013, 198, 459–465).

4.3 Oxyfluoride-based UC for DSSCs application

In addition to the fluoride or oxide-based phosphors, oxyfluoride crystals have widely been considered as host materials due to their comparative low phonon energy corresponding to fluorides and the retained chemical and physical properties of oxides. Yu et al.⁸¹ combined the up-conversion property of rare earth ions and the F-doping effect together, an efficient improvement of photovoltaic performance for DSSCs was then achieved. The results indicate that the Er³⁺ and Yb³⁺ co-doped TiO_2−xF_x could increase response in visible light region via spectra conversion and enhance the luminescence efficiency of rare earth ions with F-doping, as well as lower the band gap energy, leading to a faster electrons injection and a lower electron–hole recombination rate. What's more, the bilayer structured photoelectrode UC-TiO_2−xF_x and P25 with different size as a light-scattering layer could enhance light harvesting and reduce the light loss. In addition, Yb³⁺ Er³⁺ doped yttrium oxyfluoride (YOF:Yb³⁺,Er³⁺) acted as luminescence medium is introduced into the TiO₂ electrode.⁸² Fig. 9(a) shows the Mott–Schottky (MS) plots of TiO₂ electrodes with different amount of RE dopant. It is clear that the curves show a positive slope at negative potentials, consisting with the expected n-type semiconductor characteristics and a flat band potential of −0.44 V was obtained for pure TiO₂ electrode. And the negative shift of flat-band was noticed with the increasing amounts of RE³⁺ dopant. Besides, as a p-type dopant, YOF:Yb³⁺,Er³⁺ act as a p-type doping will lead to an upward shift of conduction band edge and an improvement Fermi level of TiO₂. Owing to the effect of up-conversion and the shift of the conduction band edge of TiO₂, the photocurrent and photovoltage of the DSSC was enhanced (Fig. 9(b)).

Fig. 9 (a) MS plots of YOF:Yb³⁺,Er³⁺/TiO₂ systems; (b) J–V curves of the DSSC with and without YOF:Yb³⁺,Er³⁺ dopant (7 wt%) under an infrared light irradiation of 33 mW cm⁻². Reproduced by the permission of Elsevier (ref. 82: Electrochim. Acta, 2012, 70, 131–135).

5. DC phosphors for DSSCs applications

To reduce the energy loss in UV light is another important issue for improving the efficiency of DSSCs. In the case of downconversion, one high energy photon is split into two lower energy photons that can both be absorbed by the solar cell. Besides, for the practical use of DSSCs, the chemical stability is as important as its conversion efficiency. Irreversible electrochemical and thermal degradation of the dye or electrolyte components, originating from UV irradiation affect the chemical stability of DSSCs. The common strategy to avoid the UV light is utilizing an additional UV filter to absorb UV light and convert it to visible light, which is absorbed by dye. Researches on the application of DC materials in photovoltaic field have been investigated for decades. The research results confirmed that the incorporation of the DC materials into the DSSCs can not only increase incident light harvest and photocurrent via conversion luminescence and light scatter, but improve the photovoltage according to the doping effect. Also, the lifetime of the solar cell could be greatly enhanced by the absorption of UV light. Therefore, the usage of DC phosphors is an available option to enhance the performance of DSSCs. Besides, DC phosphors with different host materials for DSSCs applications are summarized in Table 4.

Table 4 The DC phosphors for DSSCs applications

Sample	JSC (mA cm^−2)	VOC (V)	FF	η (%)	Reference
TiO2:Eu^3+	9.61	0.77	0.69	5.16	88
TiO2:Sm^3+	10.9	0.81	0.67	5.81	88
Y3Al5O12:Ce^3+	14.91	0.74	0.71	7.91	100
YVO4:Eu^3+,Bi^3+@SiO2	13.44	0.69	0.64	5.9	91
LaVO4:Dy^3+	11.4	0.68	0.41	3.7	96
SrAl2O4:Eu^2+,Dy^3+	12.84	0.80	0.66	6.85	101
Y2O3:Eu^3+	13.26	0.79	0.62	6.52	102
Gd2O3:Sm^3+	12.93	0.77	0.67	6.72	87

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Recently, the exploration of DC oxide phosphors based on TiO₂, Gd₂O₃ et al.^86,87 in the effect of their DC properties have been investigated. And the hybrid structure compositing with DC materials and TiO₂, ZnO and non-metal/metal doped TiO₂ have been widely used to construct the solar cells. In 2011, Hafez et al.⁸⁸ reported the fabrication of lanthanide doped TiO₂ photoelectrodes for the photovoltaic efficiency enhancement of DSSCs. The efficiency of 5.81% and 5.16% was reported for Sm³⁺ and Eu³⁺, respectively, which is higher than that of undoped TiO₂ photoelectrodes (4.23%). The higher improvement is mainly attributed to the DC luminescence characteristics of Sm³⁺ and Eu³⁺ ions. Similarly, we reported the application of DC material ZnO:Eu³⁺,Dy³⁺ on DSSCs to extend the spectral response range to the UV region.⁸⁹ With a concentration 1.75% of ZnO:Eu³⁺,Dy³⁺ to TiO₂, the performance of the DSSCs reached to the optimal values. Attempts to improve the light harvesting, one-dimensional structure combined with DC phosphors was used as photoanode in DSSCs. Hafez et al.⁹⁰ also synthesized TiO₂:Eu³⁺ nanorods by a hydrothermal method and then successfully assemble DSSCs with a TiO₂:Eu³⁺ nanorods/TiO₂ nanoparticles bilayer electrode. Compared with that of the cell consisting of undoped bilayer electrode, the light-to-electrical energy conversion efficiency of the cell with this bilayer electrode was improved to about 1% due to the down-shifting of Eu³⁺ ions from ultraviolet light to visible and the absorption increasing of the dye in the visible region. Besides, some groups have reported the introduction of light scatters with rare earth ions doped phosphors. Lai et al.⁹¹ have embedded the submicron-sized YVO₄:Eu³⁺,Bi³⁺@SiO₂ core–shell particles in the nanostructured TiO₂ layer of DSSCs. Fig. 10(a) and (b) show SEM images of the submicron-sized YVO₄:Eu³⁺,Bi³⁺ and YVO₄:Eu³⁺,Bi³⁺@SiO₂ particles. It's obvious that the surfaces of samples transfer from rough to smooth and the average diameter of particles becomes larger (about 400 nm) with the addition of SiO₂. And the excitation and emission spectra of YVO₄:Eu³⁺,Bi³⁺@SiO₂ core–shell particles in Fig. 10(c) shows a wide excitation band from 220 nm to 380 nm and a strong emission peak at 618 nm. As a result, the YVO₄:Eu³⁺,Bi³⁺@SiO₂ can not only broaden the light harvesting range to short wavelength by DSSCs via converting UV light to visible light, but also acted as light scattering particles increase the optical distance of photos in the photoanodes. And an enhancement of 64% in conversion efficiency of solar cells with the dual functional particles was achieved. In addition to the above discussion, Diau et al.⁹² have designed a novel approach to increase the light-harvesting efficiency by placing the reflective luminescent down-shifting (R-LDS) layer outside the DSSCs on either counter electrode (CE) side or working electrode (WE) side. As shown in Fig. 11, the working electrode of solar cell was illuminated in the case of the R-LDS layer on the CE side, while back illumination is used when the R-LDS layer is on the WE side, which aims to achieve the backscattering features. Due to the R-LDS layer exhibits bifunctional characteristic including the luminescent down-shifting and back reflective feature, the overall efficiencies of the CE/WE-coated LDS device has been improved to 5.0% and 4.8%, respectively.

Fig. 10 (a) SEM images of the submicron-sized YVO₄:Eu³⁺,Bi³⁺ and (b) YVO₄:Eu³⁺,Bi³⁺@SiO₂; (c) the excitation and emission spectra of the YVO₄:Eu³⁺,Bi³⁺@SiO₂ particles. Reproduced by the permission of Elsevier (ref. 91: Ceram. Int., 2014, 40, 6103–6108).

Fig. 11 The DSSCs based on (a) the R-LDS layer coated on the CE side (front illumination) and (b) the R-LDS layer coated on the WE side (back illumination). Reproduced by the permission of ACS (ref. 92: ACS Appl. Mater. Inter., 2013, 5, 5397–5402).

Another approach to enhance the light absorption is increasing oxygen vacancies on the surface of TiO₂, which could induce more dye adsorption and improve the conversion efficiency of DSSCs.⁹³ Jing et al.⁹⁴ have reported that La-doping TiO₂ could inhibit the phase transformation and increase the amount of oxygen vacancies on the TiO₂ surface, leading to higher photocatalytic activity. Similar phenomena were also observed in DSSCs by Zhang et al.⁹⁵ They synthesized La-doped TiO₂ by a sol-hydrothermal method and investigated the effect of La-doping on oxygen vacancies by Electron Paramagnetic Resonance (EPR) and Brunauer–Emmett–Teller (BET). The chemical bond strength of La–O is stronger than that of Ti–O, so more oxygen vacancies will be easier to form with La³⁺ doped onto the surface of TiO₂. In Fig. 12, the EPR result illustrates the amount of oxygen vacancies increases as the increase of dopant concentration until 1 mol%, and excessive doping causes a decrease of oxygen vacancies, consisting with BET. The higher oxygen vacancy density results in the increase of absorbed dye and a 6.72% efficiency of DSSCs with 1 mol% dopant concentration.

Fig. 12 EPR patterns of pure TiO₂ and La-doped TiO₂. Reproduced by the permission of ACS (ref. 95: J. Phys. Chem. C, 2010, 114, 18396–18400).

Moreover, considering the poor stability of DSSCs under UV radiation, DC phosphors layer can be used as an optical filter with UV light absorption.^96,97 Liu et al. have reported enhanced lifetime for DSSCs using LaVO₄:Dy³⁺ transparent film coated onto the back surface of an indium tin oxide glass substrate modified with TiO₂ as shown in Fig. 13.⁹⁶ The LaVO₄:Dy³⁺ film has ability to absorb the ultraviolet light (250–320 nm), thus it can be used as an UV filter for DSSC and enhanced its stability by deducing the degradation of the dye or electrolyte components of the cell. More importantly, the UV light could be downconverted to visible emission (450–700 nm), which is subsequently absorbed by the N3 dye. The stability of the device with the introduction of LaVO₄:Dy³⁺ luminescent film was improved as presented in Fig. 13, which can be ascribed to suppression for the degradation of dye and electrolyte under UV radiation. However, the device with a thin LaVO₄ and LaVO₄:Dy³⁺ layer show a lower I_SC than conventional cells due to the reduction of UV absorption by dye. A major limitation of using Dy³⁺-doped LaVO₄ nanocrystals is that only a small portion of the ultraviolet sunlight is harvested. Therefore, an ideal ultraviolet-absorbing luminescent converter for DSSCs should effectively capture the whole ultraviolet part of the solar spectrum and the emission band should exactly match the absorption of the dye.

Fig. 13 (a) Structure of DSSCs with the luminescent UV filtration film; (b) stability of as-prepared DSSCs. Reproduced by the permission of AIP (ref. 96: Appl. Phys. Lett., 2006, 88, 173119).

Furthermore, RE ions-doped nanostructured semiconductors may control the size or band gap engineering for energy level and electron transport. For example, Li et al.⁸⁷ prepared Gd₂O₃:Sm³⁺ nanoparticle to construct DSSCs, in which Gd₂O₃:Sm³⁺ can not only improve light harvest via a DC luminescence process, but elevate the energy level of the oxide film via p-type dopant, resulting in an enhancement of photocurrent and photovoltage of the DSSCs. However, excessive Gd₂O₃:Sm³⁺ produces defects in the oxide film, which causes the recombination of photo-induced holes and electrons and the decrease of J_SC.

In contrast, a decrease of photovoltage was observed with the introduction of Eu doped ZnO according to the results reported by Zhao and his coworkers.⁹⁸ They reported the influence of Eu³⁺ doped ZnO on the performance of DSSCs. The MS measurement in Fig. 14(c) displays a positive shift in the flat band potential of ZnO:Eu³⁺, which implies the decrease of V_OC of DSSCs, however, increases the energy gap between the conduction band of ZnO and the lowest unoccupied molecular orbital (LUMO) of dye, leading to a driving force of electron injection. And a lower charge transfer resistance and prolonged electron lifetime was obtained for DSSCs based on ZnO:Eu³⁺ in Fig. 14(d), suggesting that more efficient charge transfer through a longer distance occur in DSSCs. Results shows that 27% improvement in efficiency of solar cell with ZnO:Eu³⁺ than ZnO-based device. It is noteworthy that the ZnO:Eu³⁺ cell has a higher photoresponse than that of pure ZnO cell through the IPCE spectra (Fig. 14(b)). Furthermore, the insert in Fig. 14(a) shows that DSSCs with ZnO:Eu³⁺ possess low efficiency decay in 30 days, demonstrating an effective way to increase the cell efficiency by the doping of hierarchical ZnO nanocrystalline as well as a great potential for practical application.

Fig. 14 (a) J–V curves of DSSCs fabricated with ZnO and Eu-ZnO; (b) IPCE spectra of DSSCs fabricated with ZnO and Eu-ZnO; (c) MS plots for ZnO and Eu-ZnO films; (d) Nyquist plots of DSSCs based on ZnO and Eu-ZnO. Reproduced by the permission of Elsevier (ref. 98: Electrochem. Commun., 2013, 32, 14–17).

Oxide film modified with a RE ion layer can also suppress charge recombination reported by Yang et al.⁹⁹ They found that an energy barrier was formed when TiO₂ electrode was modified with a Ho³⁺ layer of a certain thickness. And the energy barrier effect of Ho³⁺ ion modification on photoelectrical properties of TiO₂ film was investigated. The results confirmed that the charge recombination was efficiently reduced by Ho³⁺ ion modification and a higher conversion efficiency was observed.

6. Outlooks

DSSCs have received a lot of attentions in the photovoltaic field owing to the advantages. The major challenges in DSSCs are the limited light harvesting in solar spectrum, the rate of charge recombination and the low stability. Both UC and DC materials are explored in solar cells to solve these problems. Although many researches on the enhancement of solar cell efficiency by utilizing UC or DC materials have been reported, several limitations also exist for practical application in solar cells. It is clear that the UC and DC efficiencies are still rather low and the enhancement was achieved only at high excitation densities. In order to broaden the absorption spectrum for lanthanide, a sensitizer such as quantum dots with a broad spectral absorption and a narrow emission line can be used. In addition, the utilization of plasmonic effect is also a viable option to enhance the excitation and emission of the up/down converter. Furthermore, the photo-response to whole solar spectrum is beneficial for DSSCs to achieve better performance. Herein the combination of UC and DC materials could be an available approach to improve the efficiency of solar cells via utilizing UV and NIR radiation simultaneously.

7. Conclusions

In this paper we presented different RE ions doped luminescent phosphors and their potential application in DSSCs. The introduction of lanthanide-based materials has been proved to be a promising method to achieve highly efficient solar cells. The physical mechanism and experimental results were analyzed. On one hand, UC materials can absorb NIR light and then convert it to visible light, while DC materials have ability to convert UV light to visible radiation. Thus, wide photoresponse to UV and NIR region was achieved via down-conversion and up-conversion process and then induced more electron hole pairs in DSSCs. On the other hand, the light harvesting efficiency can also be improved by combining the spectral modifier with light scattering effect or surface plasmon resonances. When depositing a DC or UC layer on the external side of DSSCs, the stability and lifetime can be enhanced by avoiding directly exposing the cells to UV or NIR light. And when the DC or UC phosphors were applied on the surface of TiO₂ film, it can serve as a barrier for solar cells to charge recombination. Furthermore, rare earth ions-doped nanostructured semiconductors may control the size or band gap engineering for energy level and electron transport. Based on these results, it's clear that the utilization of DC or UC phosphors will indeed be an available option to realize more efficient solar cells.

Acknowledgements

This work was supported by the Graduate Innovation Foundation of University of Jinan, GIFUJN, (Grant No. YCXS15006), National Natural Science Foundation of China (Grant No. 61106059, 11304120, 61504048, 21505050), the Encouragement Foundation for Excellent Middle-aged and Young Scientist of Shandong Province (Grant No. BS2014CL012), the Science-Technology Program of Higher Education Institutions of Shandong Province (Grant No. J14LA01), the Natural Science Foundation of Shandong Provience (Grant No. ZR2013AM008).

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