PEDOT-based counter electrodes for dye-sensitized solar cells: rigid, flexible and indoor light applications

Abstract

Dye-sensitized solar cells (DSSCs) are promising technology owing to their unique properties such as high transparency, good color tunability, and easy large-area fabrication, which make them attractive candidates for emerging photovoltaic applications. However, conventional DSSCs require high-temperature processing for working and counter electrodes (WEs and CEs, respectively), limiting their diverse applications. Low temperature processing for highly catalytic CEs, particularly using poly(3,4-ethylenedioxythiophene) (PEDOT) as a conducting and catalytic replacement for platinum, shows potential for increased efficiency under various light conditions. Despite the high catalytic activity of PEDOT, its limited solubility and processing technologies (e.g., electrochemical deposition and spin-coating) have necessitated the interest in composites of PEDOT either with poly(styrene sulfonate), metal compounds, or in combination with carbon materials, aiming to overcome these limitations. With the combined properties of high conductivity, catalytic activity, porosity, and low temperature processability, these CEs based on PEDOT have higher scientific and industrial prospects. Moreover, the highly transparent PEDOT-based CEs can also be used for bifacial application in DSSCs. To continuously draw interest to further research on these materials, this review provided an overview of PEDOT-based CEs for rigid, flexible, and indoor applications of DSSCs. Additionally, we discuss the changes in electronic, chemical, and stability properties associated with the formation of each type of composite material. The challenges and prospects of PEDOT-based materials are further highlighted, which pave the way for performance improvements in the future, as well as identifying other potential applications in the semiconductor industry.

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Cheng Chen

Chen Cheng received his BS degree in Polymer Materials and Engineering from Jiaxing University in 2018, and his master's degree in engineering from Dongguk University, Korea, in 2024. He is currently a PhD student under the supervision of Prof. Jae-Joon Lee at the Department of Energy and Materials Engineering, Dongguk University, South Korea. His research focuses on the development of counter electrode materials for dye-sensitized solar cells and electrochemistry.

Francis Kwaku Asiam

Francis Kwaku Asiam obtained his BSc degree in chemistry from Kwame Nkrumah University of Science and Technology (KNUST), Ghana, in 2017, and his master's degree in engineering from Dongguk University, Korea, in 2022. He is currently a PhD student under the supervision of Prof. Jae-Joon Lee in the Department of Energy & Materials Engineering, Dongguk University, Korea. His research focus is on developing materials for solar energy (dye-sensitized solar cells) applications and their related fundamental kinetic and thermodynamic properties.

Ashok Kumar Kaliamurthy

Ashok Kumar Kaliamurthy has been working as an Assistant Professor in the Department of Energy & Materials Engineering at Dongguk University in Seoul, Republic of Korea, since 2023. He was a Korea Research Fellow (KRF) with the same department at Dongguk University (2019–2023). He was a National Post-Doctoral Fellow (NPDF) in the Department of Chemistry at Anna University in Chennai, India (2017–2019). He received his PhD and MPhil in Physics at the Department of Nuclear Physics, University of Madras, Tamil Nadu, India. His current research interests are focused on the development of indoor photovoltaics (dye-sensitized and perovskite solar cells) by utilizing nanophosphor, and flexible supercapacitors.

Md. Mahbubur Rahman

Md. Mahbubur Rahman has held the position of Associate Professor at Konkuk University in South Korea since September 2021. He completed his MS degree in Chemistry at the University of Dhaka, Bangladesh, in 2005, followed by his PhD in electrochemistry from Konkuk University in South Korea in 2014. Additionally, he served as a postdoctoral scholar at Incheon National University, South Korea. His research is primarily focused on the development of nanomaterials for various electrochemical and photo-electrochemical applications, including solar cells, energy storage systems, and electrochemical sensors.

Muhammad Sadiq

Muhammad Sadiq obtained his Bachelor degree in Polymer Engineering from University of Engineering and Technology (UET) Lahore, Pakistan, in 2018. He is currently a Master student under the supervision of Prof. Jae-Joon Lee in the Department of Energy & Materials Engineering, Dongguk University, Korea. His research focus is on developing materials for dye-sensitized solar cells.

Jae-Joon Lee

Jae-Joon Lee has been a Professor with the Department of Energy & Materials Engineering, Dongguk University, Republic of Korea, since 2016. He was a professor with the Department of Applied Life Science, Konkuk University, Korea, from 2004 to 2015. He received his MS in chemistry from Seoul National University. He received his PhD in chemistry from Case Western Reserve University and worked as a postdoctoral scholar with the California Institute of Technology, California. His research interests include dye-sensitized solar cells and perovskite solar cells as well as the development of electrochemical energy conversion systems and biosensors based on surface modification of electrodes.

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1. Introduction

With the continuous advances in science and technology, fossil fuels such as oil, coal, and natural gas remain vital to the global energy supply.¹ However, the heavy reliance on these non-renewable resources has caused a host of environmental issues.² Urgent action is needed to protect the environment and mitigate these problems, which highlights the critical need for sustainable and clean energy sources that can reduce carbon emissions.³ Solar power is a promising solution to the energy crisis and environmental pollution issues. Solar panels are among the most efficient devices to harness energy from the sun, converting it directly into electrical energy. Dye-sensitized solar cells (DSSCs) are a promising and cost-effective option for photovoltaic devices.⁴

The structure of the DSSC primarily features a sandwich configuration, which includes a dye-attached photoanode, an electrolyte, and a counter electrode (CE). The fabrication process of DSSCs is relatively straightforward. It typically begins with the doctor blading method, which is used to uniformly coat the TiO₂ paste onto the fluorine-doped tin oxide (FTO) substrate. This coated substrate is then annealed at high temperatures to form a mesoporous-TiO₂ film. Subsequently, the TiO₂ electrode is immersed in a dye solution for a designated period, allowing the dye molecules to attach to the TiO₂ surface, creating a dye-sensitized photoanode. Following this, the photoanode and the CE are assembled using a sealing film, such as surlyn, to ensure a tight and stable connection. The final step involves injecting the electrolyte into the assembled device and sealing it, thereby completing the fabrication of a typical DSSC.

The initial development of DSSC photosensitizers prominently featured with ruthenium-based dyes due to their remarkable efficiency and stability.⁵ Among these, the N3 and N719 dyes stand out as ideal examples, demonstrating an exceptional photovoltaic performance with a PCE reaching 10–11% under standard sunlight conditions (AM 1.5G).^6,7 To further enhance the light absorption capabilities of ruthenium-based dyes, the N749 dye (black dye) was developed.^8,9 This dye has a broad absorption spectrum that extends into the near-infrared region, covering a significant range from 400 nm to 800 nm. Such an extensive absorption spectrum enables the black dye to capture a wider range of sunlight, thereby optimizing its efficiency and reaches up to 11%.⁸ Moreover, the amphiphilic dye Z907 has been recognized for its outstanding stability.¹⁰ The hydrophobic side chains of Z907 play a crucial role in preventing water adsorption on the TiO₂ surface. This characteristic is vital for enhancing the long-term stability of the DSSC.

However, the industrial application of DSSCs faces significant hurdles due to the high cost and limited availability of ruthenium metal. To overcome these challenges, extensive research efforts have been directed towards developing ruthenium-free dyes capable of achieving high performance in DSSC technology. Among the various alternatives explored, donor–π-bridge–acceptor (D–π–A) dyes and porphyrin dyes have emerged as particularly promising candidates. D–π–A dyes offer a versatile advantage due to their structurally tunable nature, allowing to fine-tune the light-absorption properties and energy levels by adjusting the composition of donors, π-conjugated bridges, and acceptors within the molecule. This precise tuning has led to the development of several high-performance D–π–A dyes, including C281 and SGT137, which have demonstrated impressive PCE exceeding 12%.^11,12 Push–pull structured porphyrin dyes leverage a molecular structure akin to natural chlorophyll, offering inherent advantages in light absorption and charge transfer capabilities. Traditional development of porphyrin dyes progressed slowly until the use of the push–pull structure, which accelerated advancements in their efficiency.¹³ Noteworthy examples include dyes like SM315 and GY50, which have achieved remarkable PCEs approaching 13%.^14,15 In addition to the advancements in individual dye formulations, co-sensitization strategies have proven effective in further enhancing the DSSC performance. These strategies involve using multiple sensitizers to broaden the absorption spectrum and improve charge transfer kinetics within the cell. For instance, DSSCs prepared with the co-sensitizers SL9 and SL10 have exhibited a record-high PCE of 15.2%.¹⁶

However, the conventional architecture has the photoanode and counter electrode prepared under high temperature conditions (∼500 °C), which limit the scope of application of the devices. More importantly is that the era of Internet-of-Things (IoTs) is here with a projected expansion in coming years. This means that the components should be processable under ambient conditions.¹⁷ The CE is crucial in regenerating the oxidized electrolyte during the operation of DSSCs. Hence, easily processable carbon materials like one-dimensional (1D) carbon nanotubes (CNTs) and polymers like poly(3,4-ethylenedioxythiophene) (PEDOT) have been reported for use as CE materials in DSSCs. Conventional platinum CEs are primarily fabricated via thermal decomposition based on high-temperature processing, which limits their application to glass substrates. Therefore, conducting polymers, renowned for their flexibility and stretchability, have emerged as preferred materials for diverse electrodes of DSSCs.

PEDOT is one of the most used conducting polymers and possesses superior conductivity, flexibility, transparency, and stability compared to other materials.¹⁸ With conductivity values exceeding 1000 S cm⁻¹ in specially treated PEDOT:poly(styrene sulfonate) (PSS) films obtained by gas-phase polymerization,¹⁹ PEDOT is considered an ideal material for replacing platinum CEs. Furthermore, PEDOT and PEDOT:PSS are extensively utilized across diverse fields (Fig. 1a), spanning energy applications such as solar cells,²⁰ supercapacitors,²¹ thermoelectric generators,²² and fuel cells,²³ as well as electronic applications including stretchable field-effect transistors,²⁴ electrochromic devices,²⁵ electronic textiles,²⁶ and soft actuators.²⁷ Moreover, they find utility in biological contexts such as biosensors,²⁸ electronic skin,²⁹ neural probes,³⁰ and soft sensors.³¹ On the DSSC side (Fig. 1b), PEDOT has garnered considerable attention. This is because it can be used both for counter electrodes and gel electrolytes of DSSCs.^32,33 PEDOT also exhibits excellent electrocatalytic performance not only in I⁻/I₃⁻electrolyte but also in redox media such as cobalt- and copper-based complexes.^34,35 Due to this advantage, the current record power conversion efficiency (PCE) of DSSCs (15.2%) was possible by employing the same effective PEDOT as CE material. This shows the competitiveness of this material when compared to platinum (Pt).³⁶

Fig. 1 (a) Applications for PEDOT in the energy, electronics and biology fields. “Solar cells”, “Supercapacitors”, and “Electrochromic devices” image was reprinted (adapted) with permission from ref. 20, 21 and 25, respectively. Copyrights 2023, 2022, 2021 American Chemical Society. “Fuel cells” image adapted from ref. 23. Copyright 2019, Springer Science Business Media, LLC, part of Springer Nature. “Stretchable field-effect transistors”, “Biosensors” image reproduced from ref. 24,28, respectively, with permission from Elsevier B. V., Copyrights © 2021, 2021. “Thermoelectric generators”, “Electronic textiles”, “Electronic skins”, “Neural probes”, “Soft sensors” image was adapted from ref. 22, 26 and 29–31. “Soft Actuators” image was reproduced with permission from ref. 27, Copyright © 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. (b) The number of publications with the keywords “PEDOT” and “PEDOT & dye-sensitized solar cells” from 2014 to 2023. Source: Web of Science.

Shown in Fig. 2 are the issues of importance to the use of PEDOT-based electrodes, with recent advances in PEDOT-based CEs for DSSCs also added. Additionally, this paper systematically summarizes the extensive applications of PEDOT-based CEs in DSSCs, highlighting changes in electronic, chemical, and stability properties resulting from structural modifications. Moreover, it analyzes the advantages, challenges, and prospects of PEDOT-based materials for photovoltaics, potentially opening new avenues for developing additional applications of the presented materials.

Fig. 2 A flow chart of the main issues covered in this paper.

2. Properties and synthesis of PEDOT and PEDOT:PSS

PEDOT was first synthesized in the 1980s at Bayer Laboratories. It's remarkable electrical conductivity and stability have made it a popular research subject in the scientific community. Moreover, the unique properties of PEDOT have stimulated its investigation in a variety of fields, including electronics, energy storage, and biomedicine.³⁷

2.1. Properties

PEDOT is a conducting polymer with a 3,4-ethylenedioxythiophene repeat unit, as shown in Fig. 3a;³⁸ its insolubility in most solvents makes its preparation and use challenging.¹⁸ To address this issue, water-soluble PEDOT:PSS was developed by employing PSS, as a solubilizer during the polymerization process, as shown in Fig. 3b.^39,40 The resulting aqueous solution of PEDOT:PSS facilitated the industrialization and commercialization of PEDOT. Additionally, doping with the high-work function (WF) PSS increases the overall WF of PEDOT, making it more suitable as a hole transport material for organic and perovskite solar cells (OSCs and PSCs).⁴¹

Fig. 3 Structure of (a) PEDOT and (b) PEDOT:PSS.

PEDOT:PSS exhibits a very low conductivity (<1 S cm⁻¹) after doping with the insulator PSS, which severely limits its application scope and affects the overall efficiency of solar cells using it as a counter electrode material.⁴² This is due to the formation of a core–shell structure resulting from the ionic interaction between PEDOT and PSS. Since PSS has a much higher molecular weight than PEDOT, it tends to curl and form a shell outside the conductive PEDOT core.⁴³ The discontinuous distribution of conductive PEDOT in the core–shell structure hinders charge transfer and is considered the main cause of the poor conductivity of PEDOT:PSS.⁴⁴ Two main methods have been developed to increase its conductivity: secondary doping and removal of PSS.^45–47 The latter method involves using concentrated sulfuric acid, concentrated nitric acid, or polar solvents such as dimethyl sulfoxide (DMSO) to remove the PSS insulator and enhance the overall conductivity. Commercial PEDOT:PSS solutions contain an excess of PSS, with a PEDOT:PSS ratio of 1 : 2.5 or even 1 : 20, causing it to form a barrier layer to charge transfer when the film is spin-coated.⁴³ Previous studies have shown that the conductivity of PEDOT:PSS films increase with the acid concentration rather than the treatment time, and their thickness decreases because of PSS removal. For instance, Yeon et al.⁴⁷ treated PEDOT:PSS films with 3 M and 14 M nitric acid at room temperature, which resulted in their conductivity increasing to 1600 and 3964 S cm⁻¹, respectively. Shi et al.⁴⁶ also reported that the conductivity of PEDOT:PSS films significantly increased from 4.2 to 5355.3 S cm⁻¹ after a two-step acid treatment using 9.2 and 18.4 M concentrated sulfuric acid. The one-step treatments with 9.2 and 18.4 M sulfuric acid also increased the conductivity of the films to 871.7 and 2383.7 S cm⁻¹, respectively, while reducing the film thickness. Overall, the removal of PSS from PEDOT:PSS films is an effective method for increasing their conductivity and broadening their application scope.

However, the process of removing PSS from the film creates safety concerns, owing to the use of concentrated sulfuric and nitric acids. Moreover, it has been observed that the removal of PSS significantly reduces the overall WF, which can adversely affect the use of the film as a hole transport layer.⁴¹ Secondary doping methods have been proposed to overcome these limitations. These approaches involve the addition of ionic liquids to the PEDOT:PSS solution, which helps to modify the distribution of the two components, resulting in an improved overall conductivity. In fact, the conductivity of a PEDOT:PSS film has been reported to reach 1280 S cm⁻¹ by the addition of 1-ethyl-3-methylimidazolium tetracyanoborate (EMIM TCB), which is more than 1000 times that of the original PEDOT:PSS (∼1 S cm⁻¹).⁴⁸ Additionally, Song et al.⁴³ have shown that the addition of hydroquinone to a PEDOT:PSS solution can significantly increase the electrical conductivity of the PEDOT:PSS films from 0.7 to 1394 S cm⁻¹. Notably, this increase in conductivity was achieved with only a slight decrease in WF from 5 to 4.89 eV, which was higher than that (4.74 eV) of the film after washing with sulfuric acid. These findings highlight the potential of secondary doping methods to improve the performance of PEDOT:PSS films, while mitigating the safety concerns associated with the removal of PSS.

2.2. Synthesis of PEDOT

The first synthesis of PEDOT was achieved through oxidative chemical polymerization of 3,4-ethylenedioxythiophene (EDOT). Currently, polymerization methods of EDOT can be mainly grouped into two, as illustrated in Fig. 4: chemical and electrochemical polymerization.⁴⁹ These methods present different advantages and disadvantages, making them suitable for specific applications.

Fig. 4 EDOT polymerization methods: chemical and electrochemical polymerization.

2.2.1. Chemical polymerization of EDOT. This is the original and most widely used method for synthesizing PEDOT.⁵⁰ The oxidative polymerization mechanism of EDOT involves two main steps: (1) the EDOT monomer is oxidized to cationic radicals, which then undergo radical dimerization and deprotonation processes to produce an active neutral dimer; (2) the neutral dimer is doped by an oxidant, whose anion neutralizes and stabilizes the charged dimer.⁵¹ This process then continues with increasing molecular weight to yield a polymer, as shown in Fig. 5. Typically, iron(III) is used as oxidant in the chemical polymerization of EDOT.⁵² However, other metal ions such as cerium(IV),⁵³ manganese(IV),⁵⁴ and copper(II)⁵⁵ have also been employed.in this process.

Fig. 5 Chemical polymerization of EDOT to PEDOT (with iron(III) tosylate). OTs denote a tosylate anion and Im denotes imidazole. Image was reproduced with permission from ref. 56, Copyright © 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

2.2.2. Electrochemical polymerization of EDOT. This method is like chemical polymerization, with the key difference being that no oxidant is required in the former. Instead, EDOT is oxidized through an applied electric potential and then polymerized at the electrode.⁴⁹ The initial mechanism involves the oxidation of EDOT when the potential is applied as shown in step 1 of Fig. 6, followed by dimerization (step 2) and growth (step 3) to yield the PEDOT. It is typically conducted using a three-electrode system comprising a working, a reference, and a counter electrode, in addition to an EDOT solution.^57,58 The latter typically contains acetonitrile or aqueous solvents. Since EDOT is only soluble in acetonitrile, a surfactant, such as sodium dodecyl sulfate is added to enhance its solubility for aqueous mixtures. The techniques used for making the PEDOT include cyclic voltammetry as well as constant–voltage, constant–current, and constant–voltage pulse methods. These methods are well-established and have been extensively studied.^59–62

Fig. 6 Electrochemical polymerization mechanism of EDOT. Image reproduced from ref. 63 with permission from Elsevier B. V., Copyrights © 2018.

2.3. Characterization of PEDOT-based CEs

2.3.1. Scanning electron microscopy. Scanning electron microscopy (SEM) serves as a highly effective tool for the characterization of PEDOT-based CEs. It offers detailed imaging capabilities that enable researchers to discern the morphology of the material with clarity and evaluate its uniformity. This aspect is pivotal for comprehending the physical attributes that influence CE performance. Morphological variances within PEDOT-based CEs (such as surface roughness) directly correlate with alterations in specific surface area. A rougher surface typically signifies an expanded contact interface with the electrolyte and heightened active sites, thereby enhancing charge transfer dynamics between the electrode and electrolyte and augmenting electrocatalytic efficiency.

2.3.2. Cyclic voltammetry. Cyclic voltammetry (CV) is an essential analytical technique for studying PEDOT-based CEs. It is conducted using a three-electrode system, comprising of a working electrode (PEDOT CE), a reference electrode, and a counter electrode (typically platinum). The fundamental principle of CV involves applying a triangular waveform pulse voltage (Fig. 7b) to the closed circuit formed by the working and counter electrodes.⁶⁴ This process modulates the potential at the working electrode/electrolyte interface at a constant rate (from A to D to G within one cycle), inducing oxidation and reduction reactions of the analyte at the electrode while continuously monitoring the current. The resulting electrode potential and the corresponding response current are recorded to generate a current–potential curve, known as the CV curve. A typical cyclic voltammogram for a potassium ferricyanide system is depicted in Fig. 7a, where points C and F denote the peak reduction potential (E_pc) and peak oxidation potential (E_pa), respectively. The potential difference between the two peaks (E_pp) is a critical metric for assessing the catalytic performance of the electrode.⁶⁵ A smaller E_pp indicates a faster redox reaction and higher electrocatalytic activity.

Fig. 7 (a) Typical cyclic voltammogram (potassium ferricyanide system), (b) applied potential as a function of time, with the initial (A), transition (D), and termination potentials (G), respectively.⁶⁴

Additionally, the oxidation peak current (I_pa) and reduction peak current (I_pc) are significant indicators. For reversible redox pairs, the values of I_pa and I_pc are nearly identical, and the peak shapes are comparable. In DSSCs, the primary function of the CEs is to reduce the oxidized electrolyte. Consequently, the I_pc value is particularly crucial. A higher I_pc value signifies superior electrocatalytic performance. This makes the material more suitable for use as a CE for DSSC. Moreover, CV is frequently employed to evaluate the stability of PEDOT-based CEs. A stable CE typically exhibits a consistent CV curve shape across multiple cycles, indicating reliable performance.

2.3.3. Electrochemical impedance analysis. Electrochemical impedance spectroscopy (EIS) is a widely utilized technique for analyzing the interfacial charge transport kinetics in the complex electrochemical systems.⁶⁶ To evaluate the charge transport properties of PEDOT CEs, a symmetric dummy cell is typically fabricated using two identical electrodes. By performing an EIS study on this dummy cell, the resistance of the CE and the charge transfer between the CE/electrolyte can be assessed.⁶⁷ A typical Nyquist plot for a dummy cell with PEDOT CEs features two semicircles: the starting point in the high-frequency region represents the series resistance (R_s), and the first semicircle at high-frequency region represents the charge transfer resistance (R_ct) at the CE/electrolyte interface, and the semicircle at low-frequency region corresponds to the diffusion resistance (Z_w) of the redox pair. Kim et al.⁶⁸ conducted EIS studies on various counter electrodes, including Pt CEs, graphene nanoplatelets (GnPs) CEs, PEDOT:PSS (PP) CEs, and GnPs/PEDOT:PSS (PPG) composite CEs (Fig. 8). Among the other CE materials, the PPG electrode exhibited the lowest R_ct value which is advantageous for the rapid reduction of the oxidized electrolyte, thereby accelerating the charge transfer kinetics within the DSSC.

Fig. 8 Nyquist plots of symmetric dummy cells with Pt, GnP, PP, and PPG4 CEs; the inset shows an enlarged Nyquist plot of PPG4 dummy cells in the high-frequency region and the equivalent circuit model used to fit the EIS data.⁶⁸

3. PEDOT-based materials for DSSC counter electrodes

In a DSSC, the CE is responsible for the reduction of the oxidized electrolyte.⁶⁹ The proper functioning of the overall reaction relies on the ability of the CE to reduce the oxidized electrolyte at a rate equal to or greater than that of dye oxidation. Therefore, the CE plays a vital role in enhancing the performance of DSSCs. Current DSSCs typically employ an I⁻/I₃⁻ mixture as the electrolyte, and platinum as CEs because it exhibits excellent catalytic performance in the reduction of I₃⁻.⁷⁰ The widespread use of Pt in DSSC CEs can be attributed to its superior electrical conductivity. However, it is worth noting that Pt is susceptible to corrosion in I⁻/I₃⁻ electrolytes and may be oxidized and dissolved to form PtI₄ or H₂PtI₆.⁷¹ For this reason, it is essential to take into account the durability of the CE when selecting materials for DSSCs.

Owing to its scarce and non-renewable natural resources, the high cost of Pt limits its widespread commercial application. Therefore, developing Pt-free DSSCs is crucial to facilitate their commercialization. As shown in Fig. 10, current research efforts focus on identifying alternative materials to Pt, including carbon materials, transition metal oxides (TMO), transition metal chalcogenides (TMC), and conducting polymers.⁶⁹

Carbon materials are naturally abundant and can be easily produced, making carbon-based CEs a hot research topic in the field of DSSCs.⁷² These materials offer several advantages, including low cost, high conductivity, excellent catalytic performance, large specific surface area, and good resistance to iodine corrosion. Consequently, carbon materials are very suitable for replacing expensive platinum in DSSC CEs. Various carbon materials, such as graphene, carbon nanotubes, graphite, and carbon black have been successfully developed for DSSCs CEs.^73–75 However, despite their promising performance, carbon materials face certain limitations. One major issue is the poor adhesion of carbon-based materials to the substrate, which can compromise the stability and durability of DSSCs. Additionally, achieving optimal catalytic activity often requires a high dosage of carbon material, leading to low transmittance. This reduced transmittance contradicts the inherent advantage of DSSCs' translucency, thereby limiting the potential of carbon materials when used alone.⁶⁵

Besides, TMC and TMO are also popular candidates for Pt-free CEs in DSSCs due to their versatility, high catalytic activity, and ease of modification. TMC mainly includes sulfides and selenides. Compared to Pt, some TMC exhibit higher electrocatalytic activity and lower charge transfer resistance. Currently, materials such as NiS, MoS₂, NiSe, NiCoSe, FeNiCoSe, WO₃ and CuO have been developed as CEs for DSSCs.^76–82 However, TMC faces challenges such as poor electrical conductivity and limited resistance to iodine corrosion. Conducting polymers, discovered in 1977, represent the fourth generation of polymers, and have since been extensively developed and used in a broad range of applications.^83,84 The most studied conducting polymers include polyaniline (PANI), polypyrrole (PPy), polythiophene (PT), and its derivative, poly(3,4-ethylenedioxythiophene) (PEDOT). Although PANI has been widely studied because of its multifunctional properties and low cost, its instability and carcinogenic properties limit its potential applications.^85,86 Additionally, PANI has lower conductivity (only 0.1–5 S cm⁻¹)⁸⁷ than other conducting polymers and is therefore not an ideal material for CEs. On the other hand, PPy represents a promising candidate to replace platinum CEs, owing to its easy synthesis, low cost, and environmental stability.⁸⁸ However, PPy faces challenges associated with its high R_ct and low conductivity (10–50 S cm⁻¹).⁸⁷ Thus, PEDOT stands out for its superior conductivity and safety compared to other conducting polymers, making it a common choice for designing CEs in DSSCs.

In the area of CEs, the active surface area stands as a crucial metric for evaluating performances. A larger active surface area inherently implies an increase in the number of active sites available for electrochemical reactions, as well as an extended contact area with the electrolyte. This augmentation typically translates to enhanced electrocatalytic performance, which is particularly significant for DSSC CEs. The morphology of PEDOT films can be strategically modified by adjusting the synthesis method to obtain a larger surface area, thereby improving the catalytic activity of PEDOT films and the overall performance of DSSCs. Xiao et al.⁶⁰ fabricated PEDOT films for DSSC CEs using CV, constant current, and pulse potentiostatic electrodeposition methods. As shown in Fig. 9a, PEDOT CEs fabricated by the CV method exhibited flaky and inhomogeneous surfaces. In contrast, using the constant current and pulse potentiostatic methods, the PEDOT CE displayed a uniform, and continuous layers composed of numerous PEDOT particles. Notably, the pulse potentiostatic technique resulted in smaller PEDOT particles and more uniform and continuous films. This suggests an increase in the active surface area, resulting in a PCE of the DSSC with the pulse potentiostatic PEDOT CE (6.4%), surpassing those based on PEDOT CEs fabricated using the CV and constant current methods. In addition to optimizing the synthesis method, various PEDOT films with different morphologies can be prepared by using sacrificial templates such as polymethyl methacrylate (PMMA) microspheres and ZnO nanowires, etc. This method is effective in increasing the active surface area of the PEDOT films. Trevisan et al.⁶¹ used ZnO nanowires as template to synthesize PEDOT nanotube (NT) films on FTO substrates. As shown in Fig. 9b the high active surface area of the NT structure was obtained, and this enhanced the contact with the electrolyte solution, leading to an efficiency of 8.3%, which was comparable to that of a Pt-based CE. Li et al.,⁶² later utilized PMMA microspheres as a sacrificial template to prepare honeycomb structured PEDOT films (Fig. 9c). Increasing the PMMA concentration resulted in the generation of more honeycomb holes, thereby improving the transmittance and active surface area of the film. Benefiting from this, the final efficiency of a DSSC based on the honeycomb-structured PEDOT CE (9.12%) was higher than that of a DSSC based on a flat-PEDOT CE.

Fig. 9 SEM images of (a) PEDOT CEs made by CV, I–t and pulse potentiostatic; (b) PEDOT NT CE (top and cross section views); (c) flat-PEDOT and honeycomb-structured PEDOT CE. “(a)” and “(c)” image reproduced from ref. 60,62, with permission from Elsevier B. V., Copyrights © 2012, 2017. “(b)”image was reproduced with permission from ref. 61, Copyright © 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Fig. 10 PEDOT and composite materials employed for DSSC counter electrodes.

Hence, incorporating materials with large surface areas, such as carbon and metal sulfides, through composite formation, is also an effective strategy to enhance the surface area of PEDOT films. Composite PEDOT films offer additional advantages compared to pure PEDOT films, including improved catalytic ability, enhanced conductivity, and greater stability. Therefore, composite PEDOT films designed using composite technology are widely employed as CEs in DSSCs (Table 1).

Table 1 Classification of PEDOT-based electrodes for DSSCs reported in recent years and corresponding PCE values under AM 1.5G illumination

Class	CE catalyst	Dye	Redox couple	V oc (mV)	J sc (mA cm^−2)	FF (%)	PCE (%)	V ^Ptoc (mV)	J ^Ptsc (mA cm^−2)	FF^Pt (%)	PCE^Pt (%)	Method	Ref.
PEDOT or PEDOT:PSS	PEDOT(SSP)	N719	I^−/I3^−	0.71	16.26	61	7.04	0.72	16.47	62	7.3	Solid-state polymerization	89
	PEDOT(CV)-I	N719	I^−/I3^−	0.751	14.6	67.5	7.4	0.757	15	70.7	8.02	Electrodeposition	90
	PEDOT NT	N719	I^−/I3^−	0.72	16.24	70	8.3	0.74	16.21	71	8.5	Electrodeposition	61
	PEDOT(VPP)	N719	I^−/I3^−	0.64	20.24	65	8.42	0.64	21.47	61	8.38	Vapor-phase polymerization	19
	PEDOT(CV)-Co	Y123	[Co(bpy)3]^2+/3+	0.872	14.5	68.4	8.65	0.855	12.1	57.4	5.94	Electrodeposition	90
	Honeycomb-like PEDOT	N719	I^−/I3^−	0.768	17.72	67	9.12	0.75	16.75	64	8.05	Electrodeposition	62
PEDOT or PEDOT:PSS and metallic compounds	PEDOT-Ag3PO4	N719	I^−/I3^−	0.762	11.08	44.1	3.731	0.751	12.14	69	6.307	Spin coating	91
	PEDOT-Co3(PO4)2	N719	I^−/I3^−	0.724	12.05	69.6	6.109	0.751	12.14	69	6.307	Spin coating	91
	PEDOT-Ni3(PO4)2	N719	I^−/I3^−	0.746	12.21	70.3	6.412	0.751	12.14	69	6.307	Spin coating	91
	PEDOT@TiO2	MK-2	[Co(bpy)3]^2+/3+	0.809	11.222	72.87	6.62	0.811	10.161	73.58	6.07	Electrodeposition	92
	Pt/PEDOT	N719	I^−/I3^−	0.74	13.12	69.7	6.77	0.76	12.74	65	6.27	Electrodeposition	93
	PEDOT-Co2P	N719	I^−/I3^−	0.75	13.61	68	6.85	0.75	13.85	68	7.09	Spin coating	94
	PEDOT-Ni2P	N719	I^−/I3^−	0.75	13.91	68	7.14	0.75	13.85	68	7.09	Spin coating	94
	PEDOT/Fe3O4	N719	I^−/I3^−	0.74	18.6	63	8.69	0.742	17.9	63.1	8.38	Spin coating	95
	PEDOT/Pt	N719	I^−/I3^−	0.757	15.9	74.5	8.97	0.765	15.99	82.1	8.82	Electrodeposition	96
	PEDOT/Ag–CuO	N719	I^−/I3^−	0.76	17.79	67	9.06	0.75	16.18	66	8.01	Electrodeposition	97
PEDOT or PEDOT:PSS and transition metal sulfides	MoS2/PEDOT	N719	I^−/I3^−	0.743	13.73	68	7	0.767	14.28	60	6.58	Electrodeposition	98
	Si3N4/MoS2-PEDOT:PSS	N719	I^−/I3^−	0.73	14.28	69	7.16	0.75	14.32	71	7.5	Spin coating	99
	MoS2/NC-PEDOT:PSS-1	N719	I^−/I3^−	0.75	14.69	69.32	7.67	0.75	14.65	71.65	7.86	Spin coating	100
	MoS2/NC-PEDOT:PSS-2	N719	[Co(bpy)3]^2+/3+	0.857	13.36	71.21	7.87	0.817	12.95	71.65	7.49	Spin coating	100
	NiS(NPs)/PEDOT:PSS	N719	I^−/I3^−	0.76	16.05	67	8.18	0.71	16.88	71	8.62	Doctor blading	101
PEDOT/carbon-based materials	SWCNH/PEDOT:PSS	N719	I^−/I3^−	0.64	15.42	47	4.7	0.7	15.75	50	5.55	Spin coating	102
	PEDOT:PSS/SWCNH	N719	I^−/I3^−	0.68	14.06	52	5.1	0.69	15.74	51	5.53	Spin coating	103
	Carbon/PEDOT	Z907	T^−/T2	0.642	15.8	56.8	5.76	0.621	14	45.35	3.96	Electrodeposition	104
	DMSO-PEDOT:PSS/Carbon	N3	I^−/I3^−	0.78	14.05	53	5.81	0.68	14.64	57	5.66	Spin coating	105
	PEDOT/rGO-150s	N719	I^−/I3^−	0.73	15.82	67	7.79	0.78	17.03	63	8.33	Electrodeposition	106
	YSm-SiO2@NC-PEDOT:PSS	N719	I^−/I3^−	0.75	17.28	62.51	8.26	0.74	15.74	72.47	8.5	Spin coating	107
	GnP/PEDOT:PSS	Y123	[Co(bpy)3]^2+/3+	0.92	13.64	67.01	8.33	0.92	13.27	65.52	7.99	Electrospray	108
	PEDOT/MWCNT	N719	I^−/I3^−	0.792	17.09	67	9.07	0.76	15.6	65	7.71	Electrodeposition	109
	PEDOT/CB	Y123	[Cu(dmby)2]^+/2+	1.05	13.51	71	10.08	1.07	12.12	53.1	6.89	Electrodeposition	20

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3.1. PEDOT and PEDOT:PSS CEs

PEDOT is a conducting polymer insoluble in various solvents, with the commercially available form mainly being PEDOT:PSS aqueous solution. The main fabrication methods of PEDOT or PEDOT:PSS CEs include chemical or electrochemical polymerization, as well as spin coating the PEDOT:PSS solution on transparent conductive oxide (TCO). However, the excess non-conductive PSS in the PEDOT:PSS solution results in a low conductivity of the CEs, approximately 2 S cm⁻¹,¹⁰⁵ which limits its application in DSSCs.

Conductivity is a crucial indicator for evaluating the performance of PEDOT in DSSCs. In a DSSC system, PEDOT not only reduces the oxidized electrolyte but also serves as the channel connecting the external circuit to the internal electrolyte. Therefore, high conductivity PEDOT can facilitate rapid electron transfer from the external circuit, minimizing resistance loss and supporting effective charge collection. Similarly, high conductivity PEDOT can reduce the interfacial resistance between the CEs and the electrolyte. Low interfacial resistance improves charge transfer efficiency between electrodes and reduces energy loss, thereby enhancing the overall efficiency of the device.

To enhance the conductivity of PEDOT:PSS, Chen et al.¹⁰⁵ treated it with four organic solvents, namely, dimethyl acetamide (DMAc), dimethyl formamide (DMF), dimethyl carbonate (DMC), and DMSO. The PEDOT:PSS films were then dip coated on FTO. A comparison of the room-temperature conductivity of the PEDOT:PSS films treated with the different solvents showed that the conductivity of the DMSO-treated film reached 85 S cm⁻¹, which was more than 40 times higher than that of the bare PEDOT:PSS film (2 S cm⁻¹). The final power conversion efficiency (PCE) of a DSSC assembled with the DMSO-treated PEDOT:PSS CE was found to be 2.41%, which was still lower than that of a DSSC based on a Pt CE (5.66%), but higher than that of a bare PEDOT:PSS CE-based DSSC (1.9%).

Yin et al.⁸⁹ presented a method for fabricating highly conductive PEDOT CEs using a 2,5-dibromo-3,4-ethylenedioxythiophene (DBEDOT) monomer solution. The solution was spin-coated onto FTO/glass and ITO/polyethylene naphthalate (PEN) substrates and sintered at 80 °C for 6 h, resulting in a PEDOT film with excellent conductivity. The efficiency of a DSSC based on the best PEDOT/FTO CE reached 7.04%, which was comparable to that of the thermally deposited Pt/FTO CE. In another study, Kouhnavard et al.¹⁹ fabricated a highly conductive PEDOT film on FTO using rust-based vapor-phase polymerization. As shown in Fig. 11, the process involved depositing a layer of Fe₂O₃ solid oxidant precursor using physical vapor deposition on a clean FTO substrate, followed by heating at 140 °C for 1.5 h with HCl and EDOT solution in chlorobenzene. The resulting PEDOT film was purified in 6 M HCl to remove iron impurities, and exhibited an exceptional conductivity of 1120 S cm⁻¹. Following CV analysis, the J_pc value of PEDOT was much higher than that of Pt, and the peak potential separation (ΔE_pp) was smaller. The final PCE of a DSSC assembled from the PEDOT CE reached 8.42%, which was slightly higher than that of a DSSC based on the Pt CE. These results show that PEDOT can be a promising alternative to Pt in DSSCs and highlights the potential of novel methods for enhancing the conductivity of PEDOT.

Fig. 11 (a) Schematic illustration of PEDOT film synthesis. (b) SEM and (c) XRD images of PEDOT films. (d) Cyclic voltammograms of Pt and PEDOT CEs with a scan rate of 50 mV s⁻¹. (e) J–V characteristics of DSSCs with PEDOT and Pt CE under AM 1.5G illumination. Images reproduced from ref. 19 with permission from Elsevier B. V., Copyrights © 2020.

The thickness of the PEDOT CEs significantly influences its electrocatalytic activity and the device performance. Thin PEDOT films generally exhibit good electrical conductivity due to shorter electron transport paths. However, their limited catalytic material results in fewer active sites, hindering the reduction of the oxidized electrolyte and lowering the overall efficiency of the DSSC. Conversely, thicker PEDOT films offer more catalytic material and active sites, enhancing catalytic activity. However, their conductivity tends to decrease with increasing thickness due to more structural defects and irregularities.¹¹⁰ Additionally, electrons and ions must travel longer paths in thicker films, slowing down reaction kinetics and impacting overall catalytic performance and response time.¹¹¹ Furthermore, as PEDOT film thickness increases, the intermolecular attraction between PEDOT chains surpasses the adhesion between the PEDOT and the electrode. This can lead to poor mechanical stability at the PEDOT-to-electrode interface, affecting the overall durability of the DSSC. Thus, optimizing the performance of PEDOT-based CEs requires balancing the benefits and drawbacks of film thickness. An optimal PEDOT film should combine high conductivity, high catalytic activity, stability, and good transmittance. Therefore, electrochemical polymerization has emerged as a popular technique for producing PEDOT films, owing to advantages such as short processing time, ease of operation, and ability to control the film thicknesses by varying the polymerization conditions.

Venkatesan et al.¹¹¹ deposited PEDOT films with various thicknesses on FTO glass substrates using the constant–current method (Fig. 12). The PEDOT CEs were then tested under indoor light and 1 sun conditions with cobalt electrolyte. Interestingly, the PEDOT CE with the highest transmittance (70%) was obtained through a short polymerization time of only 5 s. The highest efficiency achieved by a double-sided DSSC assembled with this counter electrode reached an impressive 26.93% under 1000 lx illumination. Notably, a DSSC based on a thin PEDOT CE maintained 100% of its initial efficiency after 500 h, while the efficiency retention of a DSSC based on a thick PEDOT CE was only 91%. These findings highlight the significance of optimizing the thickness and polymerization time of PEDOT films to achieve DSSCs with high efficiency and stability.

Fig. 12 (a) Schematic illustration of a double-sided DSSC; (b) SEM images and (c) transmittance profiles of bare FTO and PEDOT layers prepared using polymerization times of 5, 10, 50, 90 and 130 s; (d) stability of DSSCs based on PEDOT layers prepared using polymerization times of 5 and 130 s. Images were reprinted (adapted) with permission from ref. 111. Copyright 2022 American Chemical Society.

3.2. Composites of PEDOT and metallic compounds

Composites of metallic compounds with PEDOT have been extensively studied as potential materials for CEs of DSSCs, to enhance the surface area to volume ratio of the catalytic CE. Owing to this property, Yii et al.⁹¹ investigated the impact of excessive contents of metal phosphates on the performance of PEDOT. They mixed three phosphates [Ni₃(PO₄)₂, Co₃(PO₄)₂, and Ag₃PO₄] with the EDOT solution, which was then spin-coated onto a FTO glass substrate and cured to obtain the CEs. Among the three phosphates, the highest PCE value (6.41%) was obtained when Ni₃(PO₄)₂ was 50 mg mL⁻¹, slightly higher than that of a Pt-based CE (6.31%). Additionally, Wu et al.¹¹² doped PEDOT with four transition metal ions (Mn²⁺, Co²⁺, Ni²⁺, and Cu²⁺) via a simple electrochemical method. The R_ct values at the CE/electrolyte interface obtained from Nyquist plots of electrochemical impedance spectroscopy data) (Fig. 13) decreased in the order PEDOT-Mn²⁺ < PEDOT-Ni²⁺ < PEDOT-Co²⁺ < PEDOT-Cu²⁺ < PEDOT. Owing to its low R_ct value, PEDOT-Mn²⁺ achieved the highest PCE (5.19%) among the doped materials, which was also much higher than that of pure PEDOT (3.26%).

Fig. 13 (a) SEM images, (b) cyclic voltammograms, (c) Nyquist plots, and (d) J–V curves of PEDOT-Mn²⁺, PEDOT-Ni²⁺, PEDOT-Co²⁺, PEDOT-Cu²⁺, and PEDOT electrodes. Images reproduced from ref. 112 with permission from Elsevier B. V., Copyrights © 2017.

Considering the doping effects, Mohammad et al.⁹⁷ synthesized Ag–CuO/PEDOT composite electrodes by reducing AgNO₃/Cu(NO₃)₂ with sodium borohydride and annealing at 400 °C for 3 h, followed by electrochemical deposition. The CV curves in Fig. 14b show that Ag–CuO/PEDOT displayed a stronger reduction peak and a narrower redox potential difference than PEDOT and Pt, indicating a stronger catalytic activity for I⁻/I₃⁻ reduction. A DSSC based on the Ag–CuO/PEDOT CE achieved a PCE of 9.06%, which was higher than that of Pt (8.01%). Long-term stability tests (Fig. 14d) revealed that the PCE of the Pt CE-based DSSC device decreased sharply after 10 days, whereas that of the Ag–CuO/PEDOT CE-based DSSC device remained stable for 30 days. Since Pt has a higher conductivity, Xu et al.⁹³ prepared Pt/PEDOT CEs by electrochemical polymerization on Pt electrodes. Upon optimizing the amounts of Pt and PEDOT, the efficiency of the DSSC based on Pt-10 mg/PEDOT CE was higher than that of the Pt-30 mg CE-based DSSC. This highlighted the significance of synergy in the use of both materials.

Fig. 14 (a) FE-SEM images (top view) of PEDOT as well as Ag–CuO and PEDOT/Ag–CuO nanocomposites. (b) Cyclic voltammograms of I⁻/I₃⁻ redox electrolyte catalyzed by PEDOT, PEDOT/Ag–CuO nanocomposite, and Pt-based CEs obtained in a three-electrode system at a scan rate of 50 mV s⁻¹ from −0.6 to 1.4 V. (c) J–V plots of DSSCs recorded at 1 sun illumination. (d) Long-term stability tests of DSSCs based on CEs with PEDOT/Ag–CuO nanocomposite, PEDOT, and Pt. The images are from ref. 97.

3.3. PEDOT or PEDOT:PSS and transition metal sulfides

Molybdenum disulfide (MoS₂, with a two-dimensional layered structure like that of graphene) has attracted significant attention among different metal sulfides, owing to its good electrical and catalytic properties.¹¹³ Due to its 2D-structure and transparency, Xu et al.⁹⁸ prepared a transparent composite CE by depositing MoS₂ nanoparticles onto FTO through a hydrothermal method, followed by coating PEDOT on the MoS₂ nanoparticles via electrochemical polymerization (Fig. 15). The MoS₂/PEDOT composite CE exhibited a different morphology from pure PEDOT, with a certain roughness that led to a larger contact area with the electrolyte. The average transmittance of the composite electrode exceeded 60% in the wavelength range of 400–800 nm. The front and rear PCEs of a double-sided DSSC based on these CEs (7% and 4.82%, respectively) were higher than those of Pt (6.58% and 4.78%).

Fig. 15 (a) Schematic illustrations of bifacial DSSC assembled with a MoS₂/PEDOT CE, and J–V curves of DSSCs based on Pt, MoS₂, PEDOT, and MoS₂(2L)/PEDOT CEs under front and back illumination. (b) Cyclic voltammograms of Pt, MoS₂, PEDOT, and MoS₂(2L)/PEDOT CEs obtained at a scan rate of 50 mV s⁻¹. (c) Tafel polarization curves and (d) Nyquist plots for symmetrical cells based on Pt, MoS₂, PEDOT, and MoS₂(2L)/PEDOT CEs. (e) Enlargement of Nyquist plots, with equivalent circuit shown in the inset. The images are from ref. 98.

Similarly, Abdelaal et al.¹¹⁴ successfully grew several layers of MoS₂ on nitrogen-doped carbon (NC) derived from a zeolite imidazolium ester backbone (ZIF-8). They prepared MoS₂/NC-PEDOT:PSS composite CEs using the spin coating method. The front and rear PCEs of a double-sided DSSC based on the 5% MoS₂/NC-PEDOT:PSS composite CE in the I⁻/I₃⁻ electrolyte (7.67% and 4.54%, respectively) were slightly lower than those of Pt (7.86% and 4.78%). They also investigated the performance of this PEDOT composite CE in the [Co(bpy)₃]^2+/3+ electrolyte; the PCE values of a two-sided DSSC based on the MoS₂/NC-PEDOT:PSS CE in the [Co(bpy)₃]^2+/3+ electrolyte was 7.87% (front) and 5.34% (rear), slightly higher than those of Pt (7.49% and 5.5%). Therefore, the composite electrode showed good electrocatalytic performance for both iodine- and cobalt-based electrolyte systems.

With the electron-rich property of the sulfur in the MoS₂ structure giving it high catalytic property, Maiaugree et al.¹⁰¹ synthesized NiS nanoparticles via the hydrothermal method and prepared NiS:PEDOT:PSS pastes by mixing them with PEDOT:PSS. Then, they prepared composite CEs by scraping. The maximum efficiency (8.18%) was obtained at a NiS content of 0.3 g; this efficiency was much higher than that of pure NiS (5.9%) and PEDOT:PSS (5.27%), and close to that of Pt (8.62%). Ma et al.¹¹⁵ also prepared transparent NiS/PEDOT CEs using electrochemical deposition and in situ polymerization and assembled double-sided DSSCs using aluminum foil as a reflector. The PCE measured in the presence of the aluminum foil reflector (8.47%) was higher than that of the DSSC with the Pt CE (7.3%). These strategies showed that sulfur-based compounds have good catalytic activity because of the high number of lone-pair electrons on the atom. Despite the improvement observed in their work, it can be inferred from the other reports that Pt gives higher PCEs with I⁻/I₃⁻ (although close enough to PEDOT composites), while PEDOT-based composites give better PCE and stability with other redox couples. This results from the suitable redox positioning and prevention of side reactions with the CE.

3.4. PEDOT or PEDOT:PSS and carbon-based materials

While PEDOT has good conductivity among conducting polymers, its conductivity is still lower than that of Pt, which limits the overall performance of DSSCs based on PEDOT CEs.¹¹⁶ Thus, compounding PEDOT with carbon-based materials has been explored as a potential strategy for improving its electrical conductivity and catalytic activity. Various carbon-based materials, such as carbon, graphene, graphene oxide (GO), reduced graphene oxide (rGO), and carbon nanotubes (CNTs), have been reported to enhance the performance of PEDOT.

Wang et al.¹⁰⁴ reported the fabrication of a carbon/PEDOT composite CE for DSSCs. The authors first spin-coated carbon ink onto an FTO glass substrate, and then heated it to obtain the FTO/carbon electrode. PEDOT was deposited onto the FTO/carbon electrode by CV method to obtain an FTO/carbon/PEDOT bilayer composite CE. The device used the Z907Na dye as the sensitizer. The carbon nanoparticles adopted spherical shapes on the FTO substrate, which provided a large surface area for PEDOT electrodeposition. Therefore, PEDOT was deposited uniformly onto the carbon nanoparticles, and the contact area with the electrolyte increased, providing a higher number of catalytic reduction sites. The final PCE of the device was 5.76%, which was significantly higher compared to that of Pt (3.96%).

Graphene is a two-dimensional material composed of a single layer of carbon atoms arranged in a honeycomb lattice structure.¹¹⁷ Each carbon atom in graphene is covalently bonded to three neighboring C atoms, forming a sp²-hybridized carbon network. The overlapping of these orbitals leads to the formation of a delocalized π-electron system that extends throughout the entire graphene sheet. This unique electronic structure endows graphene with exceptional electrical conductivity and excellent catalytic activity.¹¹⁸ In addition to these properties, the near-perfect transparency of the individual graphene sheets makes it a promising material for optoelectronic applications.

Kim et al.¹⁰⁸ developed a GnP/PEDOT:PSS hybrid solution by dispersing GnPs into an aqueous solution of PEDOT:PSS via ultrasonic treatment. As shown in Fig. 16, electronic spraying was used to deposit the solution onto FTO glass substrates.¹¹⁹ Ultrasonication was found to break down the GnPs into smaller pieces, which were then deposited onto the substrate along with PEDOT:PSS. The doping of the PEDOT:PSS interlayer into the GnPs (PPGx, x = 1–4) increased the compactness of the film. The transmittance of PPG4 was found to be lower (0.04%) than that of GnP (34%), even though the film thickness of PPG4 was lower than that of GnP under the same deposition conditions. A DSSC was fabricated using Y123 sensitizer and Co(bpy)₃^2+/3+ redox couple. The PCE of the DSSC based on 1 wt% PPG4 CE (8.33%) was higher than that of Pt (7.99%).

Fig. 16 (a) Schematic illustration of the fabrication of GnP, PP, and GnP-PP (PPGx) composite thin films using the e-spray technique; (b) transmittance of PP and PP/GnPs (PPGx, x = 1–4) thin films on FTO/glass substrate; (c) photographs of e-sprayed GnP, PP, and PPG4 thin films deposited on FTO/glass substrates; (d) SEM images of e-sprayed GnP, PP, and PPG4 thin films deposited on FTO/glass substrates (the insets show the corresponding cross-section images). The images are from ref. 108.

Liu et al.¹²⁰ deposited different concentrations of PEDOT/GO on FTO glass substrates using the constant–voltage method. The deposition potential was fixed at 1.2 V for 120 s. As shown in Fig. 17, the PEDOT/GO composite film showed absorption peaks at 253 and 304 nm shifted to longer wavelengths and had higher intensities compared to the characteristic absorption peaks of pure PEDOT. The best efficiency (6.23%) was obtained for PEDOT/GO at a concentration of 0.3 g L⁻¹ and was approximately 45% higher than that of pure PEDOT (4.43%).

Fig. 17 (a) SEM images of PEDOT and PEDOT/GO composite films. (b) Ultraviolet–visible absorbance spectra of GO powders, PEDOT film, and PEDOT/GO composite films. (c) Nyquist plots of PEDOT and PEDOT/GO-x (x = 1–5) symmetrical dummy cells. The images are from ref. 120.

Moreover, Ma et al.¹⁰⁶ reported the successful preparation of PEDOT/rGO composite electrodes using two electrochemical precipitation methods. First, PEDOT and GO were deposited on an FTO glass substrate by the constant–potential method, and then GO was electrochemically reduced by CV in a 0.1 M LiClO₄ solution, obtaining the PEDOT/rGO electrode. This method achieved a good chemical combination of PEDOT and rGO. Owing to the higher catalytic activity and conductivity of rGO, a DSSC based on the PEDOT/rGO composite electrode showed a PCE of 7.79%, which was higher than that of pure PEDOT (6.88%) and close to that of the Pt CE (8.33%).

CNTs are allotropes of carbon with a cylindrical nanostructure, resembling a tube. They consist of one or more cylindrical layers of graphene, rolled up to form a seamless tube with diameter ranging from few nanometers to several micrometers.¹²¹ CNTs possess unique properties such as high tensile strength, stiffness, and flexibility, along with excellent electrical and thermal conductivities, and a high surface area to volume ratio. These properties make them promising materials for various applications, including nanoelectronics, energy storage, and biomedical devices. Additionally, CNTs exhibit high catalytic activity owing to their large surface area and exposure of edge sites, making them suitable materials for catalysis and chemical sensing applications.^122–124

Li et al.¹⁰⁹ reported the fabrication of transparent honeycomb-like PEDOT (h-PEDOT)/multi-walled CNT (MWCNT) electrodes using PMMA nanospheres as sacrificial templates and applied them in double-sided DSSCs. Four types of electrodes were prepared in their study: flat PEDOT (f-PEDOT), flat PEDOT/MWCNT (f-PEDOT/MWCNT), h-PEDOT, and h-PEDOT/MWCNT. The normal electrodeposited PEDOT and the PEDOT deposited using the PMMA template exhibited a honeycomb shape, as shown in Fig. 18. The incorporation of MWCNTs increased the thickness of the counter electrode and provided more catalytic sites, enhancing the electrocatalytic activity. Additionally, h-PEDOT/MWCNT showed transmittance and diffuse reflectance values of 58% and 26%, respectively. The efficiencies of front- and back-illuminated DSSCs based on h-PEDOT/MWCNT CEs were 9.07% and 5.62%, respectively, which were higher than the corresponding values for f-PEDOT (7.51% and 3.49%) and Pt (7.7% and 3.75%) CE-based DSSCs.

Fig. 18 (a) Top-view FESEM images, (b) schematic illustration of electron transport paths, and UV-visible (c) transmittance and (d) reflectance spectra of DSSCs based on f-PEDOT, f-PEDOT/MWCNT, h-PEDOT, and h-PEDOT/MWCNT CEs. Images reproduced from ref. 109 with permission from Elsevier B. V., Copyrights © 2017.

Single-walled carbon nanohorns (SWCNHs) are a novel type of nanomaterial, structurally like single-walled carbon nanotubes (SWCNTs).¹²⁵ SWCNHs have a conical shape made up of a graphite structure, with a cone angle of 20°, a tube diameter ranging between 2 and 5 nm, and a tube length of 40–50 nm. Typically, 1000–2000 SWCNHs cluster together to form a sphere of 25–150 nm diameter,¹²⁶ exhibiting a large specific surface area, high porosity, and good catalytic activity.¹²⁷ Susmitha et al.¹⁰² developed a SWCNH/PEDOT:PSS CE by spin coating different concentrations of SWCNHs and PEDOT:PSS mixtures on FTO glass substrates. The authors reported that DSSCs based on 0.4 wt% SWCNH/PEDOT:PSS CE achieved a higher PCE (4.7%) than other SWCNH/PEDOT:PSS electrodes. However, the PCE was still lower than that of a DSSC based on Pt electrodes (5.55%), as determined by the analysis of J–V curves. On the other hand, Kasi Reddy et al.¹⁰³ prepared SWCNH/PEDOT:PSS CEs using the double spin coating method. The composite electrode exhibited a R_ct (25.9 Ω), indicating good adhesion between SWCNH/PEDOT:PSS and the FTO substrate. The authors reported an average surface roughness of 3.0587 nm for the SWCNHs/PEDOT:PSS CE, which was higher than that of SWCNH (1.5008 nm) and PEDOT:PSS (1.1737 nm), as shown in Fig. 19. The higher surface roughness suggests that the counter electrode had a larger contact area with the electrolyte, indicating the availability of more abundant catalytic sites. Moreover, the bright peaks observed in the AFM images of the SWCNH/PEDOT:PSS counter electrode indicate a high conductivity compared to SWCNH and PEDOT:PSS. Owing to the high roughness and conductivity, the PCE of DSSC based on the SWCNH/PEDOT:PSS CE (5.10%) was much higher than those of devices based on PEDOT:PSS (3.87%) and SWCNH (1.88%) CEs, and comparable to that of a Pt CE-based cell (5.53%).

Fig. 19 (a) Schematic illustration of the fabrication of bilayer SWCNH/PEDOT:PSS CEs. (b) 3D AFM images of PEDOT:PSS, SWCNH, SWCNH/PEDOT:PSS, and Pt CEs. (c) Cyclic voltammetry plots of SWCNH, PEDOT:PSS, SWCNH/PEDOT:PSS, and Pt CEs, along with schematic illustration of faster tri-iodide reduction at the surface of a bilayer SWCNH/PEDOT:PSS CE-based DSSC. (d) J–V characteristics of fabricated DSSCs. Images adapted from ref. 103. Copyright 2020, Springer Nature.

Not all composites of PEDOT or PEDOT:PSS with carbon structures of different morphologies indicate the superiority of carbon, which indicates that aside the good stability of the composite, it is important to consider the conductivity and morphology of the carbon material (especially catalytic sites). Additionally, functionalization of either PEDOT or carbon materials also gives better catalytic properties, highlighting the versatility of this category of composites.

3.5. Transparent PEDOT-based counter electrodes

PEDOT is frequently employed to produce transparent CEs for DSSCs, owing to its excellent average visible transmittance (AVT). PEDOT has proved to be more suitable for cobalt-based electrolytes (which are also transparent) than conventional Pt CEs. Consequently, the combination of organic dye, cobalt-based electrolyte, and a PEDOT CE emerges as one of the effective solutions for fabricating highly efficient and transparent DSSCs. Currently, the transmittance of PEDOT composite electrodes falls within the range of 60% to 65%; adjusting the synthesis method and film thickness can achieve higher transmittances in pure PEDOT electrodes. As discussed above, the MoS₂/NC-PEDOT:PSS CE fabricated by Abdelaal et al.¹¹⁴ demonstrated excellent efficiencies for both I⁻/I₃⁻ and [Co(bpy)₃]^2+/3+ electrolytes, with an AVT of 62.5%. In a separate study, Li et al.¹⁰⁹ employed PMMA sacrificial templates to produce a honeycomb-like PEDOT CE, achieving an AVT of 65%. Li et al.¹²⁸ successfully developed a highly transparent CE through the deposition of island shaped PEDOT on FTO using AC current (Fig. 20a and b). This unique island-shaped structure not only enhances transmittance but also exhibits commendable catalytic activity. As shown in Fig. 20c and d they employed the transparent CE in the fabrication of a bifacial DSSC. Through meticulous optimization of the electrodeposition time, the optimized device (4 s) yielded a PCE of 9.1% under front side illumination and 7.8% under rear side illumination. Impressively, the PCE ratio between the two sides soared to an impressive 86%. This research underscores the potential of PEDOT transparent CEs in enhancing the efficiency and versatility of solar cells.

Fig. 20 Morphologies of the optimized PEDOT CE (4s): (a) a top-view SEM image with an inset illustrating the distribution of PEDOT aggregates, and (b) a cross-sectional SEM image. Photographs of (c) PEDOT CE with different electrodeposition times and Pt CE, (d) bifacial DSSC based on optimized PEDOT CE. Images are reproduced with permission from ref. 128, Copyright © 2023 Royal Society of Chemistry.

3.6. Stability of DSSCs with PEDOT-based CEs

PEDOT-based CEs offer a range of advantageous properties for DSSCs. They are renowned not only for their excellent catalytic properties and good transparency but also for their remarkable stability. Though traditional platinum CEs are effective electrocatalyst, they suffer from long-term stability issues. This is primarily due to the chemical reaction between Pt and the iodine-based electrolyte, which leads to the formation of PtI₄. Such reactions degrade the performance of the DSSC over time, reducing its durability. In contrast, PEDOT exhibits superior corrosion resistance against the I⁻/I₃⁻ redox couple and excellent thermal stability. These attributes make PEDOT a more robust alternative for CEs in DSSCs. Furthermore, PEDOT-based CEs have demonstrated enhanced efficiency when paired with other types of electrolytes, such as those based on cobalt or copper redox species. These combinations not only preserve the stability of the device but also contribute to improve the overall efficiency. Consequently, DSSCs incorporating PEDOT-based CEs typically exhibit good long-term stability and performance.

The evaluation of stability in DSSCs is predominantly based on electrochemical stability and the long-term stability of the CEs. Electrochemical stability is typically assessed through continuous cycling of CV. The stability is measured by observing changes in parameters such as the E_pp, J_red, and R_ct after successive CV cycles. Smaller changes in these parameters indicate better stability. Yi Di et al.^91,94 demonstrated the remarkable electrochemical stability of PEDOT-Ni₃(PO₄)₂ and PEDOT-Ni₂P composite CEs. Their CV curves remained virtually unchanged after 50 cycles in an iodine-based electrolyte solution, highlighting the robustness of these composites. Similarly, Carbas B. B et al.¹²⁹ synthesized doped PEDOT via a straightforward one-step electropolymerization process using the 1-ethyl-3-methylimidazole hydrogen sulfate (EMIMHSO₄) ionic liquid. The doped PEDOT CEs retained 94% of their initial catalytic performance after 50 consecutive CV cycles in an iodine-based electrolyte solution, outperforming pure PEDOT electrodes, which retained 87%. In addition to their outstanding stability in iodine-based electrolytes, PEDOT CEs also exhibit excellent stability in other types of electrolytes, such as cobalt-based electrolytes. Previous research from our laboratory evaluated the stability of graphene/PEDOT:PSS and Pt CEs in cobalt-based electrolytes using the EIS analysis after successive CV cycles.¹⁰⁸ The results revealed that the R_ct of the Pt CE increased from 3.17 Ω cm⁻² to 7.38 Ω cm⁻² after 1000 consecutive CV cycles. In contrast, the R_ct of the graphene/PEDOT:PSS CE only increased from 0.1 Ω cm⁻² to 0.3 Ω cm⁻², demonstrating that the stability of the graphene/PEDOT:PSS CE in cobalt-based electrolytes is significantly superior to that of the Pt CE.

The long-term stability of a DSSC is determined by comparing the performance of the fresh device with that of an aged device over a period. Smaller changes indicate better long-term stability, which is a crucial parameter for evaluating the viability of solar cells for practical applications. Masud et al.¹³⁰ demonstrated the impressive long-term stability of DSSCs employing PEDOT CEs prepared through chemical polymerization. The PCE of these DSSCs decreased only marginally (11.56% to 11.11%) after 200 hours. Similarly, S Venkatesan et al.²⁰ fabricated high-performance PEDOT/carbon black (CB) CEs for copper(I)/(II)-mediated DSSCs. Their studies showed that the PCE of DSSCs based on both pure PEDOT and PEDOT/CB CEs essentially unchanged after 500 hours. These findings further corroborate the excellent stability of PEDOT-based CEs. Numerous other reports have also highlighted the superior long-term stability of PEDOT CEs compared to traditional Pt CEs. Various studies have consistently shown that PEDOT-based CEs exhibit minimal degradation in performance over extended periods, whereas Pt CEs often suffer more significant performance losses due to issues like corrosion and interaction with the electrolyte.^91,94,96,97 These results collectively emphasize the potential of PEDOT CEs as a more durable and stable alternative to Pt CEs in DSSCs.

4. Flexible PEDOT-based counter electrodes

Flexibility is important when considering multiple applications of photovoltaics like in wearable IoTs. To realize cost-effective processes for manufacturing these electronics, simple preparation methods at low temperature and with solution processability is required. Electrochemical polymerization is a simple and fast method for fabricating flexible PEDOT CEs, which does not require high temperatures or additional processing. The recent literature on the performance of flexible PEDOT CEs is summarized in Table 2. Pringle et al.¹³¹ successfully deposited PEDOT onto ITO-PEN substrates using an acetonitrile solution of EDOT. The films were prepared using the constant–current method, and the effect of different deposition times on their growth was investigated. The morphology of the PEDOT film, consisting of a dense base layer and a rough particle layer, changed with increasing deposition time. Electrodeposition times of 5 s were sufficient to produce PEDOT films with high transmittance and electrocatalytic activity. The PCE of a device based on the PEDOT/ITO CE (8.0%) was higher than that of Pt/FTO CE (7.9%).

Table 2 PCEs of DSSCs with PEDOT-based flexible counter electrodes measured under AM 1.5G in recent years, along with corresponding fabrication methods

Class	CE catalyst	Dye	Redox couple	V oc (mV)	J sc (mA cm^−2)	FF (%)	PCE (%)	V ^Ptoc (mV)	J ^Ptsc (mA cm^−2)	FF^Pt (%)	PCE^Pt (%)	Method	Ref.
Flexible PEDOT-based CE	f-PEDOT(SSP)	N719	I^−/I3^−	0.726	11.90	54	4.65	0.724	12.83	58	5.38	SSP	89
	CDs-PEDOT:PSS	N3	I^−/I3^−	0.787	8.7	70	4.84	0.79	10.7	50	4.41	Ultrasonic spraying	132
	GDs/PEDOT:PSS	N719	I^−/I3^−	0.698	12.08	58	4.91	0.672	7.08	36	1.7	Dipping	133
	f-PEDOT/Pt	N719	I^−/I3^−	0.75	9.33	72	5.04	0.76	8.8	71	4.73	Electrodeposition	134
	PEDOT/EFG	N719	I^−/I3^−	0.77	10.2	72	5.7	0.79	9.01	63	4.49	Electrodeposition	135
	PEDOT(SDS)	LEG4	[Co(bpy)3]^2+/3+	0.862	10.4	69	6.2	0.865	10.3	57	5	Electrodeposition	136
	PEDOT/Ti	N719	I^−/I3^−	0.718	7.06	68	6.33	0.72	6.55	68	6.13	Electrodeposition	137
	PEDOT	N719	I^−/I3^−	0.787	14.1	73	8	0.805	13.8	61	6.79	Electrodeposition	131

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Ellis et al.¹³⁶ also successfully fabricated PEDOT films on a large-sized FTO glass and ITO/PET substrates (9 cm × 9 cm) using only aqueous EDOT and sodium dodecyl sulfate solutions. Fig. 21 fabricated PEDOT/Pt CEs by loading a small amount of Pt onto the PEDOT film.¹³⁴ The PCE of the DSSC based on this PEDOT/Pt CE was enhanced by 25% compared to that based on a pure PEDOT CE.

Fig. 21 Photographs of (a) PEDOT/FTO glass and (b) PEDOT/ITO/PEN. SEM images of PEDOT films grown on FTO substrates with different deposition times: (c) PEDOT-A (175 s), (d) PEDOT-B (250 s), and (e) PEDOT-C (400 s). (f) J–V curves of DSSCs based on different electrodes under 1 sun illumination, along with molecular structure of the LEG4 dye. Images reproduced from ref. 136 with permission from Elsevier B. V., Copyrights © 2013.

In addition to depositing PEDOT on ITO/plastic substrates, some groups have used metal foil or even A4 printing paper to fabricate flexible CEs. Xiao et al.¹³⁷ demonstrated the successful deposition of PEDOT on titanium meshes via CV. The resulting PEDOT/Ti CE exhibited good electrocatalytic activity and enabled the corresponding DSSC to reach a PCE of 6.33%, higher than that achieved with a Pt/Ti CE (6.13%). Similarly, Nagarajan et al.¹³⁵ fabricated a flexible electrode by electrochemical deposition of PEDOT onto a conductive exfoliated graphite substrate. Compared to the commonly used FTO substrate, exfoliated graphite exhibited better compatibility with PEDOT, resulting in a higher PCE of the solid-state DSSC based on the PEDOT/exfoliated graphite CE (5.78%) than those based on PEDOT/FTO (4.81%) and Pt/FTO (4.49%) CEs.

In another study, Lee et al.¹³³ successfully fabricated a flexible CE by using A4 paper and a graphene dot (GD)/PEDOT:PSS composite. They investigated the effects of the PEDOT:PSS thickness and GD content on the photovoltaic performance of the resulting DSSC, ultimately identifying a PEDOT:PSS thickness of 5.25 μm and a GD content of 30% as optimal conditions. The authors then immersed the paper in a 30 vol% GD/PEDOT:PSS solution for 10 min to create paper-based flexible CEs using the GD/PEDOT:PSS composite, and sputtered Pt to prepare Pt paper-based flexible CEs. As shown in Fig. 22, the resulting CE was highly flexible, lightweight, and easy to cut. The corresponding cell exhibited a higher PCE and bending stability than the flexible DSSC made with the Pt-based CE. The efficiency of the flexible DSSC based on GD/PEDOT:PSS remained almost unchanged after 150 bending cycles, whereas that of sputtered Pt decreased by 45%. These reports have demonstrated that flexible applications and large-scale production can be realized for photovoltaics with PEDOT by simple techniques, especially by electrodeposition.

Fig. 22 (a) Schematic illustration of GD/PEDOT:PSS paper electrode. (b) Photographs of flexible CE. (c) SEM images of GD/PEDOT:PSS and sputtered Pt paper-based electrodes. (d) J–V curves of flexible DSSCs based on the two CEs, along with PCE values of each flexible device over 150 bending cycles. Images reproduced from ref. 133 with permission from Elsevier B. V., Copyrights © 2017.

5. PEDOT counter electrode under Indoor light conditions

IoTs are usually employed under various diffuse and low light illumination settings, hence the performance and development of DSSCs for these applications with self-powering potential is crucial. In general, the light harnessing efficacy of DSSC under indoor light conditions is limited due to the constrained spectral range of indoor light spectrum. Under such low intensity conditions, the number of excited electrons is significantly lower compared to 1-sun conditions, which directly affects the photovoltaic parameters, hence more material defects can easily lower the open-circuit voltage (V_oc) of the cell. Additionally, the redox potential of the electrolyte also plays a crucial role in determining the V_oc of these devices under illumination. To address these challenges, Co complexes and Cu-based redox couples have been sought to get high V_oc in the DSSCs operating under low-intensity conditions. However, considering the potential market for indoor photovoltaics and the compatibility of redox electrolytes with catalytic materials, PEDOT CEs have emerged as efficient and conductive materials for bifacial applications owing to their high transparency and excellent catalytic performance. Several reports have demonstrated that PEDOT-based CEs exhibit outstanding electrocatalytic performance in DSSCs and have even achieved world-record device performances under both indoor and 1-sun conditions (Table 3).

Table 3 Literature review on the performance of PEDOT-based CEs in DSSC under low-intensity light conditions

CE	Coating method	Dyes	Redox couple	1-Sun illumination				P in (μW cm^−2) or lux	Indoor illumination					Ref.
				V oc (mV)	J sc (mA cm^−2)	FF (%)	PCE (%)		V oc (mV)	J sc (mA cm^−2)	FF (%)	PCE (%)	P out (μW cm^−2)
PEDOT:PSS:GNP	Spin coating	YD2-o-C8	[Co(bpy)3]^2+/3+	0.902	15.32	75	10.4	1000 lx	0.724	0.146	80	29.7	—	138
PEDOT	Electrodeposition	D35 + XY1b/XY1b + Y123	[Co(bpy)3]^2+/3+	—	—	—	—	642 μW cm^−2 (2000 lx)	0.856	0.356	76.3	36.27	232.83	139
PEDOT	Electrodeposition	C106	[Cu(dmby)2]^+/2+	0.744	2.05	52	0.79	1000 lx	0.525	0.077	61	7.9	24.6	140
PEDOT	Electrodeposition	XY1:L1	[Cu(dmby)2]^+/2+	—	—	—	—	303.1 μW cm^−2 (1000 lx)	0.91	0.147	77	34	103.1	141
PEDOT:PSS	Spin coating	YD2-o-C8	[Co(bpy)3]^3+/2+	0.942	16.1	72	11	275 μW cm^−2 (1000 lx)	0.759	0.14	83	32	88.1	142
PEDOT	Electrodeposition	XY1 + D35	[Cu(dmby)2]^+/2+	—	—	—	—	306.6 μW cm^−2 (1000 lx)	0.797	0.138	80	28.9	88.5	143
PEDOT	Electrodeposition	XY1b + Y123	[Cu(dmby)2]^+/2+	—	—	—	—	318.2 μW cm^−2 (1000 lx)	0.878	0.149	77.3	31.8	101.1	144
PEDOT	Electrodeposition	MS5 + XY1b	[Cu(dmby)2]^+/2+	1.046	15.84	81.3	13.5	318.2 μW cm^−2 (1000 lx)	0.98	0.138	81.5	34.5	109.8	145
PEDOT	Electrodeposition	XY1 + 5T	[Cu(tmby)2]^+/2+	1.05	11.4	76	9.1	303.1 μW cm^−2 (1000 lx)	0.86	0.131	78	29.2	88.5	146
PEDOT/CB	Electrodeposition	Y123	[Cu(dmby)2]^+/2+	1.05	13.51	71	10.08	488 μW cm^−2 (1500 lx)	0.945	0.181	74.7	26.21	127.88	20
PEDOT/Pt	Electrodeposition	N719	I^−/I3^−	0.757	15.9	74.5	8.97	200 lx	0.583	0.025	76	15.72	11.03	147
PEDOT	Electrodeposition	XY1 + L1	[Cu(tmby)2]^+/2+	1.07	12.7	70.5	9.47	1000 lx	0.995	0.147	77.8	38	115.4	148
PEDOT	Electrodeposition	D35 + XY1b	[Co(bpy)3]^2+/3+	—	—	—	—	329 μW cm^−2 (1000 lux)	0.77	0.138	78.6	25.3	83.2	149
PEDOT	Electrodeposition	D35 + XY1	[Cu(dmp)2Cl]^+/[Cu(dmp)2]^+	0.975	12.8	64.2	8	283.4 μW cm^−2 (1000 lux)	0.905	0.135	82.6	35.6	100.8	150

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Gratzel et al.¹⁴³ first tested DSSCs under indoor illumination by employing a combination of Cu(II/I)(tmby)₂ as a redox shuttle, PEDOT as CE, and co-sensitization of XY1 and D35 organic dyes. This innovative device architecture exhibited remarkable improvements in V_oc under ambient light conditions (930 warm-white, fluorescent light-1000 lux), achieving 28.9% PCE and a power output of 88.5 μW cm⁻². Expanding on their work in 2018, the same research group introduced the concept of a type II junction formed by an n-type semiconductor and a p-type polymeric semiconductor. By utilizing a Cu(II)/Cu(I) redox-based electrolyte and PEDOT CE as a hole collector, they achieved outstanding PCE values of 13.1% and 32% under 1-sun and indoor (warm white fluorescent light conditions at 1000 lux) illumination, respectively.¹⁴⁴ These advancements originated from reducing the diffusion length of the redox mediator to the mesoporous TiO₂ layer and decreasing the Warburg impedance, consequently enhancing device performance. Additionally, continuous research efforts focused on enhancing V_oc and PCE of DSSCs under indoor light conditions have been successful, particularly through the utilization of the Cu redox couple and co-sensitization strategies.¹⁴⁵ Notably, the synthesis of the novel MS5 organic dye [N-(2′,4′-bis(dodecyloxy)-[1,1′-biphenyl]-4-yl)-2′,4′-bis(dodecyloxy)-N-phenyl-[1,1′-biphenyl]-4-amine] and the electron acceptor 4-(benzo[c][1,2,5]thiadiazol-4-yl)benzoic acid)] resulted in high V_oc of 1.24 V and PCE of 13.5% under 1-sun conditions, and V_oc of 0.98 V and PCE of 34.5% under ambient light conditions.

Mendes et al.¹³⁸ introduced a novel approach by employing a transparent (65%) CE composed of PEDOT:PSS and graphene nanocomposite, spin coated onto FTO for DSSC applications. This configuration realized efficiencies of around 10.4% under 1-sun conditions and 29.7% under LED light illumination (1000 lux) when combined with a TiO₂ reflecting layer. For the bifacial application, the DSSC featuring two layers of m-TiO₂ as a photoanode realized the device PCEs of 21.9 and 18.2% with the front and rear illumination, respectively. The same research group utilized a [Co(bpy)₃]^3+/2+ redox electrolyte with PEDOT:PSS CE to maintain the transparency of DSSC, achieving an impressive PCE of 32% under LED-1000 lux illumination.¹⁴² Notably, this device also recorded a PCE of 27.3% from rear-side illumination by maintaining 85.3% PCE.

Venkatesan et al.¹³⁹ investigated the effect of organic dye-sensitization (specifically, D35 + XY1b and D35 + Y123) in tandem structures with PEDOT CEs. This tandem structure exhibited a remarkable PCE of approximately 36.27% under ambient light illumination, with a power input of 642 μW cm⁻² (2000 lx). This performance surpassed that of tandem DSSCs using only the Y123 dye with PEDOT CEs, which achieved a PCE of 33%. To enhance the catalytic behavior of the CEs, PEDOT and Pt were deposited layer by layer using an electro-deposition technique. This approach improved electron transport properties at the CE/electrolyte interface, resulting in significant PCE improvements under 1-sun (8.97%) and room-light conditions (15.35%) at 200 lux intensity.¹⁴⁷ To reduce the cost of CE materials, Pt was substituted with carbon black (CB), and composites with PEDOT were prepared for CE applications.²⁰ Utilizing a copper(I)/(II)-mediated redox couple combined with PEDOT/CB CE proved effective as a charge transport interface with enhanced electro-catalytic activity. This approach achieved a PCE of 10.08% under 1-sun conditions and 26.21% under indoor light conditions (1500 lux).

Ghaddar et al.¹⁴⁰ highlighted the significance of Cu based aqueous electrolyte for designing long-term stable DSSCs. Employing the C106 dye, Cu^I(dmby)₂/Cu^II(dmby)₂ redox electrolyte and PEDOT CE in aqueous based DSSC, the PCEs of 0.79 and 7.9% were realized under 1-sun and indoor fluorescent lamp illumination (1000 lux), respectively. Furthermore, they determined that a concentration of 3.35 M HNMBI⁺OAc⁻ served as the optimized Lewis base concentration to balance PCE performance under indoor and outdoor conditions.

Moreover, Freitag et al.¹⁴¹ demonstrated the feasibility of ambient light-harvesting self-powered smart IoT devices. By employing a strategy of co-sensitizing XY1 and L1 dyes, they achieved an improved PCE of 34% under 1000 lux intensity of warm white, fluorescent lamp illumination when integrated with PEDOT CEs and a copper(II/I) electrolyte. Additionally, they employed a cost-effective approach by co-sensitizing XY1 with 5T dye for ambient light harvesting, resulting in an impressive PCE of 29.2% (under 1000 lux) with PEDOT CEs.¹⁴⁶ These dye combinations were tested with a Cu^I/II(tmby)₂ electrolyte under 1-sun/0.1-sun illumination conditions, achieving device PCEs of 9.1% and 9.4%, respectively. The same group set a world record PCE of 38% by employing XY1 + L1 organic dye-co-sensitization, a copper(II/I) redox electrolyte and electro-polymerized PEDOT CE for self-powered IoT device applications (Fig. 23).¹⁴⁸ The self-powered IoT device comprises an array of seven ambient photovoltaic cells with a total area of 22.4 cm², which power the FireBeetle ESP32 microcontroller. Additionally, the device is equipped with two 1.5 F supercapacitors and a diode to ensure continuous power supply during periods of darkness. The usability of the device was evaluated in various scenarios, including home, office, and factory environments, demonstrating the potential of ambient photovoltaics and artificial intelligence as effective power sources for IoT applications.

Fig. 23 (a) Ambient light harvesting of DSSC for self-powered IoT devices, and (b) J–V curve of record PCE under low-intensity light conditions. Images are reproduced from ref. 148.

Lee et al.,¹⁴⁹ reported a remarkable PCE of 25.3% for solid state-DSSCs under ambient light condition (1000 lux; P_in = 329 μW cm⁻²) by utilizing Co complex-PVDF-HFP/PMMA gel type redox electrolyte and D35/XY1b co-sensitized strategy, while maintaining 96% of initial device PCE after 2000 hours of operation. To enable bifacial application suitability, PEDOT CEs were introduced as highly transparent electrodes, resulting in rear illumination PCE values surpassing approximately 91% of those obtained from front-side illumination. Furthermore, modifying the coordination environment of the Cu(II) center using the 2,9-dimethyl-1,10-phenanthroline (dmp) ligand allowed tailoring of the recombination lifetime due to asymmetric redox behavior. This modification facilitated an improvement in device PCE up to 35.6% under warm white CFL illumination (1000 lux) based on co-sensitized (D35:XY1) dyes and PEDOT CEs.¹⁵⁰

It is evident that DSSCs with PEDOT-based CEs are much sought after for indoor and diffuse light applications because of the stable, suitable work-function, and transparent properties of the resulting CEs. Further developments in this area are therefore necessary, to widen the powering ability of DSSCs.

6. Outlook, summary and future direction

Exploring the potential of PEDOT and related composites as presented in Fig. 24, it is important to realize that metal compounds give the advantage of a wide range of morphologies to enhance surface area of the CE but have conductivity limitations. Composites with carbon materials and regular PEDOT show the highest PCEs, because of the high conductivity they possess. Therefore, to realize a combination of high catalytic activity arising from the synergy between conductivity and high surface area-to-volume ratio, developing more advanced composites by employing efficient carbon materials with different morphologies is necessary. Hence, PEDOT or PEDOT:PSS composites with carbon materials seem to have the greatest potential.

Fig. 24 Comparison of PCE values of DSSCs based on PEDOT and composite CEs.

The development of inexpensive CE materials with high catalytic activity and low-temperature fabrication processes is crucial for advancing the large-scale production and application of DSSCs. This review focuses on PEDOT and its composites as CE for DSSC and their potential use for next-generation photovoltaics, such as flexible, stretchable, and wearable solar cells, particularly for the growing market of IoTs application. PEDOT has emerged as a promising CE material for DSSCs; composite CEs consisting of PEDOT, and other materials exhibit impressive PCEs (>8%) when compared to Pt under the same experimental conditions, with other desirable properties (like high conductivity and flexibility). Various PEDOT-based composites with carbon materials, metal sulfides, and metal compounds have been extensively investigated to achieve high surface area, catalytic activity and electrical conductivity as compared to bare PEDOT. The additional stability (free from corrosion) of these electrodes makes them very suitable for IoTs, where power generation under ambient conditions is required for electronics and sensors. Interestingly, DSSCs under ambient illumination have reached a PCE of 38% with PEDOT CE, much higher than those with Pt CEs.

Nevertheless, PEDOT itself has solubility issues which makes creating a solution of pure PEDOT for commercialization challenging. The current methods for fabricating pure PEDOT electrodes are predominantly limited to in situ polymerization techniques, such as electrochemical polymerization and chemical in situ polymerization. However, for large-area electrodes, electrochemical polymerization is hindered by the difficulty of solubilizing the EDOT monomer in water and achieving uniform distribution of the PEDOT film. Meanwhile, chemical in situ polymerization is time-consuming and unsuitable for continuous production. Therefore, improving the polymerization methods to enable the continuous manufacturing of uniform PEDOT electrodes is crucial for the industrialization of PEDOT. Currently, commercial PEDOT:PSS inks are widely utilized in various fields. However, due to the non-conductive nature of PSS, PEDOT:PSS exhibits lower conductivity compared to pure PEDOT, which results in inferior performance of PEDOT:PSS electrodes. Identifying new soluble conductive dopants to replace PSS is a promising approach to commercialize high-performance PEDOT electrodes. Moreover, the exceptional properties of PEDOT highlight its significant potential in diverse applications, ranging from photovoltaics to wearable electronics. For flexible PEDOT electrodes, developing a well-designed roll-to-roll fabrication process is essential for the rapid production of PEDOT counter electrodes.

Finally, since the thickness of the PEDOT film has minimal impact on its performance, composite PEDOT films formed from very thin PEDOT layers combined with other high-performance and highly transparent materials (such as graphene or transition metal chalcogenides) can be employed for bifacial DSSC CEs. These composite PEDOT films typically exhibit high transmittance and excellent catalytic activity, enabling the achievement of high PCE. In conclusion, PEDOT-based CEs have several advantages and prospects as alternatives to conventional Pt-based CEs for further development of DSSCs as next-generation photovoltaic technologies.

Author contributions

Cheng Chen: conceptualization, writing – original draft, visualization, validation. Francis Kwaku Asiam, Ashok Kumar Kaliamurthy, Md. Mahbubur Rahman: writing – review & editing, visualization. Muhammad Sadiq: Review. Jae-Joon Lee: funding acquisition, supervision, validation, writing – review & editing.

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research was supported by the Korean National Research Foundation, funded by the Ministry of Science, ICT & Future Planning (grant NRF-2016M1A2A2940912 and NRF-2021R1A2C2094554).

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