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Review

A Review on Transparent Electrodes for Flexible Organic Solar Cells

by
Yiyun Li
,
Mengzhen Sha
and
Shufen Huang
*
School of Physics, Shandong University, Jinan 250100, China
*
Author to whom correspondence should be addressed.
Coatings 2024, 14(8), 1031; https://doi.org/10.3390/coatings14081031
Submission received: 19 July 2024 / Revised: 10 August 2024 / Accepted: 13 August 2024 / Published: 14 August 2024
(This article belongs to the Section Surface Engineering for Energy Harvesting, Conversion, and Storage)
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Abstract

:
Flexible organic solar cells (FOSCs) represent a promising and rapidly evolving technology, characterized by lightweight construction, cost-effectiveness, and adaptability to various shapes and sizes. These advantages render FOSCs highly suitable for applications in diverse fields, including wearable electronics and building-integrated photovoltaics. The application scope of FOSCs necessitates electrodes with properties such as high optical transmittance, low electrical resistivity, and exceptional mechanical strength, where their selection significantly influences the overall device performance. This review explores several materials, focusing on polymers, carbon nanomaterials, and metal nanowires, highlighting their unique advantages and challenges in FOSC applications. Through this thorough review, we would like to elucidate the relationship between electrode materials and device performance, thereby inspiring further improvements and developments in FOSCs and broadening their application range.

1. Introduction

Flexible organic solar cells (FOSCs) represent an innovative and rapidly advancing technology with significant potential, offering advantages such as lightweight construction and lower costs compared to conventional silicon-based solar cells [1,2,3,4,5,6,7]. FOSCs are adaptable to various shapes and sizes, making them ideal for a wide range of applications, from wearable devices to outdoor equipment and flexible sheets, with extensive application potential and demand. Additionally, their ease of fabrication and compatibility with roll-to-roll printing processes contribute to reduced production expenses, positioning them as a feasible alternative to traditional solar cells. Despite desirable merits, FOSCs face challenges such as comparatively lower power conversion efficiency (PCE) when compared to silicon-based counterparts. Also, external strains during device application will inevitably cause cracks in the morphology of FOSCs (as shown in Figure 1), exacerbating film defects. Experimental results have shown that devices with lower flexibility exhibit poorer PCE stability [8]. Addressing the flexibility issues of FOSCs is also an important direction for expanding their application range. Researchers are actively addressing these challenges through innovative material designs and manufacturing techniques aimed at enhancing their efficiency and stability [9,10,11].
Unlike conventional silicon-based solar cells, FOSCs have the unique capability to be fabricated on flexible substrates like plastic or metal foils. This characteristic enables the creation of lightweight, bendable, and adaptable solar panels. Such flexibility broadens the scope of potential applications, spanning from wearable electronics to building-integrated photovoltaics, portable power sources, and various unconventional form factors. To expedite their application, researchers are actively investigating diverse types of FOSCs, aiming to enhance their PCE and photothermal stability. This continuous endeavor highlights the exciting and promising path of FOSC technology in the field of renewable energy.
The flexible transparent electrode (FTE) serves as a critical component of FOSCs, directly influencing their overall performance [12]. Achieving a high-performance transparent conducting electrode necessitates characteristics such as high transmittance, low resistivity, minimal roughness, substantial strength and toughness, robust interface adhesion, and exceptional thermal stability. Only when these criteria are optimized can transparent conductive electrodes of high performance be attained, thus improving the PCE of FOSCs. Various materials have been utilized as FTEs in FOSCs fabrication, with indium tin oxide (ITO) being a prevalent choice due to its notable optical transparency and electrical conductivity [13,14,15,16,17,18]. However, ITO is plagued by several significant drawbacks, including nonuniform absorption, susceptibility to chemical and mechanical instability, high cost, and limited mechanical flexibility. To address these challenges, researchers have explored alternative materials to ITO [19,20,21,22,23] (see Table 1), with conductive polymers (such as PEDOT), carbon nanotubes, and metallic nanowire networks emerging as promising candidates. These materials offer significant advantages, such as enhanced mechanical flexibility, improved chemical and mechanical stability, increased environmental friendliness, and potentially reduced production costs [24,25,26]. Herein, we present a comprehensive review elucidating the materials employed for FTEs in recent years, focusing on three main categories: conducting polymer, carbon nanomaterials, and metal nanowires. We expect our work could inspire further exploration of FTEs and promote the development of FOSCs, potentially increasing renewable energy generation and reducing our reliance on fossil fuels [27,28,29,30].

2. Conducting Polymer

Throughout the past century, significant breakthroughs have been achieved in understanding the electrical conductivity of certain polymers, such as polythiophene, polypyrrole, and polyaniline, achieved through straightforward chemical doping processes [31,32]. Extensive research has been conducted on these materials, with the conductive polymer poly (3,4-ethylenedioxythiophene) (PEDOT), particularly in its complex with poly (styrene sulfonate) (PEDOT:PSS), becoming one of the most prominent conductive materials due to its remarkable properties such as high transparency and flexibility [33,34,35,36]. Compared to traditional ITO-based electrodes, PEDOT not only offers significant advantages in terms of transparency and flexibility, but also achieves conductivity comparable to ITO through a simple chemical doping process. Additionally, it avoids issues such as oxygen and indium release into the active layer and does not require high-temperature processing. These characteristics make organic-based electrode materials a promising approach to addressing the limitations associated with ITO, thereby advancing the development of flexible substrate devices.
Zhang et al. were among the first to investigate the feasibility of employing metallic conducting polymers as anodes to substitute ITO in various applications, shedding light on challenges related to current densities and voltage drop within conducting polymer layers, especially in large-area devices like passive displays and organic photovoltaic (OPV) devices [37]. Their research specifically focuses on using flexible polymers (PEDOT:PSS) as an alternative anode material in plastic photovoltaic cells. They found that when the anode is changed from ITO to PEDOT:PSS, the Voc of devices with ITO anodes (0.3–0.43 V) is always smaller than that of devices with PEDOT:PSS anodes (0.46–0.75 V). Additionally, PEDOT:PSS demonstrates advantages in forming uniform transparent films and achieving micron or nanoscale patterning. Their work not only demonstrates the potential use of polymer anodes for photodiodes, but also presents a straightforward approach for fabricating other organic electronic devices.
Chung-Ki Cho et al. successfully fabricated 100 nm thick PEDOT:PSS electrodes on PET substrate via gravure printing [38], utilizing an aqueous solution [39]. Employing a roll width of 300 mm, they examined the mechanical flexibility using a custom-built bending test system. The gravure-printed PEDOT:PSS electrodes exhibited remarkable flexibility in various types of flexibility tests, including outer/inner bending, twisting, and stretching. Notably, these electrodes demonstrated a consistent resistance change (ΔR/R0) within an outer and inner bending radius of 10 mm. Additionally, when stretched, the PEDOT:PSS electrode displayed a relatively stable ΔR/R0 of up to 4%, surpassing the resistance change of conventional amorphous ITO electrodes. However, during the twisting test, the resistance of the PEDOT:PSS electrode began to increase at an angle of 361 due to delamination of the film from the PET substrate. Despite the relatively high sheet resistance of the PEDOT:PSS electrode, the FOSCs fabricated on it exhibited a respectable PCE of 2%, with a fill factor (FF) of 44.9%, open-circuit voltage (VOC) of 0.495 V, and short-circuit current (JSC) of 9.1 mA/cm2, underscoring the potential of gravure-printed PEDOT:PSS as a flexible and transparent electrode for printing-based FOSCs (see Figure 2).
Seok-In Na et al. pioneered the fabrication of ITO-free organic solar cells (OSCs) utilizing an organic-based electrode material to entirely replace ITO [40]. They explored a PEDOT:PSS formulation, specifically Baytron PH 500, as a polymer anode in plastic solar cells. The PCEs of ITO-free OSCs on glass and flexible plastic substrates, measuring 3.27% and 2.8%, respectively, under 100 mWcm2 illumination with AM 1.5G spectrum, were comparable to those of ITO-based devices on glass and flexible substrates, achieving 3.66% and 2.9%, respectively, under identical conditions. ITO-free OSCs using PEDOT anodes on a flexible substrate exhibited better mechanical robustness compared to ITO-based cells in the flexibility test. The tests revealed the potential of highly conductive polymer materials as viable alternatives to costly and fragile ITO.
Despite the widespread attention and rapid development of the conductive polymer PEDOT:PSS due to its unique properties, its conductivity performance is still inferior to that of ITO electrodes. Therefore, enhancing the conductivity of PEDOT:PSS is a key focus of research. Currently, the primary method to improve its performance is by combining PEDOT:PSS with additives. Beyond its inherent properties, PEDOT:PSS can be enhanced by combining it with additives to further improve its performance. By incorporating PEDOT:PSS into composites, researchers aim to enhance efficiency, stability, and flexibility, addressing some of the limitations associated with the pure form of PEDOT:PSS. Michael Vosgueritchian et al. achieved highly conductive and transparent PEDOT:PSS films through the use of the fluorosurfactant zonyl as an additive, enhancing the conductivity of the film and facilitating multilayer deposition on both conventional and stretchable substrates [41]. The treatment of PEDOT:PSS films with fluorosurfactant resulted in a significant 35% enhancement in sheet resistance (Rs) compared to untreated films (see Figure 3). Moreover, the application of fluorosurfactant enabled the deposition of PEDOT:PSS solutions onto hydrophobic surfaces, including pre-deposited, annealed films of PEDOT:PSS, to achieve thick, highly conductive multilayer films, as well as stretchable poly (dimethylsiloxane) (PDMS) substrates, enabling the development of stretchable electronics. Impressively, four-layer PEDOT:PSS films exhibited an Rs of 46 Ω per square while maintaining 82% transmittance at 550 nm. These films, when deposited on a pre-strained PDMS substrate and subjected to buckling, demonstrated excellent reversibly stretchable properties, with no alteration in Rs even after undergoing over 5000 cycles of 0 to 10% strain. Leveraging the remarkable conductivity of multilayer PEDOT:PSS films as anodes, alternative OPVs were fabricated without the use of ITO, demonstrating comparable PCEs to conventional devices employing ITO as the anode. This indicates that PEDOT has excellent potential as transparent electrodes in new devices.
Current research on the conductive polymer PEDOT:PSS not only leverages its unique advantages to address the limitations of traditional ITO electrodes in flexible devices, but also focuses on overcoming its drawbacks, such as low conductivity, through methods like additive doping and improved fabrication processes, thereby broadening its application scope in the field of organic photovoltaic cells. Organic photovoltaic cells using PEDOT:PSS as the anode have demonstrated efficiencies comparable to those with ITO, confirming the feasibility of replacing ITO with PEDOT:PSS. This significantly enhances the potential application of conductive polymers like PEDOT:PSS in fabricating flexible organic solar cells. Future research should focus on optimizing these composites to balance transparency, conductivity, and mechanical robustness, paving the way for the widespread application of conductive polymers in conventional and stretchable electronic devices.

3. Carbon Materials

Carbon nanomaterials have gained widespread attention in the field of FOSCs, thanks to their remarkable combination of high conductivity, long-term stability, exceptional transparency, and excellent mechanical flexibility. Among these carbon nanomaterials, carbon nanotubes (CNTs) and graphene stand out as the most quintessential representatives, offering superior flexibility and durability compared to traditional ITO. The utilization of CNTs and graphene represents a pivotal advancement in this field, driving the development of next-generation FOSCs with enhanced efficiency, longevity, and adaptability to diverse applications [42,43,44,45,46,47].

3.1. Carbon Nanotubes

CNTs exhibit a distinctive tubular structure composed of single-layer or multi-layer graphite sheets coiled at a specific spiral angle, resulting in seamless nanotubes. Each graphite sheet is SP2-hybridized, forming a hexagonal plane cylinder, which imparts exceptionally high Young’s modulus and fracture strength to carbon nanotubes. As a result, CNTs are resilient to bending and deformation, making them highly robust materials [44].
Bharti Sharma et al. employed the SCAPS 1-D software (3.3.09) to simulate the performance of NFA-BHJ-OSCs utilizing a transparent electrode composed of carbon CNTs doped with molybdenum trioxide (MoO3) (see Figure 4) [23]. The results indicated significant enhancements in various photovoltaic parameters for OSCs utilizing CNTs as contacts compared to those based on ITO electrodes. This improvement was also reflected in the external quantum efficiency (EQE) spectra, with CNTs exhibiting a larger EQE value over a wider wavelength range. Optimized PCE of 22.71% was achieved with an FF of 63.14%, short-circuit current density (JSC) of 38.38 mA/cm2, and open-circuit voltage (VOC) of 0.9371 V by varying the band gap of MoO3-doped carbon nanotubes. Moreover, by employing alternative hole transport layers (HTLs) such as CuI, Cu2O, and CuSCN, superior device performance was attained. Notably, the highest PCE of 27.57% was achieved with an FF of 70.88%, JSC of 37.12 mA/cm2, and VOC of 1.048 V in the CuSCN case, demonstrating significant potential as a transparent conductive electrode (TCE) for enhancing the device performance of OSCs in the future.

3.2. Graphene

Graphene, with its two-dimensional honeycomb lattice structure formed by sp2-hybridized carbon atoms, stands as a marvel in material science. The unique atomic arrangement endows it extraordinary properties, ranging from exceptional optical transparency to remarkable electrical conductivity and outstanding mechanical strength. These properties make graphene an incredibly enticing candidate for a wide array of applications, particularly in the field of FOSC devices [48,49,50].
Jing Kong’s group utilized graphene as both the anode and cathode for low-bandgap polymer devices (see Figure 5), employing a room temperature dry-transfer technique to transfer graphene onto the top of active layers [51]. By combining these highly TCEs with organic materials that primarily absorb in the ultraviolet and near-infrared region, they achieved an impressive optical transmittance of up to 61% across the visible spectrum, with a PCE ranging from 2.8% to 4.1%. This study also demonstrated the production of these devices on various flexible substrates such as plastic and paper, showcasing superior resilience to bending compared to devices utilizing ITO electrodes. This research marks a significant advancement in the development of graphene-based electronics, paving the way for the creation of advanced, flexible FOSCs devices with enhanced performance and durability.
PCE plays a crucial role in assessing the performance of OSCs. While significant strides have been made in improving the PCE of rigid OSCs, their flexible counterparts still lag behind. Efforts to enhance the device performance of FOSCs have been extensive. Hyesung Park et al. fabricated FOSCs with graphene-based anodes and cathodes, achieving PCEs of 6.1% and 7.1%, respectively [52]. The high PCEs were attained through thermal treatment of the MoO3 hole transport layer (HTL) and direct deposition of the ZnO electron transporting layer onto graphene (see Figure 6). Numerous endeavors have been undertaken to further enhance the PCE of graphene-based FOSCs. Donghwan Koo et al. developed a highly flexible and durable electrode by directly integrating polyimide (PI) with graphene [21]. The PI-assisted graphene electrode displayed an exceptionally clean surface, optical transmittance exceeding 92%, a sheet resistance of 83 Ω/sq, and remarkable thermal stability. In their study, FOSCs achieved an impressive PCE of 15.2%, along with outstanding mechanical robustness. The proposed electrode holds promise for various optoelectronic devices requiring high efficiency and flexibility.
Despite their acknowledged excellence as electrode materials, CNTs and graphene still require further advancements in both conductivity and transmittance. This enhancement is crucial for CNTs and graphene to effectively compete with other electrode materials, such as ITO, and meet the requirements of FOSCs. Therefore, ongoing research and development efforts are essential to improve the conductivity and transmittance of CNTs and graphene, thereby strengthening their position as indispensable components in the realm of flexible electronics. These endeavors include exploring novel synthesis techniques, refining material properties, and optimizing fabrication processes to unlock the full potential of CNTs and graphene-based electrodes, thus facilitating their widespread adoption in FOSCs.

4. Metal Nanowires

Metal nanowires exhibit remarkably low sheet resistance (∼0.01 Ω/) and superior mechanical properties compared to alternative conductive materials such as conductive polymers and carbon-based materials. This unique combination of high electrical conductivity and mechanical resilience makes metal nanowires highly promising for various applications in flexible electronics and optoelectronic devices. Their excellent conductivity enables efficient charge transport across devices, while their inherent flexibility ensures durability against mechanical stress and bending, essential for flexible and wearable electronics. Moreover, the ability to tailor the properties of metal nanowires, including length, diameter, and composition, offers opportunities for customized performance in specific applications. Advances in metal nanowire technology have the potential to bring about transformative changes in fields such as wearable electronics, transparent conductive films, flexible displays, and biomedical devices [53,54,55,56]. Due to these advantages, metal nanowire electrodes are widely recognized as the primary candidate material for replacing ITO.
Takehiro Tokuno et al. devised a room temperature method for preparing silver nanowire (AgNW) transparent conductive electrodes (TCEs) [57]. They synthesized AgNW TCEs using a polyol method and improved the electrical conductivity of the AgNW electrodes by mechanically pressing them at 25 MPa for 5 s at room temperature. This simple procedure achieved a low sheet resistance of 8.6 Ω/square and a transparency of 80.0%, comparable to properties exhibited by AgNW electrodes heated at 200 °C. To evaluate the potential practical application of AgNW transparent electrodes, organic solar cells (OSCs) were fabricated on ITO electrodes and on the heat-treated and pressed AgNW electrodes on glass substrates, resulting in favorable device performance.
Yanna Sun et al. proposed a method to produce grid-like, smooth, flexible water-processed silver nanowire (AgNW) electrodes [58]. They fabricated flexible transparent electrodes (FTEs) in a single step without any post-treatment by blending a water suspension of AgNWs with a PSSNa aqueous solution. The resulting electrodes showed promising optoelectric properties, as well as uniformity, smoothness, and flexibility in the film. Subsequently, these electrodes were employed to fabricate flexible organic solar cells (FOSCs). The findings revealed that FOSC devices on the water-processed AgNW electrodes exhibited improved performance comparable to devices on commercial ITO glass electrodes.
The electrical properties of AgNW films are significantly impacted by the resistance at wire–wire junctions within the films. Currently, the main method to reduce this resistance involves nano-welding or soldering techniques. However, these methods have drawbacks, including the need for specialized equipment, additional materials like “solders”, and potential adverse effects on the AgNW films and substrates. Yuan Liu et al. introduced capillary force to cold weld AgNWs, enhancing junction contact [59]. They employed a moisture-treatment approach to induce capillarity, thereby improving the performance of AgNW networks. The experimental process of moisture treatment involved two steps: moisture application and drying (refer to Figure 7). Moisture was applied to the AgNW film electrode using a humidifier or simply by breathing on it for 1–3 s. Subsequently, the sample was allowed to air dry naturally for 30 to 40 s, aided by gentle fanning to facilitate the drying process. Without any further treatment, they successfully reduced the sheet resistance and enhanced the mechanical flexibility of the AgNW films, while observing no noticeable changes in optical transmittance.
Apart from AgNW, copper nanowire (CuNW) has emerged as a prominent electrode material due to its outstanding optoelectrical performance, flexibility, and cost-effectiveness. In recent years, transparent conducting films based on CuNWs have achieved requisite levels of conductivity and transmittance for practical applications. However, despite these advancements, several challenges persist, particularly related to oxidation and dispersion. Strategies such as surface modification or encapsulation can be employed to mitigate oxidation and enhance the long-term performance of CuNW-based electrodes. Additionally, innovative fabrication techniques such as solution processing methods could enhance CuNW dispersion and film homogeneity. By addressing these challenges, CuNW-based transparent conducting films can fully realize their potential across various applications, including flexible electronics.
Zhenxing Yin et al. introduced a method for synthesizing oxidation-resistant, single-crystalline CuNWs, suitable for producing solution-processable, flexible, foldable, and free-standing electrodes [60]. The oxidation-resistant CuNWs were synthesized using a salt-assisted polyol reduction method, where structure-induced factors such as capping ratio and nucleation rate were meticulously controlled. The findings showed that the CuNWs produced in this study exhibited exceptional resistance to oxidation and could be easily dispersed without requiring antioxidants. This outstanding property is attributed to the robust stabilization provided by amine and bromide molecules present in the synthesis process. This research marks a significant advancement in materials science and offers promise for realizing transparent, flexible, and efficient CuNW-based electronic devices across various applications.

5. Conclusions

Over the past decade, the rapid expansion of flexible organic solar cells (FOSCs) has driven vigorous research into materials and processes essential for their production. Traditional indium tin oxide (ITO) electrodes fail to meet the ideal properties of high flexibility, conductivity, and transparency required for FOSCs. The development of innovative electrode materials with high electrical conductivity, transparency, and mechanical flexibility is crucial for enhancing the performance and durability of these emerging photovoltaic technologies.
This review summarizes three common types of novel materials used for FOSCs electrodes, detailing their characteristics that meet the requirements for FOSC applications as transparent electrode materials. It highlights their excellent performance post application in flexible photovoltaic devices and summarizes effective treatments currently employed to address certain shortcomings of these materials. Enhancing the conductivity of PEDOT:PSS through additive combinations allows it to fully utilize properties such as high transparency and flexibility. The unique structure of carbon materials imparts excellent mechanical stability and can achieve high PCE in FOSCs through modification. Metal nanowires, controlled for their electrical properties via physical properties, offer prospects for scalable production after mitigating issues like contact resistance and stability.
However, challenges persist, such as balancing the optical, electrical, and mechanical properties of electrode materials, and optimizing their integration into FOSCs. Compared to traditional silicon-based solar cells, the power conversion efficiency of FOSCs still has significant room for improvement. Future research efforts should prioritize addressing these challenges and further exploring novel electrode materials capable of meeting the rigorous requirements of FOSCs. Moreover, scalability and cost-effectiveness considerations are important for the commercialization of electrode materials. At the same time, when integrating with emerging technologies such as wearable electronics and building-integrated photovoltaic systems, materials should be specifically selected and modified based on the usage environment and requirements. With continued efforts and innovations, significant progress can be expected in this area, leading to more efficient, durable, and cost-effective FOSCs in the future.

Author Contributions

Investigation, Y.L. and S.H.; Writing-original draft preparation, Y.L.; Writing-review and editing, S.H. and M.S.; Visualization, Y.L. and S.H.; Project administration, S.H. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

No data were used for the research described in the article.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (a) Photographs of fabricated is-OSCs. (b) Optical images of transparent electrode EG:PH1000 under different strains, scale bar: 200 μm. (c) Normalized PCE of is-OSCs as a function of test cycles under strain. Reprinted with permission from ref. [8]. Copyright 2024 American Chemical Society.
Figure 1. (a) Photographs of fabricated is-OSCs. (b) Optical images of transparent electrode EG:PH1000 under different strains, scale bar: 200 μm. (c) Normalized PCE of is-OSCs as a function of test cycles under strain. Reprinted with permission from ref. [8]. Copyright 2024 American Chemical Society.
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Figure 2. (a) Comparison between OSCs fabricated on rigid ITO/glass and flexible PEDOT:PSS/PET substrate. (b) Current density–voltage (J-V) characteristics of OSCs fabricated on rigid ITO/glass and flexible PEDOT:PSS/PET substrate. Reprinted with permission from ref. [39] (copyright Elsevier 2011). (c) Picture of pilot R2R printing machine [38]. Reprinted with permission from ref. [38] (copyright John Wiley and Sons 2019).
Figure 2. (a) Comparison between OSCs fabricated on rigid ITO/glass and flexible PEDOT:PSS/PET substrate. (b) Current density–voltage (J-V) characteristics of OSCs fabricated on rigid ITO/glass and flexible PEDOT:PSS/PET substrate. Reprinted with permission from ref. [39] (copyright Elsevier 2011). (c) Picture of pilot R2R printing machine [38]. Reprinted with permission from ref. [38] (copyright John Wiley and Sons 2019).
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Figure 3. DC to optical conductivity ratios (σdc/σop) and sheet resistance (Rs) vs. zonyl concentration in wt% for four samples per condition across two batches. All samples were one layer and with 5% DMSO, except where noted. Reprinted with permission from ref. [41] (copyright John Wiley and Sons 2012).
Figure 3. DC to optical conductivity ratios (σdc/σop) and sheet resistance (Rs) vs. zonyl concentration in wt% for four samples per condition across two batches. All samples were one layer and with 5% DMSO, except where noted. Reprinted with permission from ref. [41] (copyright John Wiley and Sons 2012).
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Figure 4. Structure of the simulated OSCs. Reprinted with permission from ref. [23] (copyright Elsevier 2022).
Figure 4. Structure of the simulated OSCs. Reprinted with permission from ref. [23] (copyright Elsevier 2022).
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Figure 5. (a) Device structure with approximate layer thicknesses. (b) Energy level diagram for the different active layers. (c) Absorption spectra of PDTP-DFBT, PC60BM, and PC70BM spin-coated device. (d) Photograph taken through a PC60BM device. Dotted lines outline the corners of the device. (e) Cumulative absorption spectra of a PDTP:PC60BM device as each layer is added from the bottom (cathode) to the top (anode). Shaded regions indicate absorption contribution of each layer within the visible regime. Percentages in the legend refer to the total visible absorption of the layer. The cross pattern underneath the MoO3 spectrum (purple) indicates wavelengths where absorption decreases as a result of the MoO3 film. Reprinted with permission from ref. [51] (copyright John Wiley and Sons 2016).
Figure 5. (a) Device structure with approximate layer thicknesses. (b) Energy level diagram for the different active layers. (c) Absorption spectra of PDTP-DFBT, PC60BM, and PC70BM spin-coated device. (d) Photograph taken through a PC60BM device. Dotted lines outline the corners of the device. (e) Cumulative absorption spectra of a PDTP:PC60BM device as each layer is added from the bottom (cathode) to the top (anode). Shaded regions indicate absorption contribution of each layer within the visible regime. Percentages in the legend refer to the total visible absorption of the layer. The cross pattern underneath the MoO3 spectrum (purple) indicates wavelengths where absorption decreases as a result of the MoO3 film. Reprinted with permission from ref. [51] (copyright John Wiley and Sons 2016).
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Figure 6. Graphene electrode-based flexible solar cell and power conversion efficiencies of graphene anode- and cathode-based flexible PSCs. Reprinted with permission from ref. [52] (copyright American Chemical Society 2014).
Figure 6. Graphene electrode-based flexible solar cell and power conversion efficiencies of graphene anode- and cathode-based flexible PSCs. Reprinted with permission from ref. [52] (copyright American Chemical Society 2014).
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Figure 7. Capillary-force-induced cold welding of AgNWs. (a) Schematic of moisture treatment for capillary-force-induced cold welding of AgNWs. (b) Schematic of the mechanism of capillary interaction between two particles connected with a liquid bridge. (c–f) Two sets of SEM images of Ag wire–wire junctions before and after moisture treatment. The thickness (t) of the top nanowire is significantly smaller than the original diameter (d) due to welding. Scale bar: 200 nm. (g) SEM image of a relatively large area of AgNWs showing well-welded wire–wire junctions and relatively smooth surface caused by the moisture treatment. Scale bar: 1 μm. Reprinted with permission from ref. [59] (copyright American Chemical Society 2019).
Figure 7. Capillary-force-induced cold welding of AgNWs. (a) Schematic of moisture treatment for capillary-force-induced cold welding of AgNWs. (b) Schematic of the mechanism of capillary interaction between two particles connected with a liquid bridge. (c–f) Two sets of SEM images of Ag wire–wire junctions before and after moisture treatment. The thickness (t) of the top nanowire is significantly smaller than the original diameter (d) due to welding. Scale bar: 200 nm. (g) SEM image of a relatively large area of AgNWs showing well-welded wire–wire junctions and relatively smooth surface caused by the moisture treatment. Scale bar: 1 μm. Reprinted with permission from ref. [59] (copyright American Chemical Society 2019).
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Table 1. Different electrode materials and device PCEs.
Table 1. Different electrode materials and device PCEs.
ElectrodeActive LayerPCEYear
ITOP3HT:PCBM3.3%2008
ITOPTB7-TH:IEIC2.26%2016
Su-8/ITOPBDTTT-OFT:IEICO-4F:PC71BM13%2020
PEDOT:PSSP3HT:PCBM2.8%2008
SOCl2-treated PEDOT:PSSPM6:Y613.69%2022
ITO and graphenePM6:Y615.2%2020
CNTs(MoO3)PBDB-T:ITIC27.57%2022
AgNWs(Al/AZO)PBDB-T-2F:Y615.21%2020
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Li, Y.; Sha, M.; Huang, S. A Review on Transparent Electrodes for Flexible Organic Solar Cells. Coatings 2024, 14, 1031. https://doi.org/10.3390/coatings14081031

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Li Y, Sha M, Huang S. A Review on Transparent Electrodes for Flexible Organic Solar Cells. Coatings. 2024; 14(8):1031. https://doi.org/10.3390/coatings14081031

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Li, Yiyun, Mengzhen Sha, and Shufen Huang. 2024. "A Review on Transparent Electrodes for Flexible Organic Solar Cells" Coatings 14, no. 8: 1031. https://doi.org/10.3390/coatings14081031

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