The Critical Choice of PEDOT:PSS Additives for Long Term Stability of Roll‐to‐Roll Processed OPVs

The impact of additives mixed with poly(3,4‐ethylenedioxythiophene):polysty­renesulfonate (PEDOT:PSS) on the stability of organic photovoltaic modules is investigated for fully ambient roll‐to‐roll (R2R) processed indium tin oxide free modules. Four different PEDOT:PSS inks from two different suppliers are used. The modules are manufactured directly on barrier foil without a UV filter to accelerate degradation and enable completion of the study in a reasonable time span. The modules are subjected to stability testing following well‐established protocols developed by the international summit on organic photovoltaic stability (ISOS). For the harsh indoor test (ISOS‐L‐3) only a slight difference in stability is observed between the different modules. During both ISOS‐L‐3 and ISOS‐D‐3 one new failure mode is observed as a result of tiny air inclusions in the barrier foil and a R2R method is developed to detect and quantify these. During outdoor operation (ISOS‐O‐1) the use of ethylene glycol (EG) as an additive is found to drastically increase the operational stability of the modules as compared to dimethylsulfoxide (DMSO) and a new failure mode specific to modules with DMSO as the additive is identified. The data are extended in an ongoing experiment where DMSO is used as additive for long‐term outdoor testing in a solar park.     

Carbon: The Ultimate Electrode Choice for Widely Distributed Polymer Solar Cells

As mass‐produced, low‐cost organic electronics enter our everyday lives, so does the waste from them. The challenges associated with end‐of‐life management must be addressed by careful design and carbon‐based electrodes are central to these developments. Here, the reproducible production of vacuum‐, indium tin oxide (ITO)‐, and silver‐free solar cells in a fully packaged form using only roll‐to‐roll processing is reported. Replacing silver with carbon as electrode material significantly lowers the manufacturing cost and makes the organic photovoltaic (OPV) modules environmentally safe while retaining their flexibility, active area efficiency, and stability. The substitution of silver with carbon does not affect the roll‐to‐roll manufacturing of the modules and allows for the same fast printing and coating. The use of carbon as electrode material is one step closer to the wide release of low‐cost plastic solar cells and opens the door to new possible applications where silver recycling is not manageable.     

Vacuum‐Assisted Growth of Low‐Bandgap Thin Films (FA0.8MA0.2Sn0.5Pb0.5I3) for All‐Perovskite Tandem Solar Cells

All‐perovskite multijunction photovoltaics, combining a wide‐bandgap (WBG) perovskite top solar cell (E_G ≈1.6–1.8 eV) with a low‐bandgap (LBG) perovskite bottom solar cell (E_G < 1.3 eV), promise power conversion efficiencies (PCEs) >33%. While the research on WBG perovskite solar cells has advanced rapidly over the past decade, LBG perovskite solar cells lack PCE as well as stability. In this work, vacuum‐assisted growth control (VAGC) of solution‐processed LBG perovskite thin films based on mixed Sn–Pb perovskite compositions is reported. The reported perovskite thin films processed by VAGC exhibit large columnar crystals. Compared to the well‐established processing of LBG perovskites via antisolvent deposition, the VAGC approach results in a significantly enhanced charge‐carrier lifetime. The improved optoelectronic characteristics enable high‐performance LBG perovskite solar cells (1.27 eV) with PCEs up to 18.2% as well as very efficient four‐terminal all‐perovskite tandem solar cells with PCEs up to 23%. Moreover, VAGC leads to promising reproducibility and potential in the fabrication of larger active‐area solar cells up to 1 cm².     

Evaporation‐Free Nonfullerene Flexible Organic Solar Cell Modules Manufactured by An All‐Solution Process

To ensure laboratory‐to‐industry transfer of next‐generation energy harvesting organic solar cells (OSCs), it is necessary to develop flexible OSC modules that can be produced on a continuous roll‐to‐roll basis and to apply an all‐solution process. In this study, nonfullerene acceptors (NFAs)‐based donor polymer, SMD2, is newly designed and synthesized to continuously fabricate high‐performance flexible OSC modules. Also, multifunctional hole transport layers (HTLs), WO₃/HTL solar bilayer HTLs, are developed and applied via an all‐solution process called “ProcessOne” into inverted structure. SMD2, the donor terpolymer, has a deep highest occupied molecular orbital (HOMO) level and can achieve a power conversion efficiency (PCE) of 11.3% with NFAs without any pre‐/post‐treatment because of its optimal balance between crystallinity and miscibility. Furthermore, the integration of multifunctional HTLs enables the recovery of the drop in open circuit voltage (V_OC) caused by a mismatch in energy levels between the deep HOMO level of the NFAs‐based bulk‐heterojunction layer and the solution‐processed HTLs. Also, the photostability under ultraviolet‐exposure necessary for “ProcessOne” is greatly improved because of the integration of multifunctional HTLs. Consequently, because of the synergistic effects of these approaches, the flexible OSC modules fabricated in an industrial production line have a PCE of 5.25% (P_max = 419.6 mW) on an active area of 80 cm².     

Roll‐to‐Roll Printed Large‐Area All‐Polymer Solar Cells with 5% Efficiency Based on a Low Crystallinity Conjugated Polymer Blend

The challenge of continuous printing in high‐efficiency large‐area organic solar cells is a key limiting factor for their widespread adoption. A materials design concept for achieving large‐area, solution‐coated all‐polymer bulk heterojunction solar cells with stable phase separation morphology between the donor and acceptor is presented. The key concept lies in inhibiting strong crystallization of donor and acceptor polymers, thus forming intermixed, low crystallinity, and mostly amorphous blends. Based on experiments using donors and acceptors with different degree of crystallinity, the results show that microphase separated donor and acceptor domain sizes are inversely proportional to the crystallinity of the conjugated polymers. This methodology of using low crystallinity donors and acceptors has the added benefit of forming a consistent and robust morphology that is insensitive to different processing conditions, allowing one to easily scale up the printing process from a small‐scale solution shearing coater to a large‐scale continuous roll‐to‐roll (R2R) printer. Large‐area all‐polymer solar cells are continuously roll‐to‐roll slot die printed with power conversion efficiencies of 5%, with combined cell area up to 10 cm². This is among the highest efficiencies realized with R2R‐coated active layer organic materials on flexible substrate.     

Self‐Assembly Atomic Stacking Transport Layer of 2D Layered Titania for Perovskite Solar Cells with Extended UV Stability

A novel atomic stacking transporting layer (ASTL) based on 2D atomic sheets of titania (Ti_1?δO₂) is demonstrated in organic–inorganic lead halide perovskite solar cells. The atomically thin ASTL of 2D titania, which is fabricated using a solution‐processed self‐assembly atomic layer‐by‐layer deposition technique, exhibits the unique features of high UV transparency and negligible (or very low) oxygen vacancies, making it a promising electron transporting material in the development of stable and high‐performance perovskite solar cells. In particular, the solution‐processable atomically thin ASTL of 2D titania atomic sheets shows superior inhibition of UV degradation of perovskite solar cell devices, compared to the conventional high‐temperature sintered TiO₂ counterpart, which usually causes the notorious instability of devices under UV irradiation. The discovery opens up a new dimension to utilize the 2D layered materials with a great variety of homostructrual or heterostructural atomic stacking architectures to be integrated with the fabrication of large‐area photovoltaic or optoelectronic devices based on the solution processes.     

High‐Performance Large‐Area Organic Solar Cells Enabled by Sequential Bilayer Processing via Nonhalogenated Solvents

While the performance of laboratory‐scale organic solar cells (OSCs) continues to grow over 13%, the development of high‐efficiency large area OSCs still lags. One big challenge is that the formation of bulk heterojunction morphology is an extremely complicated process and the formed morphology is also a highly delicate balance involving many parameters such as domain size, purity, miscibility, etc. The morphology control becomes much more challenging when the device area is scaled up. In this work, a highly efficient (12.9%) nonfullerene organic solar cell processed using a sequential bilayer deposition method from nonhalogenated solvents, is reported. Using this bilayer processing method, the organic solar cells can be scaled up to a larger area (1 cm²) while maintaining a high performance of 11.4% using doctor‐blade‐coating technique. Moreover, as the acceptor is hidden behind the polymer donor, the possibility of degradation by sunlight is lessened. Thus, improved photostability is observed in the bilayer structure device when compared with the bulk heterojunction device. This method offers a truly compatible processing technique for printing large‐area OSC modules.     

Corrugation Architecture Enabled Ultraflexible Wafer‐Scale High‐Efficiency Monocrystalline Silicon Solar Cell

Advanced classes of modern application require new generation of versatile solar cells showcasing extreme mechanical resilience, large‐scale, low cost, and excellent power conversion efficiency. Conventional crystalline silicon‐based solar cells offer one of the most highly efficient power sources, but a key challenge remains to attain mechanical resilience while preserving electrical performance. A complementary metal oxide semiconductor‐based integration strategy where corrugation architecture enables ultraflexible and low‐cost solar cell modules from bulk monocrystalline large‐scale (127 × 127 cm²) silicon solar wafers with a 17% power conversion efficiency. This periodic corrugated array benefits from an interchangeable solar cell segmentation scheme which preserves the active silicon thickness of 240 µm and achieves flexibility via interdigitated back contacts. These cells can reversibly withstand high mechanical stress and can be deformed to zigzag and bifacial modules. These corrugation silicon‐based solar cells offer ultraflexibility with high stability over 1000 bending cycles including convex and concave bending to broaden the application spectrum. Finally, the smallest bending radius of curvature lower than 140 µm of the back contacts is shown that carries the solar cells segments.     

Lateral Organic Solar Cells with Self‐Assembled Semiconductor Nanowires

Solution‐processable organic semiconductor nanowires (NWs) offer a potentially powerful strategy for producing large‐area printed flexible devices. Here, the fabrication of lateral organic solar cells (LOSC) using solution‐processed organic NW blends on a flexible substrate to produce a power source for use in flexible integrated microelectronics is reported. A high photocarrier generation and an efficient charge sweep out are achieved by incorporating 1D self‐assembled poly(3‐hexylthiophene) NWs into the active layer, and an MoO₃ interfacial layer with high work function is introduced to increase the built‐in potential. These structures significantly increase the carrier diffusion/drift length and overall generated photocurrent in the channel. The utility of the LOSCs for high power source applications is demonstrated by using interdigitated electrode patterns that consist of multiple devices connected in parallel or in series. High photovoltage‐producing LOSC modules on plastic substrates for use in flexible optoelectronic devices are successfully fabricated. The LOSCs described here offer a new device architecture for use in highly flexible photoresponsive energy devices.     

Slot‐Die and Roll‐to‐Roll Processed Single Junction Organic Photovoltaic Cells with the Highest Efficiency

The record efficiency of the state‐of‐the‐art polymer solar cells (PSCs) is rapidly increasing, due to the discovery of high‐performance photoactive donor and acceptor materials. However, strong questions remain as to whether such high‐efficiency PSCs can be produced by scalable processes. This paper reports a high power conversion efficiency (PCE) of 13.5% achieved with single‐junction ternary PSCs based on PTB7‐Th, PC₇₁BM, and COi8DFIC fabricated by slot‐die coating, which shows the highest PCE ever reported in PSCs fabricated by a scalable process. To understand the origin of the high performance of the slot‐die coated device, slot‐die coated photoactive films and devices are systematically investigated. These results indicate that the good performance of the slot‐die PSCs can be due to a favorable molecule‐structure and film‐morphology change by introducing 1,8‐diiodooctane and heat treatment, which can lead to improved charge transport with reduced carrier recombination. The optimized condition is then used for the fabrication of large‐area modules and also for roll‐to‐roll fabrication. The slot‐die coated module with 30 cm² active‐area and roll‐to‐roll produced flexible PSC has shown 8.6% and 9.6%, respectively. These efficiencies are the highest in each category and demonstrate the strong potential of the slot‐die coated ternary system for commercial applications.     

Slot‐Die Coating of a High Performance Copolymer in a Readily Scalable Roll Process for Polymer Solar Cells

Copolymers based on dithieno[3,2‐b:2′,3′‐d]silole (DTS) and dithienylthiazolo[5,4‐d]thiazole (TTz) are synthesized and tested in an all‐solution roll process for polymer solar cells (PSCs). Fabrication of polymer:[6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM) solar cells is done on a previously reported compact coating/printing machine, which enables the preparation of PSCs that are directly scalable with full roll‐to‐roll processing. The positioning of the side‐chains on the thiophene units proves to be very significant in terms of solubility of the polymers and consequently has a major impact on the device yield and process control. The most successful processing is accomplished with the polymer, PDTSTTz‐4 , that has the side‐chains situated in the 4‐position on the thiophene units. Inverted PSCs based on PDTSTTz‐4 demonstrate high fill factors, up to 59%, even with active layer thicknesses well above 200 nm. Power conversion efficiencies of up to 3.5% can be reached with the roll‐coated PDTSTTz‐4 :PCBM solar cells that, together with good process control and high device yield, designate PDTSTTz‐4 as a convincing candidate for high‐throughput roll‐to‐roll production of PSCs.     

Hybrid Perovskite‐Organic Flexible Tandem Solar Cell Enabling Highly Efficient Electrocatalysis Overall Water Splitting

Perovskite‐organic tandem solar cells are attracting more attention due to their potential for highly efficient and flexible photovoltaic device. In this work, efficient perovskite‐organic monolithic tandem solar cells integrating the wide bandgap perovskite (1.74 eV) and low bandgap organic active PBDB‐T:SN6IC‐4F (1.30 eV) layer, which serve as the top and bottom subcell, respectively, are developed. The resulting perovskite‐organic tandem solar cells with passivated wide‐bandgap perovskite show a remarkable power conversion efficiency (PCE) of 15.13%, with an open‐circuit voltage (V_oc) of 1.85 V, a short‐circuit photocurrent (J_sc) of 11.52 mA cm^?2, and a fill factor (FF) of 70.98%. Thanks to the advantages of low temperature fabrication processes and the flexibility properties of the device, a flexible tandem solar cell which obtain a PCE of 13.61%, with V_oc of 1.80 V, J_sc of 11.07 mA cm^?2, and FF of 68.31% is fabricated. Moreover, to demonstrate the achieved high V_oc in the tandem solar cells for potential applications, a photovoltaic (PV)‐driven electrolysis system combing the tandem solar cell and water splitting electrocatalysis is assembled. The integrated device demonstrates a solar‐to‐hydrogen efficiency of 12.30% and 11.21% for rigid, and flexible perovskite‐organic tandem solar cell based PV‐driven electrolysis systems, respectively.     

Perovskite Solar Cells on the Way to Their Radiative Efficiency Limit – Insights Into a Success Story of High Open‐Circuit Voltage and Low Recombination

Inorganic‐organic lead‐halide perovskite solar cells have reached efficiencies above 22% within a few years of research. Achieved photovoltages of >1.2 V are outstanding for a material with a bandgap of 1.6 eV – in particular considering that it is solution processed. Such values demand for low non‐radiative recombination rates and come along with high luminescence yields when the solar cell is operated as a light emitting diode. This progress report summarizes the developments on material composition and device architecture, which allowed for such high photovoltages. It critically assesses the term “lifetime”, the theories and experiments behind it, and the different recombination mechanisms present. It attempts to condense reported explanations for the extraordinary optoelectronic properties of the material. Amongst those are an outstanding defect tolerance due to antibonding valence states and the capability of bandgap tuning, which might make the dream of low‐cost highly efficient solution‐processed thin film solar cells come true. Beyond that, the presence of photon recycling will open new opportunities for photonic device design.     

Efficient Non‐Fullerene Organic Photovoltaic Modules Incorporating As‐Cast and Thickness‐Insensitive Photoactive Layers

In this work, a new combination of a wide bandgap polymer poly[4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)benzo[1,2‐b:4,5‐b′]‐dithiophene‐alt‐N‐(2‐hexyldecyl)‐5′5‐bis[3‐(decylthio)thiophene‐2‐yl]‐2′2‐bithiophene‐3′3‐dicarboximide] (PBTIBDTT) and a non‐fullerene small molecule acceptor based on a bulky seven‐ring fused core (indacenodithieno[3,2‐b]thiophene) end‐capped with 2‐(3‐oxo‐2,3‐dihydroinden‐1‐ylidene)malononitrile groups with one fluorine substituent (ITIC‐F) is proposed, and as‐cast non‐fullerene organic solar cells (NFOSCs) with 11.2% efficiency are achieved. The device efficiencies are also insensitive to the variation of photoactive layer (PAL) thickness and can maintain over 9% efficiency as PAL thickness increases to 350 nm, which is one of the best results for as‐cast organic solar cells. More importantly, non‐fullerene organic photovoltaic (OPV) modules are demonstrated via laser ablation technique for the first time, which delivers a record efficiency of 8.6% (with active area of 3.48 cm²) among large‐area OPV modules. Furthermore, the morphology and performance evolutions of the as‐cast NFOSCs and the ones processed with solvent additive are systematically investigated. The results demonstrate the great advantage of as‐cast solar cells in achieving constant morphology and high performance with thick PALs. The NFOSCs fabricated with simple procedure, insensitive to film thickness and compatible with large‐area OPV modules, show significant potential for application the future.     

Fabricating High Performance,Donor–Acceptor Copolymer Solar Cells by Spray‐Coating in Air

We report the fabrication of high performance organic solar cells by spray‐coating the photoactive layer in air. The photovoltaic blends consist of a blend of carbazole and benzothiadiazole based donor–acceptor copolymers and the fullerene derivative PC₇₀BM. Here, we formulate a number of photovoltaic inks using a range of solvent systems that we show can all be deposited by spray casting. We use a range of techniques to characterize the structure of such films, and show that spray‐cast films have comparable surface roughness to spin‐cast films and that vertical stratification that occurs during film drying reduces the concentration of PCBM towards the underlying PEDOT:PSS interface. We also show that the active layer thickness and the drying kinetics can be tuned through control of the substrate temperature. High power conversion efficiencies of 4.3%, 4.5% and 4.6% were obtained for solar cells made from a blend of PC₇₀BM with the carbazole‐based co‐polymers PCDTBT, P2 and P1. By applying a low temperature anneal after the deposition of the cathode, the efficiency of spray‐cast solar‐cells based on a P2:PC₇₀BM blend is increased to 5.0%. Spray coating holds significant promise as a technique capable of fabricating large‐area, high performance organic solar cells in air.     

Fully Plastic Dye Solar Cell Devices by Low‐Temperature UV‐Irradiation of both the Mesoporous TiO2 Photo‐ and Platinized Counter‐Electrodes

The application of UV irradiation processes are successfully proposed for the first time in the fabrication of both of the two plastic electrodes in flexible dye solar cells (DSCs) and modules. For the realization of the photo‐electrode, a customized TiO₂ paste formulation and UV processing method was developed which yields 134% (48%) performance enhancement with respect to the same (binder‐free) paste treated at 120 °C. UV treatment induces both complete removal of organic media and more efficient charge collection. Significantly, highly catalytic platinized flexible counter‐electrodes are also obtained via UV photo‐induced reduction of screen‐printed platinum precursor pastes based on hexachloroplatinic acid. Using both UV‐processed electrodes, a fully plastic DSC is fabricated with a conversion efficiency of 4.3% under 1 Sun (semitransparent) and 5.3% under 0.2 Sun (opaque). Performance is within 10% of the efficiency of a glass‐based DSC prepared with the same materials but with conventional high temperature processes. The material formulations and processes are simple, and easily up‐scaled over large areas, even directly and simultaneously applicable to the preparation of both the photo‐and counter‐electrode on the same substrate which enabled us to demonstrate the first module on plastic realized with a W series interconnection.     

Overcoming the “Light‐Soaking” Issue in Inverted Organic Solar Cells by the Use of Al:ZnO Electron Extraction Layers

A common phenomenon of organic solar cells (OSCs) incorporating metal‐oxide electron extraction layers is the requirement to expose the devices to UV light in order to improve device characteristics – known as the so‐called “light‐soaking” issue. This behaviour appears to be of general validity for various metal‐oxide layers, various organic donor/acceptor systems, and regardless if single junction devices or multi stacked cells are considered. The requirement of UV exposure of OSCs may impose severe problems if substrates with limited UV transmission, UV blocking filters or UV to VIS down‐conversion concepts are applied. In this paper, we will demonstrate that this issue can be overcome by the use of Al doped ZnO (AZO) as electron extraction interlayer. In contrast to devices based on TiO_x and ZnO, the AZO devices show well‐behaved solar cell characteristics with a high fill factor (FF) and power conversion efficiency (PCE) even without the UV spectral components of the AM1.5 solar spectrum. As opposed to previous claims, our results indicate that the origin of s‐shaped characteristics of the OSCs is the metal‐oxide/organic interface. The electronic structures of the TiO_x/fullerene and AZO/fullerene interfaces are studied by photoelectron spectroscopy, revealing an electron extraction barrier for the TiO_x/fullerene case and facilitated electron extraction for AZO/fullerene. These results are of general relevance for organic solar cells based on various donor acceptor active systems.     

Targeting Ideal Dual‐Absorber Tandem Water Splitting Using Perovskite Photovoltaics and CuInxGa1‐xSe2 Photocathodes

Efficient sunlight‐driven water splitting devices can be achieved by pairing two absorbers of different optimized bandgaps in an optical tandem design. With tunable absorption ranges and cell voltages, organic–inorganic metal halide perovskite solar cells provide new opportunities for tailoring top absorbers for such devices. In this work, semitransparent perovskite solar cells are developed for use as the top cell in tandem with a smaller bandgap photocathode to enable panchromatic harvesting of the solar spectrum. A new CuIn_xGa_1‐xSe₂ multilayer photocathode is designed, exhibiting excellent performance for photoelectrochemical water reduction and representing a near‐ideal bottom absorber. When pairing it below a semitransparent CH₃NH₃PbBr₃‐based solar cell, a solar‐to‐hydrogen efficiency exceeding 6% is achieved, the highest value yet reported for a photovoltaic–photoelectrochemical device utilizing a single‐junction solar cell as the bias source under one sun illumination. The analysis shows that the efficiency can reach more than 20% through further optimization of the perovskite top absorber.     

Room‐Temperature Vapor Deposition of Cobalt Nitride Nanofilms for Mesoscopic and Perovskite Solar Cells

Organic/inorganic hybrid solar cells, typically mesoscopic and perovskite solar cells, are regarded as promising candidates to replace conventional silicon or thin film photovoltaics. There have been intensive investigations on the development of advanced materials for improved power conversion efficiencies, however, economical feasibilities and reliabilities of the organic/inorganic photovoltaics are yet to reach at a sufficient level for practical utilizations. In this study, cobalt nitride (CoN) nanofilms prepared by room‐temperature vapor deposition in an inert N₂ atmosphere, which is a facile and highly reproducible procedure, are proposed as a low‐cost counter electrode in mesoscopic dye‐sensitized solar cells (DSCs) and a hole transport material in inverted planar perovskite solar cells (PSCs) for the first time. The CoN film successfully replaces conventional Pt in DSCs, resulting in a power conversion efficiency comparable to the ones based on Pt. In addition, PSCs employing the CoN manifest high efficiency even up to 15.0%, which is comparable to state‐of‐the‐art performance in the cases of PSCs employing inorganic hole transporters. Furthermore, flexible solar cell applications of the CoN are performed in both mesoscopic and perovskite solar cells, verifying the advantages of the room‐temperature deposition process and feasibilities of the CoN nanofilms in various fields.     

Efficient,Large Area,and Thick Junction Polymer Solar Cells with Balanced Mobilities and Low Defect Densities

The high power conversion efficiencies (PCEs) of laboratory‐scale polymer‐based organic solar cells are yet to translate to large area modules because of a number of factors including the relatively large sheet resistance of available transparent conducting electrodes (TCEs), and the high defect densities associated with thin organic semiconductor junctions. The TCE problem limits device architectures to narrow connected strips (<1 cm) causing serious fabrication difficulties and extra costs. Thin junctions are required because of poor charge transport (imbalanced mobilities) in the constituent organic semiconductors. These issues are addressed using a combination of approaches to create thick junctions conformally coated on low sheet resistance metal grid TCEs. An essential feature of these thick junctions is balanced carrier mobilities, which affords high fill factors and efficient carrier extraction. Conformal coating is achieved by promoting enhanced intermolecular interactions in the coating solution using a high molecular weight polymeric semiconductor and appropriate solvent system. This combination of balanced mobilities, conformal coating and metallic grid TCEs is a simple and generic approach to the fabrication of defect‐free large area organic solar cells (OSCs). The approach is demonstrated with 25 cm² monolithic devices possessing aperture‐corrected power conversion efficiencies of 5% and fill factors exceeding 0.5.