Recent advances in the power conversion efficiency (PCE) of organic solar cells (OSCs) have greatly enhanced their commercial viability. Considering the technical standards (e.g., mechanical ...robustness) required for wearable electronics, which are promising application platforms for OSCs, the development of fully stretchable OSCs (f‐SOSCs) should be accelerated. Here, a comprehensive overview of f‐SOSCs, which are aimed to reliably operate under various forms of mechanical stress, including bending and multidirectional stretching, is provided. First, the mechanical requirements of f‐SOSCs, in terms of tensile and cohesion/adhesion properties, are summarized along with the experimental methods to evaluate those properties. Second, essential studies to make each layer of f‐SOSCs stretchable and efficient are discussed, emphasizing strategies to simultaneously enhance the photovoltaic and mechanical properties of the active layer, ranging from material design to fabrication control. Key improvements to the other components/layers (i.e., substrate, electrodes, and interlayers) are also covered. Lastly, considering that f‐SOSC research is in its infancy, the current challenges and future prospects are explored.
A comprehensive overview of fully stretchable organic solar cells (f‐SOSCs), including essential studies to make each layer of an f‐SOSC stretchable and efficient is provided. Various strategies to simultaneously enhance the photovoltaic and mechanical properties of the active layer, ranging from material design to fabrication control, are emphasized.
A novel approach to fabricate flexible organic solar cells is proposed without indium tin oxide (ITO) and poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) using junction‐free metal ...nanonetworks (NNs) as transparent electrodes. The metal NNs are monolithically etched using nanoscale shadow masks, and they exhibit excellent optoelectronic performance. Furthermore, the optoelectrical properties of the NNs can be controlled by both the initial metal layer thickness and NN density. Hence, with an extremely thin silver layer, the appropriate density control of the networks can lead to high transmittance and low sheet resistance. Such NNs can be utilized for thin‐film devices without planarization by conductive materials such as PEDOT:PSS. A highly efficient flexible organic solar cell with a power conversion efficiency (PCE) of 10.6% and high device yield (93.8%) is fabricated on PEDOT‐free and ITO‐free transparent electrodes. Furthermore, the flexible solar cell retains 94.3% of the initial PCE even after 3000 bending stress tests (strain: 3.13%).
A poly(3,4‐ethylenedioxythiophene)‐free and indium tin oxide (ITO)‐free junction‐free AgNN electrode with high optoelectrical properties is proposed for flexible organic solar cells (FOSCs). The electrical sheet resistance and optical transmittance can be controlled by both initial metal thickness and NN density; even a very thin Ag layer with appropriate NN density can show high transmittance and low sheet resistance, yielding a highly efficient FOSC.
Environmentally friendly colloidal nanocrystals (NCs) are promising materials for next‐generation solar cells because of their low cost, solution processability, and facile bandgap tunability. ...Recently, silver bismuth disulfide (AgBiS2) has attracted considerable attention owing to its appreciable power conversion efficiency (PCE) of 6.4%. However, issues such as the low open‐circuit voltage (VOC) compared to the bandgap of the AgBiS2 NCs and the unoptimized energy level structure at the AgBiS2 NC/PTB7 hole‐transporting layer (HTL) interface should be resolved to enhance the performance of solar cells. In this study, a design strategy to obtain efficient energy level structure in AgBiS2 NC/organic hybrid solar cells is proposed. By selecting PBDB‐T‐2F as an HTL with a lower highest occupied molecular orbital level than that of PTB7, the VOC of the device is increased. Furthermore, iodide‐ and thiolate‐passivated AgBiS2 NC surfaces are generated using tetramethylammonium iodide (TMAI) and 2‐mercaptoethanol (2‐ME), which leads to the energy level optimization of NCs for efficient charge extraction. This improves the PCE from 3.3% to 7.1%. In addition, the polymer is replaced with a PBDB‐T‐2F:BTP‐4Cl blend to achieve a higher short‐circuit current density through complementary absorption. Accordingly, an AgBiS2 NC‐based solar cell with a PCE of 9.1% is fabricated.
A structural design of the energy levels for efficient AgBiS2 colloidal nanocrystal solar cells is proposed. By lowering the highest occupied molecular orbital level of the polymer hole‐transporting layer and the valence band of nanocrystals using iodide and thiolate dual passivation, a well‐aligned energy level structure is formed, leading to higher open circuit voltage and power conversion efficiency.
Perovskite light-emitting diodes have recently broken the 20% barrier for external quantum efficiency. These values cannot be explained with classical models for optical outcoupling. Here, we analyse ...the role of photon recycling (PR) in assisting light extraction from perovskite light-emitting diodes. Spatially-resolved photoluminescence and electroluminescence measurements combined with optical modelling show that repetitive re-absorption and re-emission of photons trapped in substrate and waveguide modes significantly enhance light extraction when the radiation efficiency is sufficiently high. In this manner, PR can contribute more than 70% to the overall emission, in agreement with recently-reported high efficiencies. While an outcoupling efficiency of 100% is theoretically possible with PR, parasitic absorption losses due to absorption from the electrodes are shown to limit practical efficiencies in current device architectures. To overcome the present limits, we propose a future configuration with a reduced injection electrode area to drive the efficiency toward 100%.
Silver nanowire (AgNW) random meshes have attracted considerable attention as flexible and high‐performance transparent electrodes. Notably, post‐treatment of the AgNW random meshes, such as thermal ...annealing, is usually required to guarantee comparable optical transparency and electrical conductivity to commercial indium tin oxide (ITO). Here, the integral elements of preparing a high‐performance, large‐area AgNW random mesh network are discussed. High‐performance nanostructured transparent electrodes can be obtained without any post‐treatment, thereby relieving the restrictions related to the substrate. Solvent washing and a large‐area spray‐coating method effectively reduce the wire–wire contact resistances, thus reducing or eliminating the requirement for post‐treatment.
Solvent washing and spraying of silver nanowires decrease the sheet resistance of silver nanowire networks dramatically. The newly suggested methods eliminate post treatments such as annealing and pressing, which have been considered essential processes to reduce the sheet resistance and obtain ITO‐comparable transparent electrodes.
Blends of polymer donors (PDs) and small molecule acceptors (SMAs) have afforded highly efficient polymer solar cells (PSCs). However, most of the efficient PSCs are processed using toxic halogenated ...solvents, and they are mechanically fragile. Here, a new series of PDs by incorporating a hydrophilic oligo(ethylene glycol) flexible spacer (OEG‐FS) is developed, and efficient PSCs with a high power conversion efficiency (PCE) of 17.74% processed by a non‐halogenated solvent are demonstrated. Importantly, the incorporation of these OEG‐FSs into the PDs significantly increases the mechanical robustness and ductility of resulting PSCs, making them suitable for application as stretchable devices. The OEG‐FS alleviates excessive backbone rigidity of the PDs while enhancing their pre‐aggregation in the non‐halogenated solvent. In addition, the OEG‐FS in the PDs enhances PD‐SMA interfacial interactions and improves blend morphology, resulting in efficient charge generation and mechanical stress dissipation. The resulting PSCs demonstrate a superior PCE (17.74%) and high crack‐onset strain (COS = 10.50%), outperforming the PSCs without OEG (PCE = 15.64% and COS = 2.99%). Importantly, intrinsically stretchable (IS) PSCs containing the PD featuring OEG‐FS exhibit a high PCE (12.05%) and stretchability (maintaining 80% of the initial PCE after 22% strain), demonstrating their viability for wearable applications.
New polymer donors (PDs) featuring hydrophilic oligo(ethylene glycol) (OEG) spacers are designed to demonstrate intrinsically stretchable (IS) and toluene‐processed polymer solar cells (PSCs). The high power conversion efficiency (PCE, 17.7%) and crack‐onset strain (COS, 10.5%) of the PSC are achieved with the OEG‐containing PDs, outperforming the PSC featuring a PD without the OEG spacer (PCE = 15.6% and COS = 3.0%).
We report a comprehensive study of transparent and conductive silver nanowire (Ag NW) electrodes, including a scalable fabrication process, morphologies, and optical, mechanical adhesion, and ...flexibility properties, and various routes to improve the performance. We utilized a synthesis specifically designed for long and thin wires for improved performance in terms of sheet resistance and optical transmittance. Twenty Ω/sq and ∼80% specular transmittance, and 8 ohms/sq and 80% diffusive transmittance in the visible range are achieved, which fall in the same range as the best indium tin oxide (ITO) samples on plastic substrates for flexible electronics and solar cells. The Ag NW electrodes show optical transparencies superior to ITO for near-infrared wavelengths (2-fold higher transmission). Owing to light scattering effects, the Ag NW network has the largest difference between diffusive transmittance and specular transmittance when compared with ITO and carbon nanotube electrodes, a property which could greatly enhance solar cell performance. A mechanical study shows that Ag NW electrodes on flexible substrates show excellent robustness when subjected to bending. We also study the electrical conductance of Ag nanowires and their junctions and report a facile electrochemical method for a Au coating to reduce the wire-to-wire junction resistance for better overall film conductance. Simple mechanical pressing was also found to increase the NW film conductance due to the reduction of junction resistance. The overall properties of transparent Ag NW electrodes meet the requirements of transparent electrodes for many applications and could be an immediate ITO replacement for flexible electronics and solar cells.
Liquid metals are attractive materials for stretchable electronics owing to their high electrical conductivity and near‐zero Young's modulus. However, the high surface tension of liquid metals makes ...it difficult to form films. A novel stretchable film is proposed based on an over‐layered liquid‐metal network. An intentionally oxidized interfacial layer helps to construct uninterrupted indium and gallium nanoclusters and produces additional electrical pathways between the two metal networks under mechanical deformation. The films exhibit gigantic negative piezoresistivity (G‐NPR), which decreased the resistance up to 85% during the first 50% stretching. This G‐NPR property is due to the rupture of the metal oxides, which allows the formation of liquid eutectic gallium‐indium (EGaIn) and the connection of the over‐layered networks to build new electrical paths. The electrodes exhibiting G‐NPR are complementarily combined with conventional electrodes to amplify their performance or achieve some unique operations.
The fabrication of thin films with liquid metal is proposed using thermal evaporation. The oxide film on the liquid metal surface forms a unique structure as indium/oxide/gallium (InOG). The oxide film helps form conductivity and exhibits negative piezoresistivity by rupturing. InOG electrode exhibits controllable electrical properties and mechanical stability. InOG overcomes the limitations of conventional applications through unique properties.
Organic solar cells (OSCs) are promising wearable/stretchable power sources, but the development of high‐performance intrinsically stretchable OSCs (IS‐OSCs) has rarely been reported. Herein, IS‐OSCs ...exhibiting high power conversion efficiencies (PCEs) (>12%) and excellent stretchability are developed by constructing efficient and mechanically robust active layers via the addition of a high‐molecular weight polymer acceptor (PA) to polymer donor:small‐molecule acceptor blends. PA addition significantly enhances the stretchability and PCEs of the blends as the long PA chains function as molecular bridges between different domains, effectively dissipating mechanical stresses and improving charge transport. The IS‐OSCs with 20 wt% PA content exhibit a high PCE of 11.7% and excellent stretchability, retaining 84% of the initial PCE after 100 cycles of repetitive stretching/releasing at a 15% strain. To the best of the authors’ knowledge, the device represents the best IS‐OSC performance reported to date in terms of PCE and stretchability, demonstrating the great potential of IS‐OSCs as an efficient and wearable power generator.
Herein, high‐performance, intrinsically stretchable‐organic solar cells, which maintain 84% of the initial power conversion efficiency after 100 cycles of stretching/releasing under 15% strain are realized. High performance is achieved by efficient and mechanically robust active layers constructed by adding a high‐molecular weight polymer acceptor into polymer donor:small‐molecule acceptor blends.
Intrinsically stretchable organic solar cells (IS‐OSCs), consisting of all stretchable layers, are attracting significant attention as a future power source for wearable electronics. However, most of ...the efficient active layers for OSCs are mechanically brittle due to their rigid molecular structures designed for high electrical and optical properties. Here, a series of new polymer donors (PDs, PhAmX) featuring phenyl amide (N1,N3‐bis((5‐bromothiophen‐2‐yl)methyl)isophthalamide, PhAm)‐based flexible spacer (FS) inducing hydrogen‐bonding (H‐bonding) interactions is developed. These PDs enable IS‐OSCs with a high power conversion efficiency (PCE) of 12.73% and excellent stretchability (PCE retention of >80% of the initial value at 32% strain), representing the best performances among the reported IS‐OSCs to date. The incorporation of PhAm‐based FS enhances the molecular ordering of PDs as well as their interactions with a Y7 acceptor, enhancing the mechanical stretchability and electrical properties simultaneously. It is also found that in rigid OSCs, the PhAm5:Y7 blend achieves a much higher PCE of 17.5% compared to that of the reference PM6:Y7 blend. The impact of the PhAm‐FS linker on the mechanical and photovoltaic properties of OSCs is thoroughly investigated.
Efficient, intrinsically stretchable organic solar cells (IS‐OSCs) are developed by designing a new series of polymer donors (PDs, PhAm) featuring hydrogen‐bonding‐capable flexible spacers. High power conversion efficiency (PCE = 12.7%) and stretchability (PCE retention of > 80% at 32% strain) are demonstrated, which represent the best performances in terms of both PCE and stretchability among the IS‐OSCs reported to date.
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