tremendous progress in power conversion efficiency (PCE) and flexibility. [1a,2b,3] Although flexible OSCs exhibit high durability against bending, most of them are not suitable for application in wearable electronics due to the large tensile stress exerted on the devices by human body movements. Specifically, the human body consists of movable joints and elastic skin, which can exert a high stretchability of over 50% strain. [4] Consequently, it is imperative to develop stretchable OSCs beyond the flexible devices reported thus far.Although stretchable OSCs prepared via structural device engineering (i.e., a buckling method) have been reported by a few groups, [5] their manufacturing processes are typically complicated and expensive. Moreover, the stretching is limited to only one allowed direction, and it depends on the pattern of the wrinkled structures. [6] Intrinsically stretchable OSCs (IS-OSCs) have also been developed, utilizing electrodes from organic materials and liquid metal. All the constituent layers in the IS-OSCs can be stretched in every direction, distinguishable from the structurally engineered stretchable devices. [7] For example, Lipomi et al. reported IS-OSCs for the first time consisting of poly(3-hexylthiophene) (P3HT) and phenyl-C 61 -butyric acid methyl ester (PCBM) active layers on stretchable poly(dimethylsiloxane) substrates. [7c] Subsequently, Chen et al. prepared stretchable polymeric charge-transport layers consisting of poly[(9,9-bis(3′-(N,N-dimethylamino) propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN) and nitrile butadiene rubber (NBR) to construct IS-OSCs. The resulting device exhibited an initial PCE of 3% and maintained 94% of the performance under a 10% strain. [7a] Recently, we developed IS-OSCs with a higher PCE (≈11%) by using a thermoplastic polyurethane (TPU)-based substrate, a poly(3,4-ethylened ioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)-based transparent anode, and a liquid metal-based cathode. [7b] Nevertheless, the normalized PCEs of the PM6:Y7-based IS-OSCs rapidly decreased to ≈52% under 15% strain during in situ stretching cycles. [7b] This stretchability falls short of the requirements for practical application as wearable devices on human bodies.The limited stretchability of IS-OSCs is mainly due to the use of a mechanically brittle active layer containing small-molecule acceptors (SMAs). The use of strong light-absorbing SMAs and 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 (P A ) to polymer donor:smallmolecule acceptor blends. P A addition significantly enhances the stretchability and PCEs of the blends as the long P A chains function as molecular bridges ...

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.

Polythiophenes (PTs) have attracted considerable interest for application in organic solar cells (OSCs) owing to their simple molecular structures and low‐cost synthesis. However, the power conversion efficiencies (PCEs) of PT‐based OSCs are lower than those of state‐of‐the‐art OSCs. Herein, the development of two sequentially fluorinated PT donors (PT‐2F and PT‐4F) is reported for realizing highly efficient OSCs. PT‐2F and PT‐4F are designed to contain two and four fluorine atoms, respectively, per repeating unit to decrease their highest occupied molecular orbital energy levels and increase the open‐circuit voltages of the OSCs. Importantly, the PT‐4F polymers exhibit high backbone rigidity and the desired temperature‐dependent aggregation behavior, affording well‐developed crystalline structures in thin films for efficient charge transport. These beneficial features promote the construction of an optimal blend morphology of PT‐4F:small‐molecule acceptor with a suitable energy offset and low energetic disorder. Thus, the PT‐4F‐based binary and ternary OSCs achieve high PCEs of 15.6% and 16.4%, respectively.

Herein, the impacts of the selective halogenation at two different positions of dicyanomethylene‐3‐indanone (IC) end groups and inner side chains of small molecular acceptors (SMAs) on the PD:SMA interfacial interactions, blend morphology, and resulting photovoltaic properties are described. In this study, four different SMAs (A1, A2, A3, and A4) with the same molecular backbone, but with different degrees of halogenation, are synthesized. The IC end groups on the backbones of the A1 and A3, and A2 and A4 SMAs are chlorinated and fluorinated, respectively; in addition, 6‐phenoxyhexyl inner side chains of the A3 and A4 are chlorinated. The SMAs are paired with a chlorinated PBDT‐Cl PD to construct organic solar cells (OSCs). The PBDT‐Cl:A4‐based OSC exhibits the highest power conversion efficiency of 17.2%, outperforming the PBDT‐Cl:A1‐(13.3%), PBDT‐Cl:A2‐(15.6%), and PBDT‐Cl:A3‐based OSC (16.5%). The Cl atoms on the side chains in the A3 and A4 SMAs enhance the molecular/energetic interactions at the PD:SMA interfaces and improve the blend morphology in terms of domain purity and spacing. These effects lead to the improved fill factors and reduced voltage loss of the PBDT‐Cl:A3‐ and PBDT‐Cl:A4‐based OSCs. This study demonstrates the importance of appropriate halogenation of SMAs in optimizing the blend morphology, reducing voltage loss, and improving OSC performance.

Intrinsically stretchable organic solar cells (IS‐OSCs) have been recently spotlighted for their omnidirectional stretchability, seamless integrability to any surface, and facile fabrication. Due to these attributes, IS‐OSCs are ideal off‐grid power sources, especially for wearable electronics in real‐life. However, under human body elongation as high as ≈40%, cracks in IS‐OSCs are considered inevitable, and the origin of the mechanical failure is rarely identified. Herein, the crack‐initiation and the propagation mechanism are first clarified. Based on this, a crack‐free substrate/transparent electrode platform for stretchable electronics is also suggested. A double‐locking scheme, which reinforces the physical/chemical adsorption within the most mechanically fragile layer, a poly(3,4‐ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) and also with thermoplastic polyurethane substrate, is introduced. As a result, the crack‐onset strain of double‐locked IS‐OSCs surpasses 40%, while that of pristine ones is less than 20%. The IS‐OSCs with the double‐locked system exhibits an efficient power conversion efficiency of 10.2%, and the absence of cracks allows the IS‐OSCs to maintain 79.7% of the initial PCE at 40% strain.