Development of conjugated polymers for high-performance and intrinsically stretchable organic solar cells

Abstract

As the market size of the wearable electronics rapidly expands, the demand for wearable power generators is increasing. Organic solar cells (OSCs) have been spotlighted as next generation wearable power generators due to their lightweight and flexibility. Although flexible OSCs (f-OSCs) have been developed by many research groups and exhibited excellent durability against bending, most of the f-OSCs are not suitable for wearable applications considering the large tensile strain required for the human body movements. In this regard, intrinsically stretchable OSCs (IS-OSCs), in which all the consisting layers are stretchable, have been recently developed. However, power conversion efficiency (PCE) and mechanical robustness of the reported IS-OSCs fall shorts of the requirements for the commercialization levels. Therefore, development of efficient and durable IS-OSCs is highly demanded to realize wearable applications of the OSCs. Both PCE and stretchability of the entire IS-OSCs are mainly governed by properties of active layers which generate electricity from the sunlight. The active layers of the OSCs consist of two different materials

electron donor and acceptor. As most of the donor and acceptor materials have dissimilar molecular structures, they often produce phase separated blend morphology due to unfavorable molecular interactions at their interfaces. The phase separated blend morphology produces narrow and weak interfaces between the donor and acceptor domains, which result in low photovoltaic and mechanical properties of the resulting OSCs. Besides, most efficient conjugated polymers (CPs) have been designed to have rigid molecular structures to ensure high electrical properties. However, the rigid backbones of the CPs generally facilitate to form hard and large crystallites, which significantly deteriorate stretchability of the IS-OSCs. Therefore, 1) producing well-mixed blend morphology by enhancing miscibility between donor and acceptor materials and 2) suppressing the formation of excessive crystallites by alleviating backbone rigidity of the CPs are important to realize highly efficient and stretchable IS-OSCs. In this context, we suggest a series of molecular design strategies for conjugated polymers, which include 1) improvement of blend miscibility by controlling interfacial interactions of donor and acceptor materials, 2) alleviation of excessive backbone rigidity of the CPs by incorporation of flexible spacers in the CP backbones, and 3) development of ultra-stretchable CPs by incorporation of hydrogen bonding spacers capable of reversible formation of the dynamic bonding. Consequently, we aim to resolve the abovementioned challenges by realizing highly efficient and stretchable IS-OSCs. It is hoped that the above series of studies can serve as important guidelines for the design of conjugated polymers for highly efficient and durable IS-OSCs.