Increased elasticity of a low-bandgap conjugated copolymer by random segmentation for mechanically robust solar cells†
Despite the necessity of organic electronic materials to undergo large deformations in flexible, ultra-thin, and stretchable applications, many high-performance organic semiconductors are mechanically fragile. This paper describes an approach to increase the elasticity of low-bandgap conjugated polymers by statistical incorporation of unlike monomers. The material under study is PDPP2FT, an alternating copolymer. Synthesized by the Stille polymerization, it comprises an N-alkylated diketopyrrolopyrrole (DPP) unit flanked by two furan rings (2F) alternating with thiophene (T). In the modified (“segmented”) polymer, PDPP2FT-seg-2T, the DPP is exchanged for a tail-to-tail coupled unit of two 3-hexylthiophene rings (bithiophene, 2T) in an average of one of approximately five repeat units. 1H NMR spectroscopy, ultraviolet-visible spectroscopy, and gel-permeation chromatography confirm the presence and covalent incorporation of the 2T units within the conjugated backbone of the segmented polymer. The tensile modulus of the segmented polymer, 0.93 ± 0.16 GPa, is lower than that of the homopolymer, 2.17 ± 0.35 GPa. When blended with PC61BM, the segmented material produces devices with power conversion efficiencies of 2.82 ± 0.28%, which is similar to that of PDPP2FT, 2.52 ± 0.34%. These results suggest that it is possible to increase the mechanical resiliency of semiconducting polymers for solar cells without having a deleterious effect on the photovoltaic properties.