Ternary Blend Organic Solar Cells Incorporating Ductile Conjugated Polymers with Conjugation Break Spacers

A broad family of ductile semirandom donor-acceptor (D-A) copolymers with 8-carbon alkyl conjugation break spacer (CBS) units were incorporated into ternary blend organic solar cells in order to determine their impact on the electrical metrics of solar cell performance. The goal of this study was to elucidate potential co-optimization strategies for photovoltaic and mechanical properties in organic solar cells. The ternary blended active layers were based on two polymer donors and the acceptor [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM). In all cases, the majority polymer donor component was the previously reported fully conjugated semirandom polymer P3HTT-ehDPP-10%, comprised of 80% 3-hexylthiophene, 10% diketopyrrolopyrrole (DPP) with 2-ethylhexyl (eh) side chains, and 10% thiophene. As the second donor, three different classes of CBS polymers were used, where the spacer length was kept constant at 8 methylene units. The mechanical properties of these polymers are quite notable with moduli as low as 8.54 MPa and fracture strains as high as 432%. However, it was found that as ductility increased, hole mobility decreased. In this study, we observed that the hole mobilities of the ternary active layers generally increased upon increasing the content of the CBS polymer up to 15% of the overall donor fraction. The higher carrier mobilities likely contribute to the higher J(SC) observed in many of the ternary devices. The as-cast ternary solar cells made in ambient environment without any pre/post treatment gave strong performance up to 25% of CBS polymer loading. This work demonstrates that introducing highly stretchable CBS polymers with poor charge mobility does not adversely affect solar cell performance, offering insights into the development of ternary strategies for flexible/stretchable organic solar cells.