Improving the efficiency of all-polymer solar cells through morphology control via a combination approach: polar solvent additives and external electric field

Abstract

Controlling the morphology of bulk heterojunction (BHJ) films, which are the optoelectrical active layer, in all-polymer solar cells (all-PSCs) is both delicate and challenging in the pursuit of high-performance organic solar cells. In this study, we achieved improvement in the efficiency of all-PSCs by integrating two straightforward processes. First, we induced appropriate pre-aggregation by adding a polar solvent additive to a polymer solution. Subsequently, an external electric field was applied to the polymer film to promote crystal expansion. This combination approach differs from the previously reported BHJ film morphology control processes by leveraging the polarity differences between the donor and acceptor materials to selectively enhance crystallinity. The active materials used were PTB7-Th as the donor and P(NDI2OD-T2) as the acceptor, and acetonitrile (ACN) was applied as a polar solvent additive. As a result, suitable aggregation differences led to an interpenetrated structure, forming a well-defined fibrillar morphology. This structure facilitated the creation of an efficient charge transport pathway, resulting in increased short-circuit current (JSC) and fill factor (FF). Consequently, a 35.7% increase in power conversion efficiency (PCE) was achieved. This study is significant as it demonstrates the effective control of the morphology of BHJ films through the application of a novel combination of two processes.