Molecular ordering and phase segregation induced by a volatile solid additive for highly efficient all-small-molecule organic solar cells

Abstract

Morphology control remains a major challenge for all-small-molecule organic solar cells (ASM OSCs), mainly reflecting in the elusive trade-off between the molecular ordering and phase separation of the active layer. In this study, a novel volatile solid additive IC-FI, namely dihalogenated 1,1-dicyanomethylene-3-indanone, whose chemical structure is analogous to the end-group moiety of the representative non-fullerene acceptor, has been prepared and applied in BTR-Cl:N3-based ASM OSCs. The intrinsic structural feature of IC-IF enables the self-assembly of N3 along with the enhanced intermixing between BTR-Cl and N3 in the subsequent thermal annealing process. This offers nano-scale phase separation with a predominant face-on oriented molecular packing in the blend film, thereby boosting the effective charge transport and extraction process in the device. Consequently, a superior power conversion efficiency (PCE) of 14.43% with an outstanding fill factor (FF) of 73.53% was achieved in the IC-FI processed device. These results indicate that the use of the volatile solid additive is a simple and practical strategy for the optimization of the small-molecule bulk-heterojunction morphology toward highly efficient ASM OSCs.