Achieving High-Performance Organic Photovoltaics by Morphology Optimization of Active Layers Using Fluorene-Based Solid Additives

Organic solar cells (OSCs) have drawn lots of attention because of their rapid development and great potential in large-area flexible electronics. Recently, volatile solid additives have been widely used in optimizing morphologies of active layers and improving device performances for nonfullerene (NF)-based OSCs. Most solid additives, however, still suffer severe problems such as unsuitable volatile temperatures and requirement of extra solvent additives. Herein, a new solid additive 2,7-dibromo-9,9-dimethylfluorene (DBDMF) with a high crystallinity and suitable volatile temperature as an additive for NF-based OSCs is designed. DBDMF can suppress the overaggregation of the nonfullerene acceptors (NFAs) and improve the material rearrangements after thermal annealing because of the good miscibility with the NFAs. As a result, DBDMF-treated OSC devices display more favorable film morphologies and phase separation, well-balanced charge mobilities, higher electron transfer rates, and better device stability. Consequently, the PM6:BTP-BO-4F binary system shows an outstanding power conversion efficiency of 17.2% from 15.3% with a simultaneous increase in the fill factor from 71.4 to 77.1%. Furthermore, DBDMF has been applied to other two active layers, manifesting the general applicability. This study demonstrates a feasible and promising approach to develop volatilizable solid additives for improving performance and stability of NF-based OSCs.