High crystalline small molecule manipulates polymer-fullerene morphology and enables 20% improvement in fill factor and device performance

A morphology of bi-continuous donor-acceptor network absorber is critical to achieve high-performance organic solar cells and the rational control of the morphology is hard to realize. In this work, two high crystalline acceptor-donor-acceptor (A-D-A) conjugated small molecules (ITDCN, ITDCF) were synthesized and employed as the third component for PBDB-T/PC₇₁BM system. Although ITDCN and ITDCF do not have favorable complementary absorption and cascade energy level alignment to PBDB-T and PC₇₁BM, addition of ITDCN or ITDCF into PBDB-T:PC₇₁BM binary system significantly improves device fill factor (up to 20%) and open circuit voltage (20–40 mV). Characterization using space-charge limited current (SCLC), AFM and TEM shows addition of ITDCN or ITDCF manipulates absorber film phase separation which leads to improved and balanced charge carrier mobility. A power conversion efficiency of 6.68% has been achieved in PBDB-T:ITDCN:PC₇₁BM ternary solar cell with 15% ITDCN which is 24% higher than binary device. Our results suggest introducing high crystalline small molecule into polymer-fullerene system is an effective strategy to rationally control film morphology and improve polymer/fullerene solar cell performance.