Quaternary polymer solar cells with over 13% efficiency enabled by improving film-morphologies via binary mixed fullerene additive

Abstract

In this article, we report two new cases of quaternary polymer solar cells with over 13% efficiency. By introducing bis-PC₇₁BM:PC₆₁BM or bis-PC₇₁BM:IC₆₀BA into a known nonfullerene system – PBDB-T:IT-M, the quaternary solar cells significantly outperform the nonfullerene binary and ternary (PBDB-T:IT-M:fullerene) cells with a significant increase in short-circuit current-density (18.2 vs. 16.5 and 16.8–17.4 mA cm⁻², respectively) and fill-factor (0.73 vs. 0.67 and 0.705–0.726, respectively) and hence large power conversion efficiencies (13.3% for the quaternary vs. 11% for the binary and 12% for the ternary solar cells). Compared with the clear phase-separated domains in the bis-PC₇₁BM based ternary blend, the PC₆₁BM based film showed the finest morphologies and the IC₆₀BA based film had the largest domains. In sequence, the bis-PC₇₁BM:PC₆₁BM-based quaternary blend showed finer morphology than the bis-PC₇₁BM:IC₆₀BA based blend, while both the quaternary blends showed clear phase-separated bright and dark domains. One-dimensional grazing incidence X-ray diffraction (1D GIXRD) data indicated that the addition of PC₆₁BM and IC₆₀BA helped more IT-M molecules to form compacted π–π stacks at the out-of-plane directions, which resulted in the increase in electron mobilities. Our results indicate that the use of the fourth fullerene component provides more choices and more mechanisms than the ternary systems for tuning the photon-to-electron conversion; therefore, this study sheds light on the realization of high-efficiency polymer solar cells by designing multi-acceptor component energy levels.