Achieving over 11% power conversion efficiency in PffBT4T-2OD-based ternary polymer solar cells with enhanced open-circuit-voltage and suppressed charge recombination

Abstract

Fabricating ternary solar cells (TSCs) is a promising strategy to improve the power conversion efficiency of organic photovoltaics without introducing sophisticated processing procedures. We report in this work high efficiency TSCs with the maximum PCE over 11% by introducing a medium band gap conjugated polymer PCDTBT8 into the PffBT4T-2OD:PC71BM binary photovoltaic system. Morphological investigation shows that the third component PCDTBT8 locates at the interface between PffBT4T-2OD and PC71BM without disrupting the crystallization of PffBT4T-2OD to maintain decent charge mobility, and loosens the fullerene aggregation networks to facilitate exciton dissociation. The efficient Förster energy transfer from PCDTBT8 to PffBT4T-2OD enables the ternary devices to retain a high short-circuit current density despite the slightly decreased light absorption. Device physics studies suggest that the addition of PCDTBT8 can enhance the built-in voltage, prolong the carrier lifetime, reduce the defect density and suppress the trap-assisted charge recombination, leading to an improved FF and VOC to enhance the efficiency of ternary devices.