Morphology and device physics of ternary polymer solar cells with power conversion efficiencies over 11%.*

Fabricating ternary solar cells (TSCs) is a promising strategy to improve the device efficiency of organic photovoltaics without introducing sophisticated processing procedures. We report high efficiency TSCs with PCEs over 11% in a few polymer solar cell systems. We demonstrate contrasting effects of two crystallizable polymers in determining efficiency improvements in PTB7-Th:PC₇₁BM host blends when they are added in. Whilst negligible charge transfer was determined in binary blends of each polymer mixture, effective energy transfer was identified from PffBT4T-2OD to PTB7-Th that contributes to an improvement in ternary device efficiency. In contrast, energy transfer from PTB7-Th to PDPP2TBT worsened the device efficiency. In another ternary system, the thrid component PCDTBT8 was found locating at the donor/acceptor interface without disrupting the crystallization of PffBT4T-2OD to maintain decent charge mobility, and loosens the fullerene networks to facilitate exciton dissociation. Device physics studies support that the additive can enhance the built-in voltage and suppress the trap-assisted charge recombination, leading to improved FF and V_OC to boost efficiency.