Enhanced Charge Separation in Ternary Bulk-Heterojunction Organic Solar Cells by Fullerenes

Carrier generation dynamics in binary PTB7-Th:CO8DFIC (1:1.5) and ternary PTB7-Th:CO8DFIC:PC₇₁BM (1:1.05:0.45) composites were investigated to identify the origins of high power conversion efficiencies (PCEs) in ternary bulk-heterojunction (BHJ) organic solar cells. Steady-state photoluminescence and time-resolved photoinduced absorption spectroscopic analyses revealed that the ternary composite exhibited faster hole transfer from CO8DFIC to PTB7-Th (8 ps compared to 21 ps in the binary composite), which led to an improved exciton separation yield in CO8DFIC (94% compared to 68% in the binary composite). Improved intermixing of the component materials and efficient electron transfer from CO8DFIC to PC₇₁BM facilitated enhancement in the hole transfer rate. The CO8DFIC-to-PC₇₁BM electron transfer promoted an electron transport cascade over PTB7-Th, CO8DFIC, and PC₇₁BM, which efficiently deactivated back-electron transfer (carrier recombination loss) from CO8DFIC to PTB7-Th at ∼160 ps and assisted in improving the PCE of the ternary BHJ cell (13.4% compared to 10.5% in the binary BHJ cell).