Enhanced efficiency of polymer solar cells through synergistic optimization of mobility and tuning donor alloys by adding high-mobility conjugated polymers

A diketopyrrolopyrrole-based small bandgap polymer (DPPT-TT) with high mobility is introduced as an additive to D-A1-D-A2 type thieno[3,4-b]thiophene-based random copolymer (P3):(6,6)-phenyl-C-70-butyric acid methyl ester (PC71BM) polymer solar cells (PSCs). The average power conversion efficiencies (PCEs) were improved from 6.15% to 8.30% with the addition of 0.5% DPPT-TT. The photocurrent density versus effective voltage (J(ph)-V-eff) curves, short-circuit current density (J(SC)) and open circuit voltage (VOC) as functions of incident light intensity, photoluminescence (PL) and time-resolved transient PL (TRTPL) spectra were investigated, and the results certified the effect of DPPT-TT as the third component material in terms of efficient exciton dissociation and weakened charge carrier recombination. The relationship between VOC and the weight ratio of DPPT-TT was explained with density functional theory (DFT) calculations and the electron density of states of unit mass (N-e), indicating the formation of a polymer alloy in ternary blend. With proper addition of DPPT-TT, the mobility of electrons and holes becomes more balanced and the efficiency of exciton utilization is improved due to the existence of Forster resonance energy transfer (FRET), which also contributes to the enhanced J(SC) and PCEs. Our work demonstrates that appropriate donor polymers forming a polymer alloy in blend is a rational strategy to improve photovoltaic performance.