Over 13% Efficiency Ternary Nonfullerene Polymer Solar Cells with Tilted Up Absorption Edge by Incorporating a Medium Bandgap Acceptor

Efficient ternary polymer solar cells (PSCs) are prepared with poly-[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′] dithiophene-co-3fluorothieno[3,4-b]thiophene-2-carboxylate] (PTB7-Th):CO_i8DFIC as host system and medium bandgap material BDTThIT-4F as the third component. The power conversion efficiency of PSCs can be increased from 11.47% to 13.08% by incorporating 20 wt% BDTThIT-4F in acceptors, along with the simultaneously improved three key photovoltaic parameters. The absorption edge of ternary blend films can be tilted up in long wavelength range by incorporating appropriate BDTThIT-4F, although the bandgap of BDTThIT-4F is wider than that of CO_i8DFIC, leading to the extended external quantum efficiency spectra of ternary PSCs. The tilted up absorption edge of blend films should be attributed to the variation of CO_i8DFIC molecular arrangement, which can be well demonstrated from the transient and steady absorption spectra of blend films with different donors and acceptors. A new ground state bleach signal can be clearly observed in transient absorption spectra of the optimized ternary blend films, which may be due to the varied CO_i8DFIC molecular energy levels by incorporating BDTThIT-4F. Meanwhile, the lifetimes on excited states are increased in the ternary blend films, which is beneficial to exciton dissociation for improving the performance of ternary PSCs.