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(i)8DFIC 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(i)8DFIC, 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(i)8DFIC 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(i)8DFIC 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.