The contradiction between enlarging the offset between energy levels of donor/acceptor and the required driving force for exciton split leads to a trade-off between open circuit voltage (V OC ) and short circuit current density (J SC ), which is a big challenge for development of high performance polymer solar cells (PSCs). Some advanced works reported the PSCs with low photon energy loss (E loss ) and small driving force, but the correlation of molecular structures of light-harvesting system and driving force is still unclear. In this work, a new alkylsilyl functionalized copolymer donor PBDS-T (PBDST: poly[(2,6trialkylsilyl thiophen2yl)benzo[1,2b:4,5b′]dithiophene))alt(5,5(1′,3′ di2thienyl5′,7′bis(2ethylhexyl)benzo[1′,2′c:4′,5′c′]dithiophene4,8dione))]) with low-lying energy levels was designed for efficient PSCs. By monitoring the Photoluminescence quenching of the bulk and bilayer heterojunctions, small driving forces, ∆E HOMO of 0.15 eV and ∆E LUMO of 0.22 eV were founded to allow for efficient charge transfer, which were observed to correlate with the crystalline PBDS-T and the optimal morphology in PBDS-T:ITIC (ITIC: 3,9bis (2methylene (3(1,1dicyanomethylene)indanone))5,5,11,11tetrakis(4hexy lphenyl) dithieno[2,3d:2′,3′d′]sindaceno[1,2b:5,6b′]dithiophene). Simultaneously improved V OC , J SC and small Eloss boosted the PCE over 11%, which is one of the highest values for annealing-free device. These results shield a light on precise design of a light-harvesting system with small driving force to simultaneously improve the V OC and J SC for highly efficient PSCs.
