Alkylsilyl Functionalized Copolymer Donor for Annealing‐Free High Performance Solar Cells with over 11% Efficiency: Crystallinity Induced Small Driving Force

Abstract

AbstractThe 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 (VOC) and short circuit current density (JSC), 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 (Eloss) 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, ∆EHOMO of 0.15 eV and ∆ELUMO 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(4hexylphenyl)dithieno[2,3d:2′,3′d′]sindaceno[1,2b:5,6b′]dithiophene). Simultaneously improved VOC, JSC 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 VOC and JSC for highly efficient PSCs.