Rational design of asymmetric benzodithiophene based photovoltaic polymers for efficient solar cells

Abstract

Extending π-conjugation in the benzodithiophene (BDT) side chains has been proven useful to improve the efficiencies of the BDT-based polymer solar cells (PSCs). Herein, combined with a symmetry-breaking strategy of a BDT unit, we further designed a new asymmetric 1D–2D (one dimensional–two dimensional) monomer asy-BDTBP with an alkoxyl group as the 1D part and a π-extending alkoxybiphenyl as the 2D substituted group. Medium band-gap donor–acceptor (D–A) conjugated polymer P1 was synthesized with asy-BDTBP and 4,7-di(4-(2-ethylhexyl)-2-thienyl)-5,6-difluoro-2,1,3-benzothiadiazole (DTffBT) as the donor and acceptor unit, respectively. Encouragingly, P1 blended with PC₇₁BM exhibited an obviously enhanced power conversion efficiency (PCE) compared to the reported symmetric analogue PBDTBP–DTffBT (6.70%). The PCE increased to 8.45% with an open-circuit voltage (V_OC) of 0.838 V, a short-circuit current density (J_SC) of 14.35 mA cm⁻² and a fill factor (FF) of 70.27%. However, P1 coupled with a classical non-fullerene acceptor ITIC revealed a relatively poor efficiency of 6.35% due to the bad complementarity of absorption spectra. To match the absorption of ITIC, a wide band-gap D–A polymer P2 was further designed with a weak electron-withdrawing group benzo[1,2-c:4,5-c′]dithiophene-4,8-dione (BDD) instead of DTffBT as the acceptor unit. As a result, P2 possessed a complementary absorption spectrum with ITIC, and the resulting devices presented an excellent photovoltaic performance. The optimal efficiency boosted to 10.04% with V_OC of 0.873 V, J_SC of 17.60 mA cm⁻² and FF of 65.37%. This work demonstrates the great potential of asymmetric BDTs for high efficient PSCs and the importance of the rational design of polymers for different types of PSCs.