Alkylthienyl substituted asymmetric 2D BDT and DTBT-based polymer solar cells with a power conversion efficiency of 9.2%

Abstract

An alkyl thiophene unit was employed for the first time as a side chain substituent on an asymmetric benzodithiophene (BDT) building block in the design of novel light-harvesting polymers. A new D–A type polymer (PBDT_Th–DT_ffBT) based on alkylthienyl asymmetric BDT and the well-known 4,7-di(thiophen-2-ethylhexyl)-5,6-difluoro-2,1,3-benzothiadiazole (DT_ffBT) was synthesized through a palladium catalyzed Stille coupling reaction. Efficient bulk heterojunction solar cells were fabricated by blending PBDT_Th–DT_ffBT with PC₇₁BM. The optimized PBDT_Th–DT_ffBT-based photovoltaic device exhibits an open circuit voltage (V_OC) of 0.87 V, a short-circuit current density (J_SC) of 15.06 mA cm⁻², a fill factor (FF) of 70.4% and a high power conversion efficiency (PCE) of 9.22%. The PCE was higher than the polymers which have the same backbone structure but also have the 1D or 2D symmetric BDT unit. In addition, a PCE of 7.83% with a V_OC of 0.91 V, a J_SC of 13.62 mA cm⁻², and an FF of 63.2% was obtained for PBDT_Th–DT_ffBT/ITIC, which is also impressive for non-fullerene PSCs. The results indicate that our design strategy of introducing an alkylthienyl unit as side chain to fabricate asymmetric BDT-based polymer is efficient in improving the photovoltaic performance.