Dithieno[3,2-b:2′,3′-d]silole-based low band gap polymers: the effect of fluorine and side chain substituents on photovoltaic performance

Three alkyl-thiophene p-bridged polymers, PDTS-hDTFBT (P-hF), PDTS-hDTDFBT (P-hDF) and PDTS-ehDTDFBT (P-ehDF), with different number of F atoms and side chain substituents are synthesized through a palladium catalyzed Stille coupling reaction. P-hF, P-hDF and P-ehDF show a narrow band gap of 1.56, 1.56 and 1.60 eV with deep lying highest-occupied molecular orbital (HOMO) energy levels of -5.17, -5.21 and -5.35 eV, respectively. The optimized P-hDF-based photovoltaic device exhibits an open circuit voltage of 0.593 V, a short-circuit current density of 15.98 mA cm(-2), a fill factor of 64.8% and a high energy conversion efficiency of 6.14%, which is partially ascribed to the deep HOMO energy level and good coplanarity. The performance is among the highest reported ones in devices based on polymers with dithieno[3,2-b:2',3'-d]silole (DTS) as the electron-rich unit and 2,1,3-benzothiadiazole (BT) derivatives as the electron-deficient unit.