Synthesis and characterization of D-A-A type regular terpolymers with narrowed band-gap and their application in high performance polymer solar cells

Two novel D-A-A type regular terpolymers of PBDT-DTQ and PIDT-DTQ were designed and synthesized, in which benzodithiophene and indacenodithiophene building blocks were employed as the D unit, and quinoxaline building block was introduced as the A unit. Owing to the strong intra-molecular charge transfer effect in polymer backbone, much broader absorption spectra covering from 320 to 800 nm with the narrowed band-gap were obtained for the developed D-A-A type polymers in contrast to their corresponding D-A type polymers. On the other hand, compared with PBDT-DTQ, PIDT-DTQ exhibited a deeper HOMO energy level and also higher charge carrier mobility. To investigate the photovoltaic properties of PBDT-DTQ and PIDT-DTQ in detail, bulk hetero-junction polymer solar cells with a structure of ITO/PEDOT: PSS/Active Layer/Ca/Al were fabricated. PBDT-DTQ-based solar cells exhibited a moderate PCE value of 3.84%, however, an increased J_sc of 11.42 mA/cm², V_oc of 0.86 V and FF of 65.77% was achieved for PIDT-DTQ-based device, leading to the maximum PCE up to 6.41%. Our results here demonstrated that using the regular terpolymers as electron donor materials could be an efficient way to broaden the absorption of polymers and improve the photovoltaic performance of PSCs.