Improving photovoltaic properties of linear small molecules with TPA–DPP segment by tuning their frameworks

Three low band-gap small molecules (SMs) with D–A, D–A–D and D–A–B–A–D frameworks, namely TPA–DPP, TPA2DPP and (TPADPP)2AN, were designed and synthesized, in which triphenylamine (TPA), 3,6-dithienyldiketopyrrolopyrrole (DPP) and 9,10-anthracene (AN) were used as donor (D), acceptor (A) and bridge (B) segments, respectively. Their optical, thermal, electrochemical and photovoltaic properties were investigated. Significant influence of different frameworks on these properties was exhibited for three SMs. Strongest absorption ability, lowest HOMO energy level and highest hole-mobility make (TPADPP)2AN with D–A–B–A–D frame-work exhibit best photovoltaic property in organic solar cells. The maximum power conversion efficiency (PCE) of 1.94% and short-circuit current density (Jsc) of 6.15 mA/cm2 were observed in the (TPADPP)2AN/PC71BM based cell. This PCE value is about 2 and 5 times the corresponding values of the TPA2DPP/PC71BM and TPA–DPP PC71BM based devices, respectively.