Molecular engineering of Diketopyrrolopyrrole-based photosensitizer for solution processed small molecule bulk heterojunction solar cells

New symmetrical low band-gap small molecule materials, DPP-bis[ter-3HT-TPA] and DPP-bis[ter-3HT] as novel derivatives of Diketopyrrolopyrrole–thiophene with/without triphenylamine (TPA) end group have been synthesized and characterized. And the effects of TPA moiety were investigated. Compared to DPP-bis[ter-3HT], DPP-bis[ter-3HT-TPA] shows red-shifted absorption and significantly higher molar absorption coefficient. And the HOMO level of DPP-bis[ter-3HT-TPA] is elevated than DPP-bis[ter-3HT]. Moreover, DPP-bis[ter-3HT-TPA] exhibited one order higher hole mobility than DPP-bis[ter-3HT], suggesting that TPA contributes to a better hole mobility. The bulk-heterojunction photovoltaic devices with DPP-bis[ter-3HT-TPA] showed better efficiencies than DPP-bis[ter-3HT], showing the best power-conversion efficiency (PCE) of 1.5% (±0.12) under 100 mW/cm2 with a short-circuit current (Jsc)=5.73 mA/cm2, a fill factor (FF)=0.45, and an open-circuit voltage (Voc)=0.59 mV.