Simultaneously enhancing the dielectric constant, photo-response and deepening HOMO levels of benzo[1,2-b;4,5-b']dithiophene derivatives-based conjugated polymers

An alternating conjugated polymer (CPs) named as PBDT-TS-DPP, derived from 2-ethylhexylthiothiophen-2-yl-flanked (TS) benzo[1,2-b;4,5-b']dithiophene (BDT-TS) and diketopyrrolopyrrole derivatives (DPP), was prepared by Palladium-catalyzed Stille coupling reaction. The comparison investigations of the opto-electronic properties and aggregation characteristics between the PBDT-TS-DPP and PBDT-T-DPP, which was derived from 2-ethylhexylthiophen-2-yl-flanked (T) benzo[1,2-b;4,5-b']dithiophene (BDT-T) and DPP, were implemented. The results indicated that the PBDT-TS-DPP not only presented narrower optical band gap, deeper HOMO energy levels, similar charger transporting and aggregation characteristics etc., but also held enhanced relative dielectric constant (εr) in relative to that for the PBDT-T-DPP. To our knowledge, it is firstly to verify that the replacement of T with TS flanks would result in the enhancing εr besides the deepening HOMO energy levels and broadening the light response range for the BDT based CPs. Beyond that, the inverted photovoltaic cells (i-OPVs) with power conversion efficiencies (PCEs) of 5.48% and 4.78%, open circuit voltage (VOC) of 0.78 V and 0.74 V, short current density (JSC) of 11.52 mA/cm2 and 9.46 mA/cm2, fill factors (FF) of 61.0% and 68.3%, were respectively presented in PBDT-TS-DPP and PBDT-T-DPP based i-OPVs. Meanwhile, it has also been certified that the i-OPVs from PBDT-TS-DPP exhibited higher exciton dissociation probability, charge carrier density, charger carriers lifetime, and lower recombination rate co-efficiency etc., which might be attributed to the higher dielectric constant of PBDT-TS-DPP as compared with PBDT-T-DPP, thus devote to the additional improvement of the PCEs, VOC and JSC for the PBDT-TS-DPP based i-OPVs, except that the improvement of the these parameters were compensated from the deepening HOMO energy level and broadening light response of the PBDT-TS-DPP.