Significant improvement of photovoltaic performance by embedding thiophene in solution-processed star-shaped TPA-DPP backbone

Abstract

Solution-processed star-shaped triphenylamine (TPA) derivatives and dialkylated diketopyrrolopyrrole (DPP)-based small molecules have been widely studied because they both yield promising photon-to-electron conversion. However, the power conversion efficiency (PCE) of covalent star-shaped TPA-DPP derivatives is still very low. To design star-shaped TPA-DPP derivatives with better photovoltaic performance, we embedded a thiophene ring in between the TPA and DPP units, namely TPA-T-DPP, and reported the comparative studies of the optoelectronic and photovoltaic properties of TPA-DPP and TPA-T-DPP. Benefiting from the covalent thiophene bridges, compared to the TPA-DPP solid film, the TPA-T-DPP film showed enhanced light-harvesting ability, for instance, an improved absorptivity (Abs. = 1.72/100 nm vs. 1.23/100 nm), a broader absorption band (131 nm vs. 107 nm) and a narrower band gap (1.86 eV vs. 1.91 eV), from cyclic voltammetry. Studies on the photovoltaic properties revealed that the best TPA-T-DPP:PC71BM based device showed a dramatically enhanced PCE of 2.95%, increased by 2.14 times with respect to the efficiency of the best TPA-DPP based device (1.38%). The improvement of PCE also was observed in the small molecule:PC61BM based devices (1.81% vs. 1.13%). Test of the hole mobilites of the active layer provided further insight into the impact of the embedded thiophene units. The hole mobility of the TPA-T-DPP:PC71BM blended films was higher by about one order of magnitude (1.16 × 10⁻² cm² V⁻¹ s⁻¹) than that of the TPA-DPP:PC71BM blended films (3.85 × 10⁻³ cm² V⁻¹ s⁻¹). These results clearly indicated that embedding the thiophene ring enlarged the conjugation, thus enhanced the light-harvesting ability and hole mobility, while further significantly improving the device performance. Additionally, TPA-T-DPP was also used as the electron-acceptor material, and the best P3HT:TPA-T-DPP based device exhibited a very high open-circuit voltage (1.14 V), which was among the highest values reported for single-layered OSC devices.