Enhancing the power conversion of the perovskite solar cells via structural tuning of BTT(DPP)3-based low bandgap hole transporting material

Three solution-processed star-shaped small molecules (SMs) based on benzo[1,2-b:3,4-bꞌ:5,6-bꞌꞌ]trithiophene (BTT) and 1,4-diketo-3,6-dithienylpyrrolo[3,4-c]pyrrole (DPP) building blocks have been synthesized and used as hole transport material (HTM) in perovskite solar cells (PSCs). The physical, optical and electrical properties of the SMs are studied in detail. The fine tuning of molecular structures have made to achieve the power conversion efficiency (PCE) of PSCs up to 14.13% for BTT(DPP-Th)3-EH over the other two (BTT(DPP)3-C8 (8.65%) and BTT(DPP)3-EH (8.39%) SMs. The enhanced solar cell performance majorly attributed to the high hole mobility (1.26 × 10−3 cm2 V−1 s−1) through nanofibril structured morphology of BTT(DPP-Th)3-EH. The molecular orientation and crystalline properties of SMs in the solid-state are investigated and correlated with their solar cell properties, by employing grazing incidence X-ray diffraction (GIXRD) measurements. In addition, light intensity (I) dependent current density-voltage (J-V) studies are also carried out to understand the effect of second-order bimolecular recombination and extent of the trap-assisted recombination in the PSC devices.