Band engineering of a PTAA hole transporting layer in the n–i–p architecture of MAPbI3-based perovskite solar cells†

In this work, we report the synthesis of poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) with different chemical skeletal structures, resulting in varied highest occupied molecular orbital (HOMO) positions. During the synthesis of PTAA derivatives we have attempted to introduce the bulkier ethyl groups, and the benzene ring with the substituent in the triphenylamine moiety is twisted, so the ionization potential becomes larger, leading to deeper-lying HOMO levels. The effect of HOMO positions of different PTAA structures on the power conversion efficiency (PCE) of perovskite solar cells (PSCs) within the n–i–p architecture is studied. We show that changes in the HOMO level positions affect the PCE. The analysis of the influence of the HOMO level of PTAA derivatives on the PCE of PSCs is carried out by incorporating them as a hole transport material (HTM) in PSC fabrication. The enhanced photovoltaic parameters of PSCs with deeper HOMO levels are due to reduction in the band offset between the HTM and the perovskite light absorber as well as due to the improved hole extraction/collection resulting from the effective charge transfer at the HTM/perovskite interface. When the HOMO level of PTAA is deepened from −5.23 to −5.39 eV, the PCE increases. The solar cell devices having a PTAA derivative as the HTM with a HOMO level of −5.39 eV shows the best PCE of 14.6% compared to the devices with the shallower HOMO positions of HTMs, outperforming the other CH₃NH₃PbI₃ (MAPbI₃)-based PSCs with PTAA as the HTM.