Low-cost carbazole and phenothiazine based trimer molecules as hole transporting materials for inverted perovskite solar cells

Phenothiazine (P) and Carbazole (C) are low-cost scaffolds widely used in the synthesis of Hole Transporting Materials (HTMs) for Perovskite Solar Cells (PSCs). So far, these compounds have been assembled together forming HTMs applied exclusively in direct cell architectures with the necessity of dopants to improve power conversion efficiencies (PCEs). In this work, two trimer molecules with opposite structure, namely CPC and PCP, have been designed, synthesized and implemented without any dopant in inverted PSCs. We assessed the impact of the molecular design on the final device performances, in view of the different intrinsic features. Both HTMs optoelectronic properties have been investigated along with a computational study by DFT. For optimizing the implementation in PSCs, thermal annealings and decrease of HTM concentration have been considered. PCP outperformed CPC, with an average PCE of 14.1 % against 10.4 %, achieving comparable performances to PTAA reference device (14.5 %). No hysteresis was observed for all devices and high FF reaching almost 80 % were obtained with PCP-based devices. The performances of the HTMs were correlated with the electronic behaviour observed by synchrotron-based soft X-ray photoelectron spectroscopy (PES). Secondary electron cut-off analysis highlighted a favourable work-function modification and the presence of high intermolecular interaction along with better energetic alignment for PCP, which contributed to its enhanced performances. Complementary characterizations by transient photocurrent and transient photovoltage confirmed the positive effect of decreasing the HTM concentration as observed in devices. Steady state and time resolved photoluminescence experiments corroborated the improved charge carrier dynamics and recombination features for PCP.