Low-cost asymmetric structured hole transport material for perovskite solar cells

Hole transport material (HTMs) is a key component for perovskite solar cells (PSCs) to achieve high photovoltaic performance and good long-term stability. In this study, we developed a series of efficient and cost-effective asymmetric small-molecule HTMs with 9-(4-methoxyphenyl)-9H-3,9 '-bicarbazole (BCz) as the core unit, termed BCz-DM and BCz-NP. The design of asymmetric molecular chemical structure for these type HTMs raises up the dipole moment, which is beneficial for enhancing the intermolecular push-pull effect and rendering the HTM higher hole mobility. Moreover, combining with the It-conjugation expansion strategy, asymmetric HTM BCz-NP demonstrates compatible energy levels, along with superior hole mobility and conductivity. Consequently, the PSC device employing BCz-NP as HTM yields an impressive efficiency of 23.58 %, while the PCE of the reference device based on Spiro-OMeTAD is merely 22.77 %. Furthermore, BCz-NP based devices exhibit excellent stability, remaining 86 % of its initial PCEs after 1000 h aging test. These findings provided a promising approach for the improvement of photovoltaic performance for PSCs from the perspective of HTM design.