Triphenylimidazole substituted D-π-D porphyrin based dopant-free hole transport materials for perovskite solar cells

Hole-transport materials (HTMs) contribute an important function in perovskite solar cells (PSCs) for achieving favourable photovoltaic performance with durability. In this work, we designed and synthesized a series of porphyrin based hydrophobic HTMs based on donor-π-donor (D-π-D) tactic in which triphenylimidazole as donor and either free-base porphyrin (H2LD) or metalloporphyrin (Cu, CuLD or Zn, ZnLD) as π-spacer for PSCs. All three HTMs were characterized by a variety of spectroscopic and electrochemical methods. Optical and electrochemical properties propose that the highest occupied molecular orbital (HOMO) energy levels were healthily aligned with the valence band of MAPbI₃ perovskite. Among these HTMs, CuLD exhibited higher hole mobility of 7.39 × 10^-5 cm²V^-1s^-1, when compared to ZnLD 5.77 × 10^-5 cm²V^-1s^-1, and H2LD 2.7 × 10^-5 cm²V^-1s^-1. We fabricated PSCs with these HTMs and among them the CuLD based PSCs showed highest power conversion efficiency of 12.44%. The better PCE for the PSCs fabricated with CuLD could be attributed to the higher conductivity and better hole mobility which could be correlated to the better film formation.