Dopant-Free Spiro-OMe Imidazole-Based Hole-Transporting Material for Stable and Low-Cost Organic-Inorganic Perovskite Solar Cell.

The engineering of charge transport materials, with electronic characteristics that result in effective charge extraction and transport dynamics, is pivotal for the realization of efficient perovskite solar cells (PSCs). Herein, we elucidate the critical role of terminal substituent methoxy groups (-OCH) on the bandgap tuning of the spiro-like hole transport materials (HTMs) to realize performant and cost-effective PSCs. By considering spiro-OMeTAD as the benchmark HTM, we kept the backbone of spiro while replacing diphenylamine with phenanthrenimidazole. This approach significantly decreases the cost of spiro-OMeTAD by reducing the cost of the ancillary group from 0.051 to 0.012 $/g. By increasing the number of methoxy groups on the ancillary ligand from four to eight, the power conversion efficiency (PCE) of the corresponding PSCs containing dopants passed from 17.10% to 18.70%, approaching the value achieved using spiro-OMeTAD containing dopants (PCE = 19.26%). Remarkably, the devices based on dopant-free spiro-OMeTAD have shown a significant loss of PCE, which decreased from 12.9% to 10.1% after 300 h (to 8.2% after 600 h) of light soaking at an open circuit voltage. On the contrary, the cells based on the designed dopant-free HTM demonstrated optimal PCE retention, experiencing a minor drop from 14.4% to 14.1% and 13.2% after 300 and 600 h, respectively, of light soaking at open-circuit voltage.