Dopant-Free Spiro-OMe<sub>2</sub> Imidazole-Based\nHole-Transporting Material for Stable and Low-Cost Organic–Inorganic\nPerovskite Solar Cell

The engineering of charge transport materials, with electronic\ncharacteristics that result in effective charge extraction and transport\ndynamics, is pivotal for the realization of efficient perovskite solar\ncells (PSCs). Herein, we elucidate the critical role of terminal substituent\nmethoxy groups (−OCH<sub>3</sub>) on the bandgap tuning of\nthe spiro-like hole transport materials (HTMs) to realize performant\nand cost-effective PSCs. By considering spiro-OMeTAD as the benchmark\nHTM, we kept the backbone of spiro while replacing diphenylamine with\nphenanthrenimidazole. This approach significantly decreases the cost\nof spiro-OMeTAD by reducing the cost of the ancillary group from 0.051\nto 0.012 $/g. By increasing the number of methoxy groups on the ancillary\nligand from four to eight, the power conversion efficiency (PCE) of\nthe corresponding PSCs containing dopants passed from 17.10% to 18.70%,\napproaching the value achieved using spiro-OMeTAD containing dopants\n(PCE = 19.26%). Remarkably, the devices based on dopant-free spiro-OMeTAD\nhave shown a significant loss of PCE, which decreased from 12.9% to\n10.1% after 300 h (to 8.2% after 600 h) of light soaking at an open\ncircuit voltage. On the contrary, the cells based on the designed\ndopant-free HTM demonstrated optimal PCE retention, experiencing a\nminor drop from 14.4% to 14.1% and 13.2% after 300 and 600 h, respectively,\nof light soaking at open-circuit voltage.