Coplanar phenanthro [9,10-d] imidazole based hole-transporting material enabling over 19%/21% efficiency in inverted/regular perovskite solar cells

Hole-transporting materials (HTMs) play a crucial role in achieving highly efficient and stable perovskite solar cells (PSCs). Currently, the vast majority of HTMs reported are applicable only for either n-i-p structured regular cells (r-PSCs) or p-i-n structured inverted devices (i-PSCs) (e.g. Spiro-OMeTAD shows an impressive efficiency in r-PSCs, while a poor performance was observed in i-PSCs). The restricted application of HTMs greatly causes waste of materials and increases the research cost of photovoltaic devices, which is harmful to industrial large-scale application and sustainable development. Here, we provide an effective and efficient approach to improve the performance of both r-PSCs and i-PSCs using HTM (PI-2) featuring with a phenanthro [9,10-d]imidazole core via rational pi-extension and lower symmetry. The other HTM, DI-1, using non-hybrid imidazole as the central core, was designed as control. Comparing with DI-1, the coplanar pi-extended phenanthro[9,10-d]imidazole based PI-2 endows enriched intermolecular interactions as well as enhanced pi-pi stacking, thus achieving a higher hole mobility. It also exhibits matched energy levels and high thermal stability for application in PSCs. Moreover, a decreased symmetrical conformation of PI-2 contributes to an amorphous film with superior morphological uniformity. Consequently, the p-i-n structured devices with PI-2 realize a champion power conversion efficiency (PCE) of 19.11% and the n-i-p structured devices based on PI-2 achieve an encouraging champion PCE of 21.65%, representing one of the best results among universal HTMs in both i-PSCs and r-PSCs.