Tris(pentafluorophenyl)borane–water complex doped Spiro-TTB for high-efficiency and stable perovskite solar cells

Abstract

Bis(trifluoromethane)sulfonimide lithium salt (Li-TFSI) and 4-tert-butylpyridine (t-BP) that could work as dopants to modify the hole transport layer (HTL) have been widely used for constructing high-efficiency perovskite solar cells (PSCs). However, these dopants usually suffer from the drawback of environmental sensitivity, significantly degrading photovoltaic performance. Here, to solve this issue, we used tris(pentafluorophenyl)borane (BCF) p-type dopant to prepare a high-quality HTL of 2,2′,7,7′-tetrakis-(N,N,-di-p-methylphenylamino)-9,9′-spirobifluorene (Spiro-TTB). The doping process between BCF and Spiro-TTB through electron transfer was studied on the basis of Lewis acid and Brønsted acid doping. Interestingly, the water molecule promoted Spiro-TTB protonation for high-efficiency doping, then Brønsted-acid-doped Spiro-TTB yielded a high hole mobility (1.22 × 10⁻² cm² V⁻¹ s⁻¹). The corresponding PSC achieved a champion power conversion efficiency (PCE) of 20.86% based on a 1.65 eV bandgap perovskite, while a four-terminal perovskite/silicon tandem solar cell yielded a superior PCE of 27.82%. The modified devices also presented much higher long-term stability under various conditions, maintaining 92% of their original PCEs after storage for over 1500 h in 25 ± 5% RH ambient air, and 93% of their original PCE after 350 h at 85 °C under an N₂ atmosphere without encapsulation. Therefore, these results demonstrated that Brønsted acid doping is a promising and low-cost approach to construct efficient doped HTLs for high-performance PSCs.