Neglected acidity pitfall: boric acid-anchoring hole-selective contact for perovskite solar cells

The spontaneous formation of self-assembly monolayer (SAM) on various substrates represents an effective strategy for interfacial engineering of optoelectronic devices. Hole-selective SAM is becoming popular among high-performance inverted perovskite solar cells (PSCs), but the presence of strong acidic anchors (such as -PO3H2) in state-of-the-art SAM is detrimental to device stability. Herein, we report for the first time that acidity-weakened boric acid can function as an alternative anchor to construct efficient SAM-based hole-selective contact (HSC) for PSCs. Theoretical calculations reveal that boric acid spontaneously chemisorbs onto indium tin oxide (ITO) surface with oxygen vacancies facilitating the adsorption progress. Spectroscopy and electrical measurements indicate that boric acid anchor significantly mitigates ITO corrosion. The excess boric acid containing molecules improves perovskite deposition and results in a coherent and well-passivated bottom interface, which boosts the fill factor (FF) performance for a variety of perovskite compositions. The optimal boric acid-anchoring HSC (MTPA-BA) can achieve power conversion efficiency close to 23% with a high FF of 85.2%. More importantly, the devices show improved stability: 90% of their initial efficiency is retained after 2400 h of storage (ISOS-D-1) or 400 h of operation (ISOS-L-1), which are 5-fold higher than those of phosphonic acid SAM-based devices. Acidity-weakened boric acid SAMs, which are friendly to ITO, exhibits well the great potential to improve the stability of the interface as well as the device.