Surface fluorination of ALD TiO₂ electron transport layer for efficient planar Perovskite solar cells

Perovskite solar cells (PSCs) are emerging among the photovoltaic (PV) technologies due to their high power conversion efficiency (PCE) in combination with potentially low cost manufacturing processing. In this contribution, the fabrication of efficient planar n-i-p PSCs by the modification of the electron transport layer (ETL) adopted as n-type contact is demonstrated. Specifically, a fluorine-based plasma treatment prior to perovskite deposition leads to surface fluorination of the TiO₂ ETL. The presence of fluorine on the TiO₂ surface drastically improves the adhesion between the ALD layer and the methylammonium lead iodide perovskite film, and leads to a more favourable energy band alignment, accompanied by a faster electron carrier extraction at the interface. As consequence of surface fluorination, we observe a significant reduction in the current density-voltage curve hysteresis with respect to the ALD based reference sample, as well as a remarkable improvement in power conversion efficiency from 4% up to a stable 14.8%.