Improving the Electron Transport Performance of TiO₂ Film by Regulating TiCl₄ Post‐Treatment for High‐Efficiency Carbon‐Based Perovskite Solar Cells

Abstract Titanium oxide (TiO 2 ) has been widely used as an electron transport layer (ETL) in perovskite solar cells (PSCs). Typically, TiCl 4 post‐treatment is indispensable for modifying the surfaces of TiO 2 ETL to improve the electron transport performance. However, it is challenging to produce the preferred anatase phase‐dominated TiO 2 by the TiCl 4 post‐treatment due to the higher thermodynamic stability of the rutile phase. In this work, a mild continuous pH control strategy for effectively regulating the hydrolysis process of TiCl 4 post‐treatment is proposed. As the weak organic base, urea has been demonstrated can maintain a moderate pH decrease during the hydrolysis process of TiCl 4 while keeping the hydrolysis process relatively mild due to the ultra‐weak alkalinity. The improved pH environment is beneficial for the formation of anatase TiO 2 . Consequently, a uniform anatase‐dominated TiO 2 surface layer is formed on the mesoporous TiO 2 , resulting in reduced defect density and superior band energy level. The interfacial charge recombination is effectively suppressed, and the charge extraction efficiency is improved simultaneously in the fabricated solar cells. The efficiency of the fabricated carbon electrode‐based PSCs (C‐PSCs) is improved from 16.63% to 18.08%, which is the highest for C‐PSCs based on wide‐bandgap perovskites.