Band alignment and carrier recombination roles on the open circuit voltage of ETL‐passivated perovskite photovoltaics

Summary Titanium dioxide (TiO2) is the most often utilised electron transport layer (ETL) in perovskite solar cells (PSCs). Owing to its inherent limitations such as low electron mobility, inadmissible charge accumulation and recombination, many efforts have been devoted to the surface alteration of TiO2 to enhance the performance of PSCs. However, the key mechanism of this improvement is still unclear. Here, we study the mesoporous TiO2 (ms‐TiO2) ETL passivated by Nb2O5 and Ta2O5. The ETL modified PSCs show increased open‐circuit voltage (VOC). Power conversion efficiency (PCE) of 18.70%, 20.71%, and 20.96% were obtained for the devices with ms‐TiO2, ms‐TiO2/Nb2O5 and ms‐TiO2/Ta2O5 ETL. Furthermore, the hysteresis index (HI) of pristine ms‐TiO2 (HI = 4.91%) based devices are much higher than the Nb2O5 (HI = 2.75%) and Ta2O5 (HI = 1.19%) based devices. Our characterization results reveal that the carrier recombination is the main factor in increasing the VOC rather than energy band alignment once we passivate the surface of the ETL. This work is helpful in providing new insights into the surface modification of the mesoscopic ETL. The surface of mesoscopic‐TiO2 is passivated by Nb2O5 and Ta2O5 which leads to increased open‐circuit voltage (VOC). The results revealed that recombination plays dominant role over energy band‐alignment in increasing VOC when the surface is passivated. Thus, this study provides new insights in deciphering the role of recombination and band alignment in surface modification of TiO2.