Hf Doping for Defect and Carrier Management in Magnetron-Sputtered Tin Oxide Electron Transport Layers for Perovskite Solar Cells

The performance of perovskite solar cells (PSCs) with magnetron-sputtered tin oxide (SnO_x) electron transport layers (ETLs) is strongly influenced by the optical and electrical characteristics of the SnO_x. However, magnetron-sputtered SnO_x typically exhibits oxygen-vacancy (V_O)-related point defects. This leads to significant interface charge recombination, which restricts both the open-circuit voltage (V_OC) and fill factor (FF) of PSCs using SnO_x ETLs. In this study, a Hf-doping strategy is proposed to enhance the transmittance of SnO_x ETLs, reduce V_O defects, and modulate the carrier density. The introduction of Hf dopants into SnO_x successfully minimized V_O-defect formation, as confirmed by Hall-effect measurements, X-ray absorption spectroscopy, and X-ray photoelectron spectroscopy, leading to a reduced carrier density in SnO_x. Density functional theory simulations corroborated these experimental findings, revealing the mechanism behind V_O suppression. PSCs incorporating HTO ETLs demonstrated marked improvements in key performance parameters, including short-circuit current density, V_OC, and FF. Optimized HTO-based PSCs achieved an average power-conversion efficiency (PCE) of 18.23%, exhibiting a 14.2% increase compared with undoped SnO_x-based devices. Additionally, the best-performing PSCs utilizing HTO ETLs achieved an optimal PCE of 21.2% under reverse scan and 19.9% under forward scan.