Bulk Passivation of Lead Halide Perovskites: The Key to High‐Performance Indoor Photovoltaics at Very Low‐Light Intensities

Lead halide perovskites (LHPs) are outstanding light harvesters for indoor photovoltaics (IPVs), enabling the highest power conversion efficiency (PCE) among emerging IPV technologies. However, their performance under very low‐light intensity (≤200 lux) is rarely assessed, despite this being a common condition in indoor environments. At low‐light intensities, reducing bulk traps in the absorber layer is paramount. In this study, we investigate the impact of bulk passivation on the performance of IPVs under various illumination intensities. We select sodium thioglycolate (STG) as a bifunctional passivator added to a triple‐cation LHP precursor solution. Our results indicate that incorporating STG leads to devices with improved performance under low‐intensity white LED illumination (≤200 lux), while efficiency improvements are negligible at higher intensity of 1000 lux. Notably, under low‐light intensities (100 and 200 lux), the PCE increase is driven by enhancements in fill factor (FF) and open‐circuit voltage ( V OC ), indicating effective bulk passivation by STG. Detailed analyses, including transient photovoltage and photocurrent measurements, light intensity dependence of FF and V OC , and impedance spectroscopy, confirm reduced trap‐assisted recombination and accelerated charge extraction in the STG‐passivated IPVs. This work demonstrates the crucial role of bulk trap passivation for efficient low‐intensity indoor light harvesting with LHPs.