• Sub-micrometer-resolved photocurrent mapping in operational perovskite solar cells reveals enhanced photocurrent at grain boundaries compared to grain interiors.
• Local pump-probe femtosecond transient absorption and Kelvin probe force microscopy measurements corroborate the presence of a built-in electric field near grain boundaries that promotes electron–hole separation and subsequent charge collection.
• Grain boundaries in high-efficiency perovskite solar cells function as carrier transport channels, contrary to the traditional view that they are detrimental.
• This study provides valuable insights for the rational design of high-efficiency perovskite solar cells by highlighting the beneficial role of grain boundaries.