Carbon-based perovskite solar cells (C-PSCs) are widely accepted as stable, cost-effective photovoltaics. However, C-PSCs have been suffering from relatively low power conversion efficiencies (PCEs) due to severe electrode-related energy loss. Herein, we report the application of a single-atom material (SAM) as the back electrode in C-PSCs. Our Ti-1-rGO consists of single titanium (Ti) adatoms anchored on reduced graphene oxide (rGO) in a well-defined Ti1O4-OH configuration capable of tuning the electronic properties of rGO. The downshift of the Fermi level notably minimizes the series resistance of the carbon-based electrode. By combining with an advanced modular cell architecture, a steady-state PCE of up to 20.6% for C-PSCs is finally achieved. Furthermore, the devices without encapsulation retain 98% and 95% of their initial values for 1,300 h under 1 sun of illumination at 25 degrees C and 60 degrees C, respectively. Carbon materials are promising for perovskite solar cells but suffer from poor interfacial energy level alignment. Now, Zhang et al. show that Ti atomically dispersed in reduced graphene reduces energy losses improving device performance.