Natrium Doping Pushes the Efficiency of Carbon-Based CsPbI₃ Perovskite Solar Cells to 10.7

The CsPbI₃ inorganic perovskite is a potential candidate for fabricating long-term operational photovoltaic devices owing to its intrinsic superb thermal stability. However, the carbon-based CsPbI₃ perovskite solar cells (C-PSCs) without hole transport material (HTM) are currently disadvantaged by their relatively low power conversion efficiency resulting from the poor grain quality and mismatched energy band levels of the as-made CsPbI₃ films. Herein we demonstrate that by doping Na into the CsPbI₃ lattice, the grain quality is significantly improved with low defect density, and also, the energy band levels are better matched to the contact electrodes, affording a higher built-in potential. Consequently, the V_oc of the C-PSCs is drastically increased from 0.77 to 0.92 V, and the efficiency from 8.6% to 10.7%, a record value for the CsPbI₃ PSCs without HTM. Moreover, the non-encapsulated device showed virtually no performance degradation after 70 days of storage in air atmosphere.