All-inorganic perovskite solar cells (PVSCs) have drawn widespread attention for its superior thermal stability. Carbon-based devices are promising to demonstrate excellent long-term operational stability due to the hydrophobicity of carbon materials and the abandon of organic hole-transporting materials (HTMs). However, the difficulty to control the crystallinity process and the poor morphology leads to serious non-radiative recombination, resulting in low V_OC and power conversion efficiency (PCE). In this article, the crystal formation process of all-inorganic perovskites is controlled with a facile composition engineering strategy. By bromide incorporation, high-quality perovskite films with large grain and fewer grain boundaries are achieved. As-prepared perovskite films demonstrate longer carrier lifetime, contributing to lower energy loss and better device performance. Fabricated carbon-based HTM-free PVSCs with CsPbI_2.33Br_0.67 perovskite realized champion PCE of 12.40%, superior to 8.80% of CsPbI₃-based devices, which is one of the highest efficiencies reported for the carbon-based all-inorganic PVSCs to date. The high V_OC of 1.01 V and FF of 70.98% indicate the significance of this composition engineering method. Moreover, fabricated carbon-based devices exhibit excellent stability, and unencapsulated device retains over 90% of its initial efficiency under continuous one sun illumination for 250 h in N₂ atmosphere and keeps ~84% of its original value after stored in ambient environment with RH 15–20% for 200 h. This work provides a facile way to fabricate high-performance and stable carbon-based all-inorganic PVSCs.