Simple CsI doping outperforms complex organic additives in carbon-based perovskite solar cells

Carbon-based hole-transport-layer (HTL)-free perovskite solar cells (C-PSCs) have emerged as promising candidates for commercialization, owing to their low cost, high stability, and simple device architecture. Nevertheless, the performance of C-PSCs is still limited by inconsistent perovskite film quality and high defect density. While cesium iodide (CsI) doping has been explored in perovskite systems, its application in HTL-free C-PSCs remains relatively underinvestigated and often yields only limited performance improvements, particularly in single-cation systems or with the aid of additional interlayers or additives. In this work, we report a facile yet effective strategy of incorporating CsI into MA/FA dual-cation perovskite films via a one-step spin-coating process-without any other interlayer or organic passivation additives. The incorporation of Cs⁺ significantly enhances crystallization quality and induces lattice contraction which synergistically suppresses the formation of intrinsic defects and non-perovskite phases, mitigates ion migration, and improves phase stability. As a result, the champion CsI-doped C-PSC achieves a remarkable power conversion efficiency (PCE) of 17.31%, substantially outperforming the control device (13.25%) fabricated under identical conditions. Furthermore, the CsI-incorporated devices demonstrate exceptional long-term operational stability, retaining 73% of their initial PCE after 30 days of storage in ambient air. This work highlights the potential of inorganic cation engineering in realizing high-performance, low-cost, and durable C-PSCs through a simple and scalable process.