Perovskite solar cells have led the new surge of solar energy research. However, their instability is a pressing issue mostly attributed to the perovskite interface with charge-selective transport layers. In this work, diethylammonium iodide (DEAI) surface treatment is used to mitigate interfacial non-radiative recombination losses by forming a mixed phase of layered perovskite on the surface. This results in enhanced device performance with the power conversion efficiency of 23.3% and improved operational stability under thermal stress. Moreover, the DEAI treatment facilitates interfacial hole transfer, enabling a carbon-based hole transport layer-free perovskite solar cell with a power conversion efficiency of 15.6%.|Graetzel and co-workers report a perovskite surface treatment using diethylammonium iodide (DEAI) that significantly reduces non-radiative recombination losses at the interface via the formation of a mixed phase of layered perovskite on the surface, resulting in a remarkable improvement in performance, with a power conversion efficiency (PCE) of 23.3% and improved stability. Additionally, the DEAI treatment promotes interfacial hole transfer enabling efficient carbon-based HTM-free PSCs, achieving a PCE of15.6%.image