Contact Engineering and In Situ Formation of 2D Perovskite Via Solid‐Phase Growth for Efficient Hole‐Transport‐Layer‐Free Perovskite Solar Cells

Carbon‐based perovskite solar cells (C‐PSCs) without hole transport layer have piqued the interest of researchers due to their low‐cost fabrication processes and exceptional stability. However, the efficiency of C‐PSCs lags far behind that of state‐of‐the‐art metal‐based devices, owing to an energy‐level mismatch and poor interfacial contact at the interface between carbon electrode and perovskite layer. Herein, a simple method for performing a solid‐phase reaction at the perovskite/carbon interface using a hot‐pressing process is presented. Unlike the solution processes commonly used in PSCs to obtain 2D halide perovskite layers, butylammonium iodide (BAI) is introduced into carbon electrodes, which are then hot pressed onto 3D perovskite in this work. During hot pressing, BAI in the carbon electrode reacts with the 3D perovskite layer to form a 2D perovskite layer at the perovskite/carbon interface through a solid‐phase reaction, resulting in a contiguous contact perovskite/carbon interface and better energy‐level alignment. Charge recombination at the perovskite/carbon interface is greatly reduced thanks to the seamless contact and the in situ formed 2D perovskite layer as passivation and electron blocking layer. This strategy results in a high power conversion efficiency of up to 16.86% and significantly improved the device stability for C‐PSCs.