Two-Step Physical Deposition of a Compact CuI Hole-Transport Layer and the Formation of an Interfacial Species in Perovskite Solar Cells

A simple and practical approach is introduced for the deposition of CuI as an inexpensive inorganic hole‐transport material (HTM) for the fabrication of low cost perovskite solar cells (PSCs) by gas–solid phase transformation of Cu to CuI. The method provides a uniform and well‐controlled CuI layer with large grains and good compactness that prevents the direct connection between the contact electrodes. Solar cells prepared with CuI as the HTM with Au electrodes displays an exceptionally high short‐circuit current density of 32 mA cm−2, owing to an interfacial species formed between the perovskite and the Cu resulting in a long wavelength contribution to the incident photon‐to‐electron conversion efficiency (IPCE), and an overall power conversion efficiency (PCE) of 7.4 %. The growth of crystalline and uniform CuI on a low roughness perovskite layer leads to remarkably high charge extraction in the cells, which originates from the high hole mobility of CuI in addition to a large number of contact points between CuI and the perovskite layer. In addition, the solvent‐free method has no damaging side effect on the perovskite layer, which makes it an appropriate method for large scale applications of CuI in perovskite solar cells. Interfacial synergy: A simple and practical approach is used for the deposition of a CuI thin film by gas–solid phase transformation of Cu to CuI. The deposition of the CuI layer by this method shows an exceptionally high short‐circuit current density of 32.72 mA cm−2, owing to the quality of the deposited CuI layer and to the formation of a new species that contributes to the long‐wavelength incident photo‐to‐electron conversion efficiency.