Self‐Assembled Ionic Liquid for Highly Efficient Electron Transport Layer‐Free Perovskite Solar Cells

Abstract Electron transport layer (ETL)‐free perovskite solar cells (PSCs) are attractive because they have fewer layers and hence are lower in cost, but their inferior photovoltaic performance, as compared to ETL‐containing PSCs, greatly restricts their practical application. This study concerns the design and synthesis of a hydroxyethyl‐functionalized imidazolium iodide ionic liquid, the determination of its single crystal structure, and its self‐assembly on a conductive substrate for ETL‐free PSCs. The self‐assembly of the ionic liquid on the conductive substrate is found to lower the work function of the conductive substrate and enhance interfacial electron extraction while retarding interfacial charge recombination. As a consequence, the power conversion efficiency is improved remarkably from 9.01 % to 17.31 % upon self‐assembly of the ionic liquid on the conductive substrate. This finding provides a new way to assemble highly efficient ETL‐free PSCs.