Solution ALD Processing of Halide Perovskite Thin Films Yields Superior Functional Quality and Stability Than Classical Processing

Abstract

The range of materials accessible by classical ALD is limited by the volatility requirement applied to the precursors. However, solution ALD (sALD) transfers the self-limiting surface reaction principle of ALD into the solution processing realm and allows for the same level of control in the deposition of ionic or molecular solids, such as polymers, metal-organic frameworks, and halide perovskites, which cannot be delivered from the gas phase.We have developed an sALD procedure for the direct deposition of the prototypical halide perovskite compound, methylammonium triiodoplumbate(IV), (H3CNH3)(PbI3), also referred to as methylammonium lead iodide or MAPI. The process saturates upon dosage variation to a self-limiting growth rate. The material is obtained in crystalline and highly pure, stoichiometric form at room temperature. All analytical techniques confer to indicate the absence of any lead iodide trace. The MAPI layers generated by sALD can be integrated into planar heterojunction stacks and yield functional solar cells.The direct comparison of films of several thickness values prepared in pairs by sALD and by a state-of-the-art spin-coating method shows that the sALD films always overperform their spin-coated counterparts both in terms of charge carrier lifetimes and in terms of stability towards decomposition under thermal, radiative or vacuum stress.Thus, experimental atomic-level control of solution-processed semiconductors is accessible for the first time. It can now be applied to the accurate investigation of halide perovskite photophysics and photochemistry fundamentals as they pertain to transport and interface phenomena.