Deleterious Effects of Halides and Solvents used in Electronic Device Fabrication on the Integrity of Copper Iodide Thin‐Films**

Copper iodide (CuI) is a promising material for use as hole-transport layers in electronic devices due to their solution processability and efficient hole conductivity. CuI has a rich chemistry with halide salts and solvents to which it may be exposed during device fabrication. Thus, care must be taken during device fabrication when CuI is used. We present a study using CuI as a hole transport layer in a p-i-n perovskite photovoltaic architecture. We studied how each of the components present in the perovskite precursor solution impacts the integrity of CuI films using power x-ray diffraction, UV-vis spectroscopy and impedance spectroscopy. Based on these studies, we show that DMSO, mixtures of γ-butyrolactone:DMSO (v/v 7 : 3) and DMF:DMSO (v/v 8 : 2), and iodide ions can dissolve the CuI layer. We also how that by coating a layer of copper(II) acetate and utilizing the known Cu(II)/Cu(I) redox chemistry with iodide ions, we can preserve CuI in the presence of halide salts and solvents.