Deleterious Effects of Halides and Solvents on the Integrity of Copper Iodide Hole Extraction Layers in Hybrid Perovskite Photovoltaics

Copper iodide (CuI) is a promising material for use as a hole transport layer in perovskite solar cells due to its optical transparency, low-cost fabrication, and efficient electronic (hole) conductivity. Various reports of perovskite solar cells that utilize CuI have shown impressive solar cell performance and improved device stability. Despite these observations, we found no clear experimental evidence that the CuI hole transport layer is retained in perovskite p-i-n solar cells after device fabrication. Using powder X-ray diffraction (PXRD), UV-vis spectroscopy, and impedance spectroscopy, we studied how each of the components present in the precursor solution for fabricating the perovskite active layer impacts the integrity of CuI films. Based on these data, we establish the deleterious effects of halide ions and solvents such as dimethyl sulfoxide (DMSO). We also show that we can fabricate stable CuI material in situ during perovskite deposition by taking advantage of a known redox chemistry of Cu(II)/Cu(I) and halides.