Three-type Precursors for Low-Temperature Solution-Processed Void-and-Crack-Free CuI Films: Comparison of Electrical Conductivities and Optical Transparency

Abstract Copper(I) iodide is a wide-bandgap (colorless) p-type semiconductor with a high Seebeck coefficient. Although copper(I) iodide is promising for fabricating transparent thermoelectric devices and hole-transfer layers of solar cells, the insolubility in common solvents due to 3-dimensional coordination networks has been a drawback to constructing low-temperature solution-processed thin films. Moreover, it is challenging to fabricate void-and-crack-free copper(I) iodide thin films through a convenient spin-coating process. In limited solvents of acetonitrile and diethyl sulfide, copper(I) iodide is dissolved by forming soluble copper(I) iodide complexes; however, void-and-crack-free copper(I) iodide thin films have never been prepared. In this study, we report that copper(I) iodide–alkanolamine complexes are soluble in alcohols and the spin-coated complexes undergo thermal decomposition to a copper(I) iodide thin film at moderately low temperatures until 150 °C. We discover that the copper(I) iodide–alkanolamines show different properties such as solubility and melting/decomposition temperatures depending on their structures. Specifically, by using 1-amino-2-propanol, we obtain void-and-crack-free and transparent copper(I) iodide thin films with controlled thicknesses of >50 nm. The conductivity, carrier density, mobility, and Seebeck coefficient of the copper(I) iodide thin film are 9.35 S·cm−1, 6.38 × 1019 cm−3, 0.96 cm2·V−1·S−1, and 192 µV·K−1, respectively.