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

Copper iodide (CuI) is a wide-bandgap (colorless) p-type semiconductor with a high Seebeck coefficient. Although CuI 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 CuI thin films through a convenient spin-coating process. In limited solvents of acetonitrile and diethyl sulfide, CuI is dissolved by forming soluble CuI complexes; however, void-and-crack-free CuI thin films have never been prepared. In this study, we report that CuI-alkanolamine complexes are soluble in alcohols and the spin-coated complexes undergo thermal decomposition to a CuI thin film at moderately low temperatures until 150 °C. We discover that the CuI-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 CuI thin films with controlled thicknesses of >50 nm. The conductivity, carrier density, mobility, and Seebeck coefficient of the CuI 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.