Vapor Deposition of Transparent, p-Type Cuprous Iodide Via a Two-Step Conversion Process

Photovoltaic devices require p-type layers with high optical transparency and electrical conductivity. One promising material is cuprous iodide, CuI, thin films of which have hole mobilities in the 1-12 cm(2)/V.s range. However, despite adequate electrical properties in many CuI thin films, most deposition processes afford only rough films that have poor continuity and low optical transparency, hampering the final device performance. We now report an all-vapor method, amenable to large-scale processing, for preparation of CuI thin films with near record optical and electrical properties. In this process, thin films of Cu(2-x)S (x = 0-0.1) or Cu2O grown by chemical vapor deposition from bis(N,N'-di-sec-butylacetamidinato)dicopper(I) in combination with hydrogen sulfide or water, respectively, were converted to gamma-CuI upon exposure to dilute hydrogen iodide vapor. The rate of this iodide-for-chalcogenide anion exchange reaction is controlled by the concentration and delivery rate of HI. The nucleation rate of the nascent CuI may be modified by dosing with iodine vapor (for Cu(2-x)S) or with vapors of thiodiglycol or ethylene glycol (for Cu2O). By balancing the rates of nucleation and conversion, we are able to prepare smooth, continuous thin films possessing optical and electrical properties approaching those of the best native p-type CuI films. We believe that the underlying chemical and materials science reasoning leading to these high-quality films will prove instructive in other thin-film systems. Furthermore, based on the measured band positions and carrier mobilities we anticipate high utility for these smooth CuI films as hole-transport layers in Earth-abundant, inexpensive thin-film photovoltaics.