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

Abstract

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 Cu 2 O 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 γ-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 Cu 2 O). 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.

Introduction of TiO2 in CuI for Its Improved Performance as a p-Type Transparent Conductor

“…[13][14][15][16] Furthermore, CuI can be synthesized and heavily doped at low temperatures (<100 o C) to achieve a hole conductivity of σ > 280 S/cm while maintaining a transparency of more than 70%. [14][15]17 Owing to aforementioned qualities, CuI has been used to achieve high solar cell efficiencies, [18][19][20][21][22] high rectification ratio diodes (rectification ratios larger than 10 9 ), [23][24] photodetectors, 25 piezoelectric enhancement, 26 flexible transparent thin film transistors (TFTs), 27 and light emitting diodes [28][29] . In addition, CuI has most recently been used as a transparent thermoelectric material to achieve a large thermoelectric figure of merit of ZT=0.21 at 300 C, which is three orders of magnitude higher compared with state-ofthe-art p-type transparent materials.…”

Versatile, Rapid, and Plasma‐Assisted Synthesis of Cuprous Halide Composites at Room Temperature and Pressure

“…[8] In addition to the compositional variation, disorderrelated effects contribute to the changes in the electronic and optical structure in these solid solutions. [9] With these electronic applications in mind, the synthesis of CuX films has become a hot topic in recent years, with a focus on chemical vapor deposition [10][11][12] and vacuum deposition techniques such as molecular beam epitaxy, [13,14] reactive sputtering, [15] and thermal evaporation. [8,16,17] At the same time, the cuprous halides are promising candidates for other applications, such as the use of CuBr in sensing, [18][19][20] or the use of CuCl as cathode in Mg primary batteries.…”

Atomic Structure and Work Function Modulations in Two-Dimensional Ultrathin CuI Films on Cu(111) from First-Principles Calculations

“…The black circles and red squares stand for ΔΦ of on-surface adsorption models (I/Cu(111)) and ultrathin CuI layers on Cu(111) substrate, respectively. The measured IP of the CuI{111} surface with respect to the experimental value of Φ of Cu(111), i.e., IP CuI – Φ Cu(111) (5.4 ± 0.2 eV), is denoted by a blue triangle symbol with the error bar. The solid line connecting the calculated values is a guide to the reader’s eye.…”

“…However, the ΔΦ values of H2L A and H3L A are almost the same, implying the saturation of ΔΦ. In Figure 5, the experimentally measured ionization potential (IP) using X-ray photoelectron spectroscopy (XPS) of the preferentially ⟨111⟩oriented CuI surface (5.4 ± 0.2 eV) 53 is denoted with respect to the experimental value of Φ of Cu(111), 54 i.e., IP CuI − Φ Cu(111) . The ΔΦ values of H2L A and H3L A are located within the error bar of this quantity, indicating that the internal electrostatic potential of H2L A and H3L A is already close to that of γ-CuI(1̅ 1̅ 1̅ ).…”