Chemical Vapor Deposition of the Transparent p-Type Semiconductor Cuprous Iodide for Application in Flexible Thin-Film Photovoltaics

Abstract

Transparent p- and n-type semiconductors are necessary for a wide variety of applications including transparent electronics, photovoltaics (PV), flat panel displays, light emitting diodes, touchscreens, liquid-crystal displays, and smart windows. Compared with their n-type counterparts, transparent p-type semiconductors are rare and have comparatively low performance. Cuprous iodide is a relatively high-performing p-type semiconductor and, while its performance lags behind that of the leading transparent n-type semiconductors, development of deposition techniques yielding device-quality films of CuI may enable advances in a number of device applications. CuI is of particular interest as a hole transport material in thin film PV utilizing metal halide perovskite light absorbers, which have the potential to provide lightweight, inexpensive, and easily transportable flexible PV when coated on polymer substrates. This dissertation details chemical vapor deposition (CVD) techniques yielding thin films of CuI on substrates of interest for application in flexible thin film PV. Because CVD is a scalable deposition technique capable of depositing device-quality films over large areas, it is our hope that the advances reported here will assist in the commercial-scale deployment of next-generation flexible thin film PV. In Chapter 2, CuI thin films are deposited from CVD reaction between vinyltrimethylsilane(hexafluoroacetylacetonato)copper(I) [Cu(hfac)(vtms)] and anhydrous hydrogen iodide. This method yields pure, stoichiometric zincblende CuI films on silicon dioxide, silicon nitride, fluorine-doped tin oxide, and vitreous carbon substrates. An alternative iodine precursor, trimethylsilyliodide (TMSI), is introduced in Chapter 3. In contrast to the hydrogen halides, TMSI is not expected to react with certain substrates or damage vacuum equipment. CVD reaction between Cu(hfac)(vtms) and TMSI yields pure, stoichiometric zincblende CuI at temperatures at least as low as 50 degrees C, providing the possibility of deposition atop thermally sensitive substrates including flexible polymer films and metal halide perovskite light absorbers. This CVD method is applied to yield continuous CuI films on silicon dioxide, silicon nitride, quartz, indium tin oxide on glass or poly(ethylene terephthalate), fluorine-doped tin oxide, vitreous carbon, Kapton, low-density polyethylene, and poly(ethylene terephthalate). Finally, in Chapter 4, the CVD reaction between Cu(hfac)(vtms) and TMSI is applied to deposit CuI directly atop the metal halide perovskite absorber material methylammonium lead trisiodide (MAPbI3), as well as NaCl(100), KCl(100), and KBr(100) substrates. CVD atop perovskite absorber materials is challenging and typically results in some degradation of the absorber due to a combination of elevated deposition temperatures and detrimental reaction between the perovskite and one or more CVD precursors. Here we demonstrate dramatic reduction in perovskite degradation during CVD through the new chemistry offered by TMSI, as well as careful control and optimization of deposition parameters. These results provide convenient routes to device-quality CuI thin films on a wide variety of substrates including flexible polymers and perovskite absorbers, and show a promising advancement in the field of vapor deposited flexible thin film PV.