CuInS2 Films Deposited by Aerosol-Assisted Chemical Vapor Deposition Using Ternary Single-Source Precursors

Photovoltaics are an important power source for both off-grid terrestrial and extraterrestrial use. Thin film polycrystalline materials have been studied extensively for solar cell applications partially because their polycrystalline nature allows their formation on many different types of substrates including glass, metal foil, and lightweight flexible polymer substrates. For example, monolithically integrated Cu(In,Ga)Se2 (CIGS) modules were recently realized on both metal foils and polymers with cell efficiencies up to 13.8 percent. Using polymer substrates will particularly benefit space missions by reducing the power requirement for the payload of the spacecraft. However, existing polymer substrates can only be processed under limited conditions because of their limited thermal durability. For example, Kapton, a well-known space-qualified polymer material, can be processed at temperature up to 400 °C. Previously we have developed new single-source precursors (SSPs) for chalcopyrite thin film formation. One important prerequisite for SSPs is a lower decomposition temperature Polycrystalline CuInS2 films were deposited by aerosol-assisted chemical vapor deposition using both solid and liquid ternary single-source precursors (SSPs) which were prepared in-house. Films with either (112) or (204/220) preferred orientation, had a chalcopyrite structure, and (112)-oriented films contained more copper than (204/220)-oriented films. The preferred orientation of the film is likely related to the decomposition and reaction kinetics associated with the molecular structure of the precursors at the substrate. Interestingly, the (204/220)-oriented films were always In-rich and were accompanied by a secondary phase. From the results of post-growth annealing, etching experiments, and Raman spectroscopic data, the secondary phase was identified as an In-rich compound. On the contrary, (112)-oriented films were always obtained with a minimal amount of the secondary phase, and had a maximum grain size of about 0.5 μm. Electrical and optical properties of all the films grown were characterized. They all showed p-type conduction with an electrical resistivity between 0.1 and 30 Ω·cm, and an optical band gap of approximately 1.46 eV ± 0.02, as deposited. The material properties of deposited films revealed this methodology of using SSPs for fabricating chalcopyrite-based solar cells to be highly promising.