Atmospheric Pressure Chemical Vapor Deposition of Fluorine-Doped Tin Oxide and Tin-Germanium Oxide: Photovoltaic and Energy Storage Applications

Abstract

This dissertation addresses renewable energy in two ways: energy storage technology that eases the integration of large-scale wind and solar power into the electric grid, and an earth- abundant solar cell layer intended to increase cell efficiency. Flow batteries employing bromine-hydrobromic acid positive electrolytes are promising energy storage options, but the destruction of graphite flow plates by bromine limits battery lifetimes. Accordingly, atmospheric pressure chemical vapor-deposited (APCVD) SnO2(F) protective coatings were developed to stabilize stainless steel flow plates against attack by bromine and hydrobromic acid. By varying the ratio of bromotrifluoromethane to tetramethyltin delivered during deposition, the cell resistance contribution of SnO2(F) was minimized. The considered SnO2(F) film was stable in heated electrolyte, with suitable resistance for flow batteries, and continuous films were achieved on sufficiently smooth substrates. Metal nitride films were also explored as a protective flow plate coating: the considered VN, WN, and TiN films had cell resistance contributions competitive with those of SnO2(F) and graphite, but insufficient chemical stability to bromine and/or hydrobromic acid. The chemical stability of other battery components was studied by extended exposure to heated bromine/hydrobromic acid electrolyte. Nafion proton exchange membranes showed a possible decrease in conductivity. The dry cell resistance and appearance of carbon paper electrodes and pyrolytically sealed graphite flow plates were stable. Finally, tin-germanium oxide alloys were evaluated as electron transport layers (ETLs) for solar cells. A (Sn,Ge)O2 APCVD method was developed using tetramethyltin, tetramethylgermanium, and oxygen at nominal deposition temperatures of 475-520 C. Up to 8 at. % germanium incorporation was achieved. Resistivity decreases with increasing germanium incorporation, due to increasing carrier concentration. Optical absorption spectra are consistent with crystalline-direct and amorphous-direct optical transitions. Optical and XRD data suggest crystalline films with a decreasing fraction of well-ordered material with increasing germanium incorporation. For the first time, the band structure of (Sn,Ge)O2 is reported. Modest tuning of the conduction band position (ca. 0.6 eV) was observed at low germanium concentrations. The high carrier concentration and likely presence of defects tailing into the bandgap may preclude the use of this deposition method for (Sn,Ge)O2 ETLs for all but the lowest germanium concentrations.