UC Berkeley

Solution deposition is a scalable method to fabricate transparent conducting and semiconducting oxide films that could enable low-cost large-area optoelectronic devices, including solar cells and electrochromic windows. However, high temperatures (>500 °C) are typically required to remove excess counterions and ligands from solution-deposited films. We report the synthesis of reactive F-modified tin(II) hydroxide nitrate nanoscale cluster precursors from the controlled dissolution of SnO and SnF2 in minimal nitric acid (∼1.6 mol HNO3 per mol Sn). After spin-casting to form films, heating at temperatures < 100 °C consumes the nitrate counterions likely by oxidation of the tin(II) precursor to form amorphous F:SnO2. The optical, electrical, structural, and morphological properties of F-doped and undoped SnO2 thin films are reported as a function of annealing temperature on both glass and polyimide (Kapton) substrates. Electron microscopy and X-ray reflectivity results show that the films are uniform and crack-free with <10% thickness reduction after annealing at temperatures up to 450 °C. X-ray diffraction shows the formation of crystalline SnO2 at >300 °C. Quartz-crystal microbalance, X-ray photoelectron spectroscopy, and infrared spectroscopy show near-complete removal of nitrate counterions and hydroxide/water by 200 °C, leading to an approximate 30% mass loss from the as-deposited film. Secondary-ion mass spectrometry shows that the F concentration in the annealed films scales with the concentration in the precursor solution. The F-doped films are conductive when annealed at ≥250 °C in H2/N2 gas and at ≥350 °C in air. The lowest electrical resistivity (ρ) of 1.5 × 10–4 Ω·m was obtained from 10 atom % F-doped SnO2 films annealed at 600 °C in air. These films had a Hall mobility of 4.2 cm2 V–1 s–1 and a carrier concentration of 9.5 × 1019 cm–3. Films deposited directly on polyimide sheets were crack-free after annealing at temperatures below 350 °C. The films exhibited ρ ≈ 10–2 Ω·m after H2/N2 annealing and were stable to more than 200 bending cycles at angles up to 90°, demonstrating flexibility.