Correlated effects of fluorine and hydrogen in fluorinated tin oxide (FTO) transparent electrodes deposited by sputtering at room temperature

Highlights: • FTO films conductivity varies four orders with F and H₂ contents. • F and H₂ cause the reduction of Sn⁴⁺ and generate oxygen vacancies. • O vacancies control electrical and optical properties and promote amorphization. • RT-deposited electrodes with R_S ~20 Ω/□ and T > 90% on organic substrates. The optical and electrical properties of fluorinated tin oxide (FTO) films deposited at room temperature by sputtering have been investigated varying the fluorine content and the hydrogen atmosphere. The complex behavior of the obtained films is disclosed using a wide set of characterization techniques that reveals the combined effects of these two parameters on the generated defects. These defects control the electrical transport (carrier density, mobility and conductivity), the optical properties (band gap and defects-related absorption and photoluminescence) and finally promote the amorphization of the samples. H₂ in the sputtering gas does not modify the H content in the films but induces the partial reduction of tin (from Sn⁴⁺ to Sn²⁺) and the consequent generation of oxygen vacancies with shallow energy levels close to the valence band. A variation of up to four orders of magnitude in electrical conductivity is reported in samples with the appropriate fluorine doping and hydrogen fraction in the sputtering gas, maintaining excellent optical transparency. Optimized room temperature grown electrodes reach sheet resistance ~20 Ω/□ and transparency >90%. This room temperature deposition process enables film preparation on flexible organic substrates, such as polyethylene terephthalate (PET), with identical performance of doubtless interest in flexible and large scale electronics.