Structural, optical and electrical properties of SnO₂:F thin films deposited by spray pyrolysis for application in thin film solar cells

Fluorine doped tin oxide (FTO) thin films with adequate properties to be used as transparent electrical contact for PV solar cells were synthesised using the spray pyrolysis technique, which provides a low cost operation. The deposition temperature and the fluorine doping have been optimized for achieving a minimum resistivity and maximum optical transmittance. No post-deposition annealing treatments were carried out. The X-ray diffraction study showed that all the FTO films were polycrystalline with a tetragonal crystal structure and preferentially oriented along the (200) direction. The grain size ameliorates with the increase in substrate temperature. The samples deposited with the substrate temperature at 440 °C and fluorine content of 20 wt % exhibited the lowest electrical resistivity (1.8 × 10⁻⁴ Ω cm), as measured by four-point probe. Room-temperature Hall measurements revealed that the 20 wt% films are degenerate and exhibit n-type electrical conductivity with carrier concentration of ~4.6 × 10²⁰ cm⁻³, sheet resistance of 6.6 Ω/□ and a mobility of ~25 cm² V⁻¹ s⁻¹. In addition, the optimized growth conditions resulted in thin films (~500 nm thickness) with average visible transmittance of 89 % and optical band-gap of 3.90 eV. The electrical and optical characteristics of the deposited films revealed their excellent quality as a TCO material.