Structure and optical band gap of ZnO1−xSx thin films synthesized by chemical spray pyrolysis for application in solar cells

Abstract

Highly crystalline c-axis oriented and homogeneous ZnO1−xSx films with sulfur composition 0.05 ≤ x ≤ 0.9 without phase separation were deposited using spray pyrolysis of aqueous precursors. A mechanism of film growth is proposed which envisages a slower kinetics of ZnO precursor decomposition and its specific by-products combined with S-precursor decomposition which enables homogeneous ZnO1−xSx phase formation over a wide S-composition range 0.05 ≤ x ≤ 0.9. This is achieved by controlling the substrate temperature to ≤ 300 °C and the spray rate at ∼ 3 ml/min. The ZnO1−xSx films primarily form by S2− substitution at the O2− lattice sites which is confirmed by detailed analysis of the Zn2p, S2p and O1s x-ray photoelectron spectroscopy peaks. With the increasing of S-content, a structural transformation is observed in ZnO1−xSx films; for x < 0.3, the ZnO1−xSx films in the oxygen-rich phase are in ZnO wurtzite crystal structure and for x ≥ 0.44, the ZnO1−xSx films lie in the sulfur-rich phase with β-ZnS structure. At threshold x = 0.3, in the structural transition state, diffraction peaks corresponding to both structural phases are observed. The optical transmission spectra at the band gap transition energy position shift to the red side for 0.05 ≤ x < 0.52 and towards the blue side with the further increase in S-content in the 0.52 < x ≤ 0.9 range. Optical band gap energy determined for ZnO1−xSx films show strong band gap bowing over the entire S-composition range, 0.05 ≤ x ≤ 0.9. The band gap modulation with increased S-content is inconsistent with the use of a single bowing parameter. A bowing parameter of 2.5 eV shows a closer fit for 0.05 ≤ x < 0.52. By accounting for additional effect of the strain energy on the band gap due to addition of excess S-content, a closer fit for the observed variation in the band gap is explained in the 0.52 < x ≤ 0.9 composition range. Electrical resistivity variation with S-content is consistent with the compensating effect of S2− substitution at the O2− sites in ZnO1−xSx films.