Cadmium Oxysulfide Cd(S,O) as Novel High Transmittance Buffer Layer Formed by Surfactant Mediated Chemical Bath Deposition for Thin-Film Heterojunction Solar Cells

Abstract

ABSTRACTPolycrystalline <100 nm thin CdS films deposited by the chemical bath deposition (CBD) method are used as n-type window layer in high efficiency heterojunction CuInSe2, Cu2ZnSnS4, and CdTe solar cells. The major shortcoming of the CdS films is its stability and high sub-bandgap absorption. In the present work, an alternative window layer for the heterojunction solar cells based on cadmium oxysulfide Cd(S,O) thin films formed by the CBD method is described. The ∼100 nm thin Cd(S,O) films formed by co-deposition of CdS with CdO show high >85% optical transmittance over 550-1000 nm region compared to 70% transmission in CdS films. Cd(S,O) film formed with highly conformal coverage showed as direct optical band gap of 2.19-2.09 eV depending on the CdO fraction in the film. The CBD growth of CdS is via controlled release of S2- and Cd2+ ions in an alkaline medium through complexing to prevent CdS or Cd(OH)2 as colloids in the solution phase. In the present method, by altering the substrate conditions we carried out controlled Cd(OH)2 deprotonation reaction to form CdO over the substrate concurrently with CdS to deposit Cd(S,O) films. These films are polycrystalline having wurtzite structure identified by (002), (110) and (112) x-ray diffractions. Formation of Cd(S,O) phase is confirmed by Raman spectra which besides the characteristic CdS 1LO and 2LO modes at 300 and 600 cm-1, also showed Raman lines from CdO at 1094 cm-1 and 943 cm-1 assigned as overtone of 2LO phonon modes at the L or Γ points of the Brillouin zone of CdO.