Electrochemical Deposition Of Cadmium Selenide Films And Their Photoelectrochemical Performance

Cadmium selenide（CdSe）is a kind of attractive semiconductor for solar energy conversion because its proper band gap and its stable chemical property.In this work,CdSe films were prepared on the FTO conductive glass by electrochemical deposition in electrolyte containing cadmium sulfate（CdSO₄）and selenium dioxide（SeO₂）.The electrochemical deposited CdSe films were treated with chemical etching,annealing,or annealing add chemical etching,and the photoelectrochemical performance of these films were improved.The results are as following:（1）Se⁴⁺in H₂SeO₃ was reduced to Se^2-,and then combined with Cd²⁺to form CdSe film on the surface of the FTO conductive glass during the electrochemical deposition.A CdSe film with cubic sphalerite structure and the band gap of 1.79 eV was deposited at-0.65 V vs SCE for 2400 s in pH 2.50 solution.The obtained film shows a surface with dense grains and clear grain boundaries.（2）The photoelectrochemical performance of CdSe films was significantly enhanced with chemical etching,annealing,or annealing add chemical etching.When the CdSe film was chemically etched in a 7 mol dm^-3 HCl solution for 15 min,the grain sizes became more uniform and the crystallinity was improved.The carrier density of CdSe film increased from 8.005×10¹⁷ cm^-3 to 3.373×10²⁰ cm^-3 and the charge transfer resistance at the interface of the CdSe film and the FTO conductive glass was reduced.The photocurrent density under zero bias of the chemical etched CdSe film was 3.10 mA cm^-2,which was about 24times higher than that of the untreated CdSe film(0.13 mA cm^-2),and power conversion efficiency was 0.21%.For the CdSe film was annealed at 500°C for 90 min in air,the crystal structure of CdSe was converted from metastable cubic sphalerite to hexagonal wurtzite with good crystallinity,the band gap reduced from 1.79 eV to 1.71 eV.However,annealing did not affect the carrier density and impedance of CdSe film very much.It was also found that the photoelectrochemical performance of the annealed CdSe film was lower than chemically etched one,and the photocurrent density under zero bias of annealed CdSe film was 1.50 mA cm^-2 and power conversion efficiency was 0.19%.The low efficiency was because of the evaporation and oxidation of Se and the recombination center was formed on the surface during the annealing.To solve the problem,the annealed CdSe film was chemically etched in a 7 mol dm^-3 HCl solution for 15 min.The photocurrent density under zero bias of annealed CdSe film with chemical etching was 5.90 mA cm^-2,and power conversion efficiency was 1.13%.The improving of photoelectrochemical performance for the annealed CdSe film can attributed to the enhanced conductivity and the enhanced separation and transporation of photogenerated carriers after chemically etching.