4. ZnS Deposition and Characterisation

Zinc sulphide (ZnS) layers have been used as buffer layers in the solar cells described in this book, and this chapter provides an insight into the electrodeposition of ZnS layers. Electrodeposition of zinc sulphide (ZnS) was achieved from an electrolytic bath containing zinc sulphate monohydrate (ZnSO₄ ·H₂O) and ammonium thiosulphate ((NH₄)₂S₂O₃) in a two-electrode electroplating configuration. Cyclic voltammetric studies show that ZnS layers can be electroplated between 1350 and 1550 mV. The grown layers were characterised for their structural, optical, morphological and electronic properties using X-ray diffraction (XRD) and Raman spectroscopy, UV-visible spectrophotometry, scanning electron microscopy (SEM), photoelectrochemical (PEC) cell and DC conductivity measurements, respectively. The structural analyses show that crystalline ZnS can be deposited within a narrow cathodic deposition range between 1420 and 1430 mV. UV-visible spectrophotometry shows that the bandgap of both as-deposited and heat-treated ZnS films is in the range of ~(3.70 and 3.90) eV. The SEM shows small grains in the ZnS layer and the full coverage of the underlying substrate by the film. PEC results show that the electroplated ZnS layers grown below 1425 mV are p-type and above 1425 mV are n-type under both as-deposited and heat-treated condition. DC conductivity shows that the highest resistivity is at the inversion growth voltage (V _i) for the ZnS layers.