The influence of ZnS crystallinity on all-electroplated ZnS/CdS/CdTe graded bandgap device properties

Electrodeposition of zinc sulphide (ZnS) was achieved from electrolytic bath containing zinc sulphate monohydrate (ZnSO4·H2O) and ammonium thiosulphate ((NH4)2S2O3) in a two-electrode electroplating configuration. The cyclic voltammetric studies show that ZnS layers can be electroplated between (1350 and 1550) mV. The grown layers were characteristically explored 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 analysis shows that crystalline ZnS can be deposited within a narrow cathodic deposition range between (1420 and 1430) mV. The UV–Visible spectrophotometry shows that the bandgap of both as-deposited and heat-treated ZnS films are in the range of ~(3.70 and 3.90) eV. The SEM shows small grains depicting the wetting property of ZnS. The PEC results show that the electroplated ZnS below 1425 mV is p-type and above 1425 mV is n-type under both as-deposited and heat treated condition. The DC conductivity shows that the highest resistivity is at the inversion growth voltage (Vi) for the ZnS layers. The glass/FTO/n-ZnS/n-CdS/n-CdTe/Au devices were fabricated using crystalline-ZnS and amorphous-ZnS buffer layers. The devices were explored using current-voltage (I-V) and capacitance-voltage (C-V) techniques. As expected, devices fabricated with c-ZnS show improved device parameters (ideality factor n=1.60, depletion width W=1092 nm, open-circuit voltage Voc=730 mV, short-circuit current density Jsc=34.1 mAcm-2, fill factor FF=0.57, conversion efficiency η=14.2%) when compared to device parameters (n=1.85, W=900 nm, Voc=720 mV, Jsc=29.9 mAcm-2, FF=0.52, η=11.2%) of these devices fabricated with a-ZnS buffer layers.