Engineering of electroplated materials for multilayer next generation graded bandgap solar cells
The work presented in this thesis aims to reduce cost per capita and increase the conversion efficiency of CdS/CdTe-based solar cells using multilayer graded bandgap configurations. To effect economic viability, electroplating technique was utilised as the low-cost semiconductor deposition technique. CdS and CdTe are the primary materials grown and explored in this thesis. These layers were characterised for their structural, morphological, compositional, optical, and electrical properties using X-ray diffraction, Raman spectroscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, UV-Vis spectroscopy, photoelectrochemical cell measurement, current-voltage and capacitance-voltage measurement techniques. The precursor (thiourea) utilised in the deposition of CdS solves the problem of sulphur precipitation during electroplating. Very few publications with explicit exploration on this subject are available in the literature. Further to this, the nitrate and chloride based CdTe precursors explored is an alternative to the sulphate based norm as reported in the literature. The effect of extrinsic doping of CdTe incorporating F, Cl, I and Ga was also systematically explored to improve both the material and electronic properties of CdTe. Doping of CdTe during growth was studied by adding the above dopants to the electrolyte. The inclusion of F and Ga into the regular CdCl2 post-growth treatment (PGT) and the effect of pH were also explored with improved results as compared to the regular CdCl2-PGT. The inclusion of Ga into the regular CdCl2 post-growth treatment (PGT) and the effect of pH were also explored with the improved result as compared to the regular CdCl2 PGT. The solar cell device configurations explored include n-n+large Schottky barrier (SB), n-p, n-n-n+SB and n-n-p incorporating CdS/CdTe-base semiconductors. Based on all experimental findings as explored within the limit of this thesis, the most promising of the configurations examined are the glass/FTO/n-CdS/n-CdTe/p-CdTe/Au with thicknesses of glass/FTO/(120 nm) n-CdS/(1200 nm) n-CdTe/(30 nm) p-CdTe/(100 nm) Au. The highest conversion efficiencies observed for two separate batches were 15.3% and 18.4%. The devices with the 18.4% efficiency showed some instability and therefore require further investigation. The glass/FTO/n-ZnS/n-CdS/n-CdTe/Au configuration with thicknesses of glass/FTO/50 nm n-ZnS/(65 nm) n-CdS/(1200 nm) n-CdTe/(100 nm) Au also show promising results with the highest efficiency achieved being 14.1% owing to bandgap grading strengths.
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