Optoelectronic devices based on graded bandgap structures utiliising electroplated semiconductors.

The main aim of the work presented in this thesis is to develop low-cost multi-junction graded bandgap solar cells using electroplated semiconductors. The semiconductor materials explored in this research are CdSe, ZnTe, CdS, CdMnTe and CdTe thin films. These layers were characterised for their structural, compositional, morphological, optical, and electrical features using XRD, Raman spectroscopy, EDX, SEM, UV-Vis spectroscopy, PEC cell, C-V, I-V and UPS measurement techniques respectively. The summary of the results depict that CdSe and CdS semiconductors have hexagonal crystal structures and are mainly n-type in electrical conduction within the explored range of deposition potentials. The crystal structures of ZnTe thin films are hexagonal and the electroplated ZnTe thin films have both n- and p-type electrical conduction. In the literature, the electrical conductivity type of ZnTe thin films has been reported to be p-type. In this work, the developments of n-type ZnTe thin films have been successfully achieved for the first time by using intrinsic doping. Also, the fabrication of p-n homo-junction diodes from intrinsically doped electroplated ZnTe layers have been developed for the first time. Results from analytical techniques showed that CdTe and CdMnTe thin films have cubic crystal structures and can exist as n- and p-type materials. The semiconductor materials investigated in this work have been used for solar cells fabrication. Some of the device structures explored are based on p-n hetero-junction solar cells fabricated from CdS/ZnTe hetero-structure and combination of n-n hetero-junction plus large Schottky barrier (n-n+SB) solar cells fabricated from CdS/CdTe hetero-structure. The highest efficiency obtained for the p-n junction solar cell with device structure glass/FTO/n-CdS/p-ZnTe/Au was -5.3% while the highest efficiency reported in this work for n-n+SB solar cells with device structure glass/FTO/n-CdS/n-CdTe/Au was -7.6%. Multi-junction graded bandgap solar cells with different device structures were also fabricated in this research work. The two most important solar cells in this category are n-n-n plus large Schottky barrier (n-n-n+SB) solar cells fabricated from glass/FTO/n-ZnS/n-CdS/n-CdTe/Au device structures and n-n-p solar cells fabricated from glass/FTO/n-CdS/n-CdTe/p-CdTe/Au device structures. The n-n-p device structure is a combination of one n-n hetero-junction and one n-p homo-junction interface. The experimental investigations carried out on the effect of thickness of p-CdTe on n-n-p device structures showed that thinner p-CdTe layer of -35 nm is most appropriate to be used in the n-n-p solar cells device configuration. The highest efficiency obtained for the n-n-p solar cell device structures was -10.9% while the highest efficiency obtained for the n-n-n+SB solar cell device structures was -12.8% with an active area of -0.031 cm[2].