Thin film solar cells using all-electrodeposited ZnS, CdS and CdTe materials.

The urgent global need for affordable alternative and clean energy supply has triggered extensive research on the development of thin-film solar cells since the past few decades. This has necessitated the search for low-cost, scalable and manufacturable thin-film semiconductor deposition techniques which in turn has led to the research on electrodeposition technique as a possible candidate for the deposition of semiconductor materials and the fabrication of thin-film solar cells using these materials.Electronic quality ZnS, CdS, and CdTe thin layers have been successfully electrodeposited from aqueous solutions on glass/fluorine-doped tin oxide (FTO) substrates, using simplified two-electrode system instead of the conventional three-electrode system. This process was also carried out in a normal physical chemistry laboratory instead of the conventional cleanroom that is very expensive to maintain. The electrodeposited materials were characterised for their structural, optical, electrical, morphological and compositional properties using x-ray diffraction, optical absorption, photoelectrochemical cell, current-voltage, scanning electron microscopy and energy dispersive x-ray techniques respectively. The results show that amorphous n-type and p-type ZnS layers were deposited by varying the concentrations of Zn[2+] and S[2-] in the deposition electrolyte. The CdS layers show hexagonal structure with n-type electrical conduction while CdTe layers show cubic structure with n-type electrical conduction, in the cathodic deposition potential range explored.Using CdTe as the main absorber material, fully fabricated solar cell structures of the n-n hetero-junction + large Schottky barrier type were fabricated instead of the conventional p-n junction type structure. Conventional post-deposition CdCl[2] treatment of CdTe rather carried out with a mixture of CdCl[2] and CdF[2], resulted in pronounced improvement of all the device parameters. Characterisation of the fully fabricated solar cells was done using current-voltage and capacitance-voltage techniques. Promising device parameters were obtained for the best devices, with barrier heights greater than (1.00 - 1.13) eV, short-circuit current densities of (20 - 48) mAcm[-2], open-circuit voltages of (500 - 670) mV, fill factors of (0.33 - 0.47) and overall conversion efficiencies of (5.0 - 12.0)%. Remarkably, the two highest efficiency figures of 10.4% and 12.0% came up for solar cells involving ZnS as buffer layer and window layer with the structures, glass/FTO/n-ZnS/n-CdS/n-CdTe/Au and glass/FTO/n-ZnS/n-CdTe/Au, respectively. At present, the reproducibility and consistency of these devices is poor, but these results demonstrate that these devices structures have the potential to achieve efficiency values over 20% when fully optimised.