The development of cuIn1-xALSe2 thin films for use in photovoltaic solar cells

Nasikkar, Paresh (2009) The development of cuIn1-xALSe2 thin films for use in photovoltaic solar cells. Doctoral thesis, Northumbria University.

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Abstract

The aim of the work presented in this thesis was to develop CuInSe2 (CIS) and CuIni_„Al„Se2 (CIAS) thin films for application in photovoltaic (PV) solar cells. The purpose of the addition of aluminium (Al) in CIS thin films was to modify the energy band gap of the thin films to be nearer to the optimum for PV energy conversion and to replace the less abundant element, gallium (Ga) in CuIni_,,Ga,Se2 (CIGS) solar cells. This also makes possible the production of tandem solar cells using CIAS to make the wide energy band gap top cell and the CIS to make the narrow energy band gap lower cell. The use of very thin CIS and CIAS absorber layers in solar cell structures was also investigated; the aim was to reduce the amount of indium (In) in cell production. The CIS and CIAS absorber films were prepared by a sequential two step method in which Cu-In and Cu-In-Al precursor layers were magnetron sputter deposited onto Mo-coated soda lime glass (SLG) substrates; the CIS or CIAS was then formed by heating in a selenium (Se) containing environment. Thin film solar cells were developed in the substrate configuration and had the structure Ni-Al/Indium tin oxide (ITO)/i-ZnO/CdS/CIAS/Mo/SLG. In order to achieve high efficiency solar cells it is an important to optimse the back contact molybdenum (Mo) layer, the absorber layer, the CdS buffer layer, the window layer and top contact layers. The work described in this thesis focused on the optimisation of the back contact and absorber layers. The thin films were characterised mainly using X-ray diffraction (XRD), energy dispersive X-ray analysis (EDS), scanning electron microscopy (SEM), secondary ion mass spectroscopy (MiniSIMS), atomic force microscopy (AFM) and using spectroscopy measurements to investigate the effect of processing conditions on the composition, crystal structure, surface morphology and the optical properties of the films. The solar cells were characterised by current-voltage (/- V) and incident photon-to-photocurrent conversion efficiency (IPCE) measurements. Both Mo single and bilayer structures were investigated. It was found that single layers had better properties than Mo bilayers. The optimisation of the Mo deposition sputtering process yielded Mo layers which had good adherence and were conformal to the glass substrates, had low resistivity (29 if .cm), were pin hole free and had good crystallinity. The influence of Cu-In precursor layers with thicknesses in the range 90-400 nm on the microstructure of the CIS thin films (thicknesses in the range 400-1600 nm) was investigated. Solar cells fabricated from the CIS films of thicknesses 500 nm and 900 nm yielded highest cell conversion efficiencies of 4.3% and 8.2%, respectively. The selenisation of the magnetron sputter deposited Cu-In-Al precursor layers was carried out at a temperature of 550°C. Films were poor in surface quality and adhesion. Films prepared from the precursor layer with n [(Al/(Al+In))] = 0.21 had a non-uniform Al depth profile towards the bottom of the film. Although the film was found to be photoactive its effective energy band gap was 0.98 eV suggesting the properties of CIS. This confirmed incomplete mixing of Al in the thin films which was considered to be segregated at the bottom of the film. The thinner layers of Cu-In-Al precursors with thicknesses in the range 0.55¬1.00 gm and n [(Al/Al+In)] in the range 0.28-0.54 were magnetron sputter deposited. The precursor layers showed the prominent binary A1Cu4 compound with a uniform distribution of Al in the layer. Thin films converted from these precursor layers of thicknesses in the range 1.3-2.0 pm were fairly uniform in surface structure. Films with x 0.2 were found to have an energy bandgap of 1.10 eV and were also photoactive. Solar cells fabricated from this absorber film yielded a highest cell efficiency of 4.9%. Environmental impact assessments have been made on materials and the processes used in the fabrication of CIS and CIAS and...

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