Process development and optimisation for efficient and cost-effective Cu(In,Ga)Se2 thin film solar cells
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Chalcopyrite copper indium gallium diselenide (Cu(In,Ga)Se2 or CIGS) solar cells have achieved the highest laboratory power conversion efficiency among thin film PV technologies, and have the potential to be manufactured cost-effectively at large scale. In this thesis, detailed studies on the growth of CIGS solar cells by means of a pilot scale inline co-evaporation system have been performed to explore the beneficial deposition techniques to successfully transfer the research achievements into cost-effective industrial production. The work involved the development and optimisation of several new processes along with the design of the inline pilot scale system. Effects of the thin absorber layers (with thickness of ~1 μm) on material quality and the device performance have been investigated using (i) a two-step process with room temperature evaporated metal precursors, (ii) a three-stage process at constant substrate temperature and (iii) a three-stage process at varied substrate temperatures. Changing the copper and gallium compositions of the CIGS layers was observed to have a strong impact on the structural and electronic properties and hence on the performance of the solar cells. In one study, simple mechanical compression was employed on a batch of low quality porous CIGS films to significantly improve the surface morphological and the optoelectronic properties. Photoluminescence measurements revealed the band-to-tail defect-related recombination that was detrimental to the quality of as-grown and compressed films. In another study, rapid thermal processing (RTP) was applied to the low temperature Cu-In-Ga-Se precursor layers in an attempt to reduce the cost and optimise the reaction mechanism of the chalcopyrite material. After fine tuning of the process conditions, the CIGS layers with larger grain size exhibited power conversion efficiencies up to 10.8%. While these results are promising, the device performance was mainly limited by the low open circuit voltage and the low fill factor. It has been found that the Cu-In-Ga-Se precursors doped with low concentrations of sodium were beneficial for re-crystallisation of the CIGS films due to the reduced grain size and crystallinity of the precursors. An in-situ X-ray diffraction technique was used to investigate the phase evaluation in both Na-free and Na-doped Cu-In-Ga-Se precursor layers as a function of temperature. The results confirmed that the formation of both CIGS as well as secondary phases in the layer with the Na-doped precursors started at higher temperatures compared to Na-free precursors. It is therefore expected that further improvements in the solar cell efficiency might be achieved following this RTP and low-temperature precursor process.