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Recrystallization of Cu(In,Ga)Se₂ thin films by metal halide vapor treatments
Palmiotti, Elizabeth C.
Palmiotti, Elizabeth C.
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2021
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Abstract
CuIn_(1-x)Ga_xSe_2 (CIGS) thin film photovoltaic module performance and manufacturing have shown significant successes; however, cost has limited a large-scale commercial presence. Significant reductions in absorber deposition time and temperature are necessary to minimize expenses to compete as a single junction technology. Due to a tunable band gap, an alternative future for CIGS is using a wide band gap alloy, CuGaSe_2 (CGS) as a top cell in a tandem structure with silicon. However, wide gap CIGS materials produce devices which are typically of poor quality due to kinetically-limited growth which results in secondary phases, small grains, and inhomogeneities. In this thesis these problems are addressed through novel metal halide vapor treatments to recrystallize CIGS thin films.
In this work, metal halide compounds used for treatment are selected such that the cation is already a species in the (Cu,Ag)(In,Ga)Se_2 alloy system and the anion is a halide. Metal halide vapor treatments to CIGS films deposited at 350°C result in large grain growth and improved crystallinity. Composition variation occurs depending on the metal halide used, though, gallium depletion is common for all. It is also demonstrated that the metal halide treatments catalyze and accelerate the crystallization kinetics for CIGS using in-situ high-temperature x-ray diffraction (HT-XRD). An evaluation of metal halide thermodynamic properties is presented and compared to experimental results. Based on bond dissociation energies, CuBr, CuI, AgBr, and AgI are recommended as metal halide source materials with a preference to AgBr and AgI due to the known benefits of Ag alloying. It is proposed that growth occurs by vapor phase transport induced by a halide transport agent. The halide anion forms volatile compounds with metal cations on the surface of one grain, which etch and re-deposit onto the surface of a neighboring grain. This causes the observed grain growth and defect passivation. For such volatile compounds with very high vapor pressures, such as the Ga-halide compounds, re-deposition to a neighboring grain surface is too slow and results in the preferential etching of such metallic species.
The beneficial treatments are applied to CGS films to replace a long, high-temperature deposition procedure necessary to fabricate uniform films. Instead, the deposition is interrupted and AgBr evaporated during growth, reducing deposition time by ~50%. This results in large grains, phase homogeneity, and device efficiency improvements due to open-circuit voltage and fill factor.
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