Abstract
Cu(InGa)Se2 (CIGS) thin film absorbers are prepared using sputtering and selenization processes. The CuGa/In precursors are selenized during rapid thermal annealing (RTA), by the deposition of a Se layer on them. This work investigates the effect of the Cu content in precursors on the structural and electrical properties of the absorber. Using X-ray diffraction, field emission scanning electron microscopy, Raman spectroscopy, and Hall effect measurement, it is found that the CIGS thin films produced exhibit facetted grains and a single chalcopyrite phase with a preferred orientation along the (1 1 2) plane. A Cu-poor precursor with a Cu/() ratio of 0.75 demonstrates a higher resistance, due to an increase in the grain boundary scattering and a reduced carrier lifetime. A Cu-rich precursor with a Cu/() ratio of 1.15 exhibits an inappropriate second phase () in the absorber. However, the precursor with a Cu/() ratio of 0.95 exhibits larger grains and lower resistance, which is suitable for its application to solar cells. The deposition of this precursor on Mo-coated soda lime glass substrate and further RTA causes the formation of a MoSe2 layer at the interface of the Mo and CIGS.
Highlights
The use of polycrystalline Cu(InGa)Se2 (CIGS) thin films as the absorber material for thin film solar cells allows easier commercial production because of the suitability of its bandgap and its high absorption coefficient for solar radiation [1]
Ga accumulation near the Mo side of the substrate/Mo/CIGS structure is often observed during the selenization of Cu–Ga–In precursors, yielding phase-separated CuInSe2 (CIS) and CuGaSe2 (CGS), which subsequently result in lower open-circuit voltages [9]
This study investigates the effect of stacked CuGa/In precursors with different Cu contents on the growth of CIGS thin films
Summary
The use of polycrystalline Cu(InGa)Se2 (CIGS) thin films as the absorber material for thin film solar cells allows easier commercial production because of the suitability of its bandgap and its high absorption coefficient for solar radiation [1]. For a two-stage method, the compositional uniformity and surface morphology of metallic precursors affect the quality of the absorber layer. To ensure the compositional uniformity of precursor films, many procedures using stacked metal or alloy layers, such as In/CuGa, CuGa/In, CuGa/In/CuGa, or In/CuGa/In films, have been proposed [7, 8]. In traditional two-stage growth processes, Cu–Ga–In metallic precursors are selenized in an elemental Se vapor or a H2Se/Ar gas mixture to form CIGS. Compared to the other selenization methods, the progressive RTA treatment minimizes the defect density in the CIGS thin film by reducing the thermal budget and reaction time; the conversion efficiency of solar cells is remarkably improved [12]. Good control of precursor structure, process parameters, and reaction routes can improve the quality of photovoltaic absorber [13, 14]. The precursors are selenized during RTA, by deposition of a 2 μm thick Se layer on them
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