Promising results have been obtained using silver as dopant or even zinc as alloy in CIS (Werner et al., 2000, Enzenhofer et al., 2006). A further increase in the efficiency is expected from doping or alloying of the CIS absorber material with isovalent and/or non-isovalent substitutional elements. Significant improvement could not be achieved and open circuit voltage values did not typically exceed 730 mV. Though CIS-based solar cell have a strong potential of achieving high efficiency due to its almost ideal band gap energy of 1.5 eV (Klenk et al., 2005), debates have been going on regarding the limited efficiency and open circuit voltages of the cells remained below the predicted theoretical values. CuInS 2 (CIS) and its related quaternary chalcopyrite materials (incorporating gallium) have received further great attention as photovoltaic absorber materials (Scheer et al., 2004). However, it is approaching the theoretical limit (Repins et al., 2008). Among the chalcopyrite photovoltaic (PV) materials, Cu(In,Ga)Se 2 (CIGSe) has become the promising absorber material for thin film solar cells in recent years and a power conversion efficiency of 20.8% was achieved (Jackson et al., 2014). The flat band potential and the carrier concentration of obtained CIGS thin films increase with negative shift of deposition potential.Ĭhalcopyrite semiconductors have a huge potential as absorber layer in thin film solar cell applications owing to their advantages of high power conversion efficiency, direct band gap, high absorption coefficient, low toxicity and excellent stability (Merdes et al., 2013). Impedance spectroscopy test demonstrates the semiconductor property of the synthesized CIGS polycrystalline thin films are p-type. The thickness of film deposited at −1.1 V is about 2 μm which is in the range of the optimum thickness of CIGS thin film solar cell.
The thickness of CIGS films tends to increase slightly with negative shift of deposited potential. A relative positive deposition potential results in a lower Ga/(Ga + In) ratio and a lager crystal size. The Cu/(Ga + In) ratio decreases with the deposition potential benefiting the formation of a pure crystallized CIGS thin film. A relative negative shift of deposition potential results in the increasement of Ga content and the decrease of In and Cu content in quaternary CIGS structure. The influence of deposition potential on the crystalline phase, morphology, composition and carrier concentration of the films are investigated. Pure quaternary chalcopyrite CIGS phase in good polycrystalline structure without secondary phase is obtained after annealing. Cu(In,Ga)S 2 (CIGS) solar energy thin film is fabricated with the one-step electrodeposition of Cu-Ga precursors on indium tin oxide (ITO) substrate from deep eutectic solvent to eliminate the interference of hydrogen evolution reaction (HER) followed by thermal annealing treatment to incorporate In, diffused from the ITO substrate.
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