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[en] This thesis focuses on the electrodeposition of metal precursors from a deep eutectic based ionic liquid electrolyte and their annealing for Cu(In,Ga)(S,Se)2 thin film solar cells. Ionic liquids are ionic compounds which are liquid at temperatures below 100 °C and contain no water. Therefore, by using an ionic liquid electroplating bath, the electrolysis of water is avoided and elements which are normally difficult to electrodeposit, such as gallium, can be easily deposited. In this sense, the electroplating current efficiency of the process (i.e. the energy efficiency of the process) is significantly improved by using an ionic liquid electrolyte instead of an aqueous electroplating bath.
The objectives of this work are i) to electrodeposit Cu(In,Ga) metal precursors with high electroplating current efficiency, ii) to control the chemical composition of these precursors and iii) form Cu(In,Ga)(S,Se)2 absorber layers with adequate chemical composition and morphology for solar cell fabrication. In the frame of the third objective, it is intended to obtain absorber layers with a continuous gallium distribution as well. This last point is due to the fact that, during thermal annealing of metal precursors, gallium often segregates to the back of the absorber layer. Ultimately, this uneven gallium distribution can hinder the performance of solar cells. To meet these objectives, the co-electrodeposition of a) copper and indium, b) indium and gallium and c) copper, indium and gallium from an ionic liquid electrolyte is studied. The ionic liquid used in this work results from the 1:2 molar mixture of choline chloride and urea.
From this work, the electrodeposition of Cu(In,Ga) metal precursors with an electroplating current efficiency above 75% was achieved. It was observed that the morphology of the electrodeposited precursors depended on the chemical composition of the electrolyte. In this frame, Cu(In,Ga) layers with dendritic or compact morphology were obtained. Precursors with dendritic morphology are not adequate, since this morphology persists after thermal annealing and ultimately results in devices with no efficiency. The chemical composition of the metal precursors can be controlled as well. Specifically, the gallium content of the metal precursor, which influences the optoelectronic properties of the absorber layer, was accurately tuned. The gallium content is usually expressed as the concentration ratio [Ga]/([Ga]+[In]) and could be tuned between 0.1 and 0.9. Therefore, the electrodeposition of Cu(In,Ga) metal precursors with high gallium content was achieved for the first time and would not have been possible without the use of an ionic liquid electrolyte. After subjecting the metal precursors to a thermal annealing, absorber layers with adequate chemical composition and morphology were obtained. Additionally, by employing a specific annealing routine developed at the Institute of Energy Conversion in the University of Delaware, absorbers with a continuous gallium profile and different gallium contents were obtained. It was observed that the performance of the solar cells was limited by the thermal annealing step and a maximum solar cell efficiency of 9.8% was achieved. In general, it can be concluded that different precursors require different thermal annealing routines in order to form quality absorber layers.