dc.description.abstract
Concentrator photovoltaic (CPV) is a cost-effective method for energy generation with a high photovoltaic conversion rate. Highly efficient solar cells, which are based on III-V semiconductor materials, are used for CPV applications present a higher cost with respect to other material-based solar cells. To achieve a price reduction in this technology and, therefore, a higher integration and expansion of this renewable source of energy, efforts must be made to simplify each element contained in a CPV device. Every single element, from the optical system to the cooling device through the solar cell material, must be modified to reduce its cost without decreasing the overall performance of the solar cell. In this work, thanks to the funding from the Helmholtz-Association for Young Investigator groups within the Initiative and Networking fund (VH-NG-928) and the funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement n° 609788, micrometer-sized chalcopyrite-based solar cells, specifically, Cu(Inx,Ga1-x)Se2 (CIGSe), were investigated in detail to reduce the requirements of this technology for concentration purposes.
In the first part of this work, FEM simulations were employed to predict the heat management of micrometer-sized CIGSe solar cells in order to verify the benefits of cell minimization under concentrated light. These novel findings were obtained by varying different structure elements to find out the best configuration for high concentration. High concentration factors and high material saving, up to 105x, are feasibly to be applied on micrometer-sized CIGSe solar cells. In the second part of this work, based on these investigative thermal simulations, micrometer-sized CIGSe solar cells were fabricated via “top-down approach” and characterized by different techniques. The morphology as well as the elemental composition and distribution were investigated in order to characterize the quality of the fabricated solar cells. In addition, electrical characterizations were carried out, specifically, photoluminescence, current-voltage characteristic curve as a function of the temperature and as a function of the concentrated light, to determine the main opto-electronic parameters of the micrometer-sized CIGSe solar cells. These properties were dramatically affected by the fabrication method as the active area was reduced due to a higher SRH recombination mechanism. Higher recombination and the temperature increment of the solar cell during I-V measurements resulted in a maximum power conversion to electricity, i.e. efficiency, under concentrated light in the range of 20x to 50x and in the range of 20x to 30x for shaded and etched cells, respectively.
Finally, a 3D thermal-opto-electronic (TOE) model was successfully validated and employed to simulate the experimental setup as well as the ideal one, where the active area of the solar cell is only illuminated. These simulations predicted the output parameters of micrometer-sized CIGSe solar cells under all considered concentration factors and beam profiles in this work. The results extracted from the TOE model were compared with those of the experiments, and therefore, assessing and verifying such model. These simulations exhibited a good correlation under STC-20°C and under concentrated light with respect to those obtained experimentally. These simulations forecast the application of higher concentration factors, above 100x, to micrometer-sized CIGSe solar cells without lessening the overall performance by taking advantage of cell miniaturization.
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