Abstract :
[en] The worrying progression of climate change is urging for scientists and engineers to rapidly deploy renewable energy technologies and to develop pedagogic methods to explain to the public the urgent need for changing our energy infrastructure and potentially even our way of life. Photovoltaics are a cheap and reliable solution that can convert sunlight into electricity and greatly contribute to decarbonize the current world energy mix. Thin film Cu(In,Ga)Se2 (CIGSe) solar cells are a mature technology that is well-known for using 100-fold less absorber material compared to silicon solar cells. Furthermore, CIGSe solar cells were demonstrated to be compatible with micro-concentrator photovoltaics (micro-CPV), which combines optical lenses, to concentrate more sunlight, with an array of miniaturized solar cells to generate more electricity, i.e. increase the solar cell’s power conversion efficiency (PCE). This allows to simultaneously achieve higher PCEs from the same active area and considerable semiconductor material savings. As an example, a 100X light concentration would lead to a 100-fold material savings. However, so far, only material wasteful methods have produced CIGSe micro solar cells with PCEs similar to the world record CIGSe solar cells (23.6%). To minimize the use of materials, material efficient deposition methods have been proven to effectively produce arrays of CIGSe micro solar cells. However, a large gap in PCE still exists compared to material wasteful methods. To understand the reasons for this discrepancy, both material wasteful and material efficient synthesis methods are investigated in this work, with the aim of growing CIGSe on patterned substrates, containing the arrays of holes that define the micro solar cells. Firstly, since each array contains a high number of individual future micro solar cells, a simple methodology was developed to characterize each individual cell and to statistically compare them. From optical and topographic images, acquired with confocal microscopy at each step of the synthesis, four conclusions were drawn: (i) that the morphology of the precursor layers play a major role in determining both the morphology and phase formation of the respective absorber. (ii) a new optical method to measure elemental composition in sequential processes, (iii) which combined with the phase diagram, allowed to spatially predict which phases would form at the end of the synthesis process. (iv) the ability to quickly differentiate phases in a material. Secondly, a reference co-evaporation growth method was used to investigate whether the use of a SiO2 patterned substrate itself influences the growth of CIGSe. It was found that the patterned SiO2 layer, acts as a diffusion barrier layer for alkali dopants, from the substrate, which redirected and enhanced the diffusion of sodium through the holes meant for the micro solar cells. This led to the formation of a Na(In,Ga)3Se5 secondary phase and to a poor adhesion between the CIGSe film and the molybdenum back contact. Three distinct methods were studied to control the sodium diffusion and the most effective was the implementation of a sodium barrier, grown directly on the sodalime glass substrate. Further comparison with the reference growth method unveiled that the selenium partial pressure, during CIGSe formation, regulates the sodium diffusion from the patterned substrate, and influences the morphology and composition homogeneity of the resulting CIGSe absorber. Thirdly, a novel material efficient synthesis method was demonstrated to yield micro solar cells with PCEs up to 5% at 1 Sun, which is the highest PCE reported for island-shaped micro solar cells. It was observed that a fine control of the selenium supply is crucial to optimize the morphology, phase purity and PCE of the CIGSe devices. Also, the design of the substrate pattern was proven to three dimensionally shape the CIGSe absorber and the diameter of the holes has an influence in the formation mechanism of CIGSe and adhesion to the back contact. Finally, a new pedagogic tool was developed to explain the abstract concept of energy, involved in every citizen's lifestyle, without visible calculations. Here, a description of the design and involved calculations are detailed with the aim of demonstrating that complex topics can be conveyed in widely-known terms, such as dimensions, weight and area. The optical method developed for the characterization of micro solar cells, can be applied to other systems, in particular for sequential processes, in order to study morphology, relative composition, diffusion processes, all with statistical weight. Furthermore, confocal microscopy was demonstrated to be a diagnosis tool to monitor the progression of a process or to highlight possible issues, allowing to intervene at an early stage. The examples shown in this work widen the range of applications for confocal microscopy and confirms its applicability for thin film characterization. Regarding the material efficient synthesis of CIGSe micro solar cells, this work has highlighted issues, inherent to the method, and has laid out solutions to circumvent them. This allowed to highlight the relevant experimental parameters to reproduce CIGSe micro solar cells with higher PCE. Nevertheless, further optimization of the experimental parameters (temperature, precursor composition, selenium partial pressure, alkali post-deposition treatment) is expected to result in even higher PCEs, decreasing the gap to the material wasteful methods. Last but not least, the developed pedagogic tool demonstrates a method to popularize a complex topic, which can be used to easily inform the regular citizen about current problematics or research, and in this case, hopefully trigger further interest and momentum in the fight against climate change.
