Band gap grading strategies for high efficiency kesterite based thin film solar cells

Abstract

The main subject of this work focuses on the development of advanced technological strategies for bandgap profile engineering on Earth-abundant and eco-friendly kesterite thin film solar cells which potentially optimize and enhance the energy power conversion efficiency of solar cell devices. By exposing the contemporneuous world energy consumption hassles and its direct implication with the heating imbalance produced by the current greenhouse gas emissions; it is doubtlessly notified that renewable energy supplies, mainly based on thin film solar cells, and focused on sustainable materials such as ‘kesterite’ (CZTS), could successfully perform in a wide variety of energy application scenarios. This is due to its potential to be deposited on flexible substrates, its aesthetics and selective transparency for integrations in construction and automotive sectors. As well as its use in new concepts of energy portability like the Internet of Things, even enhanced when combined with its nanostructured form as a potential thermoelectric material. However, the actual kesterite thin film solar cell devices hinder the actual energy conversion efficiencies of a single absorber layer PN junction. This fact is primordially demonstrated with a theoretical numerical modeling simulation (SCAPS-1D) of first front graded bandgap profile attempts in CZTSSe. Consequently, the effect of the front sulfurization of CZTSe solar cell with a realistic experimental compositional profile is analyzed and discussed. In light of this, it possibly to demonstrate that the next generation of kesterite (and chalcopyrite) solar cells power energy conversion efficiency improvements could be remarkably enhanced with the development of novel and more strategic methodologies for collecting photon energy. In this way, the graded bandgap profiling in kesterites is proposed as a sustainable strategy to improve the utilization of the solar spectrum, through the generation of quasi-electric internal fields along the thin films, increasing the drift and diffusion lengths of minority charge and finally improving the power conversion efficiency. First of all, by developing a novel and disruptive chalcogenization process for the fabrication of CZTSSe solar cells enabling the generation of a superficial graded compositional profile. Hence, controlling several front-graded bandgap profiles along the CZTSSe absorber thin film layer thickness. Furthermore, by means of generating a rear bandgap graded profile strategy mainly based on the spontaneous cationic substitution during the kesterite (CZTGSe) alloy synthesis, it was possible to reduce the effect of deep defect (SnCu) formation and impose an additional drift (back surface) field within the quasi-neutral region. Additionally, this improves the crystallization quality of the absorber material by generating metallic Ge liquid phase fluxes. Thus, controlling several rear-graded bandgap profiles along the CZTGSe absorber thin layer film thickness. Finally, assembling together the abovementioned strategies in order to simultaneous generate both anionic and cationic compositional grading profiles inside the same kesterite matrix structure. In this way providing for the first time a demonstration of the joint synergy between defect passivation and interface energetics-modification, as a result of applying bandgap grading strategies in kesterite-based thin film solar cells. In the case of this work studied kesterite alloyed material (CZTGSSe), the band energy offset can be independently controlled trough the sulfur (S) and germanium (Ge) contents, which is explained by a double U-Shaped graded bandgap model. Consequently, this Thesis develops advanced material synthesis techniques and surface characterization, which, when integrated with the structural complexity of kesterite (CZTGSSe), allow Nature to reveal several new and disruptive properties of matter, deliberately manipulable when working out of thermodynamic equilibrium conditions. Last but not least, by optimizing the synthesis conditions, an absolute increase in bare energy conversion efficiency is obtained for the champion kesterite-based thin film solar cell device (> 10%) without any antireflective coating (ARC) nor metallic grid.

Los suministros de energía renovable basados en celdas solares de película delgada/fina, y enfocados en materiales sostenibles, tales como la ‘kesterita’ (CZTS), podrían desenvolverse de manera muy exitosa en una amplia variedad de escenarios de aplicaciones energéticas. Esto se debe a su potencial para ser depositadas sobre substratos flexibles, su estética y transparencia selectiva para integraciones en sectores como el de construcción y la automoción. Así como su uso en nuevos conceptos de portabilidad energética, tales como el Internet de las cosas. Las celdas solares actuales de kesterita y de una sola capa absorbente con perfiles de banda prohibida no variables limitan la plenitud en la obtención de mejores eficiencias de conversión energética para una unión PN. Ante esto, esta Tesis demuestra que la eficiencia de conversión energética de la próxima generación de celdas solares de kesterita (y calcopiritas) puede verse potenciada tras desarrollar nuevas metodologías más estratégicas de recolección de energía fotónica. Por tanto, se propone el graduado del perfil el de banda prohibida como una estrategia sostenible para mejorar la utilización del espectro solar, mediante la generación de campos internos cuasi-eléctricos a lo largo de las películas finas, consiguiendo incrementar las longitudes de deriva y difusión de los portadores de carga minoritarios y finalmente aumentar la eficiencia. De esta manera, esta Tesis desarrolla técnicas avanzadas de síntesis y caracterización de superficies, que al integrarse con la complejidad estructural de la kesterita (CZTGSSe), permiten a la Naturaleza revelarnos varias novedosas y disruptivas propiedades de la materia, deliberadamente manipulables cuando se trabaja en condiciones fuera del equilibrio termodinámico. Por último, al optimizar las condiciones de síntesis, se obtiene un notable incremento en la eficiencia absoluta de conversión energética en celdas solares a película delgada basadas en kesterita mayores al 10%, esto sin depositar recubrimiento anti-reflectante (ARC), ni rejilla metálica alguna.