Advanced Characterization of Interfaces of the Chalcogenide-Based Absorbers for the Thin Film Photovoltaic Technologies

Abstract

[eng] The thesis Advanced Characterization of Interfaces of the Chalcogenide-Based Absorbers for the Thin Film Photovoltaic Technologies is based on six articles that are published in high impact factor peer-reviewed journals from the first quartile, and its main goal consists in the identification of the main physicochemical mechanisms that occur at the interfaces of the chalcogenide absorber layers used in the thin film solar cells and that have an influence on the device performance. It has been carried out at the Catalonia Institute for Energy Research (IREC) in the Solar Energy Materials and Systems (SEMS) research group during the period 2018-2022. The thesis is divided into four chapters: introduction, methodology, results, and conclusions. The first chapter starts with an explanation of the problems associated with fossil fuels, which illustrates the need to develop further renewable energy sources such as photovoltaics. Then, the fundamental processes that undergo in a solar cell and their main parameters are described. The different generations of solar cells are briefly presented with a main focus on chalcopyrite and kesterite-based thin film solar cells. After this, the main principle of different characterization techniques suitable for analyzing the interfaces of solar cells are explained. Finally, the first chapter is concluded with the objectives of the thesis. The second chapter presents the details about the samples preparation, techniques and characterization methodologies employed during the development of the thesis. First, a brief description of the solar cells synthesis is provided. Then, a detailed description of the equipment and the experimental conditions is offered for each characterization technique. The third chapter collects the main results obtained during the thesis preparation. A general introduction is provided to contextualized the experiments carried out, followed by a summary of the obtained results and a general discussion. Then, the six scientific articles published in first quartile journals are provided. The first article presents a detailed analysis of a high efficiency Cu2ZnSnSe4 device using a combination of advanced electron microscopy and spectroscopy techniques, so a full picture of the different defects present at the interfaces of the device and in the bulk of the absorber is achieved. The second article describes a method to probe the depth morphology, defect profile, and possible secondary phases in Cu2ZnSnSe4 absorbers based on a controlled Methanol-Br2 chemical etching and the employment of different characterization techniques. The third article focuses on an innovative approach promoting the formation at the rear interface of wide-bandgap CuGaSe2, which acts as an efficient electron reflector and functions as

an effective interlayer improving the kesterite crystallinity at the back interface. The fourth article presents a simple and reliable technique that allows to achieve anionic compositional profiles and gradients for Cu2ZnSn(S1-xSex)4 and Cu2ZnGe(S1-xSex)4 kesterite absorbers. The fifth article performs a high statistics analysis on high efficiency CuIn1-xGaxSe2 solar cells submitted to different RbF post-deposition treatments conditions, revealing a redistribution of defects between the ordered vacancy compound and the chalcopyrite phases at the absorber surface. Additionally, industry-compatible methodologies for the assessment of the RbF process and prediction of the open-circuit voltage of the final devices are developed. The sixth article presents a detailed study of the stability of different indium tin oxide-based back contacts under the co-evaporation processes of CuIn1-xGaxSe2 solar cells, showing that the application of functional layers on the surface of indium tin oxide is required to avoid a degradation of the optical transparency and of the device performance at high co-evaporation process temperatures. The fourth and final chapter of the thesis presents the conclusions and outlook of the work.

[cat] La tesi “Advanced Characterization of Interfaces of the Chalcogenide-Based Absorbers for the Thin Film Photovoltaic Technologies” està basada en sis articles publicats en revistes d’alt factor d’impacte del primer quartil i el seu principal objectiu consisteix a identificar els mecanismes fisicoquímics que ocorren a les interfícies de les capes absorbidores calcogenures en cèl·lules solars de capa prima i que influeixen al rendiment del dispositiu. Aquesta tesi es divideix en quatre capítols: introducció, metodologia, resultats i conclusions.

El primer capítol explica els problemes associats als combustibles fòssils i demostra la necessitat de desenvolupar fonts d'energia renovable com la fotovoltaica. Després, es descriuen els processos fonamentals de les cèl·lules solars i els seus paràmetres principals. Les diferents generacions de cèl·lules solars es presenten amb un enfocament en les de capa fina basades en calcopirites i kesterites. A continuació, s'explica el principi bàsic de diferents tècniques de caracterització adients per a l'anàlisi d'interfícies de cèl·lules solars. Finalment, es presenten els objectius de la tesi. El segon capítol presenta els detalls sobre la preparació de mostres, les tècniques i les metodologies de caracterització emprades durant el desenvolupament d'aquesta tesi. El tercer capítol recol·lecta els resultats principals mitjançant sis articles científics. El primer article presenta una anàlisi detallada d'un dispositiu Cu2ZnSnSe4 d'alta eficiència utilitzant una combinació de tècniques avançades d'espectroscòpia i de microscòpia electrònica. El segon article descriu un mètode per a analitzar en profunditat la morfologia, els defectes i les fases secundàries en absorbidors Cu2ZnSnSe4 basat en un gravat químic de metanol-Br2. Al tercer es promou la formació de CuGaSe2 a la interfície posterior, que actua com a reflector d'electrons i millora la cristal·linitat de la kesterita. El quart presenta una tècnica simple i segura que permet aconseguir perfils composicionals aniònics per a absorbidors de kesterita. El cinquè investiga l’impacte del tractament de postdeposició de RbF a la superfície de dispositius CuIn1-xGaxSe2 d'alta eficiència. El sisè estudia l'estabilitat de contactes posteriors basats en òxid d'estany i indi a diferents temperatures. El quart i últim capítol d'aquesta tesi presenta les conclusions i els pronòstics del treball.