Wide bandgap chalcogenide thin films onto transparent substrates

Abstract

This PhD thesis explores the use of wide band gap chalcopyrite (CIGS) material and its application in thin film solar cells onto transparent back contacts, as a way to promote the use of thin film technologies in novel and advanced photovoltaic (PV) applications. A brief introduction summarizes the challenges faced by humanity regarding the reduction of carbon dioxide emissions, and describes how photovoltaic energy can help achieving the necessary energy transition within the next two decades. Among the industrially viable thin film materials used as solar absorbers, CIGS appears is the technology having demonstrated the best ratio in term of performance/processing cost, and it has reached a high degree of commercial maturity. An important asset of these materials is the possibility to tune the bandgap by substituting In by Ga or Se by S, thus offering opportunities for novel and advanced PV applications where semi-transparency is required. This work focuses on wide bandgap absorbers fabricated on transparent substrates. A theoretical part introducing the PV basics including semiconductor device physics such as thin film materials and devices processing/characterization is presented. In the next part, the results regarding the optimization of a baseline process for Ga-rich CIGS on (fluorine doped tin oxide) FTO-based substrate based on a sequential process route, including the deposition of metallic stack precursors by sputtering and thermal evaporator, and followed by reactive annealing under chalcogen atmosphere, are described. This has allowed to set-up a baseline process for Ga- rich synthesis with optimized compositional ratios and processing conditions. In particular, it has been demonstrated that the optimal copper on gallium + indium (CGI) and gallium on gallium + indium (GGI) ratios are both comprised between 0.7 and 0.8, and the highest efficiency was achieved using a temperature annealing of 550 °C, which improved the Ga incorporation and material quality of the absorbers. Several layers were also investigated as alternative buffers to CdS, In2S3 leading to high Voc devices. In a last section, the effect of the introduction of alkali atoms (Na, K, Cs, Rb) and their influence on the CIGS material quality and device performance is evaluated. First, a pre-deposition treatment strategy involving Na incorporation was designed (through NaF layers deposited before the synthesis of the CIGS), leading to absorber layers with a better Ga incorporation, and

improved device efficiencies, mainly due to an enhanced photocarriers’ collection from low energy photons, linked to an optimum back interface quality. The best results were achieved using 15 nm NaF thick layer deposited onto the metallic precursor before the thermal reactive annealing, leading to a device efficiency of 10.15% and a bandgap of 1.42eV. Finally, a study of post-deposition treatments involving K, Cs and Rb was carried out. A clear improvement in the crystallinity of the absorber and electrical device parameters was reported, especially with CsF post deposition treatment, leading to a record device efficiency of 11%.

Esta tesis doctoral explora el uso del material calcopirita (CIGS) de energía de banda prohibida ancha y su aplicación en células solares de capa delgada con contactos traseros transparentes. En una primera parte se presentan los resultados respecto la optimización de un proceso de fabricación de capas CIGS ricas en Ga sobre un sustrato basado en FTO, utilizando un proceso secuencial (deposición de precursor metálico seguido de un recocido reactivo en atmósfera de calcógeno). Esto permite establecer un proceso de base para la síntesis de capas, con relaciones de composición y condiciones de procesamiento optimizadas. Las proporciones óptimas de cobre sobre galio + indio (CGI) y galio sobre galio + indio (GGI) están comprendidas entre 0.7 y 0.8, y se logra mayor eficiencia en dispositivos utilizando una temperatura de recocido de 550 °C. También se investiga varias capas “buffers” como alternativas al CdS, In2S3 conduciendo a dispositivos con mayor Voc. En un último apartado, se evalúa el efecto de la introducción de elementos alcalinos y su influencia sobre la calidad del material y eficiencia de los dispositivos. En primer lugar, se diseña una estrategia de tratamiento pre-deposición (antes del precursor) que involucraba la incorporación de Na, que mejora tanto la incorporación de Ga como las eficiencias de los dispositivos, debido a una mejor colección de fotoportadores provenientes de fotones de baja energía, ligado a una mayor calidad de la interficie trasera. Los mejores resultados se logran utilizando una capa gruesa de 15 nm de NaF depositada sobre el contacto trasero, con una eficiencia de 10.15 % (con energía de banda prohibida 1.42 eV). Finalmente, se lleva a cabo un estudio de tratamientos post-deposición (sobre el absorbedor) con elementos alcalinos K, Cs y Rb, demostrando una mejora de la calidad cristalina en el caso de CsF, y eficiencias del 11%.