Development of RTP processing for kesterite solar cells

Abstract

This thesis is devoted to the development of new annealing strategies via Rapid Thermal Processing (RTP) for Earth-abundant Cu2ZnSn(Sx,Se1-x) (CZTSSe) kesterite solar cells, which show a clear consensus amongst the scientific community as one of the most suitable production techniques for a posterior industrial implementation. RTP contains several advantages, mostly summarized in: the employment of fast heating halogen lamps in common with the industry, better suited for obtaining an accurate control of the process and the obtainment of high Se vapour pressures since the very beginning of the annealing, which in this thesis have been proven to have strong impacts on the final device characteristics. This thesis is englobed into the so-called sequential processes where the precursor and the absorber are obtained in different steps. It has been developed and optimized an annealing process for the baseline established at IREC, where the precursor is synthesized via sputtering and later annealed in a furnace. The aim of the developed annealing strategy has consisted on obtaining the fastest annealing possible without compromising the final efficiency and properties of the solar cell. Thanks to the annealing procedure stablished in this thesis, it could be reached up to 8.3% efficiency in an only 12 minutes annealing process. Once an RTP annealing procedure was developed and optimized, it was of key importance to deeply understand and comprehend the insights into the process and the formation pathways to synthesize the kesterite on it. In order to do so, the process was stopped at several intermediate key steps and strongly characterized, depicting the very early formation of the kesterite and that the kesterite synthesis process, for the RTP annealing procedure at IREC, it is driven by a competition between two formation mechanisms (ruled either via ternary or binary compounds). In this thesis is stated and commented why the formation mechanism driven by the ternary compound is preferred over the one ruled via binaries. In this thesis it is also provided a study about the kinetics present on the RTP process, where the kinetic orders for the different formation mechanisms to synthesize the kesterite are analysed, complemented by a deep and extensive analyse of the phases present through the process. By the combination of the kinetic and phase analysis, it is concluded that even the less favourable formation mechanism driven by binaries compounds can be reduced to a marginal extent yet it can never be completely overcome as it is always present. This process might end into detrimental secondary phases for the devices final performing, endorsing the necessity of extra processing steps such as chemical etchings to counteract their effects. Finally, in order to study the robustness of the annealing process stablished through this thesis, the RTP annealing process has been tested onto precursors synthesized by electrodeposition, instead of sputtering as usual in IREC’s standard baseline. Electrodeposited precursors have been obtained by the co-electrodeposition approach and later submitted to the RTP annealing. Efficiencies up to 5.2% have been achieved by this combination, which are the highest reported by this sequence so far, proving the robustness of the RTP annealing procedure developed and optimized in this thesis.

Esta tesis estudia el desarrollo de nuevas estrategias de recocido por Rapid Thermal Procesing (Procesos de Recocido Rápidos) para celdas solares de kesterita, Cu2ZnSn(Sx,Se1-x) (CZTSSe), gracias a que las celdas de kesterita únicamente poseen en su estructura materiales abundantes en la corteza terrestre y además, la producción por RTP es una de las técnicas que posee mayor consenso entre la comunidad científica para una plausible aplicación industrial posterior. Para la síntesis de la kesterita, la línea desarrollada en IREC se basa en los procesos secuenciales. En ellos se obtiene el precursor por sputtering y el absorbedor mediante hornos en etapas claramente diferenciadas. En esta tesis se presenta el estudio del desarrollo y optimización de un proceso de recocido basado en dos etapas por RTP, donde se busca minimizar el tiempo de recocido, sin comprometer las propiedades de la celda solar. Con susodicho proceso, se han llegado a obtener celdas con eficiencias de hasta 8.3% con un tiempo de recocido de solamente 12 minutos. A continuación, se presenta un estudio en profundidad sobre los entresijos del proceso de recocido, ayudando a esclarecer las partes intermedias del proceso, concluyendo que el proceso de síntesis de la kesterita procede por una competición entre mecanismos: el mecanismo dirigido por compuestos ternarios y el segundo por compuestos binarios. En la presente tesis se provee además con un estudio sobre la cinética de reacción para la síntesis de la kesterita por RTP, presentando los órdenes de reacción y concluyendo mediante un exhaustivo análisis de fases que no es posible la completa erradicación de los compuestos binarios para la síntesis de la kesterita. Finalmente, para estudiar la robustez del proceso de recocido desarrollado por RTP, ha sido extendido a precursores obtenidos por otra técnica de síntesis, en este caso la técnica escogida ha sido la del electrodepósito. Mediante el uso de precursores obtenidos por co-electrodepósito y su posterior recocido por RTP, se ha demostrado la robustez del proceso desarrollado en esta tesis al obtener eficiencias de hasta el 5.2%, la más alta reportada con esta secuencia.