Advanced strategies for high efficiency kesterite thin film solar cells

Abstract

The main objective of this thesis was to develop high efficiency thin film solar cells based on low-toxic and earth-abundant kesterite Cu2ZnSnSe4 (CZTSe) absorbers through the implementation of innovative doping strategies. Special focus is put on the optimization of reactive thermal processes, followed by the screening of possible doping elements and further analysis of the most promising ones. Additionally, deeper investigations were carried out in order to improve understanding of main loss mechanisms that can degrade device performance and the use of small amounts of Ge to mitigate some of them, including the possible interactions with alkali elements and the effect of post-deposition annealing treatments on devices properties. Most of the results obtained in this thesis have been published as articles in high impact peer-reviewed journals, included in text. In the first part of the thesis, previous study and optimization of the thermal processes were presented, identifying and varying the most critical parameters in a conventional tubular furnace selenization, in order to establish the best performing treatment for our particular precursors and set-up. Following this, after a preliminary screening of possible doping elements in the CZTSe system (including Ag, In, Si, Ge and Pb), In and Ge were both selected as the most promising/interesting ones to further analyze their doping properties. Although In doping did not show any performance improvement, it was demonstrated that CZTSe absorbers can tolerate rather high quantities of this element without significant modifications of their properties, confirming the possibility of using In-containing layers in kesterite CZTSe-based devices. On the other hand, with regard to Ge, a remarkable improvement of solar cells performance (from about 7% efficiency for Ge-free reference samples to more than 10% efficiency for Ge-doped ones) was presented, based on the introduction of nanometric Ge layers into the metallic stack precursors. Several reasons were proposed to explain the great efficiency improvement in spite of the observed Ge loss. In the following optimization, we determined the optimum Ge thickness range, achieving a 10.6% efficiency and open-circuit voltage values around 490 mV for pure selenide CZTSe, leading to voltage deficits around 0.56 V, which are among the best values reported for kesterites. Furthermore, a detailed microstructural analysis of high efficiency Ge-doped CZTSe solar cells was presented, revealing the presence of two distinct types of grain boundaries: one type is meandering in nature and grows largely parallel to the substrate, and denotes the boundary between two CZTSe layers with differing Cu/Zn ratios; and the second type is Cu-enriched and more straight, and predominates in the upper layer. After that, a new approach for obtaining high quality CZTSe layer was presented, by introducing extremely thin Ge nanolayers below and above metallic stack precursors. This strategy led to eliminate the previously characterized meandering horizontal grain boundaries and to obtain huge CZTSe grains. In addition, a deep study of the effect of Ge on the selenization process revealed that Ge strongly affects the in-depth elemental distribution and the phases formation, ultimately, modifying the reaction pathways of CZTSe. Through the optimization of the quantity and location of Ge, a record efficiency of 11.8% was achieved. In the last part of the thesis, the complex Ge-Na interaction was investigated, and a detailed analysis of low temperature post-deposition annealings by multi-wavelength Raman spectroscopy was also presented. On the whole, the work presented in this thesis provides meaningful results and innovative strategies to boost the efficiency of kesterite solar cells, by tackling some limiting factors of this promising material.

El objetivo principal de esta tesis es el desarrollo de células solares de capa fina de alta eficiencia basadas en absorbedores compuestos de elementos de baja toxicidad y abundantes en la corteza terrestre (kesterita, Cu2ZnSnSe4 (CZTSe)), mediante la implementación de estrategias innovadoras de dopaje. En particular, se ha desarrollado un método secuencial basado en el depósito por sputtering de capas metálicas seguido por un proceso térmico reactivo. A través de la optimización de los procesos y la implementación y análisis de diferentes elementos dopantes, se han investigado los mecanismos de pérdida de eficiencia más relevantes en kesteritas, contribuyendo a desarrollar soluciones alternativas. Los resultados obtenidos han sido publicados como artículos en revistas internacionales de alto factor de impacto. En la primera parte de la Tesis, se han estudiado profundamente los procesos térmicos reactivos, variando los parámetros más críticos para adaptarlos a las características particulares de los precursores metálicos. Seguidamente, se ha realizado un análisis de diferentes elementos dopantes (Ag, Si, Ge, Pb e In); In y Ge se han seleccionado como los más prometedores. De estos, Ge ha mostrado excelentes propiedades como dopante, incrementando la eficiencia de los dispositivos desde 7% hasta más de 10%, mediante la introducción de capas nanométricas (10 nm aprox.) en el precursor metálico. A través de una optimización profunda del proceso de dopado con Ge, se ha obtenido una eficiencia de conversión máxima de 11.8% y un déficit de voltaje de alrededor de 0.56 V, que representa uno de los mejores valores reportados para esta tecnología. Estos progresos se han acompañado de una profunda caracterización micro-estructural, que ha facilitado la identificación de importantes características de las kesteritas, como por ejemplo la presencia de dos tipos diferentes de fronteras de grano con distinta composición. Adicionalmente, Ge induce una modificación en los mecanismos de formación de kesteritas, que ha sido clave para mejorar las propiedades de los dispositivos fotovoltaicos basados en estas tecnologías. En resumen, los resultados obtenidos en la presente Tesis han servido para comprender, implementar y demostrar soluciones innovadoras para conseguir avances significativos en el desarrollo de tecnologías fotovoltaicas sostenibles basadas en kesteritas.