Alternative Substrates for Sustainable and Earth-abundant Thin Film Photovoltaics

Abstract

The development of sustainable energy sources with a high energy return on energy investment (EROI) that can substitute fossil fuels is a must in order to avoid the collapse of our current civilization. In this context, this work explores the feasibility of fabricating efficient Cu2ZnSn(Sx,Se1-x)4 (kesterite) solar cells on three strategic substrates: polyimide, ceramic and SnO2:F-coated soda-lime glass (SLG/FTO). These substrates present several advantages with respect to the standard SLG/Mo. Polyimide is compatible with roll-to-toll production processes and easily integrable in many applications thanks to its light weight and flexibility, ceramics have a direct application in building-integrated photovoltaics as solar tiles and the transparency of SLG/FTO enables advanced photovoltaic concepts like bifacial and tandem solar cells as well as the fabrication of semi-transparent devices. Their combination with a sustainable thin film photovoltaic technology based on Earth-abundant materials like kesterites has the potential of decreasing the energy fabrication cost and, thus, of increasing the EROI of photovoltaics through: 1) high throughput production, 2) integration and 3) advanced applications and functionality. However, these substrates also present several drawbacks. Alkalis, especially Na, are fundamental to achieve high efficiency devices but polyimide and ceramics are alkali-free materials. Likewise, FTO acts as a barrier for alkalis. In addition, polyimide presents a low thermal robustness that limits process temperatures below 500ᵒC, ceramics are very rough and possess detrimental impurities and the use of FTO as back contact leads to a non-optimum p-kesterite/n-FTO interface. This work focuses on the implementation of specific strategies to adapt the kesterite solar cell fabrication process to the characteristics of the different substrates. A combination of alkaline doping and low-temperature annealings is studied for the fabrication of Cu2ZnSnSe4 solar cells on polyimide. While doping with NaF and KF is found to lead to critical improvements, working at low temperature is linked to the formation of SnSe2. This phase decreases the open-circuit voltage of the devices and is the main factor controlling their performance. Further experimentation leads to a 4.9% efficiency record device by combining NaF and Ge doping and a 480ᵒC annealing. In the case of ceramic, vitreous enamels with controlled amounts of Na2O in their composition are used as surface smoothers, Na sources and impurity barriers, simultaneously. However, large amounts of Na2O in the enamel composition result in high densities of surface defects: undulations, pinholes and cracks. While undulations and pinholes are rather benign, cracks strongly deteriorate the back contact. In addition, the annealing time needs to be controlled to avoid the formation of SnSe2. Besides these issues, enamelled ceramic substrates are observed to behave similarly to SLG yielding a record Cu2ZnSnSe4 device with a 7.5% efficiency. Regarding SLG/FTO, the addition of transition metal oxides (TMOs) and/or Mo:Na nanolayers is studied as an approach to improve the back interface of the devices. Mo:Na is found to alleviate shunting and recombination issues and to protect FTO from degradation during annealing which leads to highly improved devices, especially for Cu2ZnSnS4. On the other hand, TMOs introduce a severe current blockage. However, the combination of the TiO2 and V2O5 with Mo:Na is observed to boost the beneficial effects of the latter in Cu2ZnSnSe4 and Cu2ZnSn(S,Se)4 devices. Although these multi-layered back interfaces exhibit a complex behaviour, this approach results in record efficiencies of 6.2%, 6.1% and 7.9% for Cu2ZnSnSe4, Cu2ZnSnS4 and Cu2ZnSn(S,Se)4 devices, respectively. These results represent the highest efficiencies ever reported for kesterite solar cells fabricated on polyimide, ceramic and transparent substrates and give proof of their large potential for sustainable kesterite-based photovoltaics.

Esta tesis estudia el desarrollo de células solares de Cu2ZnSn(Sx,Se1-x)4 (kesterita) sobre sustratos de poliimida, cerámica y vidrio recubierto con SnO2:F (SLG/ITO). Estos sustratos presentan una serie de ventajas frente al estándar SLG/Mo como su compatibilidad con procesos de fabricación rollo a rollo (poliimida), con fotovoltaica integrada en edificios (todos) y/o la posibilidad de aplicaciones y funcionalidades avanzadas (poliimida y SLG/FTO). Sin embargo, también poseen características que dificultan la fabricación de células solares de kesterita como la falta de elementos alcalinos (todos), su baja resistencia térmica (poliimida), su alta rugosidad (cerámica) o su conductividad tipo n (SLG/FTO). Este trabajo se centra en la implementación de estrategias para la fabricación de células solares de kesterita de alta eficiencia atendiendo a las características de cada sustrato. En el caso de la poliimida, se utiliza una combinación de dopaje alcalino y procesos de recocido a baja temperatura la cual permite obtener un dispositivo Cu2ZnSnSe4 record con una eficiencia del 4.9% combinando dopaje con NaF y Ge y una temperatura de 480ᵒC. Los sustratos cerámicos se recubren con un esmalte vítreo que contiene una cantidad controlada de Na2O en su composición y que actúa como reductor de la rugosidad superficial, fuente de Na y barrera para impurezas. A pesar de que altas concentraciones de Na2O provocan defectos superficiales, los sustratos cerámicos esmaltados presentan un comportamiento muy similar al vidrio con un dispositivo Cu2ZnSnSe4 record de 7.5% de eficiencia. En cuanto a los sustratos SLG/FTO, se estudia el depósito de nanocapas de óxidos de metales de transición (TMOs) y de Mo:Na para mejorar la interficie trasera kesterita-p/FTO-n de los dispositivos. Los TMOs inducen una fuerte barrera eléctrica mientras el Mo:Na se muestra fundamental para la fabricación de dispositivos eficientes. Sin embargo, el uso conjunto de Mo:Na y TiO2 o V2O5 amplifica los efectos beneficiosos del Mo:Na lo que permite obtener un dispositivo Cu2ZnSn(Sx,Se1-x)4 record con una eficiencia de 7.9%. Estas son las eficiencias más altas reportadas para células solares de kesterita sobre poliimida, cerámica y SLG/FTO y demuestran el potencial de estos sustratos para el desarrollo de una fotovoltaica sostenible basada en kesterita.