Raman scattering based strategies for assessment of advanced chalcopyrite photovoltaic technologies: Characterisation of electrodeposited Cu(In,Ga)(S,Se)2 solar cells

Abstract

The main objective of this thesis is the development of Raman scattering based methodologies for the analysis of advanced electrodeposition-based CIGS technologies, with the identification and characterization of parameters relevant for the efficiency of the solar cells and modules that can be used for quality control and process monitoring applications. The work aims to propose methodologies and tools that can be implemented for the monitoring of the processes at on-line level, contributing to increase the yield and reliability of the processes involved in the fabrication of these devices. The thesis is structured around five papers that have been published in peer-reviewed journals, according to the requirements for the achievement of the degree of Doctor in the Doctoral Program of Engineering and Advanced Technologies in the University of Barcelona.

The thesis is structured in seven chapters. The first chapter is an introduction into Chalcopyrite photovoltaic technologies, including their background, current production strategies and optical characterization by Raman scattering based techniques. The main processes used for the fabrication of the CIGS solar cells and modules are described. Later in the introduction Raman scattering is presented as the main technique used in this work and the approach of this thesis to develop process monitoring techniques to assess parameters for each of the layers in the devices is described. Raman spectra are sensitive to chemico-physical and structural parameters of the layers that determine the device efficiency, as the crystalline quality and presence of defects, the chemical composition, as well as stress and strain effects and presence of secondary phases. The second chapter is dedicated to the disclosure of the Raman experimental set-ups that have been developed and that have been used to obtain the data presented in this work and the experimental conditions chosen to ensure reliability in the measurements. A more detailed description of the application of Raman spectroscopy are disclosed in the following chapters, that address the detection of secondary phases in the absorber layers that are relevant for device performance in high efficiency devices (Chapter 3), the chemical characterization of the surface region of the absorbers (Chapter 4), the assessment of the thickness of the CdS buffer layers (Chapter 5) and the electrical conductivity of the window layers (Chapter 6).

The secondary phases studied in Chapter 2 are the OVC ones in Cu(In,Ga)Se2 alloys and CuAu polytypes in CuInS2 based cells; determining and clarifying their impact on the optoelectronic characteristics of the cells. This work reports for the first time in the literature clear experimental evidences of the impact of the presence of the OVC phases on the optoelectronic characteristics of the cells. Although the efficiencies achieved within CuInS2 (CIS) solar cells are lower (12.7% ) than the yielded by CIGS devices, the larger bandgap of the CIS semiconductor (1.55 eV) gives interest to these devices for the increase of the open circuit voltage Voc. In this work, the role of the presence of CuAu polytypic domains in advanced cells made by electrochemical processes is investigated. The fourth chapter addresses the development of methodologies for the quantitative analysis of the chemical composition of the surface region of the absorber layers, including the Ga/(In+Ga) relative content in Cu(In,Ga)Se2 absorbers and the S/(S+Se) relative content in Cu(In,Ga)(S,Se)2 absorbers. These are the parameters that allow suitable control of the value of the band gap in the surface region of the absorbers. Next chapters address the Raman scattering assessment of the CdS buffer and Al-doped ZnO window layers, being the control of the thickness of the buffer and conductivity of the window layer are relevant for the device efficiency. The final chapter summarizes the main conclusions

El principal objetivo de esta tesis es el desarrollo de metodologías basadas en la espectroscopia Raman para el análisis de tecnologías fotovoltaicas Cu(In,Ga)(S,Se)2 avanzadas basadas en procesos electroquímicos, con la identificación y caracterización de parámetros relevantes para la eficiencia de las celdas solares y módulos. El trabajo desarrolla y propone metodologías y herramientas que pueden ser implementadas para aplicaciones de control de calidad y monitorización de procesos a nivel "on-line", contribuyendo a incrementar el rendimiento y la fiabilidad de los procesos involucrados en la fabricación de estos dispositivos.

La tesis está estructurada en siete capítulos. El primer capítulo es una introducción a las tecnologías fotovoltaicas de la calcopirita, incluyendo las estrategias actuales de producción y su caracterización óptica por medio de técnicas basadas en espectroscopia Raman; más tarde en el capítulo se presenta la espectroscopia Raman como la técnica principal utilizada en el trabajo y se describe el enfoque abordado en la tesis para el desarrollo de técnicas para la monitorización de los procesos. Los espectros Raman permiten obtener información de parámetros estructurales y químico-físicos de las diferentes capas en la estructura que determinan la eficiencia del dispositivo, como la calidad cristalina y presencia de defectos, composición química, estrés y presencia de fases secundarias. El Segundo capítulo describe los sistemas experimentales que se han desarrollado en el trabajo, y las condiciones experimentales determinadas para garantizar la fiabilidad de las medidas. Una descripción más detallada de la aplicación de la espectroscopia Raman se describe en los siguientes capítulos, que abordan la detección de fases secundarias en la capa del absorbedor que son relevantes para la eficiencia de la celda en dispositivos de alta eficiencia (Capítulo 3), la caracterización química de la región superficial de las capas absorbedoras (Capítulo 4), el espesor de las capa buffer de CdS (capítulo 5) y la conductividad eléctrica de las capas ventana (capítulo 6). El último capítulo de la tesis resume las principales conclusiones del trabajo.