Role of Nanoscale Inhomogeneities in Cu(In,Ga)Se₂ Thin Film Solar Cells

As a consequence of the growing global energy consumption and broader awareness about environmental pollution, the interest in photovoltaic (PV) has been steadily increasing over the last few years. Amongst different PV technologies, thin film solar cells based on Cu(In, Ga)Se2 (CIGS) absorbers have demonstrated high conversion efficiencies of 22.3% on rigid glass substrates and of 20.4% on flexible polyimide (PI) foils. Therefore, CIGS thin film solar cells manifest a promising potential for the development of highly efficient, flexible, low-cost solar cells. Even though such high efficiencies have been reached, many fundamental properties governing the performance are not yet clearly understood. The interfacial regions and also the bulk of the polycrystalline CIGS absorber layer are complex and inhomogeneous materials whose quality crucially affects the solar cell’s efficiency. Next to grain boundaries, which represent the most common structural defects in CIGS, also dislocations, stacking faults and point defects are present in bulk CIGS. In general, such structural defects are accompanied by chemical inhomogeneities: an enrichment or depletion of Cu, In, Ga or Se as well as the additional presence of the impurity atoms Na, K and O is typically observed. The controlled introduction of an anticorrelated Ga and In gradient across the layer thickness and a slight variation of the Cu content allows for tuning the electronic band alignment in a beneficial way. However, structural and chemical inhomogeneities that occur within shorter length scales are provoking fluctuations of the band structure, too. The electronic inhomogeneities can be divided into two main groups: The spatially independent fluctuations of the valence and conduction band first and secondly their parallel fluctuations. The spatially independent fluctuations lead to local fluctuations of the band gap energy. Variations of the Ga concentration and the Cu concentration represent possible sources for the generation of such band gap fluctuations. Parallel fluctuations of the band edges are induced by doping atoms, impurities or charge accumulations which exist at microstructural defects for instance. Even though the band gap energy is not affected by the second group of electronic inhomogeneities, these impact the charge carrier transport in CIGS anyway. It is commonly believed that the presence of both types of electronic inhomogeneities impair the solar cell’s performance and may even represent an efficiency limiting characteristic. This thesis addresses the impact of chemical and structural inhomogeneities on the local electronic properties in CIGS by means of advanced transmission electron microscopy (TEM) methods. The research focuses specifically on the two advanced analytic TEM techniques valence electron energy-loss spectroscopy (VEELS) and electron holography. These exhibit a promising potential for characterizing the electronic properties of a material on nanoscale. VEELS provides access to the dielectric function of the probed material and thus allows for measuring the band gap energy at high spatial resolutions. Electron holography reveals information about electrostatic potential fluctuations of a material at high spatial resolution, by means of accessing the phase shift of the electron wave. However,