Element redistribution and defect formation at the CdS/CIGS interface

Cu(In1−x, Gax)Se2 (CIGS) based solar cells are one of the most promising thin-film technologies for large scale applications. Despite their high energy conversion efficiency of 22.6 % the theoretically predicted limit of ~ 30 % is far beyond achievement, leaving a large field for further device development. One of the challenges is the ability to control the CdS/CIGS interface composition and its electrical behaviour. Unfortunately, commonly employed characterization techniques such as secondary ion mass spectrometry (SIMS) or x-ray photoelectron spectroscopy (XPS) can only provide an average composition, but cannot reveal the true complexity of the interface and its role for carrier transport. In this thesis atom probe tomography (APT) is used to determine for the first time the concentration profile across the CdS/CIGS interface with sub-nanometer spatial resolution. For the APT mass spectrum, a special analysis technique has been developed in order to disentangle overlapping signals that belong to different elements, which provided high accuracy concentration profiles across the CdS/CIGS interface and inside the CdS and CIGS phases. The interfacial composition is correlated with the electrical properties of the device as obtained from external quantum efficiency (EQE) and temperature dependent current density-voltage(JV (T)) measurements. The measurements were performed on a set of solar cells from an annealing series in order to investigate subsequent steps of element redistribution and its influence on the cell characteristics. All cells were deposited in the same production process and annealed after deposition for 30 minutes but at different temperatures, namely 150 °C, 200 °C, 250 °C, and 300 °C. The electrical characterizations reveal change of the dominant recombination mechanism from space charge region recombination to interface recombination taking place between 200 °C and 300 °C of annealing. In addition the energy conversion efficiency and the fill factor systematically dropped, with increasing temperature of annealing. APT measurement demonstrated that the CdS/CIGS interface is atomistically rough - in addition to the geometric roughness well known from transmission electron microscopy - forming an interface band (2 nm to 6.5 nm wide) within which the composition changes gradually from CdS to CIGS. Regardless of applied heat treatment the APT reveal local composition fluctuates both within the CdS/CIGS interface band and inside the CdS and CIGS bulks. Beyond the local variation, the concentrations show a general trend towards Cu and Ga depletion at the CIGS side of the interface zone as well as enhanced Cd concentration on the CIGS side of the interface band for low Ga/(Ga+In) ratio. This finding indicated profound effect of V_{Ga} on Cd interdiffusion process inside the CIGS phase. On the other hand the long-range concentration profiles showed an increasing Cd concentration inside the CIGS bulk with increasing the temperature of annealing. The Cd diffusion profiles exhibit an unusual intermediate shoulder which is interpreted as change of sublattice site. In addition for elevated temperatures of annealing an increased Cu concentration was found in the near interface region that was associated with V_{Cu} mediated deep penetration of Cd inside the p-type CIGS bulk. The increased Cu concentration is expected to influence theband alignment in the near interface region by upward shift of the CIGS valence band edge, increasing recombination probability in this region. This model agrees well with the recombination mechanism observed in JV (T) measurements