Nano-Characterization of Interface Defects in Chalcopyrite Thin Film Solar Cells

Basic data for this project

Description

Since 2013, chalcopyrite thin film solar cells on the basis of Cu(In,Ga)Se2 (CIGS) have shown remarkable efficiency gains. With the latest world record of 22.6%, these solar cells even outperform the ones from conventional silicon photovoltaics. Nevertheless, CIGS solar cells are by far not as well understood as the latter. In fact, many open questions are related to the complex defect physics of the chacopyrite absorber material. Due to increased recombination losses, the properties of the interface between the p-type absorber and the n-type window layer are of particular concern. This is emphasized by the recent efficiency gains which were accomplished by introducing alkali metals at this interface where the beneficial effects are still largely unclear. Therefore, a further knowledge based optimization of these solar cells requires a profound characterization of electronic defect levels at this interface. Usually, electronic defect levels in semiconductors are investigated by integral spectroscopic methods, which do not allow to assign spectral signatures to the volume material or an interface in a straight forward way. Furthermore, polycrystalline chalcopyrite materials show pronounced lateral inhomogeneities impeding such analyses. As shown by preceding work, experiments by scanning tunneling spectroscopy provide a powerful alternative approach. This method allowed to achieve a comprehensive knowledge about spectral defect distributions and lateral band bending effects on intrinsic chalcopyrite surfaces with a lateral resolution in the nanometer regime. These results serve now as the basis for the project focussing on systematic investigations of the formation of the absorber/window interface. Hereby, the scanning tunneling spectroscopy experiments are complemented by local contact potential- and photovoltage measurements based on non-contact atomic force microscopy. With this methodology it is possible to correlate the local defect level density with the local photovoltaic properties. Moreover, the corresponding global compositional and electronic properties are analyzed by photoelectron spectroscopy. Initially, one of the major goals of the project is to understand the beneficial effects of the alkali post-deposition treatments. In the further course, effects due to the deposition of the window layer, which is mediated by a thin (30-50 nm) CdS layer, are investigated. The proposed experiments will give important insights to the locally resolved interface properties of CIGS thin film solar cells and will provide feedback to improve the technological relevant growth and deposition processes of the involved materials.