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

• Applicant: Privatdozent Dr. Harry Mönig
• Subject Area: Synthesis and Properties of Functional Materials
• Term: since 2017
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 387562434

In recent years, chalcopyrite thin film solar cells on the basis of Cu(In,Ga)Se2 (CIGSe) and Cu(In,Ga)(Se,S)2 (CIGSSe) have shown remarkable efficiency gains. With the latest world record of nearly 23.4%, these solar cells even outperform the ones from conventional silicon photovoltaics. Nevertheless, these 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 chalcopyrite absorber material. Due to its sensitivity for recombination losses, the properties of the interface between the p-type absorber and the n-type window layer, which is mediated by a thin buffer 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 manner. Furthermore, polycrystalline chalcopyrite materials show pronounced lateral inhomogeneities impeding such analyses. As shown by the results of the previous funding period, 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 the CIGSe absorber system after various (e.g. heat- and wet-chemical) treatments relevant for the device fabrication. These results serve now as basis for the project extension focusing on the more complex mixed selenide/sulphide absorber system CIGSSe, which has been used for the most recent record solar cells. Hereby, the main analytical tool of scanning tunneling spectroscopy is combined with photoelectron spectroscopy techniques, which, due to very similar information depths, provide strongly correlated results. The major goal of the project is now to understand the beneficial effects of the alkali post-deposition treatments in conjunction with Zn(O,S) buffer layers. Such buffer layers have become of crucial importance to mediate the p/n-junction formation and avoid interface recombination losses in technologically relevant CIGSSe-based devices. The proposed experiments will give important insights to the local interface properties of the solar cells and will provide valuable feedback to improve the growth and deposition processes of the involved materials.

• DFG Programme: Research Grants