Investigation of the chemistry of structural defects in Cu(In,Ga)Se₂ solar cell material

Cu(In,Ga)Se₂ (CIGSe) based solar cells fabricated with multi-stage co-evaporation show power-conversion efficiencies of more than 20%. However, in some cases efficiencies fall below this value, in particular when the solar cell is manufactured using low-temperature processing without a Cu-rich stage. The reasons for this efficiency loss, as well as the limitations for further efficiency increase, are not fully understood. In this project, we analyse structural defects in CIGSe absorber layers to gain a better understanding of the defects’ chemical characteristics, which are known to affect the photovoltaic performance of the material. We primarily use electron energy loss spectroscopy (EELS) combined with high-resolution scanning transmission electron microscopy (HR-STEM).

Initial results show interesting chemical characteristics for a number of observed defects in Cu-poor CIGSe thin films. The chemistry of twin boundaries, which are very frequent in these samples, is identical to that in the grain interiors, with a homogeneous distribution of all the elements. By contrast, Cu enrichment in combination with In depletion was observed within complex defects closely related to stacking faults, as well as at grain boundaries. Finally, Cu_xSe-rich channels seem to form immediately outside (not within) dislocation cores, suggesting these defects play a role in the photovoltaic properties of the material.

This project is part of the Network Project: Virtual Institute with Helmholtz-Zentrum Berlin für Materialien und Energie, Institute of Technology "Mikrostrukturkontrolle in Dünnschichtsolarzellen".