Characterization of Interface Between Accurately Controlled Cu-Deficient Layer and Cu(In,Ga)Se2 Absorber for Cu(In,Ga)Se2 Solar Cells

We recently succeeded in controlling a Cu-deficient Cu(In,Ga)Se-2 layer (CDL) on a Cu(In,Ga)Se-2 (CIGS) surface by introducing an Se irradiation after the completion of the second stage in a three-stage process during CIGS growth. The CDL on the surface causes the formation of a valence band offset (E-V) between the CDL and CIGS because the Cu vacancies decrease the valence band maximum of the CDL. Therefore, we can expect the suppression of recombination at the CdS/CIGS interface in CIGS solar cells due to the repelling of holes by E-V. The amount of knowledge regarding the properties of CDL/CIGS interfaces is observed to be quite small because a control technique for CDL has not been developed so far. In this study, the compositional and structural properties of an accurately controlled CDL/CIGS interface are investigated in detail. The composition of the interface between the CDL and CIGS is observed using an energy dispersive X-ray spectroscopy with the help of a transmission electron microscope. Using nanobeam electron diffraction and Fourier transfer mapping analysis, it is confirmed that the (112) plane in the CDL continuously grows on the (112) plane in CIGS. Further, these results indicate that a high-quality interface is formed between the CDL and CIGS, which contains only a small amount of dangling bonds. Finally, a high conversion efficiency of 19.4% is achieved in the CIGS solar cell, which can be attributed to the formation of CDL and effect of Delta E-V.