J. Seeger, J. Grutke, N. Weber, S. Schützhoff, X. Jin, R. Schneider, D. Gerthsen, W. Witte, D. Hariskos, O. Kiowski, M. Schweiger, H. Kalt, and M. Hetterich

Appl. Phys. Lett. 115, 263901 (2019)

Currently, the use of Zn(O,S) as buffer material for Cu(In,Ga)Se₂ (CIGS) solar cells is intensely studied in order to further boost the performance of these devices. In this context, nondestructive analytical tools are needed that enable the determination of buffer bandgap energies in the complete device. To this end, we developed a spectroscopic approach based on electroreflectance (ER). From a set of measured angle-resolved ER (ARER) spectra, an averaged modulus spectrum is numerically calculated. This method suppresses the commonly observed detrimental line-shape distortions due to interference effects in the layered device structure and thus enables the determination of bandgap energies even for thin buffer layers. To verify the working principle of ARER, we first apply it to CIGS absorber and CdS buffer layers. Then, we utilize it to investigate CIGS solar cells with Zn(O,S) buffers. All ARER results are compared to the results of diffuse ER, a technique previously developed for the suppression of interference fringes. We demonstrate that ARER is the superior ER method for nondestructive bandgap determination of thin buffer layers in complete CIGS solar cells. Moreover, a Cu containing compound was determined as a secondary phase in the Zn(O,S) buffer by combined ARER studies, scanning transmission electron microscopy, and energy-dispersive X-ray spectroscopy.