Aleksandra Nikolaeva1 Maximilian Krause1 Wolfram Witte2 Dimitrios Hariskos2 Daniel Abou-Ras1

1, Helmholtz-Zentrum Berlin, Berlin, , Germany
2, Zentrum für Sonnenenergie- und Wasserstoff-Forschung Baden-Württemberg (ZSW), Stuttgart, , Germany

Light-induced enhancement of the performance as a result of an increase of the open-circuit voltage and fill factor is widely observed in Cu(In,Ga)Se2 (CIGS) solar cells with Zn-compound buffer layers [1,2]. However, the reason for the improved efficiency is still under discussion. The present study was conducted in order to investigate the microscopic effects of light-soaking (LS) in Cu(In,Ga)Se2 solar cells with solution-grown CdS and Zn(O,S) buffer layers. Electron-beam induced current (EBIC) measurements were carried out before and after the LS under AM 1.5G illumination, as well as after keeping samples in the dark. In addition, cathodoluminescence and energy-dispersive X-ray spectrometry analyses were performed in order to gain insight into possible effects on the local radiative recombination and the elemental distribution. By evaluating EBIC profiles extracted perpendicular to the p-n junction, values for the widths of the space-charge regions, which are (roughly) proportional to NA-0.5 [3], and for the diffusion lengths of minority charge carriers were determined [4]. It is revealed that for solar cells with both, CdS/CIGS and Zn(O,S)/CIGS heterojunctions, the diffusion length increases significantly already after 30 min LS treatment. The doping density slightly increases during the LS process. In addition, the influence of electron-beam irradiation and heat-light soaking was investigated.

1. T. Kobayashi, H. Yamaguchi, T. Nakada, “Effects of combined heat and light-soaking on device performance of Cu(In,Ga)Se2 solar cells with ZnS(O,OH) buffer layer”, Prog. Photovolt.: Res. Appl. 22, pp. 115-121 (2014).
2. W. Witte, D. Hariskos, M. Powalla, “Comparison of charge distributions in CIGS thin-film solar cells with ZnS/(Zn,Mg)O and CdS/i-ZnO buffers”, Thin Solid Films 519, pp.7549-7551 (2011).
3. R. Scheer and H.-W. Schock, “Chalcogenide Photovoltaics: Physics, Technologies, and Thin Film Devices” (Wiley-VCH, Weinheim, 2011).
4. C. Donolato, “Evaluation of diffusion lengths and surface recombination velocities from electron beam induced current scans”, Appl. Phys. Lett. 43(1), pp. 120-122 (1989).