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Cu(In,Ga)Se2 thin film solar cells are known to show a metastable behavior of their open circuit voltage when exposed to light. Time resolved measurements of the photoluminescence decay after pulsed excitation (TRPL) reveal that also the photoluminescence decay (PL) changes with conditioning, yielding shorter effective lifetimes after annealing under illumination (light soaking) than after annealing in the dark. By using time resolved device simulations we applied a model that explains the TRPL metastable behavior. For this we used a model of capture and reemission of minority charge carriers via two characteristic trap states near the conduction band, while also accounting for changes in trap density and doping concentration caused by light soaking. To further understand the mechanisms behind PL decay we analyse the influence that the charge carrier dynamics play in TRPL decay by a variation of select simulation parameters.

In this study strong evidence for an illumination-induced change in minority charge carrier diffusion length is given for Cu(In,Ga)Se 2 solar cells. After annealing under illumination (light soaking) the cells show the well-known metastable increase in open circuit voltage, but also a metastable reduction in current collection efficiency (which can be reversed by annealing in the dark). Partly, this can be attributed to an increase in doping density causing a reduced space charge region width as verified by capacitance-voltage profiling. Nevertheless, by using time-resolved photoluminescence and electron-beam-induced current measurements we found that the changes in doping density and space charge region width are not sufficient to describe the modification in current collection efficiency. Additionally there seems to be a reduction in minority carrier diffusion length and lifetime after white light soaking. This can be explained by a metastable change of electronic defects as found in temperature-dependent admittance spectroscopy. Device simulations confirm the impact of the found defects on the photocurrent.

The constitution of secondary phases in kesterite Cu2ZnSnSe4 (CZTSe) thin films is still a limiting factor for their application in solar cells. Therefore an enhanced understanding of phase formation processes during the fabrication of CZTSe films is required. In this study we present a temperature and film-depth dependent phase analysis of Zn/Sn/Cu precursors on ZnO substrates selenized at different temperatures. A special sample preparation step using a focused ion beam was applied to prepare shallow angle cross sections for depth-resolved Raman profiling of the thin films. At low selenization temperatures multiphase structures are demonstrated and a first formation of CZTSe besides secondary phases at only 250 °C is detected. At high selenization temperatures an accumulation of ZnSe at the interface of CZTSe and ZnO substrates is observed. Furthermore indications for the formation of a thin SnO2 interface layer were found by X-ray diffraction, secondary electron microscopy and energy dispersive X-ray spectrometry.

We present a study on the temperature and film depth dependent phase formation in Cu-Zn-Sn-Se thin films. Zn/Sn/Cu precursors were selenized at different temperatures, followed by depth-resolved Raman profiling. A high depth resolution for Raman analysis was achieved by a special sample preparation step using a focused ion beam to prepare shallow angle cross sections (SACS) of the absorber. Multi-phase structures were observed at low selenization temperatures with a first formation of the quaternary Cu 2 ZnSnSe 4 at only 250 °C and the existence of Sn x O y in films annealed at 330 °C. At high selenization temperatures up to 560 °C Cu 2 ZnSnSe 4 was the main phase with some traces of ZnSe and a MoSe 2 interface layer at the back contact. Furthermore, compositional gradients were investigated by scanning electron microscopy and energy dispersive X-ray spectroscopy measurements on sample cross sections. The combination of these results allows for observation of both elemental composition and phase composition of the films and their dynamics during the annealing procedure.