CuGaSe2–CuGa3Se5 phase transition in CCSVT-grown thin films

Abstract

The system CuGaSe 2 -CuGa 3 Se 5 in thin films has been investigated. Layer synthesis was carried out by chemical close-spaced vapour transport (CCSVT) using Cu precursors on Mo/soda-lime glass substrates. The extension of deposition times in a two-step process led to final film compositions with [Ga]/[Cu] ratios ranging from 1 to 3, allowing the study of the phase transition mentioned above. Films showing chalcopyrite (1:1:2), OVC (1:3:5) and twophase were grown. X-ray emission spectroscopy and x-ray diffraction (XRD) techniques have been combined for a compositional and structural study of this material system probing both bulk and near surface properties of the films. This analysis was also extended to the rearsurface investigation of selected two-phase thin films and complemented with surface sensitive photoelectron spectroscopy (PES). From these results a growth model is presented for CuGa 3 Se 5 formation in gallium-rich, CCSVT-grown CuGa x Se y -films.

Synchrotron-based spectroscopy for the characterization of surfaces and interfaces in chalcopyrite thin-film solar cells

“…In contrast, the intensity of the measured front-surface spectrum clearly exceeds that of the computed one. This strengthens the assumption of a bi-layer structure formation, which was further supported by X-ray diffraction measurements [21]. In order to investigate in more detail whether in CGSe thin films relevant for solar cell devices, i.e.…”

“…The transition region between these two phases moves towards the rear side with increasing gallium content, until the complete layer shows a 1:3:5 structure for a [Ga]/[Cu] ratio of 3.0 [21]. For slightly Cu-rich samples (bulk [Ga]/[Cu] ratio¼0.93) the chemical surface structure agrees with a 1:1:2 composition.…”

“…Hence, while XES probes the near-surface bulk, XPS probes the surface structure of a sample. The investigated CuGa x Se y (''CGSe'') thin films were grown by chemical closed-space vapor transport (CCSVT, [20]) in a compositional range of 1oxo3 (see [20,21] and the corresponding contribution in this issue for more details).…”

“…The bulk [Ga]/[Cu] composition of the analyzed CGSe films was determined by X-ray fluorescence analysis (XRF) [21]. The corresponding Cu L 3 and Ga L 3 XES spectra are shown in Fig.…”

“…As expected, one can observe that the Ga L 3 emission intensity increases with a higher bulk [Ga]/[Cu] ratio. In order to test for compositional depth gradients in the CGSe samples, XES spectra were additionally recorded from the back surface after lift-off (for more details about the lift-off process see [21][22][23][24][25]). The spectra (again normalized to the Cu L 3 line intensity) are compared to the front-surface spectra in Fig.…”