Spectroscopic investigation of the deeply buried Cu(In,Ga)(S,Se)2∕Mo interface in thin-film solar cells

Abstract

The Cu͑In, Ga͒͑S,Se͒ 2 / Mo interface in thin-film solar cells has been investigated by surface-sensitive photoelectron spectroscopy, bulk-sensitive x-ray emission spectroscopy, and atomic force microscopy. It is possible to access this deeply buried interface by using a suitable lift-off technique, which allows us to investigate the back side of the absorber layer as well as the front side of the Mo back contact. We find a layer of Mo͑S,Se͒ 2 on the surface of the Mo back contact and a copper-poor stoichiometry at the back side of the Cu͑In, Ga͒͑S,Se͒ 2 absorber. Furthermore, we observe that the Na content at the Cu͑In, Ga͒͑S,Se͒ 2 / Mo interface as well as at the inner grain boundaries in the back contact region is significantly lower than at the absorber front surface.

Characterization of the Electronic and Chemical Structure at the Thin Film Solar Cell Interfaces: June 2005 -- June 2009

“…We find strong indications for different intermixing processes at the CIGSe/Mo interface, as will be discussed in the following. While only trace amounts of In and (within the detection limit of the experiment) no Cu is found, the Se signal increases at the Mo front side, pointing towards the formation of a MoSe 2 compound, as was found before for Cu(In,Ga)(S,Se) 2 absorbers [18]. This finding is corroborated by the Mo M 4,5 XES spectra shown in Fig.…”

“…In consequence, the comparatively large intensities of the photoemission and Auger lines of S and Se observed on the Mo Front point to the formation of a Mo(S,Se) 2 and MoSe 2 layer at the back contact for the CIGSSe and CIGSe sample, respectively. This was similarly reported/suggested in the past [20][21][22][23][24][25]. However, as shown in Fig.…”

Chemical structures of theCu(In,Ga)Se2/MoandBär1, Weinhardt2, Heske3 et al. 2008Phys. Rev. BSelf Cite313180View full textAdd to dashboardCite

“…In order to derive the fraction of CIG͑S͒Se crystallites remaining on the Mo-side samples ͑in our previous experiments, 20 we derived that almost half of the Mo side was covered with absorber crystallites after lift-off͒, we analyzed the respective XPS detail spectra of the most prominent absorber elements ͑Ga 2p 3/2 , Cu 2p 3/2 , and In 3d 3/2 ͒. The corresponding spectra of the Mo-side samples are shown in Fig.…”

“…While the presence of a Cu-poor surface composition in high-efficiency CIG͑S͒Se solar cells 27 is widely accepted, only the pioneering work of Scheer and Lewerenz 11 and our own work report a Cu-poor stoichiometry at the CuInS 2 or CIGSe back side. 20 It is, however, not clear whether the absorber back side is inherently Cu-poor or whether the Cu-poor surface composition is only developed after cleavage.…”

“…10,11 In earlier publications we have demonstrated that this approach is suitable to make the buried CIG͑S͒Se/Mo interface accessible for investigation by surface-sensitive techniques and/or techniques that are sensitive to the surface-near bulk. 20,21 In this paper, we systematically investigate the impact of S incorporation in the CIG͑S͒Se on the CIG͑S͒Se/ Mo-interface formation by x-ray photoelectron spectroscopy ͑XPS͒ and x-ray-excited Auger-electron spectroscopy ͑XAES͒ and paint a comprehensive picture of the chemical structure of the CIG͑S͒Se/Mo interface.…”