Jörg Palm

Impact of environmental conditions on the chemical surface properties of Cu(In,Ga)(S,Se)2 thin-film solar cell absorbers

Environmentally driven aging effects play a crucial role in thin-film solar cells based on Cu(In,Ga)(S,Se)2, both for long-term stability and short air exposure during production. For a better understanding of such effects, Cu(In,Ga)(S,Se)2 absorber surfaces were investigated by x-ray photoelectron and Auger electron spectroscopy after exposure to different environmental conditions. Identical absorbers were stored in a nitrogen atmosphere, in damp heat, and under ambient conditions for up to 14 days. We find varying degrees of diffusion of sulfur, copper, and sodium towards the surface, with potential impact on the electronic surface structure (band gap) and the properties of the interface to a buffer layer in a solar cell device. Furthermore, we observe an oxidation (in decreasing order) of indium, copper, and selenium (but no oxidation of sulfur). And finally, varying amounts of carbon- and oxygen-containing adsorbates are found. In particular, the findings suggest that, for ambient air exposure, sodium carbonate is formed at the surface.

Direct determination of the band alignment at the (Zn,Mg)O/CISSe interface

The electronic and chemical properties of the (Zn1−x,Mgx)O/CuIn(S,Se)2 interface, prepared by sputtering of thin (Zn,Mg)O layers, were investigated with direct and inverse photoelectron spectroscopy on in situ prepared samples. With the combination of both techniques we have determined the band alignment at this interface as a function of Mg-content in the range 0≤x≤0.30. We find that the band alignment at the interface can be tailored between a “cliff” (downward step) in the conduction band for pure ZnO and a “spike” (upward step) for high Mg-contents. A direct influence of the band alignment modifications on the solar cell parameters is found.

Chemical structure of the (Zn1−x,Mgx)O/CuIn(S,Se)2 interface in thin film solar cells

The formation of the interface between a magnetron sputtered (Zn1−x,Mgx)O buffer layer and a CuIn(S,Se)2 absorber in thin film solar cells has been investigated by x-ray photoelectron spectroscopy and x-ray induced Auger electron spectroscopy. Detailed analysis of the spectra shows the incorporation of Zn into the absorber surface in the initial stage of the deposition process forming ZnS and/or ZnSe bonds. As a result we find the buffer layer to be Zn-depleted near the interface.

Band-Gap Widening at the Cu(In,Ga)(S,Se)2 Surface: A Novel Determination Approach Using Reflection Electron Energy Loss Spectroscopy

Using reflection electron energy loss spectroscopy (REELS), we have investigated the optical properties at the surface of a chalcopyrite-based Cu(In,Ga)(S,Se)2 (CIGSSe) thin-film solar cell absorber, as well as an indium sulfide (InxSy) buffer layer before and after annealing. By fitting the characteristic inelastic scattering cross-section λK(E) to cross sections evaluated by the QUEELS-ε(k,ω)-REELS software package, we determine the surface dielectric function and optical properties of these samples. A comparison of the optical values at the surface of the InxSy film with bulk ellipsometry measurements indicates a good agreement between bulk- and surface-related optical properties. In contrast, the properties of the CIGSSe surface differ significantly from the bulk. In particular, a larger (surface) band gap than for bulk-sensitive measurements is observed, providing a complementary and independent confirmation of earlier photoelectron spectroscopy results. Finally, we derive the inelastic mean free path λ for electrons in InxSy, annealed InxSy, and CIGSSe at a kinetic energy of 1000 eV.

Lateral inhomogeneity of the Mg/(Zn+Mg) composition at the (Zn,Mg)O/CuIn(S,Se)2 thin-film solar cell interface revealed by photoemission electron microscopy

Spatial variations in the chemical composition of the (Zn,Mg)O/CuIn(S,Se)2 thin-film solar cell interface were studied by photoemission electron microscopy (PEEM). Energy filtered PEEM images indicate significant differences in the magnesium and zinc distribution. Local photoemission measurements reveal a relative difference in the derived Mg/(Zn+Mg) composition of the (Zn,Mg)O material of up to (11.4 ± 0.7)%, which can be expected to induce band gap fluctuations of (60 ± 30) meV. Furthermore, local areas with significant accumulations of sodium could be observed.