By combining ultraviolet and x-ray photoelectron spectroscopy with inverse photoemission spectroscopy, we find that the conduction-band alignment at the CdS/CuInSe2 thin-film solar- cell heterojunction is flat (0.0+/-0.2 eV). Furthermore, we observe a valence-band offset of 0.8+/-0.2 eV. The electronic level alignment is dominated by (1) an unusually large surface band gap of the CuInSe2 thin film (1.4 eV), (2) by a reduced surface band gap of the CdS overlayer (2.2 eV) due to intermixing effects, and (3) by a general influence of the intermixing on the chemical state near the interface

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Articles you may be interested inLateral 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

This paper summarizes recent material and device results obtained at the Institute of Phy sical Electronics at Stuttgart University (lP£). Properties of the mat erial system Cu(ln, Ga)(S, Se h were analy sed and wherever possible a correlation bet ween the material properties and the device characteristics is made. Different high-vacuum techniques of absorber preparation are presented and compared. The f ormation of different alloy s of the family Cu(ln, Ga)(S, S eh is possible for the co-evaporation and at least one of the sequential evaporation techniques. The model f or Cu-rich grow th of Cul nSe2 kno wn from the co-evaporation process can also be used for the Cu-rich growth in the sequential evaporation processes. The surf ace composition of slightly (In, Gas-rich bulk compositions is always determined to be the defect chalcopyrite Cu(/n, GahSe5. Solar cells prepared with different processes and therefore different morphologie s yielded similar device performance. An exponential decay of the density of states from the valence and conduction bands was obtained. Improved cell performance is achieved using absorber layers with higher carrier concentrations. The carrier concentration can be increased by using Na-containing substrates or by utilizing a new Cd-free buffer layer. Device efficiencies in the range of 15% were achieved using the Cd-free buffer layer.1 -5 and 12% for CulnS2.6 But, not only for single cells on a laboratory scale, the material systems passed the status of only being a promising candidate for solar cell applications. Modules with more than 40 W have been realized." Submodules of 50 em? revealed efficiencies of more than 10%7 and the upscaling process to full-size modules is ongoing. To overcome the actual limitations of these cells it is very important to understand the correlation between crystallographical, electrical and optical properties and the achieved solar cell data. This paper tries to illustrate some of these connections.

The electronic and chemical structure of Cu(In,Ga)(S,Se) 2 (CIGSSe) thin film surfaces and of relevant interfaces in CIGSSe-based thin film solar cells is investigated with a combination of X-ray emission spectroscopy (XES) and photoelectron spectroscopy. Examples of sulfur L 2,3 XES spectra of CdS and CIGSSe are discussed in view of resonant excitation, surface oxidation, and chemical bonding. The combination of the two techniques proves to be a powerful tool to identify spectral features correlated to certain chemical states or bonds. By monitoring these features in interface formation sequences, chemical and electronic information about buried interfaces can be obtained, which will be discussed in detail for the ZnO/CIGSSe interface. The experimental results provide valuable information on the CIGSSe surface and the ZnO/CIGSSe interface and, in general, demonstrate some of the spectroscopic advantages of X-ray emission spectroscopy.