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

Room-temperature recombination dynamics has been investigated in a large set of different Cu(In,Ga)Se2 absorber films and compared to the electrical device characteristics of the respective solar cell modules. For a given cell preparation process, a characteristic relation between the low-injection minority-carrier lifetime of the absorber layers and the conversion efficiency of the solar cells is observed: Long lifetimes correlate with high open circuit voltages and conversion efficiencies, while no significant influence of the lifetime on the short circuit current is found.

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A combination of x-ray emission spectroscopy and x-ray photoelectron spectroscopy using high brightness synchrotron radiation has been employed to investigate the electronic and chemical structure of the buried CdS/Cu(In, Ga)Se2 interface, which is the active interface in highly efficient thin film solar cells. In contrast to the conventional model of an abrupt interface, intermixing processes involving the elements S, Se, and In have been identified. The results shed light on the electronic structure and interface formation processes of semiconductor heterojunctions and demonstrate a powerful tool for investigating buried interfaces in general.

X-ray and UV photoelectron spectroscopy measurements of Cu(InGa)Se2 thin films grown on Mo coated soda-lime glass show segregation from the substrate and formation of two different Na species. One of these species is also identified after deliberate deposition of metallic Na. Moreover, the adsorption (or segregation) of this species reduces the native oxide SeO2, while the other, reacted Na species coexists with SeO2. Small amounts of Na (≤0.05 Å) induce a band bending and reduce the surface dipole. These findings reveal a positive influence of segregated Na on the morphology and electrical characteristics of Na-enriched films, improving the overall performance of the solar cells.