͑͒The chemical structure of the interface between a nominal In 2 S 3 buffer and a Cu͑In, Ga͒Se 2 ͑CIGSe͒ thin-film solar cell absorber was investigated by soft x-ray photoelectron and emission spectroscopy. We find a heavily intermixed, complex interface structure, in which Cu diffuses into ͑and Na through͒ the buffer layer, while the CIGSe absorber surface/interface region is partially sulfurized. Based on our spectroscopic analysis, a comprehensive picture of the chemical interface structure is proposed. ͓͔ Cu͑In, Ga͒Se 2 ͑CIGSe͒ thin-film solar cells with an n + -ZnO/i-ZnO/CdS/CIGSe/Mo/glass device structure have reached efficiencies of 20%.1 To replace the CdS layer by a nontoxic, more transparent buffer, and the conventionally used chemical bath deposition by a technique allowing inline processing, In 2 S 3 layers have been deposited by physical vapor deposition, 2 sputtering, 3 atomic layer deposition, 4 and spray ion layer gas reaction.
5The In 2 S 3 /CIGSe interface has been previously investigated by different destructive depth-profiling techniques, 2,6 high-resolution transmission electron microscopy and energy dispersive x-ray analysis, 7 and x-ray photoelectron spectroscopy ͑XPS͒. 4,8,9 At ͑post-͒deposition annealing temperatures necessary for high device efficiencies ͑200-250°C͒, a pronounced diffusion of Cu and Na from the CIGSe/Mo/glass substrate into the nominal In 2 S 3 buffer layer was found in these studies. However, a complete picture of the chemical interface structure is still missing. In this paper, we will report on the characterization of the In 2 S 3 /CIGSe interface by a combination of nondestructive techniques ͓XPS and soft x-ray emission spectroscopy ͑XES͔͒, deliberately varying the probing depth. Our measurements result in a depth-resolved picture of the interface in unprecedented detail.In 2 S 3 /CIGSe structures were prepared at IMN on Mo/ glass substrates. 10 The absorber layers were dipped in NH 3 solution ͑1 M, room temperature, 1 min͒ prior to the In 2 S 3 buffer layer deposition by thermal coevaporation of elemental indium and sulfur at 200°C substrate temperature. To vary the In 2 S 3 thickness, different deposition times were used. The standard 80 nm buffer used in solar cells is prepared in 10 min ͑called "1/1" in the following͒. For reference, an In 2 S 3 layer, different In 2 S 3 : Cu standards, and a CuInS 2 ͑CIS͒ absorber 11 were deposited on Mo/glass substrates. After preparation, all samples were sealed in polyethylene bags filled with dry N 2 and desiccant for transport. At UNLV the samples were transferred into the analysis chamber ͑base pressure Ͻ5 ϫ 10 −10 mbar͒ without air exposure. XPS was performed using Mg K ␣ and Al K ␣ excitation and a Specs PHOIBOS 150 MCD electron analyzer ͑calibrated according to Ref. 12͒. Subsequently, XES was performed at the ALS using the soft x-ray fluorescence endstation of Beamline 8.0.XPS survey spectra ͑not shown͒ show all expected absorber photoemission lines, Na-related peaks, and only minor spectral contributions of C-and O-co...