Nondestructive depth-resolved spectroscopic investigation of the heavily intermixed In2S3/Cu(In,Ga)Se2 interface

Abstract

͑͒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...

Surface and Interface Properties in Thin‐Film Solar Cells: Using Soft X‐rays and Electrons to Unravel the Electronic and Chemical Structure

“…The XPS data of the as‐deposited samples with varying buffer layer thickness with and without the addition of Na points towards an interface with a small degree of Cu diffusion from the absorber into the buffer layer . Our XPS/XES study of In 2 S 3 layers from the Institut des Matériaux Jean Rouxel (IMN, University of Nantes) revealed a similar diffusion of Cu into the buffer …”

RIXS investigations of liquids, solutions, and liquid/solid interfaces

“…Soft X-ray techniques have proven to be very powerful for the investigation of applied systems such as thin-film solar cells [66][67][68][69][70][71], since they give detailed information about the local chemical environment of specific elements and the band gap in the surface-near bulk region [72]. In such applied systems, it would be especially interesting to perform in situ investigations, e.g., during wet-chemical treatment or deposition steps, of catalysts in operation, of batteries during charge/discharge cycles, and many more.…”

Improving performance by Na doping of a buffer layer—chemical and electronic structure of the InxSy:Na/CuIn(S,Se)2 thin‐film solar cell interface

“…Solar cells with an indium sulfide buffer, as well as its interface with CIGSSe absorbers, have previously been well investigated . For undoped indium sulfide, the highest efficiencies have been reported after a thermal treatment that leads to a strong copper and sodium diffusion from the absorber into the buffer .…”