Annealing-Induced Effects on the Chemical Structure of the In2S3

Article pubs.acs.org/JPCC

Annealing-Induced Effects on the Chemical Structure of the In2S3/ CuIn(S,Se)2 Thin-Film Solar Cell Interface D. Hauschild,† F. Meyer,† A. Benkert,†,‡ D. Kreikemeyer-Lorenzo,‡ S. Pohlner,§ J. Palm,§ M. Blum,∥ W. Yang,⊥ R. G. Wilks,# M. Bar̈ ,∥,#,¶ C. Heske,‡,∥,○,∇ L. Weinhardt,*,‡,∥,○,∇ and F. Reinert†,◆ †

Experimental Physics VII, University of Würzburg, Am Hubland, 97074 Würzburg, Germany Institute for Photon Science and Synchrotron Radiation, Karlsruhe Institute of Technology (KIT), Hermann-v.-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany § AVANCIS GmbH, Otto-Hahn-Ring 6, 81739 Munich, Germany ∥ Department of Chemistry, University of Nevada, Las Vegas (UNLV), 4505 Maryland Parkway, Las Vegas, Nevada 89154-4003, United States ⊥ Advanced Light Source, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, United States # Renewable Energy, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Hahn-Meitner-Platz 1, 14109 Berlin, Germany ¶ Institut für Physik und Chemie, Brandenburgische Technische Universität Cottbus-Senftenberg, Platz der Deutschen Einheit 1, 03046 Cottbus, Germany ○ Institute for Chemical Technology and Polymer Chemistry, Karlsruhe Institute of Technology (KIT), Engesserstraße 18/20, 76128 Karlsruhe, Germany ∇ ANKA Synchrotron Radiation Facility, Karlsruhe Institute of Technology (KIT), Hermann-v.-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany ◆ Gemeinschaftslabor für Nanoanalytik, Karlsruhe Institute of Technology (KIT), 76021 Karlsruhe, Germany ‡

ABSTRACT: We have investigated the impact of heat treatments on the chemical structure of the In2S3/CuIn(S,Se)2 thin-film solar cell interface using X-ray photoelectron and soft Xray emission spectroscopy. As-grown, we find the formation of a sulfur-poor (indium-rich) In2S3 surface, an abrupt interface, and sulfur atoms in both In2S3 and CuIn(S,Se)2 chemical environments (as expected for an abrupt interface and a thin overlayer). After a heat treatment at 200 °C to simulate subsequent process steps, a strong copper and sodium diffusion into the In2S3 layer is observed. This diffusion extends throughout the layer, indicating the formation of a copper−indium−sulfide phase.

I. INTRODUCTION

performed at lower temperatures, the subsequent cell processes often require a high-temperature step which leads to an improved solar cell performance.12 In this paper, we present an X-ray photoelectron spectroscopy (XPS) and soft X-ray emission spectroscopy (XES) study of indium sulfide layers grown with a dry low-temperature process on CuIn(S,Se)2 (CISSe) absorber surfaces and find that diffusion does not occur during deposition, but is induced by a subsequent heat treatment. The latter was conducted by annealing at temperatures up to 200 °C under UHV conditions.

Chalcopyrite-based thin-film solar cells are a low-cost, highefficiency alternative to the well-established silicon-based solar cells. High-efficiency chalcopyrite based solar cells are processed either with a CdS or a Zn(O,S) buffer layer.1−3 In2S3 offers another promising Cd-free alternative that can easily be integrated in a dry inline production line, and efficiencies of more than 16%3−5 have been demonstrated. Indium sulfide buffer layers for chalcopyrite-based solar cells have been processed by a variety of deposition methods, including spray-ion-layer gas reaction (ILGAR),6,7 coevaporation,8 physical vapor deposition (PVD),9,10 and atomic layer deposition (ALD).11 In cases with a high substrate temperature during the indium sulfide deposition step, sometimes a strong diffusion of copper and sodium into the buffer layer was found.12−14 Likewise, even if the indium sulfide deposition is © 2015 American Chemical Society

Received: February 17, 2015 Revised: April 10, 2015 Published: April 15, 2015 10412

DOI: 10.1021/acs.jpcc.5b01622 J. Phys. Chem. C 2015, 119, 10412−10416

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The Journal of Physical Chemistry C

II. EXPERIMENTAL SECTION Cu(In,Ga)(S,Se)2 (CIGSSe) absorbers were grown by stacked elemental layer−rapid thermal processing (SEL-RTP).15,16 Note that Ga at the absorber surface is below the XPS detection limit17,18 and thus the surface is better described as a Ga-free CISSe. Indium sulfide layers were grown by a dry physical vapor deposition (PVD) process (a detailed description can be found elsewhere5) on the CISSe/ molybdenum/glass substrate, which was heated to temperatures below 150 °C at the pilot line of AVANCIS GmbH in Munich (note that we will, in this paper, designate the buffer layer as “In2S3”, even though actual compositions vary, especially at the surface). Different thicknesses of the buffer layer were deposited, ranging between 0.5 and 80 nm, as estimated from the deposition time. All samples were packed under nitrogen and sent to Würzburg for the XPS measurements. The transfer into vacuum was realized with a nitrogen filled glovebag mounted at the load lock of the UHV chamber. All samples were only exposed to air for a short duration (approximately 15 min) at AVANCIS GmbH. The XPS measurements were carried out in UHV (base pressure below 2 × 10−10 mbar) using a VG CLAM4 electron analyzer and a nonmonochromatized X-ray source with Mg Kα excitation. After the initial XPS data set was taken, a sample with an 80 nm thick In2S3 buffer layer was annealed under UHV conditions, first at 100 °C (30 min) and then at 200 °C (for a further 30 min), to simulate a heat treatment as it occurs during subsequent process steps. In addition, a second sample set (with slightly different buffer layer thicknesses than the first one) was sealed under nitrogen atmosphere and shipped to Beamline 8.0.1.1 of the Advanced Light Source for XES measurements. Note that this paper focuses on the In2S3/CISSe heterostructure; further studies will be needed to also shed light on the impact of additional cover layers, in particular the nZnO/i-ZnO contact structure of the completed device.

Figure 1. Na 1s, Cu 2p3/2, and Se 3d XPS spectra (Mg Kα excitation) of a CISSe absorber surface, In2S3/CISSe samples with varying buffer layer thicknesses, and the 80 nm In2S3/CISSe sample after annealing at 200 °C.

core levels with respect to the Fermi level due to the absorberbuffer junction formation. For a quantitative evaluation, we have determined the composition of the samples by analyzing the intensities of the different elemental peaks (i.e., In 4d, Se 3d, Cu 3p, S 2p, and Se 3p). By comparing peaks with similar kinetic energies, the differences in attenuation lengths and analyzer transmission can be minimized. All peaks of the same type were fitted simultaneously by Voigt profiles with the same peak widths and shapes throughout the thickness series. The peak area ratio of the spin−orbit split peaks was fixed according to their multiplicity (2j + 1). The surface composition for different In2 S3 thicknesses was then determined by taking the corresponding cross sections,20 the inelastic mean free path (calculated with the QUASES-IMFP-TPP2M21 code based on ref 19) and an analyzer transmission function of T ∼ Ekin−0.6 (the exponent was determined after ref 22) into account. Figure 2 displays the thus derived chemical composition for Cu, In, S, and Se for each sample. Note that, in particular for thin overlayers, the In and S composition includes contributions from both the CISSe substrate and the In2S3 overlayer. The composition for Na is not shownat the CISSe surface it is about 1.0 (±0.8) %. Overall, we find an increase of the In and

III. RESULTS AND DISCUSSION In the XPS survey spectra (not shown), all expected photoemission and Auger emission lines of the absorber elements are detected, and a small amount of carbon and oxygen adsorbates are visible. This indicates that the transfer of the samples into the UHV system while keeping the exposure to air at a minimum was successful. Figure 1 shows the Na 1s, Cu 2p3/2, and Se 3d detail spectra of the pristine CISSe absorber and of the buffer/absorber samples with increasing In2S3 buffer layer thickness. A linear background was subtracted for all spectra. With increasing In2S3 buffer layer thickness, all indium and sulfur peaks increase in intensity (not shown), whereas selenium, copper, and sodium peaks decrease. Because of the small 1/e attenuation length λ (0.7 nm19) of the Na 1s as well as of the Cu 2p3/2 electrons (λ = 0.9 nm19), the intensity of these two lines decreases stronger than that of the Se 3d signal (λ = 2.5 nm19). The extinction of all three signals is a first hint that, initially (i.e., for the asdeposited In2S3/CISSe interface), the overlayer is closed and sufficiently thick to attenuate all substrate signals, and that furthermore no strong diffusion of sodium, copper, and/or selenium into the In2S3 buffer layer occurs. In addition, a small shift of the absorber-related peaks toward higher binding energies for increasing buffer thickness can be seen in Figure 1. Since all (except the 0.5 nm sample) investigated samples shift in the same direction this suggests a relative shift of all bands/

Figure 2. Composition analysis of Cu, In, S, and Se as a function of overlayer thickness. Also, the expected theoretical attenuation of substrate signals is displayed as a dashed line. The box on the righthand side shows the composition of the annealed 80 nm In2S3/CISSe sample; for comparison, the straight horizontal lines illustrate a Cu:In:S = 1:5:8 composition. 10413

DOI: 10.1021/acs.jpcc.5b01622 J. Phys. Chem. C 2015, 119, 10412−10416

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The Journal of Physical Chemistry C

S 2p transition at about 147 eV (not shown). Four main spectral features appear in the spectrum of the pristine absorber (a). Three of them (at ∼155.0 eV, ∼156.3 eV, and ∼157.7 eV) originate from In 5s-derived valence band states (1)26 decaying into S 2p core holes, while the fourth peak (∼160.0 eV) originates from the relaxation of Cu 3d-derived states (2).27 While the former indicate the presence of S−In bonds, the latter is indicative of S−Cu interactions. With increasing In2S3 thickness (e.g., at 10 nm, b), the CISSe-related features weaken, until, at 80 nm (c), the spectrum largely resembles that of a thick In2S3 reference (d). Note that the latter only shows the three In 5s → S 2p transitions, but not the Cu 3d-derived peak at 160 eV. In contrast, after annealing (Figure 3 e), the Cu 3d-related emission (2) returns, indicative of the formation of S−Cu bonds due to Cu diffusion. As expected, the spectrum of the 80 nm thick buffer layer strongly resembles that of the In2S3 reference showing the presence of In−S bonds. However, the spectral features are slightly different indicating small variations in the chemical environment. In order to analyze the XES S L2,3 spectra in detail, we fitted all spectra as a sum of the spectra of the pristine CISSe absorber and the thick In2S3 reference film. This procedure was done for all XES spectra and is illustrated in Figure 4 for the as-deposited

S concentration and a decrease of the Cu and Se concentration with increasing overlayer thickness. The decrease of the Cu and Se concentration is approximately exponential (dashed line in Figure 2), as would be expected for a homogeneous layer-bylayer growth of the indium sulfide buffer with no (significant) diffusion of absorber elements. The compositions of In and S increase for thin overlayers (as expected), and remain constant at and above a layer thickness of 12.5 nm (46.0 ± 3.5% for In and 54.0 ± 4.0% for S, corresponding to an In 6 S 7 stoichiometry). Compared to the nominal In2S3 composition (40% In and 60% S), this indicates that the indium sulfide layer is sulfur-poor (and indium-rich). After the annealing at 200 °C, we find a distinct copper and sodium diffusion, which manifests itself in increasing Na 1s and Cu 2p3/2 signals for the 80 nm buffer layer sample, as shown in Figure 1, top spectra. Note that we do not find an increase in the Se signal, ruling out the possibility of having created an inhomogeneous film, e.g., by island formation, or the sublimation of indium sulfide (or its components) from the surface. The right box of Figure 2 shows the surface composition of the annealed 80 nm buffer layer. The prominent enhancement of the copper signal observed in Figure 1 corresponds to a surface concentration of (5 ± 3) %. In parallel, a small decrease in the sulfur concentration and a more pronounced decrease in the indium concentration are found (in comparison with the thickest as-deposited buffer layer). In earlier studies, the formation of a buffer with a CuIn5S8 composition was suggested.12,25 This composition is indicated as straight horizontal lines in the box in Figure 2. The observed Cu intensity agrees with this model, while the In composition is somewhat higher and the S composition somewhat lower than the model. This might point to a more indium-rich/sulfur-poor composition of the buffer layer surface than predicted by the 1:5:8 model. For completion, the Na concentration, as derived from the Na 1s signal in Figure 1, is 1.0 (±0.8) %, identical to the one found for the bare absorber surface. As proposed in ref 25, this could be explained by the formation of a Na film or NaxS islands at the surface. In addition to the surface-sensitive XPS, we also used the more bulk-sensitive XES to characterize the In2S3/CISSe interface. In Figure 3, a portion (energy region from 153 to 163 eV) of the S L2,3 XES spectra of selected In2S3/CISSe samples are displayed. All spectra are normalized to the S 3s →

Figure 4. Fit of the 10 (a, top) and 80 (b, bottom) nm S L2,3 XES spectra, using a superposition of the spectra of the CISSe film and the In2S3 reference (0.55 × “CISSe” + 0.45 × “In2S3” and 0.4 × “CISSe” + 0.6 × “In2S3”, respectively). The residuals are shown at the bottom of each panel.

10 nm In2S3/CISSe sample (top) and the annealed 80 nm In2S3/CISSe sample (bottom). As expected, the as-deposited 10 nm In2S3/CISSe sample can be well reproduced by a superposition of the CISSe spectrum (scaled with a factor of 0.55) and the In2S3 spectrum (scaled with a factor of 0.45), since the spectrum combines the signal from both the thin overlayer and the underlying substrate. All spectra of the asdeposited In2S3/CISSe samples can be nicely reproduced (not shown), with varying weight factors (e.g., from 0.82 to 0.02 for the “CISSe factor” for 2.8 nm up to 80 nm In2S3 thickness, respectively). The spectrum of the 80 nm In2S3/CISSe sample after annealing can be well described by a superposition of 0.4 × “CISSe” + 0.6 × “In2S3” (Figure 4, bottom), with only small deviations visible in the residual. The clear increase of the

Figure 3. S L2,3 (hνexc. = 200 eV) XES spectra of a CISSe film (a), two as-deposited In2S3/CISSe interface structures with (b) 10 and (c) 80 nm buffer layer thickness, an In2S3 reference (d), and the 80 nm In2S3/ CISSe sample after annealing at 200 °C (e). The features labeled (1) and (2) are discussed in the text. 10414

DOI: 10.1021/acs.jpcc.5b01622 J. Phys. Chem. C 2015, 119, 10412−10416

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The Journal of Physical Chemistry C CISSe fraction after annealing corroborates the XPS finding of copper diffusion and the formation of a Cu−In−S compound. On the basis of the experimental findings described above, we can now understand the properties of the In2S3/CISSe interface as follows. For the here-employed low-temperature PVD process (without additional annealing), an indium sulfide film with a stoichiometry close to In6S7 is formed. While such a phase (or a corresponding mixture of InS and In2S3 phases) is reported to be n-conductive (i.e., favorable for its use as a buffer), we speculate that the band gap of the as-deposited film is significantly smaller than In2S3 (e.g., below 1 eV for In6S7)23,24 This possibly leads to a reduction of the photocurrent due to absorption in the buffer layer. However, upon annealing, a strong Cu diffusion into the buffer layer is observed. We speculate that in the buffer layer a Cu−In−S phase is formed which increases the buffer band gap in comparison to e.g., In6S7. Simultaneously, the CISSe surface region becomes Cu depleted enlarging its band gap. This might lead to a reduced absorption in the buffer layer and a reduced recombination rate at the interface, respectively.

(6) Bär, M.; Allsop, N.; Lauermann, I.; Fischer, C.-H. Deposition of In2S3 on Cu(In,Ga)(S,Se)2 Thin Film Solar Cell Absorbers by Spray Ion Layer Gas Reaction: Evidence of Strong Interfacial Diffusion. Appl. Phys. Lett. 2007, 90 (13), 132118−132118−3. (7) Allsop, N. A.; Schönmann, A.; Belaidi, A.; Muffler, H.-J.; Mertesacker, B.; Bohne, W.; Strub, E.; Röhrich, J.; Lux-Steiner, M. C.; Fischer, C.-H. Indium Sulfide Thin Films Deposited by the Spray Ion Layer Gas Reaction Technique. Thin Solid Films 2006, 513 (1−2), 52−56. (8) Barreau, N.; Mokrani, A.; Couzinie-Devy, F.; Kessler, J. Bandgap Properties of the Indium Sulfide Thin-Films Grown by CoEvaporation. Thin Solid Films 2009, 517 (7), 2316−2319. (9) Gall, S.; Barreau, N.; Harel, S.; Bernède, J. C.; Kessler, J. Material Analysis of PVD-Grown Indium Sulphide Buffer Layers for Cu(In,Ga)Se2-Based Solar Cells. Thin Solid Films 2005, 480−481, 138− 141. (10) Maksoud, H. A.; Igalson, M.; Spiering, S. Influence of PostDeposition Heat Treatment on Electrical Transport Properties of In2S3-Buffered Cu(In,Ga)Se2 Cells. Thin Solid Films 2013, 535, 158− 161. (11) Sterner, J.; Malmström, J.; Stolt, L. Study on ALD In2S3/ Cu(In,Ga)Se2 Interface Formation. Prog. Photovolt: Res. Appl. 2005, 13 (3), 179−193. (12) Abou-Ras, D.; Kostorz, G.; Strohm, A.; Schock, H.-W.; Tiwari, A. N. Interfacial Layer Formations between Cu(In,Ga)Se2 and InxSy Layers. J. Appl. Phys. 2005, 98 (12), 123512−123512−7. (13) Laurencic, C.; Arzel, L.; Couzinié-Devy, F.; Barreau, N. Investigation of Cu(In,Ga)Se2/In2S3 Diffuse Interface by Raman Scattering. Thin Solid Films 2011, 519 (21), 7553−7555. (14) Naghavi, N.; Spiering, S.; Powalla, M.; Cavana, B.; Lincot, D. High-Efficiency Copper Indium Gallium Diselenide (CIGS) Solar Cells with Indium Sulfide Buffer Layers Deposited by Atomic Layer Chemical Vapor Deposition (ALCVD). Prog. Photovolt: Res. Appl. 2003, 11 (7), 437−443. (15) Palm, J.; Probst, V.; Brummer, A.; Stetter, W.; Tölle, R.; Niesen, T. P.; Visbeck, S.; Hernandez, O.; Wendl, M.; Vogt, H.; et al. CIS Module Pilot Processing Applying Concurrent Rapid Selenization and Sulfurization of Large Area Thin Film Precursors. Thin Solid Films 2003, 431−432, 514−522. (16) Palm, J.; Probst, V.; Stetter, W.; Toelle, R.; Visbeck, S.; Calwer, H.; Niesen, T.; Vogt, H.; Hernández, O.; Wendl, M.; et al. CIGSSe Thin Film PV Modules: From Fundamental Investigations to Advanced Performance and Stability. Thin Solid Films 2004, 451− 452, 544−551. (17) Heske, C.; Richter, G.; Chen, Z.; Fink, R.; Umbach, E.; Riedl, W.; Karg, F. Influence of Na and H2O on the Surface Properties of Cu(In,Ga)Se2 Thin Films. J. Appl. Phys. 1997, 82 (5), 2411. (18) Morkel, M.; Weinhardt, L.; Lohmüller, B.; Heske, C.; Umbach, E.; Riedl, W.; Zweigart, S.; Karg, F. Flat Conduction-Band Alignment at the CdS/CuInSe2 Thin-Film Solar-Cell Heterojunction. Appl. Phys. Lett. 2001, 79 (27), 4482−4484. (19) Tanuma, S.; Powell, C. J.; Penn, D. R. Calculations of Electron Inelastic Mean Free Paths. V. Data for 14 Organic Compounds over the 50−2000 eV Range. Surf. Interface Anal. 1994, 21 (3), 165−176. (20) Yeh, J. J.; Lindau, I. Atomic Subshell Photoionization Cross Sections and Asymmetry Parameters: 1 ⩽ Z ⩽ 103. Atomic Data and Nuclear Data Tables 1985, 32 (1), 1−155. (21) Tougaard, S. QUASES http://www.quases.com/home/ (accessed Mar 23, 2015). (22) Hesse, R.; Streubel, P.; Szargan, R. Improved Accuracy of Quantitative XPS Analysis Using Predetermined Spectrometer Transmission Functions with UNIFIT 2004. Surf. Interface Anal. 2005, 37 (7), 589−607. (23) Seyam, M. A. M. Optical and Electrical Properties of Indium Monosulfide (InS) Thin Films. Vacuum 2001, 63 (3), 441−447. (24) Ho, C.-H.; Wang, Y.-P.; Huang, Y.-S. Optical Characterization of Band-Edge Property of In6S7 Compound. Appl. Phys. Lett. 2012, 100 (13), 131905.

IV. CONCLUSIONS We have presented an XPS/XES study of the chemical structure at the In2S3/CISSe interface before and after annealing in UHV. We find no diffusion of absorber elements into the buffer layer for the here-employed low-temperature deposition process. Our XPS data reveals that the surface of the buffer layer is sulfur-poor/indium-rich (in comparison with the nominal In2S3 stoichiometry). After 200 °C annealing, a distinct diffusion of copper and sodium into the buffer and to its surface is observed. The XPS/XES data suggests that both penetrate through the complete buffer and that the copper also diffuses into the In2S3 buffer “bulk”, forming a new phase. In contrast to earlier findings suggesting a CuIn5S8 stoichiometry, we find the surface to be somewhat more In-rich and S-poor.

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AUTHOR INFORMATION

Corresponding Author

*(L.W.). Address: Hermann-v.-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany. Telephone: +49 721 608-29242. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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DOI: 10.1021/acs.jpcc.5b01622 J. Phys. Chem. C 2015, 119, 10412−10416