Reliable wet-chemical cleaning of natively oxidized high-efficiency Cu(In,Ga)Se2 thin-film solar cell absorbers: Journal of Applied Physics: Vol 116, No 23

Abstract

Currently, Cu-containing chalcopyrite-based solar cells provide the highest conversion efficiencies among all thin-film photovoltaic (PV) technologies. They have reached efficiency values above 20%, the same performance level as multi-crystalline silicon-wafer technology that dominates the commercial PV market. Chalcopyrite thin-film heterostructures consist of a layer stack with a variety of interfaces between different materials. It is the chalcopyrite/buffer region (forming the p-n junction), which is of crucial importance and therefore frequently investigated using surface and interface science tools, such as photoelectron spectroscopy and scanning probe microscopy. To ensure comparability and validity of the results, a general preparation guide for “realistic” surfaces of polycrystalline chalcopyrite thin films is highly desirable. We present results on wet-chemical cleaning procedures of polycrystalline Cu(In_1-xGa_x)Se₂ thin films with an average x = [Ga]/([In] + [Ga]) = 0.29, which were exposed to ambient conditions for different times. The hence natively oxidized sample surfaces were etched in KCN- or NH₃-based aqueous solutions. By x-ray photoelectron spectroscopy, we find that the KCN treatment results in a chemical surface structure which is – apart from a slight change in surface composition – identical to a pristine as-received sample surface. Additionally, we discover a different oxidation behavior of In and Ga, in agreement with thermodynamic reference data, and we find indications for the segregation and removal of copper selenide surface phases from the polycrystalline material.

References

1. ZSW press release (22 September 2014), http://www.zsw-bw.de/uploads/media/pr12-2014-ZSW-WorldrecordCIGS.pdf. Google Scholar
2. D. Schmid, M. Ruckh, and H.-W. Schock, Appl. Surf. Sci. 103, 409 (1996). https://doi.org/10.1016/S0169-4332(96)00099-2, Google ScholarCrossref, CAS
3. Y. Hashimoto, N. Kohara, T. Negami, M. Nishitani, and T. Wada, Jpn. J. Appl. Phys. Part 1 35, 4760 (1996). https://doi.org/10.1143/JJAP.35.4760, , Google ScholarCrossref, CAS
4. H. Iwakuro, C. Tatsuyama, and S. Ichimura, Jpn. J. Appl. Phys. Part 1 21, 94 (1982). https://doi.org/10.1143/JJAP.21.94, , Google ScholarCrossref, CAS
5. C. Heske, D. Eich, R. Fink, E. Umbach, T. van Buuren, C. Bostedt, S. Kakar, L. J. Terminello, M. M. Grush, T. A. Callcott, F. J. Himpsel, D. L. Ederer, R. C. C. Perera, W. Riedl, and F. Karg, Surf. Interface Anal. 30, 459 (2000). https://doi.org/10.1002/1096-9918(200008)30:1<459::AID-SIA757>3.0.CO;2-L, , Google ScholarCrossref, CAS
6. R. Wuerz, A. Meeder, D. Fuertes Marrón, and T. Schedel-Niedrig, Phys. Rev. B 70, 205321 (2004). https://doi.org/10.1103/PhysRevB.70.205321, , Google ScholarCrossref
7. D. Cahen, P. J. Ireland, L. L. Kazmerski, and F. A. Thiel, J. Appl. Phys. 57, 4761 (1985). https://doi.org/10.1063/1.335341, , Google ScholarScitation, CAS
8. M. Bär, I. Repins, M. Contreras, L. Weinhardt, R. Noufi, and C. Heske, Appl. Phys. Lett. 95, 052106 (2009). https://doi.org/10.1063/1.3194153, , Google ScholarScitation
9. M. Bär, J. Klaer, L. Weinhardt, R. G. Wilks, S. Krause, M. Blum, W. Yang, C. Heske, and H.-W. Schock, Adv. Energy Mater. 3, 777 (2013). https://doi.org/10.1002/aenm.201200946, , Google ScholarCrossref
10. S. Sadewasser, T. Glatzel, S. Schuler, S. Nishiwaki, R. Kaigawa, and M. Lux-Steiner, Thin Solid Films 431–432, 257 (2003). https://doi.org/10.1016/S0040-6090(03)00267-0, , Google ScholarCrossref, CAS
11. C.-S. Jiang, F. Hasoon, H. Moutinho, H. Al-Thani, M. Romero, and M. Al-Jassim, Appl. Phys. Lett. 82, 127 (2003). https://doi.org/10.1063/1.1534417, , Google ScholarScitation, CAS
12. M. J. Romero, C.-S. Jiang, R. Noufi, and M. Al-Jassim, Appl. Phys. Lett. 87, 172106 (2005). https://doi.org/10.1063/1.2119422, , Google ScholarScitation
13. S. Sadewasser, Phys. Status Solidi A 203, 2571 (2006). https://doi.org/10.1002/pssa.200669573, , Google ScholarCrossref, CAS
14. H. Mönig, Y. Smith, R. Caballero, C. A. Kaufmann, I. Lauermann, M. C. Lux-Steiner, and S. Sadewasser, Phys. Rev. Lett. 105, 116802 (2010). https://doi.org/10.1103/PhysRevLett.105.116802, , Google ScholarCrossref, CAS
15. L. L. Kazmerski, O. Jamjoum, P. J. Ireland, S. K. Deb, R. A. Mickelsen, and W. Chen, J. Vac. Sci. Technol. 19, 467 (1981). https://doi.org/10.1116/1.571040, , Google ScholarCrossref, CAS
16. L. C. Olsen, F. W. Addis, L. Huang, W. N. Shafarman, P. Eschbach, and G. J. Exarhos, in Conference Record of the 28th IEEE Photovoltaic Specialists Conference, Piscataway, NJ (IEEE, 2000), p. 458. , Google Scholar
17. C. A. Kaufmann, A. Neisser, R. Klenk, and R. Scheer, Thin Solid Films 480–481, 515 (2005). https://doi.org/10.1016/j.tsf.2004.11.067, Google ScholarCrossref, CAS
18. C. A. Kaufmann, R. Caballero, T. Unold, R. Hesse, R. Klenk, S. Schorr, M. Nichterwitz, and H.-W. Schock, Sol. Energy Mater. Sol. Cells 93, 859 (2009). https://doi.org/10.1016/j.solmat.2008.10.009, , Google ScholarCrossref, CAS
19. M. P. Seah, Surf. Interface Anal. 14, 488 (1989). https://doi.org/10.1002/sia.740140813, , Google ScholarCrossref, CAS
20. A. Kylner, J. Electrochem. Soc. 146, 1816 (1999). https://doi.org/10.1149/1.1391849, , Google ScholarCrossref, CAS
21. G. Schön, J. Electron Spectrosc. Relat. Phenom. 2, 75 (1973). https://doi.org/10.1016/0368-2048(73)80049-0, , Google ScholarCrossref, CAS
22. P. A. Bertrand, J. Vac. Sci. Technol. 18, 28 (1981). https://doi.org/10.1116/1.570694, , Google ScholarCrossref, CAS
23. C. Wagner and P. Biloen, Surf. Sci. 35, 82 (1973). https://doi.org/10.1016/0039-6028(73)90205-7, , Google ScholarCrossref, CAS
24. M. Ruckh, D. Schmid, M. Kaiser, R. Schäffler, T. Walter, and H.-W. Schock, Sol. Energy Mater. Sol. Cells 41–42, 335 (1996). https://doi.org/10.1016/0927-0248(95)00105-0, , Google ScholarCrossref
25. D. W. Niles, K. Ramanathan, F. Hasoon, R. Noufi, B. J. Tielsch, and J. E. Fulghum, J. Vac. Sci. Technol. A 15, 3044 (1997). https://doi.org/10.1116/1.580902, , Google ScholarCrossref, CAS
26. C. Heske, G. Richter, Z. Chen, R. Fink, E. Umbach, W. Riedl, and F. Karg, J. Appl. Phys. 82, 2411 (1997). https://doi.org/10.1063/1.366096, , Google ScholarScitation, CAS
27. C. D. Wagner, L. H. Gale, and R. H. Raymond, Anal. Chem. 51, 466 (1979). https://doi.org/10.1021/ac50040a005, , Google ScholarCrossref, CAS
28. C. D. Wagner, Discuss. Faraday Soc. 60, 291 (1975). https://doi.org/10.1039/dc9756000291, , Google ScholarCrossref
29.Note that while selenium oxides are soluble in water and hence should be removed by both, the aqueous NH₃ as well as the aqueous KCN solution, copper selenides are only soluble in a KCN solution.
30. J. H. Scofield, J. Electron Spectrosc. Relat. Phenom. 8, 129 (1976). https://doi.org/10.1016/0368-2048(76)80015-1, Google ScholarCrossref, CAS
31. QUASES-IMFP-TPP2M software (http://www.quases.com) applying the TPP2M algorithm described in S. Tanuma, C. J. Powell, and D. R. Penn, Surf. Interface Anal. 21, 165 (1994). https://doi.org/10.1002/sia.740210302, , Google ScholarCrossref
32. N. S. McIntyre and M. G. Cook, Anal. Chem. 47, 2208 (1975). https://doi.org/10.1021/ac60363a034, , Google ScholarCrossref, CAS
33.As the kinetic energy here considered photoelectrons' changes from ≈369 eV for Ga 2p_3/2, over ≈ 555 eV for Cu 2p_3/2 and ≈1035 eV for In 3d_3/2 to ≈1432 eV for Se 3d_5/2, the inelastic mean free path (IMFP) in a stoichiometric chalcopyrite absorber probed with Al K_α excitation increases: IMFP (Ga 2p_3/2) ≈ 1.2 nm < IMFP (Cu 2p_3/2) ≈1.6 nm < IMFP (In 3d_3/2) ≈ 2.6 nm < IMFP (Se 3d_5/2) ≈3.4 nm (IMFPs calculated according to Ref. 3131. QUASES-IMFP-TPP2M software (http://www.quases.com) applying the TPP2M algorithm described in S. Tanuma, C. J. Powell, and D. R. Penn, Surf. Interface Anal. 21, 165 (1994). https://doi.org/10.1002/sia.740210302). In consequence, the information depth of the Ga 2p_3/2, Cu 2p_3/2, In 3d_3/2, and Se 3d_5/2 core levels is different and thus might influence the derived oxidation prevalence. However, as the prevalence of oxides formed on an air-exposed CuIn_0.71Ga_0.29Se₂ surface was derived to be in the order of Ga₂O₃, In₂O₃, Se-O_x, and Cu₂O/CuO, the impact of the information depth can, however, not be dominant.
34. A. F. Holleman and E. Wiberg, in Lehrbuch der anorganischen Chemie, edited by E. Wiberg ( Walter de Gruyter & Co, 1971). Google Scholar
35. O. Knacke and O. Kubaschewski, in Thermochemical Properties of Inorganic Substances, edited by K. Hesselmann ( Springer, Heidelberg, 1991). Google Scholar
36.By normalization of the intensities of the photoelectron spectra, it is possible to gain information about the surface composition. Assuming a homogeneous distribution of the element concentration, the normalized intensity can be derived from
${Int}_{norm}^{X} = \frac{{Int}_{det .}^{X}}{λ^{CIGSe} (E_{kin}) \cdot \frac{d σ^{X}}{Ω (h ν)} \cdot T (E_{kin}) \cdot Z},$
where ${Int}_{norm}^{X}$ is the normalized intensity of a spectral line X, ${Int}_{det}^{X}$ the intensity of the same line measured by the detector, $λ^{CIGSe} (E_{kin})$ the inelastic mean free path for photoelectrons in CuIn_0.71Ga_0.29Se₂ with energy $E_{kin}$ ,³¹31. QUASES-IMFP-TPP2M software (http://www.quases.com) applying the TPP2M algorithm described in S. Tanuma, C. J. Powell, and D. R. Penn, Surf. Interface Anal. 21, 165 (1994). https://doi.org/10.1002/sia.740210302 $\frac{d σ^{X}}{Ω (h ν)}$ the partial photoionization cross section of spectral line X with excitation energy hν,³⁰30. J. H. Scofield, J. Electron Spectrosc. Relat. Phenom. 8, 129 (1976). https://doi.org/10.1016/0368-2048(76)80015-1 $T (E_{kin})$ the transmission function of the analyzer for electrons with energy $E_{kin}$ , and Z the number of scans. The transmission function was determined according to the survey spectra approach by recording a gold spectrum and adjusting it with a reference spectrum [M. P. Seah and G. C. Smith, Surf. Interface Anal. 15, 751 (1990); G. C. Smith and M. P. Seah, ibid. 16, 144 (1990)].
37. J. R. Tuttle, D. S. Albin, and R. Noufi, Sol. Cells 30, 21 (1991). https://doi.org/10.1016/0379-6787(91)90034-M, Google ScholarCrossref, CAS
38. D. Schmid, M. Ruckh, F. Grunwald, and H.-W. Schock, J. Appl. Phys. 73, 2902 (1993). https://doi.org/10.1063/1.353020, , Google ScholarScitation, ISI, CAS
39. D. Liao and A. Rockett, Appl. Phys. Lett. 82, 2829 (2003). https://doi.org/10.1063/1.1570516, , Google ScholarScitation, CAS
40. H. Mönig, Ch.-H. Fischer, R. Caballero, Ch. A. Kaufmann, N. Allsop, M. Gorgoi, R. Klenk, H.-W. Schock, S. Lehmann, M. C. Lux-Steiner, and I. Lauermann, Acta Mater. 57, 3645 (2009). https://doi.org/10.1016/j.actamat.2009.04.029, , Google ScholarCrossref
41. X. Song, R. Caballero, R. Félix, D. Gerlach, C. A. Kaufmann, H.-W. Schock, R. G. Wilks, and M. Bär, J. Appl. Phys. 111, 034903 (2012). https://doi.org/10.1063/1.3679604, , Google ScholarScitation
© 2014 AIP Publishing LLC.

Article Metrics

Views

123

Citations

Crossref 19

Web of Science

ISI 17

Reliable wet-chemical cleaning of natively oxidized high-efficiency Cu(In,Ga)Se₂ thin-film solar cell absorbers

References

Article Metrics

Altmetric

Select Your Access

Purchase

Resources

General Information

123

References

Article Metrics

Altmetric

Select Your Access

Individual Access

Institutional Access

Purchase