Improving Cu(In,Ga)Se2 solar cell absorbers based on atomic-level modeling

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School of Science | Doctoral thesis (article-based) | Defence date: 2019-06-28
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Authors

Date

2019

Department

Teknillisen fysiikan laitos
Department of Applied Physics

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Mcode

Degree programme

Language

en

Pages

69 + app. 47

Series

Aalto University publication series DOCTORAL DISSERTATIONS, 120/2019

Abstract

Cu(In, Ga)Se2 (CIGS)-based solar cells are among the most promising candidates to replace crystalline silicon solar cells, thanks to their high efficiencies and low costs. The defect microstructure of the CIGS light absorber layer influences the optical and electronic properties of CIGS solar cells. The recent progress in their efficiency (up to 23.35 %) is mainly due to the incorporation of different alkali metal atoms into the absorber layer. As efficiency increases towards the Shockley-Queisser limit (31 % for a cell with a band gap of 1.3 eV), it becomes more difficult to improve. Thus, knowledge about native point defects and impurities, as well as about the formation of secondary phases in the CIGS absorber layer, is among essential information for optimizing CIGS solar cell performance. In this thesis, the choice of computational methods and their details strongly affects even the qualitative features of the obtained results. Therefore, the effects of the most important computational parameters are studied carefully in the thesis. Native point defects, native point defect complexes, and alkali metal impurities in the CIGS absorber layer are investigated using first-principles calculations within density functional theory (DFT) in order to understand their effect on the electronic structure. Moreover, based on DFT calculations, a mechanism for secondary phase (e.g. alkali indium selenide) formation is suggested. Calculating defects in CIGS is not straightforward. It is impossible to model defects directly in CIGS because In and Ga randomly occupy the same sites. Therefore all the calculations presented in this thesis are performed for CuInSe2 and CuGaSe2. In this thesis, calculations of native point defect formation energies in CuInSe2 provide information about the abundances of acceptors and donors for materials of different Cu concentrations. Moreover, it is shown that light alkali metal atoms prefer to accumulate on the Cu sublattice as impurities, and incorporation of heavy alkali metal atoms contributes mostly by phase separation. The formation of alkali indium/gallium selenide secondary phases during the post-deposition treatment is predicted by considering possible reactions between CuInSe2/CuGaSe2 and different alkali metal compounds by calculating their formation enthalpies. Interfaces between the secondary phases and the CuInSe2 absorber layer are studied in terms of band offsets. Finally, comparisons between the hard X-ray photoelectron spectroscopy (HAXPES) data and the density of states calculations with potassium post-deposition treatment (PDT) as a case study reveal the formation of the KInSe2 phase on the CIGS absorber surface after heavy potassium post-deposition treatment. In summary, the results in this thesis give information about the energetic characteristics of native point defects, native point defect complexes, alkali metal impurities, and alkali metal secondary phases. The results help to analyse already existing experimental observations of the abundances of point defects, migration mechanisms, and the formation of secondary phases. 

Description

Supervising professor

Puska, Martti, Prof., Aalto University, Department of Applied Physics, Finland

Thesis advisor

Havu, Ville, Dr., Aalto University, Department of Applied Physics, Finland
Komsa, Hannu-Pekka, Dr., Aalto University, Department of Applied Physics, Finland

Keywords

solar cells, DFT, Cu(In,Ga)Se2, alkali post-deposition treatment

Other note

Parts

  • [Publication 1]: M. Malitckaya, H.-P. Komsa, V. Havu, M.J. Puska. First-principles modeling of point defects and complexes in thin-film solar-cell absorber CuInSe2. Advanced electronic materials, 2017, 3, 1600353(1-9).
    Full text in Acris/Aaltodoc: http://urn.fi/URN:NBN:fi:aalto-201806183320
    DOI: 10.1002/aelm.201600353 View at publisher
  • [Publication 2]: M. Malitckaya, H.-P. Komsa, V. Havu, M.J. Puska. Effect of alkali metal atom doping on the CuInSe2-based solar cell absorber. The Journal of Physical Chemistry C, 2017, 121,15516-15528,
    Full text in Acris/Aaltodoc: http://urn.fi/URN:NBN:fi:aalto-201805222398
    DOI: 10.1021/acs.jpcc.7b03083 View at publisher
  • [Publication 3]: M. Malitckaya, T. Kunze, H.-P. Komsa, V. Havu, E. Handick, R.G. Wilks, M. Bär, M.J. Puska. Alkali post-deposition treatment-induced changes of the chemical and electronic structure of Cu(In,Ga)Se2 thin-film solar cell absorbers: A first-principle perspective. ACS Applied Materials and Interfaces, 2019, 11(3), 3024-3033,
    Full text in Acris/Aaltodoc: http://urn.fi/URN:NBN:fi:aalto-201902252046
    DOI: 10.1021/acsami.8b18216 View at publisher
  • [Errata file]: Errata in P1, P2

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https://urn.fi/URN:ISBN:978-952-60-8617-0

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[diss] Perustieteiden korkeakoulu / SCI