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Reactively sputtered films in the CuxS–ZnS–SnSy system: From metastability to equilibrium
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.ORCID iD: 0000-0002-0501-8969
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.ORCID iD: 0000-0003-2679-2387
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2015 (English)In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 582, 208-214 p.Article in journal, Meeting abstract (Refereed) Published
Abstract [en]

Cu2ZnSnS4 is a promising photovoltaic absorber containing earth abundant elements. Using a two stage process, low temperature reactive co-sputtering followed by heat treatment, we have previously achieved a 7.9% efficient solar cell. Because the sputtered precursors contain non-equilibrium phases with unusual crystal structures, it is crucial to understand their nature and their conversion into Cu2ZnSnS4 (and secondary phases) during heat treatment. In this study, we report phase analysis of reactively sputtered binary and ternary sulfides in the CuxS–ZnS–SnSy system before and after annealing. In the as deposited films, Raman spectroscopy with 532 and 325 nm excitation wavelengths reveals expected phases for the binaries (CuS, ZnS and SnS2) and the ternary (Cu2SnS3), and unique metastable phases for the Cu–Zn–S and Zn–Sn–S precursors. Upon annealing, the non-equilibrium phases disappear, accompanied by additional chemical changes. Excess S content in the films is removed, and in the Sn–S and Zn–Sn–S films, further S loss from decomposition of SnSx (x > 1) and CuS respectively generates SnS and CuxS (x > 1). Due to the presence of SnS vapor, Cu2ZnSnS4 is generated from the Cu–Zn–S precursor. Additionally, the range of sulfur partial pressure in the annealing process is estimated according to the temperature–pressure phase diagram. This gives us useful insight allowing better control of annealing conditions.

Place, publisher, year, edition, pages
2015. Vol. 582, 208-214 p.
Keyword [en]
Copper zinc tin sulfide; Thin film solar cells; Reactive sputtering; Secondary phases; Raman scattering; X-ray diffraction; Kesterite
National Category
Other Materials Engineering Other Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Engineering Science with specialization in Electronics
Identifiers
URN: urn:nbn:se:uu:diva-236734DOI: 10.1016/j.tsf.2014.10.076ISI: 000352225900045OAI: oai:DiVA.org:uu-236734DiVA: diva2:765294
Funder
EU, FP7, Seventh Framework Programme, 316488 (KESTCELLS)
Available from: 2014-11-21 Created: 2014-11-21 Last updated: 2017-12-05Bibliographically approved
In thesis
1. Annealing of Cu2ZnSn(S,Se)4 Thin Films: A Study of Secondary Compounds and Their Effects on Solar Cells
Open this publication in new window or tab >>Annealing of Cu2ZnSn(S,Se)4 Thin Films: A Study of Secondary Compounds and Their Effects on Solar Cells
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Kesterite Cu2ZnSnS4 (CZTS) is interesting as a sustainable photovoltaic technology due to its earth-abundant elements and suitable semiconducting properties. To date, a record efficiency of 12.6% has been achieved but further improvements are required to reach high efficiency for industrial implementation. Among the limiting issues is the understanding of the annealing process, which is crucial in promoting high material quality. In particular, the knowledge of the effects of segregated secondary compounds on solar cell performance is lacking.

In contrast to formation of ZnS particles throughout CZTS film, it is notable that SnS forms and usually segregates on the CZTS top and rear surfaces. The influence of SnS on CZTS solar cells was studied by electron beam induced current measurements. It is found that SnS presence on the CZTS surfacecan introduce “dead area”, whereas it seems beneficial for solar cell current when accumulates on the CZTS rear. For SnS passivation and from investigation of the passivation effect from an Al2O3 thin layer at the CZTS rear, improvement in overall device performance could not be demonstrated, due to either poor CZTS bulk or non-optimal device structure. The limitation in CZTS bulk quality was shown from a thickness study where carrier collection saturated already about 700-1000 nm CZTS thickness.

Formation of SnS alongside CZTS implies the anneal is limited by a deficient sulfur partial pressure (PS2). By looking into Sn-S phase transformations in SnS2 films after annealing, we find that PS2 drops rapidly over the annealing time, which could be well-correlated to a series of changes in CZTS material quality including secondary phase formations and defect modifications. It is shown that annealing CZTS under sufficiently high PS2 is critical for CZTS solar cells with high open circuit voltage (upto 783mV was reached), possibly due to the defect modification.

Besides SnS, it is observed that NaxS compounds are also readily formed on CZTS surfaces, due to Na diffusion from the glass substrate during annealing. NaxS negatively affects the formation of the CdS/CZTS interface during chemical bath deposition. It can be removed by an oxidation process or wet chemical etching.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2017. 85 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1476
Keyword
Annealing, sputtering, thin film, CZTS, secondary phases, solar cell
National Category
Engineering and Technology
Research subject
Engineering Science
Identifiers
urn:nbn:se:uu:diva-314975 (URN)978-91-554-9817-7 (ISBN)
Public defence
2017-03-31, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 13:15 (English)
Opponent
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Available from: 2017-03-10 Created: 2017-02-07 Last updated: 2017-03-20

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Ren, YiScragg, Jonathan J.Ericson, ToveKubart, TomasPlatzer-Björkman, Charlotte

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