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Evolution of Na-S(-O) compounds on the Cu2ZnSnS4 absorber surface and their effects on CdS thin film growth
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.
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2016 (English)In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 8, no 28, 18600-18607 p.Article in journal (Refereed) Published
Abstract [en]

Formation of Na-containing surface compounds is an important phenomenon in the Cu2ZnSnS4 (CZTS) quaternary material synthesis for solar cell applications. Still, identification of these compounds and the understanding of their potential influence on buffer layer growth and device performance are scarce. In this work, we discovered that the evolution of Na-S(-O) compounds on the CZTS surface substantially affect the solution/CZTS interface during the chemical bath deposition of CdS buffer film. We showed that Na2S negatively affects the growth of CdS, and that this compound is likely to form on the CZTS surface after annealing. It was also demonstrated that the Na2S compound can be oxidized to Na2SO4 by air exposure of the annealed CZTS surface or be removed using water dipping instead of the commonly used KCN etching process, resulting in significantly better quality of the CdS layer. Lastly, 6.5% CZTS solar cells were fabricated with air exposure treatment without incorporation of the KCN etching process. This work provides new insight into the growth of the CdS/CZTS interface for solar cell applications and opens new possibilities for improving likewise Cd-free buffer materials that are grown with a similar chemical bath deposition process.

Place, publisher, year, edition, pages
2016. Vol. 8, no 28, 18600-18607 p.
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:uu:diva-298673DOI: 10.1021/acsami.6b04978ISI: 000380298400096PubMedID: 27356214OAI: oai:DiVA.org:uu-298673DiVA: diva2:946840
Funder
EU, FP7, Seventh Framework Programme, 316488Knut and Alice Wallenberg FoundationSwedish Foundation for Strategic Research Swedish Research Council
Available from: 2016-07-06 Created: 2016-07-06 Last updated: 2017-02-07Bibliographically 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)
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Supervisors
Available from: 2017-03-10 Created: 2017-02-07 Last updated: 2017-03-20

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