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Zinc-Tin-Oxide Buffer Layer and Low Temperature Post Annealing Resulting in a 9.0% Efficient Cd-Free Cu2ZnSnS4 Solar Cell
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. (Ångström Solar Center)
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.ORCID iD: 0000-0002-4125-4002
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2017 (English)In: Solar RRL, ISSN 2367-198X, Vol. 1, no 5, article id 1700001Article in journal (Refereed) Published
Abstract [en]

Zn1−xSnxOy (ZTO) has yielded promising results as a buffer material for the full sulfur Cu2ZnSnS4 (CZTS), with efficiencies continuously surpassing its CdS-references. ZTO can be deposited by atomic layer deposition (ALD), enabling tuning of the conduction band position through the choice of metal ratio or deposition temperature. Thus, an optimization of the conduction band alignment between ZTO and CZTS can be achieved. The ZTO bandgap is generally larger than that of CdS and can therefore yield higher currents due to reduced losses in the short wavelength region. Another advantage is the possibility to omit the toxic Cd. In this study, the ALD process temperature was varied from 105 to 165 °C. Current-blocked devices were obtained at 105 °C, while the highest open-circuit voltage and device efficiency was achieved for 145 °C. The highest fill factor was seen at 165 °C. The best efficiency reached in this study was 9.0%, which, to our knowledge, is the highest efficiency reported for Cd-free full-sulfur CZTS. We also show that the effect of heat needs to be taken into account. The results indicate that part of the device improvement comes from heating the absorber, but that the benefit of using a ZTO-buffer is clear.

Place, publisher, year, edition, pages
2017. Vol. 1, no 5, article id 1700001
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
URN: urn:nbn:se:uu:diva-321006DOI: 10.1002/solr.201700001OAI: oai:DiVA.org:uu-321006DiVA, id: diva2:1091772
Funder
Swedish Energy Agency, 32787Swedish Foundation for Strategic Research , FFL12-0178Swedish Research Council, 2015-04558Knut and Alice Wallenberg Foundation, KAW 2012.0144Available from: 2017-04-27 Created: 2017-04-27 Last updated: 2018-08-17Bibliographically approved
In thesis
1. Sputtering of Precursors for Cu2ZnSnS4 Solar Cells and Application of Cadmium Free Buffer Layers
Open this publication in new window or tab >>Sputtering of Precursors for Cu2ZnSnS4 Solar Cells and Application of Cadmium Free Buffer Layers
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The aim of this thesis is to understand the influence of the deposition process and resulting film properties on Cu2ZnSnS4 (CZTS) thin film solar cells. Two main aspects are studied, namely formation of absorber precursors by sputtering, and alternative Cd-free buffer materials with improved band alignment.

Reactive sputtering is used to grow dense and homogeneous precursor films containing all elements needed for CZTS absorbers. The addition of H2S gas to the inert Ar sputter atmosphere leads to a drastic decrease of Zn-deposition rate due to the sulfurization of the target. Sulfurization also leads to instabilities for targets made of CuSn, Cu and Cu2S, while sputtering from CuS gave acceptable process stability.

The H2S/Ar-ratio also affects film morphology and composition. Precursors with sulfur content close to stoichiometric CZTS have a columnar, crystalline structure. Materials analysis suggests a non-equilibrium phase with a cubic structure, where each S atom is randomly surrounded by 2:1:1 Cu:Zn:Sn-atoms, respectively. Substrate heating during sputtering is shown to be important to avoid cracks in the annealed films while stress in the precursor films is not observed to affect the absorber or solar cell quality.

Sputtering from compound targets in Ar-atmosphere yields precursor properties similar to those from reactive sputtering at high H2S/Ar-ratios and both types can be processed into well-performing solar cells.

Additionally, a low temperature treatment of CZTS absorbers in inert atmosphere prior to buffer layer growth is shown to affect the device properties, which indicates that the thermal history of the CZTS absorber is important.

The alternative buffer system ZnO1-xSx is found to yield lower efficiencies than expected, possibly due to inferior interface or buffer quality. The Zn1-xSnxOy (ZTO) buffers instead give better performance than their CdS references. For optimized parameters, the activation energy for recombination coincides with the energy of the photoluminescence peak of the absorber. This can be interpreted as a shift of dominant recombination path from the interface to the CZTS bulk. A well-performing CZTS-ZTO device with antireflective coating yielded an efficiency of 9.0 %, which at the time of publication was the highest value published for a Cd-free pure-sulfide CZTS solar cell.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2018. p. 102
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1707
Keywords
solar cells, thin film, buffer layer, sputtering, reactive sputtering, CZTS, kesterite, zinc tin oxide
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Engineering Science with specialization in Electronics
Identifiers
urn:nbn:se:uu:diva-357354 (URN)978-91-513-0414-4 (ISBN)
Public defence
2018-10-05, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 13:15 (English)
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Supervisors
Available from: 2018-09-13 Created: 2018-08-17 Last updated: 2018-10-02

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Ericson, ToveLarsson, FredrikTörndahl, TobiasFrisk, ChristopherLarsen, JesKosyak, VolodymyrHägglund, CarlLi, ShuyiPlatzer Björkman, Charlotte

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