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Zn(O,S)/Cu(In,Ga)Se2 solar cells: band alignment and sulfur gradient
Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Technology, Department of Engineering Sciences.
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Article in journal (Refereed) Submitted
URN: urn:nbn:se:uu:diva-93920OAI: oai:DiVA.org:uu-93920DiVA: diva2:167560
Available from: 2006-01-13 Created: 2006-01-13Bibliographically approved
In thesis
1. Band Alignment Between ZnO-Based and Cu(In,Ga)Se2 Thin Films for High Efficiency Solar Cells
Open this publication in new window or tab >>Band Alignment Between ZnO-Based and Cu(In,Ga)Se2 Thin Films for High Efficiency Solar Cells
2006 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Thin-film solar cells based on Cu(In,Ga)Se2 contain a thin buffer layer of CdS in their standard configuration. In order to avoid cadmium in the device for environmental reasons, Cd-free alternatives are investigated. In this thesis, ZnO-based films, containing Mg or S, grown by atomic layer deposition (ALD), are shown to be viable alternatives to CdS.

The CdS is an n-type semiconductor, which together with the n-type ZnO top-contact layers form the pn-junction with the p-type Cu(In,Ga)Se2. From device modeling it is known that a buffer layer conduction band (CB) position of 0-0.4 eV above that of the Cu(In,Ga)Se2 layer is consistent with high photovoltaic performance. For the Cu(In,Ga)Se2/ZnO interface this position is measured by photoelectron spectroscopy and optical methods to –0.2 eV, resulting in increased interface recombination. By including sulfur into ZnO, a favorable CB position to Cu(In,Ga)Se2 can be obtained for appropriate sulfur contents, and device efficiencies of up to 16.4% are demonstrated in this work. From theoretical calculations and photoelectron spectroscopy measurements, the shift in the valence and conduction bands of Zn(O,S) are shown to be non-linear with respect to the sulfur content, resulting in a large band gap bowing.

ALD is a suitable technique for buffer layer deposition since conformal coverage can be obtained even for very thin films and at low deposition temperatures. However, deposition of Zn(O,S) is shown to deviate from an ideal ALD process with much larger sulfur content in the films than expected from the precursor pulsing ratios and with a clear increase of sulfur towards the Cu(In,Ga)Se2 layer.

For (Zn,Mg)O, single-phase ZnO-type films are obtained for Mg/(Zn+Mg) < 0.2. In this region, the band gap increases almost linearly with the Mg content resulting in an improved CB alignment at the heterojunction interface with Cu(In,Ga)Se2 and high device efficiencies of up to 14.1%.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2006. 80 p.
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 136
Electronics, solar cells, Cu(In Ga)Se2, atomic layer deposition, ZnO, Zn(O S), (Zn Mg)O, band alignment, photoelectron spectroscopy, Elektronik
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
urn:nbn:se:uu:diva-6263 (URN)91-554-6435-1 (ISBN)
Public defence
2006-02-03, Häggsalen, Ångströmslaboratoriet, Uppsala, 09:30 (English)
Available from: 2006-01-13 Created: 2006-01-13 Last updated: 2011-11-10Bibliographically approved

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