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XPS/UPS monitoring of ALCVD ZnO growth on Cu(In,Ga)Se2 absorbers
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Materials Science.
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Materials Science.
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Materials Science.
2002 (English)In: Seventeenth European Photovoltaic Solar Energy Conference: proceedings of the international conference held in Munich, Germany, 22-26 October 2001 / [ed] McNelis, B, 2002, Vol. II, 1118-1121 p.Conference paper, Published paper (Refereed)
Place, publisher, year, edition, pages
2002. Vol. II, 1118-1121 p.
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:uu:diva-93914ISBN: 3-936338-08-6 (print)ISBN: 88-900442-3-3 (print)OAI: oai:DiVA.org:uu-93914DiVA: diva2:167554
Conference
Seventeenth European Photovoltaic Solar Energy Conference, Munich, Germany, 22-26 October 2001
Available from: 2006-01-13 Created: 2006-01-13 Last updated: 2011-11-11Bibliographically 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.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 136
Keyword
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
Identifiers
urn:nbn:se:uu:diva-6263 (URN)91-554-6435-1 (ISBN)
Public defence
2006-02-03, Häggsalen, Ångströmslaboratoriet, Uppsala, 09:30 (English)
Opponent
Supervisors
Available from: 2006-01-13 Created: 2006-01-13 Last updated: 2011-11-10Bibliographically approved
2. ALD Buffer Layer Growth and Interface Formation on Cu(In,Ga)Se2 Solar Cell Absorbers
Open this publication in new window or tab >>ALD Buffer Layer Growth and Interface Formation on Cu(In,Ga)Se2 Solar Cell Absorbers
2004 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Cu(In,Ga)Se2 (CIGS) thin film solar cells contain a thin layer of CdS. To avoid toxic heavy-metal-containing waste in the module production the development of a cadmium-free buffer layer is desirable. This thesis considers alternative Cd-free buffer materials deposited by Atomic Layer Deposition (ALD). Conditions of the CIGS surface necessary for ALD growth are investigated and the heterojunction interface is characterized by band alignment studies of ZnO/CIGS and In2S3/CIGS interfaces. The thesis also includes investigations on the surface modification of the CIGS absorber by sulfurization.

According to ALD theory the growth process is limited by surface saturated reactions. The ALD growth on CIGS substrates shows nucleation failure and generally suffers from surface contaminations of the CIGS layer. The grade of growth disturbance varies for different ALD precursors. The presence of surface contaminants is related to the substrate age and sodium content. Improved growth behavior is demonstrated by different pretreatment procedures.

The alignment of the energy bands in the buffer/absorber interface is an important parameter for minimization of the losses in a solar cell. The valence band and conduction band offsets was determined by in situ X-ray and UV photoelectron spectroscopy during layer by layer formation of buffer material. The conduction band offset (ΔEc) should be small but positive for optimal solar cell electrical performance according to theory. The conduction band offset was determined for the ALD ZnO/CIGS interface (ΔEc = -0.2 eV) and the ALD In2S3/CIGS interface (ΔEc = -0.25 eV).

A high temperature process for bandgap grading and a low temperature process for surface passivation by post deposition sulfurization in H2S were investigated. It is concluded that the high temperature sulfurization of CuIn(1-x)GaxSe2 leads to phase separation when x>0. The low temperature process did not result in enhanced device performance.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2004. 51 p.
Series
Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1104-232X ; 942
Keyword
Electronics, thin film, Cu(In, Ga)Se2, CIGS, chalcopyrite, ALD, sulfurization, buffer layer, XPS, UPS, electron spectroscopy, band alignment, atomic layer deposition, Elektronik
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:uu:diva-4009 (URN)91-554-5883-1 (ISBN)
Public defence
2004-03-12, Polhemssalen, Ångströmslaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:30 (English)
Opponent
Supervisors
Available from: 2004-02-20 Created: 2004-02-20 Last updated: 2011-11-11Bibliographically approved

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