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The effect of Zn1-xMgxO buffer layer deposition temperature on Cu(In,Ga)Se2 solar cells: A study of the buffer/absorber interface
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, Experimental Physics. (Elektronmikroskopi och Nanoteknologi)
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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2009 (English)In: Progress in Photovoltaics, ISSN 1062-7995, E-ISSN 1099-159X, Vol. 17, no 2, 115-125 p.Article in journal (Refereed) Published
Abstract [en]

The effect of atomic layer deposition temperature of Zn1-xMgxO buffer   layers for Cu(In,Ga)Se-2 (CIGS) based solar cell devices is evaluated.   The Zn1-xMgxO films are grown using diethyl zinc, bis-cyclopentadienyl   magnesium and water as precursors in a temperature range of 105 to 180   C High efficiency devices are produced in the region front 105 up to   135 degrees C. At a Zn1-xMgxO deposition temperature of 120 C, a   maximum cell efficiency of 15.5% is reached by using a Zn1-xMgxO layer   with an x-value of 0.2 and a thickness of 140 inn. A significant drop   in cell efficiency due to large losses in open circuit voltage and fill   factor is observed for devices grown at temperatures above 150 C. No   differences in chemical composition, structure and morphology of the   samples are observed, except for the samples prepared at 105 and 120 C   that show elemental selenium present at the buffer/absorber interface.   The selenium at the interface does not lead to major degradation of   the,solar cell device efficiency. Instead, a decrease in Zn1-xMgxO   resistivity by more than one order of magnitude at growth temperatures   above 150 C may explain the degradation in solar cell performance. From   energy filtered transmission electron microscopy, the width of the   CIGS/Zn1-xMgxO chemical interface is found to be thinner than 10 not   without any areas of depletion for Cu, Se, Zn and O.

Place, publisher, year, edition, pages
2009. Vol. 17, no 2, 115-125 p.
Keyword [en]
Zn1-xMgxO, Cu(In, Ga)Se-2, interface, selenium, atomic layer deposition, resistivity
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:uu:diva-111352DOI: 10.1002/pip.859ISI: 000263896800002OAI: oai:DiVA.org:uu-111352DiVA: diva2:280766
Available from: 2009-12-11 Created: 2009-12-11 Last updated: 2017-12-12Bibliographically approved
In thesis
1. Cadmium Free Buffer Layers and the Influence of their Material Properties on the Performance of Cu(In,Ga)Se2 Solar Cells
Open this publication in new window or tab >>Cadmium Free Buffer Layers and the Influence of their Material Properties on the Performance of Cu(In,Ga)Se2 Solar Cells
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

CdS is conventionally used as a buffer layer in Cu(In,Ga)Se2, CIGS, solar cells. The aim of this thesis is to substitute CdS with cadmium-free, more transparent and environmentally benign alternative buffer layers and to analyze how the material properties of alternative layers affect the solar cell performance. The alternative buffer layers have been deposited using Atomic Layer Deposition, ALD. A theoretical explanation for the success of CdS is that its conduction band, Ec, forms a small positive offset with that of CIGS.

In one of the studies in this thesis the theory is tested experimentally by changing both the Ec position of the CIGS and of Zn(O,S) buffer layers through changing their gallium and sulfur contents respectively. Surprisingly, the top performing solar cells for all gallium contents have Zn(O,S) buffer layers with the same sulfur content and properties in spite of predicted unfavorable Ec offsets. An explanation is proposed based on observed non-homogenous composition in the buffer layer.

This thesis also shows that the solar cell performance is strongly related to the resistivity of alternative buffer layers made of (Zn,Mg)O. A tentative explanation is that a high resistivity reduces the influence of shunt paths at the buffer layer/absorber interface. For devices in operation however, it seems beneficial to induce persistent photoconductivity, by light soaking, which can reduce the effective Ec barrier at the interface and thereby improve the fill factor of the solar cells.

Zn-Sn-O is introduced as a new buffer layer in this thesis. The initial studies show that solar cells with Zn-Sn-O buffer layers have comparable performance to the CdS reference devices.

While an intrinsic ZnO layer is required for a high reproducibility and performance of solar cells with CdS buffer layers it is shown in this thesis that it can be thinned if Zn(O,S) or omitted if (Zn,Mg)O buffer layers are used instead. As a result, a top conversion efficiency of 18.1 % was achieved with an (Zn,Mg)O buffer layer, a record for a cadmium and sulfur free CIGS solar cell.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2010. 75 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 787
Keyword
Cu(In, Ga)Se2, Solar cells, Thin film, Buffer layer, Window layer, ZnO, Zn(O, S), (Zn, Mg)O, Zn-Sn-O
National Category
Condensed Matter Physics
Research subject
Engineering Science with specialization in Solid State Physics
Identifiers
urn:nbn:se:uu:diva-133112 (URN)978-91-554-7944-2 (ISBN)
Public defence
2010-12-16, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 13:00 (English)
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Felaktigt tryckt som Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 717Available from: 2010-11-25 Created: 2010-11-02 Last updated: 2011-03-21Bibliographically approved

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Törndahl, TobiasCoronel, ErnestoHultqvist, AdamPlatzer-Björkman, CharlotteLeifer, KlausEdoff, Marika

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