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Deposition temperature induced conduction band changes in zinc tin oxide buffer layers for Cu(In,Ga)Se2 solar cells
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.
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics. (Ångström Solar Center, FTE)
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics. (Ångström Solar Center, FTE)
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2016 (English)In: Solar Energy Materials and Solar Cells, ISSN 0927-0248, E-ISSN 1879-3398, Vol. 144, 684-690 p.Article in journal (Refereed) Published
Abstract [en]

Thin film Cu(In,Ga)Se2 solar cells with ALD-deposited Zn1-xSnxOy buffer layers were fabricated and the solar cell properties were investigated for varying ALD deposition temperatures in the range from 90 °C up to 180 °C. It was found that a process window exists between 105 °C and 135 °C, where high solar cell efficiency can be achieved. At lower ALD deposition temperatures the solar cell performance was mainly limited by low fill factor and at higher temperatures by low open circuit voltage. Numerical simulations and electrical characterization were used to relate the changes in solar cell performance as a function of ALD deposition temperature to changes in the conduction band energy level of the Zn1-xSnxOy buffer layer. The Zn1-xSnxOy films contain small ZnO or ZnO(Sn) crystallites (~10 nm), resulting in quantum confinement effects influencing the optical band gap of the buffer layer. The ALD deposition temperature affects the size of these crystallites and it is concluded that most of the changes in the band gap occur in the conduction band level.

Place, publisher, year, edition, pages
2016. Vol. 144, 684-690 p.
Keyword [en]
Zinc tin oxide (ZTO); Atomic layer deposition (ALD); Buffer layer; Thin film photovoltaics; CIGS; Conduction band line-up;
National Category
Engineering and Technology Environmental Engineering
Identifiers
URN: urn:nbn:se:uu:diva-260879DOI: 10.1016/j.solmat.2015.09.048ISI: 000366223900087OAI: oai:DiVA.org:uu-260879DiVA: diva2:848649
Funder
Swedish Energy Agency, 2012-004591VINNOVA, 2013-02199StandUp
Available from: 2015-08-25 Created: 2015-08-25 Last updated: 2017-12-04Bibliographically approved
In thesis
1. Atomic layer deposition of zinc tin oxide buffer layers for Cu(In,Ga)Se2 solar cells
Open this publication in new window or tab >>Atomic layer deposition of zinc tin oxide buffer layers for Cu(In,Ga)Se2 solar cells
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The aim of this thesis is to provide an in-depth investigation of zinc tin oxide, Zn1-xSnxOy or ZTO, grown by atomic layer deposition (ALD) as a buffer layer in Cu(In,Ga)Se2 (CIGS) solar cells. The thesis analyzes how changes in the ALD process influence the material properties of ZTO, and how these in turn affect the performance of CIGS solar cells.

It is shown that ZTO grows uniformly and conformably on CIGS and that the interface between ZTO and CIGS is sharp with little or no interdiffusion between the layers. The band gap and conduction band energy level of ZTO are dependent both on the [Sn]/([Zn]+[Sn]) composition and on the deposition temperature. The influence by changes in composition is non-trivial, and the highest band gap and conduction band energy level are obtained at a [Sn]/([Zn]+[Sn]) composition of 0.2 at 120  °C. An increase in optical band gap is observed at decreasing deposition temperatures and is associated with quantum confinement effects caused by a decrease in crystallite size. The ability to change the conduction band energy level of ZTO enables the formation of suitable conduction band offsets between ZTO and CIGS with varying Ga-content.

It is found that 15 nm thin ZTO buffer layers are sufficient to fabricate CIGS solar cells with conversion efficiencies up to 18.2 %. The JSC is in general 2 mA/cm2 higher, and the VOC 30 mV lower, for cells with the ZTO buffer layer as compared to cells with the traditional CdS buffer layer. In the end comparable efficiencies are obtained for the two different buffer layers. The gain in JSC for the ZTO buffer layer is associated with lower parasitic absorption in the UV-blue region of the solar spectrum and it is shown that the JSC can be increased further by making changes to the other layers in the traditional CdS/i-ZnO/ZnO:Al window layer structure. The ZTO is highly resistive, and it is found that the shunt preventing i-ZnO layer can be omitted, which further increases the JSC. Moreover, an additional increase in JSC is obtained by replacing the sputtered ZnO:Al front contact with In2O3 deposited by ALD. The large gain in JSC for the ZTO/In2O3 window layer stack compensates for the lower VOC related to the ZTO buffer layer, and it is demonstrated that the ZTO/In2O3 window layer structure yields 0.6 % (absolute) higher conversion efficiency than the CdS/i-ZnO/ZnO:Al window layer structure. 

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2015. 104 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1277
Keyword
CIGS; Solar cells; Thin film; Buffer layer; TCO; Window layer; Zinc tin oxide; ZTO; Indium oxide
National Category
Engineering and Technology Materials Engineering
Research subject
Engineering Science with specialization in Electronics
Identifiers
urn:nbn:se:uu:diva-260882 (URN)978-91-554-9313-4 (ISBN)
Public defence
2015-10-16, Häggsalen, Lägerhyddsvägen 1, Uppsala, 13:00 (English)
Opponent
Supervisors
Funder
Swedish Energy Agency, 2012-00-4591
Available from: 2015-09-24 Created: 2015-08-25 Last updated: 2015-10-01

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Lindahl, JohanKeller, JanDonzel-Gargand, OlivierSzaniawski, PiotrEdoff, MarikaTörndahl, Tobias

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