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Ga-grading and Solar Cell Capacitance Simulation of an industrial Cu(In,Ga)Se2 solar cell produced by an in-line vacuum, all-sputtering process
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.ORCID iD: 0000-0002-4125-4002
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2017 (English)In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 636, 367-374 p.Article in journal (Refereed) Published
Abstract [en]

Cadmium-free Cu(In,Ga)Se-2 (CIGS) solar cells are fabricated on stainless steel substrate using an industrial, inline vacuum, all sputtering process. The absorber layer is deposited from compound CIGS targets and crystallized simultaneously by high temperature processing. In-depth compositional and structural characterization of the chalcopyrite material is conducted and a Solar Cell Capacitance Simulator (SCAPS) model for the complete device is set-up. Ga-grading of the absorber through the successive use of different CIGS target compositions and resulting in solar cell performance enhancement is shown. At the research and development scale, efficiency values of 15.1% and 13.2% are reported for 1 cm(2) and 225 cm(2) total area solar cells, respectively. Successful transfer to production is also demonstrated. A series of a hundred 225 cm(2) solar cells produced following an optimized process including the Ga grading studied in the present contribution average at 14.8% total area efficiency.

Place, publisher, year, edition, pages
2017. Vol. 636, 367-374 p.
Keyword [en]
Copper indium gallium selenide, In-line vacuum, Sputtering, Cadmium-free, SCAPS, Gallium-grading
National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Electronics
Identifiers
URN: urn:nbn:se:uu:diva-329771DOI: 10.1016/j.tsf.2017.06.031ISI: 000408037800053OAI: oai:DiVA.org:uu-329771DiVA: diva2:1143238
Available from: 2017-09-21 Created: 2017-09-21 Last updated: 2017-11-20Bibliographically approved
In thesis
1. Sputtering-based processes for thin film chalcogenide solar cells on steel substrates
Open this publication in new window or tab >>Sputtering-based processes for thin film chalcogenide solar cells on steel substrates
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Thin film chalcogenide solar cells are promising photovoltaic technologies. Cu(In,Ga)Se2 (CIGS)-based devices are already produced at industrial scale and record laboratory efficiency surpasses 22 %. Cu2ZnSn(S,Se)4 (CZTS) is an alternative material that is based on earth-abundant elements. CZTS device efficiency above 12 % has been obtained, indicating a high potential for improvement.

In this thesis, in-line vacuum, sputtering-based processes for the fabrication of complete thin film chalcogenide solar cells on stainless steel substrates are studied. CIGS absorbers are deposited in a one-step high-temperature process using compound targets. CZTS precursors are first deposited by room temperature sputtering and absorbers are then formed by high temperature crystallization in a controlled atmosphere. In both cases, strategies for absorber layer improvement are identified and implemented.

The impact of CZTS annealing temperature is studied and it is observed that the absorber grain size increases with annealing temperature up to 550 °C. While performance also improves from 420 to 510 °C, a drop in all solar cell parameters is observed for higher temperature. This loss is caused by blisters forming in the absorber during annealing. Blister formation is found to originate from gas entrapment during precursor sputtering. Increase in substrate temperature or sputtering pressure leads to drastic reduction of gas entrapment and hence alleviate blister formation resulting in improved solar cell parameters, including efficiency.

An investigation of bandgap grading in industrial CIGS devices is conducted through one-dimensional simulations and experimental verification. It is found that a single gradient in the conduction band edge extending throughout the absorber combined with a steeper back-grading leads to improved solar cell performance, mainly due to charge carrier collection enhancement.

The uniformity of both CIGS and CZTS 6-inch solar cells is assessed. For CZTS, the device uniformity is mainly limited by the in-line annealing process. Uneven heat and gas distribution resulting from natural convection phenomenon leads to significant lateral variation in material properties and device performance. CIGS solar cell uniformity is studied through laterally-resolved material and device characterization combined with SPICE network modeling. The absorber material is found to be laterally homogeneous. Moderate variations observed at the device level are discussed in the context of large area sample characterization.

Power conversion efficiency values above 15 % for 225 cm2 CIGS cells and up to 5.1 % for 1 cm2 CZTS solar cells are obtained.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2017. 107 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1564
Keyword
CIGS, CZTS, solar cell, photovoltaics, thin film, sputtering, annealing, gallium grading, blister, uniformity, stainless steel
National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Electronics
Identifiers
urn:nbn:se:uu:diva-329778 (URN)978-91-513-0078-8 (ISBN)
Public defence
2017-11-09, Polhemssalen, Ångström laboratory, Lägerhyddsvägen 1, Uppsala, 09:15 (English)
Opponent
Supervisors
Funder
Swedish Foundation for Strategic Research Swedish Energy Agency
Available from: 2017-10-17 Created: 2017-09-21 Last updated: 2017-11-13

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Bras, PatriceFrisk, ChristopherPlatzer Björkman, Charlotte

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