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Light-enhanced reverse breakdown in 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, FTE)
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. (Ångström Solar Center)
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics. (Ångström Solar Center)
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2013 (English)In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 535, 326-330 p.Article in journal (Refereed) Published
Abstract [en]

Partial shading of solar modules can subject shaded cells to significant reverse bias, often large enough toforce them into electrical breakdown, possibly resulting in irreversible damage. Therefore, better understandingof reverse current–voltage characteristics might lead to improvements in the design of solar modules. Thefocus of this study is the breakdown behavior of Cu(In,Ga)Se2 (CIGS) cells in darkness and under illumination.Two series of CIGS cells were investigated, with CdS and Zn–Sn–O buffer layers of varying thickness. Electricalbreakdown was found to be highly dependent on the buffer layer. Under blue illumination a remarkable decreasein breakdown voltage was observed for both buffer types. Metastable defects in the buffer/CIGS interfaceregion are tentatively proposed as the source of this effect and tunnelling is suggested as the mainmechanism responsible for breakdowns.

Place, publisher, year, edition, pages
Elsevier, 2013. Vol. 535, 326-330 p.
Keyword [en]
copper indium gallium selenide, photoelectric properties, reverse breakdown, defects
National Category
Engineering and Technology Engineering and Technology
Research subject
Engineering Science with specialization in Electronics
Identifiers
URN: urn:nbn:se:uu:diva-187534DOI: 10.1016/j.tsf.2012.09.022ISI: 000318973600073OAI: oai:DiVA.org:uu-187534DiVA: diva2:575060
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Available from: 2012-12-07 Created: 2012-12-07 Last updated: 2017-12-07
In thesis
1. From Light to Dark: Electrical Phenomena in Cu(In,Ga)Se2 Solar Cells
Open this publication in new window or tab >>From Light to Dark: Electrical Phenomena in Cu(In,Ga)Se2 Solar Cells
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In Cu(In,Ga)Se2 (CIGS) solar cells the CIGS layer serves as the light absorber, growing naturally p-type. Together with an n-type buffer layer they form a p-n heterojunction. Typically, CdS is used as a buffer, although other, less toxic materials are investigated as alternatives. The intrinsic p-type doping of CIGS layers is the result of complex defect physics. Defect formation energies in CIGS are very low or even negative, which results in extremely high defect concentrations. This leads to many unusual electrical phenomena that can be observed in CIGS devices. This thesis mostly focuses on three of these phenomena: light-soaking, light-on-bias, and light-enhanced reverse breakdown.

Light-soaking is a treatment that involves illuminating the investigated device for an extended period of time. In most CIGS solar cells it results in an improvement of open-circuit voltage, fill factor, and efficiency that can persist for hours, if not days. The interplay between light-soaking and the remaining two phenomena was studied. It was found that light-soaking has a strong effect on light-on-bias behavior, while the results for light-enhanced breakdown were inconclusive, suggesting little to no impact.

Light-on-bias is a treatment which combines simultaneous illumination and application of reverse bias to the studied sample. Illuminating CdS-based samples with red light while applying a reverse bias results in a significant increase in capacitance due to filling of traps. In many cases, this is accompanied by a decrease in device performance under red illumination. Complete recovery is possible by illuminating the treated sample with blue light, which causes hole injection from the CdS buffer. In samples with alternative buffer layers, there is little distinction between red and blue illumination, and the increase in capacitance is milder. At the same time, there is little effect on device performance.

Reverse breakdown can occur when a sufficiently large reverse bias is applied to a p-n junction, causing a large reverse current to flow through the device. In CIGS solar cells, the voltage at which breakdown occurs in darkness decreases in the presence of blue illumination. A model explaining the breakdown in darkness was proposed as a part of this thesis. The model assumes that all voltage drops on the buffer layer in darkness and on the CIGS layer under blue illumination. The high electric field in the buffer facilitates Poole-Frenkel conduction and Fowler-Nordheim tunneling between the absorber and the buffer.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2017. 83 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1501
Keyword
Solar cells, Photovoltaics, Cu(InGa)Se2, CIGS, Electrical characterization
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Engineering Science with specialization in Electronics
Identifiers
urn:nbn:se:uu:diva-319454 (URN)978-91-554-9884-9 (ISBN)
Public defence
2017-06-01, Häggsalen, 10132, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 13:15 (English)
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Available from: 2017-05-11 Created: 2017-04-04 Last updated: 2017-05-23

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Szaniawski, PiotrLindahl, JohanTörndahl, TobiasZimmermann, UweEdoff, Marika

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