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Advancing the understanding of 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.
Warsaw Univ Technol, Fac Phys, PL-00662 Warsaw, Poland.
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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2017 (English)In: IEEE Journal of Photovoltaics, ISSN 2156-3381, E-ISSN 2156-3403, Vol. 7, no 4, 1136-1142 p.Article in journal (Refereed) Published
Abstract [en]

Reverse breakdown is investigated in multiple Cu(In,Ga)Se-2 solar cells with varying buffer layer thicknesses. A method to extract transition voltage, which marks the change of conduction mechanism that leads to electrical breakdown, is described as an alternative to the often less-meaningful breakdown voltage. Transition voltages for samples with CdS and ZnxSn1-xOy buffers are extracted from breakdown measurements performed in darkness and under illumination. The electric field is calculated for ZTO-based samples measured in darkness, and its implications for the energy band structure are examined. Fowler-Nordheim tunneling and Poole-Frenkel conduction are considered as candidates for the main breakdown mechanism in darkness. A model combining the two conduction mechanisms is proposed, and fits for experimental data are presented and discussed. Involvement of defects is debated, and defect-andbreakdown- related phenomena are showcased.

Place, publisher, year, edition, pages
2017. Vol. 7, no 4, 1136-1142 p.
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
URN: urn:nbn:se:uu:diva-319442DOI: 10.1109/JPHOTOV.2017.2699860ISI: 000404258900027OAI: oai:DiVA.org:uu-319442DiVA: diva2:1086833
Available from: 2017-04-04 Created: 2017-04-04 Last updated: 2017-09-12Bibliographically approved
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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