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Influence of Varying Cu Content on Growth and Performance of Ga-Graded Cu(In,Ga)Se-2 Solar Cells
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
Int Iberian Nanotechnol Lab, LaNaSC, P-4715330 Braga, Portugal..
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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2015 (English)In: IEEE Journal of Photovoltaics, ISSN 2156-3381, E-ISSN 2156-3403, Vol. 5, no 6, 1775-1782 p.Article in journal (Refereed) Published
Abstract [en]

Cu(In,Ga)Se-2 thin-film solar cells with Ga-graded absorber layers and a [Cu]/([In] + [Ga]) ratio varying between 0.5 and 1.0 were prepared by coevaporation and investigated. Except for the sample with a final [Cu]/([In] + [Ga]) ratio of 1.0, the samples were Cu-poor at all times during the evaporation. The variation in copper was found to influence the material properties in several ways: 1) Changing the Cu content had a strong impact on In and Ga interdiffusion, resulting in decreased Ga gradients in samples with large Cu deficiency; 2) the Cu-poor Cu(In, Ga)(3)Se-5 phase was detected in absorbers with [Cu]/([In] + [Ga]) ratios of 0.65 and below; and 3) the grain size changed significantly with the Cu variation. We observe a trend of reduced solar cell efficiencies for [Cu]/([In] + [Ga]) ratios of 0.65 and below, with an efficiency of 13.4% for the sample with a [Cu]/([In] + [Ga]) ratio of only 0.5, i.e., far from stoichiometry. We tentatively attribute the efficiency loss to a high concentration of point defects caused by the Cu deficiency.

Place, publisher, year, edition, pages
2015. Vol. 5, no 6, 1775-1782 p.
Keyword [en]
Coevaporation, Cu(In, Ga)Se-2 (CIGS), grading, interdiffusion
National Category
Physical Sciences Engineering and Technology
Identifiers
URN: urn:nbn:se:uu:diva-268416DOI: 10.1109/JPHOTOV.2015.2478033ISI: 000364098400041OAI: oai:DiVA.org:uu-268416DiVA: diva2:877208
Funder
Swedish Energy AgencyStandUpEU, FP7, Seventh Framework Programme, 327367
Available from: 2015-12-06 Created: 2015-12-04 Last updated: 2017-12-01Bibliographically 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)
Opponent
Supervisors
Funder
StandUp
Available from: 2017-05-11 Created: 2017-04-04 Last updated: 2017-05-23

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Szaniawski, PiotrFjällström, ViktorTörndahl, TobiasZimmermann, UweEdoff, Marika

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