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From Light to Dark: Electrical Phenomena in Cu(In,Ga)Se2 Solar Cells
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
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 [en]
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: urn:nbn:se:uu:diva-319454ISBN: 978-91-554-9884-9 (print)OAI: oai:DiVA.org:uu-319454DiVA: diva2:1086886
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
List of papers
1. Light-enhanced reverse breakdown in Cu(In,Ga)Se2 solar cells
Open this publication in new window or tab >>Light-enhanced reverse breakdown in Cu(In,Ga)Se2 solar cells
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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
Keyword
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:nbn:se:uu:diva-187534 (URN)10.1016/j.tsf.2012.09.022 (DOI)000318973600073 ()
Funder
StandUp
Available from: 2012-12-07 Created: 2012-12-07 Last updated: 2017-04-04
2. Influence of Varying Cu Content on Growth and Performance of Ga-Graded Cu(In,Ga)Se-2 Solar Cells
Open this publication in new window or tab >>Influence of Varying Cu Content on Growth and Performance of Ga-Graded Cu(In,Ga)Se-2 Solar Cells
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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.

Keyword
Coevaporation, Cu(In, Ga)Se-2 (CIGS), grading, interdiffusion
National Category
Physical Sciences Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-268416 (URN)10.1109/JPHOTOV.2015.2478033 (DOI)000364098400041 ()
Funder
Swedish Energy AgencyStandUpEU, FP7, Seventh Framework Programme, 327367
Available from: 2015-12-06 Created: 2015-12-04 Last updated: 2017-04-04Bibliographically approved
3. A Systematic Study of Light-On-Bias Behavior in Cu(In,Ga)Se2 Solar Cells With Varying Absorber Compositions
Open this publication in new window or tab >>A Systematic Study of Light-On-Bias Behavior in Cu(In,Ga)Se2 Solar Cells With Varying Absorber Compositions
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2017 (English)In: IEEE Journal of Photovoltaics, ISSN 2156-3381, E-ISSN 2156-3403, Vol. 7, no 3, 882-891 p.Article in journal (Refereed) Published
Abstract [en]

Light-on-bias effects were investigated in multiple Cu(In, Ga)Se2 solar cells with varying absorber layer compositions. A strong link between deformations caused by red-on-bias treatments in current-voltage (IV ) and capacitance-voltage (CV) characteristics was demonstrated. Similarly to red-on-bias, blue-on-bias leads to a local increase in static negative charge, but in samples with CdS buffers this increase is shifted away from the interface and has no impact on device performance. IV characteristics of samples with Cd-free buffers are not affected by any light-on-bias treatments, suggesting that CdS plays a vital role in the decreased performance after red-on-bias. A statistical approach was used to search for compositional trends in red-on-bias behavior. Deformation factors were defined for IV and CV characteristics before and after the treatment. While there is a strong relationship between the deformations observed in both types of measurements, the degree to which red-on-bias affects IV and CV curves can vary dramatically. These variations cannot be attributed to changes in composition, since no clear compositional trends were found. Rather, other factors related to sample manufacturing and to the buffer layer seem to have major impact on red-on-bias behavior.

Keyword
Capacitance;Charge measurement;Current measurement;Light emitting diodes;Lighting;Photovoltaic cells;Temperature measurement;Capacitance-voltage characteristics;current-voltage characteristics;degradation;deposition and characterization of thin film PV absorbers;photovoltaic cells;photovoltaic effects;thin film PV device properties and modeling
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:uu:diva-319426 (URN)10.1109/JPHOTOV.2017.2655148 (DOI)000399992000023 ()
Available from: 2017-04-04 Created: 2017-04-04 Last updated: 2017-05-30Bibliographically approved
4. Advancing the understanding of reverse breakdown in Cu(In,Ga)Se2 solar cells
Open this publication in new window or tab >>Advancing the understanding of reverse breakdown in Cu(In,Ga)Se2 solar cells
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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.

National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:uu:diva-319442 (URN)10.1109/JPHOTOV.2017.2699860 (DOI)000404258900027 ()
Available from: 2017-04-04 Created: 2017-04-04 Last updated: 2017-09-12Bibliographically approved

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