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Potential-Induced Degradation and possibilities for recovery of CuIn1-xGaxSe2 thin film solar cells
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
2015 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

The long-term performance of solar modules is of key importance to achieve profitable solar power installations. In this work, the degradation mechanism potential-induced degradation (PID) was investigated for CuIn1-xGaxSe2 (CIGS) thin film solar cells. PID is caused by a combination of certain system voltage situations and environment conditions, such as temperature and humidity. The conditions for PID were reproduced in the lab, using small test cells. A voltage was applied between the solar cell back contact and the rear side of the glass substrate while heating the samples to a temperature of 85°C.

Similar to crystalline silicon technology, CIGS solar cells were found to be susceptible to PID. One critical parameter for the degradation behavior is the choice of substrate and its ability to release Na during applied bias. The degradation was found to be linked with Na migration from the substrate into the devices. Solar cells, which were fully deteriorated in terms of electrical performance by PID, were found to have a substantially increased Na concentration. However, solar cells grown on Na free and high resistivity substrates were observed to be PID-resilient.

The degradation was shown to partly be non-permanent. Fully degraded CIGS solar cells could recover electrical performance to a certain degree. Three different recovery methods were applied (i) a passive recovery in darkness at room temperature, (ii) accelerated recovery with a reversed bias as compared to the PID treatment and (iii) etching and replacement of the top window layers followed by reversed bias. Recovery to over 90% of the initial efficiency was possible. However, the recovery rate varied depending on the recovery method. The accelerated method was found to reduce the concentration of Na in the buffer layer and interface volumes. The etch recovery method, which consists of renewing window and buffer layers further strengthen the hypothesis that a major part of the degradation could be attributed to the buffer layer and its interface to CIGS.

The importance of the buffer layer in PID was further highlighted in the experiment where the standard CdS buffer layer was substituted with Zn(O,S). Both types of solar cells degrade in the PID conditions. Zn(O,S) cells exhibited ohmic current-voltage relationship (no diode characteristics) in the degraded state, while the CdS counterpart had some degree of diode behavior. During recovery with the accelerated method, the CdS cells restored both current-voltage and capacitance-voltage behavior to larger extent than the Zn(O,S) cells. For the latter, the efficiency stayed close to zero throughout the recovery period.

Place, publisher, year, edition, pages
Uppsala: Uppsala universitet, 2015.
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Engineering Science with specialization in Electronics
Identifiers
URN: urn:nbn:se:uu:diva-265141OAI: oai:DiVA.org:uu-265141DiVA: diva2:862624
Presentation
2015-09-28, Å80121, Ångströmlaboratoriet, 15:00 (English)
Opponent
Supervisors
Available from: 2015-10-30 Created: 2015-10-23 Last updated: 2015-10-30Bibliographically approved
List of papers
1. Potential-Induced Degradation of CuIn1-xGaxSe2 Thin Film Solar Cells
Open this publication in new window or tab >>Potential-Induced Degradation of CuIn1-xGaxSe2 Thin Film Solar Cells
Show others...
2013 (English)In: IEEE Journal of Photovoltaics, ISSN 2156-3381, Vol. 3, no 3, 1090-1094 p.Article in journal (Refereed) Published
Abstract [en]

The use of Na-free or low Na content glass substrates is observed to enhance the resiliency to potential-induced degradation, as compared with glass substrates with high Na content, such as soda lime glass (SLG). The results from stress tests in this study suggest that degradation caused by a combination of heat and bias across the SLG substrate is linked to increased Na concentration in the CdS and Cu(In,Ga)Se-2 (CIGS) layers in CIGS-based solar cells. The degradation during the bias stress is dramatic. The efficiency drops to close to 0% after 50 h of stressing. On the other hand, cells on Na-free and low Na content substrates exhibited virtually no efficiency degradation. The degraded cells showed partial recovery by resting at room temperature without bias; thus, the degradation is nonpermanent and may be due to Na migration and accumulation rather than chemical reaction.

Keyword
Cu(In, Ga)Se-2 (CIGS), photovoltaics, potential-induced degradation, thin film solar cells
National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Electronics
Identifiers
urn:nbn:se:uu:diva-204264 (URN)10.1109/JPHOTOV.2013.2253833 (DOI)000320862500024 ()
Available from: 2013-07-30 Created: 2013-07-29 Last updated: 2015-10-30Bibliographically approved
2. Recovery After Potential-Induced Degradation of CuIn1-xGaxSe2 Solar Cells With CdS and Zn(O,S) Buffer Layers
Open this publication in new window or tab >>Recovery After Potential-Induced Degradation of CuIn1-xGaxSe2 Solar Cells With CdS and Zn(O,S) Buffer Layers
Show others...
2015 (English)In: IEEE Journal of Photovoltaics, ISSN 2156-3381, E-ISSN 2156-3403, Vol. 5, no 2, 664-669 p.Article in journal (Refereed) Published
Abstract [en]

This study deals with potential-induced degradation (PID) of Cu(In,Ga)Se-2-based solar cells and different approaches to subsequent recovery of efficiency. Three different recovery methods were studied: 1) etch recovery, 2) accelerated recovery, and 3) unaccelerated recovery. After being completely degraded, the solar cells with CdS buffer layers recovered their efficiencies at different rates, depending on the method which was used. On the other hand, if Zn(O,S) was used as a buffer layer instead of CdS, the recovery rate was close to zero. The buffer layer type clearly influenced the sodium distribution during PID stressing and recovery, as well as the possibilities for recovery of the electrical performance.

Keyword
Buffer layer, Cu(In, Ga)Se-2 (CIGS), potential-induced degradation (PID), thin-film solar cells
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:uu:diva-253087 (URN)10.1109/JPHOTOV.2014.2384839 (DOI)000353524800028 ()
Available from: 2015-06-02 Created: 2015-05-20 Last updated: 2017-12-04Bibliographically approved

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