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Fjällström, Viktor
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Publications (10 of 26) Show all publications
Szaniawski, P., Olsson, J., Frisk, C., Fjällström, V., Ledinek, D., Larsson, F., . . . Edoff, M. (2017). A Systematic Study of Light-On-Bias Behavior in Cu(In,Ga)Se2 Solar Cells With Varying Absorber Compositions. IEEE Journal of Photovoltaics, 7(3), 882-891
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, p. 882-891Article 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
Salome, P. M. P., Fjällström, V., Szaniawski, P., Leitao, J. P., Hultqvist, A., Fernandes, P. A., . . . Edoff, M. (2015). A comparison between thin film solar cells made from co-evaporated CuIn1-xGaxSe2 using a one-stage process versus a three-stage process. Progress in Photovoltaics, 23(4), 470-478
Open this publication in new window or tab >>A comparison between thin film solar cells made from co-evaporated CuIn1-xGaxSe2 using a one-stage process versus a three-stage process
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2015 (English)In: Progress in Photovoltaics, ISSN 1062-7995, E-ISSN 1099-159X, Vol. 23, no 4, p. 470-478Article in journal (Refereed) Published
Abstract [en]

Until this day, the most efficient Cu(In,Ga)Se-2 thin film solar cells have been prepared using a rather complex growth process often referred to as three-stage or multistage. This family of processes is mainly characterized by a first step deposited with only In, Ga and Se flux to form a first layer. Cu is added in a second step until the film becomes slightly Cu-rich, where-after the film is converted to its final Cu-poor composition by a third stage, again with no or very little addition of Cu. In this paper, a comparison between solar cells prepared with the three-stage process and a one-stage/in-line process with the same composition, thickness, and solar cell stack is made. The one-stage process is easier to be used in an industrial scale and do not have Cu-rich transitions. The samples were analyzed using glow discharge optical emission spectroscopy, scanning electron microscopy, X-ray diffraction, current-voltage-temperature, capacitance-voltage, external quantum efficiency, transmission/reflection, and photoluminescence. It was concluded that in spite of differences in the texturing, morphology and Ga gradient, the electrical performance of the two types of samples is quite similar as demonstrated by the similar J-V behavior, quantum spectral response, and the estimated recombination losses. 

Keyword
photovoltaics, thin film solar cells, Cu(In, Ga)Se-2 (CIGS), co-evaporation, 3-stage (three-stage), multistage, one-stage, in-line
National Category
Other Engineering and Technologies
Identifiers
urn:nbn:se:uu:diva-258843 (URN)10.1002/pip.2453 (DOI)000351178400007 ()
Funder
StandUp
Available from: 2015-07-23 Created: 2015-07-20 Last updated: 2017-12-04Bibliographically approved
Jacobsson, T. J., Fjällstrom, V., Edoff, M. & Edvinsson, T. (2015). A theoretical analysis of optical absorption limits and performance of tandem devices and series interconnected architectures for solar hydrogen production. Solar Energy Materials and Solar Cells, 138, 86-95
Open this publication in new window or tab >>A theoretical analysis of optical absorption limits and performance of tandem devices and series interconnected architectures for solar hydrogen production
2015 (English)In: Solar Energy Materials and Solar Cells, ISSN 0927-0248, E-ISSN 1879-3398, Vol. 138, p. 86-95Article in journal (Refereed) Published
Abstract [en]

Photo-driven catalytic (PDC) water splitting, using either photoelectrochemical cells (PEC-cells), PV-electrolyzers, or some hybrid system in-between, has attracted a lot of attention. In single-cell device architectures for solar hydrogen production, based on single band gap photoabsorbers, there is a fundamental efficiency problem originating from the energy distribution of the solar spectrum and the thermodynamic and kinetic requirements for splitting water. The minimum band gap for a single-junction device in order to withhold unbiased overall water splitting is considered to be at least 2.0 eV. This is far from the 1.35 eV which is the optimal band gap of a semiconductor for maximum power conversion of light in the solar spectrum. This discrepancy has been termed as the solar spectrum mismatch problem (the SSM-problem). The standard solution to this problem is to construct tandem devices, whereas an alternative is to interconnect several one band gap cells in series, side by side. Both approaches enable the use of low energy photons in the solar spectrum while still providing a sufficiently high photopotential for driving the full reaction, without seriously compromising with the area efficiency. In this paper, the tandem and serial architectures for handling the SSM-problem are analyzed and compared. The analysis is focused towards differences in the limits of optical absorption, the optimal number of optical. absorbers, and their corresponding band gaps. Taking losses due to charge carrier separation and catalysis into account, the maximum STH-efficiency for a series interconnected solar splitting device was found to be 24.6%, compared to 32.0% for an optimum tandem device at 1 Sun (air mass 1.5, 1000 W m(-2)). This can be compared with the maximum efficiency of 18.0% for an ideal single band gap photoabsorber in single junction device. The analysis shows that the maximum STH efficiency limits for series interconnected architectures for unassisted solar water splitting are not particularly far behind the more commonly studied tandem devices. They could then be an interesting alternative given the simplicity and versatility of series interconnected device architectures. The analysis also compares how tandem devices and series interconnected devices can differ in terms of charge carrier separation, charge carrier transport, catalysis, overall efficiency, device architecture, and expected cost.

Keyword
The SSM-problem, IPDC, PDC, PEC, PV, CIGS
National Category
Environmental Engineering
Identifiers
urn:nbn:se:uu:diva-253223 (URN)10.1016/j.solmat.2015.02.023 (DOI)000353747600012 ()
Available from: 2015-06-22 Created: 2015-05-25 Last updated: 2017-12-04Bibliographically approved
Jacobsson, T. J., Fjällstrom, V., Edoff, M. & Edvinsson, T. (2015). CIGS based devices for solar hydrogen production spanning from PEC-cells to PV-electrolyzers: a comparison of efficiency, stability and device topology. Solar Energy Materials and Solar Cells, 134, 185-93
Open this publication in new window or tab >>CIGS based devices for solar hydrogen production spanning from PEC-cells to PV-electrolyzers: a comparison of efficiency, stability and device topology
2015 (English)In: Solar Energy Materials and Solar Cells, ISSN 0927-0248, E-ISSN 1879-3398, Vol. 134, p. 185-93Article in journal (Refereed) Published
National Category
Physical Chemistry Materials Chemistry Condensed Matter Physics Engineering and Technology
Research subject
Engineering Science with specialization in Electronics
Identifiers
urn:nbn:se:uu:diva-244294 (URN)10.1016/j.solmat.2014.11.041 (DOI)000349728100023 ()
Available from: 2015-02-13 Created: 2015-02-13 Last updated: 2017-12-04Bibliographically approved
Vermang, B., Wätjen, J. T., Fjällström, V., Rostvall, F., Edoff, M., Gunnarsson, R., . . . Flandre, D. (2015). Highly reflective rear surface passivation design for ultra-thin Cu(In,Ga)Se2 solar cells. Thin Solid Films, 582, 300-303
Open this publication in new window or tab >>Highly reflective rear surface passivation design for ultra-thin Cu(In,Ga)Se2 solar cells
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2015 (English)In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 582, p. 300-303Article in journal (Refereed) Published
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Engineering Science with specialization in Electronics
Identifiers
urn:nbn:se:uu:diva-235577 (URN)10.1016/j.tsf.2014.10.050 (DOI)000352225900064 ()
Available from: 2014-11-06 Created: 2014-11-06 Last updated: 2017-12-05Bibliographically approved
Kotipalli, R., Vermang, B., Fjällström, V., Edoff, M., Delamare, R. & Flandre, D. (2015). Influence of Ga/(Ga plus In) grading on deep-defect states of Cu(In, Ga)Se-2 solar cells. Physica Status Solidi. Rapid Research Letters, 9(3), 157-160
Open this publication in new window or tab >>Influence of Ga/(Ga plus In) grading on deep-defect states of Cu(In, Ga)Se-2 solar cells
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2015 (English)In: Physica Status Solidi. Rapid Research Letters, ISSN 1862-6254, E-ISSN 1862-6270, Vol. 9, no 3, p. 157-160Article in journal (Refereed) Published
Abstract [en]

The benefits of gallium (Ga) grading on Cu(In, Ga) Se-2 (CIGS) solar cell performance are demonstrated by comparing with ungraded CIGS cells. Using drive-level capacitance profiling (DLCP) and admittance spectroscopy (AS) analyses, we show the influence of Ga grading on the spatial variation of deep defects, free-carrier densities in the CIGS absorber, and their impact on the cell's open-circuit voltage V-oc. The parameter most constraining the cell's Voc is found to be the deep-defect density close to the space charge region (SCR ). In ungraded devices, high deep-defect concentrations (4.2 x 1016 cm(-3)) were observed near the SCR, offering a source for Shockley Read-Hall recombination, reducing the cell's Voc. In graded devices, the deep-defect densities near the SCR decreased by one order of magnitude (2.5 x 1015 cm(-3)) for back surface graded devices, and almost two orders of magnitude (8.6 x 1014 cm(-3)) for double surface graded devices, enhancing the cell's Voc. In compositionally graded devices, the free-carrier density in the absorber's bulk decreased in tandem with the ratio of gallium to gallium plus indium ratio GGI = Ga/(Ga + In), increasing the activation energy, hindering the ionization of the defect states at room temperature and enhancing their role as recombination centers within the energy band.

Keyword
gallium, deep levels, defects, admittance spectroscopy, CuInGaSe2, solar cells
National Category
Environmental Engineering
Research subject
Engineering Science with specialization in Electronics
Identifiers
urn:nbn:se:uu:diva-252029 (URN)10.1002/pssr.201510024 (DOI)000351674600001 ()
Available from: 2015-04-30 Created: 2015-04-28 Last updated: 2017-12-04Bibliographically approved
Szaniawski, P., Salome, P., Fjällström, V., Törndahl, T., Zimmermann, U. & Edoff, M. (2015). Influence of Varying Cu Content on Growth and Performance of Ga-Graded Cu(In,Ga)Se-2 Solar Cells. IEEE Journal of Photovoltaics, 5(6), 1775-1782
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, p. 1775-1782Article 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: 2017-12-01Bibliographically approved
Fjällström, V. (2015). Potential-Induced Degradation and possibilities for recovery of CuIn1-xGaxSe2 thin film solar cells. (Licentiate dissertation). Uppsala: Uppsala universitet
Open this publication in new window or tab >>Potential-Induced Degradation and possibilities for recovery of CuIn1-xGaxSe2 thin film solar cells
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:nbn:se:uu:diva-265141 (URN)
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
Fjällström, V., Szaniawski, P., Vermang, B., Salome, P. M. P., Rostvall, F., Zimmermann, U. & Edoff, M. (2015). Recovery After Potential-Induced Degradation of CuIn1-xGaxSe2 Solar Cells With CdS and Zn(O,S) Buffer Layers. IEEE Journal of Photovoltaics, 5(2), 664-669
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
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2015 (English)In: IEEE Journal of Photovoltaics, ISSN 2156-3381, E-ISSN 2156-3403, Vol. 5, no 2, p. 664-669Article 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
Kapilashrami, M., Conti, G., Zegkinoglou, I., Nemsak, S., Conlon, C. S., Törndahl, T., . . . Himpsel, F. J. (2014). Boron Doped diamond films as electron donors in photovoltaics: An X-ray absorption and hard X-ray photoemission study. Journal of Applied Physics, 116(14), 143702
Open this publication in new window or tab >>Boron Doped diamond films as electron donors in photovoltaics: An X-ray absorption and hard X-ray photoemission study
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2014 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 116, no 14, p. 143702-Article in journal (Refereed) Published
Abstract [en]

Highly boron-doped diamond films are investigated for their potential as transparent electron donors in solar cells. Specifically, the valence band offset between a diamond film (as electron donor) and Cu(In,Ga)Se-2 (CIGS) as light absorber is determined by a combination of soft X-ray absorption spectroscopy and hard X-ray photoelectron spectroscopy, which is more depth-penetrating than standard soft X-ray photoelectron spectroscopy. In addition, a theoretical analysis of the valence band is performed, based on GW quasiparticle band calculations. The valence band offset is found to be small: VBO = VBMCIGS -VBMdiamond = 0.3 eV +/- 0.1 eV at the CIGS/Diamond interface and 0.0 eV +/- 0.1 eV from CIGS to bulk diamond. These results provide a promising starting point for optimizing the band offset by choosing absorber materials with a slightly lower valence band maximum. 

National Category
Physical Sciences Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-238570 (URN)10.1063/1.4897166 (DOI)000343988000019 ()
Available from: 2014-12-17 Created: 2014-12-14 Last updated: 2017-12-05Bibliographically approved
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