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Modelling Band Gap Gradients and Cd-free Buffer Layers in Cu(In,Ga)Se2 Solar Cells
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics. (Solar Cell Group)
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

A deeper understanding of Cu(In,Ga)Se2 (CIGS) solar cells is important for the further improvement of these devices. This thesis is focused on the use of electrical modelling as a tool for pursuing this aim. Finished devices and individual layers are characterized and the acquired data are used as input in the simulations. Band gap gradients are accounted for when modelling the devices. The thesis is divided into two main parts. One part that treats the influence of cadmium free buffer layers, mainly atomic layer deposited (Zn,Mg)O, on devices and another part in which the result of CIGS absorber layer modifications is studied. Recombination analysis indicates that interface recombination is limitting the open circuit voltage (Voc) in cells with ZnO buffer layers. This recombination path becomes less important when magnesium is introduced into the ZnO giving a positive conduction band offset (CBO) towards the CIGS absorber layer. Light induced persistent photoconductivity (PPC) is demonstrated in (Zn,Mg)O thin films. Device modelling shows that the measured PPC, coupled with a high density of acceptors in the buffer-absorber interface region, can explain light induced metastable efficiency improvement in CIGS solar cells with (Zn,Mg)O buffer layers. It is shown that a thin indium rich layer closest to the buffer does not give any significant impact on the performance of devices dominated by recombination in the CIGS layer. In our cells with CdS buffer the diffusion length in the CIGS layer is the main limitting factor. A thinner CIGS layer improves Voc by reducing recombination. However, for thin enough absorber layers Voc deteriorates due to recombination at the back contact. Interface recombination is a problem in thin devices with Zn(O,S) buffer layers. This recombination path is overshadowed in cells of standard thickness by recombination in the CIGS bulk. Thin cells with Zn(O,S) buffer layers have a higher efficiency than CdS cells with the same absorber thickness.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2012. , p. 70
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 900
Keywords [en]
CIGS, Thin film solar cells, Electrical modelling, Alternative buffer layers, Gallium gradients, Simulations, Electrical characterization, Metastabilities, Light-soaking, Hall measurements, Persistent photoconductivity
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering Materials Engineering Physical Sciences
Research subject
Engineering Science with specialization in Electronics
Identifiers
URN: urn:nbn:se:uu:diva-168618ISBN: 978-91-554-8280-0 (print)OAI: oai:DiVA.org:uu-168618DiVA, id: diva2:501452
Public defence
2012-03-30, Polhemssalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 13:15 (English)
Opponent
Supervisors
Available from: 2012-03-09 Created: 2012-02-13 Last updated: 2013-04-08Bibliographically approved
List of papers
1. CuGaSe2 solar cells using atomic layer deposited Zn(O,S) and (Zn,Mg)O buffer layers
Open this publication in new window or tab >>CuGaSe2 solar cells using atomic layer deposited Zn(O,S) and (Zn,Mg)O buffer layers
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2009 (English)In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 517, no 7, p. 2305-2308Article in journal (Refereed) Published
Abstract [en]

The band gap of Zn(O,S) and (Zn,Mg)O buffer layers are varied with the objective of changing the conduction band alignment at the buffer layer/CuGaSe2 interface. To achieve this, alternative buffer layers are deposited using atomic layer deposition. The optimal compositions for CuGaSe2 solar cells are found to be close to the same for (Zn,Mg)O and the same for Zn(O,S) as in the CuIn0.7Ga0.3Se2 solar cell case. At the optimal compositions the solar cell conversion efficiency for (Zn,Mg)O buffer layers is 6.2% and for Zn(O,S) buffer layers it is 3.9% compared to the CdS reference cells which have 5-8% efficiency.

Keywords
Solar cells, CuGaSe2, Buffer layer, (Zn, Mg)O, Zn(O, S), ALD
National Category
Physical Sciences Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-110983 (URN)10.1016/j.tsf.2008.10.109 (DOI)000263847300047 ()
Note

0040-6090 doi: DOI: 10.1016/j.tsf.2008.10.109

Available from: 2009-12-01 Created: 2009-12-01 Last updated: 2017-12-12Bibliographically approved
2. Temperature-dependent current-voltage and lightsoaking measurements on Cu(In,Ga)Se2 solar cells with ALD-Zn1-xMgxO buffer layers
Open this publication in new window or tab >>Temperature-dependent current-voltage and lightsoaking measurements on Cu(In,Ga)Se2 solar cells with ALD-Zn1-xMgxO buffer layers
2009 (English)In: Progress in Photovoltaics, ISSN 1062-7995, E-ISSN 1099-159X, Vol. 17, no 7, p. 460-469Article in journal (Refereed) Published
Abstract [en]

In this paper, lightsoaking and temperature-dependent current-voltage (JVT) measurements on Cu(In,Ga)Se2 solar cells with atomic layer deposited Zn1-xMgxO buffer layers are presented. A range of Mg concentrations are used, from pure ZnO (x=0) to 26% Mg (x=0·26). Since this kind of solar cells exhibit strong metastable behaviour, lightsoaking is needed prior to the JVT-measurements to enable fitting of these to the one-diode model. The most prominent effect of lightsoaking cells with Mg-rich buffer layers is an increased fill factor, while the effect on cells with pure ZnO buffer is mainly to increase Voc·. The activation energy is extracted from JVT-measurement data by applying three different methods and the ideality factors are fitted to two different models of temperature-dependence. A buffer layer consisting either of ZnO or Zn1-xMgxO with a minor Mg content gives solar cells dominated by interface recombination, which probably can be related to a negative conduction band offset. A relatively high Mg content in the buffer layer (x=0·21) leads to solar cells dominated by recombination in the space charge region. The recombination is interpreted as being tunnelling-enhanced. The situation in between these Mg concentrations is less clear. Before lightsoaking, the sample with x=0·12 has the highest efficiency of 15·3%, while after lightsoaking the sample with x=0·21 holds the best efficiency, 16·1%, exceeding the value for the CdS reference. The Jsc values of the Zn1-xMgxO cells surpass that of the reference due to the larger bandgap of Zn1-xMgxO compared to CdS.

Keywords
CIGS, ZnMgO, thin film solar cells, recombination, lightsoaking
National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-109854 (URN)10.1002/pip.912 (DOI)000270774600003 ()1099-159X (ISBN)
Available from: 2009-10-28 Created: 2009-10-28 Last updated: 2017-12-12Bibliographically approved
3. Measurements of photo-induced changes in the conduction properties of ALD-Zn1−xMgxO thin films
Open this publication in new window or tab >>Measurements of photo-induced changes in the conduction properties of ALD-Zn1−xMgxO thin films
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2010 (English)In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. T141, p. 014010-1-014010-4Article in journal (Refereed) Published
Abstract [en]

Resistivity and Hall measurements are conducted on atomic layer deposited Zn1−xMgxO thin films of different thicknesses and compositions. It is found that the films exhibit persistent photoconductivity after UV-light exposure. The effect is more pronounced for thinner films with higher magnesium content. These are also the films with the highest resistivity. Light-induced excess conductivity is still present in some of the films after weeks of dark storage. Conductivity relaxation is faster at higher temperatures. From Hall measurements, it is observed that conductivity changes are a combined effect of changes in the mobility and concentration of free carriers.

Keywords
solar cells, ZnMgO, thin films, ALD, Hall measurements, persistent photoconductivity
National Category
Physical Sciences Engineering and Technology
Research subject
Engineering Science with specialization in Electronics
Identifiers
urn:nbn:se:uu:diva-134626 (URN)10.1088/0031-8949/2010/T141/014010 (DOI)000284694500011 ()
Available from: 2010-11-30 Created: 2010-11-30 Last updated: 2017-12-12Bibliographically approved
4. Baseline model of graded-absorber Cu(In,Ga)Se2 solar cells applied to cells with Zn1−xMgxO buffer layers
Open this publication in new window or tab >>Baseline model of graded-absorber Cu(In,Ga)Se2 solar cells applied to cells with Zn1−xMgxO buffer layers
2011 (English)In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 519, no 21, p. 7476-7480Article in journal (Refereed) Published
Abstract [en]

A baseline parameter set for electrical modelling of Cu(In,Ga)Se2 solar cells with compositionally graded absorber and CdS buffer layer is established. The cases with and without Fermi level pinning as well as withand without a surface defect layer are considered. Simulations with a defect layer are observed to give the best correspondence to measurements. Zn1−xMgxO buffer layers are introduced and initial modelling of the lightsoaking behaviour is performed. Simulation results are compared with experimental data.

Place, publisher, year, edition, pages
Elsevier, 2011
Keywords
Electrical modelling, CIGS solar cells, Buffer layers, Graded absorber, Light-soaking, Metastabilities
National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Electronics
Identifiers
urn:nbn:se:uu:diva-157808 (URN)10.1016/j.tsf.2010.12.141 (DOI)000295347700082 ()
Available from: 2011-08-23 Created: 2011-08-23 Last updated: 2017-12-08Bibliographically approved
5. Surface engineering in Cu(In,Ga)Se2 solar cells
Open this publication in new window or tab >>Surface engineering in Cu(In,Ga)Se2 solar cells
2013 (English)In: Progress in Photovoltaics, ISSN 1062-7995, E-ISSN 1099-159X, Vol. 21, no 4, p. 561-568Article in journal (Refereed) Published
Abstract [en]

Surface modifications of 3-stage co-evaporated Cu(In,Ga)Se2 (CIGS) thin films are investigated by finishing the evaporation with gallium-free (CuInSe2, CIS) stages of various lengths. We find substantial interdiffusion of indium and gallium, smearing out the Ga/(Ga+In) profile so that the addition of a CIS layer merely lowers the gallium content at the surface. For the thinnest top layer, equivalent to 20 nm of pure CIS, we cannot detect any compositional difference compared to the reference device. The modification are evaluated both by electrical characterization of actual solar-cell devices and by electrical modelling, using the latest version of SCAPS-1D. The best solar-cell device from this series is obtained for the 20 nm top layer, with an efficiency of 16.3 % after anti­reflective coating. However, we observe a trend of decreasing open-circuit voltage for increasing top-layer thicknesses, and we do not find direct evidence that the lowering of the gallium concentration at the CIGS surface should generally be expected to improve the device performance. A simulated device with reduced bulk and interface defect levels achieves 20 % efficiency, but the trends concerning the CIS top layer remain the same.

Keywords
CIGS, CIS, interface, SIMS, XPS, electrical modelling
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Engineering Science with specialization in Electronics
Identifiers
urn:nbn:se:uu:diva-151405 (URN)10.1002/pip.1229 (DOI)000319425900016 ()
Available from: 2011-04-11 Created: 2011-04-11 Last updated: 2017-12-11Bibliographically approved
6. Electrical modeling of Cu(In,Ga)Se2 cells with ALD-Zn1xMgxO bufferlayers
Open this publication in new window or tab >>Electrical modeling of Cu(In,Ga)Se2 cells with ALD-Zn1xMgxO bufferlayers
2012 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 111, no 1, p. 014509-014509Article in journal (Refereed) Published
Abstract [en]

Electrical modeling of Cu(In,Ga)Se2 solar cells with Zn1-xMgxO buffer layers is performed. A number of  different  device  models  are  implemented  and  tested  by  comparing  simulation  results  and measurement data. Room temperature light-soaking and dark-light cross-over behavior as well aslow-temperature characteristics of these cells are studied. The light-soaking improvements in the solarcell  parameters  are  attributed  to  an  increase  in  buffer  donor  density,  due  to  persistent  photoconductivity, that counteracts charged acceptors in the absorber-buffer region. Dark-light JV-curvecross-over is explained by deep acceptor defects with small electron capture cross-section, in thebuffer. Best correspondence to measurements on ZnO and Zn0.83Mg0.17O cells is obtained with models including absorber-buffer interface acceptor states. No wideband-gap surface defect layer is needed to reproduce measurement data.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2012
Keywords
CIGS solar cells, electrical modeling, alternative buffer layers, metastabilities
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Engineering Science with specialization in Electronics
Identifiers
urn:nbn:se:uu:diva-166753 (URN)10.1063/1.3672813 (DOI)000299127200108 ()
Available from: 2012-01-13 Created: 2012-01-13 Last updated: 2017-12-08Bibliographically approved
7. Infuence of absorber thickness on Cu(In,Ga)Se2 solar cells with different buffer layers
Open this publication in new window or tab >>Infuence of absorber thickness on Cu(In,Ga)Se2 solar cells with different buffer layers
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

The purpose of this work is to investigate the interplay between the absorber layer of Cu(In,Ga)Se2 solar cells and the contacts, including the buffer layer, of these devices. With this in mind Cu(In,Ga)Se2 devices with gallium-graded absorber layers of different thicknesses and  different  types  of  buffer  layers are fabricated. Absorbers are co-evaporated in-line with the substrate speed determining the layer thickness. Absorber layers and finished devices are characterized. Voc and FF optima are found for cells with 0.8 µm to 0.9 µm thick absorber layers but the highest efficiencies are found for standard devices with 1.6 µm absorbers due to a high Jsc. Cu(In,Ga)Se2 cells with Zn(O,S) buffer layers are found to be more efficient than CdS reference devices for the same absorber thickness due to a higher Jsc caused mainly by less light absorption in the alternative buffer layer. For cells with absorber layers thinner than normal, a better QE was also observed at longer wavelengths. Electrical simulations are used to reproduce the behaviour of the devices. It is found that recombination at the back contact limits the thinner devices with CdS buffer layers while the thin Zn(O,S) devices also have a problem with interface recombination. These recombination paths are over-shadowed in the standard thick devices by recombination in the CIGS layer.

Keywords
Solar cells, CIGS, Alternative buffer layers, Electrical modelling
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering Materials Engineering
Research subject
Engineering Science with specialization in Electronics
Identifiers
urn:nbn:se:uu:diva-168615 (URN)
Available from: 2012-02-13 Created: 2012-02-13 Last updated: 2012-03-29

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