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CuGaSe2 solar cells using atomic layer deposited Zn(O,S) and (Zn,Mg)O buffer layers
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics. (Solcellsgruppen)
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics. (Solcellsgruppen)
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics. (Solcellsgruppen)
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics. (Solcellsgruppen)
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2009 (English)In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 517, no 7, 2305-2308 p.Article 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.

Place, publisher, year, edition, pages
2009. Vol. 517, no 7, 2305-2308 p.
Keyword [en]
Solar cells, CuGaSe2, Buffer layer, (Zn, Mg)O, Zn(O, S), ALD
National Category
Physical Sciences Engineering and Technology
Identifiers
URN: urn:nbn:se:uu:diva-110983DOI: 10.1016/j.tsf.2008.10.109ISI: 000263847300047OAI: oai:DiVA.org:uu-110983DiVA: diva2:279071
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
In thesis
1. Cadmium Free Buffer Layers and the Influence of their Material Properties on the Performance of Cu(In,Ga)Se2 Solar Cells
Open this publication in new window or tab >>Cadmium Free Buffer Layers and the Influence of their Material Properties on the Performance of Cu(In,Ga)Se2 Solar Cells
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

CdS is conventionally used as a buffer layer in Cu(In,Ga)Se2, CIGS, solar cells. The aim of this thesis is to substitute CdS with cadmium-free, more transparent and environmentally benign alternative buffer layers and to analyze how the material properties of alternative layers affect the solar cell performance. The alternative buffer layers have been deposited using Atomic Layer Deposition, ALD. A theoretical explanation for the success of CdS is that its conduction band, Ec, forms a small positive offset with that of CIGS.

In one of the studies in this thesis the theory is tested experimentally by changing both the Ec position of the CIGS and of Zn(O,S) buffer layers through changing their gallium and sulfur contents respectively. Surprisingly, the top performing solar cells for all gallium contents have Zn(O,S) buffer layers with the same sulfur content and properties in spite of predicted unfavorable Ec offsets. An explanation is proposed based on observed non-homogenous composition in the buffer layer.

This thesis also shows that the solar cell performance is strongly related to the resistivity of alternative buffer layers made of (Zn,Mg)O. A tentative explanation is that a high resistivity reduces the influence of shunt paths at the buffer layer/absorber interface. For devices in operation however, it seems beneficial to induce persistent photoconductivity, by light soaking, which can reduce the effective Ec barrier at the interface and thereby improve the fill factor of the solar cells.

Zn-Sn-O is introduced as a new buffer layer in this thesis. The initial studies show that solar cells with Zn-Sn-O buffer layers have comparable performance to the CdS reference devices.

While an intrinsic ZnO layer is required for a high reproducibility and performance of solar cells with CdS buffer layers it is shown in this thesis that it can be thinned if Zn(O,S) or omitted if (Zn,Mg)O buffer layers are used instead. As a result, a top conversion efficiency of 18.1 % was achieved with an (Zn,Mg)O buffer layer, a record for a cadmium and sulfur free CIGS solar cell.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2010. 75 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 787
Keyword
Cu(In, Ga)Se2, Solar cells, Thin film, Buffer layer, Window layer, ZnO, Zn(O, S), (Zn, Mg)O, Zn-Sn-O
National Category
Condensed Matter Physics
Research subject
Engineering Science with specialization in Solid State Physics
Identifiers
urn:nbn:se:uu:diva-133112 (URN)978-91-554-7944-2 (ISBN)
Public defence
2010-12-16, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 13:00 (English)
Opponent
Supervisors
Note
Felaktigt tryckt som Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 717Available from: 2010-11-25 Created: 2010-11-02 Last updated: 2011-03-21Bibliographically approved
2. Modelling Band Gap Gradients and Cd-free Buffer Layers in Cu(In,Ga)Se2 Solar Cells
Open this publication in new window or tab >>Modelling Band Gap Gradients and Cd-free Buffer Layers in Cu(In,Ga)Se2 Solar Cells
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. 70 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 900
Keyword
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:nbn:se:uu:diva-168618 (URN)978-91-554-8280-0 (ISBN)
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

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Publisher's full texthttp://www.sciencedirect.com/science/article/B6TW0-4TW11GP-H/2/3d0c0e1dd3897c66c46ed6523b4622eb

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Hultqvist, AdamPlatzer-Björkman, CharlottePettersson, JonasTörndahl, TobiasEdoff, Marika

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