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Infuence of absorber thickness on Cu(In,Ga)Se2 solar cells with different buffer layers
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics. (Solar Cell Group)
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics. (Solar Cell Group)
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics. (Solar Cell Group)
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics. (Solar Cell Group)
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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.

Keyword [en]
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: urn:nbn:se:uu:diva-168615OAI: oai:DiVA.org:uu-168615DiVA: diva2:500457
Available from: 2012-02-13 Created: 2012-02-13 Last updated: 2012-03-29
In thesis
1. 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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