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Development of gallium gradients in three‐ stageCu(In,Ga)Se2 co‐evaporation processes
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics. (Solar Cells)
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics. (Solar Cells)
Royal Institute of Technology, Stockholm.
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics. (Solar Cells)
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2012 (English)In: Progress in Photovoltaics, ISSN 1062-7995, E-ISSN 1099-159X, Vol. 20, no 3, 284-293 p.Article in journal (Refereed) Published
Abstract [en]

We use secondary-ion mass spectrometry, X-ray diffraction and scanning electron microscopy to investigate the development over time of compositional gradients in Cu(In,Ga)Se2 thin films grown in three-stage co-evaporation processes and suggest a comprehensive model for the formation of the well-known ‘notch’ structure. The model takes into account the need for compensating Cu diffusion by movement of group-III ions in order to remain on the quasi-binary tie line and indicates that the mobilities of In and Ga ions differ. Cu diffuses towards the back in the second stage and towards the front in the third, and this is the driving force for the movement of In and Ga. The [Ga]/[In + Ga] ratio then increases in the direction of the respective Cu movement because In has a higher mobility at process conditions than has Ga. Interdiffusion of In and Ga can be considerable in the (In,Ga)2Se3 film of the first stage, but seems largely to cease in Cu(In,Ga)Se2 and shows no signs of being boosted by the presence of a Cu2Se layer.

Place, publisher, year, edition, pages
John Wiley & Sons, 2012. Vol. 20, no 3, 284-293 p.
Keyword [en]
CIGS, three‐stage process, gradient, SIMS
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Engineering Science with specialization in Electronics
Identifiers
URN: urn:nbn:se:uu:diva-151408DOI: 10.1002/pip.1134ISI: 000302946900005OAI: oai:DiVA.org:uu-151408DiVA: diva2:409799
Available from: 2011-04-11 Created: 2011-04-11 Last updated: 2016-06-10Bibliographically approved
In thesis
1. ZrN Back-Contact Reflectors and Ga Gradients in Cu(In,Ga)Se2 Solar Cells
Open this publication in new window or tab >>ZrN Back-Contact Reflectors and Ga Gradients in Cu(In,Ga)Se2 Solar Cells
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Solar cells constitute the most direct way of converting solar energy to electricity, and thin-film solar-cell technologies have lately been growing in importance, allowing the fabrication of less expensive modules that nonetheless have good power-conversion efficiencies. This thesis focuses on solar cells based on Cu(In,Ga)Se2, which is the thin-film technology that has shown the highest conversion efficiency to date, reaching 20.3 % on the laboratory scale. Solar modules still have some way to go to become entirely competitive with existing energy technologies, and there are two possible paths to this goal: Firstly, reducing their manufacturing costs, for instance by minimizing the material usage per module and/or by increasing the throughput of a given factory; and secondly, increasing the power output per module in other words, the module efficiency. The subject matters of this thesis are related to those two approaches.

The first issue investigated is the possibility for reducing the thickness of the Cu(In,Ga)Se2 layer and compensating for lost absorption by using a ZrN back reflector. ZrN layers are fabricated by reactive sputtering and I present a method for tuning the sputtering parameters so as to obtain a back reflector with good optical, electrical and mechanical properties. The reflector layer cannot be used directly in CIGS devices, but relatively good devices can be achieved with a precursor providing a homogeneous supply of Na, the addition of a very thin sacrificial Mo layer that allows the formation of a film of MoSe2 passivating the back contact, and optionally a Ga gradient that further keeps electrons away from the back contact.

The second field of study concerns the three-stage CIGS coevaporation process, which is widely used in research labs around the world and has yielded small-area cells with highest efficiencies, but has not yet made it to large scale production. My focus lies on the development and the effect of gradients in the [Ga]/[In+Ga] ratio. On the one hand, I investigate 'intrinsic' gradients (ones that form autonomously during the evaporation), and present a formation model based on the differing diffusivity of Ga and In atoms in CIGS and on the development along the quasi-binary tie line between (In,Ga)2Se3 and Cu2Se. On the other hand, I determine how the process should be designed in order to preserve 'extrinsic' gradients due to interdiffusion. Lastly, I examine the electrical effects of Ga-enhancement at the back and at the front of the absorber and of In-enhancement at the front. Over a wide range, In-rich top layers prove to have no or a weakly beneficial effect, while Ga-rich top regions pose a high risk to have a devastating effect on device performance.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2011. 66 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 830
Keyword
Solar cells, CIGS, ZrN, three-stage process, multi-stage process, grading, SIMS, electrical modelling
National Category
Natural Sciences
Research subject
Engineering Science with specialization in Electronics
Identifiers
urn:nbn:se:uu:diva-151402 (URN)978-91-554-8086-8 (ISBN)
Public defence
2011-05-31, room 80101, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:30 (English)
Opponent
Supervisors
Available from: 2011-05-10 Created: 2011-04-11 Last updated: 2011-07-01Bibliographically approved

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Schleussner, SebastianTörndahl, TobiasZimmermann, UweWätjen, TimoEdoff, Marika
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