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Surface engineering in Cu(In,Ga)Se2 solar cells
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)
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)
2013 (English)In: Progress in Photovoltaics, ISSN 1062-7995, E-ISSN 1099-159X, Vol. 21, no 4, 561-568 p.Article 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.

Place, publisher, year, edition, pages
2013. Vol. 21, no 4, 561-568 p.
Keyword [en]
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: urn:nbn:se:uu:diva-151405DOI: 10.1002/pip.1229ISI: 000319425900016OAI: oai:DiVA.org:uu-151405DiVA: diva2:409790
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
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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Schleussner, SebastianPettersson, JonasTörndahl, TobiasEdoff, Marika
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