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ZrN Back-Contact Reflectors and Ga Gradients 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)
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 [en]
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: urn:nbn:se:uu:diva-151402ISBN: 978-91-554-8086-8OAI: oai:DiVA.org:uu-151402DiVA: diva2:411435
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
List of papers
1.
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2. Reactively sputtered ZrN for application as reflecting back contact in Cu(In,Ga)Se-2 solar cells
Open this publication in new window or tab >>Reactively sputtered ZrN for application as reflecting back contact in Cu(In,Ga)Se-2 solar cells
2009 (English)In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 517, no 18, 5548-5552 p.Article in journal (Refereed) Published
Abstract [en]

We investigate reactively sputtered films of zirconium nitride, ZrN, for use as highly reflecting back contacts in Cu(In,Ga)Se2 (CIGS) devices with sub-micrometer absorbers. We identify the nitrogen flow and the sputter current as the decisive parameters for the composition, and demonstrate a method for determining the nitrogen flow at which the transition from metallic to compound sputtering mode occurs for a given current. Films prepared at this working point consist of stoichiometric ZrN with a low resistivity, a high reflectance for red and infrared light, and have a fairly high sputter rate. Calculations show that the reflectance at the ZrN/CIGS interface is significantly superior to that at the standard Mo/CIGS interface.

Place, publisher, year, edition, pages
Elsevier, 2009
Keyword
ZrN, Reactive DC magnetron sputtering, Optical properties, CIGS
National Category
Engineering and Technology
Research subject
Materials Science
Identifiers
urn:nbn:se:uu:diva-102584 (URN)10.1016/j.tsf.2009.03.196 (DOI)000267182700028 ()
Note
Correction in: Thin Solid Films, 2010, vol. 518, issue 10, p. 2924, doi: 10.1016/j.tsf.2009.06.023 Available from: 2011-11-23 Created: 2009-05-08 Last updated: 2011-12-02Bibliographically approved
3. Effect of gallium grading in Cu(In,Ga)Se2 solar-cell absorbers produced by multi-stage coevaporation
Open this publication in new window or tab >>Effect of gallium grading in Cu(In,Ga)Se2 solar-cell absorbers produced by multi-stage coevaporation
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2011 (English)In: Solar Energy Materials and Solar Cells, ISSN 0927-0248, Vol. 95, no 2, 721-726 p.Article in journal (Refereed) Published
Abstract [en]

We investigate Cu(In,Ga)Se2 thin films grown in multi-stage coevaporation processes and solar cells fabricated from such absorbers. Despite some interdiffusion during film growth, Ga/(Ga+In) gradients defined via evaporation-profile variations in the process are to a good part retained in the finished film. This indicates that the bandgap can be engineered in this type of process by varying the evaporation profiles, and consequently also that unintended profile variations should be noted and avoided. With front-side gradients the topmost part of many grains seems to be affected by a higher density of lattice defects due to the strong change of gallium content under copper-poor growth conditions. Electrically, both back-side gradients and moderate front-side gradients are shown to yield an improvement of device efficiency. If a front-side gradient is too wide, though, it causes strong voltage-dependent collection and the fill factor is severely reduced.

Keyword
CIGS, Coevaporation, Multi-stage process, Three-stage process, Gradients
National Category
Physical Sciences Engineering and Technology
Research subject
Engineering Science with specialization in Electronics; Engineering Science with specialization in Materials Science
Identifiers
urn:nbn:se:uu:diva-132556 (URN)10.1016/j.solmat.2010.10.011 (DOI)000287006900048 ()
Available from: 2011-11-23 Created: 2010-10-21 Last updated: 2013-08-30Bibliographically approved
4. Technological and economical aspects on the influence of reduced Cu(In,Ga)Se2 thickness and Ga grading for co-evaporated Cu(In,Ga)Se2 modules
Open this publication in new window or tab >>Technological and economical aspects on the influence of reduced Cu(In,Ga)Se2 thickness and Ga grading for co-evaporated Cu(In,Ga)Se2 modules
2011 (English)In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 519, no 21, 7530-7533 p.Article in journal (Refereed) Published
Abstract [en]

Reducing the Cu(In,Ga)Se2 (CIGS) thickness is one way of improving the throughput and capacity in existing production, provided that the efficiency can be kept at a high level. Our experimental results from an in-line co-evaporation process show that it is possible to produce CIGS solar cells with good efficiency at a CIGS thickness of less than 1 µm. An efficiency of 14.4% was obtained for an evaporation time of 8 min and a resulting CIGS thickness of only 0.8 [mu]m. The quantum efficiency measurements show only a minor reduction of the collection in the infrared region that can be related to losses caused by reduced absorption. Passivation of the back contact has been found to be important for thin devices and one way of obtaining good back contact properties, or to reduce the impact of back contact recombination is to use an increased Ga content near the back contact. We have found that Ga grading is feasible also in the three stage process, i.e. a Ga-rich layer near the back contact from stage one is to a high degree retained also after stages two and three. In this paper we discuss the implication of efficiency reduction for the economy of the production and how high efficiency loss that can be tolerated, provided that the output is doubled at equal production cost for the CIGS layer.

Place, publisher, year, edition, pages
Elsevier, 2011
Keyword
Solar cells, PV modules, CIGS, Thin absorber, Ga-grading, Cost
National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Electronics
Identifiers
urn:nbn:se:uu:diva-151407 (URN)10.1016/j.tsf.2011.01.369 (DOI)000295347700094 ()
Available from: 2011-04-11 Created: 2011-04-11 Last updated: 2016-04-20Bibliographically 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, 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.

Keyword
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: 2016-06-10Bibliographically approved
6. Development of gallium gradients in three‐ stageCu(In,Ga)Se2 co‐evaporation processes
Open this publication in new window or tab >>Development of gallium gradients in three‐ stageCu(In,Ga)Se2 co‐evaporation processes
Show others...
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
Keyword
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:nbn:se:uu:diva-151408 (URN)10.1002/pip.1134 (DOI)000302946900005 ()
Available from: 2011-04-11 Created: 2011-04-11 Last updated: 2016-06-10Bibliographically approved

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