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Microscopic Characterisation of Solar Cells: An Electron Microscopy Study of Cu(In,Ga)Se2 and Cu2ZnSn(S,Se)4 Solar Cells
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The sun provides us with a surplus of energy convertible to electricity using solar cells. This thesis focuses on solar cells based on chalcopyrite (CIGSe) as well as kesterite (CZTS(e)) absorber layers. These materials yield record efficiencies of 20.4 % and 11.1 %, respectively. Especially for CZTS(e), the absorber layers often do not consist of one single desired phase but can exhibit areas with deviating material properties, referred to as secondary phases. Furthermore, several material layers are required for a working solar cell, each exhibiting interfaces. Even though secondary phases and interfaces represent a very small fraction of the solar cell they can have a profound influence on the over-all electrical solar cell characteristics. As such, it is crucial to understand how secondary phases and interfaces influence the local electrical characteristics.

Characterising secondary phases and interfaces is challenging due to their small sample volume and relatively small differences in composition amongst others. This is where electronmicroscopy, especially transmission electron microscopy, offers valuable insight to material properties on the microscopic scale. The main challenge is, however, to link these material properties to the corresponding electrical characteristics of a solar cell.

This thesis uses electron beam induced current imaging and introduces a new method for JV characterisation of solar cells on the micron scale. Combining microscopic structural and electrical characterisation techniques allowed identifying and characterising local defects found in the absorber layer of CIGS solar cells after thermal treatment. Furthermore, CZTSe solar cells in this thesis exhibited a low photo-current density which is traced to the formation of a current blocking ZnSe secondary phase at the front contact interface. The electron microscopy work has contributed to an understanding of the chemical stability of CZTS and has shown the need for an optimised back contact interface in order to avoid chemical decomposition reactions and formation of detrimental secondary phases. With this additional knowledge, a comprehensive picture of the material properties from the macroscopic down to the microscopic level can be attained throughout all required material layers.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2013. , xii + 70 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1053
Keyword [en]
TEM, SEM, FIB, solar cell, CIGS, CZTS, Alternative buffer layers, Gallium gradients, microscopic electrical characterisation, Secondary Phases
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering Physical Sciences Materials Engineering
Research subject
Engineering Science with specialization in Electronics
Identifiers
URN: urn:nbn:se:uu:diva-199432ISBN: 978-91-554-8692-1 (print)OAI: oai:DiVA.org:uu-199432DiVA: diva2:620967
Public defence
2013-09-06, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:30 (English)
Opponent
Supervisors
Available from: 2013-06-05 Created: 2013-05-04 Last updated: 2013-08-30Bibliographically approved
List of papers
1. The effect of Zn1−xSnxOy buffer layer thickness in 18.0% efficient Cd-free Cu(In,Ga)Se2 solar cells
Open this publication in new window or tab >>The effect of Zn1−xSnxOy buffer layer thickness in 18.0% efficient Cd-free Cu(In,Ga)Se2 solar cells
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2013 (English)In: Progress in Photovoltaics, ISSN 1062-7995, E-ISSN 1099-159X, Vol. 21, no 8, 1588-1597 p.Article in journal (Refereed) Published
Abstract [en]

The influence of the thickness of atomic layer deposited Zn1−xSnxOy buffer layers and the presence of an intrinsic ZnO layer on the performance of Cu(In,Ga)Se2 solar cells are investigated. The amorphous Zn1−xSnxOy layer, with a [Sn]/([Sn] + [Zn]) composition of approximately 0.18, forms a conformal and in-depth uniform layer with an optical band gap of 3.3 eV. The short circuit current for cells with a Zn1−xSnxOy layer are found to be higher than the short circuit current for CdS buffer reference cells and thickness independent. On the contrary, both the open circuit voltage and the fill factor values obtained are lower than the references and are thickness dependent. A high conversion efficiency of 18.0%, which is comparable with CdS references, is attained for a cell with a Zn1−xSnxOy layer thickness of approximately 13 nm and with an i-ZnO layer.

Keyword
zinc tin oxide, CIGS, ALD, buffer layer, i-ZnO
National Category
Other Physics Topics Engineering and Technology
Research subject
Engineering Science with specialization in Electronics
Identifiers
urn:nbn:se:uu:diva-182449 (URN)10.1002/pip.2239 (DOI)000327260800004 ()
Available from: 2012-10-10 Created: 2012-10-10 Last updated: 2017-12-07Bibliographically approved
2. 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, E-ISSN 1879-3398, 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: 2017-12-12Bibliographically approved
3. Microanalysis of laser micro-welded interconnections in CIGS PV modules
Open this publication in new window or tab >>Microanalysis of laser micro-welded interconnections in CIGS PV modules
2012 (English)In: Solar Energy Materials and Solar Cells, ISSN 0927-0248, E-ISSN 1879-3398, Vol. 98, 172-178 p.Article in journal (Refereed) Published
Abstract [en]

Laser micro-welding is a technique that can be used for interconnective (P2) patterning of Cu ( In , Ga ) Se 2 (CIGS) thin film solar cells after deposition of the ZnO:Al window layer. The micro-welding process transforms CIGS into a conductive compound that forms a contact between the ZnO:Al front contact and the Mo back contact. In this work we present a study on the influence of process parameters on the interconnectivity of the laser micro-weld as well as an investigation of the morphology of the micro-weld zone. We found that the current method results in an overlap of conditions where electrical connection is achieved and where Mo/glass damage is caused by laser action. Stability testing shows that interconnections processed without appreciable damage are stable. The connecting region contained metal rich zones and segregated CuxSe that we believe account for its conductivity.

Keyword
Laser micro-welding, Laser patterning, CIGS, Module technology
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Engineering Science with specialization in Electronics
Identifiers
urn:nbn:se:uu:diva-142582 (URN)10.1016/j.solmat.2011.07.032 (DOI)000300536500022 ()
Available from: 2011-01-14 Created: 2011-01-14 Last updated: 2017-12-11Bibliographically approved
4. Microanalysis of post-deposition annealing of Cu(In,Ga)Se2 solar cells
Open this publication in new window or tab >>Microanalysis of post-deposition annealing of Cu(In,Ga)Se2 solar cells
2012 (English)In: Solar Energy Materials and Solar Cells, ISSN 0927-0248, E-ISSN 1879-3398, Vol. 107, 396-402 p.Article in journal (Refereed) Published
Abstract [en]

The influence of selenium background pressure during post-deposition annealing of Cu(In,Ga)Se 2 (CIGS) is investigated. Solar cells made from samples post-annealed with selenium showed the same solar cell parameters as references without any annealing treatment. Dark JV measurements of microscopic devices with sizes of 10μm×10μm from the sample annealed with selenium showed good agreement with the corresponding macroscopic solar cells. Samples annealed without selenium showed degradation in terms of open circuit voltage and fill factor. Electron beam induced current (EBIC) imaging for these degraded solar cells revealed patches of reduced current. Microscopic JV measurements showed that the deterioration is not limited to these patches. Cross-sectional transmission electron microscopy analysis showed phase decomposition of the CIGS absorber in areas of the patches toward the back contact. We conclude that in addition to the local phase decomposition of the CIGS leading to patches in the EBIC image the anneal in vacuum without selenium background pressure also leads to other modifications of the CIGS layer influencing the interface region on a macroscopic scale.

Keyword
CIGS, Gradients, Microscopic JV-characterization, Post-deposition annealing, TEM, Three-stage process, Post deposition annealing, Electric currents, Gallium, Open circuit voltage, Selenium, Solar cells, Transmission electron microscopy, Annealing
National Category
Natural Sciences Engineering and Technology
Research subject
Engineering Science with specialization in Electronics
Identifiers
urn:nbn:se:uu:diva-184888 (URN)10.1016/j.solmat.2012.07.026 (DOI)000311270000053 ()
Available from: 2012-11-20 Created: 2012-11-15 Last updated: 2017-12-07Bibliographically approved
5. Direct evidence of current blocking by ZnSe in Cu2ZnSnSe4 solar cells
Open this publication in new window or tab >>Direct evidence of current blocking by ZnSe in Cu2ZnSnSe4 solar cells
2012 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 100, no 17, 173510- p.Article in journal (Refereed) Published
Abstract [en]

Thin films of polycrystalline Cu2ZnSnSe4 were made by selenization of co-sputtered metallic precursors and processed to solar cells. Electron beam induced current (EBIC) in combination with microscopic scale IV characterization is used to investigate lateral inhomogeneities in electrical performance across the solar cell area. Transmission electron microscopy relates areas with low EBIC response to the formation of a ZnSe phase at the absorber surface resulting in a current blocking behavior and a reduced short-circuit current density for the solar cells. Areas without ZnSe have a high EBIC response and result in high quality diodes well suited for solar cells.

National Category
Physical Sciences Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Engineering Science with specialization in Electronics
Identifiers
urn:nbn:se:uu:diva-174929 (URN)10.1063/1.4706256 (DOI)000303340300094 ()
Available from: 2012-05-30 Created: 2012-05-30 Last updated: 2017-12-07Bibliographically approved
6. Secondary compound formation revealed by transmission electron microscopy at the Cu2ZnSnS4/Mo interface
Open this publication in new window or tab >>Secondary compound formation revealed by transmission electron microscopy at the Cu2ZnSnS4/Mo interface
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2012 (English)In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 535, 31-34 p.Article in journal (Refereed) Published
Abstract [en]

One promising candidate considered for solar cell absorber layers is Cu2ZnSnS4 (CZTS). Transmission electron microscopy (TEM) investigations of such solar cells to date are scarce. We present microanalysis results on our fully processed CZTS solar cells based on absorber layers deposited by reactive sputtering of a precursor layer followed by a short anneal. The initially small grain size for precursor layers increases rapidly due to annealing, typically spanning the entire absorber layer thickness. Energy dispersive X-ray spectroscopy in a TEM clearly reveals the formation of secondary compounds containing Zn-, Cu- or Sn-sulfides located at the Mo/CZTS back contact interface after annealing. Simultaneously a MoS2 layer is formed at the back contact. The extent to which secondary compounds and MoS2 form scales with annealing time, indicating that Mo is not stable when in contact with CZTS. Understanding the chemical reactions at the back contact is considered to be essential to limit the secondary phase formation during annealing.

Keyword
Copper zinc tin sulfide; Reactive sputtering; Secondary compounds; Molybdenum disulfide; Transmission electron microscopy
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering Other Physics Topics
Research subject
Engineering Science with specialization in Electronics
Identifiers
urn:nbn:se:uu:diva-190042 (URN)10.1016/j.tsf.2012.11.079 (DOI)000318973600009 ()
Available from: 2013-01-07 Created: 2013-01-07 Last updated: 2017-12-06Bibliographically approved
7. A Detrimental Reaction at the Molybdenum Back Contact in Cu2ZnSn(S,Se)4 Thin-Film Solar Cells
Open this publication in new window or tab >>A Detrimental Reaction at the Molybdenum Back Contact in Cu2ZnSn(S,Se)4 Thin-Film Solar Cells
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2012 (English)In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 134, no 47, 19330-19333 p.Article in journal (Refereed) Published
Abstract [en]

Experimental proof is presented for a hitherto undetected solid-state reaction between the solar cell material Cu2ZnSn(S,Se)4 (CZTS(e)) and the standard metallic back contact, molybdenum. Annealing experiments combined with Raman and transmission electron microscopy studies show that this aggressive reaction causes formation of MoS2 and secondary phases at the CZTS|Mo interface during thermal processing. A reaction scheme is presented and discussed in the context of current state-of-the-art synthesis methods for CZTS(e). It is concluded that alternative back contacts will be important for future improvements in CZTS(e) quality.

National Category
Materials Chemistry Engineering and Technology
Research subject
Engineering Science with specialization in Electronics
Identifiers
urn:nbn:se:uu:diva-186976 (URN)10.1021/ja308862n (DOI)000311521500006 ()
Available from: 2012-11-30 Created: 2012-11-30 Last updated: 2017-12-07Bibliographically approved
8. Cu out-diffusion in kesterites: A transmission electron microscopy specimen preparation artifact
Open this publication in new window or tab >>Cu out-diffusion in kesterites: A transmission electron microscopy specimen preparation artifact
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2013 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 102, no 5, 051902- p.Article in journal (Refereed) Published
Abstract [en]

Solar cells based on Cu2ZnSn(S,Se)(4) absorber layers have received a growing amount of interest. Typically a Mo(S,Se)(2) layer is formed at the Cu2ZnSn(S,Se)(4)/Mo interface during processing. Transmission electron microscopy (TEM) analyses showed the presence of Cu in the Mo(S,Se)(2) which was thought to cause secondary phase formation at the back contact. However, preparing TEM samples can induce artifacts leading to false conclusions. It is therefore of great importance to identify such artifacts. In this work, we show that the Cu presence in the Mo(S, Se) 2 stems solely from TEM sample preparation and does not occur as part of the synthesis process. 

National Category
Natural Sciences Engineering and Technology
Research subject
Engineering Science with specialization in Electronics
Identifiers
urn:nbn:se:uu:diva-196537 (URN)10.1063/1.4790282 (DOI)000314770300025 ()
Available from: 2013-03-13 Created: 2013-03-11 Last updated: 2017-12-06Bibliographically approved

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