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The Multiple Faces of Interfaces: Electron microscopy analysis of CuInSe2 thin-film solar cells
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences. Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics. (Solid State Electronics)ORCID iD: 0000-0002-2101-3746
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The CIS solar cell family features both a high stability and world-class performances. They can be deposited on a wide variety of substrates and absorb the entire solar spectrum only using a thickness of a few micrometers. These particularities allow them to feature the most positive Energy returned on energy invested (EROI) values and the shortest Energy payback times (EPBT) of all the main photovoltaic solar cells. Using mainly electron microscopy characterization techniques, this thesis has explored the questions related to the interface control in thin-film photovoltaic solar cells based on CuInSe2 (CIS) absorber materials. Indeed, a better understanding of the interfaces is essential to further improve the solar cell conversion efficiency (currently around 23%), but also to introduce alternative substrates, to implement various alloying (Ga-CIS (CIGS), Ag-CIGS (ACIGS)…) or even to assess alternative buffer layers.

The thread of this work is the understanding and the improvement of the interface control. To do so, the passivation potential of Al2O3 interlayers has been studied in one part of the thesis. While positive changes were generally measured, a subsequent analysis has revealed that a detrimental interaction could occur between the NaF precursor layer and the rear Al2O3 passivation layer. Still within the passivation research field, incorporation of various alkali-metals to the CIS absorber layer has been developed and analyzed. Large beneficial effects were ordinarily reported. However, similar KF-post deposition treatments were shown to be potentially detrimental for the silver-alloyed CIGS absorber layer. Finally, part of this work dealt with the limitations of the thin-barrier layers usually employed when using steel substrates instead of soda-lime glass ones. The defects and their origin could have been related to the steel manufacturing process, which offered solutions to erase them.

Electron microscopy, especially Transmission electron microscopy (TEM), was essential to scrutinize the local changes occurring at the different interfaces within a few nanometers. The composition variation was measured with both Electron energy loss spectroscopy (EELS) and Energy dispersive X-ray spectroscopy (EDS) techniques. Finally, efforts have been invested in controlling and improving the FIB sample preparation, which was required for the TEM observations in our case.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2018. , p. 85
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1701
Keywords [en]
Electron microscopy, TEM, STEM, EELS, EDS, solar cells, CIGS, ACIGS, CZTS, post deposition treatment, KF, RbF, buffer layers, interfaces, inter layers, barrier layers, passivation layers
National Category
Energy Systems Other Electrical Engineering, Electronic Engineering, Information Engineering Other Materials Engineering
Research subject
Engineering Science with specialization in Materials Science; Engineering Science with specialization in Electronics
Identifiers
URN: urn:nbn:se:uu:diva-357127ISBN: 978-91-513-0402-1 (print)OAI: oai:DiVA.org:uu-357127DiVA, id: diva2:1238134
Public defence
2018-09-28, Polhemssalen, The Angstrom laboratory, Lägerhyddsvägen 1, Uppsala, 09:30 (English)
Opponent
Supervisors
Available from: 2018-09-07 Created: 2018-08-12 Last updated: 2018-10-02
List of papers
1. On the assessment of CIGS surface passivation by photoluminescence
Open this publication in new window or tab >>On the assessment of CIGS surface passivation by photoluminescence
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2015 (English)In: Physica Status Solidi. Rapid Research Letters, ISSN 1862-6254, E-ISSN 1862-6270, Vol. 9, no 5, p. 288-292Article in journal (Refereed) Published
Abstract [en]

An optimized test structure to study rear surface passivation in Cu(In,Ga)Se-2 (CIGS) solar cells by means of photoluminescence (PL) is developed and tested. The structure - illustrated in the abstract figure - is examined from the rear side. To enable such rear PL assessment, a semi-transparent ultrathin Mo layer has been developed and integrated in place of the normal rear contact. The main advantages of this approach are (i) a simplified representation of a rear surface passivated CIGS solar cell is possible, (ii) it is possible to assess PL responses originating close to the probed rear surface, and (iii) a stable PL response as a function of air exposure time is obtained. In this work, PL measurements of such structures with and without rear surface passivation layers have been compared, and the measured improvement in PL intensity for the passivated structures is associated with enhanced CIGS rear interface properties. [GRAPHICS] Transmission electron microscope (TEM) bright field cross-section image of the rear illuminated test structure fabricated for PL characterization.

Keywords
solar cells, thin films, CuInGaSe2, surface passivation, photoluminescence
National Category
Environmental Engineering Physical Sciences
Identifiers
urn:nbn:se:uu:diva-256541 (URN)10.1002/pssr.201510081 (DOI)000354888300002 ()
Available from: 2015-06-25 Created: 2015-06-24 Last updated: 2018-08-12Bibliographically approved
2. Surface defect passivation by a thin metallic barrier for Cu(InxGa1-x)Se2 co-evaporation on Cr-steel substrates
Open this publication in new window or tab >>Surface defect passivation by a thin metallic barrier for Cu(InxGa1-x)Se2 co-evaporation on Cr-steel substrates
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2016 (English)In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 619, p. 220-226Article in journal (Refereed) Published
Abstract [en]

The use of Cr-steel substrates for the fabrication of Cu(In,Ga)Se2 (CIGS) solar cells is highly desirable and is a topic of considerable research interest. However, solar cells on non-treated steel substrates often exhibit decreased performance compared to their homologues on soda lime glass substrates. This is partly attributed to out-diffusion of steel components (Fe, Cr, Mn, etc.) into the solar cell. To avoid this contamination, thin film barriers can be added on top of the steel surface, but they do not always prevent the diffusion completely. In this paper we study the potential of using Cr and Ti as thin barrier layers. We find that local surface defects on the steel, several micrometers in height, lead to cracks in the back contact as well as in the barrier layers. Advanced transmission electron microscopy (TEM) techniques reveal that elemental diffusion and chemical reactions occur at these openings during heat treatments in Se atmosphere. TEM-energy dispersive X-ray spectroscopy (TEM-EDX) analysis in combination with calculation of the solid state diffusion coefficient demonstrate that a Cr-barrier sacrificially protects the Cr-steel substrate, blocking most of the Fe out-diffusion, whereas a Ti-barrier is less efficient.

Place, publisher, year, edition, pages
Elsevier, 2016
Keywords
Back contact; Thin film; Solar cell; Steel substrate; CuInGaSe2; Electron microscopy; Diffusion; Barrier
National Category
Metallurgy and Metallic Materials Condensed Matter Physics Energy Systems
Research subject
Engineering Science with specialization in Materials Analysis; Engineering Science with specialization in Materials Science
Identifiers
urn:nbn:se:uu:diva-307368 (URN)10.1016/j.tsf.2016.10.063 (DOI)000389610900033 ()
Available from: 2016-11-14 Created: 2016-11-14 Last updated: 2018-08-12Bibliographically approved
3. Rear Surface Optimization of CZTS Solar Cells by Use of a Passivation Layer With Nanosized Point Openings
Open this publication in new window or tab >>Rear Surface Optimization of CZTS Solar Cells by Use of a Passivation Layer With Nanosized Point Openings
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2016 (English)In: IEEE Journal of Photovoltaics, ISSN 2156-3381, E-ISSN 2156-3403, Vol. 6, no 1, p. 332-336Article in journal (Refereed) Published
Abstract [en]

Previously, an innovative way to reduce rear interface recombination in Cu(In, Ga)(S, Se)(2) (CIGSSe) solar cells has been successfully developed. In this work, this concept is established in Cu-2(Zn, Sn)(S, Se)(4) (CZTSSe) cells to demonstrate its potential for other thin-film technologies. Therefore, ultrathin CZTS cells with an Al2O3 rear surface passivation layer having nanosized point openings are fabricated. The results indicate that introducing such a passivation layer can have a positive impact on open-circuit voltage (V-OC; +17% rel.), short-circuit current (J(SC); +5% rel.), and fill factor (FF; +9% rel.), compared with corresponding unpassivated cells. Hence, a promising efficiency improvement of 32% rel. is obtained for the rear passivated cells.

Keywords
Aluminum oxide, Cu(In, Ga)(S, Se)(2), Cu-2(Zn, Sn)(S, Se)(4), nanosized point contacts, solar cells, surface passivation layer, thin-film
National Category
Environmental Engineering
Identifiers
urn:nbn:se:uu:diva-274909 (URN)10.1109/JPHOTOV.2015.2496864 (DOI)000367251900048 ()
Funder
Swedish Research CouncilSwedish Energy AgencyEU, FP7, Seventh Framework Programme, 300998EU, FP7, Seventh Framework Programme, 327367
Available from: 2016-01-27 Created: 2016-01-26 Last updated: 2018-08-12Bibliographically approved
4. Deep surface Cu depletion induced by K in high-efficiency Cu(In,Ga)Se2 solar cell absorbers
Open this publication in new window or tab >>Deep surface Cu depletion induced by K in high-efficiency Cu(In,Ga)Se2 solar cell absorbers
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2018 (English)In: Progress in Photovoltaics, ISSN 1062-7995, E-ISSN 1099-159X, Vol. 26, no 9, p. 730-739Article in journal (Refereed) Published
Abstract [en]

In this work, we used K‐rich glass substrates to provide potassium during the coevaporation of Cu(In,Ga)Se2 (CIGS) absorber layers. Subsequently, we applied a postdeposition treatment (PDT) using KF or RbF to some of the grown absorbers. It was found that the presence of K during the growth of the CIGS layer led to cell effi- ciencies beyond 17%, and the addition of a PDT pushed it beyond 18%. The major finding of this work is the observation of discontinuous 100‐ to 200‐nm‐deep Cu‐ depleted patches in the vicinity of the CdS buffer layer, correlated with the presence of K during the growth of the absorber layer. The PDT had no influence on the forma- tion of these patches. A second finding concerns the composition of the Cu‐depleted areas, where an anticorrelation between Cu and both In and K was measured using scanning transmission electron microscopy. Furthermore, a steeper Ga/(In+Ga) ratio gradient was measured for the absorbers grown with the presence of K, suggesting that K hinders the group III element interdiffusion. Finally, no Cd in‐diffusion to the CIGS layer could be detected. This indicates that if CdCu substitution occurs, either their concentration is below our instrumental detection limit or its presence is contained within the first 6 nm from the CdS/CIGS interface.

Keywords
CIGS, Cu depletion, EELS, OVC, Raman, solar cell, TEM
National Category
Other Materials Engineering Energy Systems
Identifiers
urn:nbn:se:uu:diva-357120 (URN)10.1002/pip.3010 (DOI)000442501000004 ()
Funder
Swedish Research CouncilSwedish Energy AgencyEU, Horizon 2020, 720887
Available from: 2018-08-12 Created: 2018-08-12 Last updated: 2018-11-13Bibliographically approved
5. Surface Modification And Secondary Phase Formation From a High Dose KF-Post Deposition Treatment of (Ag,Cu)(In,Ga)Se2 Solar Cell Absorbers
Open this publication in new window or tab >>Surface Modification And Secondary Phase Formation From a High Dose KF-Post Deposition Treatment of (Ag,Cu)(In,Ga)Se2 Solar Cell Absorbers
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(English)In: Progress in Photovoltaics, ISSN 1062-7995, E-ISSN 1099-159XArticle in journal (Refereed) Submitted
Abstract [en]

In this study we assessed the potential of KF-Post Deposition Treatment (PDT) performed on a silver-alloyed Cu(Inx,Ga1-x)Se2 (ACIGS) solar absorber. ACIGS absorbers with Ag/Ag+Cu ratio (Ag/I) close to 20% were co-evaporated on a Mo-coated glass substrate and exposed to in-situ KF-PDT of various intensities. The current-voltage characteristics indicated that an optimized PDT can be beneficial, increasing in our study the median Voc and efficiency values by +48 mV and +0.9 %abs (from 728 mV and 16.1 % efficiency measured for the sample without PDT), respectively. However, an increased KF-flux during PDT resulted in a net deterioration of the performance leading to median Voc and efficiency values as low as 503 mV and 4.7 %. The chemical composition analysis showed that while the reference absorber without any PDT was homogeneous, the KF-PDT induced a clear change within the first 10 nm from the surface. Here, the surface layer composition was richer in K and In with an increased Ag/I ratio, and its thickness seemed to follow the KF exposure intensity. Additionally, high-dose KF-PDT resulted in substantial formation of secondary phases for the ACIGS. The secondary phase precipitates were also richer in Ag, K and In, and Electron and X-ray diffraction data match with the monoclinic C 1 2/c 1 space group adopted by the Ag-alloyed KInSe2 (AKIS) phase. It could not be concluded whether the performance loss for the solar cell devices originated from the thicker surface layer or the presence of secondary phases, or both for the high-dose KF-PDT sample.

Keywords
TEM, EDS, ACIGS, XPS, KF-PDT, KInSe
National Category
Energy Systems Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:uu:diva-357122 (URN)
Available from: 2018-08-12 Created: 2018-08-12 Last updated: 2018-08-13Bibliographically approved
6. Effect of different Na supply methods on thin Cu(In,Ga)Se2 solar cells with Al2O3 rear passivation layers
Open this publication in new window or tab >>Effect of different Na supply methods on thin Cu(In,Ga)Se2 solar cells with Al2O3 rear passivation layers
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2018 (English)In: Solar Energy Materials and Solar Cells, ISSN 0927-0248, E-ISSN 1879-3398, Vol. 187, no 1, p. 160-169Article in journal (Refereed) Published
Abstract [en]

In this work, rear-contact passivated Cu(In,Ga)Se2 (CIGS) solar cells were fabricated without any intentional contact openings between the CIGS and Mo layers. The investigated samples were either Na free or one of two Na supply methods was used, i) a NaF precursor on top of the Al2O3 rear passivation layer or ii) an in situ post- deposition treatment with NaF after co-evaporation of the CIGS layer. The thickness of the ALD-Al2O3 passi- vation layer was also varied in order to find an optimal combination of Na supply and passivation layer thickness. Our results from electrical characterization show remarkably different solar cell behavior for different Na supplies. For up to 1nm thick Al2O3 layers an electronically good contact could be confirmed independently of Na deposition method and content. When the Al2O3 thickness exceeded 1 nm, the current was blocked on all samples except on the samples with the NaF precursor. On these samples the current was not blocked up to an Al2O3 layer thickness of about 6 nm, the maximum thickness we could achieve without the CIGS peeling off the Al2O3 layer. Transmission electron microscopy reveals a porous passivation layer for the samples with a NaF precursor. An analysis of the dependence of the open circuit voltage on temperature (JVT) indicates that a thicker NaF precursor layer lowers the height of the hole barrier at the rear contact for the passivated cells. This energy barrier is also lower for the passivated sample, compared to an unpassivated sample, when both samples have been post-deposition treated.

Keywords
Alkali, Back contact, CIGS, Passivation, Thin films, Rear contact, Tunneling
National Category
Electrical Engineering, Electronic Engineering, Information Engineering Energy Systems Other Materials Engineering
Identifiers
urn:nbn:se:uu:diva-357123 (URN)10.1016/j.solmat.2018.07.017 (DOI)000445308300019 ()
Funder
Swedish Research Council, 43523-1StandUpEU, Horizon 2020, 720887
Available from: 2018-08-12 Created: 2018-08-12 Last updated: 2019-08-08Bibliographically approved

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