Change search
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Annealing of Cu2ZnSn(S,Se)4 Thin Films: A Study of Secondary Compounds and Their Effects on Solar Cells
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics. (Thin Film Solar Cell)ORCID iD: 0000-0002-0501-8969
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Kesterite Cu2ZnSnS4 (CZTS) is interesting as a sustainable photovoltaic technology due to its earth-abundant elements and suitable semiconducting properties. To date, a record efficiency of 12.6% has been achieved but further improvements are required to reach high efficiency for industrial implementation. Among the limiting issues is the understanding of the annealing process, which is crucial in promoting high material quality. In particular, the knowledge of the effects of segregated secondary compounds on solar cell performance is lacking.

In contrast to formation of ZnS particles throughout CZTS film, it is notable that SnS forms and usually segregates on the CZTS top and rear surfaces. The influence of SnS on CZTS solar cells was studied by electron beam induced current measurements. It is found that SnS presence on the CZTS surfacecan introduce “dead area”, whereas it seems beneficial for solar cell current when accumulates on the CZTS rear. For SnS passivation and from investigation of the passivation effect from an Al2O3 thin layer at the CZTS rear, improvement in overall device performance could not be demonstrated, due to either poor CZTS bulk or non-optimal device structure. The limitation in CZTS bulk quality was shown from a thickness study where carrier collection saturated already about 700-1000 nm CZTS thickness.

Formation of SnS alongside CZTS implies the anneal is limited by a deficient sulfur partial pressure (PS2). By looking into Sn-S phase transformations in SnS2 films after annealing, we find that PS2 drops rapidly over the annealing time, which could be well-correlated to a series of changes in CZTS material quality including secondary phase formations and defect modifications. It is shown that annealing CZTS under sufficiently high PS2 is critical for CZTS solar cells with high open circuit voltage (upto 783mV was reached), possibly due to the defect modification.

Besides SnS, it is observed that NaxS compounds are also readily formed on CZTS surfaces, due to Na diffusion from the glass substrate during annealing. NaxS negatively affects the formation of the CdS/CZTS interface during chemical bath deposition. It can be removed by an oxidation process or wet chemical etching.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2017. , p. 85
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1476
Keywords [en]
Annealing, sputtering, thin film, CZTS, secondary phases, solar cell
National Category
Engineering and Technology
Research subject
Engineering Science
Identifiers
URN: urn:nbn:se:uu:diva-314975ISBN: 978-91-554-9817-7 (print)OAI: oai:DiVA.org:uu-314975DiVA, id: diva2:1072439
Public defence
2017-03-31, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 13:15 (English)
Opponent
Supervisors
Available from: 2017-03-10 Created: 2017-02-07 Last updated: 2018-09-14
List of papers
1. Reactively sputtered films in the CuxS–ZnS–SnSy system: From metastability to equilibrium
Open this publication in new window or tab >>Reactively sputtered films in the CuxS–ZnS–SnSy system: From metastability to equilibrium
Show others...
2015 (English)In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 582, p. 208-214Article in journal, Meeting abstract (Refereed) Published
Abstract [en]

Cu2ZnSnS4 is a promising photovoltaic absorber containing earth abundant elements. Using a two stage process, low temperature reactive co-sputtering followed by heat treatment, we have previously achieved a 7.9% efficient solar cell. Because the sputtered precursors contain non-equilibrium phases with unusual crystal structures, it is crucial to understand their nature and their conversion into Cu2ZnSnS4 (and secondary phases) during heat treatment. In this study, we report phase analysis of reactively sputtered binary and ternary sulfides in the CuxS–ZnS–SnSy system before and after annealing. In the as deposited films, Raman spectroscopy with 532 and 325 nm excitation wavelengths reveals expected phases for the binaries (CuS, ZnS and SnS2) and the ternary (Cu2SnS3), and unique metastable phases for the Cu–Zn–S and Zn–Sn–S precursors. Upon annealing, the non-equilibrium phases disappear, accompanied by additional chemical changes. Excess S content in the films is removed, and in the Sn–S and Zn–Sn–S films, further S loss from decomposition of SnSx (x > 1) and CuS respectively generates SnS and CuxS (x > 1). Due to the presence of SnS vapor, Cu2ZnSnS4 is generated from the Cu–Zn–S precursor. Additionally, the range of sulfur partial pressure in the annealing process is estimated according to the temperature–pressure phase diagram. This gives us useful insight allowing better control of annealing conditions.

Keywords
Copper zinc tin sulfide; Thin film solar cells; Reactive sputtering; Secondary phases; Raman scattering; X-ray diffraction; Kesterite
National Category
Other Materials Engineering Other Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Engineering Science with specialization in Electronics
Identifiers
urn:nbn:se:uu:diva-236734 (URN)10.1016/j.tsf.2014.10.076 (DOI)000352225900045 ()
Funder
EU, FP7, Seventh Framework Programme, 316488 (KESTCELLS)
Available from: 2014-11-21 Created: 2014-11-21 Last updated: 2017-12-05Bibliographically approved
2. Influence of the Cu2ZnSnS4 absorber thickness on thin film solar cells
Open this publication in new window or tab >>Influence of the Cu2ZnSnS4 absorber thickness on thin film solar cells
2015 (English)In: Physica status solidi. A, Applied research, ISSN 0031-8965, E-ISSN 1521-396XArticle in journal (Refereed) Published
National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-265420 (URN)DOI: 10.1002/pssa.201532311 (DOI)
Available from: 2015-10-28 Created: 2015-10-28 Last updated: 2017-12-01
3. Investigation of the SnS/Cu2ZnSnS4 interfaces in Kesterite Thin-Film Solar Cells
Open this publication in new window or tab >>Investigation of the SnS/Cu2ZnSnS4 interfaces in Kesterite Thin-Film Solar Cells
Show others...
2017 (English)In: ACS Energy Letters, E-ISSN 2380-8195, Vol. 2, no 5, p. 976-981Article in journal (Refereed) Published
Abstract [en]

Kesterite Cu2ZnSnS4 (CZTS), having only earth abundant elements, is a promising solar cell material. Nevertheless, the impact of the SnS secondary phase, which often forms alongside CZTS synthesis at high annealing temperature, on CZTS solar cells is poorly studied. We confirm, by means of X-ray diffraction, Raman scattering, and energy dispersive X-ray spectroscopy mapping, that this phase tends to segregate at both the surface and the back side of annealed CZTS films with Cu-poor and Zn-rich composition. Using electron beam-induced current measurements, it is further demonstrated that the formation of SnS on the CZTS surface is harmful for solar cells, whereas the SnS phase can be beneficial for solar cells when it segregates on the CZTS rear. This positive contribution of SnS could stem from a passivation effect at the CZTS/SnS rear interface. This work opens new possibilities for an alternative interface development for kesterite-based photovoltaic technology.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-314941 (URN)10.1021/acsenergylett.7b00151 (DOI)000401500200005 ()
Funder
EU, FP7, Seventh Framework Programme, 316488Knut and Alice Wallenberg FoundationSwedish Foundation for Strategic Research Swedish Research Council
Available from: 2017-02-07 Created: 2017-02-07 Last updated: 2017-06-30Bibliographically approved
4. 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
Show others...
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
5. Evolution of Cu2ZnSnS4 in non-equilibrium thermal processing with Quasi-in-situ Monitoring of Sulfur Partial Pressure
Open this publication in new window or tab >>Evolution of Cu2ZnSnS4 in non-equilibrium thermal processing with Quasi-in-situ Monitoring of Sulfur Partial Pressure
Show others...
2017 (English)In: Chemistry of Materials, ISSN 0897-4756, E-ISSN 1520-5002Article in journal (Refereed) Published
National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-314940 (URN)
Available from: 2017-02-07 Created: 2017-02-07 Last updated: 2017-05-09
6. Evolution of Na-S(-O) compounds on the Cu2ZnSnS4 absorber surface and their effects on CdS thin film growth
Open this publication in new window or tab >>Evolution of Na-S(-O) compounds on the Cu2ZnSnS4 absorber surface and their effects on CdS thin film growth
Show others...
2016 (English)In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 8, no 28, p. 18600-18607Article in journal (Refereed) Published
Abstract [en]

Formation of Na-containing surface compounds is an important phenomenon in the Cu2ZnSnS4 (CZTS) quaternary material synthesis for solar cell applications. Still, identification of these compounds and the understanding of their potential influence on buffer layer growth and device performance are scarce. In this work, we discovered that the evolution of Na-S(-O) compounds on the CZTS surface substantially affect the solution/CZTS interface during the chemical bath deposition of CdS buffer film. We showed that Na2S negatively affects the growth of CdS, and that this compound is likely to form on the CZTS surface after annealing. It was also demonstrated that the Na2S compound can be oxidized to Na2SO4 by air exposure of the annealed CZTS surface or be removed using water dipping instead of the commonly used KCN etching process, resulting in significantly better quality of the CdS layer. Lastly, 6.5% CZTS solar cells were fabricated with air exposure treatment without incorporation of the KCN etching process. This work provides new insight into the growth of the CdS/CZTS interface for solar cell applications and opens new possibilities for improving likewise Cd-free buffer materials that are grown with a similar chemical bath deposition process.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-298673 (URN)10.1021/acsami.6b04978 (DOI)000380298400096 ()27356214 (PubMedID)
Funder
EU, FP7, Seventh Framework Programme, 316488Knut and Alice Wallenberg FoundationSwedish Foundation for Strategic Research Swedish Research Council
Available from: 2016-07-06 Created: 2016-07-06 Last updated: 2017-11-28Bibliographically approved
7. Surface modification through air annealing Cu2ZnSn(S,Se)4 absorbers
Open this publication in new window or tab >>Surface modification through air annealing Cu2ZnSn(S,Se)4 absorbers
Show others...
2017 (English)In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 633, p. 118-121Article in journal (Refereed) Published
Abstract [en]

Recent studies demonstrate that air annealing can have a positive effect on the device performance of Cu2ZnSn(SxSe1-x)(4)[CZTSSe] solar cells. In this work air annealing of the selenium containing CZTSSe is compared to the pure sulfide CZTS. It is discovered that the selenium containing absorbers benefit from air annealing at higher temperatures than selenium free absorbers. The highest efficiency obtained utilizing the air annealing treatment on selenium containing absorbers is 9.7%. We find that the band gap is narrowed when air annealing, which is partially explained by increased Cu-Zn disorder. Furthermore Zn enrichment of the surface after etching is identified as a possible cause of enhanced device performance. It is additionally observed that elemental selenium present on the CZTSSe surface is reduced in the air annealing treatment. Selenium removal is another possible explanation for the enhanced performance caused by the air annealing treatment.

Keywords
CZTS, Kesterite, Thin film solar cells, Surface modification, Passivation
National Category
Materials Engineering Physical Sciences
Identifiers
urn:nbn:se:uu:diva-330018 (URN)10.1016/j.tsf.2016.08.030 (DOI)000404802300023 ()
Conference
Symposium V on Thin Film Chalcogenide Photovoltaic Materials held at the 13th E-MRS Spring Meeting, MAY 02-06, 2016, Lille, FRANCE
Note

Surface modification through air annealing Cu2ZnSn(S,Se)(4) absorbers

Available from: 2017-10-11 Created: 2017-10-11 Last updated: 2018-09-14Bibliographically approved

Open Access in DiVA

fulltext(2843 kB)442 downloads
File information
File name FULLTEXT01.pdfFile size 2843 kBChecksum SHA-512
69f06894171b0cbafdde74327a3856e51f15986cfde6a4875513331b2b074bb17425ce474eb2bba74428bdbb0bfc6e8dfffe4ef6a42b9facbe818f9c95650efc
Type fulltextMimetype application/pdf
Buy this publication >>

Search in DiVA

By author/editor
Ren, Yi
By organisation
Solid State Electronics
Engineering and Technology

Search outside of DiVA

GoogleGoogle Scholar
Total: 442 downloads
The number of downloads is the sum of all downloads of full texts. It may include eg previous versions that are now no longer available

isbn
urn-nbn

Altmetric score

isbn
urn-nbn
Total: 2924 hits
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf