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Reactive sputtering and composition measurements of precursors for Cu2ZnSnS4 thin film solar cells
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics. (Ångström Solar Center)
2013 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Cu2ZnSnS4 (CZTS) is a thin film solar cell material that only contains abundant elements and for which promising conversion efficiencies of 9.2 % have been shown. In this thesis composition measurements and reactive sputtering of precursors for CZTS films have been studied. These precursors can be annealed to create high quality CZTS films.

Accurate control and measurement of composition are important for the synthesis process. The composition of a reference sample was determined using Rutherford backscattering spectroscopy. This sample was thereafter used to find the composition of unknown samples with x-ray fluorescence measurements. Pros and cons with this approach were discussed.

The reactive sputtering process, and the resulting thin films, from a CuSn- and a Zn-target sputtered in H2S-atmosphere were investigated and described. A process curve of the system was presented and the influence of sputtering pressure and substrate temperature were examined. The pressures tested had little influence on the film properties but the substrate temperature affected both composition and morphology, giving less Zn, Sn and S and a more oriented film with increasingly facetted surface for higher temperatures.

The precursors produced with this method are suggested to have a disordered phase with randomized cations, giving a CZTS-like response from Raman spectroscopy but a ZnS-pattern from x-ray diffraction measurements. The films have an excellent homogeneity and it is possible to achieve stoichiometric sulfur content.

The complete steps from precursors, to annealed films, to finished solar cells were investigated for three controlled compositions and three substrate temperatures. The films sputtered at room temperature cracked when annealed and thus gave shunted solar cells. For the samples sputtered at higher temperatures the trend was an increased grain size for higher copper content and increased temperature. However, no connection between this and the electrical properties of the solar cells could be found.

Place, publisher, year, edition, pages
Uppsala: Uppsala universitet, 2013. , 44 p.
Keyword [en]
Cu2ZnSnS4, Kesterite, Reactive sputtering, Process curve, Photovoltaics, Composition measurments
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Engineering Science with specialization in Electronics
Identifiers
URN: urn:nbn:se:uu:diva-208543OAI: oai:DiVA.org:uu-208543DiVA: diva2:653003
Presentation
2013-06-03, Å2005, Ångströmlaboratoriet, Uppsala, 10:15 (English)
Opponent
Supervisors
Available from: 2013-10-16 Created: 2013-10-02 Last updated: 2013-10-16Bibliographically approved
List of papers
1. Reactive sputtering of precursors for Cu2ZnSnS4 thin film solar cells
Open this publication in new window or tab >>Reactive sputtering of precursors for Cu2ZnSnS4 thin film solar cells
2012 (English)In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 520, no 24, 7093-7099 p.Article in journal (Refereed) Published
Abstract [en]

The quaternary semiconductor Cu2ZnSnS4 (CZTS) is a possible In-free replacement for Cu(In,Ga)Se-2. Here we present reactive sputtering with the possibility to obtain homogeneous CZTS-precursors with tunable composition and a stoichiometric quantity of sulfur. The precursors can be rapidly annealed to create large grained films to be used in solar cells. The reactive sputtering process is flexible, and morphology, stress and metal and sulfur contents were varied by changing the H2S/Ar-flow ratio, pressure and substrate temperature. A process curve for the reactive sputtering from CuSn and Zn targets is presented. The Zn-target is shown to switch to compound mode earlier and faster compared to the CuSn-target. The precursors containing a stoichiometric amount of sulfur exhibit columnar grains, have a crystal structure best matching ZnS and give a broad peak, best matching CZTS, in Raman scattering. In comparing process gas flows it is shown that the sulfur content is strongly dependent on the H2S partial pressure but the total pressures compared in this study have little effect on the precursor properties. Increasing the substrate temperature changes the film composition due to the high vapor pressures of Zn, SnS and S. High substrate temperatures also give slightly denser and increasingly oriented films. The precursors are under compressive stress, which is reduced with higher deposition temperatures. (C) 2012 Elsevier B.V. All rights reserved.

Keyword
Cu2ZnSnS4, Kesterite, Reactive sputtering, Process curve, Photovoltaics, X-ray diffraction
National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Electronics
Identifiers
urn:nbn:se:uu:diva-183573 (URN)10.1016/j.tsf.2012.08.002 (DOI)000308691700011 ()
Available from: 2012-12-06 Created: 2012-10-29 Last updated: 2017-12-07Bibliographically approved
2. Annealing behavior of reactively sputtered precursor films for Cu2ZnSnS4 solar cells
Open this publication in new window or tab >>Annealing behavior of reactively sputtered precursor films for Cu2ZnSnS4 solar cells
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2013 (English)In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 535, 22-26 p.Article in journal (Refereed) Published
Abstract [en]

Reactively sputtered Cu–Zn–Sn–S precursor films are prepared and recrystallized by rapid thermal processing to generate Cu2ZnSnS4 solar cell absorber layers. We study how the film properties are affected by substrate heating and composition. The stress, density and texture in the films were measured. Compressive stress was observed for the precursors but did not correlate to the deposition temperature, and had no influence on the properties of the annealed films or solar cells. However, the substrate temperature during precursor deposition had a large effect on the behavior during annealing and on the solar cell performance. The films deposited at room temperature had, after annealing, smaller grains and cracks, and gave shunted devices. Cracking is suggested to be due to a slightly higher sulfur content, lower density or to minor differences in material quality. The grain size in the annealed films seems to increase with higher copper content and higher precursor deposition temperature. The best device in the current series gave an efficiency of 4.5%.

Place, publisher, year, edition, pages
Elsevier, 2013
Keyword
Cu2ZnSnS4, CZTS, Kesterite, Reactive sputtering, Thin film solar cell, Stress, Density
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Engineering Science with specialization in Electronics
Identifiers
urn:nbn:se:uu:diva-188166 (URN)10.1016/j.tsf.2012.10.081 (DOI)000318973600007 ()
Available from: 2012-12-13 Created: 2012-12-13 Last updated: 2017-12-06Bibliographically approved
3. 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
4. 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
5. Rapid annealing of reactively sputtered precursors for Cu2ZnSnS4 solar cells
Open this publication in new window or tab >>Rapid annealing of reactively sputtered precursors for Cu2ZnSnS4 solar cells
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2013 (English)In: Progress in Photovoltaics, ISSN 1062-7995, E-ISSN 1099-159X, Vol. 22, no 1, 10-17 p.Article in journal (Refereed) Published
Abstract [en]

Cu2ZnSnS4 (CZTS) is a promising thin-film absorber material that presents some interesting challenges in fabrication when compared with Cu(In,Ga)Se2. We introduce a two-step process for fabrication of CZTS films, involving reactive sputtering of a Cu-Zn-Sn-S precursor followed by rapid annealing. X-ray diffraction and Raman measurements of the sputtered precursor suggest that it is in a disordered, metastable CZTS phase, similar to the high-temperature cubic modification reported for CZTS. A few minutes of annealing at 550 °C are sufficient to produce crystalline CZTS films with grain sizes in the micrometer range. The first reported device using this approach has an AM1.5 efficiency of 4.6%, with Jsc and Voc both appearing to be limited by interface recombination. 

Keyword
CZTS, Cu2ZnSnS4, kesterite, reactive sputtering, sulfides, thin film solar cells
National Category
Materials Chemistry Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Engineering Science with specialization in Electronics
Identifiers
urn:nbn:se:uu:diva-180994 (URN)10.1002/pip.2265 (DOI)000328248500002 ()
Available from: 2012-09-14 Created: 2012-09-14 Last updated: 2017-12-07Bibliographically approved

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