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Secondary compound formation revealed by transmission electron microscopy at the Cu2ZnSnS4/Mo interface
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
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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.

Place, publisher, year, edition, pages
2012. Vol. 535, 31-34 p.
Keyword [en]
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: urn:nbn:se:uu:diva-190042DOI: 10.1016/j.tsf.2012.11.079ISI: 000318973600009OAI: oai:DiVA.org:uu-190042DiVA: diva2:582881
Available from: 2013-01-07 Created: 2013-01-07 Last updated: 2017-12-06Bibliographically approved
In thesis
1. Microscopic Characterisation of Solar Cells: An Electron Microscopy Study of Cu(In,Ga)Se2 and Cu2ZnSn(S,Se)4 Solar Cells
Open this publication in new window or tab >>Microscopic Characterisation of Solar Cells: An Electron Microscopy Study of Cu(In,Ga)Se2 and Cu2ZnSn(S,Se)4 Solar Cells
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
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:nbn:se:uu:diva-199432 (URN)978-91-554-8692-1 (ISBN)
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
2. Reactive sputtering and composition measurements of precursors for Cu2ZnSnS4 thin film solar cells
Open this publication in new window or tab >>Reactive sputtering and composition measurements of precursors for Cu2ZnSnS4 thin film solar cells
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
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:nbn:se:uu:diva-208543 (URN)
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

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Wätjen, Jörn TimoScragg, JonathanEricson, ToveEdoff, MarikaPlatzer-Björkman, Charlotte

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