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The effect of Zn1−xSnxOy buffer layer thickness in 18.0% efficient Cd-free Cu(In,Ga)Se2 solar cells
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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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.

Place, publisher, year, edition, pages
2013. Vol. 21, no 8, 1588-1597 p.
Keyword [en]
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: urn:nbn:se:uu:diva-182449DOI: 10.1002/pip.2239ISI: 000327260800004OAI: oai:DiVA.org:uu-182449DiVA: diva2:559809
Available from: 2012-10-10 Created: 2012-10-10 Last updated: 2017-12-07Bibliographically 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
3. Atomic layer deposition of zinc tin oxide buffer layers for Cu(In,Ga)Se2 solar cells
Open this publication in new window or tab >>Atomic layer deposition of zinc tin oxide buffer layers for Cu(In,Ga)Se2 solar cells
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The aim of this thesis is to provide an in-depth investigation of zinc tin oxide, Zn1-xSnxOy or ZTO, grown by atomic layer deposition (ALD) as a buffer layer in Cu(In,Ga)Se2 (CIGS) solar cells. The thesis analyzes how changes in the ALD process influence the material properties of ZTO, and how these in turn affect the performance of CIGS solar cells.

It is shown that ZTO grows uniformly and conformably on CIGS and that the interface between ZTO and CIGS is sharp with little or no interdiffusion between the layers. The band gap and conduction band energy level of ZTO are dependent both on the [Sn]/([Zn]+[Sn]) composition and on the deposition temperature. The influence by changes in composition is non-trivial, and the highest band gap and conduction band energy level are obtained at a [Sn]/([Zn]+[Sn]) composition of 0.2 at 120  °C. An increase in optical band gap is observed at decreasing deposition temperatures and is associated with quantum confinement effects caused by a decrease in crystallite size. The ability to change the conduction band energy level of ZTO enables the formation of suitable conduction band offsets between ZTO and CIGS with varying Ga-content.

It is found that 15 nm thin ZTO buffer layers are sufficient to fabricate CIGS solar cells with conversion efficiencies up to 18.2 %. The JSC is in general 2 mA/cm2 higher, and the VOC 30 mV lower, for cells with the ZTO buffer layer as compared to cells with the traditional CdS buffer layer. In the end comparable efficiencies are obtained for the two different buffer layers. The gain in JSC for the ZTO buffer layer is associated with lower parasitic absorption in the UV-blue region of the solar spectrum and it is shown that the JSC can be increased further by making changes to the other layers in the traditional CdS/i-ZnO/ZnO:Al window layer structure. The ZTO is highly resistive, and it is found that the shunt preventing i-ZnO layer can be omitted, which further increases the JSC. Moreover, an additional increase in JSC is obtained by replacing the sputtered ZnO:Al front contact with In2O3 deposited by ALD. The large gain in JSC for the ZTO/In2O3 window layer stack compensates for the lower VOC related to the ZTO buffer layer, and it is demonstrated that the ZTO/In2O3 window layer structure yields 0.6 % (absolute) higher conversion efficiency than the CdS/i-ZnO/ZnO:Al window layer structure. 

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2015. 104 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1277
Keyword
CIGS; Solar cells; Thin film; Buffer layer; TCO; Window layer; Zinc tin oxide; ZTO; Indium oxide
National Category
Engineering and Technology Materials Engineering
Research subject
Engineering Science with specialization in Electronics
Identifiers
urn:nbn:se:uu:diva-260882 (URN)978-91-554-9313-4 (ISBN)
Public defence
2015-10-16, Häggsalen, Lägerhyddsvägen 1, Uppsala, 13:00 (English)
Opponent
Supervisors
Funder
Swedish Energy Agency, 2012-00-4591
Available from: 2015-09-24 Created: 2015-08-25 Last updated: 2015-10-01

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Lindahl, JohanWätjen, Jörn TimoHultqvist, AdamEricson, ToveEdoff, MarikaTörndahl, Tobias

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