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Atomic layer deposition of zinc tin oxide buffer layers for Cu(In,Ga)Se2 solar cells
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics. (Ångström Solar Center)
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 [en]
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: urn:nbn:se:uu:diva-260882ISBN: 978-91-554-9313-4 (print)OAI: oai:DiVA.org:uu-260882DiVA: diva2:849194
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
List of papers
1. Inline Cu(In,Ga)Se-2 Co-evaporation for High-Efficiency Solar Cells and Modules
Open this publication in new window or tab >>Inline Cu(In,Ga)Se-2 Co-evaporation for High-Efficiency Solar Cells and Modules
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2013 (English)In: IEEE JOURNAL OF PHOTOVOLTAICS, ISSN 2156-3381, Vol. 3, no 3, 1100-1105 p.Article in journal (Refereed) Published
Abstract [en]

In this paper, co-evaporation of Cu(In,Ga)Se-2 (CIGS) in an inline single-stage process is used to fabricate solar cell devices with up to 18.6% conversion efficiency using a CdS buffer layer and 18.2% using a Zn1-xSnxOy Cd-free buffer layer. Furthermore, a 15.6-cm(2) mini-module, with 16.8% conversion efficiency, has been made with the same layer structure as the CdS baseline cells, showing that the uniformity is excellent. The cell results have been externally verified. The CIGS process is described in detail, and material characterization methods show that the CIGS layer exhibits a linear grading in the [Ga]/([Ga]+[In]) ratio, with an average [Ga]/([Ga]+[In]) value of 0.45. Standard processes for CdS as well as Cd-free alternative buffer layers are evaluated, and descriptions of the baseline process for the preparation of all other steps in the Angstrom Solar Center standard solar cell are given.

Keyword
Buffer layer, Cd-free, Cu(In, Ga)Se-2 (CIGS), inline co-evaporation, thin-film solar cells, Zn1-xSnxOy
National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Electronics
Identifiers
urn:nbn:se:uu:diva-204265 (URN)10.1109/JPHOTOV.2013.2256232 (DOI)000320862500026 ()
Available from: 2013-07-30 Created: 2013-07-29 Last updated: 2015-10-01Bibliographically approved
2. Soft X-ray characterization of Zn1-xSnxOy electronic structure for thin film photovoltaics
Open this publication in new window or tab >>Soft X-ray characterization of Zn1-xSnxOy electronic structure for thin film photovoltaics
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2012 (English)In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 14, no 29, 10154-10159 p.Article in journal (Refereed) Published
Abstract [en]

Zinc tin oxide (Zn1-xSnxOy) has been proposed as an alternative buffer layer material to the toxic, and light narrow-bandgap CdS layer in CuIn1-x,GaxSe2 thin film solar cell modules. In this present study, synchrotron-based soft X-ray absorption and emission spectroscopies have been employed to probe the densities of states of intrinsic ZnO, Zn1-xSnxOy and SnOx thin films grown by atomic layer deposition. A distinct variation in the bandgap is observed with increasing Sn concentration, which has been confirmed independently by combined ellipsometry-reflectometry measurements. These data correlate directly to the open circuit potentials of corresponding solar cells, indicating that the buffer layer composition is associated with a modification of the band discontinuity at the CIGS interface. Resonantly excited emission spectra, which express the admixture of unoccupied O 2p with Zn 3d, 4s, and 4p states, reveal a strong suppression in the hybridization between the O 2p conduction band and the Zn 3d valence band with increasing Sn concentration.

National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Engineering Science with specialization in Electronics
Identifiers
urn:nbn:se:uu:diva-178156 (URN)10.1039/c2cp41394a (DOI)000305965200011 ()
Available from: 2012-07-30 Created: 2012-07-30 Last updated: 2017-12-07
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. The effect of substrate temperature on atomic layer deposited zinc tin oxide
Open this publication in new window or tab >>The effect of substrate temperature on atomic layer deposited zinc tin oxide
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2015 (English)In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 586, 82-87 p.Article in journal (Refereed) Published
Abstract [en]

Zinc tin oxide (ZTO) thin films were deposited on glass substrates by atomic layer deposition (ALD), and the film properties were investigated for varying deposition temperatures in the range of 90 to 180 degrees C. It was found that the [Sn]/([Sn] + [Zn]) composition is only slightly temperature dependent, while properties such as growth rate, film density, material structure and band gap are more strongly affected. The growth rate dependence on deposition temperature varies with the relative number of zinc or tin containing precursor pulses and it correlates with the growth rate behavior of pure ZnO and SnOx ALD. In contrast to the pure ZnO phase, the density of the mixed ZTO films is found to depend on the deposition temperature and it increases linearly with about 1 g/cm(3) in total over the investigated range. Characterization by transmission electron microscopy suggests that zinc rich ZTO films contain small (similar to 10 nm) ZnO or ZnO(Sn) crystallites embedded in an amorphous matrix, and that these crystallites increase in size with increasing zinc content and deposition temperature. These crystallites are small enough for quantum confinement effects to reduce the optical band gap of the ZTO films as they grow in size with increasing deposition temperature.

Keyword
Zinc tin oxide (ZTO), Atomic layer deposition (ALD), Buffer layer, Mixed oxide, Thin film photovoltaics, Optical band gap
National Category
Condensed Matter Physics Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-255039 (URN)10.1016/j.tsf.2015.04.029 (DOI)000353984000014 ()
Available from: 2015-06-23 Created: 2015-06-12 Last updated: 2017-12-04
5. Deposition temperature induced conduction band changes in zinc tin oxide buffer layers for Cu(In,Ga)Se2 solar cells
Open this publication in new window or tab >>Deposition temperature induced conduction band changes in zinc tin oxide buffer layers for Cu(In,Ga)Se2 solar cells
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2016 (English)In: Solar Energy Materials and Solar Cells, ISSN 0927-0248, E-ISSN 1879-3398, Vol. 144, 684-690 p.Article in journal (Refereed) Published
Abstract [en]

Thin film Cu(In,Ga)Se2 solar cells with ALD-deposited Zn1-xSnxOy buffer layers were fabricated and the solar cell properties were investigated for varying ALD deposition temperatures in the range from 90 °C up to 180 °C. It was found that a process window exists between 105 °C and 135 °C, where high solar cell efficiency can be achieved. At lower ALD deposition temperatures the solar cell performance was mainly limited by low fill factor and at higher temperatures by low open circuit voltage. Numerical simulations and electrical characterization were used to relate the changes in solar cell performance as a function of ALD deposition temperature to changes in the conduction band energy level of the Zn1-xSnxOy buffer layer. The Zn1-xSnxOy films contain small ZnO or ZnO(Sn) crystallites (~10 nm), resulting in quantum confinement effects influencing the optical band gap of the buffer layer. The ALD deposition temperature affects the size of these crystallites and it is concluded that most of the changes in the band gap occur in the conduction band level.

Keyword
Zinc tin oxide (ZTO); Atomic layer deposition (ALD); Buffer layer; Thin film photovoltaics; CIGS; Conduction band line-up;
National Category
Engineering and Technology Environmental Engineering
Identifiers
urn:nbn:se:uu:diva-260879 (URN)10.1016/j.solmat.2015.09.048 (DOI)000366223900087 ()
Funder
Swedish Energy Agency, 2012-004591VINNOVA, 2013-02199StandUp
Available from: 2015-08-25 Created: 2015-08-25 Last updated: 2017-12-04Bibliographically approved
6. Potential gain in photocurrent generation for Cu(In,Ga)Se2 solar cells by using In2O3 as a transparent conductive oxide layer
Open this publication in new window or tab >>Potential gain in photocurrent generation for Cu(In,Ga)Se2 solar cells by using In2O3 as a transparent conductive oxide layer
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2016 (English)In: Progress in Photovoltaics, ISSN 1062-7995, E-ISSN 1099-159X, Vol. 24, no 1, 102-107 p.Article in journal (Refereed) Published
Abstract [en]

This study highlights the potential of atomic layer deposited In2O3 as a highly transparent and conductive oxide (TCO)layer in Cu(In,Ga)Se2 (CIGSe) solar cells. It is shown that the efficiency of solar cells which use Zn-Sn-O (ZTO) as an alternativebuffer layer can be increased by employing In2O3 as a TCO because of a reduction of the parasitic absorption inthe window layer structure, resulting in 1.7 mA/cm2 gain in short circuit current density (Jsc). In contrast, a degradation ofdevice properties is observed if the In2O3 TCO is combined with the conventional CdS buffer layer. The estimated improvementfor large-scale modules is discussed.

Keyword
In2O3; ALD; Zn-Sn-O; alternative window layer; CIGSe; TCO
National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-260877 (URN)10.1002/pip.2655 (DOI)000370320100011 ()
Funder
Swedish Energy AgencyVINNOVA
Available from: 2015-08-25 Created: 2015-08-25 Last updated: 2017-12-04Bibliographically approved

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  • modern-language-association-8th-edition
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  • Other locale
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Output format
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