SnS4 Absorber Materials for Monograin Layer Solar Cells

113 Pages·2017·12.41 MB·English

THESISON NATURALANDEXACTSCIENCES B239
Study of Cu (Zn,Cd)SnS Absorber
2 4
Materials for Monograin Layer
Solar Cells
MARIS PILVET
TALLINN UNIVERSITY OF TECHNOLOGY
School of Engineering
Department of Materials and Environmental Technology

This dissertation was accepted for the defence of the degree of Doctor of
Philosophy in Natural and Exact Sciences on August 03, 2017.

Supervisor: Senior Research Scientist Marit Kauk-Kuusik, Department of
Materials and Environmental Technology, Tallinn University of
Technology, Estonia

Opponents: Prof. Charlotte Platzer Björkman, Department of Engineering
Science, Solid State Electronics, Uppsala University, Sweden

Dr. Vambola Kisand, Faculty of Science and Technology,
Institute of Physics, University of Tartu, Estonia

Defense of the thesis: September 07, 2017 at 14.00
Lecture hall: U06A-229
Tallinn University of Technology, Ehitajate tee 5,
Tallinn

Declaration:
Hereby I declare that this doctoral thesis, my original investigation and
achievement, submitted for the doctoral degree at Tallinn University of
Technology, has not been submitted for any academic degree.

/Maris Pilvet/

Copyright: Maris Pilvet, 2017
ISSN 1406-4723
ISBN 978-9949-83-141-8 (publication)
ISBN 978-9949-83-142-5 (PDF)
LOODUS-JATÄPPISTEADUSED B239
Päikesepatareides kasutatavate
Cu (Zn,Cd)SnS absorbermaterjalide
2 4
uurimine
MARIS PILVET

TABLE OF CONTENTS
LIST OF PUBLICATIONS ................................................................................. 7 
AUTHOR’S CONTRIBUTION .......................................................................... 8 
LIST OF ABBREVIATIONS .............................................................................. 9 
INTRODUCTION ............................................................................................. 10 
1  LITERATURE OVERVIEW AND THE AIM OF THE STUDY ............... 12 
1.1  I -II-IV-VI compounds based solar cells ............................................. 12 
2 4
1.2  Structural properties of Cu ZnSnS and Cu CdSnS ............................ 13 
2 4 2 4
1.3  Defect structure of Cu ZnSnS and Cu CdSnS .................................... 14 
2 4 2 4
1.4  Ordering-disordering in crystal structure .............................................. 15 
1.5  Bandgap engineering ............................................................................ 17 
1.6  Monograin powder technology ............................................................. 18 
1.7  Secondary phases in Cu ZnSnS and Cu CdSnS ................................. 19 
2 4 2 4
1.8  Summary of the literature review and objectives of the research ......... 20 
2  EXPERIMENTAL ........................................................................................ 22 
2.1  Preparation of powder samples ............................................................. 22 
2.2  Post-treatments of Cu ZnSnS monograin powders .............................. 23 
2 4
2.2.1  Cooling parameters for Cu ZnSnS monograin powders ................. 24 
2 4
2.2.2  Low-temperature annealing of Cu ZnSnS monograin powder ....... 26 
2 4
2.2.3  Low-temperature annealing of Cu CdSnS polycrystalline powder 26 
2 4
2.3  Fabrication of monograin layer solar cells ............................................ 27 
2.4  Characterization of monograin and polycrystalline powders ................ 28 
2.4.1  Material characterization ................................................................. 28 
2.4.2  Solar cell characteristics .................................................................. 29 
3  RESULTS AND DISCUSSION ................................................................... 30 
3.1  Zn substitution by Cd in Cu ZnSnS monograin powders .................... 30 
2 4
3.1.1  Composition of Cu (Zn Cd )SnS monograin powders ................. 30 
2 1-x x 4
3.1.2  Influence of the Cd content on Cu ZnSnS structural properties ..... 31 
2 4
3.1.2.1  X-ray diffraction analysis ......................................................... 31 
3.1.2.2  Raman analysis ........................................................................ 33 
3.1.3  Influence of the Cd content on Cu ZnSnS optical properties ......... 34 
2 4
3.2  Disordering studies in Cu ZnSnS monograin powders ....................... 36 
2 4
3.2.1  Influence of cooling rate on Cu ZnSnS properties ......................... 36 
2 4
3.2.1.1  Compositional analysis ............................................................ 36 
3.2.1.2  Photoluminescence and Raman results .................................... 37 
3.2.2  Impact of low-temperature annealing on the CZTS monograin layer
solar cells ................................................................................................... 40 
3.3  Modification of the optoelectronic properties of Cu CdSnS through
2 4
low-temperature annealing............................................................................. 43 
3.3.1  Results of structural and compositional analysis ............................. 43 
3.3.2  Photoluminescence analysis results ................................................. 46 
CONCLUSIONS ............................................................................................... 52 
ACKNOWLEDGEMENTS ............................................................................... 54 
ABSTRACT ...................................................................................................... 55 
KOKKUVÕTE .................................................................................................. 57 
REFERENCES .................................................................................................. 59 
APPENDIX A .................................................................................................... 67 
APPENDIX B .................................................................................................... 95 

6
LIST OF PUBLICATIONS
The thesis is based on the following publications, which are referred to in the
text by Roman numerals I-IV:
I. M. Pilvet, M. Kauk-Kuusik, M. Altosaar, M. Grossberg, M. Danilson,
K. Timmo, A. Mere, V. Mikli, Compositionally tunable structure and
optical properties of Cu (Cd Zn ) SnS (0 ≤ x ≤ 1) monograin
1.85 x 1-x 1.1 4.1
powders, Thin Solid Films 582 (2015) 180−183.
II. K. Timmo, M. Kauk-Kuusik, M. Pilvet, T. Raadik, M. Altosaar,
M. Danilson, M. Grossberg, J. Raudoja, K. Ernits, Influence of order-
disorder in Cu ZnSnS powders on the performance of monograin layer
2 4
solar cells, Thin Solid Films 633 (2017) 122–126.
III. M. Pilvet, M. Kauk-Kuusik, M. Grossberg, T. Raadik, V. Mikli,
R. Traksmaa, J. Raudoja, K. Timmo, J. Krustok, Modification of the
optoelectronic properties of Cu CdSnS through low-temperature
2 4
annealing treatments, Journal of Alloys and Compounds 723 (2017)
820-825.

IV. K. Timmo, M. Kauk-Kuusik, M. Altosaar, J. Raudoja, T. Raadik,
M. Grossberg, T. Varema, M. Pilvet, I. Leinemann, O. Volobujeva,
E. Mellikov, Novel Cu CdSnS and Cu ZnGeSe absorber materials for
2 4 2 4
monograin layer solar cell application, Proceeding of EU PVSEC 2013
(2013), Wiley-Blackwell 2385−2388.

Copies of these articles are included in APPENDIX A.

7
AUTHOR’S CONTRIBUTION
The contribution by the author to the papers included in the thesis is as follows:
I. Major part of experimental work (preparation of precursors and
monograin powders, solar cell preparation), part of characterization,
analysis of the results, and major part of writing.
II. Major part of experimental work (etching and thermal treatments of the
monograins, CBD-CdS deposition, solar cell preparation), part of
characterization, analysis of the results, and minor part of writing.
III. Major part of experimental work (preparation of some precursors and
Cu CdSnS polycrystalline powders, thermal treatments of the powder),
2 4
part of characterization, analysis of the results, and major part of writing.

IV. Part of experimental work (preparation of some precursors and
Cu CdSnS monograin powders, solar cell preparation), part of
2 4
characterization, and minor part of writing.

8
LIST OF ABBREVIATIONS

Br -MeOH bromine in methanol
2
CCdTS Cu CdSnS , copper cadmium tin sulfide
2 4
CIGSe Cu(In,Ga)Se , copper (indium, gallium) diselenide
2
CZCdTS Cu Zn Cd SnS , copper zinc cadmium tin sulfide
2 1−x x 4
CZTS Cu ZnSnS , copper zinc tin sulfide
2 4
CZTSSe Cu ZnSn(S,Se) , copper zinc tin sulfoselenide
2 4
CBD chemical bath deposition
I-V current-voltage
EDX energy dispersive x-ray spectroscopy
EQE external quantum efficiency
E band gap energy
g
E * effective band gap energy
g
FF fill factor
HT high-temperature
J short circuit current
sc
LTA low-temperature annealing
MGL monograin layer
PV photovoltaic
PL photoluminescence
QE quantum efficiency
RT room temperature
SEM scanning electron microscope
T temperature annealing
ann
T critical temperature
c
V open circuit voltage
oc
 efficiency
XRD X-ray diffraction
PCE power conversion efficiency

9
INTRODUCTION
Global electricity generation was around 21 000 TWh in 2015 [1]. Most of the
electricity is produced using conventional carbon-based technologies due to
their lower price per watt. However, global fossil resources are depleting while
at the same time, the world’s population is growing rapidly. Therefore, the need
for a sustainable and environmentally friendly alternative energy source is
unavoidable.
Sunlight is the most abundant and free natural energy source, and light-to-
electricity conversion in a solar cell is one of the best-developed renewable
energy technologies [2]. The production of solar cells with reduced cost, high
efficiency and environmentally friendly processes is the current challenge for
photovoltaic research and industry for photovoltaics (PV) to be competitive
with conventional energy technologies. Currently, approximately 90% of the
world’s PV market is dominated by silicon-based technologies. The record
laboratory cell efficiency is 25.6% for mono-crystalline and 20.8% for multi-
crystalline silicon wafer-based technology [3]. Japan’s Kaneka Corporation has
reached a certified conversion efficiency of more than 26% for silicon
heterojunction solar cells [4].
In 2015, the market share of all thin film technologies amounted to about 8%
of the total annual production [3]. The most efficient thin film solar cells with
efficiencies of 22.1% [5] and 22.6% [6] are based on cadmium telluride (CdTe)
and copper (indium, gallium) diselenide (CIGSe), respectively. As production
volumes increase, the cost and scarcity of In and Te will become major issues
that may limit the utilization of widespread CdTe and CIGSe photovoltaics. In
the last decade, new absorber materials that use low cost and broadly available
elements have been proposed as an alternative. Quaternary I -II-IV-VI (I = Cu,
2 4
Ag; II = Zn, Cd; IV = Si, Ge, Sn; VI = S, Se) chalcogenides form a large group
of semiconductors with variable structural, optical and electrical properties. The
kesterite-type Cu ZnSnS (CZTS), Cu ZnSnSe (CZTSe) and their solid
2 4 2 4
solutions Cu ZnSn(S,Se) (CZTSSe) continue to excite interest as potential
2 4
earth-abundant alternatives to CIGSe for application as absorber materials in
solar cells. These materials are closely related to CIGSe as their crystal
structures are very similar and the same preparation methods can be used.
By 2014, the efficiency of CZTSSe solar cells had reached 12.7% [7], but
progress since then has been slowing down. The main limiting factor for device
performance is the open circuit voltage, which is ~200 mV lower than that of
CIGSe cells with comparable bandgaps. The reasons for the voltage losses in
kesterite solar cells are still not clear. One possible cause is enhanced
recombination due to tail states arising from Cu-Zn disorder, surface
recombination and the influence of secondary phases [8-10].
During the last decade, the power conversion efficiency of solar cells based
on pure Cu ZnSnS absorber has achieved as high as 9.5% by using Zn Cd S
2 4 1-x x
buffer layer [11]. Despite suitable properties of the absorber material, the
performance of the photovoltaic device is still poor. For high efficiency solar
10