Solution-processed Cu2 ZnSn(S,Se)4 absorbers for thin film solar cells

124 Pages·2016·29.43 MB·English

ETH Library
Solution-processed
Cu2ZnSn(S,Se)4 absorbers for
thin film solar cells
Doctoral Thesis
Author(s):
Werner, Melanie
Publication date:
2016
Permanent link:
https://doi.org/10.3929/ethz-a-010579497
Rights / license:
In Copyright - Non-Commercial Use Permitted
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DISS. ETH NO. 22997
Solution-processed Cu ZnSn(S,Se)
2 4
absorbers for thin film solar cells
A thesis submitted to attain the degree of
DOCTOR OF SCIENCES of ETH ZURICH
(Dr. sc. ETH Zurich)
presented by
MELANIE WERNER
Máster Universitario en Nanociencia y Nanotecnología, UB, Barcelona
born on 17.10.1980
citizen of Germany
accepted on the recommendation of
Prof. Dr. A. N. Tiwari, examiner
Prof. Dr. A. Pérez-Rodríguez, co-examiner
Prof. Dr. W. Wegscheider, co-examiner
Dr. Y. E. Romanyuk , co-examiner
2016
ii
“Even the least of our activities ought to have some
end in view.“
Marcus Aurelius, Meditations, written in 167
A.C.E

Contents
Abstract ix
Zusammenfassung (German) xvi
1 Introduction 1
1.1 The need for cost-effective renewable energy . . . . . . . . . . . . . . . . 1
1.2 Photovoltaic, a history of solar cells . . . . . . . . . . . . . . . . . . . . . . 2
1.3 Chalcogenide thin film solar cells . . . . . . . . . . . . . . . . . . . . . . . 3
1.4 Kesterite solar cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.4.1 Crystal structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.4.2 Electronic and optoelectronic properties . . . . . . . . . . . . . . . 6
1.4.3 Challenges of kesterite absorbers . . . . . . . . . . . . . . . . . . 7
1.5 Non-vacuum solution processing . . . . . . . . . . . . . . . . . . . . . . . 9
1.6 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2 Methodology 13
2.1 Absorber deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.1.1 Spray deposition of the precursor . . . . . . . . . . . . . . . . . . . 14
2.1.2 Spin-coating of the precursor . . . . . . . . . . . . . . . . . . . . . 15
2.2 Reactive annealing in selenium atmosphere . . . . . . . . . . . . . . . . . 17
2.2.1 Open reactor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
v
vi Contents
2.2.2 Closed reactor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.3 Material characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.3.1 Microscopy techniques . . . . . . . . . . . . . . . . . . . . . . . . 20
2.3.1.1 Scanning electron microscopy . . . . . . . . . . . . . . . 20
2.3.1.2 (Scanning) transmission electron microscopy . . . . . . . 21
2.3.2 Spectroscopy techniques . . . . . . . . . . . . . . . . . . . . . . . 21
2.3.2.1 Energy-dispersive X-ray spectroscopy . . . . . . . . . . . 21
2.3.2.2 Inductively coupled plasma mass spectrometry . . . . . . 21
2.3.2.3 X-ray fluorescence spectroscopy . . . . . . . . . . . . . . 21
2.3.2.4 Secondary ion mass spectrometry . . . . . . . . . . . . . 22
2.3.3 Diffraction techniques . . . . . . . . . . . . . . . . . . . . . . . . . 22
2.3.3.1 X-ray diffraction . . . . . . . . . . . . . . . . . . . . . . . 22
2.4 Solar cell processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
2.4.1 Substrate and back contact . . . . . . . . . . . . . . . . . . . . . . 22
2.4.2 Buffer layer and front contact . . . . . . . . . . . . . . . . . . . . . 23
2.5 Solar cell characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
2.5.1 Current-voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
2.5.2 External quantum efficiency . . . . . . . . . . . . . . . . . . . . . . 26
2.5.3 Electron beam induced current . . . . . . . . . . . . . . . . . . . . 26
3 Spray-processed Cu ZnSn(S,Se) absorbers 29
2 4
3.1 Choice of solvent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
3.2 Sequential spray deposition . . . . . . . . . . . . . . . . . . . . . . . . . . 32
3.2.1 Solar cell performance . . . . . . . . . . . . . . . . . . . . . . . . . 35
3.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
4 DMSO-solution processed Cu ZnSn(S,Se) absorbers 37
2 4
4.1 Influence of precursor thickness . . . . . . . . . . . . . . . . . . . . . . . 38
Contents vii
4.1.1 Solar cell performance . . . . . . . . . . . . . . . . . . . . . . . . . 39
4.2 Influence of reactive annealing conditions . . . . . . . . . . . . . . . . . . 40
4.2.1 Metal ratios . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
4.2.1.1 Solar cells performance . . . . . . . . . . . . . . . . . . . 43
4.2.2 Selenium pressure control . . . . . . . . . . . . . . . . . . . . . . . 44
4.2.3 Ex-situ XRD: phase formation . . . . . . . . . . . . . . . . . . . . . 46
4.2.3.1 Crystallization . . . . . . . . . . . . . . . . . . . . . . . . 47
4.2.3.2 Solar cell performance . . . . . . . . . . . . . . . . . . . 50
4.2.4 Holding temperature . . . . . . . . . . . . . . . . . . . . . . . . . . 52
4.3 Influence of chemical surface treatment . . . . . . . . . . . . . . . . . . . 54
4.4 Influence of sodium doping on CZTSSe solar cells . . . . . . . . . . . . . 56
4.4.1 Influence of Na on the crystallinity . . . . . . . . . . . . . . . . . . 58
4.4.2 Composition and Na distribution . . . . . . . . . . . . . . . . . . . 60
4.4.3 Effect of Na on the device performance . . . . . . . . . . . . . . . 62
4.4.3.1 Open reactor . . . . . . . . . . . . . . . . . . . . . . . . . 63
4.4.3.2 Closed reactor . . . . . . . . . . . . . . . . . . . . . . . . 64
4.4.4 Degradation of CZTSSe solar cells . . . . . . . . . . . . . . . . . . 66
4.5 Grain passivation with CdS . . . . . . . . . . . . . . . . . . . . . . . . . . 67
4.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
5 Conclusions and outlook 79
Acknowledgment 83
Abbreviations 85
List of Publications 107

Abstract
Solar cells for photovoltaic electricity production is an environmental friendly and su-
stainable approach to produce electricity using the solar power. The most common
solar cells are made of crystalline Silicon, however thin film solar cells are growing in
importance. The direct band gap of thin film solar cells enables film thicknesses much
lower (100-fold) to absorb most of the incident light as compared to Si-wafer based
technologies. Thin film solar cells including amorphous silicon, Cu(In,Ga)Se (CIGS),
2
CdTe, Cu ZnSn(S,Se) (CZTSSe) and perovskites can be deposited on flexible sub-
2 4
strates and show high potential for cost-effective energy conversion. CIGS represents
a very promising thin film absorber exhibiting efficiencies of over 21%, although long-
term availability of Ga and In have raised concerns in the growth of the manufacturing
volume of these solar cells.
CZTSSe - so called kesterite, an alternative chalcogenide absorber layer, replaces the
expensive In with the cheaper and more abundant Zn and Sn. For CZTSSe absorber
highest efficiencies are reached by non-vacuum fabrication. The world record of 12.6%
was obtained by IBM for CZTSSe absorbers from hydrazine-based solution, a process
known to be highly hazardous. Depositing CZTSSe by direct solution coating offers a
facile, low-cost and safe preparation route. Non-hydrazine solution processed CZTS-
Se layers however, suffer often from poor crystallinity, high surface roughness and the
presence of microscopic voids, which limit the solar cell device performance. Various
deposition methods e.g. spin-coating, electrochemical deposition and spray pyrolysis
method of kesterite films are available.
This thesis investigates solution processed CZTSSe absorbers for thin film solar cells.
TheCZTSSeabsorberswereprepareddepositingCZTSprecursorsbysequentialspray
deposition or spin-coating and converted to crystalline CZTSSe absorber layer by re-
active annealing in Se atmosphere. Spray coating offers a facile scale-up deposition
ix