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Advanced characterization of solar
energy materials and novel solar
cell concepts
Klaus Magnus JohansenHead of the Micro - and Nanofabrication lab at University of Oslo
Norwegian Micro and Nanofabrication Facility - NorFab
Outline
• Introduction to the open access national clean
room infrastructure NorFab
– Processing and characterization possibilities at UiO
• An overview and examples from the solar cell
activity at the UiO Micro and Nanotechnology
Laboratory
Norwegian Micro- and Nanofabrication Facility
NorFab in numbers:
•Project period: – NorFab I: 2010-2014
(7 M€ RCN support)
– NorFab II: 2015-2019
(14 M€ RCN-support)
•Ca. 90M€ total investment cost (60M€ buildings /30M€ equipment)
•10 M€ running cost/year
•22 engineers
•535 users/ 46476 user hours
•257 instruments
•2300m2 cleanroom area
•80 industrial users (41 companies)
www.norfab.no
Norway
Finland
Sweden
Denmark
NTNU NanoLab
Norwegian
University of
Science and
Technology
SINTEF
MiNaLab
MiNaLab
University
of Oslo
MST-lab
UC Buskerud
Vestfold
Trondheim
Oslo
Horten
Norway
Finland
Sweden
Nordic NanoLab Network (NNN)
Slide 4
MiNaLab
(SINTEF/UoO)
MST-lab
HBV
Electrumlab
KTH/Acreo
Ångström MSL
Uppsala
MC2 NFL
Chalmers
Lund Nano Lab,
Lund University
Denmark
NTNU
NanoLab
Cooperation of the national cleanroom infrastructures in the Nordic countries
Key numbers (2014):
• Open access to10 cleanrooms in 4 national
infrastructures serve over 2.000 users
• >1500 tools for micro- and nanofabrication
in over 10.000m2 cleanroom area
• Almost 300.000 user hours
• on the management level
• on the expert level
• on the user levelUniversity of Iceland, Reykjavik
Application submitted for ESFRI –European Strategy Forum on Research Infrastructures
2nd floor
UiO MiNaLab –
Novel semiconductors
Location: Oslo node
Area: 440 m2
Type: R&D, education
Staff: ~50 researchers, 3 engineers and
1 Administrator
The UiO MiNaLab facility is operated by
the LENS center of excellence at UiO.
Main competence:
•Semiconductor physics
•Ion beam modification and analysis
•Thin film manufacturing
•Electrical/chemical/optical defect
characterization
•Electronic devices
SiC
ZnO
In2O3
Si
GaN-ZnO
ZnSbCu2O
TiO2
NiO
Ga2O3
Material systems
• Wide bandgap materials:• Metal oxides• Metal oxynitrides
• Materials for LEDs, Solar cells, thermoelectrics andpower electronics
• Defect characterization• Optical • Electrical
UiO MiNaLab
Deposition- Magnetron sputtering- E-beam deposition- Thermal deposition- Metal organic vapor phase epitaxy(MOVPE)
- Atomic layer deposition (ALD)- Plasma-enhanced chemical vapor deposition (PECVD)
- Epitaxial sputter deposition
Available equipment
Processing- Ion implantation/
modification- Optical lithography- Chemical processing- Thermal processing; RTP
and furnaces up to1800°C
- Reactive ion etching (RIE)- Cross sectional polisher
- Electrical char.- Temperature dep. Hall effect
- Temperature dep.scanning probe microscopy(AFM/SPM)
- Spectrophotometer- Solar simulator
- Ellipsometry- Fourier transform infrared absorption (FTIR)
- High resolution x-ray diffraction (HRXRD)
- Chemical characterization- Rutherford backscattering spectroscopy (RBS/C)
- Cathodoluminesence
Characterization
Solar cell projects @ UiO MiNaLab
• The Norwegian Research Centre for Solar Cell Technology
(Solar United)
• Research Center for Sustainable Solar Cell Technology
(SUSOLTECH)
• Development of a Hetero-Junction Oxide-Based Solar Cell
Device (HeteroSolar)
• Longer lifetime and higher efficiency of CZTS thin-film solar
cells (PV-Life)
• Novel Semiconducting Alloys in Energy Technology (SALIENT)
• …
How does a solar cell work
• Sunlight shines on a pn-diode (i.e., with an internal electric field)
• Electron-hole pairs are generated and separated by the field
• Can be used to set up a current through an external circuit
• Key limiting factors
– The fraction of sunlight captured• Reduce reflection
• Reduce transmission
– Lifetime of the electron/hole• Avoid defect recombination
– Heat loss
– …
Solar Cell Research
@ MiNaLab UiO
Absorber
(Si)
Emitter (Si)
AR-coating
Commercial
solar cell
Solar Cell Research
@ MiNaLab UiO
AR-coating
• Silicon properties
• Defects
Commercial
solar cell
Absorber
(Si)
Emitter (Si)
Solar Cell Research
@ MiNaLab UiO
AR-coating
• Silicon properties
• Defects
• Shallow junction formation
• Diffusion
Commercial
solar cell
Absorber
(Si)
Emitter (Si)
Flash lamp annealing,
shallow B emittter formation
Riise et al. Appl. Phys. Lett.
107, 022105 (2015)
Solar Cell Research
@ MiNaLab UiO
AR-coating
• Silicon properties
• Defects
• Shallow junction formation
• Diffusion
• Replacing AR with TCO
• TCO as active emitterTCO
TCO
Commercial
solar cell
Next generation
solar cells
Absorber
(Si)
Emitter (Si)
Absorber
(Si)
Emitter (Si)
Absorber
(Si)
TCO = Transparent conductive oxide
Solar Cell Research
@ MiNaLab UiO
AR-coating
Tandem
cell
• Silicon properties
• Defects
• Shallow junction formation
• Diffusion
• Replacing AR with TCO
• TCO as active emitter
• Tandem cell concepts
TCO
TCO
Commercial
solar cell
Next generation
solar cells
Cu2O on glass ZnO/Cu2O on glass
ZnMgO:Al/Cu2O/Au on quartz
Absorber
(Si)
Emitter (Si)
Absorber
(Si)
Emitter (Si)
Absorber
(Si)
Solar Cell Research
@ MiNaLab UiO
AR-coating
Tandem
cell
• Silicon properties
• Defects
• Shallow junction formation
• Diffusion
• Replacing AR with TCO
• TCO as active emitter
• Tandem cell concepts
• Light conversion/
• Multi-carrier generation
• Novel concepts
TCO
TCO
Nano
Commercial
solar cell
Next generation
solar cells
Absorber
(Si)
Emitter (Si)
Absorber
(Si)
Emitter (Si)
Absorber
(Si)
«Why bother with solar cells beyond
Silicon?»
Si has been extremely successful:
- Cheap
- Reliable
- Well studied
- Optimized efficiencies
Image from: http://commons.wikimedia.org/ , photography by OhWeh
There are two main drawbacks with Si
• Small bandgap– Light with energy above ~1 eV
is converted to heat
– 1.34 eV would be optimal for a single pn-junction according to the Shockley–Queisser limit
– The limit for Si based cells is approximately 29 %
– Efficiency for biofuel < 1%
• Low absorption– Requires relatively thick solar cells to optimize the energy harvest.
Typically 100 – 500 µm
– No flexibility
Challenges for thin film solar cell materials
• Cost• CIGS (CuInGaS)
• InGaP / GaAs / Ge / InGaAs
• Toxicity• CdTe
• Perovskite (Often contains Pb)
• Reliability• Perovskite
• Organic
• Dye-sensitized
• Band gap
• Efficiency
Image from First Solar
CZTS (Copper Zinc Tin Sulfide)
Cu(In,Ga)Se2 (CIGS)
Indium is a rather
expensive metal
• Good absorption• 1 - 2 μm thick
Cu2ZnSnS4 (CZTS)In- free gives lower cost
• More research before industrial production
• Liquid phase non-vacuum deposition methods
are very successful → cheap production
Substrate
Mo
CdS
i-ZnO
ZnO:Al
Cu2ZnSnS4
Wide band gap
• CIGS current record device is 22.6 % (ZSW)
• CZTS current record is 13.7% with band gap grading (KIST)
Sigbjørn Grini
• Potential for tandem solar cell with a wide
band gap solar cell on top
CdS
Eg ~ 2.4 eV
Cu2ZnSn(S1-x,Sex)4
Eg ~ 1.0 - 1.5 eV
0x
Cu2ZnSnS4
CdS
Cu2ZnSn(S1-x,Sex)4
CdS x
0
Band gap grading
CdS
Band gap ~ 2.4 eV
Cu2ZnSnS4
Band gap ~ 1.5 eV
Band misalignment
Exchange sulfur with selenium
Se/S gradient?
Se atmosphere
450°C
10-30 minutes
32S
80Se
80Se
Cu2ZnSnS4
Cu2ZnSnS4
Cu2ZnSnSe4
N. Ross, J. Larsen, S. Grini, L. Vines, C. Platzer-Björkman, Practical limitations to selenium annealing of compound co-sputtered Cu2ZnSnS4 as a route to
achieving sulfur-selenium graded solar cell absorbers, Thin Solid Films, Volume 623, 1 February 2017, Pages 110-115, ISSN 0040-6090,
http://dx.doi.org/10.1016/j.tsf.2016.12.044.
Adapting the solar spectrum
Conduction band
Valence bande-
e-e-
e-e-
e-e-
e-
~1 e
V
The Si-cell
Photons ca 3 eV
Blue/UV-light
e-
Heat
We get a current, however, a
lot of the energy is lost as heat
Photonsplitting is one approach
e-e-e-e-e-e-e-e-
Conduction band
Valence bande-
e-e-
e-e-
e-e-
e-
~3 e
V
~1 e
V
Photon splitting material The solar cell material
Photons ca 3 eV
e-e-
Per-Anders Hansen,
TiO2 and europium (Eu)
The tandem concept is another approach
Cu2O – ZnO heterojunction
• Cu2O
– Cheap and reliable
– Direct band gap of
2.1 eV
– Well studied
• ZnO
– Cheap and reliable
– Direct band gap of
3.4 eV
– Well studied
Difficult to make n-type Cu2O
Heterojunction necessary
p - Cu2O n - ZnO
Lattice mismatch/Defects at interface
Kristin Bergum
Literature advances
• Until 2010, the highest efficiency was ~2%
Minami et al., Appl Phys Express, 9, 052301, 2016
8.1%
Cu2O:Na / Zn0.38Ge0.62O / AZO/MgF2
Oxidized Cu-metal sheet
Reactive magnetron sputtering
Reactive sputter deposition of Cu2O
Biccari, Francesco, «Defects and doping in Cu2O», PhD thesis
600 700 800 900
• Best solar cells (8%) are
from thermally oxidized
Cu sheets
• High crystallinity (mm-
sized grains)
– Us: ~100 - 150 nm
• Mobility of ~100 cm2/Vs
– Us: <20 cm2/Vs
• Carrier concentration of
~1013 cm-3
– Us: ~1015 cm-3
Annealing of Cu2O films
Annealing of Cu2O films –
Mobility and optical properties
400 500 600 700 800 9000
10
20
30
40
50
60
as-dep
Mob
ility
, (c
m2/V
s)
Annealing temperature, (°C)
Combination of a change in
both crystallinity and defects
Temperature dependent Hall reveals (at
least) two electrically active defects
• vCu’ and
• A still unknown defect
GaN – ZnO alloys
• Both have 3.4 eV direct bandgap
• The alloys are predicted to have a smaller
bandgap
• Of interest for, e.g.
– Water splitting
– Solar cells
Summary: Solar activity at UiO
• A modern open access cleanroom lab for
materials processing and characterization
• Activity related to several aspects of the the
Si-cell and beyond
– Transparent conductive oxides, ITO, AZO, GZO
– Oxide heterojunction cells
– CZTS-thin film cells
– Shallow emitter formation
– Defect characterization of solar grade Si
LENS – Light and electricity from novel semiconductors
The energy bandgap
• Sunlight can provide the energy to overcome the gap
• Different semiconductor materials can have different
gaps