High Efficiency Integrated Solar Energy Converter Research findings on photon enhanced thermionic emission SES2050 seminar 21.10. – 22.10.2015, Oslo, Norway Aapo Varpula, Jyrki Tervo VTT Technical Research Centre of Finland
High Efficiency Integrated Solar Energy Converter
Research findings on photon enhanced thermionic emission
SES2050 seminar 21.10. – 22.10.2015, Oslo, Norway
Aapo Varpula, Jyrki Tervo
VTT Technical Research Centre of Finland
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2Oct 21st, 2015
Project participants
VTT: Jouni Ahopelto, Aapo Varpula, Kirsi Tappura, Mika
Prunnila, Jyrki Tervo
DTU: Ole Hansen, Kasper Reck
KTH: Sebastian Lourdudos, Yanting Sun
VU: Tadas Malinauskas
Fortum Ltd: Eero Vartiainen
Picosun Ltd: Minna Toivola
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Outline
Introduction
Modelling work and results
Experimental work
Design of PETE demonstrator
SWOT analysis of PETE
Publications
Conclusions
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Inroduction
In 2010 Schwede et al.
[Schwede2010] proposed the
photon-enhanced thermionic
emission (PETE) device which is a
photovoltaic device operating at
high temperatures.
This technology can be combined
with the existing thermal solar
energy systems, thereby allowing
the solar-energy-to-electricity
conversion efficiencies above 50%
to be potentially reached.
Its main characteristics include high
temperature operation, use of high
illumination intensity (i.e., the
intensity is 100 W/cm2 at 1000
suns) and promise of efficiencies
much higher than conventional solar
cells.
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Goals of the work
To develop theoretical PETE solar cell model
To develop specific PETE solar cell designs
To evaluate materials and structures for PETE solar cells.
To evaluate the efficiency and commercial potential of PETE solar cells.
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Modelling work
The first detailed model of PETE devices developed Takes the relevant effects in semiconductors into account
Both numerical model for all calculations and analytical formulas for special cases and approximate calculations
Model published in Journal of Applied Physics in 2012 [1]
Modelled cathode material: Silicon
Analysis of space charge effects in emission current measurements
Equivalent circuit model of PETE devices developed
Extension of the model New materials
Czochralski and magnetic Czochralski silicon [2]
GaAs and InP [3,4]
Electron density dependence of the electron diffusion constant (a minor effect) [4]
[1] A. Varpula, M. Prunnila, Journal of Applied Physics 112, 044506 (2012).
[2] A. Varpula, K. Reck, M. Prunnila, O. Hansen, PETE-2014, Tel Aviv, 23-24 June 2014.
[3] A. Varpula and M. Prunnila, EU PVSEC Proceedings, p. 331, EU PVSEC 2014, Amsterdam,
22-26 Sep. 2014.
[4] A. Varpula, K. Tappura, M. Prunnila, “Si, GaAs, and InP as cathode materials for photon-
enhanced thermionic emission solar cells”, Solar Energy Materials & Solar Cells, revised
manuscript submitted.
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300 400 500 600 700 800 900 10000%
5%
10%
15%
20%
25%
30%
Cathode temperature (K)
Eff
icie
ncy
InP
GaAs
Si
Under 1000 suns
300 400 500 600 700 800 900 10000
5%
10%
14%
Cathode temperature (K)
Eff
icie
ncy
0 cm/s
102 cm/s
103 cm/s
104 cm/s
105 cm/s
Modelling results
Surface recombination and (effective) electron affinity on cathode surfaces must be controlled in order to reach high efficiencies [1–4]
Silicon is feasible cathode material for PETE devices [1,2]
Bulk recombination seems to have lower effect on the efficiency of PETE devices Standard Czochralskisilicon should be adequate [2]
GaAs and especially InP seem to be very promising for PETE because of their strong photon absorption characteristics High efficiency [3,4]
[1] A. Varpula, M. Prunnila, Journal of Applied Physics 112, 044506 (2012).
[2] A. Varpula, K. Reck, M. Prunnila, O. Hansen, PETE-2014, Tel Aviv, 23-24 June 2014.
[3] A. Varpula and M. Prunnila, EU PVSEC Proceedings, p. 331, EU PVSEC 2014, Amsterdam,
22-26 Sep. 2014.
[4] A. Varpula, K. Tappura, M. Prunnila, “Si, GaAs, and InP as cathode materials for photon-
enhanced thermionic emission solar cells”, Solar Energy Materials & Solar Cells, revised
manuscript submitted.
Comparison of cathode materials [4]
Effect of recombination on emitting surface [2]
Apparent
efficiency
from
thermally
generated
electrons
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Experimental
Materials testing and PETE
measurements were carried out in an
ultra-high vacuum (UHV) chamber at
DTU.
The chamber was fitted with a 4-axis
sample manipulator, loadlock, Ion gun for
sputter cleaning, caesium evaporator, a
copper anode, a 30 W laser source for
sample illumination, an X-ray source and
a hemispherical energy analyser for XPS
and work function measurements.
Other measurements: HR-XRD, AFM,
Raman spectroscopy, STS/STM, LITG
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Studied materials
Distance between pyramid tips 3–20 µm
Distance between 50 nm dots: ~50 nm
- highly doped p-type and n-type
silicon
- surface structured silicon
- caesiated silicon
- cesiated and oxidized silicon
- GaAs
- GaAs with InP nanodots
- C12A7
- Graphene
- cesiated graphene
- InAs/GaAs
- InxGa1-xN
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Demonstrator design
Experimental setup designed for demonstration of operation of PETE devices
Features Illumination through transparent vacuum
chamber or via an optical feedthrough(no restrictions on the type of external photon source)
Vacuum with standard vacuum pumps
Replaceable emitter and collector sample chips
Additional heater for temperature control of emitter
Cs-sources for work-function and space charge reduction
Mostly commercial components used Present design with KF flanges
Chamber can be replaced by CF-flange version for higher vacuum ( ≤10–9 mbar)
Spacers for accurate control of the gap between emitter and collector
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Operating principle
Collector
Emitter/absorber
Cs-source
Photons
ElectronsCs vapour
Cs-source
Cs vapour
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Exploded view
Glass cross
working as
vacuum
chamber
Water-cooled high-
current copper
electrical feedthrough
for collector
Standard
KF flanges
Electrical feedthroughs for
emitter (1), emitter heater (2),
and Cs-sources (2)
Collector
Standard KF
flanges
Emitter heater
SpacerEmitter
Copper block for electrical
and thermal contact
Sample holder
Sample
holder
Spacer
Alignment pins
Spacer
Optional
port
Overview
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Assembled sample holder
Side view (cross section)Perspective view
Alignment
pinAlignment
pin
Power wire 1/2
for Cs-sourcesPower wire 2/2
for Cs-sources
Emitter
heater
Alignment
pin
Contact wire for emitter
(acts also as mechanical support)
Power wire 2/2 for
emitter heater
Power wire 1/2
for Cs-sources
Power wire 1/2 for
emitter heater
Copper block for
electrical and
thermal contact
Collector
EmitterSpacer
Emitter
heater
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Illumination possibilities
Through port with optical feedthroughThrough transparent chamber wall
Light
source
Light
source
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PETE publications
[5] J.W. Schwede et al., “Photon-enhanced thermionic emission for solar concentrator systems”,
Nature Materials 9, 762 (2010).
2
8
16
2522
2010 2011 2012 2013 2014
Number of articles citing the original PETE paper [5]
Scopus 13th Nov, 2014 Our scientific article
(1st detailed model)
published in August
2012
Our publications:
2 conference presentations
1 conference paper
1 scientific article
Original paper
published in
September
HEISEC
begins
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SWOT analysis of PETE
Strengths
- Potential for high efficiency
- No moving parts
- No emissions
(greenhouse effect and pollution)
- Allows cogeneration of solar electric power
and heat
- Can be incorporated into existing solar
concentrator systems
• Low amount of materials needed
Use of exotic materials possible
• Device cost not issue
Weaknesses
- Based on non-existing materials
(problems with work function, heat
emissivity, charge-carrier recombination)
- Vacuum and high temperature differences
required
• Complex system
• Illumination more difficult
• Tight material requirements
- Maximum efficiency requires
• Light concentration
• Secondary heat engine
- Direct measurement of phenomenon
difficult
- Solution of space charge problems
requires small gap, neutralizing plasma or
other means
Opportunities
- New IPR (materials and structures)
- Markets available
• Device manufacturing
• Material deposition tools
• Power generation
Threats
- Realization difficulties
- No stable low work function materials can
be found
- Competition