High Efficiency Integrated Solar Energy Converter · photon-enhanced thermionic emission (PETE) device which is a photovoltaic device operating at high temperatures. This technology

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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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

Documents

High Efficiency Integrated Solar Energy Converter · photon-enhanced thermionic emission (PETE) device which is a photovoltaic device operating at high temperatures. This technology