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3rd Generation Solar Cells at NTNU

Turid Worren ReenaasDepartment of Physics

Norwegian University of Science and technology Turid.Reenaas@ntnu.no

100nm

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Outline

The research groupWho are we and what can we do?

Intermediate-band solar cellsWorking principleTheoretical limitPractical realization

Intermediate-band solar cell research at NTNUMaterial system and samplesMaterial characterization (preliminary results from AFM, PL, TEM)Solar cell structure Preliminary cell characterisation

Summary

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Solar cell physics group at NTNU

Department of physics

• 3 master students

• 3 PhD students Sedsel Fretheim Thomassen, Rune Strandberg and NN1 (2008)

• 1/3 + 1 Post docs Øyvind Borck, NN2 (from 2008)

• 1 + 1/2 Scientific staff Turid Worren Reenaas, Ola Hunderi, Ingve Simonsen, Randi Holmestad, Jon-Andreas Støvneng

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Solar cell physics group at NTNU

Department of Electronics and Telecommunications

• 1 master student

• 1 Post doc NN3 (starting 2008-05-01 )

• 1/4 Scientific staff Bjørn-Ove Fimland, Helge Weman

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Other groups

Department of Materials Science and Engineering

• 1 master student

• 1 postdocEdita Garskaite

• 1/4 Scientific staff Tor Grande, Mari-Ann Einarsrud

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Projects and funding

1 PhD student and 1+1/3 Post doc financed by NTNU (2005 –2010)

1 PhD student financed by The Research Council of Norway through the PhD-pool of The Centre for Renewable Energy (SFFE) (2005 – 2009)

2 Post docs (and SINTEF staff) financed by The Research Council of Norway: “3rd generation PV” (2008 – 2011)

1 PhD student financed by The Nordic Energy Research “Nordic Centre of Excellence in Photovoltaics”, (2008 – 2011) Project leader Arve Holt, IFE

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Focus areas

Department of Materials Science and EngineeringTransparent conducting oxidesMaterials for up-conversion of solar radiation

Department of physics and Department of electronics and telecommunication (IET)Materials for intermediate band solar cellsNanomaterials for 3rd generation solar cells

Material deposition and characterization Modelling (material properties and cell performance)Solar cell fabrication (NTNU Nanolab and clean room at IET)Solar cell testing

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Material growth/deposition techniques - 1

Control down to the atomic scale:

Molecular beam epitaxy (MBE)At NTNU since mid 1980’s, but limited to certain elements (Ga, In, Al, Sb, As, Te, Be)

Pulsed laser deposition (PLD)At NTNU Department of physics from 2008, essentially no element limitation (but we start with Si, Ge)

Metal-organic Chemical Vapour Deposition (MOCVD)From 2008 (SINTEF)

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Material growth/deposition techniques - 2

Less control, more suitable for large scale production?

Chemical solution deposition and sol-gelSince late 1990s

Close spaced vapour transport From 2008

Sputter deposition (SINTEF)From 2006

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Molecular beam epitaxy (MBE)

Deposition takes place in a ultra high vacuum (UHV) chamber, background pressure typ. 10-10 mbar

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Solar cell processing

Class 100 clean room of 215 m2 at Department of Electronics and Telecommunication

From 2009 (?), we will also have access to state-of-the-art processing and characterization facilities at the NTNU-NanoLab

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Outline

The research groupWho are we and what can we do?

Intermediate-band solar cellsWorking principleTheoretical limitPractical realization

Intermediate-band solar cell research at NTNUMaterial system and samplesMaterial characterization (preliminary results from AFM, PL, TEM)Solar cell structure Preliminary cell characterisation

Summary

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(Quantum Dot) Intermediate Band Solar Cells

Principal Investigator: T. Worren Reenaas

Partners NTNU

PhD students: S. Fretheim Thomassen and R. StrandbergProfessors: B.O. Fimland (MBE), O. Hunderi (semiconductor Physics), R. Holmestad and J. Walmsley (TEM), H. Weman(PL/PLE)

Chalmers University of Technology (for sample preparation)M. Sadeghi and S. Wang

Linköping University (for photoluminescence measurements)A. Larsson and P.-O. Holtz

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Energy diagram of a conventional solar cell*

Conduction band

Valence band

A smaller bandgap would result in a larger photo-generated current, but the voltage delivered by the solar cell is always smaller than the bandgap (divided by the electron charge).

*Omitting the pn-junction and contacts

Ele

ctro

n E

nerg

y

Band gap

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Intermediate band solar cell

Conduction band

Valence band

Intermediate band

Increase the photo-generated current without reducing the bandgap and thus the voltage.

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Ideal intermediate band solar cell

Assumptions: 1) Only radiative recombination2) One electron-hole pair per photon3) Constant quasi-Fermi levels4) Absorption of all photons Eg>E5) No high energy photons in low energy processes6) Maximum concentration of solar radiation

Theoretical efficiency A.S. Brown, M. A. Green and R. P. Corkish, Physica E 14 (2002) 121

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Theoretical efficiency

E12

Conduction band

Intermediate band

Valence band

ΔΔ11

E23E13

Conduction band

2nd intermediate band

1st intermediate band

Valence band

ΔΔ11

ΔΔ22

E34

E23

E12

E14

E12 = 0.71 eV

E23 = 1.24 eV

E13 = 1.97 eV

Δ1 = 0.02 eV

ηmax = 63.2 %

E12 = 0.53 eV

E23 = 0.89 eV

E34 = 1.02 eV

E14 = 2.56 eV

Δ1 = 0.07 eV

Δ2 = 0.05 eV

ηmax = 71.7 %

For a conventional single bandgap cell: ηmax= 40.7%

One intermediate band

Two intermediate bands

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Answer:

Introduce quantum dots

How can we introduce these intermediate energy levels in the bandgap?

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Quantum dots

A quantum dot is a nanometersized particle of a low band gap material surrounded by a material with larger band gap: “Artificial atom” with energy levels depending on dot size and on the bandgap difference.

If many quantum dots are placed closed to each other in a lattice one or more intermediate bands can be formed and a new semiconductor with tailored properties has been made.

~nm

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How can one fabricate quantum dots?

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Self-organized growth of quantum dots

Difference in lattice constants (ad > as) compresses the deposited layer in-plane. The deposited material relaxes (self-organizes) to form pyramidal shaped islands when the deposited layer is thicker than a critical thickness.

Quantum dots Wetting layer

Substrate

z

Lattice constant as

Deposit a few atomic layers of quantum dot material, with lattice constant ad > as.

Finally grow a layer of barrier (large bandgap) material

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Quantum dot intermediate band solar cell

p-type n-type

A single dot absorbs little solar radiation:

A high layer density of quantum dots and many layers stacked on top of each other are needed.

The quantum dots are simply placed in the pn-junction of the conventional solar cell, to form a pin-junction.

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Outline

The research groupWho are we and what can we do?

Intermediate-band solar cellsWorking principleTheoretical limitPractical realization

Intermediate-band solar cell research at NTNUMaterial system and samplesMaterial characterization (preliminary results from AFM, PL, TEM)Solar cell structure Preliminary cell characterisation

Summary

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Lattice constant (Å)

Ban

d ga

p (e

V)

InAs/Al0.35Ga0.65As

E13=1.97eV

E12 = 1.24eV

Bandgap quantum dot < bandgap barrier/substrateRequirements for self organized growth: ad > as

Quantization energy due to quantum confinement

Material system E12

Valence band

Intermediate band

Conduction band

ΔΔ11

E23 E13

Ideal gaps:

E13

E12

Quantization energy

Bandgap quantum dot semiconductor

Eg=1.86eVEg=0.36eV

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First attempts to grow (several) layers of QDs

All samples grown on GaAs substrates, since Al(Ga)As substratesare not available. GaAs and Al(Ga)As have very similar lattice constant, so the self organized growth process should be similar.

All samples fabricated at Chalmers University of Technology, Gothenburg.

Growth of InAs quantum dots on AlAs surfaces (on GaAs)Growth of InAs quantum dots on AlGaAs surfaces (on GaAs)

Growth of AlGaAs/InAs/GaAs QD intermediate band solar cells

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Atomic Force Microscopy (AFM)

1 QD layerA V B V

GaAs/AlAs/InAs/AlAs/GaAs GaAs/AlAs/GaAs/InAs/GaAs/AlAs/GaAs

Dot size typ: Diameter 100-200nm (very large)Height 2-7nm

Dot density: 1010 cm-2 (too low)

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IncreasingexcitationIntensity, Iex

1.21 eV1.12 eVP1

P2

P1

P2

P1 P2

Stronger PL than A, and E(P1B)<E(P1A).

1 QD layerA V B V

GaAs/AlAs/InAs/AlAs/GaAs GaAs/AlAs/GaAs/InAs/GaAs/AlAs/GaAs

The phonon bottleneck results in population of higher levels.

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TEM images of MBE grown QDs

InAs quantum dots in GaAs/AlAs (first set of samples)

Wetting layers

Quantum dots (form on top of each other)

The quantum dots inside the sample are smaller than the dots on the surface.AFM images should be recorded in situ, or immediately after growth.

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Elemental mapping

As

In

Ga

Al

STEM image EDS mapping

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High resolution images

HRTEM makes it possible to find structural defects

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Growth of 3 and 5 QD layers with/without barrier annealing

GaAs substrate

GaAs

Al0.35Ga0.65As

InAs

Without barrier annealing

With barrier annealing

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In-situ RHEED

In-situ Reflection High-Energy Electron Diffraction (RHEED) has been used to study the growth.

RHEED shows that the crystalline quality of the AlGaAs barriersis best for the annealed samples.

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Photoluminescence3 layers with and without annealing

PL intensity decreaseswith annealing, NOT as desired

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TEM images from the 2nd sample set

InAs quantum dots in AlGaAs

More defects and irregular wetting layer for the samples with AlGaAsbarriers.

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(Heterostructure) Quantum dot IB solar cells

GaAs n+ substrate

200 nm GaAs n+ (2 1018cm-3) buffer

100nm AlGaAs n+ (2 1018cm-3) barrier

300nm AlGaAs n (2 1017cm-3) barrier

200nm AlGaAs p (2 1018cm-3) barrier

100nm AlGaAs p+ (2 1019cm-3) barrierGaAs p+ (1019cm-3) capping

GaAs substrate

GaAs

Al0.35Ga0.65As

InAs

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Band diagram (short circuit)

GaA

s

GaA

s

AlG

aAs

AlG

aAs

QD

layers

AlG

aAs

AlG

aAs

GaA

sn i p

1.21eV1.86eV

0.65eVFermi level EF

1μm

p+n+

FrontBackside

Ideal values: 1.97eV, 1.24eV and 0.71eV

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Solar cell efficiency - Preliminary results

Very poor, unfortunatelyOnly 1-2% , even for the reference cellVery low short circuit current, open-circuit voltage also too low

Possible explanationsPartly due to poor material quality (high recombination, high series resistance), Poor electrical contacts (partially shunted cell?) and/or Poor testing equipment (test cells from IFE yielded only half ofexpected efficiency)?? Work in progress

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Summary

Materials research on III-V based quantum dot based intermediate solar cells has been initiated at NTNU (in collaboration with Chalmers University of Technology and Linköping University)

Samples with up to five layers of quantum dots have been fabricated by MBE and materials characterization has been initiated

A first set of solar cells with and without quantum dots have been fabricated and processed, but the preliminary solar cell testing gave poor result

Theoretical simulations of intermediate band solar cells have also been initiated