Eray S. Aydil

Chemical Engineering and Materials Science Department

Acknowledgements: National Science FoundationMinnesota Initiative for Renewable Energy and the Environment (IREE)

Challenges in Challenges in SolarSolar--toto--Electric Energy Conversion:Electric Energy Conversion:

an Introductionan Introduction

United States Department of Energy Report on the Basic Energy Sciences Workshop on Solar Energy Utilization by N. S. Lewis et al. (2005).

www.er.doe.gov/bes/reports/files/SEU_rpt.pdf

Ener

gy C

onsu

mpt

ion

(TW

)

0

1

2

3

4

5

6

7

5.3

4.2

3.5

0.9 0.9

0.07 0.03 0.01 0.004

oil

coal

gas

hydr

oele

ctric

nucl

ear

wind

etha

nol

geot

herm

alph

otov

olta

ic

Find ways to provide clean energy to ~ 10 billion people.

The Energy ChallengeThe Energy Challenge

Solar powerSolar power

World demand ~ 15 TWSun ~ 120,000 TW

Covering 0.125 % of earth’s surface with 10% efficient solar cells would produce enough energy to supply the annual global demand.

Global solar PV productionGlobal solar PV production

~30-50% growth

At 35% growth rate we will reach 1 TW in ~ 20 Years

StateState--ofof--thethe--art in solar cellsart in solar cells

United States Department of Energy Report on the Basic Energy Sciences Workshop on Solar Energy Utilization by N. S. Lewis et al. (2005) and from J. Crystal Growth 275, 292 (2005) by T. Surek.

Module efficiency ~ 0.5 -

0.8 × lab efficiencies

c-Si(89.6%)

CdTe(4.7%)

thin film Si(5.2%) CIGS

(0.5%)

2007 market share for various technologies2007 market share for various technologies

0

5

10

15

20

25

30

Coal Gas Oil Wind Nuclear Solar

Cos

t (¢

/kW

-hr)

1-4 ¢ 2-5 ¢ 6-8 ¢ 5-7 ¢ 6-7 ¢

20-40 ¢

~ ~ ××

5 more expensive than other sources5 more expensive than other sources

Residential

~ 40 ¢Commercial ~ 30 ¢Industrial

~ 22 ¢

M. Green Third Generation Photovoltaics” Advanced Solar Energy Conversion, Springer Verlag, Berlin (2004).

0 200 400 6000

20

40

60

80

100

Effic

ienc

y (%

)

Cost $/m2

I$ 4/W

$ 1/W

$ 0.5/W$ 0.2/W

II

Shockley-

QuessierLimit

III

Three generations of solar cellsThree generations of solar cellsI.

Crystalline Si solar cells ($ 8/W ~ 40 ¢/kWh)II.

Thin film solar cellsIII.

Advanced future structures

Installed PV System Cost = Module Cost + Balance of System (BOS)

Future projections for existing technologiesFuture projections for existing technologies

United States Department of Energy Report on the Basic Energy Sciences Workshop on Solar Energy Utilization by N. S. Lewis et al. (2005) and from J. Crystal Growth 275, 292 (2005) by T. Surek.

What do you have to do to convert photons to current?What do you have to do to convert photons to current?

Separate photogenerated

+ve

and –ve

chargesMinimize recombinationMaterial and interfacial properties control rates

A BC

e-

h+or

A Be-

h+

DoE/NREL, www.pv.unsw.edu.au

pp--nn

junction solar celljunction solar cell

Challenge is in cost reduction

New ways of making c-Si, thinner, cheaper, “solar grade”

www.semiconductor-sanyo.com

Thin film amorphous silicon solar cellThin film amorphous silicon solar cell

CuInCuIn11--xx

GaGaxx

SeSe22

(CIGS) solar cells(CIGS) solar cells

Record 19.9% efficiency achieved through empirically derived deposition

Interfaces not well understood

Relation between microstructure, composition and performance not well understood

Large area production difficult

In is scarce

Organic solar cellsOrganic solar cells

max

sc oc

PFFVI

=×

V

I

Voc

Isc

Pmax

Power

V

Pmax

max sc oc

S S

VP FF II I

η ××= =

Fill Factor

Overall Efficiency

AM1.5 2ssolar

0

I I ( ) d 1000 W / mA

∞

= λ λ ≈∫

Solar cell figures of meritSolar cell figures of merit

M. Green Third Generation Photovoltaics” Advanced Solar Energy Conversion, Springer Verlag, Berlin (2004).

0 200 400 6000

20

40

60

80

100

Ef

ficie

ncy

(%)

Cost $/m2

I$ 4/W

$ 1/W

$ 0.5/W$ 0.2/W

II

Shockley-

QuessierLimit

III

ShockleyShockley--QuessierQuessier

LimitLimit

ShockleyShockley--QuessierQuessier

LimitLimit

IncreasingEnergy

conduction band

valence band

e

h

bandgap,Eg

light

e

h

e

(1)(2)

(3)

energy lost to heat

Shockley-Queisser

limit ~ 33%

Surpassing ShockleySurpassing Shockley--QuessierQuessier

limit with limit with multijunctionmultijunction

solar cellssolar cells

www1.eere.energy.gov

Novel methods for concentratingNovel methods for concentrating

Baldo

et al. Nature 403, 750 (2000)

Design and synthesis of dyes or inorganic particles that can absorb diffuse light and reemit anisotropicaly

and efficiently

http://web.mit.edu/newsoffice/2008/solarcells-0710.html

What do you have to do to convert photons to current?What do you have to do to convert photons to current?

Separate photogenerated

+ve

and –ve

chargesMinimize recombinationMaterial and interfacial properties control rates

A BC

e-

h+or

A Be-

h+

NanostructuredNanostructured

materials are emerging as materials are emerging as potential solar cell architecturespotential solar cell architectures

Large surface and interfacial areas found in

nanostructured

materials present significant

advantages both for light absorption and for charge separation, the two critical steps in solar-to-electric energy conversion.

A

B

C

ChallengeChallenge

Find A, B and C with appropriate electronic and optical propertiesProvide means to separate charge at the A-B-C interfaceMaximize optical absorptionAssemble into high interfacial area nanostructured film Minimize premature charge recombinationCost → 0

A

B

C

HeterojunctionsHeterojunctions

between between nanostructurednanostructured

materialsmaterials

A

B

C

B

A

Some architectures that have emerged so far

Nanoparticle based dye sensitized solar cells (Gratzel, 1991)

Bulk heterojunction solar cells (Heeger, 1995 and Alivisatos, 2002)

Nanowire based dye sensitized solar cells (Baxter, 2005; Law 2005)

Nanoparticle Quantum dot sensitized solar cells (Vogel, 1990; Nozik

2003)

Nanowire quantum dot sensitized solar cells (Leschkies, 2007)

Quantum dot solar cells (Nozik, 2007, 2008)

Dye Sensitized Solar CellsDye Sensitized Solar Cells

O’Regan

& Grätzel, Nature 353, 737 (1991).Grätzel, Nature 414, 338 (2001).

Nanocrystalline, mesoporous TiO2 photoelectrode on TCO.

TiO2 is photosensitized with a monolayer of dye.

Efficient light harvesting with large dyed surface area: ~ 1000 × flat film

Heeger. Science 270, 1995.

Fulerenes blended (80 wt%) with conjugated polymer host Polymer absorbs light donates electronFullerenes are electron acceptorsBicontinuous, interpenetrating D-A heterojunction

MEH-PPV: poly[(2-methoxy-5-(2-ethyl-hexyloxy)-1,4-phenylene vinylene]

Bulk heterojunction solar cellsBulk heterojunction solar cells

•

CdSe

nanorods replace fullerenes–

semiconductor also absorbs light –

increased aspect ratio gives better transportP3HT: poly-(3-hexylthiphene) ; PEDOT: Polyethylenedioxythiophene

Tran

spar

ent e

lect

rode

met

al

Hybrid Hybrid NanorodNanorod--Polymer Solar CellPolymer Solar CellAlivisatos. Science 295, (2002).

ChallengesChallenges

•

Can we design appropriate donors and acceptors from first principles?–

molecular structure to maximize charge transport–

molecular features to phase separate at right length scales–

appropriate energy level alignments for exciton dissociation

•

Understand and determine energy level alignments at the D-A interface

•

What is the ideal interface structure that minimizes charge recombination?

Multiple exciton generation in quantum dotsMultiple exciton generation in quantum dots

conduction band

valence band

e

hhν

= 2Eg

e e

h h

ħωEgQD

Electron states

Hole states

Colloidal QDs can generate multiple electron-hole pairs per absorbed photon.

Nozik (2003) - multiple exciton generation (MEG) in quantum dots may occur with high probability

Klimov (2005) - first demonstration of MEG in PbSe quantum dots

Challenges in making Challenges in making nanostructurednanostructured solar cellssolar cells

Challenges

create high surface area nanostructured materials that enable efficient light absorption, charge separation and charge transport

establish the fundamental scientific principles that will enable novel solar cell architectures based on nanostructured materials

Synthesis Solar cellassembly

Understanding the materials & the device physics

Design