1938-1

Workshop on Nanoscience for Solar Energy Conversion

Michael GRAETZEL

27 - 29 October 2008

EPFL, IPI, Departement de ChimieCH-1015 Lausanne

Switzerland

Power from the Sun: the Advent of Mesoscopic Solar Cells

Michael Graetzel, ECOLE POLYTECHNIQUE FEDERALE DE LAUSANNE

Michael.Graetzel@epfl.ch

Power from the SunThe Advent of Mesoscopic Solar Cells

JOINT ICPT-KFAS Workshop on Nanoscience for Solar Energy Conversion

Trieste Italy October 27-29, 2008

2

PhD Students

Chen Peter

Moon Soo-Jin

Teuscher Joël

Wenger Sophie

Zhang Zhipan

Post-docs

Baranoff Etienne

El Roustom Bahaa

Evans Nicholas

Kay Andreas

Kuang Daibin

Lee Hyo Joong

Sivula Kevin

Thorsmolle Verner

Wang Deyu

Wang Mingkui

Staff Scientists

Graetzel Carole

Humphry-Baker Robin

Kalyanasundaram

Kuppuswamy

Liska Paul

Moser Jacques-E. (titled prof.)

Nazeeruddin Md. Khaja

Péchy Péter

Rothenberger Guido

Rotzinger François

Thampi Ravindranathan

Technical & Administrative Staff

Comte Pascal

Duriaux Arendse Francine

Gonthier Ursula

Gourdou Nelly

Academic Visitors

Barroso Monica

Bessho Takeru

Cevey-Ha Ngoc Le

Jang Song-Rim

Le Formal Florian

Yoneda Eiji

Yum Jun Ho

Zakeeruddin Shaik Mohammed

Professor Jacques E. Moser

Acknowledgement Present LPI Members

3

Renewable energy sources need to cover the supply gap

Solar100’000 TW at Earth surface

10,000 TW (technical value)(1.5 hr sunlight globally = 13 TW-yr)

Biomass5-7 TW

all cultivatableland not used

for food

Hydroelectric

Geothermal

Wind14 TW

1.2 TW technically feasible0.6 TW installed capacity

1.9 TW

Tide/Ocean Currents

0.7 TW

energy gap~ 14 TW by 2050~ 33 TW by 2100

from Nate Lewis

THE SOLAR CHALLENGE

!"With a projected global population of 12 billion by2050 coupled with moderate economic growth, the totalglobal energy consumption is estimated to be ~28 TW.Current global use is ~11 TW.

! To cap CO2 at 550 ppm (twice the pre-industrial level),most of this additional energy needs to come fromcarbon-free sources.

! Solar energy is the largest non-carbon-based energysource (100,000 TW).

! However, it has to be converted at reasonably lowcost.

LightFuel

Electricity

Photosynthesis

Fuels Electricity

Photovoltaics

H O

O H

2

22

sc M

e

sc

e

M

CO

Sugar

H O

O

2

2

2

Quantum Energy Conversion Strategies

Semiconductor/LiquidJunctions

7

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.00

10

20

30

40

50

60

70

80

0

10

20

30

40

50

60

70

80

So

lar

Ph

oto

n F

lux (

mA

/cm

2

.eV

)

Energy (eV)

6000K BB integrated current

AM1.5G integrated current

6000K Blackbody Spectrum

100 mW/cm2

!(E) = AM1.5G Solar Spectrum

100 mW/cm2

Inte

gra

ted

ph

oto

n f

lux (

mA

/cm

2

)Solar Spectrum and Available Photocurrent

THE SOLAR RESOURCE

8

Correct device characterisation is essential

!

" =P

max

Pin

=ISC#U

OC# FF

Pin

Characterisation Standard:

• intensity of 1000 W/m2

• spectral power distribution

corresponding to AM1.5 ! 1Sun

• temperature 298 K

Power conversion efficiency

9

The yearly shipments of PV cells are growing exponentially The market size in 2030 for the four mainPV segments is projected to be 200 bio !

Rural Electrification

60 GWp p.a.

On-Grid

150 GWp p.a.

Consumer

20 GWp p.a.

Off-Grid Industrial

70 GWp p.a.

Total 300 GWp" 200 bio !

Modul Turnover in 2030

Courtesy Dr. Winfried Hoffman, CEO, RWE, SCHOTT Solar GmbH

Production Forecasts of Solar Modules Call forContributions from Different Technologies

1,000,000

0.1

1.0

10.0

100.0

0 1 10 100 1,000 10,000 100,000

Cumulative Production (MWp)

PV

Mo

du

le P

ric

e (

20

03

$/W

p)

1976

2003

90%

80%

70%

75 GW

Disruptive PV technology approach required tomeet cost targets

From T. Surek, JCG, 2005

2007 ??

(1 TW)

Revolutionary new technology createdfrom basic science

20??

Evolutionary path

A new paradigm :

Mesoscopic solar cells

based on interpenetratingnetwork (bulk) junctions

Dye sensitized nanocrystalline solar cells lead the new PV generation

15 16

NATURE PHOTONICS WEB RELEASE OCTOBER 2008

17

The dye sensitized solar cell (DSC) is the only photovoltaic cell using amolecules that

generated charge carriers after photo-excitation without the need for excitonic transport

p-n junction photovoltaic cells dye sensited solar cells DSC

Charge separation by

electric field at the p-n

junction

Charge separation by

kinetic competition as in

photosynthesis

EF

p-Si

D/D+

e--R

n-Si

h# h#

D*/D+TiO2

Electrolyte

Dye

cb

Hole conductorElectron conductor

Dye sensitized solar cells separate light absorption from carrier transport

19

single crystal of anatase

A dye sensitized nanocrystaline film generates over 1000 times more

photocurrent than a single crystal electrode

Nanocrystalline anatase film

20

The incident photon to electrical current conversion efficiency

(external quantum efficiency) can reach close to 100 %

! =!!abs*"inj* !coll

Dye sensitized nanocrystals achieve quantitativeconversion of photons into electric current

A key question is how electrons are quantitively collectedfrom the disordered network of nanoparticles.

21

The electrons ad holes move in different phases and are separated by a phase boundary

retarding their recombination

100 nm

TiO2

dye

The collection of photo-generated electrons by the nanoparticle array is quantitative

220 10 20 30 40 500

10

20

30

40

Diameter of particles, nm

Num

ber

Titania (anatase) nanoparticles show well facetted surfaceswith preferred (101) orientation

The average particle size is ca 20 nm

23

N

N

HOOC

Ru

NCS

NCS

N

N

HOOC

COOH

COOH

Ruthenium complexes are widely used as sensitizers due totheir extraordinary performance and excellent stability

Spontaneous uptake of N3 sensitizer by a

nanocrystalline TiO2 film supported on

conducting glass

N3 (N719)

Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.; Mueller, E.; Liska, P.;

Vlachopoulos, N.; Graetzel, M. J. American Chemical Society (1993), 115(14), 6382-90.

25

Anchoring of the carboxylate groups occurs by unidentateand bridging bidentate coordination to surface titanium ions

C

R

O

Unidentate !!!!!!

Ti

O

C

R

O O

MM

Bridging bidentate

26

The RuL2 (NCS)2 sensitizer is anchored to the (101) TiO2 anatasesurface through coordinative binding of two carboxyl groups tosurface titanium ions.

Top viewSide view

27

Photo Induced Heterogeneous ElectronTransfer Cycle

N

N

Ru

C

C

O

- O

- O

O

OXIDE

Tis4+

–

–

– MLCT EXCITATION

forward reaction

Backward transfer

–

–

–

Ru (II/III)

dxy, dxz, dyz

!LIGAND *ORBITAL

h ! (" 1.7 eV)

kf

Ti 4+/3+

spatial contraction of d orbitals upon

oxidation from Ru(II) to Ru(III)

Energy

kb

28

Time dependent DFT Modeling of TiO2 nanoparticles

produces correct band gap in water

Stoichiometric anatase Ti38O76 cluster of nanometricdimensions exposing (101) surfaces

B3LYP/3-21g* : 3.20 eV, B3LYP/DZVP: 3.13 eV

Experimental gap in aqueous solutions: 3.20 – 3.30 eV

F. De Angelis, A. Tilocca, A. Selloni J. Am. Chem. Soc. 2004, 126, 15024

29

N719+ + 1 e -

N719*

N719 + h!

h!

1 e -

F. De Angelis, S. Fantacci, A. Selloni, M. Grätzel J. Am. Chem. Soc. 2007

Photoinducedinterfacial electroninjection occurson a femtosecond timescale

30

MD and DFT calculations provide importantinsight into sensitizer interaction with surface

Two prototypical configurations of N719/TiO2 were examined:

(A The two protons are located on the dye or (B) on the TiO2

A B

31

Electron injection from excited N 719 sensitizer in the conduction band of TiO2 is asssisted by surface protonation

HOMO

LUMO: 2 protons remain on N719LUMO: 2 protons transferred from N719 to TiO2 nanoparticle

AB

De Angelis, . Fantacci, S.; Selloni, A.; Nazeeruddin, M. K.; Graetzel, M JACS (2007), 129(46), 14156-14157

32

33 34

Competition "

Electron diffusion length

Dynamic CompetitionDynamic Competition

electron transport

loss mechanism:interfacial

recombination

!

Ln= "Dn $n

electron injection

dye regeneration

electron transport

interfacialrecombination

time [s]

$n: electron lifetime

Dn: electron diffusion coefficient

35

!DLn=

Typical values for high performance ( % >10% )

cells at Vmpp:

D = 10-4 cm2/s, $ = 1 s, L = 100 µm

The film thickness is less than 30 micrometer

The electron diffusion length exceedsThe electron diffusion length exceedslargely the film thicknesslargely the film thickness

The solar to electric power conversionefficiency of the DSC in full AM 1.5sun light validated by accredited PVcalibration laboratories has presentlyreached over 11 %.

Chiba, Y., Islam, A.; Watanabe, Y; Komiya, R.; Koide, N.; Han, L..Dye-sensitized solar cells with conversion efficiency of 11.1%.Japanese Journal of Applied Physics, Part 2: Letters & Express Letters (2006), 45(24-28),

37

Ongoing research

• Advanced nanostructures

• Light induced charge separation

• new sensitizers

• new redox mediators

• Solid state heterojunctions

• Quantum dot cells

• redox active ionic liquids

• tandem devices38

the work horse of the DSC is the N3 (or N719)ruthenium dye,

COOH anchoring groups

Dr. Md. K. NazeeruddinSenior Scientist andAdjoint scientifiqueLPi/EPFL Lausanne

Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.; Mueller, E.; Liska, P.;

Vlachopoulos, N.; Graetzel, M. J. American Chemical Society (1993), 115(14), 6382-90.

39

Ru

N

N

N

N

N

N

CS

CS

COO-Na

+

COOH

S

S

Ru

N

N

N

N

N

N

CS

CS

COO-

COOH

S

S

S

S

N

C101Z991

Takeru. B

40

41

!

"!

#!

$!

%!

&!

'!

(!

)!

*!

"!!

$&! %!! %&! &!! &&! '!! '&! (!! (&! )!! )&!

+,-./.012345406

789:454;

9"!"<**"=("*>?#

Figure. Comparison of IPCE curve with Z960 electrolyte from C101(blue),Z991(pink) and N719(orange) sensitizer, respectively. 8.4(DSL18)+4.8(CCIC400)

TiO2 film was used as semiconductor.

! "#

$$%& ''(

)(%& '*(

$+%, '-(

./012#345

Takeru. B

42

Jsc = 20.2 mA/cm2

In collaboration with Professor; Wu, Chun-Guey; Department of Chemistry, National Central University, Jung-Li, Taiwan

CYC-B1

43

The C101 sensitizer maintains outstanding stabiity at efficieny levels over 9percent under light soaking at 60oC with a non-volatile electrolyte

45

The New Dye “J2” developed by SIIT

Fig.2 Structure of J2 dye

J2

N719

Adsorption spectra of J2 and N719

Shimane Institute for Industrial Technology

Courtesy Professor Katsumi Yoshino

46

0

20

40

60

80

100

120

140

0 200 400 600 800 1000

Time [h]

EF

F R

ete

nti

on

[%

]

!"#$%!&'&()*)$"(+,*-./&0$#-&(+&.+*1

2)3,/.)4)(-$56.$#/7#60&.$8)00

High temperature stability reached on the module scale

Courtesy A. Yoshino, Shimane Institute of Technology

Continued Heat Stress for 1000 hours at 85 C

479:)$#:/4&()$"(*;-,-)$56.$"(+,*-./&0$9)<:(606=>$%#""91$/*$&($6.=&(/?&;6($@:6*)$',.'6*)$/*$-6$'.6A/+)$-)<:(/<&0$&**/*-&(<)$-6

<6.'6.&;6(*$/($#:/4&()$B.)5)<-,.)$-:&-$<&..>$6,-$'/6())./(=$.)*)&.<:$&(+$+)A)06'4)(-$56.$-:)$<.)&;6($65$$()@$/(+,*-./)*C

&(+$-6$@6.D$56.$-:)$/4'.6A)4)(-$$65$/(+,*-./&0$<64');;A)()**E

Development of Large Size Dye-Sensitized Solar CellModules with High Temperature Durability

Shuji NODA, Kazuhide NAGANO, Eiji INOUE,

Toshio EGI, Takeshi NAKASHIMA, Naoto IMAWAKA,

Masahiro KANAYAMA, Shiro IWATA, Kunihiro TOSHIMA, Keiko NAKADA

and Katsumi YOSHINO

Shimane Institute for Industrial Technology (JAPAN)Shimane Institute for Industrial Technology (JAPAN)

48Source: Tetsuo Nozawa Nikkei Electronics Asia -- July 2008,

Organic dyes are catching up in conversion efficiency

49

Organic push-pull sensitizers are catching up

50

51

D&'&A Dyes gaining red response

In collaboration with Peng Wang CAS Changchun

A: D-204 (n=1)C: D-205 (n=2)

52Red: D-204 , blue: D205

53

Ongoing research

• Advanced nanostructures

• Light induced charge separation

• new sensitizers

• new redox mediators

• Solid state heterojunctions

• Quantum dot cells

• redox active ionic liquids

• tandem devices54

Glass

SnO

2

Light Ho

le C

ondu

cto

r

e-

h+

Solid State Dye Sensitized PV cell

e-

Gold

TiO2 Dye OMeTAD(or other hole

conductor)

Spiro-OMeTAD

Glass transition temperature: 121 °C

Melting Point: 246 °C

Work function: 4.9 eV

(CV in CH2Cl2. 110 mV vs. Fc/Fc+)

Absorption maximum neutral:(max=372 nm ()=40100)

Radical cation:

(max=511 nm ()=37400)

Mobility ~ 1 x10-4 V/cm

55

Competition "

Electron diffusion length

Dynamic CompetitionDynamic Competition

electron transport

loss mechanism:interfacial

recombination

!

Ln= "Dn $n

electron injection

dye regeneration

electron transport

interfacialrecombination

time [s]

$n: electron lifetime

Dn: electron diffusion coefficient56

N

S

S

HOOC

CN

JK72

57

Influence of TiO2 on IPCE

0

10

20

30

40

50

60

400 500 600 700 800

wavelength [nm]

IPC

E [

%]

JK72(A)

JK72(D)

JK73(A)

JK73(D)

JK74(A)

JK74(D)

JK75(A)

JK75(D)

Gain of red response in photocurrent byformation of JK72 dye aggregates

58

59

Ongoing research

• Advanced nanostructures

• Light induced charge separation

• new sensitizers

• new redox mediators

• Solid state heterojunctions

• Quantum dot cells

• redox active ionic liquids

• tandem devices60

100 nm

TiO2

Quantumdot

Quantum dot sensitizers for mesoscopic solar cells

The electrons ad holes move in different phases and are separated by a phase boundary

61

400 450 500 550 600 650 700

0

20

40

60

IPC

E (

%)

Wavelength (nm)

CdS cell (cobalt electrolyte) PbS

CdS

400 500 600 700 800 900

0

20

40

60

80

PbS cell (cobalt electrolyte)

IPC

E (

%)

Wavelength (nm)

Quantum dot sensitizers reach 60 percent IPCE

Data : Dr. Hy-Yoong Lee62

Ongoing research

• Advanced nanostructures

• Light induced charge separation

• new sensitizers

• new redox mediators

• Solid state heterojunctions

• Quantum dot cells

• redox active ionic liquids

• tandem devices

63

Ionic Liquids (Ils )have attractive features

• Thermal Stability;• Non Flammability;• High Ionic Conductivity;• Negligible Vapor Pressure;• Wide Electrochemical Window;

Solid polymer/IL gels are formed by the addition of poly-(vinylidenefluoride-co-hexafluoropropylene) (PVDF–HFP)

Wang, P; Zakeeruddin, S. M.; Exnar, I.; Graetzel, M..

High efficiency dye-sensitized nanocrystalline solar cells

based on ionic liquid polymer gel electrolyte.

Chemical Communications (Cambridge, United Kingdom) 2002, 24, 2972-2973.

64

I-

I3-

light

The light absorption-viscosity dilemma

Due the high viscosity of the ionic liquids, diffusion limitation

of the photocurrent is expected under full sunlight.

N. Papageorgiou, Y. Athanassov, M. Armand, P. Bonhôte, H. Pettersson, A. Azam, M. Grätzel,

J. Electrochem. Soc. 1996, 143, 3099&3108

Mesoporous photoanode counterelectrode

65

N N+ I

-

NN B(CN)4

28 cP

880 cP

18 cP 17 cP 20 cP

21 cP

NN C(CN)3NN N(CN)2

NNNCS

NN

[(CF3SO2)2N]

66

• New mixed ionic liquid (three compounds) with high conductivity• Record conversion e!ciency of 8.2%• Excellent stability (1000h light soaking test)• An interesting option for "exible cells

67

Ionic liquids

EMITCB1-ethyl-3-methylimidazolium tetracyanoborate

N

N I

N

N I

N

N I

N

N I

PMII1-propyl-3-methylimidazolium iodide

EMII1-ethyl-3-methylimidazolium iodide

DMII1,3-dimethylimidazolium iodide

AMII1-allyl-3-methylimidazolium iodide

68

Tri-iodide transportFluidity (temp. dependent) vs. di#usion coe!cient for I3-

Stokes-Einstein relationfor hydrodynamic I3-

radius of 2.1Å

Melts with high iodideconcentration! Grotthus transport?

Melt with low iodideconcentration

I3- I3-I- I-

Grotthus bond exchange:enhanced di#usion coe!cent

69

Dye sensitized solar cells outperform silicon at lower light levels

70

Dye sensitized solar cells deliver high overall performance

Source: Tetsuo Nozawa, Nikkei Electronics Asia (July 2008), Data from Sony

Effect of temperature on module conversion efficiency

71

The DSC can meet future customer demands and needs

•Ease of building integration

•transparency and multicolor option (for power window application) *

•flexibility

•light weight

•Low production cost

•feedstock availability to reach terrawatt scale

•Short energy pay back time (< 1 year)

•enhanced performance under real outdoor conditions

•Bifacial cells capture light from all angles

•Tandem cell configurations boost efficiency over 15 %

•Outperforms competitors for indoor applications

* Unique selling proposition

Emerging and new applications call for:

House of the Future from InsideHouse of the Future from Inside

HOUSE

OF THE

FUTURE

73 74

75

Various colours in a series-connected dye solar cell module

Courtesy Dr. Andreas Hinsch, FHI, ISE Freiburg Germany

77

transparent, colorful, beautiful

Transparency: due to nano-sized (~20 nm) TiO2 particle filmColor: due to visible light absorption by dye

* DSC costs lower than Si solar cell; 1/4 -1/5 of Si solar cell

Courtesy Dr. Nam Gyu Park KIST

green solar cells mimic the green leave

80%

60

40

20

0

IPCE [

%]

800m700600500400

Wavelength [nm]

N

N

N

N

Zn

HO2C

CO2H

WMC 273

In collaboration with Professor David Officer, Univ.Wollongon,Australia

AISIN SEIKI CO.,LTD.

#th!SEPTEMBER. 2006SC!DEVELOPMENT!GROUP,!ENERGY! DEVELOPMENT !DEPT.

Artificial Plant with Leaves exhibited at EXPO 2005

Butterflies flutter andstop using!theelectricity generated bythis plant under theintermittent lightings.

Leaf-shaped transparent DSC with four colors

82

83

Scale-up and production

The first monolithic in series connected DSC modules showed a validated standard AM 1.5 G conversion efficiency of 5.3 %

A.Kay, M.Graetzel, Low cost PV modules

based on dye sensitized nanocrystalline

titanium dioxide and carbon powder.

Solar En. Mat. Solar Cells 1996), 44(1), 99- 17.

Module Unit Outdoor Test

Real Outdoor Test of DSC Modules!

Series connected

64 DSC cells

Kariya City at lat. 35°10’N,

Asimuthal angle: 0°

Facing due south, Tilted at 30°

DSCmade by

AISIN -SEIKI

The Toyota Dream House

!"#$%&'()*+

"#$%&'%()*

Founders of G24I: Ed Stevenson and Robert Hertzberg (64th speaker ofCalifornian State Assembly)

G24I builds 120 MeW capacity plant for flexible DSC production in Wales The G24I plant in Cardiff has started production on June 21 (solstice),2007

The first G24I product is a light weight flexible power supply for mobile

telephones

95 96

97

99

World first solar car,1912 Baker Electric Brougham

10 640 solar cells

100

http://www.teslamotors.com

Electric car: Tesla RoadsterAC induction motor, 200 kW, 35 kg,

6831 Li-cells 18650 (Ø 18 x 65 mm), 50 kWh, 450 kgrange 400 km per charge, acceleration to 100 km/h in 4 s

price $ 92.000

101 102!"#$%&&#"'()*+,-.'*/0.1123"#00%45-6%745#*#234-)48019,&)

The First Solar Car Driven by DSSCThe First Solar Car Driven by DSSC