Charge photogeneration for Solar Energy ConversionJames DurrantDepartment of Chemistry & Energy Futures Lab, Imperial College Londonwww.imperial.ac.uk/people/j.durrant

Solar Energy

There’s lots of it…….

Solar Energy

But it’s very diffuse……In UK, need ~ 40 m2 of solar cells to supply one persons average electrical demand

So solar conversion systems need to be cheap / m2!

Towards the Artificial Leaf

Renewable fuel synthesis

Storage of solar energy

Sunlight + H2O + CO2

H2 + CO2

Carbon based Fuels

Ph

oto

che

mic

al r

edu

ctio

n o

f C

O2

PV

Ph

oto

lysi

s of

H

2O

Ch

em

ica

l re

duct

ion

Other RenewableElectricity:Wind / Nuclear

Fuel Utilisation:Fuel Cells/Combustion

Ca

rbon

Re

cycl

ing

Ele

ctro

lysi

s o

f H

2O

e- + H2O + CO2

Ele

ctro

che

mic

al r

edu

ctio

n o

f C

O2

CO2

captureSunlight + H2O + CO2

H2 + CO2

Carbon based Fuels

Ph

oto

che

mic

al r

edu

ctio

n o

f C

O2

Ph

oto

che

mic

al r

edu

ctio

n o

f C

O2

PV

Ph

oto

lysi

s of

H

2OP

ho

toly

sis

of

H2O

Ch

em

ica

l re

duct

ion

Other RenewableElectricity:Wind / Nuclear

Fuel Utilisation:Fuel Cells/Combustion

Ca

rbon

Re

cycl

ing

Ele

ctro

lysi

s o

f H

2O

Ele

ctro

lysi

s o

f H

2O

e- + H2O + CO2

Ele

ctro

che

mic

al r

edu

ctio

n o

f C

O2

CO2

capture

http://www3.imperial.ac.uk/energyfutureslab/research/grandchallenges/artificialleaf

Solar Energy Conversion Technologies

Light

External circuit

Electrons

PlatinisedTCO coated glass

I- / I2 based electrolyteTCO

coated glass

NanocrystallineTiO2 film

I3-

I-

Sensitizer dye

150 nm

Challenges:Efficiency > 10%Cost < $100/m2

Robust

Dye sensitized solar cells Organic semiconductor solar cells

WaterPhotolysis

UV

e-

h+

CB

VB

Pt orAg+

2H2O

4H+ +O2

Bulk charge recombination

UV

e-

h+

CB

VB

Pt orAg+

2H2O

4H+ +O2

Bulk charge recombination

2H+

H2

Printing molecular solar cells

G24i’s production line in Cardiff, producing rolls of dye sensitised solar cells =>

Konarka’s printed organic solar cell strips

Dye sensitised solar cells

Light

External circuit

Electrons

Platinised TCO coated glass

I- / I2 based electrolyteTCO

coated glass

Nanocrystalline TiO2 film

I3-

I-

Sensitizer dye

www.iq.usp.br/geral/dyecell

different dyes(1000’s tested)

Michael Grätzel

Lab cells upto ~ 11%‘Commercial’ modules 3-4%

RuN

N NCS

NCS

N

N

OTBA O

OH

O

TBAO O

O

OH

Polymer/C60 solar cells

S

S S

O

O

P3HT~1000 tested already

PCBM~100 tested

Alan Heeger

Lab cells up to 8 %

Piggy backing on the Organic Electronics Buzz

Organic Electronics: electronic devices based on molecular or polymer semiconductorsMotivations: Harness organic chemistry to synthesis materials which are easily processable, spectrally tunable, cheap ……..Field first developed to target LED and FET applications. These are now entering market

Sony OLED tvSony flexible OLED

http://www3.imperial.ac.uk/plasticelectronics

UV

e-

h+

CB

VB

Pt orAg+

2H2O

4H+ +O2

Bulk charge recombination

UV

e-

h+

CB

VB

Pt orAg+

2H2O

4H+ +O2

Bulk charge recombination

2H+

H2

Characteristics: Disordered Nanostructures

Light

External circuit

Electrons

PlatinisedTCO coated glass

I- / I2 based electrolyteTCO

coated glass

NanocrystallineTiO2 film

I3-

I-

Sensitizer dye

SEM image of nc Fe2O3 electrode (EPFL)

150 nm

NanocrystallineTiO2

P3HT/PCBM film

Dye sensitized solar cells Organic semiconductor solar cells

WaterPhotolysis

150 nm

Interface keyLow cost material and(solution) processing

Towards Ordered Interfaces?

Donor / acceptor block co-polymers

100 nm dia TiO2 nanowires

So far disappointing device performance:• Still disordered on molecular scale (soft materials)• Loose degeneracy of pathways? • Hard work…..

Similar story for attempts to include antenna structures

Photochemical Design Considerations

1P*P+/Ph-

P+/QA-

P+/QB-

ns

s

Recombination

ps

ms / ms

Energy

High yield of long lived, energeticcharge separated states

-8

-6

-4

-2

0

2

J [m

Acm

-2

]

0.60.50.40.30.20.10.0Vcell [V]

J-V (1sun) data Simulated data

JSC

VOC

FF

High yield: CurrentEnergetic: Voltage/Optical bandgapLong lived: allows charges to transport to external circuit.

Energy versus lifetime for separated charges

Solar CellsEnergy cost of achieving a high yield / rate of charge separation

Dye sensitised solar cells

Light

External circuit

Electrons

Platinised TCO coated glass

I- / I2 based electrolyteTCO

coated glass

Nanocrystalline TiO2 film

I3-

I-

Sensitizer dye

www.iq.usp.br/geral/dyecell

different dyes

Michael Grätzel

Lab cells upto ~ 11%‘Commercial’ modules 3-4%

Energy losses in DSSCs3.0

2.5

2.0

1.5

1.0

0.5

0.0

Sto

red E

nerg

y Pe

r Ele

ctro

n,

eV

10987654321Step

Wor

k E

xtra

cted

(0

.65

eV *

0.7

QE

)

Regeneration

Transport

Injection

Ab

sorp

tion

Dye*

TiO2-e-

Dye+

TiO2-e-

I3_

SnO2-e-

I3_

at Pt

TiO2-e-

at SnO2

I3_

at Pt

Pt-e-

I3_

I_

at CE

I_

at Dye

Hot States StateEnergy State Energy * QE

O’Regan and Durrant Acc. Chem. Res. 2009

Kinetics versus energetics

G/eV

0

1

2 Dye*

Dye

Dye+ / e-TiO2

ElectronInjection~ 100 ps

Decay to ground~ 10 ns

I3- / eth

-TiO2

Hole transfer to electrolyte~ 1 ms

Recombination to dye+,ms-ms

hu

Recombination to electrolyte, ms-s

Transportms

RuN

N NCS

NCS

N

N

OTBA O

OH

O

TBAO O

O

OH

High yield of long lived, energetic charge separated states

Kinetics versus energetics: short circuit

G/eV

0

1

2 Dye*

Dye

Dye+ / e-TiO2

ElectronInjection~ 100 ps

Decay to ground~ 10 ns

I3- / eth

-TiO2

Hole transfer to electrolyte~ 1 ms

Recombination to dye+,ms

hu

Recombination to electrolyte, s

Transportms

Light

VB

CB

S0 / S+

S* / S+

h

e- I- / I3-

e-

EF

V

Charge separation in DSSCse

nerg

y

Dye

Dye*TiO2

acceptor states

Decay to Ground

k0

0kk

k

inj

injinj

kinjdoscb

exp(E/100meV)

RuN

N NCS

NCS

N

N

OHO

OH

O

HO O

O

OH

100 101 102 103 104 105

0.0

0.5

1.0

(ii)

(i)

(ii)

(i)

Inje

ctio

n Y

ield

time / picoseconds

Haque et al. J. Am. Chem. Soc. 2005Koops et al.J. Am. Chem. Soc. 2009

N719/TiO2

SolarCell

• Injection rate increases exponentially with energetics:- Increasing kinj

x10 costs ~ 300 meV• Physical origin: exponential increase of

CB density of states with energy• Impact on injection yield and device

current depends upon competition with excited state lifetime

The role of excited state lifetime:Singlet versus Triplet Injection

• Injection from both S1 and T1 states of N719 possible

• T1 energy ~ 300 meV lower than S1 => injection rate ~ 1 orders of magnitude slower.

• BUT T1 lifetime 5 orders of magnitude longer than S1

• Much easier to achieve efficient electron injection from T1 than S1 state.

• Results in injection efficiency being key limitation for singlet injector and low bandgaps dyes (e.g.: porphyrins).

G/eV

0

hν

Decay to ground~ 10 ns

1.9

1.6

S0

T1

S1

ISC~ 100 fs

RuN

N NCS

NCS

N

N

OTBA O

OH

O

TBA O O

O

OH

100 ps

1-10 ps

Koops et al. JACS 2009

Catalysis of the iodide/iodine redox couple

-Recombination rate constant krecom strongly dependent upon sensitizer dye

• -Key in determining cell voltage

O’Regan et al. J. Am. Chem. Soc. 2008

RuN

N NCS

NCS

N

N

OHO

OH

O

HO O

O

OH

I2 + 2TiO2(e-) → 2 I-Regeneration: Dye+ + 2 I- → dye + I2-

Recombination: I2 + 2TiO2(e-) → 2 I-

Counter Elec. I2 + 2FTOPt(e-) → 2 I-

2I- / I2

maximumvoltage

S / S+

S*/ S+

diffusion

electrolyte

sensitiserdyeTiO2

conductingglass

E / Vvs. NHE

1.0

0.5

0

–0.5

e–

regeneration

injection

EF

CB

cathode

e–

hν

Interface engineering to minimise recombination

a b

c d

a b

c d

Al2O3 coated

Uncoated

Haque et al.Angew. Chem. 2005

Palomares et al. JACS 2003

Haque et al. Adv Func Mat2004

10-6 10-5 10-4 10-3 10-2

0

1

2

3

mO

D

Time / Seconds

TiO 2 MFHTM

DFHTM

Dye

TiO 2 MFHTM

Li + - DFHTM

Dye

+ Li+- Li+

10-6 10-5 10-4 10-3 10-2

0

1

2

3

mO

D

Time / Seconds

TiO 2 MFHTM

DFHTM

DyeTiO 2 MFHTM

DFHTM

Dye

TiO 2 MFHTM

Li + - DFHTM

Dye

+ Li+- Li+ N N

OCH3 OCH3

n

O

OO

O

O

OO

O

Li+ Li+

Li+- DFHTM

Schmidt-Mendes et al. Nanolet. 2005

TiO2

hν

picoseconds

nanoseconds

TiO2

hν

picoseconds

nanoseconds

~ 1 s

Interfacial redox relays

~ 1 s

TiO2

hν

picoseconds

nanoseconds

TiO2

hν

picoseconds

nanoseconds

6 8 10 12 14 16 18 20-1

0

1

2

3

4

5

5

3

2

4

E

D

1C

BA

Log

(1/t

50%

) (se

cs)

spatial separation r (Angstroms)

krecom e-br , b ~ 1 Ǻ-1

Haque et al. Angew Chem. 2005

L

og (

k rec

om /

s-1)

Polymer/C60 solar cells

S

S S

O

O

P3HT PCBM

Charge photogeneration

e-

Polymer C60

huhu

Exciton separation

h+h+

LUMO

HOMO

e-e-

h+h+

Geminate recombination of bound polaron pairs (charge transfer states)

• Key consideration:

How do initially generated polaron pairs overcome their coulomb attraction and dissociate into free charges?

r

eV

r 0

2

4

Coulomb Attraction:

er = 3-4 for organics

e-e-Charge Dissociation

The energy cost of charge photogeneration

Ohkita et al. JACS 2008, Clarke et al. Adv. Func. Mat. 2009

Yie

ld o

f di

ssoc

iate

d ch

arge

s

0.6 0.7 0.8 0.9 1.01

10

100

O

D (1

s)

-GCS

rel (eV)

‘Gen 1’ Polymers (polythiophenes) blended with C60:

Efficient charge photogeneration only achieved at high energy cost (half the photon energy!)

G/eV 1P*

P

h

P+…C60-

P+ + C60-

Transportto

electrodes

(P+…C60-)hot

Thermalisation

G/eV 1P*

P

h

P+…C60-

P+ + C60-

Transportto

electrodes

(P+…C60-)hot

Thermalisation

Energy lost in charge separation

Onsager Theory

Key issue:

Electron thermalisation length (a) versus coulomb capture radius

kBT

rc a

e-hdistance

V

Recombination

Dissociation

e-

hν

Clarke and Durrant Chem Rev 2010 ACS ASAP

Adding intramolecular charge separation in the polymer appears to enable charge separation at lower energy costs

HOMO

LUMO

Clarke et al. Chem Comm 2009, Chem Rev 2010

0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.01E-6

1E-5

1E-4

1E-3

P3HTPCPDTBT (dithiol)

PCPDTBT

OD

(1

s

)

-GCS

eff (eV)

Reducing the energy cost of charge separation

CT

Cha

rge

sepa

rati

on y

ield

(a.

u.)

Energy Loss during charge separation

S

S S

Data plotted for all polymer / PCBM blend films where both DOD and JSC measured.

Remarkably good correlation between charge photogeneration yield and device JSC.

Suggests charge photogeneration is key (primary?) determinant of photocurrent rather than collection/transport (at least for polymer / PCBM blend films).

Charge Photogeneration as the key determinant of photocurrent

10-5 10-4 10-3

0.1

1

10

Cor

rect

ed J

sc (

mA

.cm

-2)

OD

( )

All optical assay of charge photogeneration in films

Devicephotocurrent

Outlier: polyfluorene based polymer-electric field dependent photogeneration?

Solar to fuels

H2OUV + nc TiO2+Pt

H2 + ½ O2

UV

e-

h+

CB

VB

Pt orAg+

2H2O

4H+ +O2

Bulk charge recombination

UV

e-

h+

CB

VB

Pt orAg+

2H2O

4H+ +O2

Bulk charge recombination

2H+

H2

Protein immobilisation

+ ZnO

TiO2

SnO2

Proteins

Protein loading of Nanomoles / cm2

Topoglidis et al.Anal Chem. 1998

Applications:BiosensingSpectroelectrochemistryArtificial Photosynthesis

Bio/inorganic electrodes for hydrogen evolution

• Long lived (200 ms) charge separation• Hydrogen evolution observed with ~

20% internal QY using visible light

• Probably never a technology, but hopefully inspiring science.

0 10 20 30 40

0

4

7

11

14

18

0

10

20

30

40

50

i / n

A

H2 /

L

Irradiation time / min

ZnCyt-c/TiO2-Pt

TiO2-Pt

Astuti et al. JACS 2005

H2

H+

Pt

D

D+

e-

e- e-

hv

TiO2 nanoparticle 1E-6 1E-5 1E-4 1E-3 0.01 0.1 1

0.0

0.2

0.4

0.6

0.8

1.0

Norm

alis

ed

OD

Time / s

Zn Cytcation

Zn Cyttriplet

Hole dynamics in TiO2

0 2 4 6 8 10 12 14 160

5

10

15

200.0 0.2 0.4 0.6 0.8 1.0 1.2

0

2

4

6

8

QY

(%

)

photon/TiO2 particle/pulse

Intensity mJ/cm2

O2

evo

lutio

n x

105 (

mo

l / s

)

•Electron scavenging by Ag+ ions results in long lived TiO2 holes (~ 200 ms)

•Oxygen QY strongly dependent upon excitation density, peaking at ~ 18% for 4 photons absorbed / nanoparticle•Consistent with needing to accumulate 4 oxidising equivalents to generate one O2

10-6 1x10-51x10-4 10-3 10-2 10-1 1000.0

0.5

1.0

1.5

2.0

2.5

3.0 460nm 800nm

mO

D

Time (s)

O2 QY data as function of excitation intensity

TiO2 hole decay dynamics

Tang, Durrant & Klug JACS 2008

UV

e-

h+

CB

VB

Pt orAg+

2H2O

4H+ +O2

Bulk charge recombination

UV

e-

h+

CB

VB

Pt orAg+

2H2O

4H+ +O2

Bulk charge recombination

Timescale:ms -seconds

150 nm

-0.2 0.0 0.2 0.4 0.6

0

50

100

150

200

J /

A

applied bias / V vs Ag/AgCl

light

dark

Hole dynamics in Fe2O3

Visible

e-

h+

CB

VB

TCO

2H2O

4H+ +O2

Bulk charge recombination

EFe -0.1 V + 0.4 V

• Photoelectrode for visible light driven water oxidation

• Photocurrent only observed under positive bias

• Positive bias increases hole lifetime from ms/ms to seconds

0 1 20.00

0.05

mO

D

time/s

-0.1V 580nm +0.4V 580nm

Lessons for the artificial leaf

• For the synthetic chemists: Key features areNanostructures, self-assembly, multi-function components

• For the photochemists: Kinetics versus thermodynamics key as for photosynthesis

• For the theoreticians: Marcus (and inorganic device physics) often less useful as a design tools than we might have hoped – appreciating the chemical complexity and impact of disorder often more important.

• For the device people: The jump from small lab devices to a scaleable, stable, low cost module can be larger! Key driver for new materials / processing etc.

Acknowledgements

Dye sensitized solar cells• Brian O’Regan, Piers Barnes, Assaf Anderson, Li Xiaoe,

Andrea Listorti, Joe Mindagaus.• plus Michael Grätzel, Nazeeruddin et al. (EPFL), David

Officer et al (Wollangong), Corus, g24i

Polymer / fullerene solar cells• Tracey Clarke, Safa Shoee, Chris Shuttle, Brian O’Regan, Rick Hamilton, Andrea Maurano, Mattias

Eng, Fiona Jamieson, Dan Credgington, Yvonne Soon. Jenny Nelson, Donal Bradley, Iain McCulloch and co-workers

plus Steve Tierney et al (Merck Chemicals), Christoph Brabec et al (Konarka), Nazario Martin et al. (Madrid), Seth Marder / Jean-Luc Bredas et al. (Georgia Tech)

Solar Fuels• Junwang Tang, Monica Barroso, Stephanie Pendlebury, Wenhua Leng, Alex Cowan, David Klug,

Steve Dennison, Geoff Kelsall, Klaus Heldgardt, plus Kevin Sivula, Michael Grätzel et al. (EPFL)

Alumini: Saif Haque, Hideo Ohkita, Emilio Palomares, Yeni Astuti, Ana Morandeira, Sara Koops etc..

Financial Support: EPSRC, EU, TSB, BP Solar, Solvay, Konarka, Carbon Trust

Lessons for the artificial leaf

• For the synthetic chemists: Key features areNanostructures, self-assembly, multi-function components

• For the photochemists: Kinetics versus thermodynamics key as for photosynthesis

• For the theoreticians: Marcus (and inorganic device physics) often less useful as a design tools than we might have hoped – appreciating the chemical complexity and impact of disorder often more important.

• For the device people: The jump from small lab devices to a scaleable, stable, low cost module can be larger! Key driver for new materials / processing etc.

Lessons from DSSCs and OPV for solar to fuels

• Yield versus lifetime versus energy determines efficiency

• Elegant structures are so far not functionally better

• Multifunctional components are painful to develop but necessary

• Real (disordered) materials properties are critical to determining function (rather than Marcus, Forster, Redfield……..)

• Catalysis and multi-electron chemistry can be exploited to aid kinetics

• A lot of the key action happens on unfashionably slow timescales

• The efficiency gap between lab scale champion cells and cheap, stable modules can be critical (and painful)