Femto- to Millisecond

Photodynamics of Porphyrins-

Based DSSCs

Maria Rosaria di Nunzio

Universidad de Castilla – La Mancha, Toledo, Spain

Source: U.S. Energy Information Administration (EIA), International Energy Outlook 2013.

Sunlight conversion

into electric power.

Dye-sensitized solar

cells (DSSCs).

Basic Aspect of Solar Energy

Conversion

Key components for a typical solar cell:

1. Light absorber (dye)

2. Hole-transport agent

3. Electron-transport agent

Three requirements:

1. Good light-harvesting capability

2. Efficient charge separation

3. Migration of the separated charges (oxidized and reduced equivalents, or holes

and electrons)

400 600 800 1000 12000.0

0.5

1.0

1.5

I , W

m-2 n

m-1

Wavelength, nm

AM 1.5

Solar intensity at AM1.5

How does it Work a DSSC?

Kinetic processes in the cell:

0. D + hν → D*

1. D* → D*+ + e-(TiO2)

2. Electron transport of e-(TiO2)

3. D+ + 2I- → S + I2·-

2I2·- → I3

- + I-

4. I3- + 2e- → 3I-

5. D* → D + hν

6. D+ + e-(TiO2) → D

7. 2e-(TiO2) + I3

- → 3I-

Photoexcitation

Charge injection (~ fs-ps)

Dye regeneration (~ μs)

Electrolyte regeneration

Dye relaxation (~ ns)

Recombination via dye

Recombination via electrolyte

energy TiO2

e-

dye

(I-/I3-)

electrolyte

e-

(D+/D)

(D+*/D*)

0

5

1

3

2

6

7

4

Fermi level

VOC

potential

Anode Cathode Dye

Aim of the Study

Dye Molecules Solar cells

Photophysical studies Femtosecond Transient Absorption Spectroscopy

Nanosecond Flash Photolysis

Estimate of efficiencies Electron injection and

regeneration

Correlation

Measurement of the I-V curves Voc, Jsc, ff, and h

Porphyrin-Based DSSC

Yella, A.; Lee, H.-W.; Tsao, H. N.; Yi, C.; Chandiran, A. K.; Nazeeruddin, M. K.; Diau, E. W.-G.; Yeh, C.-Y.;

Zakeeruddin, S. M.; Gratzel, M. Science 2011, 334, 629−634.

400 500 600 700 8000.0

0.5

1.0 YD2-o-C8 / EtOH

YD2-o-C8-TiO2-Co

2+/3+

YD2-o-C8-Al2O

3-Co

2+/3+

No

rmalized

em

issio

n

Abs.

No

rmalized

ab

so

rban

ce

Wavelength / nm

YD2-o-C8-based solar cells

Emiss.

0.0

0.5

1.0

[5,15-bis(2,6-dioctoxyphenyl)-10-(bis(4-hexylphenyl)amino)-20-(4-

carboxyphenylethynyl)porphyrinato]Zinc(II) (YD2-o-C8)

D p

A

Donor-π-bridge-acceptor (D-π-A)

dye

Long-chain alkyloxy groups

Studies in Solution

500 550 600 650-0.007

0.000

0.007

0.014

Steady-state

absorption

A

Wavelength / nm

YD2-o-C8 in EtOH

1.6 ps

180 ps

0 1 2-0.01

0.00

0.01

YD2-o-C8 in EtOH

490 nm

655 nm

A

Time / ps

= 120 fs

IRF = 50 fs

The transient absorption minimum (λmin = 653 nm) is slightly red-

shifted from the steady-state one (λmin = 641 nm) - contribution from

stimulated emission (λmax = 671 nm).

S1 = 1.6 ns

Transient Studies of Complete DSSC

500 600 700 900 1000

-0.02

0.00

0.02 YD2-o-C8-TiO2-[Co(byp)

3]3+/2+

0 ps

1.1 ps

6.5 ps

22 ps

500 ps

A

Wavelength / nm-20x10

-3

-10

0

10

20

A

200150100500

Time / ps

516 nm

656 nm

15x10-3

10

5

0

-5

-10

A

50403020100

Time / ps

516 nm

656 nm

Cobalt Electrolyte

Iodide Electrolyte

Femtosecond Transient Absorption Spectroscopy

Multi-exponential decay,

corresponding to several

overlapping bands

Effect of the conduction band edge

(Al2O3 and TiO2)

500 600 700 900 1000-0.02

0.00

0.02

(1) Al2O

3

(2) TiO2

2

1

A

Wavelength / nm

YD2-o-C8-X-[Co(byp)3]3+/2+

Delay time = 7 ps

τ (A%) YD2-o-C8-TiO2-

Co3+/2+ τ (A%)

YD2-o-C8-

Al2O3-Co3+/2+

τ1 / fs (A1%) 900 (10) τ1 / ps (A1%) 3.7 (41)

τ2 / ps (A2%) 5.2 (90) τ2 / ps (A2%) 40 (59)

0 10 20 30 40 50

0.0

0.5

1.0

(1) Al2O

3

(2) TiO2

Obs = 516 nm

YD2-o-C8-X-[Co(byp)3]3+/2+

No

rma

lize

d (

0 t

o 1

)

A

Time / ps

1

2

Multi-exponential behavior Electron injection constant

Electron injection efficiency

0 10 20 30 40 50 60 70 800.0

0.5

1.0

2

1

Obs = 1000 nm

(1) X = [Co(byp)]3+/2+

(2) X = I3

-/I

-

No

rma

lize

d (

0 t

o 1

)

A

Time / ps

YD2-o-C8-TiO2-X

Effect of the electrolyte (I3-/I- and

[Co(byp)3]3+/2+)

0 10 20 30 40 500.0

0.5

1.0

= 700 fs (12%)

= 10 ps (59%)

= 46 ps (29%)

2

1

Obs = 520 nm

(1) X = [Co(byp)]3+/2+

(2) X = I3

-/I

-

No

rma

lize

d (

0 t

o 1

)

A

Time / ps

YD2-o-C8-TiO2-X

Photodynamics

Electrolyte kei / s-1 φei

Cobalt Electrolyte 8.33 × 1011 75%

Iodide Electrolyte 1.15 × 1012 80%

Nanosecond Flash-Photolysis

600 800 1200 1500-0.003

0.000

0.003

0.006

YD2-o-C8-TiO2-[Co(byp)

3]3+/2+

1.2 s

6 s

15 s

52 s

A

Wavelength / nm1E-8 1E-6 1E-4 0.01

0.0

0.5

1.0

= 19 s

= 17 s

= 165 s

YD2-o-C8-TiO2-X

X = Co3+/2+

X= I-/I

3

-

X = no redoxNo

rma

lize

d (

0 t

o 1

)

A

Time / s

Obs = 520 nm

1E-8 1E-6 1E-4 0.01

0.0

0.5

1.0

= 470 s= 17 s

YD2-o-C8-TiO2-[Co(byp)

3]3+/2+

520 nm

1350 nm

1000 nm

No

rma

lized

(0 t

o 1

)

A

Time / s φreg = 90% for both electrolytes

1000 nm (electron recombination)

1350 nm (cation)

Cell Performances

Complete Cell Parameters

Electrolyte Jsc / mA cm-2 Voc / V ff η

Cobalt Electrolyte 10.35 0.81 0.58 4.87 %

Iodide Electrolyte 11.08 0.74 0.56 4.57 %

Conclusions

Increase of the short-circuit photocurrent density (Jsc) due to a lower recombination rate of the oxidized Iodide electrolyte species with the electrons in TiO2

NPs.

Higher open circuit voltage (Voc) for Cobalt electrolyte due to its more negative redox potenial

with respect to that of Iodide electrolyte.

η values are smaller if compared to the higher electron injection efficiencies due to photocurrent

losses (electron recombination).

Acknowledgements

Prof. Abderrazzak Douhal

Dr. Boiko Cohen

Projects: PLE2009-0015 and CYTEMA E2TP

Prof. Shuzi Hayase Dr. Shyam Pandey

Solar Energy Conversion

Efficiencies 1. Incident photon-to-current conversion efficiency (IPCE)

Where LHE(λ) = light-harvesting efficiency of the acitve materials;

Фinj = charge injection efficiency from the excited sensitizer into the TiO2 conduction band;

ηreg = efficiency for dye regeneration;

ηcol = charge carrier collection efficiency (ηcol = 1/(1 + τtrans/τrec))

2. Overall power conversion efficiency (η or PCE), which compares the total electrical energy output

with the total energy contained in the solar irradiance.

Output of a photovoltaic cell

Granada, 7th September 2011

Femtosecond Transient Absorption Spectroscopy

Variable Delay

Chopped

excite pulse

train

Probe

pulse

train

Chopper

or Fast Shutter

Slow

detector

Sample Lens

Computer

CaF2

diode laser 532 nm Ti-sapphire

oscillator 800 nm

450 mW 86 MHz - 30 fs

800 nm 1 W 1 kHz - 50 fs

5 W

Time scale: up to 3 ns FWHM: 90 fs Wavelength scale: 300-950 nm

Amplifier

OPA

Time-Tesolved Techniques

Adapted from R. Trebino

Nanosecond Flash Photolysis

Optical Parametric Oscillator (OPO)

Sample holder Xe-lamp

Lamp pulser

Monochromator

Photomultiplier

oscilloscope

Time scale: 10 ns up to ms Wavelength scale: 350-1000 nm

0.0 0.5 1.0 1.5 2.0 2.5 3.00.00

0.25

0.50

0.75

1.00

A

/ a

.u.

Time / s

SQ 26

exc

= 660 nm

Time-Resolved Techniques

Qswitched Nd:YAG laser, 1064 nm