TiO - Cnr

First Principles
Computational Modelling
of Solid/Liquid Interfaces
for Solar Energy and Solar
Fuels
Mariachiara Pastore
Computational Laboratory for Hybrid/Organic
Photovoltaics (CLHYO)
Istituto CNR di Scienze e Tecnologie Molecolari , I-06123 Perugia,
Italy
Workshop IUPAC Italia
Rome, 8th April 2014
Dye-Sensitized Solar Cells
Solar Photocatalytic Cells
Introduction
Multiscale modelling for higher efficiencies
Modeling
the molecule/semiconductor
interface are of
Dye–dye
and dye-semiconductor interactions
paramount importance in determining the overall
device conversion efficiency.
• Dye/catalyst anchoring:
photocurrent, photovoltage and
device stability
• Dye/catalyst co-adsorption and
aggregation: energy, electron/hole
transfer, excited state quenching
• Dynamical aspects
•Explicit interaction with solvents
and other cell components:
photovoltage, electronic coupling,
aggregation
Pastore, M.; De Angelis, F. J. Phys. Chem. Lett. 2013, 4, 956
Methods, models, tools
Optical/redox properties
of standalone
chromophores
Ru-complexes
Fully organic
Semiconductor cluster
models
(TiO2)38
(TiO2)82
Bipyramidal
nancorystal (TiO2)367
Dye/catalyst/semiconductor
interfaces
• Dye anchoring geometries (DFT , GGA)
• Electronic structure (DFT/implicit solvation models)
• CPMD simulations
• Absorption spectra and electronic properties
(Hybrid TDDFT)
squaraine in explicit water (90 molecules)
Modelling single dye adsorption
on the TiO2 surface
Adsorption modes on TiO2
(Newns-Anderson ) the LUMO
broadening (lorentzian) of the TiO2adsorbed dye gives the electronic
coupling
Monodentate
The anchoring mode and the extent of
electronic coupling directly influence the cell
performances: CB energy shift (Voc), electron
injection and back recombination
CB shift: 40% dye’s
electrostatic potential,
60% ground state
charge transfer
Pastore, M.; De Angelis, F.,Phys. Chem. Chem. Phys., 2012, 14, 920
Ronca, E.; Pastore, M.; Belpassi, L.; Tarantelli, F.; De Angelis,
F. Energy Environ. Sci. 2013, 6, 183
Bidentate
Modelling multiple dye
adsorption on the TiO2 surface:
aggregation
Dye Aggregation on TiO2: indoline dyes
Dye-aggregation on the
semiconductor surface is
undesired (lower IPCE
values) One rhodanin
ring: strong
aggregation
6-7%
Two rhodanin
rings: weak
aggregation
Uchida et al. Chem. Commun.
2003 - J. Am. Chem. Soc. 2004
Our strategy
Selecting dimeric arrangements on a (TiO2)82 slab
 Optimizations of selected structures
Evaluating the relative stability of the optimized dimers
Optical response simulation for the preferred arrangments
M. Pastore, F. De Angelis ACS Nano, 2010, 4, 556.
8-9%
Structures Selection and Optical Rensponse
D149
D102
Dimer
D102
D149
(0,2)
0.0
4.5
(2,2)
3.9
0.0
MP2 relative
stability
(kcal/mol)
Monomer Dimer
Dye Exc.
f
Exc.
D102 2.11 0.82 1.96
D149
2.06
0.80 1.97
Shift
0.15 0.22
0.08 0.06
*TDDFT(B3LYP)/6-31G* excitation energies in EtOH
M. Pastore, F. De Angelis ACS Nano, 2010, 4, 556.
Exp.
Co-adsorption on TiO2:
Modeling different dyes adsorption
and FRET
Exploiting FRET in DSCs
Enhancing the light
harvesting in the red
by cosensitization of
TiO2 surface with
organic dyes having
high NIR absorption
Rate of FRET
R06
1
kF =
τ0 rA − rD
6
hν
FRET
e-
Where the Foster
radius is given by
9000 × ln(10)κ 2QD
4
R =
F
(
λ
)
ε
(
λ
)
λ
dλ
D
A
∫
5 4
128π n NA
6
0
NIRERD
(AS02)
SD
(C106)
Hardin, B. E.; Sellinger, A.; Moehl, T.; Humphry-Baker, R.; Moser, J.-E.; Wang, P.; Zakeeruddin, S.
M.; Grätzel, M.; McGehee, M. D., J. Am. Chem Soc. 2011, 133, 10662.
Modeling FRET
Förster type intramolecular energy transfer mediated by
resonant dipoles
Orientation
Spectral
factor
Rate of FRET
kF =
1
R06
τ0 rA − rD
6
Where the Foster
radius is given by
9000 × ln(10)κ 2QD
R =
128π 5 n 4 NA
6
0
overlap
∫ FD (λ)ε A (λ)λ dλ
4
with NA being the Avogadro’s number and n the refractive index of the
medium. The dimensionless orientation factor κ2 can vary from 0 to 4
and is given by
κ 2 = (cos γ − 3cosα cos β )2
For randomly oriented donoracceptor dipole moments, κ2 is equal
to 2/3
Aggregates and FRET Modeling Strategy
•Anchoring geometries
onto the (TiO2)82 slab
• Selecting the closest
interacting ones
(Ti active sites grid)
+1.6
0.0
• Stability analysis
• Geometrical orientation
factor κ2 calculation
+15.6
Possible co-adsorption schemes of AS02 sorrounded by
four C106 molecules and relative energies in kcal/mol.
Pastore, M.; De Angelis, F., J. Phys. Chem. Lett., 2011, 3, 2146−2153
Calculated Κ2, Föster Radii and FRET rates
Pastore, M.; De Angelis, F., J. Phys. Chem. Lett., 2011, 3, 2146−2153
Modelling the complex cell
environment
Dye-Iodine-TiO2 Interactions
Organic dyes with
common I-/I3electrolyte generally
show lower Voc
compared to Ru-based
dyeslower electron
lifetimes
17 Å
11 Å
-1.9
kcal/mol
-4.3
kcal/mol
Oxygen atoms are the
preferred binding sites for I2
Pastore, M.; Mosconi, E; De Angelis, F. J.
Phys. Chem. C 2012, 116, 5965
Planells, M.; Pelleja, L.; Cliﬀord, J. N.;
Pastore, M.; De Angelis, F.; Lopez, N.;
-2.1
-2.5
Marder, S. R.; Palomares, E. Energy
kcal/mol
kcal/mol
Pastore, M.; Mosconi, E; De Angelis, F. J. Phys.
Chem.
2012,4,116,
5965
Environ.
Sci.C2011,
1820
Dye-ionic additives-TiO2 Interactions
Addition of Lithium salts improves
the measured photocurrents
2
2π Nacc
k = ∑k =1 | Vdk | ρ(ε k )

 h
d
inj
Calculated
CB shift
Li+
Li+
LUMO/CB
states coupling
Li+
Molecular
packing amplifies
the effect!
Injection rate
distribution
Li+
Agrawal, S.; Leijtens, T.; Ronca, E., Pastore, M.; Snaith, H.; De Angelis, F. J. Mater. Chem. A,
2013, 1, 14675-14685
Effect of TiO2 protonation on the charge generation
Surface protonation is usually
employed for improving Jsc
• CB donwnshift?
• red-shifted dye absorption?
-0.1 eV
per H+
Ronca, E.; Marotta, G.; Pastore, M.; De Angelis, F. J. Phys. Chem. C 2014, In Press. DOI:
10.1021/jp5004853
Effect of TiO2 protonation on the charge generation
Jsc
(mA/cm2)
Voc
(V)
FF
NO
5.780
0.734
0.688
2.92
YES
7.500
0.713
0.699
3.74
Acidic
treatment
Eff.
(%)
Red-shifted
absorption
Max. Jsc gain for the the spectral
red-shift is about 0.7 mA/cm2!
1H+
5H+
d
kinj
=


2
2π Nacc
|
V
|
∑
dk ρ (ε k )
k =1
h
Increased
electronic
coupling
improved
charge
generation
Dye-Sensitized Photocatalytic
Cells
Forthcoming research activity: Short term goals
• Stable anchoring on the SC in water and oxidative environment
• Optimal dye/catalyst ratio on the surface
• Optimal geometrical arrangement of the dye/catalyst assembly
• Maximizing the forward energy and electron transfer processes
• Minimizing excited state quenching and back recombination reactions
Screening of novel anchoring
groups stable in oxidative and
water environments
Simulating molecular/catalyst
aggregates in model architectures
and operative experimental
conditions (full coverage) through
QM/MM and MD simulations
J. Am. Chem Soc. 2013, 135, 4219
Energy Environ. Sci. 2011, 4, 2389
Modelling dye/catalyst/TiO2 for water splitting
(IrO2)562H2O
W. J. Youngblood, S.-H. A. Lee, Y. Kobayashi, E. A.
Hernandez-Pagan, P. G. Hoertz, T. A. Moore, A. L. Moore, D.
Gust, and T. E. Mallouk J. Am. Chem. Soc. 131, 2009, 926927
(TiO2)82
• Stability of the dye/catalys assemby
on the TiO2
• Energy levels alignment
• Electronic coupling for injection and
regeneration
Thanks to
CLHYO Perugia
Dr. Filippo De Angelis
Enrico Ronca
Gabriele Marotta
Oxford University
Prof. H. Snaith
Financial support:
FP7-NMP-2009 “SANS” - FP7-ENERGY-2010 “ESCORT”
FP7-ICT-2011 “SUNFLOWER” CNR EFOR 2011 IIT-SEED 2009
…and you for your kind attention

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