October 21, 2004 1 Exciton diffusion and interfacial charge separation in porphyrin/TiO 2 bilayers Laurens D.A. Siebbeles Delft University of Technology The Netherlands
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October 21, 2004
1
Exciton diffusion and interfacial charge separation in porphyrin/TiO2bilayers
Laurens D.A. SiebbelesDelft University of TechnologyThe Netherlands
October 21, 2004 2
Research in Delft on opto-electronic properties of materials
SS
SS
conducting polymers DNA supra-molecularassemblies
discotic liquid crystals
composite systems
inorganic nanoparticles, nanorods
October 21, 2004 3
Sample morphologies
dilute solution / gel bulk solid
+
thin film on (active) substrate heterogeneous blends
4S1
S0
CB
antenna
2 1
semiconductor3
5
VB
October 21, 2004 4
Dynamics of charge carriers and excitons
- e- accelerator - lasers
Radiation pulse
+ e-
e-+
Time-resolved detection
• electronic structure calculations (HF, DFT etc.)• quantum mechanical calcs. on charge and exciton motion• Monte Carlo simulations of hopping transport
Charge carriers• mobility, trapping, recombination• optical abs. spectra
Excitons• dissociation, diffusion and decay• polarizability, opt. abs. spectra
Formation(fsec - nsec)
Detection (microwave, THz, optical)
Theory
October 21, 2004 5
Exciton diffusion in bilayer model systems
N
NN
N
CO2H
HO2C CO2H
CO2HTiO2 porphyrin
+
-
+-
- +
hν
5,10,15,20-tetrakis(4-carboxyphenyl) porphyrin(TPPC)
M
M=H2, Pd
October 21, 2004 6
Simple bilayer dye-sensitized solar cell
+
-
+
-
- +
porphyrinTiO2
+
sunlight
Requirements:• efficient light absorption• large exciton diffusion length• efficient charge separation at interface• escape of charge from recombination• mobile charges
October 21, 2004 7
Relevant processes
4
porphyrin
2 1
TiO2 3
5
1: photo-excitation2: (non)radiative decay3: exciton diffusion and annihilation4: interfacial electron transfer5: interfacial charge recombination
CB
VB
October 21, 2004 8
Bilayer is inefficient due to small exciton diffusion length
+
-
+
-
- +
organic materialTiO2
+
sunlight
light penetration depth Λhν ~ 100 nm
exciton diffusion length ΛE =√(DE*τE) < 10 nm
only small fraction of excitons reaches interface
October 21, 2004 9
In heterogeneous structure ΛE can be short
- +
+-
Disadvantages:• percolation paths needed• recombination losses• exciton quenching on trapped charges
-+ +
-
October 21, 2004 10
Exciton diffusion and charge separation in bilayer model systems: effect of Pd
+
-
+-
- +
porphyrinTiO2
5,10,15,20-tetrakis(4-carboxyphenyl) porphyrin(TPPC)
N
NN
N
CO2H
HO2C CO2H
CO2H
M
M=H2, Pd
3 nslaser pulse
Layer preparation
quartz plate
TiO2 layer (80 nm)by EBE
porphyrin (60 nm) by spin-coating from pyridine/EtOH
October 21, 2004 11
October 21, 2004 12
Absorption spectra
TiO2
porphyrin
excitation
Selective excitation of porphyrin(400-700 nm) or TiO2 (300 nm)
October 21, 2004 13
Time-resolved microwave conductivity measurementsprobe mobile electrons in TiO2
P P-∆P
e-
e-
e-
e-
+
-
+
+
+
+
+
∆σ = A ∆PP
= enµ−
microwaves10 GHz
TiO2 porphyrin
hν
October 21, 2004 14
Measurement cell
sample
microwave
detector
source
quartz window
laser puls:iris
X-band waveguide
2.5 cm
October 21, 2004 15
Porphyrin on quartz: no photoconductivity
λ = 430 nm
H2TPPC only
October 21, 2004 16
TiO2: small short lived photoconductivity
TiO2 only
H2TPPC only
λ = 430 nm
October 21, 2004 17
Porphyrin/TiO2 bilayer: long-lived photoconductivity
TiO2 only
TiO2 / H2TPPCbilayer
H2TPPC only
λ = 430 nm
TiO2/H2TPPCbilayer
+
-
October 21, 2004 18
Porphyrin/TiO2bilayer: wavelength dependence
• No preferential charge separation at specific wavelength
• IPCSE < 0.8 % (Fa=0.6 at 430 nm)
• Exciton diffusion length only 1-2 monolayers of porphyrin
IPCSE = no. of chargesno. of incident photons
Kroeze, J. E.; Savenije, T. J.; Warman, J. M., J. Photochem. Photobiol. A-Chem. 2002, 148, 49-55.
October 21, 2004 19
In H2TPPC photoconductivity results from singlet excitons
1: photo-excitation 2: internal conversion 3: interfacial electron transfer 4: (non)radiative decay 5: intersystem crossing
5
TiO2 porphyrin
CB
VB
14
3 S1
S0
S22
T1
1
2
3
4
-1
-2
-3
0
E (V vs NHE)
(10 ps)
Singlets (τ=11 ns) undergo intersystem crossing to triplets (φ=0.78, τ=0.4 ms)which diffuse via slow Dexter mechanism
October 21, 2004 20
D* A D A*
Singlet:Förster energy transfer 'hν'
D* A D A*
Triplet:Dexter energy transfer
October 21, 2004 21
Mixing of singlet and triplet by spin orbit couling due to heavy metal
Excitons in Pd substituted porphyrin: - long lifetime due to triplet character- diffusion via singlet character
N
NN
N
CO2H
HO2C CO2H
CO2H
Pd
October 21, 2004 22
Pd substitution enhances IPCSE on longer times to 12% due to diffusion of excitons with mixed singlet/triplet character
14
12
10
8
6
4
2
0
IPC
SE
(%)
1ns 100ns 10µs time
H2TPPC
PdTPPCH2TPPC PdTPPC
τS (ns) 11 ~ 10-2
τT (ms) 0.4 0.29
φT 0.78 ~1
kP (s-1) 0.0067 61
E(S0) (eV) -5.5 [-1.1] -5.7 [-1.3]
E(S1) (eV) -3.6 [0.8] -3.5 [0.9]
E(S2) (eV) -2.6 [1.8] -2.7 [1.7]
E(T1) (eV) -4.1 [0.3] -3.9 [0.5]
2.5x1016 photons/m2
Kroeze, J. E.; Savenije, T. J.; Warman, J. M. Adv. Mater. 2002, 14, 1760-1763.
October 21, 2004 23
Enhanced diffusion length in PdTPPC
TiO2 TiO2 Pd porphyrinH2 porphyrin
+-
+
-
+-
- +
+
-
- +
October 21, 2004 24
In PdTPPC diffusion via singlet and triplet
Ψexc = cSΦS + cTΦT
Λ = Dτ
D=δ2 khop =δ2 Ps2kForster + 1− Ps( )kDexter[ ] τ = 1
Pskrad +knrad
Λ =δ2 Ps
2kForster + 1− Ps( )kDexter[ ]Pskrad + knrad
October 21, 2004 25
Effect of laser intensty on conductivity in PdTPPC/TiO2 bilayer
increasing I0
Strong influence of I0 → bimolecular exciton-exciton annihilation
October 21, 2004 26
Monte Carlo computer simulations
injection in TiO2
z
yx
• 3D excitation profile; Lambert-Beer along x
• Exciton mean lifetime τ and diffusion coefficient D
• Excitons peform 3D random walk
• Annihilation if 2 excitonswithin distance Rann
• Electron injection with chance φinj at TiO2 interface
• Output: charge separation efficiency vs time
October 21, 2004 27
Triplet exciton diffusion in PdTPPC simulated
15
10
5
0
IPC
SE
(%)
1ns 10ns 100ns 1µs 10µs
time
2.5 x 1016 m-2
86 x 1016 m-2
20
15
10
5
0
IPC
SE
(%)
1015
1016
1017
1018
1019
I0 (photons/m2/pulse)
20 ns exp 10 µs exp simulation
23 x 1016 m-2
• D=8×10-11 m2/s (5 ns hopping time for d=1.5 nm) , τ ≥ 10 µs• Exciton diffusion length ≥ 28 nm!• φinj = 0.4• Rann = 1.5 nm
Kroeze, J. E.; Savenije, T. J.; Candeias, L. P.; Warman, J. M.; Siebbeles, L. D. A. Solar Energy Mater. Solar Cells 2004, in press.
October 21, 2004 28
D = 8×10-11 m2/s for triplets in PdTPPC
literature:
D = 2.8×10-8 m2/s for singlets in lc porphyrin derivativeD = 10-8 - 10-6 m2/s for singlets in conjugated polymers
D = 10-12 m2/s for triplets in ZnTPPC
October 21, 2004 29
Conclusion
• Singlet-triplet mixing due to heavy atoms enhances exciton diffusion length
• Simple bilayer devices may become competitive with designs based on mesoporous nanocrystalline materials
Acknowledgments
Dr. J.E. Kroeze, Dr. T.J. Savenije and Dr. J.M. Warman
Netherlands Organization for Scientific Research (NWO)
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