Exciton Diffusion
•Exciton Energy transfer
•Exciton Diffusion
. Photoluminescence measurement
. Photocurrent measurement
Handout on Photocurrent Response:
Bulovic and Forrest., Chemical Physics 210, 13 (1996).
Handout on Solar Cells and Photodetectors:
Peumans, Bulovic, and Forrest., Appl. Phys. Lett. 76, 2650 (2000) & 76, 3855 (2000).
@ MIT
March 4, 2003 – Organic Optoelectronics - Lecture 8
1
Energy Transfer
dopant molecules
(generate luminescence)
host molecules
How does an exciton in the host transfer to the dopant?
Energy transfer processes:
1. Radiative transfer
2. Förster transfer
3. Dexter transfer
2
Radiative energy transfer
Transfer of excitation energy by radiative deactivation of a donor
molecular entity and reabsorption of the emitted light by an acceptor
molecular entity. The probability of transfer is given approximately
by:
Pr, t ∝ [ A]xJ
Where J is the spectral overlap integral, [A] is the concentration of
the acceptor, and x is the specimen thickness. This type of energy
transfer depends on the shape and size of the vessel utilized. Same
as trivial energy transfer.
Quoted from IUPAC Compendium of Chemical Terminology
compiled by Alan D. McNaught and Andrew Wilkinson
(Royal Society of Chemistry, Cambridge, UK).
3
Förster transfer
RO
- resonant dipole-dipole coupling
- donor and acceptor transitions
must be allowed
0Å
0
1
<
Acceptor
(dye)
Donor
very fast <10-9s
Donor*
Acceptor
Donor Acceptor*
typically singlet- singlet transitions
4
Förster transfer - Example
Alq3 LUMO
DCM LUMO
(DONOR)
Alq3
ABSORPTION
200 300
FÖRSTER
ENERGY
TRANSFER
PL
400 500 600 700 800 900
Wavelength [nm]
DCM HOMO
(ACCEPTOR)
DCM
ABSORPTION
Alq3 HOMO
DCM2 in Alq3
EL
Alq3
200
300 400 500 600 700 800
Wavelength [nm]
low DCM2
high DCM2
900
for efficient transfer
donor emission and acceptor
absorption must overlap
5
Förster excitation transfer (dipole-dipole) excitation transfer)
A mechanism of excitation transfer which can occur between molecular
entities separated by distances considerably exceeding the sum of their
van der Waals radii. It is described in terms of an interaction between
the transition dipole moments (a dipolar mechanism). The transfer rate
constant kD → A is given by:
K 2 J 8.8 × 10 −28 mol
kD → A =
n 4ϖr 6
Where K is an orientation factor, n the refractive index of the medium,
ω the radiative lifetime of the donor, r the distance (cm) between
donor (D) and acceptor (A), and J the spectral overlap (in coherent units
cm6 mol-1) between the absorption spectrum of the acceptor and the
florescence spectrum of the donor. The critical quenching radium r0, is
that distance at which kD → A is equal to the inverse of the radiative
lifetime.
Quoted from IUPAC Compendium of Chemical Terminology
compiled by Alan D. McNaught and Andrew Wilkinson
(Royal Society of Chemistry, Cambridge, UK).
6
Förster transfer – Rate Equations
2π 2
( E )dE
P
=
E ) gFAa(E)
dE
KET = (2π/ħ) |<D,A*|HDAda|D*,A>|Z2 ∫ fgdD((E)
h
Energy transfer rate (KET) depends on the overlap integral
FOR DIPOLE-DIPOLE INTERACTION:
6
⎛ 1 ⎞ ⎛ R0 ⎞
K ET ( R ) = ⎜ ⎟ ⎜ ⎟
⎝ ⎠
gA(E)
τ ⎝ R⎠
gD(E)
ALLOWED TRANSITIONS:
+ 1A → 1D + 1A*
1D* + 3A(T ) → 1D + 3A*(T )
n
m
(Adapted from Blasse & Grabmaier)
1D*
gA(E)
7
Dexter transfer
diffusion of excitons from donor to acceptor
‘Wigner-Witmer spin conservation rules’
A+B→C+D
|
|
(SC+SD),(SC+SD-1),....,|SC-SD|
total spin of reactants: (SA+SB),(SA+SB-1),...., SA-SB
total spin of products:
reaction allowed if two sequences have a number in common
only singlet-singlet, triplet-triplet allowed
speed?
Donor*
Acceptor
Donor
Donor
Acceptor*
Acceptor
( eg. phosphorescent
dye )
~ 10Å
8
Dexter excitation transfer (electron exchange excitation transfer)
Excitation transfer occurring as a result of an electron exchange
mechanism. It requires an overlap of the wave functions of the
energy donor and the energy acceptor. It is the dominant mechanism
in triplet-triplet energy transfer. The transfer rate constant, kET, is
given by:
kET ∝ [h (2π )]P 2 J exp[− 2r L]
where r is the distance between donor (D) and acceptor (A), L and P
are constants not easily related to experimentally determinable
quantities, and J is the spectral overlap integral. For this mechanism
the spin conservation rules are obeyed.
Quoted from IUPAC Compendium of Chemical Terminology
compiled by Alan D. McNaught and Andrew Wilkinson
(Royal Society of Chemistry, Cambridge, UK).
9
Nonradiative Energy Transfer
Förster,
Förster Coulombic
Dexter,
Dexter e- exchange
(long range ~30-100 Å)
(short range ~6-20 Å)
D*
A
D*
SINGLET-SINGLET
TRANSFER
A
SINGLET-SINGLET &
TRIPLET-TRIPLET TRANSFER
D
A*
10
Cascade Energy Transfer Laser
Threshold Energy ~ 0.1 µJ/cm2
Absorbance and photoluminescence spectra of
the host material and fluorescent dyes. The
host material is 2-(4-biphenylyl)-5-(4-tbutylphenyl)-1,3,4-oxadiazole(PBD); the dyes
are coumarin 490(C490), DCMII and LDS821. The
spectra for PBD are from a pure film whereas
the other spectra are from solid solutions of the
material in polystyrene.
Normalized absorbance of PBD, and stimulated
emission spectra from this material alone, and when
doped with dyes. Curve:
• (a) PBD only;
• (b) 0.6% C490 in PBD;
• (c) 0.6% C490 + 0.9% DCMII in PBD
• (d) 0.6% C490 + 0.9% DCMII + 0.6% + LDS821 in
PBD.
Adapted from Figure 1 and 2 from Berggren, et al., Nature 389, 466 (1997).
11
UV
Luminescent Organic Film
on a Si p-n Junction
ηi = 25 %
Alq3 on
Si p-n junction
d = 0.52 µm
ηηi ==0.30
± 0.05
i 0.30 ± 0.05
500
Photocurrent [a.u.]
Quantum Efficiency of Photoresponse
calculated
improvement
10.9%
400
4 5
• AR coating in Visible
• Downconverter in UV
0.1
300
Film
p-Si
6
n-Si
Coated
Uncoated
7
3
cone B
2
ηi = 35 %
0.01
1
"Escape Cone"
cone A
1
Visible
600
Wavelength [nm]
Adapted From: Garboze et al, Journal of Applied Physics, vol 80, pg 4644, 1996
Conventional Si Solar Cell
Si Cell coated with
100% PL Efficient Organic
Film
2
1
0
200
400
600
800
1000
Wavelength [nm]
12
Diffusion of Excitons
PL Internal Efficiency [%]
40
If energy transfer occurs
between the donor and
acceptor molecules of the
same species, the term
energy migration is used.
30
Alq3 exciton
diffusion length ~ 10 nm
20
PL efficiency of
Alq 3 on glass
10
0
The four general methods
used to measure diffusion
of excitons are:
bulk quenching,
surface quenching,
bimolecular recombination,
and photoconduction
0.01
0.1
1
Film Thickness [µm]
Alq3
ABSORPTION
200
300
PL
400 500 600 700
Wavelength [nm]
800
900
Small overlap between Alq3
absorption and luminescence results
in a small Förster radius of ~12 Å
Since the size of Alq3 molecules is
~10 Å Förster energy transfer can
facilitate exciton diffusion
Adapted From: Garboze et al, Journal of Applied Physics, vol 80, pg 4644, 1996
13
Photosynthetic Machinery
… of Purple Bacteria
energy transfer
often occurs in biological
systems
exciton transport
followed by
exciton dissociation
(RESULTING IN STABLE
TRANSMEMBRANE
CHARGE SEPARATION)
V. Sundström, et al., J. Phys. Chem. B 103, 2327 (1999).
Light-induced electron transport
ATP synthesis in a photosynthetic
bacterium
http://www.nobel.se/chemistry/
educational/poster/1988/
14
Schematic picture of a photosynthetic reaction
center from the bacterium Rhodopseudomonas virdis.
The polypeptide chains are drawn as ribbons of
different colors for the four different protein
subunits.
The reaction center is composed of four protein subunits. Two of
these, the L and M subunits, each form five membranespanninghelices. The structure shows the precise arrangement in
the L and M subunits of the photochemically active groups – two
chlorophyll molecules forming a dimer, two monomeric
chlorophylls, two pheophytin molecules (these lack the central
magnesium ion of chlorophyll), one quinone molecule, called QA (a
second quinone molecule, QB, is lost during the preparation of the
reaction center) and one iron ion (Fe). The L and M subunits and
their chromophores are related by a twofold symmetry axis that
passes through the chlorophyll dimer and the iron. A third subunit,
H, without active groups and located on the membrane inner
surface, is anchored to the membrane by a protein helix. The
remaining subunit, a cytochrome with four heme groups (related to
the blood pigment hemoglobin), binds at the outer surface of the
membrane.
Photosynthesis and respiration are based on the transfer of
electrons between donor and acceptor molecules bound to
biological membranes – sheet-like structures composed of lipids and
proteins which surround the cells and their inner compartments.
The photosynthetic reactions in plants take place in the inner
membranes of the chloroplasts, the organelles which contain the
chlorophyll. Some bacteria have a simpler form of photosynthesis,
to some extent similar to that in plants but without the ability to
form oxygen.
In all types of photosynthesis, the light energy absorbed by
chlorophyll is transferred to membrane-bound protein-pigment
complexes, known as reaction centers. In these complexes the light
energy initiates electron-transfer reactions which are coupled to
the translocation of hydrogen ions across the membrane. The
resulting pH gradient is utilized by another membrane-bound
protein, ATPase, to synthesize ATP, a compound used as a fuel in
energy-demanding biological processes. In cell respiration, too,
electron transport is coupled to proton translocation and ATP
synthesis.
Image from http://www.bio.anl.gov/
From
http://www.nobel.se/chemistry/educational/poster/1988/
15
Single Layer Organic PV Cells
PTCDA
V
11.96 Å
In
PTCDA
17.34 Å
ITO
Glass
z
3.21Å
y
Incident
Light
x
16
Photocurrent Generation
LIGHT ABSORPTION
EXCITON GENERATION
EXCITON DIFFUSION
EXCITON DISSOCIATION
E
CARRIER SEPARATION
CARRIER TRANSPORT
DELIVERY TO
EXTERNAL CIRCUIT
17
Photocurrent, Photovoltage, Absorption
Photocurrent measured at 0V external bias
20pA
8µV
Photo - I
5
4x10
Photo - V
5
15pA
6µV
10pA
4µV
3x10
α
5
2x10
α [cm-1 ]
Photocurrent / Photovoltage
25pA 10µV
5
5x10
500 nm PTCDA Thin Film
photon flux = 5 x 1014 cm-2 s-1
V
5pA
5
1x10
2µV
In
PTCDA
0A
0V
2.0
2.2
2.4
2.6
Energy [eV]
2.8
3.0
0
ITO
Glass
Incident
Light
18
Photocurrent Dependence on Electric Field
Different photocurrent response for positive and negative applied bias
negative field (In -, ITO +)
positive field (In +, ITO -)
-1
10
-1
V/µm
Yield [%]
Yield [%]
10
1.00
0.40
V/µm
-2
10
-2
10
0.20
0.10
0.04
-3
10
0.01
-3
1.00
0.40
0.10
0.04
0.01
0.00
10
2.0
2.5
Energy [eV]
3.0
2.0
2.5
3.0
Energy [eV]
19