Slide 1Milano, November 15-18th, 2011
FREE holes FREE electrons
= A/I
Other ways of generating carriers in organic semiconductors: -Injection from the electrodes -Doping -Thermal excitation
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SENSORISTICS
Photogeneration: a multistep process
Energy barriers at the electrodes
Excitons recombination
Beer-Lambert law:
Direct gap!
Light absorption:
a strength point of organics •Reflection losses:
We want low (n1-n2) nair=1 nglass~1.5 nSi=3.2 no~1.7
Localized states good wavefuntion overlap between initial (ground) state and final (excited) state
(normal incidence)
Conduction band
Valence band
Eg
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4
n =1 n =2
Exciton Binding Energy (EB)~10meV Exciton Radius ~100Å kT(@300K)~25meV
Thermal dissociation of the exciton into free charges
Exciton generation upon light absorption
H. Pope and C. E. Swenberg, Electronic Processes in Organic Crystals
and Polymers, 2nd ed. (Oxford, Univ. Press, 1999).
Conduction band
n =1 n =2
H. Pope and C. E. Swenberg, Electronic Processes in Organic Crystals
and Polymers, 2nd ed. (Oxford, Univ. Press, 1999).
Valence band
ORGANIC SEMICONDUCTORS:
Exciton generation upon light absorption
4
e- e h+ ( mrid, )
strongly bound exciton? Internal Conversion (IC): thermalization towards S1 (energy transferred to vibrational levels)
Molecular picture
Very efficient
Inefficient
Inefficient
Intersystem Crossing (ISC): singlet Sn converted into triplet Tn. Tn thermalizes to T1.
Autoionization (AI): competitive with IC. The “hot” electron is transferred to an adjacent molecule.
Requires wavefunction overlap of different sites
Thermalization
Dissociation
Thermalization length (Lth): spatial distance covered by the electron during thermalization
Charge Transfer State (CT)
Recombination
strongly bound exciton?
Lth
or... another kind of “extra-energy” is needed! ~0.5÷1eV

strongly bound exciton?
hole actractive potential
How to obtain free charges from a
strongly bound exciton? a) Excess energy from the incoming photon
Lth
or... another kind of “extra-energy” is needed!
qE
strongly bound exciton? J. Appl. Phys. 106, 104507, 2009.
f (r, ) = [exp(-rc / r)][1+(rc / r )B]
rc=e2 / (4 0kT)
B = (erF)(1+cos )/(2kT)
Field dependence of the ionization rate:
r=initial pair separation distance θ=angle between the radius vector and the applied electric field vector
F
r θ
Onsager-Braun model, refined by Wojcik and Tachiya (M. Wojcik and M. Tachiya, J. Chem. Phys. 130, 104107 (2009))
L. Onsager, J. Chem. Phys. 2 (1934) 599
C. L. Braun, J. Chem. Phys. 80, 4157, (1984).
Onsager-Braun model
c) Energy offset between LUMO(HOMO) levels in adjacent molecules
How to obtain free charges from a
strongly bound exciton? a) Excess energy from the incoming photon
Lth
or... another kind of “extra-energy” is needed!

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donor/acceptor interfaces
Brabec et al., Chem. Phys. Lett. 2001, 340, 232; R.H. Friend et al., Nature, 376, 498, 1995; A.J. Heeger et al., Science, 270, 1789, 1995.
• Energy requirement: E > EB hundreds meV
• Spatial requirement: excitons can travel around, moving from molecule to molecule, until either relaxation or dissociation occurs excitons must “survive” until the interface with an acceptor molecule is reached...
• Time requirement: KCT>>KREC (ps); KCT>>KCT,BACK (ms)
KCT,BACK KREC
Radiative Non-radiative
Simoultaneous double charge transition
A nearest neighbor phenomenon <20Å
Long range <10nm
Resonant dipole-dipole coupling
Exciton diffusion length (LD)<10nm
Spectral overlap between A and B required
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(a)
(b)
BULK-HETEROJUNCTION
(a)
(b)
BULK-HETEROJUNCTION
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(a)
(b)
...or not?
EXCIPLEX: hole and electron still bound by Coulomb interaction after Charge Transfer
Charges separation activation energy ~ 0.1eV Lower than what expected! ???
Screening due to dipole formation at the donor/acceptor interface
Arkhipov et al., 2003.
(a)
(b)
BULK-HETEROJUNCTION
...but poor charge transport to the electrodes!
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PL quenching
Photocurrent onset
Strong PL quenching Exciton splitting efficiency (or energy transfer?)
D/A heterojunction based active materials
A. Aharony, D. Stauffer, Introduction to Percolation Theory, Taylor & Francis, London and New York, 2nd edition, 1992.
Charges percolation
Charge separation
Charge transport
p-type, DONORS
n-type, ACCEPTORS
J. Mater. Res., Vol. 19, No. 7, Jul 2004 H. Hoppe and N. Serdar Sariciftci,
Other dissociation interfaces
• Metal-semiconductor interface at the electrodes: one charge of the couple is “captured” by the metal electrode, the other one is released inside the semiconductor where it can participate to the current flow. (C.J. Brabec and N.S. Sariciftci, Semiconducting polymers: Chemistry, physics and engineering, ch. 15, pp. 515–560, Wiley-VCH, 2000)
• Chemical defects, dopants, surface states
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• Charge Transport
To the electrodes... D. Natali
3. Trap-mediated at the D/A interface: due to the presence of foreign atoms or structural defects, introducing energy levels within the bandgap ET
LUMO
HOMO
R.A. Street, Phys. Review B 81, 205307 (2010)
1. Auger: of an exciton with a free hole or electron
but exciton lifetime ~10 100fs
Auger very unlikely to occur
rc LUMO
HOMO
4. Reverse diffusive at the contacts: free carriers diffuse against the electric field and recombine at the metal contacts
Investigating recombination losses Example. P. Schilinsky et al., Appl. Phys. Lett. 81, 20, 2002.
P3HT:PCBM bulk heterojunction
Space-charge effects: accumulation of slow carriers (either holes or electrons) in case of unbalanced mobilities μh, μe.
PI ph
Light intensity dependence of photogenerated current in case of bimolecular recombination:
Negligible bimolecular recombination (Non-geminate of CT-excitons)
... other phenomena might be playing a role!
What if <1?
Non-blocking contacts are required
but... the choice of the metal contacts is driven by other requirements: leakage current, built-in electric field, availability and cost, stability,… life at metal/semiconductor interface is far more complicated...
D. Natali
GENERAL REQUIREMENT:
nc/s