Excitons – Types, Energy Transfer Wannier exciton Charge-transfer exciton Frenkel exciton Exciton...
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Transcript of Excitons – Types, Energy Transfer Wannier exciton Charge-transfer exciton Frenkel exciton Exciton...
Excitons – Types, Energy Transfer
•Wannier exciton•Charge-transfer exciton•Frenkel exciton
•Exciton Diffusion•Exciton Energy Transfer (Förster, Dexter)
Handout (for Recitation Discusssion):J.-S. Yang and T.M. Swager, J. Am. Chem. Soc. 120, 5321 (1998)Q. Zhou and T.M. Swager, J. Am. Chem. Soc. 117, 12593 (1995)
@ MIT February 27, 2003 – Organic Optoelectronics - Lecture 7
Exciton
In some applications it is useful to consider electronic excitation as if aquasi-principle, capable of migrating, were involved. This is termed asexciton. In organic materials two models are used: the band or wave model(low temperature, high crystalline order) and the hopping model (highertemperature, low crystalline order or amorphous state). Energy transfer inthe hopping limit is identical with energy migration.
Caption from IUPAC Compendium of Chemical Terminologycompiled by Alan D. McNaught and Andrew Wilkinson(Royal Society of Chemistry, Cambridge, UK).
Wannier exciton(typical of inorganic
semiconductors)
Frenkel exciton(typical of organic
materials)
Excitons(bound
electron-holepairs)
SEMICONDUCTOR PICTURE MOLECULAR PICTURE
treat excitonsas chargeless
particlescapable ofdiffusion,
also viewthem as
excited statesof the
molecule
GROUND STATE WANNIER EXCITONGROUND STATE FRENKEL EXCITON
binding energy ~10meVradius ~100Å
binding energy ~1eVradius ~10Å
Electronic Processes in Organic Crystals and Polymers by M. Pope and C.E. Swenberg
Charge Transfer (CT)Exciton
(typical of organicmaterials)
Wannier-Mott Excitons
Columbic interaction betweenthe hole and the electron is given by
EEX = -e2/∈r
The exciton energy is then
E = EION – EEX/n2 , n = 1,2, …
EION – energy required to ionize the moleculen – exciton energy level
EEX = 13.6 eV μ/m∈
μ– reduced mass =memh / (me+mh)
n = ∞ ----------
Adapted from Electronic Processes in Organic Crystals and Polymers by M. Pope and C.E. Swenberg
An Example of Wannier-Mott Excitons
exciton progressionfits the expression
ν[cm-1] = 17,508 – 800/n2
corresponding toμ = 0.7 and = 10∈
The absorption spectrum of Cu2Oat 77 K, showing the exciton linescorresponding to several valuesof the quantum number n. (FromBaumeister 1961).
Quoted from Figure I.D.28.Electronic Processes in OrganicCrystals and Polymers by M. Popeand C.E. Swenberg
Charge Transfer Excitons
The lowest CT exciton statein the ab plane of ananthracene crystal with twoinequivalent molecules perunit cell; the plus and minussigns refer to the center ofgravity of charge distribution.The Frenkel exciton obtainswhen both (+) and (–) occupyessentially the samemolecular site.
Crystalline Organic Films
CHARGED CARRIER MOBILITYINCREASES WITH INCREASED
π−π ORBITAL OVERLAP
GOOD CARRIER MOBILITYIN THE STACKING DIRECTION
μ = 0.1 cm2/Vs – stacking directionμ = 10-5 cm2/Vs – in-plane direction
Highest mobilities obtained onsingle crystal
pentacene μ = 10 5 cm2/Vs at 10K tetracene μ = 10 4 cm2/Vs at 10K
(Schön, et al., Science 2000).
Organic Semiconducting Materials
Van der Waals-BONDEDORGANIC CRYSTALS(and amorphous films)
PTCDA monolayer on HOPG(STM scan)
HOMO of3,4,9,10- perylene tetracarboxylic dianhydride
PTCDA Solution (~ 2μM in DMSO)
PTCDA Thin Film
PTCDA Solution
(~ 2μM in DMSO)
PTCDA Thin Film
(~ 2μM in DMSO)
Solution Absorption
Absorption of Vibronic Transitions – Change with Solution Concentration
Solution Luminescence
Monomer and Aggregate Concentration in Solution
PTCDA Electron Energy Structure
Absorption Luminescence
PTCDA Solution
(~ 2μM in DMSO)
PTCDA Thin Film
(~ 2μM in DMSO)
Solution and Thin Film Fluorescence
* Thin filmfluorescence isred-shifted by0.60 eV fromsolutionFluorescence
* Minimalfluorescencebroadening dueto aggregation
* Fluorescencelifetime is longerin thin films
Thin Film Excitation Fluorescence
* Fluorescence energyand shape is notaffected by the changein excitation energy
* Fluorescenceefficiency increaseswhen exciting directlyinto CT state
Exciton Quantum Confinement in Multi Quantum Wells
So and Forrest, Phys. Rev. Lett. 66, 2649 (1991).Shen and Forrest, Phys. Rev. B 55, 10578 (1997). Exciton radius = 13 Å
Delocalized CT Exciton(a) (b) Localized CT Exciton
Electronic Processes in Molecules
Effect of Dopants on the Luminescence Spectrum
Nonradiative Energy Transfer
How does an exciton in the host transfer to the dopant?
Energy transfer processes:
1. Radiative transfer
2. Förster transfer
3. Dexter transfer