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