Presentation on theme: "Excitons – Types, Energy Transfer"— Presentation transcript:
1 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, (1995)@ MITFebruary 27, 2003 – Organic Optoelectronics - Lecture 7
2 ExcitonIn 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).
3 Excitons (bound electron-hole pairs) Wannier exciton (typical of inorganicsemiconductors)Frenkel exciton(typical of organicmaterials)treat excitonsas chargelessparticlescapable ofdiffusion,also viewthem asexcited statesof themoleculeMOLECULAR PICTURESEMICONDUCTOR PICTURECharge Transfer (CT)Exciton(typical of organicmaterials)GROUND STATE FRENKEL EXCITONGROUND STATE WANNIER EXCITONbinding energy ~10meVradius ~100Åbinding energy ~1eVradius ~10ÅElectronic Processes in Organic Crystals and Polymers by M. Pope and C.E. Swenberg
4 Wannier-Mott Excitons Columbic interaction betweenthe hole and the electron is given byEEX = -e2/∈rThe exciton energy is thenE = EION – EEX/n2 , n = 1,2, …EION – energy required to ionize the moleculen – exciton energy levelEEX = 13.6 eV μ/m∈μ– reduced mass =memh / (me+mh)Adapted from Electronic Processes in Organic Crystals and Polymers by M. Pope and C.E. Swenberg
5 An Example of Wannier-Mott Excitons exciton progressionfits the expressionν[cm-1] = 17,508 – 800/n2corresponding toμ = 0.7 and ∈ = 10The 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
6 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.
7 Crystalline Organic Films CHARGED CARRIER MOBILITYINCREASES WITH INCREASEDπ−π ORBITAL OVERLAPGOOD CARRIER MOBILITYIN THE STACKING DIRECTIONμ = 0.1 cm2/Vs – stacking directionμ = 10-5 cm2/Vs – in-plane directionHighest mobilities obtained onsingle crystalpentacene μ = 10 5 cm2/Vs at 10Ktetracene μ = 10 4 cm2/Vs at 10K(Schön, et al., Science 2000).
8 PTCDA monolayer on HOPG Organic Semiconducting MaterialsVan der Waals-BONDEDORGANIC CRYSTALS(and amorphous films)PTCDA monolayer on HOPG(STM scan)HOMO of3,4,9,10- perylene tetracarboxylic dianhydride