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1、Excitons Types, Energy Transfer,Wannier exciton Charge-transfer exciton Frenkel exciton Exciton Diffusion Exciton Energy Transfer (Frster, 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, 1259
2、3 (1995), MIT,February 27, 2003 Organic Optoelectronics - Lecture 7,Exciton,In some applications it is useful to consider electronic excitation as if a quasi-principle, capable of migrating, were involved. This is termed as exciton. In organic materials two models are used: the band or wave model (l
3、ow temperature, high crystalline order) and the hopping model (higher temperature, low crystalline order or amorphous state). Energy transfer in the hopping limit is identical with energy migration.,Caption from IUPAC Compendium of Chemical Terminology compiled by Alan D. McNaught and Andrew Wilkins
4、on (Royal Society of Chemistry, Cambridge, UK).,Wannier exciton (typical of inorganic semiconductors),Frenkel exciton (typical of organic materials),Excitons (bound electron-hole pairs),SEMICONDUCTOR PICTURE,MOLECULAR PICTURE,treat excitons as chargeless particles capable of diffusion, also view the
5、m as excited states of the molecule,GROUND STATE WANNIER EXCITON,GROUND STATE FRENKEL EXCITON,binding energy 10meV radius 100,binding energy 1eV radius 10,Electronic Processes in Organic Crystals and Polymers by M. Pope and C.E. Swenberg,Charge Transfer (CT) Exciton (typical of organic materials),Wa
6、nnier-Mott Excitons,Columbic interaction between the 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 molecule n exciton energy level EEX = 13.6 eV /m reduced mass =memh / (me+mh),n = -,Adapted from Electronic Proc
7、esses in Organic Crystals and Polymers by M. Pope and C.E. Swenberg,An Example of Wannier-Mott Excitons,exciton progression fits the expression cm-1 = 17,508 800/n2 corresponding to = 0.7 and = 10,The absorption spectrum of Cu2O at 77 K, showing the exciton lines corresponding to several values of t
8、he quantum number n. (From Baumeister 1961).,Quoted from Figure I.D.28. Electronic Processes in Organic Crystals and Polymers by M. Pope and C.E. Swenberg,Charge Transfer Excitons,The lowest CT exciton state in the ab plane of an anthracene crystal with two inequivalent molecules per unit cell; the
9、plus and minus signs refer to the center of gravity of charge distribution. The Frenkel exciton obtains when both (+) and () occupy essentially the same molecular site.,Crystalline Organic Films,CHARGED CARRIER MOBILITY INCREASES WITH INCREASED ORBITAL OVERLAP,GOOD CARRIER MOBILITY IN THE STACKING D
10、IRECTION, = 0.1 cm2/Vs stacking direction = 10-5 cm2/Vs in-plane direction,Highest mobilities obtained on single crystal,pentacene = 10 5 cm2/Vs at 10K tetracene = 10 4 cm2/Vs at 10K,(Schn, et al., Science 2000).,Organic Semiconducting Materials,Van der Waals-BONDED ORGANIC CRYSTALS (and amorphous f
11、ilms),PTCDA monolayer on HOPG (STM scan),HOMO of 3,4,9,10- perylene tetracarboxylic dianhydride,PTCDA Solution ( 2M in DMSO),PTCDA Thin Film,PTCDA Solution,( 2M in DMSO),PTCDA Thin Film,( 2M in DMSO),Solution Absorption,Absorption of Vibronic Transitions Change with Solution Concentration,Solution L
12、uminescence,Monomer and Aggregate Concentration in Solution,PTCDA Electron Energy Structure,Absorption,Luminescence,PTCDA Solution,( 2M in DMSO),PTCDA Thin Film,( 2M in DMSO),Solution and Thin Film Fluorescence,* Thin film fluorescence is red-shifted by 0.60 eV from solution Fluorescence * Minimal f
13、luorescence broadening due to aggregation * Fluorescence lifetime is longer in thin films,Thin Film Excitation Fluorescence,* Fluorescence energy and shape is not affected by the change in excitation energy * Fluorescence efficiency increases when exciting directly into CT state,Exciton Quantum Conf
14、inement 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 ,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:,Radiative transfer 2. Frster transfer 3. Dexter transfer,