Excitons Types, Energy Transfer

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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, MIT February 27, 2003 Organic Optoelectronics - Lecture 7 1

2 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 (low 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 Wilkinson (Royal Society of Chemistry, Cambridge, UK). 2

3 Wannier exciton (typical of inorganic semiconductors) SEMICONDUCTOR PICTURE CONDUCTION BAND Excitons (bound electron-hole pairs) treat excitons as chargeless particles capable of diffusion, also view them as excited states of the molecule MOLECULAR PICTURE S 1 Frenkel exciton (typical of organic materials) S 0 VALENCE BAND GROUND STATE WANNIER EXCITON binding energy ~10meV radius ~100Å Charge Transfer (CT) Exciton (typical of organic materials) GROUND STATE FRENKEL EXCITON binding energy ~1eV radius ~10Å Electronic Processes in Organic Crystals and Polymers by M. Pope and C.E. Swenberg 3

4 Wannier-Mott Excitons Columbic interaction between the hole and the electron is given by E EX = -e 2 / r n = The exciton energy is then E = E ION E EX /n 2, n = 1,2, E ION energy required to ionize the molecule n exciton energy level E EX = 13.6 ev µ/m µ reduced mass = m e m h / (m e +m h ) Adapted from Electronic Processes in Organic Crystals and Polymers by M. Pope and C.E. Swenberg 4

5 An Example of Wannier-Mott Excitons exciton progression fits the expression ν[cm-1] = 17, /n 2 corresponding to µ = 0.7 and = 10 The absorption spectrum of Cu 2 O at 77 K, showing the exciton lines corresponding to several values of the 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 5

6 Charge Transfer Excitons The lowest CT exciton state in the ab plane of an anthracene crystal with two inequivalent molecules per unit cell; the 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. 6

7 Crystalline Organic Films PTCDA PTCDA CHARGED CARRIER MOBILITY INCREASES WITH INCREASED π π ORBITAL OVERLAP GOOD CARRIER MOBILITY IN THE STACKING DIRECTION Å µ = 0.1 cm 2 /Vs stacking direction µ = 10-5 cm 2 /Vs in-plane direction Å Highest mobilities obtained on single crystal pentacene µ = 10 5 cm 2 /Vs at 10K tetracene µ = 10 4 cm 2 /Vs at 10K (Schön, et al., Science 2000). z x y substrate 3.21Å 7

8 Organic Semiconducting Materials Van der Waals-BONDED ORGANIC CRYSTALS (and amorphous films) PTCDA monolayer on HOPG (STM scan) HOMO of 3,4,9,10- perylene tetracarboxylic dianhydride 8

9 PTCDA Solution (~ 2µM in DMSO) Fluorescence Absorption Energy [ev] 9 S1 [0-3] CT [0-ST] S1 [0-1] CT [0-F] S1 [0-0] S1 [0-0] S1 [0-2] S1 [0-1]

10 PTCDA Thin Film Fluorescence Absorption CT [ST-3] CT [ST-1] CT [0-ST] CT [ST-2] CT [0-F] S1 [0-0] S1 [0-1] S1 [0-2] S1 [0-3] Energy [ev]

11 PTCDA Solution PTCDA Thin Film (~ 2µM in DMSO) 0.6 ev Fluorescence Absorption Energy [ev] 11

12 Solution Absorption PTCDA in DMSO Absorption [a.u.] AGGREGATE State 2 µm 1.6 µm µm AGGREGATE STATE ABSORPTION increases with PTCDA solution concentration Energy [ev] 12

13 Absorption of Vibronic Transitions Change with Solution Concentration S 1 [0-1]: 2.55 ev Relative Oscillator Strength r = 0.53 ± 0.02 r = 0.73 ± r = 1.00 ± 0.02 r = 1.75 ± 0.02 Absorption [a.u.] S 1 [0-0]: 2.39 ev S 1 [0-2]: 2.74 ev CT [0-ST]: 2.23 ev Measurement Fit [k] = 0.8 µm Energy [ev] PTCDA Concentration in DMSO [ µm] 13

14 Solution Luminescence I [k] µm µm Fluorescence [a.u.] Integrated Fluorescence Intensity [a.u.] Solution Concentration [k] [µ M] Energy [ev]

15 Monomer and Aggregate Concentration in Solution [µm] 1 Monomer Concentration [k m ] 0.1 Aggregate Concentration [k a ] MONOMER [k m ] [k] 0.65 AGGREGATE [k a ] [k] 1.75 rate of rise r = 1.75 Monomer and Aggregate concentrations are derived from integrated solution fluorescence efficiency by assuming that the 2.3 ev solution fluorescence is due to monomers and that the most dilute solution primarily contains monomers Nominal Solution Concentration [k] 2 [µm] 15

16 PTCDA Electron Energy Structure S 1 S CT F CT ST CT F CT ST ev 2.55 ev 2.39 ev 2.20 ev 2.12 ev S ev 2.17 ev 1.55 ev 1.70 ev 1.85 ev S 0 MONOMER TRANSITIONS THIN FILM TRANSITIONS MONOMER TRANSITIONS THIN FILM TRANSITIONS Absorption Luminescence 16

17 PTCDA Solution PTCDA Thin Film (~ 2µM in DMSO) 0.6 ev Fluorescence Absorption Energy [ev] 17

18 Solution and Thin Film Fluorescence * Thin film fluorescence is red-shifted by 0.60 ev from solution fluorescence * Minimal fluorescence broadening due to aggregation * Fluorescence lifetime is longer in thin films Fluorescence [a.u.] Normalized Counts FLUORESCENCE LIFETIME Solution τ = 4.0 ± 0.5 ns Thin Film (E ev) Thin Film τ = 10.8 ± 0.5 ns Time [ns] 0.25 µm 2.0 µm Solution Energy [ev] 18

19 Thin Film Excitation Fluorescence S 1 CT T 1 CT [0-ST] CT [0-F] S 1 [0-0] S 1 [0-1] S 1 [0-2] S 1 [0-3] S 0 * Fluorescence energy and shape is not affected by the change in excitation energy * Fluorescence efficiency increases when exciting directly into CT state Integrated Fluorescence [a.u.] nm PTCDA Thin Film Excitation Energy [ev] 19

20 Exciton Quantum Confinement in Multi Quantum Wells So and Forrest, Phys. Rev. Lett. 66, 2649 (1991). Shen and Forrest, Phys. Rev. B 55, (1997). Exciton radius = 13 Å 30 m h, = 0.18 m o E LUMO E 1s [mev] 20 m h, = 0.16 m o m h, = 0.14 m o E HOMO d 10 PTCDA NTCDA PTCDA NTCDA PTCDA d [Å] 20

21 (a) Delocalized CT Exciton (b) Localized CT Exciton Bandwidth >> V PSEUDO Bandwidth << V PSEUDO 1 ϕ ϕ 1 ϕ ϕ 0 r 0 r V [ev] -1 V [ev]

22 Electronic Processes in Molecules density of available S and T states on surrounding molecules 10 ps JABLONSKI DIAGRAM Energy FÖRSTER, DEXTER or RADIATIVE ENERGY TRANSFER INTERNAL CONVERSION INTERSYSTEM CROSSING 1-10 ns S: spin=0 (singlet) states T: spin=1 (triplet) states S 1 ABSORPTION FLUORESCENCE PHOSPHORESCENCE T 1 >100 ns S 0 22

23 Effect of Dopants on the Luminescence Spectrum 1.0 N O Normalized EL Intensity Al N O Alq 3 3 NC DCM2:Alq 3 PtOEP:Alq 3 N N Pt N N CN Wavelength [nm] 23

24 Nonradiative Energy Transfer dopant molecules (generate luminescence) host molecules How does an exciton in the host transfer to the dopant? Energy transfer processes: 1. Radiative transfer 2. Förster transfer 3. Dexter transfer 24