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 1 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 Wannier exciton Excitons Frenkel exciton (typical of inorganic (bound (typical of organic semiconductors) electron-hole materials) pairs)

treat excitons as chargeless particles capable of diffusion,

also view SEMICONDUCTOR PICTURE them as MOLECULAR PICTURE

CONDUCTION excited states BAND of the S1 molecule

S0

VALENCE BAND

GROUND STATE WANNIER EXCITON Charge Transfer (CT) GROUND STATE FRENKEL EXCITON binding energy ~10meV Exciton binding energy ~1eV (typical of organic radius ~100Å materials) radius ~10Å

Electronic Processes in Organic Crystals and Polymers by M. Pope and C.E. Swenberg 3 Wannier-Mott Excitons

n = ∞ ------Columbic interaction between the hole and the electron is given by 2 EEX = -e /∈r

The exciton energy is then

2 E = EION –EEX/n , n = 1,2, …

EION – energy required to ionize the molecule n – exciton energy level

EEX = 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 4 An Example of Wannier-Mott Excitons

exciton progression fits the expression

ν[cm-1] = 17,508 – 800/n2

corresponding to µ = 0.7 and ∈ = 10

The absorption spectrum of Cu2O 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 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 Crystalline Organic Films

PTCDAPTCDA

CHARGED CARRIER MOBILITY INCREASES WITH INCREASED π−π ORBITAL OVERLAP 11.96 Å GOOD CARRIER MOBILITY IN THE STACKING DIRECTION 17.34 Å µ = 0.1 cm2/Vs – stacking direction µ = 10-5 cm2/Vs – in-plane direction

z 3.21Å Highest mobilities obtained on single crystal

pentacene µ = 10 5 cm2/Vs at 10K y tetracene µ = 10 4 cm2/Vs at 10K x (Schön, et al., Science 2000). substrate

7 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 PTCDA Solution (~ 2µM in DMSO) [0-0] [0-0] 1 1 S S [0-1] 1 S CT [0-F] [0-2] 1 S CT [0-ST] Fluorescence

[0-3] Absorption 1 S [0-1] 1 S

1.5 2.0 2.5 3.0 3.5 Energy [eV]

9 PTCDA Thin Film [0-2] [0-1] 1 1 S CT [ST-2] S

Fluorescence [0-0] 1 [0-3] S 1 CT [0-F] S Absorption CT [0-ST] CT [ST-3] CT [ST-1]

1.5 2.0 2.5 3.0 3.5 Energy [eV]

10 PTCDA Solution PTCDA Thin Film

(~ 2µM in DMSO)

0.6 eV Fluorescence Absorption

1.5 2.0 2.5 3.0 3.5 Energy [eV]

11 Solution Absorption

PTCDA in DMSO

6 AGGREGATE 2 µM State AGGREGATE 1.6 µM STATE . . ABSORPTION . 4 increases with PTCDA solution 0.25 µM concentration

Absorption [a.u.] 2

0 1.8 2.0 2.2 2.4 2.6 2.8 3.0 Energy [eV]

12 Absorption of Vibronic Transitions – Change with Solution Concentration

S [0-1]: 2.55 eV 1 r = 0.53 ± 0.02 S [0-0]: 2.39 eV 1 1 r = 0.73 ± 0.02

S [0-2]: 2.74 eV 1 CT [0-ST]: 2.23 eV

Measurement 6 Fit 0.1 r = 1.00 ± 0.02 [k] = 0.8 µM 4

2 Relative Oscillator Strength Absorption [a.u.] 0 r = 1.75 ± 0.02 2.0 2.4 2.8 0.01 Energy [eV]

0.5 1 2 4 PTCDA Concentration in DMSO [ µM]

13 Solution Luminescence

6 4 2 µM 3 . 2 . I ∝ [k]0.65 1 4 [a.u.] Intensity .

Integrated Fluorescence 0 012 0.25 µM Solution Concentration [k] [µ M] 2 Fluorescence [a.u.]

0

1.8 2.0 2.2 2.4 Energy [eV] 14 Monomer and Aggregate Concentration in Solution

[µM]

1 MONOMER m 0.65 [k ] ∝ [k] Monomer and Aggregate Monomer concentrations are derived from Concentration integrated solution [km] fluorescence AGGREGATE efficiency by 0.1 a 1.75 assuming that [k ] ∝ [k] the 2.3 eV solution fluorescence is due to monomers and Aggregate that the most dilute Concentration rate of rise solution primarily [ka] r = 1.75 contains monomers.

0.01 0.5 1 2 [µM] Nominal Solution Concentration [k]

15 PTCDA Electron Energy Structure

S1 S1 2 2 1 1 0 0 CT F CT F CT ST CT ST 1.55 eV 2.74 eV 1.70 eV 2.55 eV 1.85 eV 2.39 eV S0 S0 2.17 eV 3 2.20 eV 3 2.32 eV 2 2.12 eV 2 1 1 0 0

MONOMER THIN FILM MONOMER THIN FILM TRANSITIONS TRANSITIONS TRANSITIONS TRANSITIONS

Absorption Luminescence

16 PTCDA Solution PTCDA Thin Film

(~ 2µM in DMSO)

0.6 eV Fluorescence Absorption

1.5 2.0 2.5 3.0 3.5 Energy [eV]

17 Solution and Thin Film Fluorescence

FLUORESCENCE LIFETIME * Thin film 103 Thin Film fluorescence is 1.0 τ = 10.8 ± 0.5 ns red-shifted by 2 10 0.60 eV from 0.8 solution 1 10 Solution fluorescence Counts Normalized τ = 4.0± 0.5 ns

0.6 0204060 * Minimal Time [ns] fluorescence broadening due 0.4 to aggregation Thin Film

Fluorescence [a.u.] (E + 0.60 eV) 0.2 0.25 µM * Fluorescence 2.0 µM Solution lifetime is longer in thin films 0.0

1.8 2.0 2.2 2.4 Energy [eV]

18 Thin Film Excitation Fluorescence

S1 T1 CT [0-1] [0-3] [0-2] [0-0] 1 1 1 1 S S CT [0-F] CT [0-ST] S S

30

650 nm PTCDA S0 20 Thin Film

* Fluorescence energy and shape is not 10 affected by the change in excitation energy 0

* Fluorescence Integrated Fluorescence [a.u.] efficiency increases when exciting directly 2.0 2.4 2.8 3.2 3.6 into CT state Excitation Energy [eV]

19 Exciton Quantum Confinement in Multi Quantum Wells

So and Forrest, Phys. Rev. Lett. 66, 2649 (1991). Exciton radius = 13 Å Shen and Forrest, Phys. Rev. B 55, 10578 (1997).

mh, ⊥ = 0.18 mo ∆E LUMO 30

mh, ⊥ = 0.16 mo ∆E HOMO 20 d [meV] 1s

E mh, ⊥ = 0.14 mo ∆

10 PTCDA PTCDA PTCDA NTCDA NTCDA

0

10 100 1000 d [Å]

20 (a)Delocalized CT Exciton (b) Localized CT Exciton

Bandwidth >> VPSEUDO Bandwidth << VPSEUDO

ϕ∗ϕ ϕ∗ϕ 1 1

0 0 r r

-1 -1 V [eV] V [eV]

-2 -2

-3 -3 21 Electronic Processes in Molecules

density of available JABLONSKI DIAGRAM S and T states on surrounding molecules 10 ps

INTERSYSTEM FÖRSTER, DEXTER CROSSING or RADIATIVE

ENERGY TRANSFER INTERNAL CONVERSION

Energy 1-10 ns S1 T1

S: spin=0 (singlet) states

ABSORPTION >100 ns

T: spin=1 (triplet) states FLUORESCENCE PHOSPHORESCENCE

S0

22 Effect of Dopants on the Luminescence Spectrum

1.0 N O

0.8 Alq3 NC CN DCM2:Alq3 N 0.6 Al PtOEP:Alq O 3 3

0.4 N N Pt N N

Normalized EL Intensity 0.2

0.0

400 500 600 700 800 Wavelength [nm]

23 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