Luminescence in Molecules and Crystals .fr

drops from an upper to a lower energy level. RADIATIVE. NON ... ENERGY TRANSFER. FÖRSTER, DEXTER ... only singlets contribute to fluorescence.
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Lecture 4 6.973 Flat Panel Display Devices

Luminescence in Molecules and Crystals • hole – electron generation recombination • exciton formation and recombination .absorption .radiative transitions .non-radiative recombination

6.973 Flat Panel Display Devices – Spring 2002

Lecture 4

Slide 1

Flat Panel Displays

Silicon Graphics 1600SW 6.973 Flat Panel Display Devices – Spring 2002

Lecture 4

Slide 2

1

Flat Panel Display Market 70

(BILLIONS OF DOLLARS)

Billion US Dollars

60 50 40 30 20 10 0

1999

2000

2001

2002

2003

2004

2005

Source: Display Search

$18 Billion annual FPD business today 20 to 30% growth per year $70 Billion business in 2005 6.973 Flat Panel Display Devices – Spring 2002

Lecture 4

Slide 3

The Next $100 Billion Market

Billion US Dollars

$600

Integrated Circuits

$500

Flat Panel Displays

$400 $300 $200 $100 0

1960

1970

1980

1990

Information provided by Lam Research

6.973 Flat Panel Display Devices – Spring 2002

2000

2010

2020 Display Works 97

Lecture 4

Slide 4

2

Question 1: How is the Radiative System Excited ?

Photoluminescence Electroluminescence Cathodoluminescence

(Plasma displays, Backlights) (Inorganic / Organic EL, LEDs) (CRTs, FEDs)

Chemoluminescence Bioluminescence

Question 2: What is the light generation mechanism ? 6.973 Flat Panel Display Devices – Spring 2002

Lecture 4

Slide 5

Radiative Transitions – Efficiency A photon can be emitted when an electron drops from an upper to a lower energy level.

η = PR / (PR+PNR) = (1/tR) / (1/tR + 1/tNR) For high efficiency devices, reduce the probability of non-radiative processes

6.973 Flat Panel Display Devices – Spring 2002

Lecture 4

RADIATIVE

The quantum efficiency of luminescense is then

excited state NON-RADIATIVE

PR = 1/tR – radiative transition probability PNR = 1/tNR – non-radiative transition probability

ground state

Slide 6

3

Electronic Transitions in Molecules MOLECULAR CONFIGURATION ENERGY DIAGRAM OF ELECTRONIC STATES S0 and S1 FOR A DIATOMIC MOLECULE

EA = absorption energy EB = emission energy rO = equilibrium distance ∆r = nuclear displacement

S1

E

a2*

c2*

a1*

dashed lines are vibrational energy levels with vibrational probability functions drawn on top

c1* c0*

a0*

hνB

EB

EA

S0

The energy shift between absorption and emission peaks is known as the Franck-Condon Shift

a2

c2

a1

c1

a0

c0

∆r r0 6.973 Flat Panel Display Devices – Spring 2002

r



Lecture 4

Slide 7

Examples of Molecular Absorption and Luminescence S1

PTCDA Solution in DMSO

3 2 1

Luminescence

0

R0

RELATIVE POSITION

Luminescence and absorption spectra of single molecules in solution show a clear series of vibronic peaks. 6.973 Flat Panel Display Devices – Spring 2002

1.5

2.0

Lecture 4

2.5 Energy [eV]

Absorption

S1 [0-3]

E

S1 [0-1]

PTCDA

S0

S1 [0-2]

0

S1 [0-1]

S1 [0-0] S1 [0-0]

3 2 1

3.0

Slide 8

4

Electronic Processes in Molecules JABLONSKI DIAGRAM

10 ps

ENERGY TRANSFER

ABSORPTION

S1

S: spin=0 (singlet) states T: spin=1 (triplet) states

INTERSYSTEM CROSSING

1-10 ns PHOSPHORESCENCE

FÖRSTER, DEXTER or RADIATIVE

FLUORESCENCE

INTERNAL CONVERSION

Energy

density of available S and T states on surrounding molecules

T1

>100 ns

S0 6.973 Flat Panel Display Devices – Spring 2002

Lecture 4

Slide 9

Combination of two Fermions

S = |↑↑> S = |↓↓> 1 S = 2 (|↑↓> + |↓↑>) S=

1 2

(|↑↓> − |↓↑>)

symmetric states ‘TRIPLETS’

antisymmetric state ‘SINGLETS’

SINGLETS COMPRISE 25% OF EXCITONS

Wavefunction

S describes the spin state of the excited electron

6.973 Flat Panel Display Devices – Spring 2002

Lecture 4

Slide 10

5

Fluorescence E

Phosphorescence

singlet

S1 excited T triplet excited state 1

E

state

S1

T1

FLUORESCENCE

S0

state S0 ground (singlet)

triplet exciton

singlet exciton

• triplet to ground state transition is not permitted

• symmetry conserved fast process ~10-9s 6.973 Flat Panel Display Devices – Spring 2002

PHOSPHORESCENCE

slow process ~ 1s Lecture 4

Slide 11

Generation of Excitons Photo generation

Electrical generation

if molecule absorbs a photon, symmetry of molecule is unchanged

if electrons and holes recombine to form an exciton, their spins are uncorrelated

⇒ only singlets

⇒ singlets and triplets

Why do we care about singlets and triplets? • only singlets contribute to fluorescence • triplets contribute to phosphorescence (low efficiency process) 6.973 Flat Panel Display Devices – Spring 2002

Lecture 4

Slide 12

6

Semiconductors – Energy Bands and Charge Carriers SEMICONDUCTOR is a crystalline or an amorphous solid whose electrical conductivity is typically intermediate between that of a metal or an insulator and can be changed significantly by altering

Motion of electrons in the conduction band (CB) and holes in the valence band (VB) at T>0 K generates electrical current

- Temperature - Impurity content - Light illumination

(from Saleh and Teich) 6.973 Flat Panel Display Devices – Spring 2002

Lecture 4

Slide 13

Semiconductors – Dopant Energy Levels

(from Saleh and Teich) 6.973 Flat Panel Display Devices – Spring 2002

Lecture 4

Slide 14

7

Semiconductors – Energy-Momentum Relations In free space energy E and momentum p of an electron are related by E = p2/2mO = ħ2k2/2mO

indirect band-gap

p – magnitude of momentum k – magnitude of wavevector mO – electron mass In semiconductors the motion of electrons is governed by -Schrödinger equation -the periodic lattice.

direct band-gap

The energy of an electron in CB, therefore, depends on both -the magnitude of its momentum -the direction in which electron is traveling in the crystal 6.973 Flat Panel Display Devices – Spring 2002

(from Saleh and Teich) Lecture 4

Slide 15

Semiconducting Materials

IIIa IVa Va VIa IIb Al

COVALENTLY-BONDED INORGANIC CRYSTALS (and amorphous films) Elemental Semiconductors Si, Ge Binary Semiconductors Formed by combining the elements in groups III and V or groups II and VI

Si

P

S

Zn Ga Ge As Se Cd Hg

In

Sn Sb Te

a section of the PERIODIC TABLE

λ (µm)

Ex: GaAs, InP, ZnS, … Ternary Semiconductors Ex: (AlXGa1-X)As Quaternary Semiconductors Ex: (InXGa1-X)(AsYP1-Y) E (eV) 6.973 Flat Panel Display Devices – Spring 2002

Lecture 4

Slide 16

8

Semiconductors – Carrier Generation Electron and Hole Generation Electrons can be thermal excited (heating) or optically excited (absorb a photon) from ground to excited state. Electrons can undergo: • Band-to-band (Interband) transitions • Impurity-to-Band transitions • Free-Carrier (Intraband) transitions • Phonon Transitions • Exciton Transitions (from Saleh and Teich) 6.973 Flat Panel Display Devices – Spring 2002

Lecture 4

Slide 17

Wannier exciton

Frenkel exciton

(typical of inorganic semiconductors)

(typical of organic materials)

SEMICONDUCTOR PICTURE

MOLECULAR PICTURE

treat excitons as chargeless particles capable of diffusion, also view them as excited states of the molecule

CONDUCTION BAND

S1

S0 VALENCE BAND

GROUND STATE

WANNIER EXCITON

binding energy ~10meV radius ~100Å 6.973 Flat Panel Display Devices – Spring 2002

Lecture 4

GROUND STATE

FRENKEL EXCITON

binding energy ~1eV radius ~10Å Slide 18

9

Semiconductors – Carrier Recombination Electron-Hole Recombination 1. Radiative (photon emitted) 2. Non-Radiative via • transfer of energy to lattice vibrations (phonons) • transfer of energy to another free electron (Auger process) • mid-gap trap levels that originate from impurities, defects due to grain boundaries, dislocations, other lattice imperfections

trap-assisted recombination

(from Saleh and Teich) 6.973 Flat Panel Display Devices – Spring 2002

Lecture 4

Slide 19

Indirect Bandgap – Generation and Recombination Electron and Hole Generation Via Photon Absorption 1. 2.

Photon generates an excited electron and a hole (vertical transition). Electron thermalizes to the bottom of the CB, hole to the top of VB, releasing their energy in the form of phonons.

Electron-Hole Recombination Requires exchange of both energy and momentum. Energy is carried away by a photon, but a phonon must also be present to conserve momentum. Æ This type of multiparticle interaction is UNLIKELY Æ low luminescence efficiency 6.973 Flat Panel Display Devices – Spring 2002

Lecture 4

(from Saleh and Teich) Slide 20

10

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

6.973 Flat Panel Display Devices – Spring 2002

Lecture 4

Slide 21

Lecture 4

Slide 22

Black Body Radiation

6.973 Flat Panel Display Devices – Spring 2002

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