(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
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
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
Journal of Crystal Growth 232 (2001) 376â386 ... monoclinic crystal structure (form B) was solved by molecular replacement using the orthorhombic crystal ... Here we present the first data on this chemistry- ... Catalytic mechanisms for the.
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An equivalent argument was followed by ReinRP: let Up be the mirror reM flexion (parity) operator which transforms leftMhanded into rightMhanded states:.
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