Nir Tessler
Microelectronic & Nanoelectronic centersElectrical Enginnering Dept.
Technion, Israel Institute of TechnologyHaifa, Israel
www.ee.technion.ac.il/nir
The Quest for Electrically Pumped Lasers
Introduction
Some of the problems
One of the ways to approach the problems
Outline
Lasers - Schawllow&Towns 1958
Organic Molecules Lasers - In Solution (Lempicki,1962) Fibre Laser (RCA, 1963) In a Matrix Energy Transfer (Morantz,1962) Triplet Laser (reported but….)Photonic Structures DBR + DFB (Kogelnik,1971) Whispering Gallery (Kuwatagonokami,1992) Conjugated Polymer Lasers &Small molecule based lasers
The issue of electrically pumped organic laser is now relevant
Historical Perspective
These materials can now be taken seriously for demanding applications
PPV
450 500 550 600 650 700 750
PL (a
.u.)
Wavelength (nm)
n
0
0.5
1
1.5
2
2.5
200 250 300 350 400 450 500 550
Abs
orpt
ion
(OD
)
Wavelength (nm)
• Stoke Shift• 4 level system (not always true)
The ”original” motivation
Technological Advantages of “Plastic” Lasers
Gain and Glueproperties
Wavelength tuning through bending
Not sensitive toSurface recombination2D Bandgap
Stamp
There is a great potential
So how come we can’t make it happen
Or at least prove that it did happen
Device structure
Material
Light - Amplifier
Mirror 1 Mirror 2
Input Power
OpticalAmplifier
OutputNoiseSource
+ X
OpticalFeedback
The most Common Laser
We are interested in molecular materials
Similar to quantum confinement based lasers
P+InGaAS
P-InP
InGaAsP
InGaAs
InGaAsP
N-InP
N-InP, Substrate
E
QW
MQW Laser Structure
N. Tessler et. al.JQE, 1993
IElectrons
IHoles
Stimulated Emission
CaptureTransport
N
P
IElectrons
IHoles
Stimulated Emission
CaptureTransport
N
P
3D
2D
Quantum Well Lasers
Many issues had to be optimized
Most of them – material related!
InGaAsP
InGaAs
Ielectrons IlHoles
Optical M
ode
Gain and Absorption In PPV
Not 4 Level System
No net Gain (with Current Drive)
Abs
orpt
ion/
Gai
n (c
m-1)
Wavelength (nm)
10
100
1000
10 4
10 5
10 6
300 400 500 600 700 800 900 1000
Absorption
ExcitonicGain
Charge InducedAbsorption
Charge absorption is plotted forExcited State Density = 1018cm-3
Charge absorption is “band to band” High cross section
T
TT
S
SS
CC
NNBdtdN
NBdtdN
NBJdtdN
2C
2C
2
43
N 41
9
3 2 1 1
18 3
10
10 ;
10
S
C
Sec
cm V Sec
N cm
Charge
SingletExciton
TripletExciton
Rate Equations
1.0N 41
SC
C
S BNN
Exciton Generation = Bottleneck
Exciton Generation
How to Enhance the Probability
1. Material with high mobility (crystals looked promising)
2. Material with low charge induced absorption
Synthesis of PolyarylaminesYamamoto Method
N
Cl Cl Cl
N
n
R1R2
R2
R1
R2+[Ni] catalyst
N
**n
R
Vary R group to optimise charge mobility
0
0.2
0.4
0.6
0.8
1
0 20 40 60 80 100
Ele
ctro
lum
ines
cenc
e (a
.u.)
Time (ns)
0
0.2
0.4
0.6
0.8
1
1.2
5 10 15 20 25
Ele
ctro
lum
ines
cenc
e (a
.u.)
Time (ns)
Fast Switching
This initial set of devices & materials requires above 20V to achieve rise time of less then 10ns. (new materials have much better mobility)
Even if we won’t make electrically pumped laser we have made the basic unit for 100MHz (500MHz) data link.
How to Enhance the Probability
1. Material with high mobility (crystals looked promising)
2. Material with low charge induced absorption
Two-Dimensional Electronic Excitations
R. Osterbacka, et. al.SCIENCE VOL 287p.839
Charge induced absorption band at the visible is reduced when chains are coupled
Are there other structural effects that can move the charge absorption oscillator strength away from the emission band?
Conduction
Valence
Split-off
Low bandgap Inorganics
Inter Valence-bandAbsorption
Conduction
Valence
Split-off
Introduce strainProblem
Anything to learn from inorganic lasers?
Hole - Polaron Exciton - Polaron
HOMO
LUMO
HOMO
LUMO
The Organic equivalent
Is there an alternative solution?
Charge absorption covers visible range and up to 1m can we take the emission band beyond 1m?
OK – Lets mix
5 nm
PbSe
20nm
InAs/ZnSe
nMeO
O
nMeO
O
Conjugatedpolymers
ZnSe
0.99
1.26
CdSe
InAs
Shell
0.46
V
Eg0.92
0.8 1.2 1.6 2.0 2.4
Absorbance intensity (a.u.)
x4
Photon Energy (eV)
Q.Y
15
20
13
0.9
~0.7 ML~1.3 ML~2.2 ML
InAs/ZnSe
1st stage = Optimising the NC Emission yield
20% PL Yield in Solution(toluene)
Best PL for Shell Thickness
between 1 and 2 monolayer
U. Banin, Hebrew University,Jerusalem
1000 1200 1400 1600 1800Wavelength (nm)
1000 1200 1400 1600 1800
Lum
ines
cenc
e (a
.u.)
Wavelength (nm)
Size A Size B Size C
InAs PbSe
>10% PL Efficiency in Solid Films
Glass
Ca\Al (cathode)
PEDOT/ITO (Anode)
nMeO
O
nMeO
O
Polymer
nanocrystal
V-
+
Current/Energy is first injected into the polymer
Energy/ChargeTransfer to the nanocrystal
Light Emission
What do we hope to achieve by mixing
0
0.2
0.4
0.6
0.8
1
400 500 600 700 800 900
Abs
orpt
ion
(OD
)
Wavelength (nm)
50V%PPV 50V%NC
NC in Polystyrene (80V%)
Polymer absorbs 70% @ ~450nm Polymer contributes <20% Energy to NC
Energy Transfer Polymer NC is Negligible
400 450 500 550 600 650 700 750 800
Em
issi
on a
t 155
0nm
(a.u
.)
Excitation Wavelength (nm)
NC in Polystyrene
20v%NC 80v%PPV
50v%NC 50v%PPV
What is the transfer mechanism?
Energy Transfer
?
Charge Transfer(trapping)
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
0 5 10 15 20 25
Cur
rent
(mA
)
Applied Voltage (V)
0
0.5
1
1.5
2
2.5
3
0 5 10 15 20 25Ele
ctro
lum
ines
cenc
e (a
.u.)
Applied Voltage (V)
Increase NC Content Increase NC Content
Experimental Efficiency-Optimization
Trapping in the governing excitation mechanism
10% 20% 33% 50%
Efficien
cy
Very High Loading Probably due to partial aggregation
Tessler et. al., Science, 2002~1% EL External-Efficiency
20nm
PPV“pin-hole”
TEM Top View of =1500nm NC in PPV
“Good” Surface Coverage
Y. Talmon
Experimental
(30v% NC)
Partial segregation
V
Optimization Requires Dedicated Modeling
V
2D Mesh with Traps (NCs)Randomly Positioned at a given density(trap depth = 0.4eV)
5V% NC
Cha
rge
Den
sity
(1018
cm-3)
Distance From Contact (nm)
V
Non-CompleteTrapping5% Loading
NC near contactSuppressInjection
The effect of trapped charges
See also A. Shik et. al.Solid. State Elect.,
46, 61,2002
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
0 2 4 6 8 10 12
Cur
rent
(mA
)
Voltage (V)
No NC 10% NC - HOMO offset=0.3eV
10% NC , offset+0.1eV Measurement
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
0 2 4 6 8 10 12
Cur
rent
(mA
)
Applied Voltage (V)
No NC10% NC
30% NC
20% NC
10% NC , offset+0.2eV
Simulation
HOMO offest ~0.3eV
Let Us Assume someone will solve all material issues
Related to Lasers
Glass
n1=1.7
n2=2
n3=1.7
Metal2
Metal1
x3
OpticalMode (Core)
(Cladding)
x1 (Cladding)
x2
Glass
n1=1.7
n2=2
n3=1.7
Metal2
Metal1
x3
OpticalMode (Core)
(Cladding)
x1 (Cladding)
x2
The structure
1
10
100
1000
0 50 100 150 200 250 300 350 400Cladding Thickness (nm)
Prop
agat
ion
Loss
(cm
-1)
Al
Ag
Consider more sophisticated structures
• Light emitting FET? (there is a talk later)
0
0.2
0.4
0.6
0.8
1
1.2
500 520 540 560 580
Wavelength (nm)
Elec
trol
umin
esce
nce
(a.u
.)
0.5-10s50s
TPPV TCTCT THS
+-
RPPV
CPPV
RCTCT
CCTCT
P THS
RIFC
Current Heating Effects
Chemistry/Materials
Device Modeling
Device Design & measure
Analysis and extraction ofproperties
New Functionalities Novel Materials
Phil MackieCupertino Domenico
Aveciapolymers
Y. Talmon TEMChem. Eng. Technion
Uri BaninChem. Hebrew U.
NC
Israel Science Foundation
European Union FW-5$
Vlad MedvedevYevgeni PreezantYohai RoichmanNoam RapaportOlga SolomeshchAlexey RazinYair GanotSagi Shaked
EE Technion
Absorption spectrum of the blends
oc10
OMe
n
oc4
oc4
*
*o
n=o=0.5
0
0.2
0.4
0.6
0.8
1
400 600 800 1000 1200 1400 1600
Abs
orpt
ion
(OD
)
Wavelength (nm)
PPV Derivative
50V%PPV 50V%NC
NC in Polystyrene (80V%)
x10
0
0.2
0.4
0.6
0.8
1
400 600 800 1000 1200 1400 1600
Abs
orpt
ion
(OD
)
Wavelength (nm)
PPV Derivative
50V%PPV 50V%NC
NC in Polystyrene (80V%)
x10
1.85
1.852
1.854
1.856
1.858
1.86
1.862
1.864
1.866
0.096
0.098
0.1
0.102
0.104
0.106
0.108
0.11
10 20 30 40 50 60 70 80
Temperature (c)
Peak
Ene
rgy
(m
-1)
Peak
Wid
th (
m-1)
Electrical Pulse Set-Up
Pulse Generator
150-200ns
45HzV
AC Current Probe
Si Photo DiodeFast APD
TemperatureControl (-170oc,70oc)
Laser Diode
0
0.2
0.4
0.6
0.8
1
1.2
450 500 550 600 650
Wavelength (nm)
Elec
trol
umin
esce
nce
(a.u
.)
20oC70oC
10 30 50 70
Temperature (C)
Ener
gy/W
idth
Current Heating Effects