Transcript Organic Lasers, ECOER 03

The Quest for Electrically Pumped Lasers
Nir Tessler
Microelectronic & Nanoelectronic centers
Electrical Enginnering Dept.
Technion, Israel Institute of Technology
Haifa, Israel
www.ee.technion.ac.il/nir
Outline
Introduction
Some of the problems
One of the ways to approach the problems
Historical Perspective
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
These materials can now be taken
seriously for demanding applications
The issue of electrically pumped
organic laser is now relevant
The ”original” motivation
PL (a.u.)
PPV
2.5
450
2
Absorption (OD)
n
500
550
600
650
700
750
Wavelength (nm)
1.5
1
0.5
0
200
250
300
350
400
Wavelength (nm)
450
500
550
• Stoke Shift
• 4 level system (not always true)
Technological Advantages of
“Plastic” Lasers
Wavelength tuning
through bending
Stamp
Gain and Glue
properties
2D Bandgap
Not sensitive to
Surface recombination
There is a great potential
So how come we can’t make it happen
Or at least prove that it did happen
The most Common Laser
Mirror 1
Mirror 2
Light - Amplifier
Optical
Feedback
Optical
Amplifier
Noise
Source
+
X
Input Power
Material
Device structure
Output
We are interested in molecular materials
Similar to quantum confinement based lasers
MQW Laser Structure
P+InGaAS
E
P-InP
InGaAsP
QW
InGaAs
InGaAsP
N-InP
N-InP, Substrate
Quantum Well Lasers
IElectrons
Transport
Capture
3D
InGaAsP
N
2D
Stimulated
Emission
InGaAs
P
IHoles
Many issues had to be optimized
Most of them – material related!
N. Tessler et. al.
JQE, 1993
Ielectrons
IlHoles
Absorption/Gain (cm-1)
Gain and Absorption In PPV
10 6
Absorption
10 5
10 4
Charge Induced
Absorption
1000 Excitonic
Gain
Charge absorption
is plotted for
Excited State
Density = 1018cm-3
100
10
300
400
500
600
700
800
900 1000
Wavelength (nm)
Not 4 Level System
No net Gain (with Current Drive)
Charge absorption is “band to band”  High cross section
Rate Equations
Charge
Singlet
Exciton
Triplet
Exciton
dNC
 J  B N C2
dt
dNS 1
NS
2
 B N C 
dt
4
S
Exciton Generation
dNT 3
NT
2
 B N C 
dt
4
T
NS  1

  B NC  S  0.1
NC  4

 S  109 Sec
  103 cm 2V 1Sec 1 ;
N C  1018 cm 3
Exciton Generation =
Bottleneck
How to Enhance the Probability
1. Material with high mobility (crystals looked promising)
2. Material with low charge induced absorption
Synthesis of Polyarylamines
Yamamoto Method
Cl
Cl
Cl
[Ni] catalyst
+
N
n
R2
N
R2
R1
R1
R
Vary R group to
optimise charge mobility
N
*
n
*
R2
Electroluminescence (a.u.)
Electroluminescence (a.u.)
Fast Switching
1
0.8
0.6
1.2
1
0.8
0.6
0.4
0.2
0
5
10
15
20
Time (ns)
0.4
Even if we won’t make electrically pumped laser
we have made the basic unit for 100MHz
(500MHz) data link.
0.2
0
0
20
40
60
80
100
Time (ns)
This initial set of devices & materials requires above 20V to achieve rise time of
less then 10ns. (new materials have much better mobility)
25
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 287
p.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?
Anything to learn from inorganic lasers?
Low bandgap Inorganics
Problem
Conduction
Introduce strain
Conduction
Valence
Valence
Split-off
Split-off
Inter Valence-band
Absorption
The Organic equivalent
Hole - Polaron
Exciton - Polaron
HOMO
HOMO
LUMO
LUMO
Is there an alternative solution?
Charge absorption covers visible range and up to 1m 
OK – Lets mix
can we take the emission band beyond 1m?
PbSe
5 nm
20nm
InAs/ZnSe
O
Conjugated
polymers
MeO
n
1st stage = Optimising the NC Emission yield
Q.Y
InAs/ZnSe
15
Absorbance intensity (a.u.)
Shell
InAs
CdSe
0.46
0.92
ZnSe
1.26
Eg
0.99
Best PL for Shell
Thickness
between 1 and 2
monolayer
20
13
20% PL Yield
in Solution
(toluene)
~2.2 ML
V
~1.3 ML
x4
0.8
1.2
0.9
~0.7 ML
1.6
2.0
2.4
Photon Energy (eV)
U. Banin, Hebrew University,Jerusalem
Luminescence (a.u.)
InAs
1000
Size A
Size B
1200
PbSe
Size C
1400
1600
Wavelength (nm)
1800
1000
1200
1400
1600
Wavelength (nm)
>10% PL Efficiency in Solid Films
1800
What do we hope to achieve by mixing
Ca\Al (cathode)
Current/Energy
is first
O
injected into the
Polymer
polymer
-
MeO
V
n
Energy/Charge
Transfer to the
nanocrystal
nanocrystal
+
Light Emission
PEDOT/ITO (Anode)
Glass
What is the transfer mechanism?
Absorption (OD)
0.8
Emission at 1550nm (a.u.)
1
50V%PPV 50V%NC
NC in Polystyrene (80V%)
0.6
0.4
0.2
0
400
500
600
700
800
900
Wavelength (nm)
Polymer absorbs 70% @ ~450nm
Energy Transfer Polymer
20v%NC 80v%PPV
50v%NC 50v%PPV
NC in Polystyrene
400 450 500 550 600 650 700 750 800
Excitation Wavelength (nm)
Polymer contributes <20%
Energy to NC
NC is Negligible
Energy Transfer
Charge Transfer
(trapping)
?
Experimental Efficiency-Optimization
Current (mA)
1.4
1.2
1
0.8
0.6
0.4
0.2
0
0
5
10
15
20
25
3
2.5
2
cy
50%
en
20% 33%
Ef
fic
i
10%
1.6
Increase NC Content
Electroluminescence (a.u.)
Increase NC Content
1.5
1
0.5
Applied Voltage (V)
0
0
5
10
15
20
25
Applied Voltage (V)
Trapping in the governing excitation mechanism
Very High Loading
Probably due to partial aggregation
~1% EL External-Efficiency
Tessler et. al., Science, 2002
Experimental
TEM Top View of =1500nm NC in PPV
(30v% NC)
Partial segregation
PPV
“pin-hole”
“Good”
Surface Coverage
Y. Talmon
20nm
Optimization Requires Dedicated Modeling
V
5V% NC
2D Mesh with Traps (NCs)
Randomly Positioned at a given
density
(trap depth = 0.4eV)
Charge Density (1018cm-3)
The effect of trapped charges
V
5% Loading
See also A. Shik et. al.
Solid. State Elect.,
46, 61,2002
NC near contact
Non-Complete
Trapping
Suppress
Injection
Simulation
No NC
1.6
10% NC - HOMO offset=0.3eV
1.2
1
10% NC ,
offset+0.1eV
0.8
Measurement
1.6
0.6
No NC
1.4
0.4
10% NC ,
offset+0.2eV
0.2
0
0
2
4
6
8
Voltage (V)
10
12
Current (mA)
Current (mA)
1.4
10% NC
20%
1.2
1
0.8
0.6
30%
0.4
0.2
0
HOMO offest ~0.3eV
0
2
4
6
8
Applied Voltage (V)
10
12
Let Us Assume someone will solve all material issues
Related to Lasers
The structure
Metal1
n3=1.7
Optical
Mode
n2=2
n1=1.7
Metal2
Glass
x3 (Cladding)
x2 (Core)
x1 (Cladding)
Propagation Loss (cm-1)
1000
Al
100
Ag
10
1
0
50
100 150 200 250 300 350
Cladding
Thickness (nm)
400
Consider more sophisticated
structures
• Light emitting FET? (there is a talk later)
Electroluminescence (a.u.)
Current Heating Effects
1.2
50s
1
0.5-10s
0.8
0.6
0.4
0.2
0
TPPV
TCTCT
RPPV
P
500
CPPV
520
THS
RCTCT
RIFC
+
-
THS
CCTCT
540
Wavelength (nm)
560
580
Chemistry/Materials
Analysis and
extraction of
properties
Device Modeling
Device Design & measure
New Functionalities
Novel Materials
EE Technion
Avecia
Phil Mackie
Cupertino Domenico
polymers
Vlad Medvedev
Yevgeni Preezant
Yohai Roichman
Noam Rapaport
Olga Solomeshch
Alexey Razin
Yair Ganot
Sagi Shaked
Chem. Eng. Technion
Y. Talmon
TEM
Chem. Hebrew U.
Uri Banin
NC
Israel Science Foundation
$
European Union FW-5
Absorption spectrum of the blends
1
oc4
PPV Derivative
Absorption (OD)
0.8
oc10
oc4
50V%PPV 50V%NC
*
n
NC in Polystyrene (80V%)
0.4
x10
0.2
0
*
OMe
0.6
400
600
800
1000 1200 1400 1600
Wavelength (nm)
o
n=o=0.5
0.11
1.864
0.108
1.862
0.106
1.86
0.104
1.858
0.102
1.856
0.1
1.854
0.098
1.852
0.096
1.85
10
20
30
40
50
60
Temperature (c)
70
80
Peak Width (m-1)
Peak Energy (m-1)
1.866
Electrical Pulse Set-Up
150-200ns
Pulse
Generator
45Hz
AC Current Probe
Si Photo Diode
Fast APD
Temperature
Control (-170oc,70oc)
V
1.2
1
0.8
Energy/Width
Electroluminescence (a.u.)
Current Heating Effects
70oC
20oC
0.6
10
30
50
70
Temperature (C)
0.4
0.2
0
450
500
550
600
Wavelength (nm)
650