Transcript Document

Phys 590B-2009-04-01
Organic Light-Emitting Devices (OLEDs):
A Brief Overview
Joe Shinar
[email protected]
Support, mostly by DOE, gratefully acknowledged
Support by NASA, NSF, & NIH also gratefully acknowledged
Current Issues in the Science & Technology of OLEDs:
 Internal quantum efficiency, at low & high current densities
 Outcoupling or extraction efficiency.
 Power efficiency, at low & high current densities.
 Stability & degradation mechanisms: Oxidation (by water),
crystallization, delamination, atomic In, Na, etc. (electro)migration.
 Is an organic injection laser (i.e., an organic diode laser) feasible?
Basic Nature of p-Conjugated Materials
A carbon-based (small) molecule or polymer with alternating
single (s) and double (s & p) bonds. Examples:
C6H13
R'
R
n
n
R
poly(p-phenylene) vinylene (PPV)
R
n
C6H13
Polyfluorene (PFO)
R
methyl-bridged ladder-type poly(p-phenylene ) (m-LPPP)
N
O
N
Al
O
N
O
tris(8-hydroxy quinoline) Al (Alq3)
4,4'-bis(2,2'-diphenylvinyl)-1,1'-biphenyl
(DPVBi)
R'
Basic Nature of p-Conjugated Materials (cont.)
pz
 Consider ethylene (C2H4):
 sp2 hybridization: three sp2
orbitals 120o to each other, pz
orbital perpendicular to the
sp2 plane.
 In benzene (C6H6) ring:
ψ2 and ψ3 are HOMO –
Valence band
ψ4*, ψ5* are LUMO –
Conduction band
Basic Nature of p-Conjugated Materials (cont.)
 PPV:
 Electrons in p orbitals have higher energy than those in s orbitals.
 The energy gap between
the highest occupied molecular orbital (HOMO)
(analogous to the top of the valence band in inorganic semiconductors)
& the lowest unoccupied molecular orbital (LUMO)
(analogous to the bottom of the conduction band in inorganic semiconductors
is 1.5 – 3.5 eV, i.e., it covers the whole visible range.
Basic structure of some (blue) OLEDs
Low-Workfunction Cathode
+
Electron-Transporting &
Emitting layer (ETL)
Hole-Transporting Layer
(HTL)
DPVBi
:
Pe:CBP
Transparent Conducting Anode
[indium tin oxide (ITO)]
Glass or Plastic Substrate
N
N
a-NPD
N
N
4,4'-bis(2,2'-diphenylvinyl)-1,1'-biphenyl (DPVBi), Perylene:4,4'-bis(9-carbazolyl)biphenyl (Pe:CBP)
N,N’-diphenyl-N,N’-bis(1-naphthyl phenyl)-1,1’-biphenyl-4,4’-diamine (a-NPD)
Basic Operation, forward bias
Lowest
unoccupied
molecular orbital
(LUMO)
eEnergy
e-
ITO Anode
HTL
Metal cathode
EF
ETL
EF
h+
Highest occupied
molecular orbital
(HOMO)
Position (from ITO anode bottom to methal cathode top
The state-of-the-art in OLEDs
• Numerous red-to-blue small-molecular- and polymer-OLEDs
• Fluorescent Green, yellow & red small molecular OLEDs with
continuous lifetimes > 300,000 hours (> 12,000 days > 33 yrs),
hext ~ 4%, power efficiency ~2%, at brightness of ~150 Cd/m2
• Blue small molecular OLEDs with
continuous lifetimes ~100,000 hours (> 4000 days > 12 yrs),
hext ~ 3%, power efficiency hP ~ 1.5%.
• Electrophosphorescent Red, green, & blue OLEDs, i.e.,
emission is from Triplet (Excitons), so efficiency can be very high --
hext ~ 18%, 19%, & 7%, hP ~ 9%, 9.5%, 3%, respectively.
Cd/m2 ????
• 1 Cd/m2 = 1 lumen/steradian = 1 lm/str = [1.46 mW @ 555 nm]/str
• For Lambertian source, 1 lm/str  total of p lm
• Typical values:
TV or computer monitor, 100 – 300 Cd/m2.
60 W incandescent light bulb, 840 lumens.
Power efficiency hP = [840 lm]/[60 W] = 14 lm/W
 [1.20 W light]/[60 W electrical] = 2%
Fluorescent tubes, 40  hP  60 lm/W, i.e., 6  hP  9 %
OLED Efficiencies
 Internal quantum efficiency IQE of fluorescent OLEDs
hint  0.25
Why? Can it be improved?
 Outcoupling or extraction efficiency hout  0.33
Why? Can it be improved?
 [Photon energy (in V)]/[Applied Voltage] = Vg / Vappl < 0.50
Why? Can it be improved?
Note: 0.250.330.50 ~ 0.04
Internal Quantum Efficiency: Carrier injection into a
luminescent p-conjugated molecule or polymer
The most basic process in all light-emitting diodes (LEDs):
Bottom of conduction band, inorganic semiconductor
LUMO, organic semiconductor
Electron e- from cathode
e-
e-
h+ & e- meet to form an exciton on a
molecule or same conjugated
segment of polymer
Photon emission
h+
Hole h+ from anode
h+
HOMO, organic semiconductor
Top of valence band, inorganic semiconductor
How many distinct SE & TE quantum states?
+
=
TE, Spin 1, Sz = +1
+
=
TE, Spin 1, Sz = 0
How many distinct SE & TE quantum states (cont.)?
+
TE, Spin 1, Sz = -1
=
SE, Spin Sz = 0
+
=
0
In summary:
 3 TE states (S = 1): () (Sz = +1), ( + )/2 (Sz = 0), () (Sz = -1).
 1 SE state (S = 0):
( - )/2 (S = 0).
 In fluorescent organic materials, only the SE decays radiatively,
to yield the photoluminescence (PL) or electroluminescence (EL).
A Fiercely Debated Question:
 Given an e- - h+ pair (i.e., a radical anion – radical cation pair)
at a given distance from each other, is
the cross section of a pair in the singlet configuration to form a SE (sSE)
equal to
the cross section of a pair in the triplet configuration to form a TE (sTE)?
 If sTE = sSE max internal quantum efficiency IQEmax of fluorescent OLEDs is 25%.
So, is sSE = sTE ?
 After ~8 years, jury is still out; i.e., there are two schools on it…
 Interestingly, the most powerful technique to explore this issue has turned
out to be optically detected magnetic resonance (ODMR), whose application
to p-conjugated polymers and OLEDs we pioneered in the late 80s.
Claiming sSE > sTE so IQEmax > 25%
1. Wohlgenannt , Vardeny, Mazumdar, et al., using ODMR [Nature
409, 494 (2001); PRL 88, 197401 (2002); PRB 66, 241201(R) (2002)]
2. Friend, Greenham, et al., using “standard” optical spectroscopy
[JAP 88, 1073 (2000); Nature 413, 828 (2001); CPL 360, 195 (2002)]
Claiming sSE ≈sTE so IQEmax ≈ 25%
1. Baldo & Forrest, using “standard” PL and EL measurements
[PRB 60, 14422 (1999); 68, 075211 (2003)]
2. Shinar, Baldo, Soos, et al., using ODMR and double-modulation ODMR
[PRL 94, 137403 (2005); PRB 71, 245201 (2005)]
Claiming spin-flip rates are very low
so sSE & sTE do not matter & IQEmax ≈ 25%
Lupton et al., using “standard” spectroscopy & time-resolved EDMR
[Nat Mat 4, 340 (2005); Nat Mat 7, xxx (2008)]
Outcoupling Efficiency Issue
 Due to total internal reflections at the various interfaces,
and resulting waveguiding of the light towards the edges of the OLED,
hout ~ 1/(2n2), where n is the organic index of refraction
[Kim et al., JAP 88, 1073 (2000)]
 So what do we do?
 Solution 1. Textured surfaces (even sandblasting helps by 20 – 100%).
 Solution 2. Microlens arrays.
 Solution 3. Top-emitting devices.
Vg / Vappl Issue
 Need to minimize voltage for given current & brightness,
in order to maximize power efficiency for that given quantum efficiency.
 Solution: Minimize voltage drop across HTL & ETL,
by maximizing their conductivity. How?
 Solution: p-dope the HTL (e.g., F4-TCNQ-doped NPB) &
n-dope the ETL (e.g., Li-, Na-, etc., doped ETL).
Conclusion on Efficiency & Outlook
 With electrophosphorescent devices, IQE max > 90%
 With textured surfaces, microlens arrays, or top-emitting devices,
outcoupling efficiency may be ~ 0.60.
 With optimized p-doped HTL & n-doped ETL, Vg / Vappl ~ 0.80.
 Then power efficiency will be ~ 0.43
 We’ll get there…
Summary of Degradation Mechanisms
 Oxidation (by water & oxygen)
Solution: Barrier coatings.
 Crystallization of amorphous layers.
Solution: Use high glass-transition temp materials.
Or fabricate cystalline layers?
 Delamination of metal cathode (leads to “black spot”)
Solution: Better organic/cathode buffer layers &
better cathode fabrication procedures.
 Atomic In, Na, etc. (electro)migration.
Solution: Use Na-free glass substrate, fabricate ITO/organic buffer layer.
But what happens at high brightness
(needed for using OLEDs for general lighting)?
 High brightness (~2000 Cd/m2) means high current density.
 High current density means high SE, polaron, & TE densities.
 High SE, polaron, & TE densities means much higher
SE-SE annihilation & quenching of SEs by polarons & TEs.
 In electrophosphorescent OLEDs, it means much higher TE-TE
annihilation to SEs.
 What can we do about these quenching processes?
 Answer: Stacked tandem OLEDs.
Stacked tandem OLEDs
Glass
Transparent anode
Organic layers
Charge generation layer
Organic layers
Charge generation layer
Etc.
Metal cathode
This way, we can get more than 1 photon per e(But we pay with higher V for lower J)
Last but not least: Is an organic injection laser feasible?
 Realized only in the fraudulent mind of J. Hendrik Schoen.
 We can observe spectrally narrowed edge emission, but no clear evidence
for optical gain [Gan et al. APL (2007)].
 Current hopes are for LED- or OLED-pumped polymer lasers
The state-of-the-art in OLEDs (cont.)
• They are appearing increasingly in commercial products (see below)
• Investment & activity growing rapidly
Cambridge Display Technology (CDT), Dow Chemical, Dupont,
Eastman Kodak, General Electric, Idemitsu Kosan, IBM,
LG, 3M, Osram-Sylvania, Philips, Samsung,
Seiko-Epson, Siemens, Sanyo, Sharp, Sony, Tohoku Pioneer,
Toshiba, Toyota, Universal Display Corp.
Actual and potential applications of OLEDs
• 7-segment alphanumeric displays for, e.g., cell phones (already obsolete).
• Displays for car stereos (Pioneer-Toyota, 1st commercial product).
• Displays for MP3 players (currently > 40% of new MP3 players).
• Full-color microdisplays (eMagin-IBM, actual).
• Full-size flat panel TVs and computer screens
(demos, up to 40” diagonal, by Sony, Samsung, etc.; )
• Light sources for fluorescent chemical & biological sensors (potential)
• Ultimate application: White OLEDs (WOLEDs) & other OLEDs for
general purpose light-sources (Solid State Lighting Initiative) (potential)
• Current OLED sales > ~$1B/yr & growing exponentially.
First commercial product:
Display for car stereo by Pioneer/Toyota (1998)
Microdisplays by eMagin-IBM joint venture (2002)
(very expensive)
Sony 11” OLED TV, commercialized in Dec 2007.
Power consumption 45 W. MSRP ~ $2,000
Contrast 1,000,000:1
Ultimate application: White & multicolor OLEDs for gen’l lighting.
• Inorganic LEDs already shining in traffic lighting, rear vehicular
lighting, flashlights.
• OLEDs, however, must perform with higher efficiency and longer
life at high brightness (> 1000 Cd/m2).
30 cm x 30 cm White OLED (WOLED) Panels,
~20 lm/W @ 1000 Cd/m2, by Kido et al. (Science 310, 1762 (2005))
Combinatorial arrays of UV-violet OLEDs; among shortest
wavelengths OLEDs
L. Zou et al., Appl. Phys. Lett.
79, 2282 (2001).
Mapping of Radiance of Devices w/CsF/Al Cathode
Extremely bright white OLEDs [Lmax > 50,000 Cd/m2);
K.-O. Cheon & J. Shinar, Appl. Phys. Lett. (2002)]
2
F (MV/cm)
1.0
0.5
0
10
2
10
10
10
-2
1.5
4
10
2
10
0
L (cd/m )
2
J (mA/cm )
10
-4
0.4
0.8
1.2
F (MV/cm)
1.6
2.0
In same OLEDs
[K.-O. Cheon & J. Shinar, Appl. Phys. Lett 83, 2073 (2003)]
…[35 nm a-NPD]/x nm 5% DCM2:a-NPD/[40 nm DPVBi/[10 nm Alq3]…
When x increases from 0 to 3.5 nm, emission changes from blue to red!
Even brighter white OLEDs [Lmax ~ 74,000 Cd/m2)];
[G. Li & J. Shinar, Appl. Phys. Lett. 83, 5359 (2003) ]
1400
2
800
2
1000
4
3
2
600
1
400
0
200
-1
0
-2
0
2
4
log10L (Cd/m )
1200
J (mA/cm )
5
Device
#1
#2
#3
#4
#5
#6
6
8
Bias (V)
10
12
14
Next time:
The most recent application of OLEDs:
Structurally-integrated OLED-based luminescent
chemical & biological sensors
Ruth Shinar
Microelectronics Research Center & ECpE Dept, ISU
Integrated Sensor Technologies, Inc. (ISTI)
Joseph Shinar
All of the aforementioned & ISTI
Support by DOE, NASA,
NSF, & NIH also gratefully acknowledged