Upload
others
View
2
Download
0
Embed Size (px)
Citation preview
Organic ElectronicsStephen R. Forrest
Organic ElectronicsStephen R. Forrest
Week 2-4Light emitters 4
Stacked WOLEDsOutcoupling
Chapter 6.5.4-6.6
1
Organic ElectronicsStephen R. Forrest
Organic ElectronicsStephen R. Forrest
2
White Phosphorescent SOLEDs
Io , Vo
Lo
3xLo
Io , 3xVo
Conventional OLED Stacked OLED (SOLED)
cathode
ITOorganic layers-
+
charge generation layer
-+
-+
-+
Lifetime and efficiency of OLEDs can be increased by vertically stacking multiple OLED units in series
Requires less current for same luminance as a single unit device• Longer lifetime at same luminance• Less current for a given luminance = reduced resistive power losses and heating
Organic ElectronicsStephen R. Forrest
SOLED Operation Principles
Blue=FlzIr Green=Ir(ppy)3Red=PQIr
Archetype stacked WOLED
CGLEBL
Two types of CGLS: Recombination vs. tunnel junctions
Kanno et al., Adv. Mater., 18, 339 (2006) 3
Organic ElectronicsStephen R. Forrest
White SOLED Panel: Efficacy vs. Luminous Emittance
Panel Area = 15 cm x 15 cm
• Efficacy approaching 50 lm/W at 10,000 lm/m2
• CRI = 83• LT70 = 4,000 hrs
1,000 cd/m2
2,000 cd/m2
ITOHIL
HTL
R:G EML
BL
ETL
Cathode
Blue EML
BL
ETL
CGLHILHTL
4
Organic ElectronicsStephen R. Forrest
White PHOLED Panel
Panel15 cm x 15 cm15 mm thick
At 1,000 cd/m2
At 3,000 cd/m2
Efficacy [lm/W] 58 49
CRI 82 83
Luminous Emittance[lm/m2] 2,580 7,740
Voltage [V] 3.8 4.3
1931 CIE (0.466, 0.413) (0.471, 0.413)
Duv 0.001 0.000
CCT [K] 2,640 2,580
EfficacyEnhancement 1.75x 1.75x
Temperature Rise [oC] 0.7 7.2
LT70 [hrs] 30,000 4,000
Cathode
AnodeHIL
Red-Green EML
Blue EML
ETL/EIL
Glass
BL
HTL
LT70 ~ 30K hrs at 1000 cd/m2
Warm White with CCT 2640K
t = 60 min3000 cd/m2
28.7oC
27.5oC
290 300 310 320 330 340 35010
100
1000
10000
Life
time
to L
T80
[hrs
]
Temperature [K]
10K Lower Temperature ~ 1.65x Longer Lifetime
Lower current at constant L⇒lower temperature⇒longer lifetime
5
Organic ElectronicsStephen R. Forrest
WOLED vs. SOLED Panel ComparisonPanel
15 cm x 15 cm 82% fill factor
Single Unit WOLED*
2 Unit WSOLED
Luminance [cd/m2] 3,000 3,000
Efficacy [lm/W] 49 48
CRI 83 86
Luminous Emittance[lm/m2]
7,740 7,740
Voltage [V] 4.3 7.4
1931 CIE (0.471, 0.413) (0.454, 0.426)
Duv 0.000 0.006
CCT [K] 2,580 2,908
Temperature [oC] 27.2 26.2
LT70 [hrs] 4,000 13,000
P.Levermore et al, SID Digest, 72.2, p.1060, 2011.
WOLED
SOLED
SOLED architecture: ~ 3x LT70 improvement vs. single unit WOLED with similar color and power efficacy
ITOHILHTL
R:G EML
BLETL
Cathode
Blue EML
ITOHILHTL
R:G EML
BL
ETL
Cathode
Blue EML
BL
ETL
CGLHILHTL
6
Organic ElectronicsStephen R. Forrest
OLEDs: Not All Light Goes to the Viewer
• Optical paths outcoupled with hemispherical lens
7
Organic ElectronicsStephen R. Forrest
• Good solutions§ Inexpensive§ Viewing angle independent§ Independent of OLED structure
• Among those things that have been tried§ Optical gratings or photonic crystals1
§ Corrugations or grids embedded in OLED2
§ Nano-scale scattering centers3
§ Dipole orientation management
8
1Y .R. Do, et al, Adv. Mater. 15, 1214 (2003).2Y. Sun and S.R. Forrest, Nat Phot. 2, 483 (2008).3Chang, H.-W. et al. J. Appl. Phys. 113, - (2013).
Getting all the photons out
Organic ElectronicsStephen R. Forrest
Organic ElectronicsStephen R. Forrest
k !
k Θ
z
dmedium 1
medium 2
Molecules are radiating dipoles in inhomogeneous media
9
Organic ElectronicsStephen R. Forrest
Cathode metal (mirror)
Organics
ITO
Glass substrate
k||
kθ
• Air modes: EQE first increases, then decreases with ETL thickness
• Waveguide modes: Only one waveguide mode TE0 due to thin ETL (<30nm). TM0 appears when >50nm.
• Surface plasmon polariton modes: Reduced with ETL thickness• Both waveguide and SPP modes are quantized• Total energy is the integral of Power Intensity x cos(θ), so SPP
not as small as it looks
Where do all the photons go?
0 0.2 0.4 0.6 0.8 1 1.20
0.5
1
1.5
2
2.5
3
3.5 x 10-3
k||/k
powe
r int
ensit
y
10nm30nm50nm70nm90nm
Air modes
Substr
atemodeswav
eguid
e
SPP
ETL thickness
(20%) (20%)
(15%) (45%)
10
Organic ElectronicsStephen R. Forrest
WOLED Outcoupling Can Yield True Color
Color balance achieved at two ETL thicknesses
ETL Thickness Controls Coupling to lossy surface plasmons
11
Organic ElectronicsStephen R. Forrest
Surface Plasmon Polariton (SPP) Modes: Major Loss Channel
ηext > 80% (incl. substrate + waveguide modes)
• Waveguided light excites lossy SPPs in metal cathode• Major loss channel partially eliminated by rapid outcoupling of waveguide modes• Most difficult to eliminate cost-effectively without impacting device structure
12
Organic ElectronicsStephen R. Forrest
Substrate Mode Outcoupling: ~2X Improvement
ηext~40%
Microlens arrays: Polymer hemispheres much smaller than pixel
Möller, S. & Forrest, S. R. 2001. J. Appl. Phys., 91, 3324.
Fabrication sequence Spectrum angle independent
⇐ Scattering and surface roughness also can reduce substrate modes
Reidel, et al., Opt. Express 18 A631 (2010) Chang, et al., J. Appl. Phys., 113 204502 (2013) 13
Organic ElectronicsStephen R. Forrest
14
Metal electrode pixel
Organics
Low-index grid
ITO
Glass substrate
Ray BRay A
Ray
C
Side view
wLIG
worg
Waveguide Mode Outcoupling:Embedded Low Index Grid
ηext~60% (incl. substrate modes)
Sun, Y. & Forrest, S. R. 2008. Nature Photon., 2, 483.
Organic ElectronicsStephen R. Forrest
15
2µm
Low Index Grid Images
• OLED >> Grid size >> Wavelength• Embedded into OLED structure• May partially decouple waveguide mode from SPPs
Organic ElectronicsStephen R. Forrest
16
400 600 8000.0
0.5
1.0
Nor
m. E
L (a
.u.)
Wavelength (nm)
Dev. 1 Dev. 4
10-6 1x10-5 1x10-4 10-3 10-2 10-1
0
10
20
30
Pow
er E
ffici
ency
(lm
/W)
Exte
rnal
Qua
ntum
Effi
cien
cy (%
)
Current Density (mA/cm2)
Dev. 4 Dev. 3 Dev. 2 Dev. 1
0
10
20
30
40
50
60
70
80
Dev. 1 x 1.32
Dev.1x1.68
Dev. 1 x 2.3
10-3 10-2 10-1 100 101 10210-3 10-2 10-1 100 101 102
Hybrid WOLED Performance Using Embedded Grids + Microlens Arrays
Device 1: ConventionalDevice 2: LIG onlyDevice 3: Microlenses onlyDevice 4: LIG + Microlenses
Method is WavelengthIndependent
Sun, Y. & Forrest, S. R. 2008. Nature Photon., 2, 483.
Organic ElectronicsStephen R. Forrest
A better approach: Sub-Anode Grid
qA multi-wavelength scale dielectric grid between glass and transparent anode (sub-anode grid)
qThe grid is removed from the OLED active region
qWaveguided light is scattered into substrate and air modes
17
Cathode
nglass=1.5
nhost
norg=1.7, nITO=1.8ngridngrid
waveguided power + dissipation
Collect substrate mode power
Qu,et al., Nature Photonics (2015), 9, 758
Organic ElectronicsStephen R. Forrest
Emission field calculations
18
WITH GRID
WITHOUT GRID
VERTICAL DIPOLE
HORIZONTAL DIPOLE
Mostly in-plane(Waveguided)
Mostly out-of-plane(Glass + air)
Qu,et al., Nature Photonics (2015), 9, 758
Organic ElectronicsStephen R. Forrest
Optical Power Distribution
Thick-ETL organic structure:
340nm grid/70nm ITO/2nm MoO3/40nm TcTa/15nm CBP: Ir(ppy)3/10nm TPBi/230nm Bphen:Li/Al
2nm MoO3/40nm CBP/15nm CBP:Ir(ppy)3/xnm TPBi/1nm LiF/Al
Qu,et al., Nature Photonics (2015), 9, 758 19
Organic ElectronicsStephen R. Forrest
Substrate Au Ag Mirror on Grid Surface
SiO2IZOOrganicAnti-reflectionlayer
Eliminating SPPs Using Sub-Anode Grid + MirrorTop Emitting OLED
3 Lift-off
Substrate Fabrication
Qu, et al. ACS Photonics, 4, 363 2017
Mirror remote from EML eliminates SPP excitation
20
Organic ElectronicsStephen R. Forrest
a. Ag
IZO/MoO3 80nmETL 60 nm
Substrate
EML 20 nmHTL 40 nm
IZO/MoO3 80nm
SiO2 65 nm
Ag
IZO/MoO3 80nmETL 60 nm
Substrate
EML 20 nmHTL 40 nm
IZO/MoO3 80nm
SiO2 245 nm
a. Ag
ETL 35 nm
Substrate
EML 15 nmHTL 30 nmAg 15nm
Sub-electrode grid modelingVariable Waveguide Widths Above Mirror Prevent Mode Propagation via Scattering
Qu, et al. ACS Photonics, 4, 363 2017
raised mirror section depressed mirror section no waveguide
SPP at k‖>1
21
Organic ElectronicsStephen R. Forrest
0 2 4 6 8 10 12 1410-5
10-4
10-3
10-2
10-1
100
101
102
Voltage (V)
Curre
nt De
nsity
(mA/
cm2)
1
10
100
1000
10000
Lum
inanc
e (Cd
/m2)
0
20
40
6080100
120
140
160
180
Con Grid
0.1 1 10 1000
5
10
15
20
25
30
hEQE (
%)
Current Density (mA/cm 2)
Performance with and without grid+mirror
Grid
No Grid
Qu, et al. ACS Photonics, 4, 363 2017 22
Organic ElectronicsStephen R. Forrest
Al OrganicITO
SEMLA
High IndexSpacer layer
Getting All the Light Out: Sub-Electrode Microlens Array (SEMLA)
Qu, Y., et al. 2018. ACS Photonics, 5, 2453. 23
Organic ElectronicsStephen R. Forrest
0 20 40 60 80 100 120 1400
20
40
60
80
100
Frac
tion
of P
ower
(%)
ETL thickness (nm)
SPPLoss
WV
SEMLA
0 20 40 60 80 100 120 1400
20
40
60
80
100
Frac
tion
of P
ower
(%)
ETL thickness (nm)
SPP
WV
SubAir
Loss
SEMLAs Change the Outcoupling Landscape
Qu, Y., et al. 2018. ACS Photonics, 5, 2453.
SPPs not completely eliminated
24
Organic ElectronicsStephen R. Forrest
0 5 1010-7
10-3
101
Con SEM LA
J (
mA
/cm2 )
Vo ltage (V )
102 103 1040
20
40
60
80 Con_air Con_IMF SEMLA_air SEMLA_MLA SEMLA_IMF Sap_IMF
h EQE (%
)
Brightness (cd/m2)
-90
-60
-300
30
60
90 L am b. C on S E M L A M LA S E M LA H S
SEMLA
SEMLA
+MLA
SEMLA
+IMF
Sap1.0
1.5
2.0
2.5
3.0
3.541±3 %45 ±4%
2 7±3 %2 0±2 % 60 ±4%
65 ±5%
47 ±4%
E F
30±3 %
SEMLA Performance
Qu, Y., et al. 2018. ACS Photonics, 5, 2453. 25
Organic ElectronicsStephen R. Forrest
Gratings Provide Efficient Waveguide Mode Outcoupling
Process for fabricating 1D and 2D gratings using optical interference resist exposure
rotate and re-expose for 2D grating
AdvantagesCan be very efficient
DisadvantagesRequires high resolution photolith over large areasVery wavelength and angle dependent
Bocksrocker et al., Opt. Express, 20, A932 (2012) 26
Organic ElectronicsStephen R. Forrest
Diffuse Reflectors: Low Cost & Simple
Teflon is the best diffuse dielectric reflector
Diffuse (Green) Mirror (Green) Diffuse (White) Mirror (White)
0.01 0.1 1 100
10
20
30
40
50
h EQE
(%)
Current Density (mA/cm2)
PHOLED Active Area
Kim, J., et al. (2018), ACS Photonics, 5, 3315. 27
Organic ElectronicsStephen R. Forrest
Pt(dbq)(acac)
Isotropic Orientation
HorizontalOrientation
Outcoupling Enhancements by Molecular Orientation
Ir(ppy)3
Prevents coupling to SPPs and waveguide modes
Planar molecules (e.g. Pt-complexes) more likely to align than octahedral (tris-Ir complexes)28
Organic ElectronicsStephen R. Forrest
Outcoupling Improvements via Alignment
Calculated dipole field
Kim et al., Adv. Funct. Mater., 23, 3896 (2013)
29
Organic ElectronicsStephen R. Forrest
Example results
0 10 20 30 40 50 60 70 80 900.0
0.5
1.0
1.5
2.0
2.5
Detector Angle [Degree]N
orm
aliz
ed In
tens
ity
Isotropic (qhor= 66.7%) Horizontal (qhor= 100%) Fit (qhor= 45.9%) CBP:Pt(dbq)(dpm) 8wt%
Pt(dbq)(dpm)
0 10 20 30 40 50 60 70 80 900.0
0.5
1.0
1.5
2.0
Detector Angle [Degree]
Nor
mal
ized
Inte
nsity
Isotropic (qhor= 66.7%) Horizontal (qhor= 100%) Fit (qhor= 65.9%) CBP:Irppy3 8wt%
Ir(ppy)3
Approach challenges• Added constraints on molecular design• Added constraints on process (growth) conditions: may not align as expected• Added constraints on device architecture• Alignment is never “perfect”: only modest improvements
Θ =
TM!
TE⊥ +TM⊥ +TM!
Ratio of light emitting by vertical to horizontal dipoles:
30
Organic ElectronicsStephen R. Forrest
Alignment is Driven by Energy and Deposition Dynamics
Alignment can be “forced” by seeding the substrate with a thin molecular template
Molecular functionalization can drive orientation: Aliphatic groups avoid substrate
Example: Ir(ppy)2acac
aliphatic acacLampe, et al. Chem. Mater. 28, 712 (2016)
Kim, et al. Adv. Mater. 31, 1900921 (2016) 31
Organic ElectronicsStephen R. Forrest
• Waveguide thickness varies due to the corrugation.
• As the thickness changes, the mode distribution changes.
• When the waveguided power travels from thin to thick areas, the k vector needs to change direction to keep “being trapped”. Otherwise, the light is extracted.
0.8 0.85 0.9 0.95 10
0.5
1
1.5
2
2.5
3
3.5 x 10-3
k||/k
powe
r int
ensit
y
10nm30nm50nm70nm90nm
TM0
TE0
ETL thickness
Waveguide
nsub/norg=0.88
W. H. Koo, et al, Nat. Photonics 2010, 4, 222.
A possible approach: Surface buckling?FFT
Al on resin
Substrate Corrugations Can Outcouple Waveguide Modes
32