electrophosphorescence for solid - state lighting...χ fraction of usable excitons η r...

Electrophosphorescence for Solid-State Lighting

Mark Thompson

Department of ChemistryUniversity of Southern California

Hole/electron recombination gives singlet and triplet excitons

Dopant

Emission

Dopant traps exciton and

emits

+

hole electron

or

singlet

tripletExperimentally determined singlet fraction for Alq3 based OLEDs = 22±3%

M.A. Baldo, et.al., Phys. Rev. B (1999)

• Expected singlet fraction = 25%

• Phosphorescence is a forbidden process, luminescent lifetimes typically min. - hours

• Luminescence lifetime must be comparable to OLED RC time constant, ca. 1 µsec

cathode

Organometallic Ir complexes in OLEDs

NIr

NIrF

S NIr

3

2

2

NIr

2

F

O

O

O

O

O

O

N

N

N

NB

pz

pzS

NIr

2

• Efficient phosphorescence with τ = 1-3 µs• Optimized OLEDs give external efficiency = 15-25% ⇒ Internal eff = 70-100%

Phosphorescenceefficiency of Ir emittersis ∼100%

0.0 0.2 0.4 0.6 0.8

0.0

0.2

0.4

0.6

0.8

y

x

Φph photoluminescent quantum efficiencies

χ Fraction of usable excitonsηr recombination efficiency (hole + electron ⇒ exciton)ηe emission coupling efficiency out of device

• Many Ir based emitters give Φph = 1

• Spin orbit coupling mixes singlet and triplet, χ = 1• Good devices can have ηr → 1

Phosphorescent OLED Efficiency

erPLEL ηχηΦ=Φ

Color Mixing to Achieve White Emission

• Color mixing with different colored OLEDs will give white light

400 500 600 7000.0

0.2

0.4

0.6

0.8

1.0

1.2 human eye RGB OLED

Inte

nsity

Wavelength (nm)

0.0 0.2 0.4 0.6 0.8

0.0

0.2

0.4

0.6

0.8

y

x

WOLED efficiencies from academic labs

From J. Ye, et. al., Adv. Mat., (2012), 24, 3410

Kido, Firpic/PQIr, all ph 2008

Kido, Ir-carbene, 3 ph 2010

Hybrid two color, blue fl dopant is the host for ph dopant (0.1%)

Konica Minolta WOLED (15 cm2)

Warm White Device Cool White Device

Device 1

Device 2CGL

K. Kato, T. Iwasaki and T. Tsujimura, J. Photopolym. Sci. Technol., 2015, 28, 335–340

• All phosphorescent• Tandem structure• Internal & external

extraction structures

OLEDWorks Bright-2

Cost: $62 for 145 cm2

J. Spindler et al. “24-2: Invited Paper: High Brightness OLED Lighting”, SID INT SYMP DIG TEC, 47 (2016)

https://www.oledworks.com/products/brite-2/

Light extraction to enhance outcoupling and stacking in increase lm/W.

Fluorescent/Phosphorescent (fl/ph) WOLED

• Singlet and triplet excitons are harvested independently:– Higher energy singlet excitons for blue emission– Remainder of lower-energy triplet excitons for green and red emission

Minimizes exchange energy losses Potential for 100% IQE

Stable color balance Enhanced stability

Comparatively simple architecture

S

S

T

HOSTExcitonformationzone

RED and GREEN

phosphorescent dopants

T

S

T

χs = 0.25

χt = 0.75

BLUEfluorescentdopant

Förster transfer

Diffusive transfer

T

Ener

gy

Total External Efficiencies

TOTAL light efficiencies at 500 cd/m2

• External Quantum Efficiency: (18.4 ±0.5)%

• Power Efficiency: (23.8 ±0.5) lm/W

• CIE = (0.40, 0.41), CRI = 85, CCT = 3750K

Y. Sun, et. al., Nature (2006)

BCzVBi:CBP

BCzVBi:CBP

ITO/Glass

NPD

BCP

LiF/Al

CBP

CBP

PQIr:CBP

Ir(ppy)3:CBP

BPhen 20nm/BPhen:Li

Singletfilter

Tripletemitter

400 500 600 700 800-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Wavelength (nm)

Norm

. EL

100mA/cm2

10mA/cm2

1mA/cm2

S

S

T

HOSTRED and GREEN

ph-dopants

T

S

T

χs = 0.25

χt = 0.75

BLUEfl-dopant

Förster transfer

Diffusive transfer

TE

nerg

y

BCzVBi

NN

Leo Device (1): Hybrid fl/ph WOLED• K. Leo, 2007, 2009: introduced neat 4P-NPD layer as

blue emitter, recombination at a single interface

N

N

4P-NPDΦPL(film) = 92% • Conductivity doped layers

• 45 lm/W at 1000 cd/m2

• CIE = (0.45, 0.43)

Hybrid fl/ph WOLED developments (cont.)

• B.-Q. Liu, et. al., Light: Sci. App. 2016 5, e16137• Simple structure

– Dual NPD/Be(Oppy)2 fl emitters– Ph dopant excited by carrier and triplet trapping

• 65 lm/W at 1000 cd/m2, CIE = (0.45, 0.49), CRI = 47– Problems: two color limits CRI, toxicity of Be, short lifetime of

NPD emitters, carrier trapping leads to color shift with intensity

erPLEL ηχηΦ=Φ

ΦEL limited by ηe

Phosphorescent OLED Efficiency

Emitter

Pulling out substrate and waveguiding modes:

D

A

Transition Dipole Moment (Vector) Dictates Direction of Light Emission

Maximum emission probability ⊥TDV

Maximum emission probability ⊥TDV

Zero emission probability ‖ TDV

Zero emission probability ‖ TDV

αSome emission probability

Emission probability ∝ sin2 𝛼𝛼

Emitter

Vertical Isotropic Horizontal

Light Outcoupling in OLEDs

B. Scholz et al., Opt. Exp. 2012, 20, A205

Only ~20% of light directly emitted for isotropically distributed dopant.

Orientation Measurement

J. Frischeisen, et. al., Org. Electron. 2011, 12, 1663

Θ = 0.6/2.6 = 0.24Θ = 1.0/3.0 = 0.33

Anisotropy factor:

Θ𝑣𝑣𝑣𝑣𝑣𝑣 =𝑝𝑝𝑧𝑧

𝑝𝑝𝑥𝑥 + 𝑝𝑝𝑦𝑦 + 𝑝𝑝𝑧𝑧=

𝑝𝑝⊥𝑝𝑝|| + 𝑝𝑝⊥

Orientation and EQE

S.-Y. Kim et al., Adv. Funct. Mater. 2013, 23 3896

Θ = 1 0.33 0Strong plasmon Weak plasmon coupling (loss) coupling

0.4 0.3 0.2 0.1 0.0

in CBP, Θ ≈ 0.05, ηEXT = 33%C. Adachi, et. al, APL, 2016

• Linear molecules– TDV along the long axis

of the molecule– Physical interactions

align dopant at the growing host surface

OLEDEQE (%)

Ir based emitters in amorphous host matrix

Emitter Host Orientation (θver)Ir(dhfpy)2(acac) NPD 0.25Ir(ppy)2(acac) CBP 0.23

TCTA/ B3PYMPM 0.24Ir(ppy)2(tmd) TCTA/ B3PYMPM 0.22Ir(MDQ)2(acac) NPD 0.24

NPD/ B3PYMPM 0.20Ir(bt)2(acac) BPhen 0.22Ir(chpy)3 NPD 0.23Ir(mphq)2(acac) NPD/ B3PYMPM 0.23Ir(phq)3 NPD/ B3PYMPM 0.30Ir(piq)3 NPD 0.22Ir(bppo)2(acac) CBP 0.22Ir(ppy)3 CBP 0.33

Graf, A. et al., J. Mater. Chem. C, 2014, 2, 10298-10304

NIr

3

Ir(ppy)3

acac tmd

O O O O

Dopant molecules present at 5-10% in an isotropic host matrix

All (C^N)2Ir(acac) give θver = 0.20-0.25

NIr

2

O

O

Ir(ppy)2(acac)

NIr

3

Ir(piq)3

NIr

3

Ir(chpy)3

Mechanisms for dopant alignment in isotropic host

• Irregular/”spherical” molecules, i.e. Ir based phosphors– Molecules are closer to spherical than linear– Vacuum/Organic boundary induces molecular orientation– Only isotropic host-dopant is observed for solution cast films– Chemical anisotropy within the molecule can drive alignment

M.J. Jurow, et al., Nat. Mater., 2015

HC N

HCIr

O

O

2

+ HOST

Vapor Deposition of

We are not measuring molecular orientation, but the orientation of the TDV. Fortunately we can get TDV from modeling.

N Ir N

OO

Ideal Orientations

• (ppy)2Ir(acac) and fac-Ir(ppy)3 emit from “Ir-ppy”– TDV should be in the Ir-ppy plane, but where?– Orientation of the transition dipole moment vectors (TDV)

measured in (ppy)Re(CO)4: Re-N-TDV = 18.5° *

* Vanhelmont, F. W. M., et. al, J. Phys. Chem. A, 1997, 101, 2946-2952

δ = 18°N

CRe(CO)4

N NN Ir

C3

fac-(C^N)3IrIf aligned θ whould

be near 0

(C^N)2Ir(L^X)θ is what we expect

C2

N Ir N

OO

High efficiency due to orientation?

NNIr

3

• Sky-blue emitter with high PLQY (Φ=0.61)

• Reported in OLEDs to have EQE’s above 30%

• Molecular frame is more planar and oblate than Ir(ppy)3 and thus might vapor deposit in a way conducive to alignment

• Candidate for molecular orientation

Kido, Adv. Mater. 2014, 26, 5062-5066

fac-Ir(mi)3

3D Representation of Ir(mi)3

• Deviation from spherical shape• Intoduction of a geometric

asymmetry from Ir(ppy)3

Ir(ppy)3Ir(mi)3

Top View – looking down the C3 axis

Side View

400 450 500 550 600 650 7000.0

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1.0

mCP : Ir(pim)3 (10wt%)

0 10 20 30 40 50 60 70 80 900.0

0.5

1.0

1.5

2.0

Nor

mal

ized

Inte

nsity

Detector Angle [Degree]

Isotropic (θhor= 33.3%) Horizontal (θhor= 0%) Fit (θhor= 26.7 1.1%) mCP : Ir(pim)3 (10wt%)

Ir(mi)3 Angle Dependent PL

Layer Anisotropyfactor Orientation

TCTA : Ir(pim)3 26 ± 3 Horizontal

TCTA: 26DCzPPy :

Ir(pim)3

25 ± 3 Horizontal

mCP : Ir(pim)3 27 ± 1 Horizontal

• Orientation is matrix independent• High EL efficiency is tied to aligned

emitters• Can we get better discrimination by

substitution?

NNIr

3

NNIr

3

NNIr

3

FFF

Diversifying Ir(pim)3 to Study Alignment

Ir(pi)3

Ir(mip)3Ir(miF)3

• Increase geometric asymmetry to more oblate

• Introduce functional group to enhance chemical asymmetry

Structure of fac-Ir(miX)3Si

de V

iew

Top

View

Ir(miF)3 Ir(mi)3 Ir(mip)3

What the orientation of the transition dipole moment?

Transition Dipole Vector (TDV) Calculations

Angle of TDV determined using ZORA calculations• Developed by Van Lenthe, Baerends, and Snijders*• Zero Order Regular Approximation (ZORA)• Implemented in Schrödinger Inc., Materials Suite

NNIr

3

fac-Ir(pi)3

33° off of Ir-N bond 33° off of Ir-N bond

NNIr

3

fac-Ir(pip)3

NNIr

F FF 3

fac-Ir(piF)3

38° off of Ir-N bond

* E. Van Lenthe; E.J. Baerends and J.G. Snijders, J. Chem. Phys., 1993, 1994, 1996

These angles put the TDV for pi and pip cpds. at 85° to the C3 axis!!

Photophysical data mi family

400 450 500 550 600 650 7000.0

0.2

0.4

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0.8

1.0

PL

Inte

nsity

(a.u

.)

Wavelength (nm)

Ir(pim)3 Ir(pimF)3 Ir(pimp)3

λem (nm) PLQY τ (μs) CIE θverIr(mi)3 470 .91 2.04 (0.20, 0.41) 0.26Ir(miF)3 484 .99 2.47 (0.19, 0.49) 0.22Ir(mip)3 472 .98 1.61 (0.16, 0.35) 0.15

Ir(pimp)3 shows substantial alignmentWith TDV 33° off of the Ir-N bond this is perfect alignment of C3 ⊥ to surface

0 10 20 30 40 50 60 70 80 900.0

0.5

1.0

1.5

2.0

Nor

mal

ized

Inte

nsity

Detector Angle [Degree]

TCTA:Irpimp3 (10 vol.%) Fit (θhor=0.150.04) θhor= 0.33 θhor= 0.00

Ir(mi)3 Ir(miF)3 Ir(mip)3

OLED Device Results

• Peak EQE

– Ir(mip)3 30.5%– Ir(mi)3 26.4%– Ir(mif)3 25.5%– Ir(ppy)3 22.3%

400 450 500 550 600 650 7000.0

0.2

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1.0

1.2

Nor

mal

ized

Spe

ctru

m

Wavelength [nm]

Ir(mi)3

Ir(miF)3

Ir(mip)3

0 3 6 9 12 1510-7

10-6

10-5

10-4

10-3

10-2

10-1

100

101

102

103

mip mi miF Irppy

J OLE

D [m

A/c

m2 ]

VOLED [V]1E-3 0.01 0.1 1 10 1000

5

10

15

20

25

30

35

mip mi miF Irppy

EQE

[%]

JOLED [mA/cm2]

Strong correlation between orientation and EQE!!!!

Summary

• Multiple solutions for white OLEDs• Alignment can be used to enhance external efficiency for

all of them, fluorescent or phosphorescent

BCzVBi:CBP

BCzVBi:CBP

ITO/Glass

NPD

BCP

LiF/Al

CBP

CBP

PQIr:CBP

Ir(ppy)3:CBP

BPhen 20nm/BPhen:Li

Singletfilter

Tripletemitter

All phosphorHybrid flour. + phos.

Acknowledgements

• Mark Thompson, John Facendola, Daniel SylvinsonDepartment of Chemistry, University of Southern California

• Wolfgang Brütting, Tobias Schmidt, Thomas LampeInstitute of Physics, University of Augsburg

• Stephen Forrest, Jongchan KimDepartments of Physics and Electrical Engineering, Univ. Michigan

Bavaria California Technology Center

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