LIGHT EXTRACTION

The hurdle, OLEDs need to take

Sebastian Reineke reineke@mit.edu

(Massachusetts Institute of Technology, Cambridge, MA)

MOTIVATION

$3.6/lm 1

[1] www.lumiotec.com

First panels entering market:

2017 projection : approx. $0.05/lm ¬ Efficiency: > 100 lm/W (10-fold increase) ¬ Cost: panel cost reduction from $355 to $55 (6.5-fold decrease)

1

High efficiency devices

Simplified, low cost devices

Next generation OLEDs

MOTIVATION

MOTIVATION

Only roughly 20% leave the

front face.

Upper limit of 40 – 50 lm/W.

EFFICIENCY

Technology lm/W Lifetime1 Comments Incandescent bulb 151 1 kh

Halogen 191 2 kh inefficient

CFL 601 10 kh moderate efficiency

Fluorescent tube 90-1002 10 kh poor light quality

Inorganic LED 1043 50 kh point source 1 Steele, Nature Photonics, 2007, 1, 25.

OLED luminous efficacy:

operating voltage light outcoupling internal efficiency

necessities for (white) OLEDs:

2 wikipedia.org, 3 leds.de, retrieved: 2012-01-23.

metal

glass substrate n = 1.51

emission layer

ITO and organics n = 1.7 – 1.8

outcoupled / air mode

substrate mode

organic mode

coupling to metal surface plasmon (SP) field

the four modes in OLEDs

LIGHT MODES

air n = 1

the substrate mode

LIGHT MODES

ideally: pyramids

Reineke et al., Nature, 2009, 459, 234.

lenses

Sun et al., Nature Photonics, 2008, 2, 483.

pillars

Moller et al., J. Applied Physics, 2002, 91, 3324.

2 mm

the organic mode

LIGHT MODES

corrugation

Koo et al., Nature Photonics, 2010, 4, 222.

mode redirection

Sun et al., Nature Photonics, 2008, 2, 483.

Koh et al., Advanced Materials, 2010, 22, 1849.

refractive index matching

Reineke et al., Nature, 2009, 459, 234.

Meerheim et al., Applied Physics Letters, 2010, 97, 253305.

substrate E

TL

interplay of organic and SP modes

LIGHT MODES

n = 1.51

EML

metal

SP field

Furno et al., Proc. SPIE, 2010, 7617, 16.

Higher order devices with refractive index matched substrates

Reineke et al., Nature, 2009, 459, 234.

LIGHT MODES

escape cone

Higher order devices with refractive index matched substrates

Reineke et al., Nature, 2009, 459, 234.

LIGHT MODES

low versus high refractive index substrate

1st versus 2nd field antinode on high index substrates

TOP EMITTING OLEDs

transparent (glass)

organic metal

B opaque (metal)

organic metal

T

dielectric

opaque (metal)

organic metal

T

Thomschke et al., Applied Physics Letters 2009, 94, 083303.

Cheap alternative with complex optics. ¬ outcoupling enhancement hard to achieve

¬ strong color changes with increasing viewing angle

¬ conventional substrate surface modification not applicable

TOP EMITTING OLEDs

10 μm

Thomschke et al., Nano Letters 2012, 12, 424.

TOP EMITTING OLEDs

Thomschke et al., Nano Letters 2012, 12, 424.

without microlens foil with microlens foil

TOP EMITTING OLEDs

Thomschke et al., Nano Letters 2012, 12, 424.

some numbers: ¬ 30 lm/W at 1000 cd/m2 ¬ CRI of 93 with CIE (0.472, 0.430)

SUMMARY

Bottom-emitting OLEDs: ¬ extracting substrate modes is trivial

¬ various concepts exist to efficiently couple out organic modes (cheap process integration is key to determine successful path) ¬ strong losses in OLEDs due to coupling to metal surface plasmon modes

• spacing EML and cathode is very efficient to suppress coupling • however, increasing ETL increases organic modes with overall small benefit • organic and SP mode extraction should be considered a coupled system

Top-emitting OLEDs: ¬ typically complex optical systems (micro-resonator) with detrimental properties for white OLEDs ¬ lamination of refractive index matched microlens film achieves efficient outcoupling enhancement and color mixing

Acknowledgements: ¬ Prof. Karl Leo and his team at IAPP, Dresden, Germany.

¬ Prof. Marc Baldo, MIT, Cambridge, USA. ¬ Deutsche Forschungsgemeinschaft for funding.

Thank you for your kind attention.

MOTIVATIONEFFICIENCYLIGHT MODESTOP EMITTING OLEDsSUMMARY