Enhanced oucoupling efficiency with nanostructured anodes

Richard Pfeifer, Karsten Fehse, Uwe Vogel, and Karl Leo

09.10.2012

© Fraunhofer COMEDD

Fraunhofer Research Institution for Organics, Materials and Electronic Devices

COMEDD

Enhanced outcoupling efficiency with

nanostructured anodes for OLED microdisplay

integration


Richard Pfeifer I 09.10.2012 I slide 2

OLED - microdisplay applications

We would like to have

• Higher luminance

• Higher power efficiency

• Longer Lifetime

Great potential for improvement is

Light outcoupling efficiency

OLED microdisplays for

• mobile video

• Head-Up-Displays

• Micro-projection

• high resolution

viewfinders



Outline

• Enhanced light outcoupling by gratings

• Integration of gratings into OLED-stacks

• Grating parameters

• Simulation model

• Results

• Conclusion



Enhanced light outcoupling by gratings

Waveguided modes

(total interal reflection)

Surface Plasmon Polaritons

(evanescent excitation)

Radiating modes

Nonradiating decays

Planar OLED: Light emitted into waveguide modes is lost

emitting layer

Cathode

Anode

Substate

Leaky Waveguide modes

(by grating coupling)

Surface Plasmon Polaritons

(evanescent excitation)

Radiating modes

Nonradiating decays

emitting layer

Cathode

Anode

Substate

Grating OLED: Waveguide modes are „leaky“



Gratings and Microdisplays: A perfect match?

Usage of gratings fabricated by optical lithography favorable because

•Small active area

•Fabrication process does already involve multiple CMOS processes

•Integration into high value products

•Demand for high luminance

•Homogeneity no strict necessity in terms of

• Angular dependance

• White light appearance



Grating integration

• Microdisplays use top-emitting OLEDs:

• How to integrate gratings into thin-film stacks?

• without disturbing electrical properties of OLED

• Exposing OLED to harmful conditions

• Use CMOS-processes already available for active

matrix backplane fabrication

Metal cathode

Organic stack

Grating: conductive high index material (e.g. Si) /

low index material (SiO2)

Planar conductive high index layer (e.g. Si)

Metal anode



Simulations • Model: dipole emitters embedded in thin-film stacks [1]

• Light emitted into different angles is independent

no coupled modes

• Fast calculation of

• power emitted into each mode at position of emitter

• power in modes above thin film structure

Get outcoupling efficiency

• Calculate ‚outcoupling enhancement factor‘ OEF along kx-direction

• Modes differing by grating vector 2π/a are coupled

• Using RCWA [2] to calculate power into coupled modes.

• choose symmetric emitter position

• integration along - direction (restrict to direction of

greatest influence)

[1] RR Chance, A Prock, R Silbey ;The Journal of Chemical Physics, 1974

[2] MG Moharam, EB Grann, DA Pommet, TK Gaylord, JOSA A, Vol.12 No 5, 1995

RCWA…”Rigorously Coupled Wave Analysis”

top view side view

kx

top view

kx

kax

ax

side view



Planar OLED vs grating OLED

best outcoupling

efficiency (OCE): 29.2%

height grating: 50nm

height planar layer: 60nm

grating period a: 280nm

Simulated stack: green p-i-n OLED at 525nm

Grating: nlow=1.48 nHigh=4.19+0.007i (k comparable to organic)

ETL

HTL

grating

planar

layer

outcoupling

enhancement factor

(OEF) : 1.43



Influence of grating period

Parameter variation

• HTL 20 … 80nm

• ETL 30 … 80nm

• Grating 40 … 70nm

• Planar 40… 70nm

• Period 225…405nm

-> 220.000 datapoints

• Correlation between grating period and optical cavity length for highest OEF

• grating period determines kt of outcoupled waveguide mode

• kt of waveguide modes determined by optical length of cavity s



Influence of absorption

• Light absorption of silicon lowers outcoupling enhancement factor in optical

wavelength range

• Silicon as available high-index material in typical CMOS-foundries has higher

exctinction coefficient

-> effect for grating: 50nm, planar: 50nm, HTL: 40nm

k=0.007 (organic) k=0.011

k=0.020 (Si@620nm) k=0.034 (Si@525nm)



Conclusion

• Gratings and Microdisplays:

• Demonstrated concept for integration of high-index-contrast grating into

thin-film stack

• Grating concept only depends on CMOS processes and materials already

available for active matrix backplane fabrication

• Used RCWA-based simulation to evaluate beneficial grating parameters

• Real world extinction coefficient of silicon lowers outcoupling efficiency

enhancement in optical region of spectrum

• More flexibility of allowed materials needed (e.g. TCOs)

• Address infrared applications



Acknowledgments

This work has been partly funded with support from the Free State of Saxony and

the European Commission (EFRE) within the project CoolProjektor.

Enhanced oucoupling efficiency with nanostructured anodes

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