Bhadri Visweswaran, Siddharth Harikrishna Mohan, William Quinn, Ruiqing (Ray) Ma, Jeff Silvernail, James Sturm, Sigurd Wagner

Electrical Engineering and Princeton Institute for the Science and Technology of Materials

Princeton University Universal Display Corporation, Ewing, New Jersey

Predicting the Lifetime of Flexible Permeation Barrier Layers for OLED Displays

1

• Introduction on permeation barrier films

• Modes of permeation of water

Bulk permeation

• Techniques for measuring diffusion of water

Secondary Ion Mass Spectrometry

Electrical Capacitance

Film stress

• Designing barrier films and predicting a display lifetime

2

Outline

Plastic film OLED

3

Why do we need permeation barrier films?

Samsung, CES 2013 LG Display, SID 2013

UDC, SID 2012

Lifetime ~𝑓𝑒𝑤 𝑚𝑖𝑛𝑢𝑡𝑒𝑠 𝑡𝑜 𝑓𝑒𝑤 𝑑𝑎𝑦𝑠

Required lifetime > 10 𝑦𝑒𝑎𝑟𝑠!

Flexible permeation barrier film

Required barrier film water vapor transmission rate: ≤ 10-6 g / (m2 day)

Organic Light Emitting Diode on Plastic film

Water permeates in four modes: 1. Through pin-holes 2. Along particles 3. Along interfaces 4. Through the bulk of the barrier layer

4

Modes of permeation through a barrier layer

P. Mandlik, et al., APL 93, 203306 (2008).

1

2

t = 0 17 h 115h 162h

Permeation along a particle 4µm film at 65°C 85% RH

3

In university research, often

Difficult to measure!

3 1 2 , , 4 ≫

Flexible permeation barrier film

Permeation along interface 6µm film at 65°C 85% RH

t = 0 863 h 1967h 2692h

OLED

Particle

Barrier

Pin-hole 4

5

Motivation for measuring bulk permeation

How does quantitative evaluation of bulk permeation help? 1. Evaluate new permeation barrier materials 2. Design new single and multilayer barrier films 3. Extrapolate and predict room temperature condition performance

from accelerated tests

I quantitatively evaluate intrinsic water diffusion using 3 techniques: 1. Secondary Ion Mass Spectroscopy (SIMS) 2. Electrical capacitance 3. Film stress

Tests on OLEDs are not quantitative!

We need new techniques! t = 0 17 h 115h 162h

Permeation along a particle 4µm film at 65°C 85% RH

6

Evaluation of diffusion profiles

𝑥 ℎ

Water side: 𝑛 = 𝑛(𝑥=0)

OLED side: 𝑛 ℎ = 0

In an ideal barrier

𝑑𝑒𝑝𝑡ℎ 𝑥

𝑡𝑖𝑚𝑒

𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 𝑛𝑥,𝑡

𝑛 𝑥, 𝑡 = 𝑛(𝑥=0)𝑒𝑟𝑓𝑐𝑥

𝐷𝑡 𝑛(0)

Water concentration profile

Permeability 𝑃 = 𝐷 × 𝑛(𝑥 = 0)

Water Vapor Transmission Rate WVTR = 𝑃/ℎ

Fundamental properties:

• Solubility of water, 𝑛(𝑥=0) • Diffusion coefficient, 𝐷

Required OLED water vapor transmission rate: ≤ 10-6 g / (m2 day)

𝑑𝑒𝑝𝑡ℎ 𝑥 (nm) 𝑎𝑡𝑜𝑚𝑠𝑐𝑐

𝐷𝑒𝑢𝑡𝑒𝑟𝑖𝑢𝑚 𝑝𝑟𝑜𝑓𝑖𝑙𝑒

100℃ 𝐷2𝑂

SIMS profile after 12 hours

1. A 660 nm thick barrier layer on a silicon wafer was boiled in heavy water, 𝐷2𝑂 for 12 hours.

2. Deuterium was determined by sputter profiling using secondary ion mass spectroscopy

𝐷𝑖𝑓𝑓𝑢𝑠𝑖𝑜𝑛 𝑐𝑜𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑡: 𝐷 = 4.2 × 10−15 𝑐𝑚2 𝑠

𝑆𝑜𝑙𝑢𝑏𝑖𝑙𝑖𝑡𝑦 𝑜𝑓 𝑤𝑎𝑡𝑒𝑟: 𝑛 0 = 1.6 × 1020𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑒𝑠 𝑐𝑚3 = 4.8𝑚𝑔 𝑐𝑚3

7

Secondary Ion Mass Spectrometry, SIMS

The deuterium follows erfc function!

100℃ 𝐷2𝑂

8

Extracting D from total dissolved water

𝑑𝑒𝑝𝑡ℎ 𝑥

𝑡𝑖𝑚𝑒

𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 𝑛𝑥,𝑡

𝑛 𝑥, 𝑡 = 𝑛(0)𝑒𝑟𝑓𝑐𝑥

𝐷𝑡

𝑛(0)

Water concentration profile

Film capacitance C Film stress σ

is proportional to 𝑁(𝑡)

Therefore C(t) and σ(t) can be used to determine D

1 2

3

𝑡𝑖𝑚𝑒 𝑡

𝑁𝑡2

Total number of dissolved molecules in the barrier

𝑁 𝑡 2 =4𝑛 𝑥=0 2

𝜋𝐷 × 𝑡

1

2

3

𝑁(𝑡) = 𝑛 𝑥, 𝑡 𝑑𝑡ℎ

0

𝑑𝑒𝑝𝑡ℎ 𝑥

𝐷𝑖𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡,𝜖𝑥,𝑡

𝑡𝑖𝑚𝑒

𝜖(0)

𝜖𝑏𝑎𝑟𝑟𝑖𝑒𝑟

9

D from Electrical Capacitance

𝐶 = 𝜀0𝜀𝐴

𝑑

𝜀𝑏𝑎𝑟𝑟𝑖𝑒𝑟 ≅ 𝜀𝑆𝑖𝑂2 = 3.9

𝜀𝑏𝑎𝑟𝑟𝑖𝑒𝑟 𝑤𝑖𝑡ℎ 𝐻2𝑂 = 3.9 + 2.6 × 10−16 𝑁(𝑡)

𝑡𝑖𝑚𝑒 𝑡 (ℎ𝑜𝑢𝑟𝑠)

1

𝐶𝑡− 1

𝐶0

2

𝑖𝑛 1

𝑝𝐹2

𝑆𝑙𝑜𝑝𝑒 =4

𝜋

1

𝐶(∞)−

1

𝐶(0)

1

ℎ

2

× 𝐷

In water at 100℃

𝐷𝑖𝑓𝑓𝑢𝑠𝑖𝑜𝑛 𝑐𝑜𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑡: 𝐷 = 5.6 × 10−15 𝑐𝑚2 𝑠

Compare D from SIMS: 4.2 × 10−15 𝑐𝑚2 𝑠

1

𝐶(𝑡)−

1

𝐶 0=

1

𝐶(∞)−

1

𝐶 0

2

ℎ 𝜋𝐷𝑡

𝐶 𝑡 = capacitance at time t 𝐶 0 = initial capacitance 𝐶(∞) = saturated final capacitance

ℎ

Capacitor structure

𝐶ℎ𝑎𝑛𝑔𝑒 𝑖𝑛 𝑠𝑡𝑟𝑒𝑠𝑠

2 𝑀𝑃𝑎2

𝑆𝑙𝑜𝑝𝑒 =4

𝜋

𝜎(∞)

ℎ

2

× 𝐷

𝑡𝑖𝑚𝑒 𝑡 (ℎ𝑜𝑢𝑟𝑠)

In water at 100℃

𝐷𝑖𝑓𝑓𝑢𝑠𝑖𝑜𝑛 𝑐𝑜𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑡: 𝐷 = 4.4 × 10−15 𝑐𝑚2 𝑠

SIMS : 4.2 × 10−15 𝑐𝑚2/𝑠 Capacitance : 5.6 × 10−15 𝑐𝑚2/𝑠

𝐷 𝑓𝑟𝑜𝑚

Average film stress:

𝜎 = 𝐸𝑊6𝑅 𝐻2

ℎ

𝑅 - Bending radius 𝐸𝑊 - Wafer elastic constant 𝐻 - Substrate thickness ℎ - Barrier thickness

Water uptake Film under stress

10

D from Stress

𝜎 𝑡 = 2 × 10−18𝑁(𝑡)

ℎ 𝑀𝑃𝑎 Stress:

𝜎 𝑡 - stress at time t 𝜎 ∞ - saturated final stress

In-diffusing water causes film expansion of the barrier layer Compressive stress

Advantages: 1. Extremely simple fabrication: 1 step! 2. Particles and defects have no impact!

11

Salient points of new techniques

Diffusion coefficient Area Barrier thickness

SIMS 𝐷 = 4.2 × 10−15 𝑐𝑚2/𝑠 0.1mmx0.1mm sputter target

660nm

Electrical Capacitance

𝐷 = 5.6 × 10−15 𝑐𝑚2/𝑠 1mmx1mm

capacitor size 200nm

Film stress 𝐷 = 4.4 × 10−15 𝑐𝑚2/𝑠 4 inch

silicon wafer 1500nm

Uniform D over different area and thickness

What about performance at room temperature?

Measured at 100°C boiling water (100°C 100% RH)

12

Solubility and Diffusion coefficient activation energies 𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑒𝑠𝑐𝑚

3𝑎𝑡𝑚

1000 𝑇 (1/𝐾)

𝑇(℃)

Solubility

𝐸𝑆 = −0.20𝑒𝑉

Measured solubility

𝑛 𝑇 = 𝑛0𝑒0.20𝑒𝑉

𝑘𝑇

Obtained from film stress measurements

1000 𝑇 (1/𝐾)

𝐷𝑖𝑓𝑓𝑢𝑠𝑖𝑜𝑛 𝐶𝑜𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑡 (𝑐𝑚2𝑠 )

𝑇(℃)

Diffusion coefficient

𝐸𝐷 = 0.71𝑒𝑉

𝐷 𝑇 = 𝐷0𝑒−0.71𝑒𝑉

𝑘𝑇

silica glass+

+Tomozawa, M., Am Ceram Soc Bull. 1985, 1337.

13

Extrapolating barrier performance to room temperature

At 100°C and 100% Relative Humidity

Solubility 1.6 × 1020𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑒𝑠 𝑐𝑚3𝑎𝑡𝑚

Diffusion coefficient 4.2 × 10−15 𝑐𝑚2 𝑠

Solubility activation energy −0.20 𝑒𝑉

Diffusion coefficient activation energy 0.71 𝑒𝑉

At 38°C and 90% Relative Humidity

Solubility 3.2 × 1019𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑒𝑠 𝑐𝑚3

Diffusion coefficient 5.4 × 10−17 𝑐𝑚2 𝑠

Water vapor transmission rate

1.5 × 10−7 𝑔 𝑚2𝑑𝑎𝑦

𝑡𝑖𝑚𝑒 𝑡 (𝑦𝑒𝑎𝑟𝑠)

𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑚𝑜𝑛𝑜𝑙𝑎𝑦𝑒𝑟𝑠

𝑜𝑓 𝑝𝑒𝑟𝑚𝑒𝑎𝑡𝑒𝑑 𝑤𝑎𝑡𝑒𝑟

Total quantity of permeated water

Performance of a 3µm barrier at 38°C and 90% Relative Humidity

3µm, 38°C and 90% RH

1 monolayer of water

(*PH2O at 38°C and 90% Relative Humidity is 0.06atm)

Permeation time for 1 monolayer

13.4 𝑦𝑒𝑎𝑟𝑠

14

Barrier design and testing

At 38°C and 90% Relative Humidity

Solubility 3.2 × 1019𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑒𝑠 𝑐𝑚3

Diffusion coefficient 5.4 × 10−17 𝑐𝑚2 𝑠

𝐵𝑎𝑟𝑟𝑖𝑒𝑟 𝑡ℎ𝑖𝑐𝑘𝑛𝑒𝑠𝑠 ℎ (𝜇𝑚)

𝑡𝑖𝑚𝑒 𝜏𝑀𝐿 (𝑦𝑒𝑎𝑟𝑠)

1 monolayer permeation time at 38°C 90% RH

𝑇𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒 (℃)

𝐴𝑐𝑐𝑒𝑙𝑒𝑟𝑎𝑡𝑖𝑜𝑛 𝑓𝑎𝑐𝑡𝑜𝑟

Acceleration factor from 38°C 90% RH to 100% RH at higher temperatures

Barrier film lifetime is not linear with thickness!

𝜏𝑀𝐿 = 2.41ℎ1.57

3µm, 𝜏𝑀𝐿 = 13.4 𝑦𝑒𝑎𝑟𝑠

15

Conclusion

Introduced simple techniques to measure diffusion coefficient of water

Electrical Capacitance

Film stress

Determined the concentration of water with SIMS, used to calibrate capacitance and film stress

The techniques are

Simple: fabrication & testing

Immune to particles and defects

With the techniques we can:

Rapidly evaluate barrier materials and films

Predict room temperature performance

16

Acknowledgements

Prof. Sigurd Wagner and group Sushobhan Avasti, Warren Rieutort-Louis, Josh Sanz-Robinson,

Lin Han, Prashant Mandlik

Prof. James Sturm

Princeton Program in Plasma Science and Technology

17

Questions?

Thanks!