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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!