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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
β’ Introduction on permeation barrier films
β’ Modes of permeation of water
Bulk permeation
β’ Techniques for measuring diffusion of water
Secondary Ion Mass Spectrometry
Electrical Capacitance
Film stress
β’ 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 ~πππ€ ππππ’π‘ππ π‘π πππ€ πππ¦π
Required lifetime > 10 π¦ππππ !
Flexible permeation barrier film
Required barrier film water vapor transmission rate: β€ 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 4Β΅m film at 65Β°C 85% RH
3
In university research, often
Difficult to measure!
3 1 2 , , 4 β«
Flexible permeation barrier film
Permeation along interface 6Β΅m film at 65Β°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 4Β΅m film at 65Β°C 85% RH
6
Evaluation of diffusion profiles
π₯ β
Water side: π = π(π₯=0)
OLED side: π β = 0
In an ideal barrier
ππππ‘β π₯
π‘πππ
πππππππ‘πππ‘πππ ππ₯,π‘
π π₯, π‘ = π(π₯=0)πππππ₯
π·π‘ π(0)
Water concentration profile
Permeability π = π· Γ π(π₯ = 0)
Water Vapor Transmission Rate WVTR = π/β
Fundamental properties:
β’ Solubility of water, π(π₯=0) β’ Diffusion coefficient, π·
Required OLED water vapor transmission rate: β€ 10-6 g / (m2 day)
ππππ‘β π₯ (nm) ππ‘πππ ππ
π·ππ’π‘ππππ’π πππππππ
100β π·2π
SIMS profile after 12 hours
1. A 660 nm thick barrier layer on a silicon wafer was boiled in heavy water, π·2π for 12 hours.
2. Deuterium was determined by sputter profiling using secondary ion mass spectroscopy
π·ππππ’π πππ πππππππππππ‘: π· = 4.2 Γ 10β15 ππ2 π
ππππ’πππππ‘π¦ ππ π€ππ‘ππ: π 0 = 1.6 Γ 1020ππππππ’πππ ππ3 = 4.8ππ ππ3
7
Secondary Ion Mass Spectrometry, SIMS
The deuterium follows erfc function!
100β π·2π
8
Extracting D from total dissolved water
ππππ‘β π₯
π‘πππ
πππππππ‘πππ‘πππ ππ₯,π‘
π π₯, π‘ = π(0)πππππ₯
π·π‘
π(0)
Water concentration profile
Film capacitance C Film stress Ο
is proportional to π(π‘)
Therefore C(t) and Ο(t) can be used to determine D
1 2
3
π‘πππ π‘
ππ‘2
Total number of dissolved molecules in the barrier
π π‘ 2 =4π π₯=0 2
ππ· Γ π‘
1
2
3
π(π‘) = π π₯, π‘ ππ‘β
0
ππππ‘β π₯
π·ππππππ‘πππ ππππ π‘πππ‘,ππ₯,π‘
π‘πππ
π(0)
ππππππππ
9
D from Electrical Capacitance
πΆ = π0ππ΄
π
ππππππππ β ππππ2 = 3.9
ππππππππ π€ππ‘β π»2π = 3.9 + 2.6 Γ 10β16 π(π‘)
π‘πππ π‘ (βππ’ππ )
1
πΆπ‘β 1
πΆ0
2
ππ 1
ππΉ2
πππππ =4
π
1
πΆ(β)β
1
πΆ(0)
1
β
2
Γ π·
In water at 100β
π·ππππ’π πππ πππππππππππ‘: π· = 5.6 Γ 10β15 ππ2 π
Compare D from SIMS: 4.2 Γ 10β15 ππ2 π
1
πΆ(π‘)β
1
πΆ 0=
1
πΆ(β)β
1
πΆ 0
2
β ππ·π‘
πΆ π‘ = capacitance at time t πΆ 0 = initial capacitance πΆ(β) = saturated final capacitance
β
Capacitor structure
πΆβππππ ππ π π‘πππ π
2 πππ2
πππππ =4
π
π(β)
β
2
Γ π·
π‘πππ π‘ (βππ’ππ )
In water at 100β
π·ππππ’π πππ πππππππππππ‘: π· = 4.4 Γ 10β15 ππ2 π
SIMS : 4.2 Γ 10β15 ππ2/π Capacitance : 5.6 Γ 10β15 ππ2/π
π· ππππ
Average film stress:
π = πΈπ6π π»2
β
π - Bending radius πΈπ - Wafer elastic constant π» - Substrate thickness β - Barrier thickness
Water uptake Film under stress
10
D from Stress
π π‘ = 2 Γ 10β18π(π‘)
β πππ Stress:
π π‘ - stress at time t π β - 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 π· = 4.2 Γ 10β15 ππ2/π 0.1mmx0.1mm sputter target
660nm
Electrical Capacitance
π· = 5.6 Γ 10β15 ππ2/π 1mmx1mm
capacitor size 200nm
Film stress π· = 4.4 Γ 10β15 ππ2/π 4 inch
silicon wafer 1500nm
Uniform D over different area and thickness
What about performance at room temperature?
Measured at 100Β°C boiling water (100Β°C 100% RH)
12
Solubility and Diffusion coefficient activation energies ππππππ’πππ ππ
3ππ‘π
1000 π (1/πΎ)
π(β)
Solubility
πΈπ = β0.20ππ
Measured solubility
π π = π0π0.20ππ
ππ
Obtained from film stress measurements
1000 π (1/πΎ)
π·ππππ’π πππ πΆππππππππππ‘ (ππ2π )
π(β)
Diffusion coefficient
πΈπ· = 0.71ππ
π· π = π·0πβ0.71ππ
ππ
silica glass+
+Tomozawa, M., Am Ceram Soc Bull. 1985, 1337.
13
Extrapolating barrier performance to room temperature
At 100Β°C and 100% Relative Humidity
Solubility 1.6 Γ 1020ππππππ’πππ ππ3ππ‘π
Diffusion coefficient 4.2 Γ 10β15 ππ2 π
Solubility activation energy β0.20 ππ
Diffusion coefficient activation energy 0.71 ππ
At 38Β°C and 90% Relative Humidity
Solubility 3.2 Γ 1019ππππππ’πππ ππ3
Diffusion coefficient 5.4 Γ 10β17 ππ2 π
Water vapor transmission rate
1.5 Γ 10β7 π π2πππ¦
π‘πππ π‘ (π¦ππππ )
ππ’ππππ ππ πππππππ¦πππ
ππ πππππππ‘ππ π€ππ‘ππ
Total quantity of permeated water
Performance of a 3Β΅m barrier at 38Β°C and 90% Relative Humidity
3Β΅m, 38Β°C and 90% RH
1 monolayer of water
(*PH2O at 38Β°C and 90% Relative Humidity is 0.06atm)
Permeation time for 1 monolayer
13.4 π¦ππππ
14
Barrier design and testing
At 38Β°C and 90% Relative Humidity
Solubility 3.2 Γ 1019ππππππ’πππ ππ3
Diffusion coefficient 5.4 Γ 10β17 ππ2 π
π΅ππππππ π‘βππππππ π β (ππ)
π‘πππ πππΏ (π¦ππππ )
1 monolayer permeation time at 38Β°C 90% RH
ππππππππ‘π’ππ (β)
π΄ππππππππ‘πππ ππππ‘ππ
Acceleration factor from 38Β°C 90% RH to 100% RH at higher temperatures
Barrier film lifetime is not linear with thickness!
πππΏ = 2.41β1.57
3Β΅m, πππΏ = 13.4 π¦ππππ
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!
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