Predicting the Lifetime of Flexible Permeation Barrier Layers for OLED Displays

Transcription

1 Predicting the Lifetime of Flexible Permeation Barrier Layers for OLED Displays 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

2 Outline 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

3 Why do we need permeation barrier films? Organic Light Emitting Diode on Plastic film Flexible permeation barrier film OLED Plastic film Lifetime ~few minutes to few days Required lifetime > 0 years! Required barrier film water vapor transmission rate: 0-6 g / (m day) LG Display, SID 03 UDC, SID 0 Samsung, CES 03 3

4 Modes of permeation through a barrier layer Flexible permeation barrier film Pin-hole 4 Particle OLED Barrier 3 Permeation along a particle 4µm film at 65 C 85% RH t = 0 7 h 5h 6h Water permeates in four modes:. Through pin-holes. Along particles 3. Along interfaces 4. Through the bulk of the barrier layer Permeation along interface 6µm film at 65 C 85% RH In university research, often,, 3 4 t = h 967h 69h P. Mandlik, et al., APL 93, (008). Difficult to measure! 4

5 Motivation for measuring bulk permeation Permeation along a particle 4µm film at 65 C 85% RH t = 0 7 h 5h 6h Tests on OLEDs are not quantitative! We need new techniques! How does quantitative evaluation of bulk permeation help?. Evaluate new permeation barrier materials. 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:. Secondary Ion Mass Spectroscopy (SIMS). Electrical capacitance 3. Film stress 5

6 concentration n x, t Evaluation of diffusion profiles Water concentration profile In an ideal barrier Water side: n = n(x=0) n(0) n x, t = n(x=0)erfc x Dt x OLED side: n h = 0 h time depth x 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: 0-6 g / (m day) 6

7 atoms cc Secondary Ion Mass Spectrometry, SIMS 00 D O SIMS profile after hours 00 D O. A 660 nm thick barrier layer on a silicon wafer was boiled in heavy water, D O for hours.. Deuterium was determined by sputter profiling using secondary ion mass spectroscopy Deuterium profile depth x (nm) The deuterium follows erfc function! Diffusion coefficient: D = cm s Solubility of water: n 0 = molecules cm 3 = 4.8 mg cm 3 7

8 concentration n x, t N t Extracting D from total dissolved water Water concentration profile x n x, t = n(0)erfc n(0) Dt Total number of dissolved molecules in the barrier N(t) = n x, t dt h 0 time 3 3 N t = 4n x=0 D t π depth x time t Film capacitance C Film stress σ is proportional to N(t) Therefore C(t) and σ(t) can be used to determine D 8

9 C t C 0 in Dielectric constant, ε x, t D from Electrical Capacitance ε(0) Capacitor structure time h ε barrier depth x pf Slope = 4 π C( ) C(0) h D C(t) C 0 = C( ) C 0 h π Dt C t = capacitance at time t C 0 = initial capacitance C( ) = saturated final capacitance In water at 00 C = ε 0 ε A d ε barrier ε SiO = 3.9 ε barrier with H O = N(t) time t (hours) Diffusion coefficient: D = cm Compare D from SIMS: cm s 9 s

10 D from Stress In-diffusing water causes film expansion of the barrier layer Compressive stress Change in stress MPa In water at 00 Water uptake Film under stress Slope = 4 π σ( ) h D Average film stress: R E W H h H σ = E W 6R h - Bending radius - Wafer elastic constant - Substrate thickness - Barrier thickness time t (hours) Diffusion coefficient: D = cm D from SIMS : cm /s Capacitance : cm /s s Stress: σ t N(t) 8 = 0 h σ t - stress at time t σ - saturated final stress MPa Advantages:. Extremely simple fabrication: step!. Particles and defects have no impact! 0

11 Salient points of new techniques SIMS Diffusion coefficient Area Barrier thickness D = cm /s Electrical Capacitance D = cm /s Film stress D = cm /s 0.mmx0.mm sputter target mmxmm capacitor size 4 inch silicon wafer 660nm 00nm 500nm Uniform D over different area and thickness Measured at 00 C boiling water (00 C 00% RH) What about performance at room temperature?

12 molecules cm 3 atm Solubility and Diffusion coefficient activation energies Diffusion Coefficient (cm s) Solubility Obtained from film stress measurements Diffusion coefficient n T = n 0 e 0.0eV kt D T = D 0 e 0.7eV kt T( ) T( ) silica glass + Measured solubility E S = 0.0eV 000 T (/K) E D = 0.7eV 000 T (/K) + Tomozawa, M., Am Ceram Soc Bull. 985, 337.

13 Extrapolating barrier performance to room temperature Number of monolayers of permeated water At 00 C and 00% Relative Humidity Solubility molecules cm 3 atm At 38 C and 90% Relative Humidity Diffusion coefficient cm s Solubility molecules cm 3 Solubility activation energy Diffusion coefficient activation energy 0.0 ev 0.7 ev Diffusion coefficient cm s (*P HO at 38 C and 90% Relative Humidity is 0.06atm) Total quantity of permeated water Performance of a 3µm barrier at 38 C and 90% Relative Humidity 3µm, 38 C and 90% RH Water vapor transmission rate g m day monolayer of water Permeation time for monolayer 3.4 years time t (years) 3

14 time τ ML (years) Acceleration factor Barrier design and testing At 38 C and 90% Relative Humidity Solubility molecules cm 3 Diffusion coefficient cm s monolayer permeation time at 38 C 90% RH Acceleration factor from 38 C 90% RH to 00% RH at higher temperatures τ ML =.4h.57 3µm, τ ML = 3.4 years Barrier thickness h (μm) Temperature ( ) Barrier film lifetime is not linear with thickness! 4

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 5

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 6

17 Questions? Thanks! 7