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Diffusion Barriers

Fig. 1: Diffusion barrier thickness dependence of WVTR in single Al2O3 layers and multilayer Al2O3/TiO2 structures.

 

Single layer Al2O3 thin films have been deposited by atomic layer deposition (ALD) and tested as diffusion barriers for organic light emitting diodes (OLED). Water vapor transmission rates (WVTR) for Al2O3 barrier layers on PPET reduce with increasing thickness of the barrier layer. WVTR of about 3·10-4 g/m2/day were demonstrated with these single layer barriers, revealing the superior barrier properties of the ALD films compared to encapsulation layers of comparable thickness realized with any other deposition technique. However, these values are still above the limit defined for OLED encapsulation.

Fig. 2: AFM image of a 5 nm thick Al2O3 layer on Ca/glass directly after deposition and 48  hours after deposition. Line scan (inset) on the two AFM images, respectively.

 

 

 

Atomic force microscopy (AFM) was performed on 5 nm thick Al2O3 barrier layers on calcium to understand the origin of defects and sub-sequent corrosion. AFM images directly after deposition and 48 hours after deposition are shown in Fig. 2. The dark spots are “open areas” without Al2O3 coverage, which are penetrable for water and causing Ca degradation. This can be seen in the right AFM image where bright areas represent Ca(OH)2 formed from the reaction of Ca with H2O. Line profiles measured before and after degradation show that trenches change into peaks after corrosion. Multilayer nano-laminates of Al2O3/TiO2 with thickness comparable to the single layer barriers show WVTR values at least one order of magnitude lower. The WVTR also improves with increasing thickness. The lowest WVTR of 1·10-5 g/m2/day is achieved at 50 nm barrier layer thickness as shown in Fig. 1.

Fig. 3: Luminescence of OLEDs coated with Al2O3/TiO2 nanolaminates in dependence on individual layer thicknesses.

 

One of our research highlights in this field is the encapsulation of OLEDs with Al2O3/TiO2 nano-laminates (Fig. 3). The analysis of the device reliability disclosed, that the luminescence drop after 1000 operation hours can be correlated to the interlayer thickness. OLEDs coated with thin interlayers (total barrier thickness constant for all samples) of 0.5 - 1 nm showed fewer pinholes compared to OLEDs coated with nano-laminates consisting of thicker interlayers in the range of 4 - 8 nm. This means, that best encapsulation layers are obtained with the thinnest interlayer thickness of 0.5 - 1 nm. In other words, encapsulation layer properties improve with increased interlayer repetition frequency.

 

 

Contact: Dr. Uwe Schroeder

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