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Transparent OLEDs offer great potential for novel applications. Preferably, the light should be emitted from one side only. This can bedone to some extent by modifying electrode thicknesses, but at the cost of reduced transparency. Here, we demonstrate a new approach tomake single side emissive OLEDs while maintaining good transparency.

Al2O3 thin films synthesized by plasma-enhanced atomic layer deposition(ALD) at room temperature (25 ºC) have been tested as water vapor per-meation barriers for OLED devices. Silicon nitride films (a-SiNx:H)deposited by plasma-enhanced chemical vapor deposition (PE-CVD) servedas reference and were used to develop Al2O3/a-SiNx:H stacks. On the basisof Ca test measurements, a very low intrinsic water vapor transmissionrate (WVTR) of ≤ 2·10-6 g·m-2·day-1 and 4·10-6 g·m-2·day-1 (20 oC/50% relative humidity) were found for 20-40 nm Al2O3 and 300 nm a-SiNx:Hfilms respectively. The cathode particle coverage was a factor four betterfor the Al2O3 films compared to the a-SiNx:H films and an average of 0.12defects per cm2 was obtained for a stack consisting of three barrier layers(Al2O3/ a-SiNx:H/ Al2O3).

Within a 500h test, ALD encapsulated poly-LEDs show approximately half the black spot density of plasma deposited a-SiNx:H (300 nm thick) encapsulated devices. The black spot density is further reduced ifthe a-SiNx:H layer is coupled with an ALD layer and such decrease is accompanied by a massive reduction of the black spot area growth in time. A selection of electroluminescence measurements of encapsulated poly-LEDs point out a very promising application of ALD Al2O3 layers in the field of LED encapsulation and will be interpreted in terms of possible mechanisms related to film growth in multi-layer structures.