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Organic light-emitting diodes degrade rapidly by means of local cathode oxidation when exposed to the ambient atmosphere, resulting in visible non-emissive areas called black spots. High performance inorganic based encapsulations are required to protect the OLED. We have applied a hybrid thin-film encapsulation stack consisting of two inorganic barrier layers of silicon nitride deposited at low temperature with an organic layer in between. The resulting water permeation mechanism into the OLED is solely by means of lateral pinhole-to-pinhole transport. With the application of CaO nanoparticles in the organic layer the lateral water transport rate is reduced and we show that black spot formation in 8 cm2 OLEDs is delayed by 6000 h at accelerated climate conditions of 60°C/90% relative humidity. This is estimated to correspond to 20 years at ambient conditions.

Local laser ablation of the cathode of OLEDs has been applied to create a population of pinholes of the same size. This enables the direct comparison at different conditions of black spots and grey spots in the emission of OLEDs as a result of water ingress into the device. We confirmed earlier observations on the operational effect of black spot growth as determined in a sequential test on particle induced pinholes. Furthermore, we measured the operational effect in parallel, providing data that are test independent. With the same population of pinholes we studied the operational effect of grey spot growth. The observed larger operational effect of grey spot growth with respect to that of black spot growth is attributed to a lateral flow of protons to the centre of the black spot. In this part of the black spot a depletion region is present which is created by transport of H3O+ to the cathode under the influence of the applied electric field. This proposal is supported by the observation of black and grey spot growth in polymer LEDs on basis of PEDOT:PSS with increased pH.

To protect organic flexible devices from the ambient, they have to be encapsulated. Depending on the application in mind (OLED lighting, PV) different thin-film encapsulation methodology can be chosen. Each encapsulation process has different requirements and fabrication process freedom might be restricted, for example by mechanical reliability requirements or the total cost of the end product. Here we will show our recent investigations into different thin-film barriers with respect to their application and the route to production.

The crack onset strain (COS) of 4-level thin film transistor (TFT) devices on both steel foils and thin polyimide (PI) films was investigated using tensile experiments carried out in situ in an optical microscope. Cracks initiated first within the SiO2 insulator layer for both types of substrates. The COS was found to be equal to 1.15% and 0.24% for steel and PI, respectively. The influence of loading direction on failure of the TFT stack with anisotropic geometry was moreover found to be considerable, leading to recommendations for backplane design. The large difference in critical strain of the SiO2 layer on the two substrates was analyzed using an energy release rate approach, and found to result from differences in layer/substrate mechanical contrast and in internal stress state. Based on this analysis a correlation between layer/substrate elastic contrast and tensile failure behavior was devised. (C) 2010 Elsevier Ltd. All rights reserved.

Nowadays two display types are dominant in the display market: the bulky cathode ray tube (CRT) and liquid crystal displays (LCD). Both types use glass as substrate material. The LCD display is the dominant player for mobile applications, in for instance mobile phones and portable computers. In the development of displays and their applications a clear interest exists to replace the rigid rectangular display cells by free-shaped, curved or even roll-up cells. These types of applications require flexible displays.

A flexible version is shown of a state of the art passive matrix LCD; a full colour STN display. By using polymer substrates, “ultra-thin” optical layers (like retarders, colour filters and polarisers) and a flexible backlight a fully flexible CSTN demonstrator is obtained, that can be bend to a radius of 40 mm during operation.

Flexible cholesteric texture liquid crystal displays are demonstrated. The 4.2″ diagonal display can be bent over a 20-mm radius of curvature, even during operation. The driving parameters and the cell-gap homogeneity are comparable to those of glass based cells. Images with 16 gray levels are shown on the display.