Herman Schoo

Stability of polymer LEDs

Abstract Polymer LEDs have a number of attractive properties that make them suitable for many applications. Operating at low voltage, bright large-area devices can be made by simple technology. One of the limitations that prohibited industrialization of polymer LEDs was their limited lifetime. An overview of the improvement of polymer LED performance at Philips is presented. The progress during the last year is reflected by lifetimes of many thousands of hours for 8 cm2 devices that operate at daylight visibility under ambient conditions. Diagnostic measurements have been performed on polymer LEDs before and after stress testing, and on the materials used in these devices. These measurements have contributed to the understanding of the nature of the degradation of polymer LEDs.

Low voltage operation of large area polymer LEDs

Continuous improvement in the materials, processing and device structure of light-emitting diodes based on fully conjugated PPV-type polymers has led to a performance level suitable for application. In this paper we report on the electrical characteristics of large area devices, and show lifetime data at room temperature of several thousands of hours. In order to come to a more convenient testing scheme, lifetimes have been measured at higher operational intensity as well as elevated temperature and relative humidity. From these data, device degradation at normal operating conditions can be extrapolated.

Stability and characterization of large area polymer light-emitting diodes over extended periods

Abstract In order to exploit the extensive potential of polymer light-emitting diodes in commercial applications a number of lifetime specifications have to be met. In this paper we report on the performance and stability of polymer light-emitting diodes based on fully conjugated PPV. Lifetime measurements have been performed on small (5 mm2) and large (8 cm2) area devices under different conditions, including variations in temperature, luminescence intensity and humidity. It will be shown that polymer LEDs can withstand extreme lifetime tests successfully. The results are compared with lifetime specifications for applications in consumer applications and are discussed in terms of the stability of the emissive polymer. Spectral measurements (IR, PL) as a function of the operational lifetime are presented.

Efficiency and stability of polymer light-emitting diodes

The operation characteristics of polymer light-emitting diodes (PLEDs) are strongly dependent on materials, processing and the structure of the device. The device structure developed at Philips Research is presented together with some typical results for brightness, efficiency, response times and stability. The PLEDs typically operate at a voltage of 3–4 V for a brightness of 100 cd m-2 and have an efficiency ranging from 2 cd A-1 for orange emitting polymers (610 nm) up to 16 cd A-1 for green emitting polymers (550 nm). The response time under conditions for display operation is determined by the charge carrier transport properties and amounts to 43 ns. Lifetimes of several thousand hours have been obtained for large orange emitting devices of 8 cm2 for daylight visibility at room temperature.

Degradation effects in poly para-phenylene vinylene derivatives due to controlled oxygen exposure

Abstract The influence of oxygen exposure on three chemically different poly para -phenylene vinylene (PPV) derivatives used in polymeric light emitting devices (PLEDs) has been investigated. During device preparation, PPV layers have been exposed to oxygen either in the dark or during the irradiation with visible light, before the cathode was applied. Device efficiency was studied by electrical and optical characterisation and correlated to oxygen depth profiles measured with Elastic Recoil Detection Analysis (ERDA). Treatment with oxygen during light exposure leads to a decrease in current, light output and efficiency. It was found that two different PPV derivatives show the same current and light output reduction with different oxygen uptake. This behaviour is explained in terms of a different chemical structure and the number of structural defects (tolane-bisbenzyl moieties (TBBs)) incorporated. For two PPV derivatives treated with oxygen in the dark a reduction in current and light output was found, while the efficiency was unchanged. Another PPV derivative, however, shows a shift in the on-set voltage of the light output accompanied by an increased oxygen level at the PPV/Ca interface. It is concluded that after exposure in the dark, oxygen is incorporated in the PPV by Van der Waals interaction. During evaporation of the cathodes, oxygen will diffuse to the cathode and will be gettered by the calcium, which results in the formation of an electron injection barrier.

Interface instabilities in polymer light emitting diodes due to annealing

In polymer light emitting diodes (PLEDS) with an (ITO/PPV/Ca) structure we observed a significant reduction of both the current and the light output at constant voltage after heat treatment for onl ...

Stability and characterization of large-area polymer light-emitting diodes over extended periods

In order to apply polymer light-emitting diodes in commercial products a number of lifetime specifications have to be met. In this paper we report on the performance and stability of polymer light-emitting diodes based on dialkoxy-substituted fully conjugated PPV. Lifetime measurements have been performed on small and large area devices under different conditions, including variations in temperature, luminescence intensity and humidity. It will be shown that polymer LEDs can withstand extreme lifetime tests successfully. The result are compared with lifetime specifications for applications in consumer applications and are discussed in terms of the stability of the emissive polymer. Spectral measurements as a function of the operational lifetime are presented.