Jeroen J. M. Vleggaar

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 OF POLYMER LIGHT-EMITTING DIODES

Abstract In this paper, we report on the lifetime of polymer LEDs fabricated at Philips Research. For single-layer LEDS, we find that the operational lifetime in nitrogen gas is limited by the stability of the indium-tin-oxide (ITO) anode. By using a polymeric capping layer for the ITO, we obtain more stable devices. In air, the lifetime is limited by black spot formation. Small pinholes in the cathode layer are the origins of the black spots. Water or oxygen may diffuse through these pinholes and react with the cathode, causing degradation. By encapsulating the devices we can prevent black spot formation. Our present 8 cm2 devices have lifetimes of many thousands of hours at daylight visibility under ambient conditions.

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.

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.