P. van de Weijer

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.

Cathode encapsulation of organic light emitting diodes by atomic layer deposited Al2O3 films and Al2O3/a-SiNx: H stacks

Al2O3 thin films synthesized by plasma-enhanced atomic layer deposition (ALD) at room temperature (25 °C) have been tested as water vapor permeation barriers for organic light emitting diode devices. Silicon nitride films (a-SiNx:H) deposited by plasma-enhanced chemical vapor deposition served as reference and were used to develop Al2O3/a-SiNx:H stacks. On the basis of Ca test measurements, a very low intrinsic water vapor transmission rate 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:H films, respectively. The cathode particle coverage was a factor of 4 better for the Al2O3 films compared to the a-SiNx:H films and an average of 0.12 defects per cm2 was obtained for a stack consisting of three barrier layers (Al2O3/a-SiNx:H/Al2O3).

21.1: Ink Jet Printing of Passive‐Matrix Polymer Light Emitting Displays

The requirement and characteristics for red, green and blue (RGB) electroluminescent polymers[1,2] suitable for fabricating monochrome and full-color passive-matrix polymer display[3] will be discussed. The controlled deposition of the light-emitting polymers is accomplished through the use of high-resolution ink jet printing[4,5], where the materials are first tested in devices prepared using spincoating techniques. The specifications for polymer materials, as well as the display design which must be taken into account to make a high-resolution passive-matrix display with low power consumption are presented.

Determination of the effective radiative lifetime of the 6 3P1 atomic mercury level in low‐pressure mercury discharges

Experiments are described in which low‐pressure mercury, mercury‐argon, and mercury‐krypton discharges were irradiated with a dye laser pulse at 405 nm, thus exciting mercury atoms from the metastable 6 3P0 level to the 7 3S1 level. The 7 3S1 level decays radiatively to the 6 3P levels. By recording the time dependence of the 6 3P1 level density at the 254‐nm fluorescence signal, the effective radiative lifetime of this level was determined. These measurements were performed for a wide range of mercury vapor densities. From the effective radiative lifetime and the 6 3P1 level density, the 254‐nm output can be calculated. Reasonable agreement is found for two discharges of which the 254‐nm output had been measured directly.

Experimental determination of 6 3P–6 3P collisional excitation cross sections for line emission in the positive column of dc mercury discharges

The absolute emission intensities of lines originating from highly excited mercury levels in the positive column of a dc mercury discharge have been determined for various discharge conditions. For the same conditions, the densities in the 6 3P states have been determined with the hook method. From these experimental data the 6 3P− 6 3P collisional excitation cross sections for line emission can be derived for nine transitions. Generally, these cross sections are much smaller than those determined in previous experiments.

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.

The pulsed optogalvanic effect in a low-pressure mercury discharge

Abstract Experiments are described in which a low-pressure mercury discharge is irradiated with a 10 ns dye laser pulse tuned resonant to one of 6 3 P-7 3 S 1 transitions. The time behaviour of the induced optogalvanic effect is found to be dependent on the value of the ballast resistance in the discharge circuit. The qualitative explanation for this phenomenon is that the ratio of the relative change in the electron temperature and the relative change in electron density depends on the value of the ballast resistance.

Determination of the oscillator strengths of the 6D→61P1 atomic energy mercury transitions

The magnitude of the oscillator strengths ( f  values) of the 6D→61P1 transitions of atomic mercury is of interest, e.g., for the determination of the 61P1 mercury density in gas discharges by optical methods. However, contradictory results for these f values have been reported in the literature. Using the accurately known radiative lifetime of the 73S1 level as a starting point, we performed combined emission and hook measurements. As a result we found f=0.28±0.04 and f=0.25±0.05 for the 577.0 nm (63D2→61P1) and 579.1 nm (61D2→61P1) line, respectively.

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.