Coen Theodorus Hubertus Fransiscus Liedenbaum

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.

Determination of the GaInP/AlGaInP band offset

Low‐temperature photoluminescence excitation spectra of disordered Ga0.5In0.5P/Al0.3Ga0.2In0.5P quantum wells lattice matched to (311)B GaAs substrates have been measured for the first time. Transition energies calculated with a k⋅p approach agree with experiment within 3 MeV, over the entire range of quantum well thicknesses (Lz=11–109 A). A conduction‐band discontinuity of 0.65±0.05 is derived.

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.

Device physics of polymer light-emitting diodes

This article reviews a device model for the current and light generation of polymer light-emitting diodes (PLEDs). The model is based on experiments carried out on poly(dialkoxy-p-phenylene vinylene) (PPV) devices. The transport properties of holes in PPV have been investigated with indium tin oxide (ITO)/PPV/Au hole-only devices. The hole current is dominated by bulk conduction properties of the PPV, in contrast to previous reports. As the hole current is space-charge limited, the hole mobility as a function of electric field E and temperature T can be directly determined. The hole mobility exhibits a field dependence ln(μ) ∼ ✓E as also has been observed from time-of-flight experiments in many molecularly doped polymers and amorphous glasses. For the zero-field hole mobility an activation energy of 0.48 eV is obtained. The electron conduction in PPV has been studied by using Ca/PPV/Ca electron-only devices. It appears that the electron current is strongly reduced by the presence of traps with a total density of 1018 cm−3. Combining the results of electron- and hole-only devices a device model for PLEDs is proposed in which the light generation is due to bimolecular recombination between the injected electrons and holes. It is calculated that the unbalanced electron and hole transport gives rise to a bias-dependent efficiency. By comparison with experiment it is found that the recombination process in PPV is for 95% nonradiative. Furthermore, the experiments reveal that the bimolecular recombination process is thermally activated with an identical activation energy as measured for the charge carrier mobility. This demonstrates that the recombination process is of the Langevin-type, in which the rate-limiting step is the diffusion of electrons and holes towards each other. The occurrence of Langevin recombination explains why the conversion efficiency (photon/carrier) of a PLED is temperature independent.

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.

Small form factor optical drive: miniaturized plastic high-NA objective and optical drive

Recent developments in portable consumer devices call for storage systems solutions using compact drive units and cheap storage media. A major advantage of conventional optical storage is the intrinsic low media cost and the ease of manufacturing of replicated ROM media. Third generation optical storage, using a blue laser and a high numerical aperture objective lens, is a perfect technology candidate for a small form factor optical (SFFO) drive. Using third generation optical storage technology 27 GBytes becomes available on a 12 cm optical disc. Using this data density for an SFFO drive, a storage capacity of over 1 Gbytes become feasible on a coin-sized disc. In this paper we report the realization of a SFFO drive, featuring 1 Gbytes on a 30 mm rewritable optical disc, with dimensions comparable to Compact Flash PCMCIA-like drives. Our main focus in this paper is on the miniaturization of the basic components of the SFFO drive, such as disc, objective and 2D-actuator. Related subjects in Philips R&D are miniature optics and low-dissipation/high-integration electronics.

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.

Neural network using longitudinal modes of an injection laser with external feedback

A new optical neural-network concept using the control of the modes of an injection laser by external feedback is described by a simple laser model. This approach uses the wavelength dispersed longitudinal modes of the laser as neurons and the amount of external feedback as connection weights. The predictions of the simple model are confirmed both with extensive numerical examples using the laser rate equations and also by experiments with GaAlAs injection lasers. The inputs and connection weights to this laser neural network are provided by external masks which control the amount of feedback reaching the laser. Stochastic learning is used to obtain weight masks for a small three-input and four-output neural net for the numerical and experimental examples. Winner-take-all and exclusive-or operations are obtained on the input set with different weight masks. Both of these operations are also obtained in experiments with a three-input/four-output laser neural network operating at an estimated speed greater than 10 GCPS. The eventual speed of this type of neural network hardware is expected to reach well within TCPS range if it is built in an optoelectronic integrated circuit with dimensions in the order of a mm. Different neural-network architectures possible with this approach are discussed.

Ordering induced splitting of light‐hole and heavy‐hole bands in GaInP grown by organometallic vapor‐phase epitaxy

The origin of the polarization of the photoluminescence in ordered InGaP is investigated. The ordering induced lowering of the cubic crystal symmetry is caused by a superlattice and/or strain effects in two of the four 〈111〉 crystal directions resulting in a splitting of the heavy‐hole and light‐hole valence bands. Using simple arguments from k⋅p theory, the difference in photon energy as well as the intensity ratio of the luminescence in the two polarization directions along the cleavage facets is explained as a function of temperature.