Transient electroluminescence of polymer light emitting diodes using electrical pulses

Abstract

Detailed experimental and theoretical analysis of the pulsed excitation of polymer light emitting diodes is presented. We find a set of universal transient features for a variety of device configurations ͑different polymers/cathodes͒ which can be reproduced using our phenomenological numerical model. We find that the temporal evolution of the electroluminescence in response to a step voltage pulse is characterized by: ͑i͒ a delay followed by; ͑ii͒ a fast initial rise at turn-on followed by; ͑iii͒ a slow rise ͑slower by at least one order of magnitude͒. The large mobility mismatch between holes and electrons in conjugated polymers allows us to separately time resolve the motion of holes and electrons. We suggest a method for extracting mobility values that takes into account the possible field-induced broadening of carrier fronts, and which is found to be compatible with mobilities determined from constant wave measurements. By using appropriate device configurations it is possible to determine the mobilities of both holes and electrons from a single device. Mobilities for holes and electrons are extracted for a poly͑p-phenylenevinylene͒ copolymer and poly͑di-octyl fluorene͒.

Thickness scaling of the space-charge-limited current in poly(p-phenylene vinylene)

“…In numerous experimental and model studies of LEDs, the possible role of this effect was not considered. 4,[8][9][10] In the present study we have investigated the thickness dependence of the SCL current, which can be used to discriminate the effect of a field from that of density-dependent mobility. The enhanced thickness dependence of the experimental SCL current provides a direct proof that the carrier-density-dependent mobility dominates the hole transport in PPV-based diodes.…”

Conjugated Zwitterionic Polyelectrolyte as the Charge Injection Layer for High-Performance Polymer Light-Emitting Diodes

“…This is the shortest response time reported for organic light-emitting diodes (OLEDs) with CPE injection layers 8 and is comparable to the response times of OLEDs with metal electrodes. [23][24][25] The short response times strongly indicate that long-range ion motion does not play a major role in the charge injection process. Because the dipoles are fixed to the polymer backbone, long distance motion is prevented, but reorientation in the electric field occurs in response to an applied electric field above a threshold of ∼3 Â 10 5 V/cm (3 V; Figure 3).…”

Asymptotic analysis of double-carrier, space-charge-limited transport in organic light-emitting diodes

“…A standard device configuration consists of a transparent anode, typically indium tin oxide, an organic semiconducting polymer film that is 10-1000 nm thick and a metallic cathode [2][3][4][5]. An applied voltage drives the injection of electrons through the cathode, and electron vacancies, or holes, through the anode into the organic thin film, where they diffuse and migrate, or drift, towards the oppositely charged electrode.…”

“…In the present case, however, the space-charge develops as a result of injection of charges into a film with no intrinsic charge, whereas the Debye length characterizes the extent of the space charge of mobile ions intrinsic to an electrolyte near a charged surface. Importantly, for space-charge-limited transport in OLEDs, the space-charge layer is often small in comparison with the width of the device, such that =λ s /L 1 [4,9,30] [30], numerical analysis of charge transport in OLEDs, = 0.0012 for ohmic and injection limited cases.…”