Space-Charge-Limited Charge Injection From ITO/PPV Into a Trap-Free Molecularly Doped Polymer
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ABSTRACT We describe bilayer structures comprised of a poly(p-phenylene vinylene) (PPV) layer and a trap-free diaryldiamine (TPD) doped in polycarbonate (PC) layer, sandwiched between indium-tin-oxide (ITO) and aluminum (Al) contacts. Two critical phenomena in the operation of polymer based electroluminescent devices, interface injection and carrier range, are investigated. It is established that the ITO/PPV contact is capable of sustaining dark current under trap-free space-charge-limited (TFSCL) conditions into a hole transporting TPD:PC layer. TFSCL currents are not observed in devices without the PPV layer. Upon increasing the thickness of the PPV layer a deviation from the TFSCL regime is observed which is attributed to trapping of the injected holes within PPV. These observations suggest a novel method for estimating the trapping mobility-lifetime product atr for holes in PPV. By this means we estimate pv-~ 10-9 cm 2/V. INTRODUCTION The discovery of electroluminescence (EL) in conjugated polymers like poly(p-phenylene vinylene) (PPV) [1] has attracted the attention of many groups around the world [2]. The current interest focuses on potential applications in large area flat panel displays as well as on fundamental understanding of phenomena associated with the operation of polymer based electroluminescent devices. In the most simple EL device, a thin film of conjugated polymer is sandwiched between two metal electrodes with work functions properly selected to match the energy levels of the polymers (-rn1*,HOMO-LUMO, etc.) where transport of holes and electrons occurs respectively. Upon the application of voltage, electrons and holes are injected from the metal contacts into the polymer and drift in opposite directions until they form bound pairs (polar-exciton pairs) which recombine to emit light. Three main requirements are critical for efficient operation: efficient injection of electrons and holes from the metal electrodes, optimized carrier range and high probability for carriers to recombine radiatively.
In this paper we describe the phenomena of hole carrier injection from ITO into PPV and the subsequent hole injection from PPV into a trap-free molecularly doped polymer (TPD:PC) in bilayer ITO/PPV/TPD:PC/AI devices. The experiments were performed under steady-state and transient spacecharge-limited (SCL) conditions [3]. We found that when positive voltage was applied on ITO the "ITO/PPV-contact" behaves as a perfect hole injector and is capable of sustaining dark current under TFSCL conditions into a trap free TPD:PC layer. Under these conditions the transit time of any excess injected carrier (hole), passing through the bulk of TPD:PC to the collecting contact (Al), is shorter than the time required to induced its neutralization (i.e. screening effect due to bulk dielectric relaxation time). The TFSCL is the maximum unipolar current that can flow through a device. Thus for an EL device, optimum injection occurs when both anode and cathode operate under trap-free space-charge-limited conditions (double
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