Alasdair J. Campbell

Space-charge limited conduction with traps in poly(phenylene vinylene) light emitting diodes

Current–voltage, impedance, and transient conductance measurements have been carried out on indium-tin-oxide/poly(phenylene vinylene)/Al light emitting diodes. In these devices injection and transport is expected to be dominated by positive carriers. Fowler–Nordheim tunneling theory cannot account for the temperature dependence, the thickness dependence, or the current magnitude of the current–voltage characteristics. Space-charge limited current theory with an exponential distribution of traps is however in extremely good agreement with all of the recorded current–voltage results in the higher applied bias regime (approximately 0.7⩽V/d⩽1.6×106u2009Vu2009cm−1). This gives a trap density Ht of 5(±2)×1017u2009cm−3 and the product of μNHOMO of between 1014 and 5×1012u2009cm−1u2009V−1u2009s−1. Assuming NHOMO is 1020u2009cm−3 gives an effective positive carrier mobility between 10−6 and 5×10−8u2009cm2u2009V−1u2009s−1. The characteristic energy Et of the exponential trap distribution is 0.15 eV at higher temperatures (190⩽T⩽290u2009K), but this decreases...

Quantifying the efficiency of electrodes for positive carrier injection into poly(9,9-dioctylfluorene) and representative copolymers

The perfect injecting contact for any semiconductor device is, by definition, an ohmic contact. When such a contact is made to an organic semiconductor the current density is limited by bulk space-charge effects. In the absence of charge carrier traps, J reaches the ultimate, trap-free, space-charge-limited value, JTFSCLC=(9/8)eμV2/d3. Knowledge of the mobility μ, permittivity e, applied bias V, and film thickness d, thus allows the maximum possible current density to be calculated. The absolute injection efficiency of any specific contact can then be quantified via a figure of merit, χ=J/JTFSCLC, namely the ratio of the actual current density to that expected for the ideal trap-free, space-charge-limited current. In this article we report on the injection efficiency of positive carriers into poly(9,9-dioctylfluorene) (PFO) and two representative copolymers, poly(9,9-diocytlfluorene-co-bis-N,N′-(4-methoxyphenyl)-bis-N,u200aN′-phenyl-1,4-phenylenediamine) (PFMO) and poly(9,9-dioctylfluorene-co-benzothiadiazole...

Bulk limited conduction in electroluminescent polymer devices

The current–voltage (J–V) characteristics of ITO/polymer film/Al or Au structures of poly(phenylene vinylene) (PPV) and a dialkoxy PPV copolymer have been recorded for a range of different film thickness d and temperatures T. At high applied bias all the characteristics can be fitted over a given range to a power law J=KVm, where m increases with decreasing T, log(K) is proportional to m, and K is proportional to d−αu2009m, where α∼2 (ITO/polymer film/Al devices) and ∼1 (ITO/polymer film/Au devices). Different single carrier space charge limited conduction theories have been used to try and explain this behavior. The analytical theory in which the carrier density is decreased by an exponential trap distribution lying below effectively isoelectronic transport states is in good agreement, but cannot explain the thickness dependence of the ITO/polymer film/Au devices and can be criticized as being physically unreasonable. A numerical analysis in which the mobility has the field and temperature dependence found f...

Transient and steady-state space-charge-limited currents in polyfluorene copolymer diode structures with ohmic hole injecting contacts

We report detailed measurements on diode structures containing the electroluminescent polyfluorene copolymer poly(9,9-dioctylfluorene-co-bis-N,N′-(4-methoxyphenyl)-bis-N,N′-phenyl-1,4 phenylenediamine). Ohmic injection of holes is achieved with an oxygen plasma cleaned indium tin oxide (ITO) electrode, untreated ITO coated with a film of poly(ethylenedioxythiophene)/polystyrenesulphonic acid (PEDOT/PSS) and plasma cleaned ITO with PEDOT/PSS. Transient dark injection and time-of-flight mobility measurements and steady state current density versus voltage measurements are then entirely consistent with a positive carrier, trap-free, space-charge-limited current. Injection limited behavior is observed, however, for untreated ITO without PEDOT/PSS and for evaporated Au contacts.

Thermally activated injection limited conduction in single layer N,N′-diphenyl-N,N′-bis(3-methylphenyl)1-1′-biphenyl-4,4′-diamine light emitting diodes

Impedance, current–voltage–luminosity and spectral measurements have been carried out on indium-tin-oxide/N,N′-diphenyl-N,N′-bis(3-methylphenyl)1-1′-biphenyl-4,4′-diamine (TPD)/Al light emitting diodes. The devices have a blue/violet emission with a spectrum peaked at 404 nm. Capacitance–voltage measurements show that at zero bias the devices are fully depleted. The impedance measurements show that the devices can be modeled on a single, frequency-independent parallel resistor-capacitor RPCP circuit with a small series resistance RS. RP changes with applied bias and temperature, while CP remains constant. The values of CP give er=3.0±0.3. Analysis of the current–voltage (J–V) characteristics show that the dominant conduction mechanism cannot be either ohmic, trap free space charge limited, or tunneling injection. The temperature and thickness dependence indicate that it must be either thermionic emission or thermally assisted tunneling, the carrier density varying from about 1010/1011 to 3×1013u200acm−3 over ...

Deep level transient spectroscopy (DLTS) of a poly(p-phenylene vinylene) Schottky diode

Abstract Deep level transient spectroscopy measurements have been carried out on ITO/poly( p -phenylenevinylene)/Al organic light emitting diodes that have a depletion region type Schottky barrier at the polymer/metal interface. The very long lived capacitance transients can be successfully described by the de-trapping of p-type majority carriers from a single energy trap level to a Gaussian distribution of transport states. The Gaussian width of 0.10±0.02 eV and trap depth of 0.75±0.05 eV are in excellent agreement with values measured from other unrelated experimental techniques.

Transient capacitance measurements of the transport and trap states distributions in a conjugated polymer

Abstract We have performed transient capacitance measurements on indium tin oxide/poly(p-phenylenevinylene)/Al diodes which have a depletion region type Schottky barrier at the polymer/Al interface. We show that both the transport and trap state distributions within the polymer can be determined from the results and present a fully consistent model that describes the observed behaviour in terms of a de-trapping of carriers from a discrete trap level to a Gaussian distribution of transport states.

Optical studies of electric fields in poly(2-methoxy-5-ethyl(2(')-hexyloxy) para-phenylene vinylene) light-emitting diodes

We report electroabsorption studies of poly(2-methoxy-5-ethyl(2′-hexyloxy) para-phenylene vinylene) light-emitting diodes. An electric field develops during operation which opposes the field of the applied bias. The counter field builds up within 5 s of turning on the device, increases in magnitude with the operating voltage, and decays exponentially with a time constant between 15 and 32 s. We attribute the counter field to bulk carrier traps and discuss its relevance to the increase of the turn-on voltage as organic light-emitting diodes degrade.

Charge trapping in polymer diodes

Abstract We report studies focusing on the nature of trap states present in single layer ITO/poly(phenylene vinylene)/Al light emitting diodes. At high applied bias the IV characteristics from 11 to 290 K can be successfully modelled by space charge limited current (SCLC) theory with an exponential trap distribution, giving a trap density H t of 4(±2) × 10 17 cm −3 , μ p , between 10 −6 and 5 × 10 −8 cm 2 V −1 s −1 and a characteristic energy E t of 0.15 eV at high temperatures. The transient conductance follows a power-law relationship with time whose decay rate decreases with decreasing temperature. This can be directly related to the emptying of the trap distribution found in the SCLC analysis. Due to variations in structure, conformation and environment, the polymer LUMO and HOMO density of states form Gaussian distributions of chain sites. The deep sites in the tail of the distributions are the observed traps for both positive and negative carriers. The same sites dominate the photo- and electroluminescence emission. This implies that the emissive layer in organic LEDs should be made as structurally disordered as possible.

Trap-free, space-charge-limited currents in a polyfluorene copolymer using pretreated indium tin oxide as a hole injecting contact

We report time-of-flight, transient dark injection (DI) and current density versus voltage measurements on polyfluorene copolymer diode structures using pretreated indium tin oxide (ITO) as a hole injecting contact. For ITO exposed to an oxygen plasma, coated in poly(ethylenedioxythiophene)/polystyrenesulphonic acid, or with both treatments, all measurements were entirely consistent with positive carrier, trap-free, space-charge-limited current theory. For untreated ITO, the behaviour is instead injection limited.