Homer Antoniadis

Dispersive electron transport in an electroluminescent polyfluorene copolymer measured by the current integration time-of-flight method

Here, we report results of both traditional current mode and current integration mode time-of-flight (TOF) measurements on the electroluminescent polyfluorene copolymer poly(9,9-dioctylfluorene-co-benzothiadiazole) (BT). Current mode TOF shows a strong but dispersive electron transport signal. The mobility derived from the current integration mode transit time (tQ) increases with decreasing film thickness as expected for dispersive transport. The fastest carriers in the photogenerated carrier packet are estimated to have mobilities of order 10−3 cm2/V s at applied fields of 0.5 MV/cm. Holes are heavily trapped close to the interface at which they are photogenerated. The TOF signals decay with repeated measurement and tQ remains constant with applied field. The transport properties of BT are thus in complete contrast to those of other polyfluorenes which show high-mobility nondispersive hole transport but weak and highly dispersive electron transport.

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, N′-phenyl-1,4-phenylenediamine) (PFMO) and poly(9,9-dioctylfluorene-co-benzothiadiazole...

Improving efficiency by balancing carrier transport in poly(9,9-dioctylfluorene) light-emitting diodes using tetraphenylporphyrin as a hole-trapping, emissive dopant

Unbalanced carrier transport is known to strongly affect the efficiency of polymer light-emitting diodes. Here, we report the results of time-of-flight (TOF), current density–voltage, and electroluminescence (EL) quantum efficiency measurements on single-layer poly(9,9-dioctylfluorene) (PFO) devices doped with the red-emitter tetraphenylporphyrin (TPP). TOF shows that PFO is a unipolar conductor, with hole transport much better than electron transport. At a field of 5×105 V/cm, a nondispersive hole mobility of 4×10−5–5×10−4 cm2/V s, dependent on sample morphology, is obtained. Upon the addition of 5% by weight TPP, hole transport becomes as highly dispersive as electron transport, having no measurable average mobility. This results in a decrease in the current for a given applied bias but an increase in the external EL quantum efficiency. TPP acts as a strong hole trap, reducing the dominant hole current and producing more balanced carrier transport. At TPP concentrations above 6%, the device characterist...

Balancing electron and hole currents in single layer poly(9,9-dioctylfluorene) light-emitting diodes

Poly(9,9-dicotylfluorene)(PFO) exhibits very good, non- dispersive hole transport but very poor electron transport. To achieve the maximum efficiency in a PFO light emitting diode it is important to balance the electron and hole currents. Here we report three schemes to achieve this in single layer devices. Firstly, by using different treatments to change the work function of the indium tin oxide anode contact, the hole current can be varied by up to 4 orders of magnitude, thus allowing it to be adjusted to the same level as the electron current. Secondly, the hole mobility can be decreased by doping PFO with a hole trapping, emissive material. Upon the addition of 5% by weight of a red-emitting tetraphenylporphyrin, hole transport in PFO becomes as highly dispersive as electron transport, resulting in a decrease in the current for a given applied bias but an increase in the electroluminescent efficiency. Thirdly, the electron mobility can be increased by doping PFO with an emissive, electron transporting material. The electroluminescent polyfluorene copolymer poly(9,9-dioctylfluorene-co-benzothiadiazole (BT) exhibits strong but dispersive electron transport. PFO devices doped with BT show very high efficiencies, high peak brightnesses and very low turn on voltages.

XGA resolution full-video microdisplay using light-emitting polymers on a silicon active matrix circuit

Capable self-emissive polymers are being developed for use as emitting materials for a variety of display applications. This paper describes the use of standard CMOS integrated circuit silicon wafer technology along with a spin-cast polyfluorene-base polymer emissive layer, to demonstrate an XGA resolution, full video microdisplay. The silicon chip drive circuitry (Analog Pixel-APIX) is described along with results from our efforts to optimize the reflective anode, the semitransparent cathode process, and emissive cell construction. The 1024 X 768 pixel display achieves 200 Cd/m2 brightness at low power (<50 mW) with fast 1 usec response times. In addition, we summarize future directions to achieve color and the need to incorporate a production- worthy seal layer on microdisplays manufactured on silicon wafers.

Passive‐matrix displays based on light‐emitting polymers

— The design architecture, product specification, and reliability of the first high-resolution commercially available passive-matrix displays based on light-emitting polymers will be presented. Also, the applications and benefits of this new low-cost display technology will be discussed.