Enhancement of polymer light-emitting diode performance through doping and improved charge injection

Abstract

Research into the performance enhancement of polymer light-emitting diodes, through techniques of doping and improved charge injection, is reported. Initial electroluminescent (EL) characterisation of all host polymers used; in particular poly(2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene) (MEH-PPV) and α,ω-Bis[N,N-di(4-methylphenyl)aminophenyl]-poly(9,9-bis(2-ethylhexyl)fluoren-2,7-diyl) (PF2/6am4) in both mono- and bi-layer configuration is presented. For all studies indium tin oxide was used as the primary anode, with either aluminium or calcium as the cathode. The polymers are all soluble and were processed from spin coating to produce films of order 100 namometres in thickness. A range of protonated polyaniline thin films are characterised for use as hole-transporting layers (HTLs) with MEH-PPV. The HTL fermi level is found to be crucial to its effectiveness in facilitating hole injection. Following this the published studies of PF2/6am4 doping with highly efficient dopants (Rubrene and three metal-porphyrin derivatives) are presented. Techniques summarised in chapter 3 are used in order to convert the measured light output into optical units of candela per metre squared and lumens per watt. The external quantum efficiency for each configuration fabricated is also calculated. Previously reported studies of the dopants are used in order to aid discussion as to the key properties that make a dopant suitable for use in donor: acceptor systems. It is noted that the dopants' excited state lifetime is directly related to the current density at which peak performance is observed, and it is recommended that in order to avoid dopant saturation the dopants' excited state lifetime should be as small as possible without sacrificing emission efficiency. Having established the considerable challenges to be overcome for energy transfer systems to be commercially viable, an alternative doping technique is presented: host perturbation. First the effect of doping both MEH-PPV and PF2/6am4 with a range of rare earth lanthanide metal: organic complexes is investigated. The effect on the inter system crossing is observed to be highly host dependent, with improvements in efficiency only for MEH-PPV. These results are compared with theoretical predictions. In all studies we draw comparison from similar studies carried out by other groups where possible.