Device physics of conjugated polymer light emitting diodes

This thesis examines the optical and electrical properties of a series of fluorenebased conjugated polymers and light-emitting devices. It addresses some of the issues related to the performance of polymer light emitting devices based on host-guest systems. The principal aim was to gain insight into the microscopic physics essential to understand device operation and hence optimize device performance. Emission from exciplex states (formed between a hole from a donor molecule and an electron from an acceptor molecule) in bilayer light-emitting diodes (LEDs) based on hole transport triarylamines and electron-transport phenylquinoxalines was examined in detail. The emission wavelength and color was found to be determined by the energy
difference between the ionization potential of the hole transport material and the electron affinity of the electron transport material. The concept of blending an emissive polymer with a hole transport material as a means of improving device performance in a single layer configuration was addressed and high performance blue and green PLEDs were demonstrated. Doped LEDs showed enhanced performance due to the significantly improved charge balance factor as a result of the improved hole injection and transport achieved by blending with a hole transporter. The effect of using LiF and PEDOTiPSS at the cathode and anode interface on the device performance was examined and the enhancement observed was mainly attributed to the
lowering of the effective barrier heights for electron and hole injection, respectively. Electrophosphorescent LEDs were prepared by blending polyfluorene with a red phosphorescent porphyrin based emitter. Direct carrier trapping and subsequent exciton
recombination on the guest molecules was found to be the dominant EL mechanism. Small area devices (with an active area of diameter 50 pm) were demonstrated to result in very high brightness LEDs that also sustained very high current densities.
Optimized devices reached a peak brightness of 6,500,000 cd/m2 and sustained peak current densities up to 5,000,000 A/m2. The enhanced device performance was attributed to the improved thermal management of the small area devices due to reduced Joule heating. The incorporation of the small area structure in a microcavity allowed the fabrication of novel microcavity LEDs with reduced emission linewidths. Finally, the issue of energy transfer in a host-guest system was addressed in detail. The large overlap of the host emission spectrum with the guest absorption spectrum gave rise to efficient energy transfer via a Forster-type mechanism and dominated the PL emission. Blend LEDs only exhibited emission from the guest for guest concentrations as low as 0.5% as a result of almost complete excitation energy transfer as well as charge trapping followed by efficient recombination on the guest sites.