Date of Award

Summer 2007

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Micro and Nanoscale Systems

First Advisor

Sandra Zivanovic

Abstract

This dissertation is devoted to the study on Polymer Light-Emitting Diodes (PLEDs) based on polyfluorenes (PFs), a promising class of semiconductive polymers for Light-Emitting Diode (LED) applications. Recently Light Emitting Polymers (LEPs) have clearly made their entry into display technology by virtue of their potential application for large-area flat-panel displays with the advantages of low cost, wide viewing angle, fast switching time, high efficiency, low driving voltage, as well as their flexibility and adaptability. Compared to inorganic materials, the efficiency of PLEDs is, however, still low. And for the application of PFs in PLEDs, an undesired longwavelength emission band is a main barrier, which occurs during device operation and results in both color instability and reduced efficiency. In this study, several types of PLED devices based on neat PFs or PF blends were designed, fabricated, and characterized: (1) PF2/6:PFB blended devices, (2) end-capped PF2/6 devices, (3) F8BT:PPB blended devices, and (4) end-capped PF2/6:F8BT blended devices. DMP-end-capped PF2/6, PFB and PPB are novel polymers. For PF2/6, only a few results have been reported although it is of great significance for study of effect of blending and end-capping on device performance and color stability.

In comparison with pure PFs, the enhanced properties were observed from blend devices although the improvement might result from different reasons. For example, DMP-end-capped PF2/6:F8BT blended devices demonstrated better performance than either of the blend components with a maximum luminance of 1074 cd/m2 at 213.6 mA/cm2, a maximum external quantum efficiency of 0.16% and a maximum luminance efficiency of 0.514 cd/A at 205.2 mA/cm2, which was assigned to efficient energy transfer of excitations.

Besides the increase in efficiencies as compared to PF2/6, long-wavelength emission is significantly suppressed for end-capped PF2/6, which dominates the EL spectrum of PF2/6. In the case of DMP-end-capping, EL spectrum peaked at 420, 445, and 485 nm, all maxima falling into violet-blue region. This might be due to the efficient hole trapping at the end-capper groups, the shift of the exciton recombination zone, or the enhancement in the polymer resistance to oxidation.

The pristine PF2/6 device was modeled by using space charge limited current theory for the hole-only case. The hole mobility is regarded as field-dependent. The calculated current density-voltage characteristics fit the experimental data very well at electric fields above 6.5 × 105V/cm. The electrical characteristics of neat PF2/6 and neat F8BT-based devices were also simulated via exploiting a commercial TCAD software package.

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