SHIN, YS; (2012) Application of molybdenum trioxide in polymer light-emitting diodes. Masters thesis , UCL (University College London).

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Abstract

The thesis investigates the application of molybdenum trioxide (MoO3-x) as hole injection layers (HILs) in polymeric light-emitting-diodes (PLEDs). Recent application of metal oxides into the PLED architecture has been motivated by the benefits of enhanced device performances, as well as, for the protection against the intrusion of oxygen and water into PLEDs. In this thesis, the performance of MoO3-x HILs in PLEDs is investigated by fabricating ITO/MoO3-x/TFB/F8BT/Ca/Al electroluminescent devices and characterising their efficiency and luminescence properties. Also the performance of the device is studied within the context of the physical properties of MoO3-x lms and electroluminescent polymers, experimentally determined by various techniques. Three different types of PLEDs are fabricated and characterised, each incorporating MoO3-x HILs with different electronic properties. Such difference in the lms are achieved by post-deposition annealing of MoO3-x lms in air and nitrogen. It is determined that annealing MoO3-x lms in air has the effect of increasing their hole concentration. This improves the performance of MoO3-x as hole injection layers in PLEDs. Upon annealing the as-deposited MoO3-x HILs in air, the maximum current efficiency of PLEDs increases from 1.27 cd/A to 1.44 cd/A. Also the maximum luminescence increases from 2723 cd/m2 to 5680 cd/m2. Kelvin probe and electroabsorption spectroscopy measurements show that annealing MoO3 x lms has the effect of decreasing their work function by 0.3 eV. Finally, using time-correlated single photon counting (TCSPC), the photoluminescence (PL) lifetime of F8BT and TFB single layers, and TFB/F8BT bilayers are determined, with and without the presence of a MoO3x contact. The PL lifetime of excitons in pure F8BT is measured to decrease from 1300 to 760 ps upon incorporation of a MoO3x contact. In contrast to F8BT, the PL lifetime of TFB is determined to be 500 40 ps, regardless of the presence of a neighbouring contact. The TFB/F8BT bilayers exhibit biexponential decay characteristics with two lifetime values; 800 50 ps and 5000 400 ps.

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