Sanjeev Singh’s Thesis

Electrophosphorescent organic light emitting diodes (EOLEDs) have a huge
potential in display applications due to its higher efficiency compared to
normal fluorescent OLEDs. In addition, when the organic materials are in
polymer form they have the added advantage of solution processing which can
substantially reduce the cost for manufacturing large area displays. Although
intrernal quantum efficiency of small molecule based EOLEDs have reached
approximately 100% mark, the EPLEDs external quantum efficiency is much
less due to difficulties in making a multilayer structure.
In part A of the thesis, a new Iridium complex bis (2-phenylpyridine-C2,
N') iridium (III) picolinate [(ppy)2&(pic)] is used to make
electrophosphorescent PLED by doping it in a hole transporting polymer
Poly(9-vinylcarbazole) (PVK). Its emission is in the green range, at 510 nm.
Photoluminescence quantum efficiency of this film (PVK + Iridium complex )
is estimated to be 25% using integrating sphere. EPLEDs made with this
Iridium complex are investigated to improve the charge balance in the device.
First we study the effect of charge injection on device efficiency for two
different device structures using aluminum and calcium/aluminum as cathode
material. The external quantum efficiency improved by two orders for Ca/Al
devices and best external quantum efficiency is 0.4%
Since PVK is a hole transporting material, we wanted to improve its
electron transporting properties by blending it with 2-(4-Biphenyl)-5~(4-tertbutylphenyl)-
l,3,4-oxadiazole (PBD), an electron transporting material.
Different ratios of 20, 40 and 60% PBD are mixed with PVK, the mixtures are
doped with 7.7% (by wt.) Iridium complex. A maximum external quantum
efficiency obtained in these experiments is 4% and luminescence is 88 cd/m2
which is achieved in case of 40% PBD-PVK mixture with Ca/Al cathode. The
results are discussed in terms of charge balance and energy transfer processes
in these devices.
In the second part of the thesis, a metal-organic interface has been studied
using surface enhanced Raman spectroscopy (SERS). Metal-organic interface
plays a crucial role in the performance of organic semiconductor devices.
Silver and polysilane interface is studied. The results show that SERS can be used to study the interface as we observe differences in (i) samples of different thicknesses of polysilane, 200 A and 1000 A, (ii) measurements using focused and defocused beam configuration, where defocused configuration is more sensitive to the interface structure, (iii) as deposited and samples annealed at 60°C