Arunandan Kumar

Enhancement of light extraction efficiency of organic light emitting diodes using nanostructured indium tin oxide

Improved outcoupling efficiency of organic light emitting diodes (OLEDs) is demonstrated by incorporating a nanostructured indium tin oxide (NSITO) film between a conducting anode and a glass substrate. NSITO film was fabricated using rf-sputtering at oblique angle (85°). Significant reduction in refractive index and improved transmission of NSITO film was observed. OLEDs were then fabricated onto NSITO film to extract the ITO-glass waveguided modes. Extraction efficiency was enhanced by 80% without introducing any detrimental effects to operating voltage, current density, and angular invariance of emission spectra of OLEDs.

Alumina nanoparticles find an application to reduce the ionic effects of ferroelectric liquid crystal

We observed that the doping of alumina nanoparticles (AL-NPs) has suppressed the undesired ionic effect in ferroelectric liquid crystals (FLCs). The pure and AL-NPs doped FLC cells were analysed by means of dielectric spectroscopy and electrical resistivity/conductivity measurements. Dielectric loss spectra confirmed the disappearance of the low-frequency relaxation peak, which appears due to the presence of ionic impurities in the FLC materials. The reduction of ionic effects has been attributed to the strong adsorption of ionic impurities on the surface of AL-NPs. The adsorption capability of AL-NPs has been studied with both the size and their concentration in FLC material. This study would be helpful to minimize the undesired ionic effects of LC-based display devices.

Energy transfer process between exciton and surface plasmon: Complete transition from Forster to surface energy transfer

The energy transfer process between surface plasmons and excitons was studied by varying the filling fraction of gold (Au) nano-clusters (NCs) and by placing a spacer of different thickness between Au NC and organic semiconductor layer. The intensity enhancement has occurred for 10%-50% filling fractions and 4-14 nm spacer thicknesses. Energy transfer mechanism was found to switch from Forster type to surface type by increase in filling fraction. Transverse electric field for Au NCs was simulated and we observed that for filling fraction 60%, they behave like 2-dimensional dipoles.

Effect of doping of 8-hydroxyquinolinatolithium on electron transport in tris(8-hydroxyquinolinato)aluminum

Effect of doping of 8-hydroxyquinolinatolithium (Liq) on the electron transport properties of tris(8-hydroxyquinolinato)aluminum (Alq3) has been investigated as a function of temperature and doping concentration by fabricating electron only devices. It has been observed that current density in the devices increases with the doping of Liq up to a doping concentration of 33 wt. % and then decreases. Current density-voltage (J-V) characteristics of 0, 15, and 33 wt. % Liq doped Alq3 devices were found to be bulk limited and analyzed on the basis of trap charge limited conduction model. The J-V characteristics of 50 and 100 wt. % Liq doped Alq3 devices were found to be injection limited and were analyzed using the Fowler-Nordheim model. The increase in current density with doping up to 33 wt. % was found to be due to an increase in electron mobility upon doping, whereas the decrease in current density above 33 wt. % was due to the switching of transport mechanism from bulk limited to injection limited type du...

Organic Light Emitting Diodes for White Light Emission

During the last few years, research based on energy saving technologies is being given high priority all over the world. General lighting is one area in which large quantity of electrical energy is being spend and substantial energy saving is possible by using energy saving technologies. Conventional light sources like incandescent filament lamps in which a major part of the energy is wasted as heat and is a less energy efficient technology is being phased out. Other technologies like gas filled electrical discharge lamps are more efficient but are polluting. Therefore there is a need for energy efficient and clean light source and solid state lighting is one of the ways to address the problem Organic light emitting diodes (OLED) is a new technology which has the potential to replace the existing lighting technologies. The attraction to organic semiconductors for lighting and display application has started during 1950-1960 because of the high fluorescence quantum efficiency exhibited by some organic molecules and their ability to generate a wide variety of colors. Study of electroluminescence (EL) in organic semiconductors have started in 1950s by Bernanose et.al (1953) using dispersed polymer films This was followed by the study of electroluminescence in anthracene single crystals by Pope et al (1963) and W.Helfrich et.al. (1965) who has studied the fundamental aspects of light generation in OLEDs. Since the single crystal based anthracence OLEDs fabricated by Pope et al (1963) were very thick and worked at very high voltages, the devices were not commercialized. In 1987, Tang and VanSlyke (1987) of Eastman Kodak has demonstrated a highly efficient multi layer OLED device based on vacuum evaporated aluminum tris 8-hydroxy quonoline (Alq3)as the emitter material. The device had different layers for hole transporting, electron transporting and light emission. Transparent Indium Tin Oxide (ITO) and aluminum metal were the anode and cathode respectively. Quantum efficiency and luminescence efficiency of 1% and 1lm/W respectively were considered enough for commercial application. This work has stimulated a very intense activity in the field of Organic electroluminescence. Numerous improvements in device structure and addition of more layers having different functionalities were incorporated and are now on the verge of commercialization. Further, the developments in conjugated polymers by Heeger, MacDiarmid, and Shirakawa in 10

Thermally activated field assisted carrier generation and transport in N,N′-di-[(1-naphthalenyl)-N,N′-diphenyl]-(1,1′ biphenyl)-4,4′-diamine doped with 2,3,5,6-tetrafluoro-7,7′,8,8′-tetracyanoquinodimethane

Current density-voltage (J-V) characteristics of N,N′-di-[(1-naphthalenyl)-N,N′-diphenyl]-(1.1′ biphenyl)-4,4′-diamine (α-NPD) doped with 2,3,5,6-tetrafluoro-7,7′,8,8′-tetracyanoquinodimethane have been studied as a function of doping concentration (0–0.8 wt %) and temperature (105–300 K). The current density was found to increase with increase in doping concentration. In the doped samples as field increases above 3.3×104 V/cm the current abruptly starts increasing at a higher rate, which is ascribed as due to increased free charge carrier generation in the bulk. The enhanced free charge carrier generation is due to field assisted thermal dissociation of donor-acceptor pairs (Poole–Frenkel process) as well as charge injection at the interface. The released carriers increase the charge carrier density which brings the Fermi level near the highest occupied molecular orbital level of the α-NPD and reduces the space charge region near the interface favoring the tunneling of charge carrier across the interface...

Chemical structure dependent electron transport in 9,10-bis(2-phenyl-1,3,4-oxadiazole) derivatives of anthracene

In this work, we present a detailed analysis on electron transport studies of 9,10-bis(2-phenyl-1,3,4-oxadiazole) derivatives of anthracene (OXD-PH, OXD-PTOL and OXD-OTOL). The effect of methyl substitution at ortho (OXD-OTOL) and para position (OXD-PTOL) on the phenyl ring on the electron transport properties was studied and the results were compared with the anthracene derivative without any substitution at the phenyl ring. Electron transport was found to be highly dependent on the methyl substitution and electron mobilities in OXD-PTOL and OXD-OTOL were found to be lower than in OXD-PH. Mobilities were also found to be different for OXD-PTOL and OXD-OTOL, which indicates that the substitution at different places did not have a similar effect on charge transport properties. Thickness dependent trap states were observed for all three molecules with thickness dependent electron mobilities. Electron mobility was found to increase in all three molecules with the decrease in thickness, which favors their use for organic electronic devices and all three molecules had a better electron transport in comparison to Alq3. These results were explained by the DFT calculation which showed a dihedral structure. The dihedral angle was found to reduce in the anionic form of these molecules. Therefore, these molecules are likely to favor a proper stacking in the solid state form.

Charge transport study in bis{2-(2-hydroxyphenyl) benzoxazolate} zinc [Zn(hpb)2]

The nature of the electrical transport mechanism for carrier transport in pure bis {2-(2-hydroxyphenyl) benzoxazolate} zinc [Zn(hpb)2] has been studied by current–voltage measurements of samples at different thicknesses and at different temperatures. Hole-only devices show ohmic conduction at low voltages and space charge conduction at high voltages. The space charge conduction is clearly identifiable with a square law dependence of current on voltage as well as the scaling of current inversely with the cube of thickness. With a further increase in voltage, the current increases with a Vm dependence with m varying with temperature typical of trap limited conduction with an exponential distribution of trap states. From the square law region the effective charge carrier mobility of holes has been evaluated as 2.5 × 10−11 m2 V−1 s−1. Electron-only devices however show electrode limited conduction, which was found to obey the Scott–Malliaras model of charge injection.