Punita Singh

Engineering fused coumarin dyes: a molecular level understanding of aggregation quenching and tuning electroluminescence via alkyl chain substitution

Simple molecular structures capable of emitting over the entire visible range are still a challenge. Planar molecular structures have the drawback of fluorescence quenching in the solid state thus limiting their application fields. Combining long range excimer/exciplex emissions with a compound emission have been used to get white light. In this work, a series of new coumarin derivatives having a planar structure have been synthesized and characterized. The effects of systematic variation in alkyl chain functionalization providing morphological variations that permit interesting solid state emitting properties have been discussed simultaneously with electrochemical behavior and OLED (organic light emitting diode) device applications. Carbon chains containing 0–16 carbon atoms have been studied in order to conclude the results that systematic changes in alkyl group substitution can be utilized as a tool to tune the emitting color of these planar coumarins. Alkyl chains were introduced by O-acylation and O-benzoylation reaction on the hydroxyl group of parent coumarin 5. Thus the present strategy is also helpful in establishing a template to control the unproductive interchromophore electronic couplings. Solid state fluorescence properties support the crystal studies. Theoretical studies are also in agreement with experimental data. Electroluminescence of Device 2 with a turn on voltage (Von) around 5–6 V having s-CBP doped with 1% of 8 having alkyl substitution of 2-carbons is found to exhibit white emission with CIE co-ordinates of (0.29, 0.34) which is close to white emission while the alkyl substitution of 14-carbons (compound 17) in Device 7 (Von = 7 V) exhibited green emission. Thus a strategy helpful to tune the electroluminescence has been discussed.

Effect of surface passivation on generation and recombination lifetimes in silicon wafer studied by impedance spectroscopy

Impedance spectroscopy is used to study the effect of surface passivation on minority carrier lifetimes. The technique allows measurement of generation and recombination lifetimes separately. Induced p+-p-n structures are prepared by depositing semitransparent layers of high and low work function metals (Pd and Al, respectively) on the two sides of silicon wafers. Hydrogen adsorption property of Pd surface has been utilized for passivation. The generation lifetimes remain almost unaffected but recombination lifetimes enhance many folds after passivations which are in agreement with values obtained by microwave photoconductive decay technique after chemical passivation. Variations are analyzed for estimation of bulk recombination lifetime.

Surface modified ZnO nanoparticles: structure, photophysics, and its optoelectronic application

Nanostructured ZnO has been synthesized by wet chemical route. Poly(vinylpyrrolidone) is used for stabilization and surface passivation of synthesized nanoparticles, thus tailoring the growth of ZnO at nanoscale. Structural characterization using X-ray diffraction, scanning electron microscopy, high-resolution transmission electron micoroscopy and Fourier transformed infrared spectroscopy of the synthesized nanoparticles confirm the evolution of nanocrystalline ZnO prevailing in hexagonal wurtzite phase. UV–Vis and photoluminescence spectroscopy studies show blue shift phenomenon in the synthesized nanoparticle in contrast to the bulk ZnO furnishing evidence in support of quantum size effect. The nanocomposites of ZnO and poly [9,9-dioctylfluorenyl-2,7-diyl] are prepared and characterized to investigate its luminescent and spectral emission effects. The nanocomposites are then incorporated in light-emitting diodes, and influence of ZnO on the device performance has been explored via electroluminescence, current density evaluation, and corresponding CIE coordinates calculation.

Packing directed beneficial role of 3-D rigid alicyclic arms on the templated molecular aggregation problem

Subtle changes in molecular structure have been used to alter the molecular packing and optical properties of organic luminophores. Thus it is important to study simple and advantageous structural modification to overcome limitations of aggregation quenching of fluorescence. Planar conjugated organic compounds are unlikely to be used in OLED devices as they suffer with luminescence weakening due to π–π cofacial stacking and excimer formation in the solid state. To avoid such critical issues, doped device architecture for OLED devices has widely been adopted which actually complicates the device fabrication process. Thus an approach for delimiting these drawbacks using simple methodologies is highly desirable for cost effective OLED device fabrication. In this context, two 3-dimensional rigid arms have been introduced into a planar benzo[h]chromen-2-one core, which suffers from aggregation caused quenching, as peripheral substituents to tune the molecular packing and subsequently their optical properties. The resultant compounds 1 and 2 have been tested for non-doped and doped OLED device application. Compound 1 was found to be highly thermally stable with a decomposition temperature of 344 °C. Single crystal XRD studies helped to elucidate that voluminous ring substituents are capable of eradicating co-facial π–π stacking by increasing the intermolecular distance and hence the luminescence in the solid state could be regenerated. This tuning of intermolecular distance between molecules helped to mitigate the close packed arrangement at a molecular level and also provided the bulk with the enhanced emission property. Electroluminescence from a pristine layer of compound 1 was possible. Thus an approach enabling planar luminophores to be used in OLED applications utilizing thin films of structurally engineered luminophores has been presented.