Chozhidakath Damodharan Sunesh

Utilization of a phenanthroimidazole based fluorophore in light-emitting electrochemical cells

An easily accessible, highly soluble, small-molecule phenanthroimidazole derivative has been synthesized and characterized. The synthesized compound shows strong luminescence in solution and exhibits good thermal stability. Single crystal X-ray crystallography studies were carried out. Correlations between the X-ray structures, photophysical properties and the performance in light-emitting electrochemical cells (LECs) are described. A yellowish green emission was achieved by using the target compound in a LEC device configuration. The constructed prototype device performance was promising with a maximum brightness of 125 cd m−2. The results suggest that the phenanthroimidazole derivative can function as an active material in LEC devices.

Blue and Blue-Green Light-Emitting Cationic Iridium Complexes: Synthesis, Characterization, and Optoelectronic Properties

Two new cationic iridium complexes, [Ir(ppy)2(phpzpy)]PF6 (complex 1) and [Ir(dfppy)2(phpzpy)]PF6 (complex 2), bearing a 2-(3-phenyl-1H-pyrazol-1-yl)pyridine (phpzpy) ancillary ligand and either 2-phenylpyridine (Hppy) or 2-(2,4-difluorophenyl)pyridine (Hdfppy) cyclometalating ligands, were synthesized and fully characterized. The photophysical and electrochemical properties of these complexes were investigated by means of UV-visible spectroscopy, emission spectroscopy, and cyclic voltammetry. Density functional theory (DFT) and time dependent DFT (TD-DFT) calculations were performed to simulate and study the photophysical and electrochemical properties of both complexes. Light-emitting electrochemical cells (LECs) were fabricated by incorporating complexes 1 and 2, which respectively exhibit blue-green (488 and 516 nm) and blue (463 and 491 nm) emission colors, achieved through the meticulous design of the ancillary ligand. The luminance and current efficiency measurements recorded for the LEC based on complex 1 were 1246 cd m(-2) and 0.46 cd A(-1), respectively, and were higher than those measured for complex 2 because of the superior balanced carrier injection and recombination properties of the former.

Non-doped deep blue light-emitting electrochemical cells from charged organic small molecules

Blue emitters are still elusive for solid-state light-emitting electrochemical cells, and limit the development of white light emitting devices for display applications. We report the photophysical, electrochemical, thermal and electroluminescence properties of two charged organic deep blue-emitters. The synthesized materials showed intense blue fluorescence with high quantum efficiencies and good thermal stabilities. Single-layered non-doped LEC devices were fabricated from solution. The fabricated non-doped LEC devices exhibited deep blue electroluminescence centered at 432 and 434 nm with the CIE coordinates of (0.15, 0.09) and (0.16, 0.10), respectively for compounds 1 and 2, which are quite close to the National Television System Committee (NTSC) standard for blue color coordinates. Electroluminescent devices operated at very low turn-on voltages reveal maximum luminance of 118 cd m−2 for compound 1 and 136 cd m−2 for compound 2. These promising results are highly desirable for the development of low cost white light-emitting devices.

Effect of Smaller Counter Anion, BF4 –, on the Electroluminescent Properties of Cationic Iridium Complex Based Light-Emitting Electrochemical Cells

A cationic iridium complex with tetrafluoroborate (BF4 -) as anion was synthesized and characterized for its potential application in display and solid state lighting devices. Light-emitting electrochemical cells (LECs) based on this complex displayed yellow electroluminescence with CIE coordinates of (0.50, 0.49). The imidazolium ionic liquids such as BMIMBF4 and EMIMBF4 were separately added to the light-emitting layer of the devices and their effects on the electroluminescent properties were studied. The presence of smaller BF4 - ion effectively facilitates the charge injection from the electrodes and hence results in rapid increase of luminance and current density of the devices with voltages.

Enhanced light absorption and charge recombination control in quantum dot sensitized solar cells using tin doped cadmium sulfide quantum dots

The photovoltaic performance of quantum dot sensitized solar cells (QDSSCs) is limited due to charge recombination processes at the photoelectrode/electrolyte interfaces. We analyzed the effect of Sn4+ ion incorporation into CdS quantum dots (QDs) deposited onto TiO2 substrates in terms of enhancing light absorption and retarding electron-hole recombination at the TiO2/QDs/electrolyte interfaces. Sensitization involved depositing CdS QDs with different Sn4+ concentrations on the surface of TiO2 using a facile and cost-effective successive ionic layer adsorption and reaction (SILAR) method. Optimized photovoltaic performance of Sn-CdS sensitized QDSSCs was explored using CuS counter electrodes (CEs) and a polysulfide electrolyte. Structural and optical studies of the photoanodes revealed that the gaps between CdS nanoparticles were partially filled by Sn4+ ions, which enhanced the light absorption of the solar cell device. Electrochemical impedance spectroscopy (EIS) and open circuit voltage decay (OCVD) tests suggested that Sn4+ ions can remarkably retard electron-hole recombination at the interfaces, stimulate electron injection into semiconductor QD layers, and provide long-term electron lifetime to the cells. We found that solar cells based on CdS photoanodes doped with 10% Sn4+ ions exhibited a superior power conversion efficiency (PCE) of 3.22%, open circuit voltage (Voc) of 0.593 V, fill factor (FF) of 0.561, and short-circuit current density (Jsc) of 9.68 mA cm-2 under an air mass coefficient (AM) 1.5 G full sun illumination. These values were much higher than those of QDSSCs based on bare CdS photoanodes (PCE = 2.16%, Voc = 0.552 V, FF = 0.471, and Jsc = 8.31 mA cm-2).

Small Molecule-Based Light-Emitting Electrochemical Cells

Organic small molecules (SM) offer excellent opportunities to construct cheap, sustainable, and efficient electroluminescent devices due to the availability of a wide range of synthetic procedures and the ability to fine-tune their photophysical and electrochemical properties. Fascinated by the high performances of SMs in organic light-emitting diodes (OLEDs), considerable amount of effort has been put forward by material scientist to find efficient methods to utilize SMs in light-emitting electrochemical cells (LEC). This chapter covers the incorporation of SM into light-emitting electrochemical cells (SM-LEC), which are categorized according to the function of the SM in the active layer of LECs. The structures and properties of the family of SM used up to date are discussed in this chapter with a special emphasis on their role in the device mechanism and the effect of the molecule charge.