Rakhi Grover

Efficiency enhancement in blue organic light emitting diodes with a composite hole transport layer based on poly(ethylenedioxythiophene):poly(styrenesulfonate) doped with TiO2 nanoparticles

Blue color organic/polymeric light emitting diodes are very important because they can be used for tri-color display applications, fluorescence imaging, and exciting yellow phosphor for generating white light for general illumination. But the efficiency of blue organic/polymeric light emitting diodes is considerably low due to their large band gap that requires higher energy for effective emission. In this paper we report the enhancement in polyfluorene blue organic light emitting diodes with a polymer nano-composite hole transport layer. Blue light emitting diode based on polyfluorene as an emissive layer and poly(3,4 ethylenedioxythiophene):poly(styrenesulfonate)–titanium dioxide nanocomposite as the hole transport layer were fabricated and studied. Different concentrations of titanium dioxide nanoparticles were doped in poly(3,4 ethylenedioxythiophene):poly(styrenesulfonate) in the hole transport layer and the performance of the devices were studied. Significant enhancement in the blue peak at 430 nm of polyfluorene has been observed with increase in concentration of TiO2 nanoparticles in the hole transport layer. The turn on voltage of the device has also been found to improve significantly with the incorporation of titanium dioxide nanoparticles in the hole transport layer. The optimized concentration of titanium dioxide in the hole transport layer for most efficient device has been found to 15 wt.%.

Multilayer thin film encapsulation for organic light emitting diodes

A transparent and effective thin film encapsulation (TFE) based on a multilayer structure is demonstrated. Alternate films of amorphous and crystalline film forming organic materials have been investigated to create complicated diffusion pathways for oxygen and water vapour, which was reflected in their increased barrier properties. These layers are further protected by an inorganic barrier coating of magnesium fluoride (MgF2) deposited by normal and glancing angle deposition methods. A significant enhancement of more than 8 times in the lifetime of organic light emitting diodes was obtained by employing this multilayer hybrid geometry. The TFE structure can be highly useful in organic opto-electronic devices requiring easy to deposit and an effective barrier layer for enhanced lifetimes.

Efficient quasi-solid state dye sensitized solar cell using succinonitrile : thiourea based electrolyte composition

The present work demonstrates the application of a composite gel based electrolyte for the fabrication of quasi solid state dye sensitized solar cells (DSSCs). Two gel electrolyte compositions were prepared using poly-ethylene glycol (PEG), titanium di-oxide (TiO2) and ionic liquids with only succinonitrile (SCN) and succinonitrileu2006:u2006thiourea (SCNu2006:u2006TU). Fabrication of DSSCs was carried out by sandwiching two gel electrolyte compositions between a dye sensitized photo anode and a platinum counter electrode. The electrolyte compositions were characterised using scanning electron microscopy (SEM), electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) studies. The presence of SCNu2006:u2006TU in the electrolyte introduced an interconnected network with titania-nano clusters embedded uniformly inside the host matrix. EIS and CV analysis demonstrated reduced charge transfer resistance and increased current density values. These results were found to be in accordance with current density–voltage measurements of DSSCs which revealed 5 times enhancement in short circuit current density and approximately 3 times enhanced efficiency of 2.38% with the addition of SCNu2006:u2006TU inside the electrolyte matrix. However, slight reduction in open circuit voltage (from 0.68 V to 0.53 V) on addition of SCNu2006:u2006TU was observed. This may be attributed to downward movement of the quasi Fermi level of titania relative to the redox potential of the electrolyte. These results were also confirmed by absorption spectroscopy analysis. The combination SCNu2006:u2006TU was found to be an efficient additive for quasi solid state electrolyte based DSSC applications.

Light out-coupling enhancement of organic light emitting devices using nano-structured substrate produced by rapid thermal processing

We report significant enhancement in light out-coupling efficiency of organic light-emitting devices (OLEDs) using a nano-structured substrate by rapid thermal processing (RTP). On the backside of the indium tin oxide (ITO) coated glass substrate a thin film of magnesium fluoride (MgF2) was coated by thermal evaporation. Nano-structured films of MgF2 and ITO were then produced by RTP on both sides of the glass substrate. Bottom-emitting OLEDs were fabricated on the ITO-coated glass substrate with ?-NPD as the hole transport layer, Alq3 as the emissive layer, and LiF and aluminum as the cathode. On the backside of the glass substrate nano-structured MgF2 were fabricated by RTP. Experimental results of enhancement of electroluminescent intensity (EL) with and without nano-structured films are presented. Results of EL intensity of OLED are also compared with the uniform MgF2 film coated on the backside of the substrate. It was found that the enhancement of EL intensity is much higher in the case of nano-porous MgF2 film than in the case of uniform MgF2 coated on the backside of the glass substrate.

White electroluminescence from hybrid organic inorganic LEDs based on thermally evaporated nanocrystals

Light-emitting diodes (LEDs) with a broad spectral emission have been fabricated using co-evaporated monolayers of cadmium sulfide (CdS) and cadmium telluride (CdTe) nanocrystals in a hybrid organic-inorganic structure. The nanocrystals of CdS and CdTe were grown by vacuum thermal evaporation technique. The incorporation of 50:50 ratio of CdS and CdTe resulted in a broad emission spectrum with CIE coordinates (0.22, 0.33). The fine tuning of the emission spectrum to achieve pure white light (CIE coordinates (0.30, 0.33)) was achieved by incorporating a small quantity of 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyan (DCM) dye in the device structure. The study indicates a promising route towards more stable and efficient light-emitting devices for lighting applications.

Interface modified thermally stable hole transporting layer for efficient organic light emitting diodes

Electrical transport in thermally stable 2, 7-bis [N, N-bis (4-methoxy-phenyl) amino]-9, 9-spirobifluorene (MeO-Spiro-TPD) thin films has been investigated as a function of temperature and organic layer thickness. ITO/MeO-Spiro-TPD interface was found to be injection limited and has been studied in detail to find barrier height for hole injection. The thickness of tetra-fluoro-tetracyano-quinodimethane thin films were optimized to be used as hole injection buffer layer which resulted in switching of charge transport mechanism from injection limited to space charge limited conduction above a critical thickness of 3 nm. Hole mobility has been measured using transient space charge limited conduction (SCLC), field dependent SCLC, and top contact transistor characteristics. The charge carrier transport in interface modified hole only devices was analysed using Gaussian disorder model. The thermal stability of MeO-Spiro-TPD has been investigated by atomic force microscopy and X-ray diffraction studies. The study indicates a thermally stable and highly efficient hole transport material for application in organic semiconductor based devices.

Solid state dye sensitized solar cells with polyaniline-thiourea based polymer electrolyte composition

The quest for cost effective and efficient solid-state electrolytes is a primary goal for dye sensitized solar cells (DSSCs). The present study describes an experimental approach of synthesizing simple composite electrolytes from iodide-triiodide ions (I−/I3−) incorporated into a polyaniline/thiourea matrix for solid-state DSSC applications. This electrolyte composition can be effectively used to decrease the probability of recombination at the TiO2/electrolyte interface and increase the catalytic process of I3− reduction at the electrolyte/counter electrode interface. The application of the optimized electrolyte in DSSC results in increased open-circuit voltage as well as short circuit current density, thereby increasing the overall efficiency by 73% as compared to the reference electrolyte. The present composition can prove to be a promising redox medium for solid state DSSCs.

Self-powered electrochromic window using green electrolyte

Electrochromic windows are considered as future smart windows for conditioning day light inside the room. Most of the electrochromic windows are fabricated using synthetic electrolytes which require an external power source and are also not always environmental friendly. Therefore there is a need for the utilization of naturally occurring electrolytes and also low power consumption devices. We report a self-powered electrochromic window based on naturally occurring gel electrolyte extracted from aloe vera. Prussian blue and aluminum electrode based electrochromic window was fabricated to test the aloe vera gel as electrolyte. A voltage of 0.8 V appeared across the two conducting electrodes of the electrochromic device and light transmittance was found to change from dark to transparent state on self-biasing. The recovery of the device from transparent to dark state with different external voltages was studied. Aloe vera gel was found to be useful as an electrolyte in smart windows, due to its advantages such as low cost, ease of availability and non–toxicity.Electrochromic windows are considered as future smart windows for conditioning day light inside the room. Most of the electrochromic windows are fabricated using synthetic electrolytes which require an external power source and are also not always environmental friendly. Therefore there is a need for the utilization of naturally occurring electrolytes and also low power consumption devices. We report a self-powered electrochromic window based on naturally occurring gel electrolyte extracted from aloe vera. Prussian blue and aluminum electrode based electrochromic window was fabricated to test the aloe vera gel as electrolyte. A voltage of 0.8 V appeared across the two conducting electrodes of the electrochromic device and light transmittance was found to change from dark to transparent state on self-biasing. The recovery of the device from transparent to dark state with different external voltages was studied. Aloe vera gel was found to be useful as an electrolyte in smart windows, due to its advantages s...

Synthesis and Properties of Inorganic Organic Chloride Based Perovskite

Here we report single step synthesis of lead chloride based organic inorganic hybrid perovskite . Thin films of the material show nanostructured morphology dependent on the fabrication method. Thin films deposited by drop casting method exhibit cubical structures. This morphology changes itself to randomly oriented platelets on spin coating. This evolution of morphology is verified by using scanning electron microscopy (SEM). However as revealed by optical microscopy both the films exhibit identical fluorescence spectra with PL peaks at around 500 nm. Such type of materials may be highly useful in photonic applications requiring varying morphological properties.

Effect of Blocking Layer on the Performance of Dye Sensitized Solar Cells

Dye Sensitized solar cells (DSSC) are a significant stepping stone in the field of photovoltaics . In DSSCs, a blocking layer is generally used in between the conducting electrode and the porous titanium dioxide layer. A compact blocking layer covers the rough surface of anode and reduces the probability of short-circuits between the electrolyte and the photoanode. However, a thick blocking layer can also act as a resistance. Therefore, efficient cells require a careful optimization of its thickness. In the present work, a compact blocking layer has been optimized for DSSCs based on a quasi solid state electrolyte and Eosin B as the sensitizer material. The porous semiconductor oxide layer and the optimized compact blocking layer have been characterized by scanning electron microscopy. In addition, the functionality of the blocking layer in DSSC is verified by solar cell characterizations. A significant enhancement has been obtained in DSSCs with blocking layer as compared to the reference cell.