Devinder Madhwal

Color tuning and improved performance of poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1, 4-phenylenevinylene]-based polymer light emitting diode using cadmium selenide/zinc sulphide core shell uncapped quantum dots as dopants

A polymer light emitting diode (PLED) based on poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene]:CdSe/ZnS core shell uncapped quantum dots (QDs) was fabricated. It was observed that the presence of QDs in the polymer tunes the emission spectrum of the PLED as their QDs concentration increases to 10% w/w and above. It was also found that the QDs present in the polymer improved the PLED luminance by ∼20 times at a typical current density of 75 mA/cm2. This was attributed to the suppression of nonradiative electrostatic energy transfer from excitons to the metallic cathode due to the insertion of a high dielectric constant QD layer. Also, the presence of QDs layer between the active layer and the cathode shifts the recombination zone away from the cathode. This reduces the diffusion of radiative excitons into the metal electrode.

Effect of thermal annealing on the efficiency of poly (3-hexylthiphone):[6,6]-phenyl-C 61 -butyric acid methyl ester bulk heterojunction solar cells

The effect of thermal annealing on the performance of bulk heterojunction poly (3-hexylthiphone) (P3HT): (6,6)-phenyl-C61-butyric acid methyl ester (PCBM) solar cells has been examined. We found that the efficiency of the solar cell increases from 0.08% to 3.81% as the annealing temperature is varied from room temperature to 120 ◦ C. This improvement in the efficiency is due to the rearrangement of polymer molecules and nanoparticles. Annealing reorders the P3HT polymer chain structure (which was ruined by the PCBM nanoparticles) by segregating out PCBM molecules from polymer. Due to annealing, movement in the P3HT and PCBM particles is induced, which reorganize and form a phase segregated 3D structure of donor and acceptor molecules enhancing the charge transfer efficiency. It also improves the surface morphology and polymer chain interconnections resulting in enhancement of hole mobility through the polymer network. C 2011 Society of Photo-Optical Instrumentation Engineers

Enhancement of efficiency of a conducting polymer P3HT:CdSe/ZnS quantum dots hybrid solar cell by adding single walled carbon nanotube for transporting photogenerated electrons

Hybrid solar cells consisting of a composite of poly (3-hexylthiophene) (P3HT), single walled carbon nanotube (SWCNT), and cadmium selenide/zinc sulphide (CdSe/ZnS) coreshell quantum dots (QDs) have been fabricated in the present work. The bulk hetrojunction has been formed from the bilayer of P3HT:SWCNT composite and QDs using inter-diffusion process. Due to low percolation limit and high conductivity of SWCNT, the photo-generated electrons are collected at the electrode very fast (within few femto-seconds) enhancing the efficiency of the solar cell. The absorption measurements on the composite film show that the addition of SWCNT in the hybrid structure increases the absorption coefficient in the near infrared region and also makes the spectrum wider as compared to that of P3HT. The photoluminescence (PL) measurements show that the PL of hybrid P3HT, SWCNT, and QDs is quenched about ∼15 times as compared to that of P3HT film. This shows that a significant charge transfer of electrons occurs through SWCN...

The effect of cadmium vacancies on the optical properties of chemically prepared CdS quantum dots

CdS quantum dots (QDs) in a polyvinyl alcohol (PVA) matrix have been grown by a chemical method and are characterized by transmission electron microscopy (TEM), UV?vis absorption, photoluminescence (PL) and energy dispersive x-ray diffraction (EDX). TEM studies of CdS films show that a nearly spherical cluster of CdS QDs with an average radius of 10?15?nm is formed. From absorption measurements, it is observed that with increasing the PVA concentration from 5 to 8?wt.%, the absorption edge shifts from 3.1 to 3.6?eV, which is attributed to an increase in quantum confinement with decreasing the QD size. PL studies in an energy range of 1.8?3.3?eV show two distinct peaks. The higher-energy peak corresponds to band edge emission, whereas the lower-energy peak corresponds to defect emission. EDX results revealed that the atomic concentration of cadmium is much lower than that of sulfur, indicating that cadmium vacancies are predominant. It was concluded that cadmium vacancies are mainly responsible for defect emission in the PL spectrum.

Development and characterization of PCDTBT:CdSe QDs hybrid solar cell

Solar cell consisting of low band gap polymer poly[N-900-hepta-decanyl-2,7-carbazole-alt-5,5-(40,70-di-2-thienyl-20,10,30-benzothiadiazole)] (PCDTBT) as donor and cadmium selenide/zinc sulphide (CdSe/ZnS) core shell quantum dots (QDs) as an acceptor has been developed. The absorption measurements show that the absorption coefficient increases in bulk heterojunction (BHJ) structure covering broad absorption spectrum (200nm–700nm). Also, the photoluminescence (PL) of the PCDTBT:QDs film is found to decrease by an order of magnitude showing a significant transfer of electrons to the QDs. With this approach and under broadband white light with an irradiance of 8.19 mW/cm2, we have been able to achieve a power conversion efficiency (PCE) of 3.1 % with fill factor 0.42 for our typical solar cell.

CHARACTERIZATION OF CdSexS1-x QUANTUM DOTS USING XRD, UV-VIS ABSORPTION AND RAMAN SPECTROSCOPY MEASUREMENTS

CdSexS1-x quantum dots (QDs) have been grown in borosilicate glass matrix using two step annealing technique and are characterized by AFM, XRD, UV-Vis absorption, HRTEM and Raman spectroscopy measurements. AFM studies show that nearly spherical clusters of about 5–10 nanocrystals were grown. From XRD measurements, it is found that the nanocrystals are grown in hexagonal phase. From HRTEM measurements, it is observed that the size of QDs increases from 5.1 to 9.5 nm with increasing annealing duration from 3 to 10 hrs. Blue shift in absorption spectrum indicates that quantum confinement increases with decreasing annealing durations. Raman spectrum of CdSexS1-x QDs shows two mode type of behavior i.e., CdSe like longitudinal optical phonon mode (LO1) and CdS like longitudinal optical phonon mode (LO2). It has been observed that with increasing the size of QDs from 5.1 to 9.5 nm, LO1 shifts from 205 to 210 cm-1 while LO2 shifts from 278 to 291 cm-1. The shift in the LO1 and LO2 Raman peaks towards high frequencies has been interpreted due to increase in the lattice compressive strain with the increase in QDs size. The effect of phonon negative dispersion on the shift of LO1 and LO2 is found to be comparatively insignificant. It is also found that the ratio of intensity of overtones to the fundamental mode increases from 0.34 to 0.48 with decreasing QD size. This indicates that the electron–phonon coupling increases with the decrease in size of QD due to increasing overlap of electron and hole wave function in small size QDs. It is further observed that the frequency of the surface optical phonon mode remains unchanged as the size of QD changes but the intensity decreases with the increase in the size of QDs finally merging in LO mode Raman spectrum.

Self-trapping mechanism in green phosphorescent dye-doped polymer light-emitting diodes

The mechanism for exciting electroluminescence (EL) in a green phosphorescent dye, iridium(III)tris(2-(4-tolyl)pyridinato-N,C2) (Ir(mppy)3), doped in a host blue-emitting conducting polymer, poly[9,9-di-n-hexyl-fluorenyl-2,7-diyl] (PFO), has been studied. Photoluminescence measurements have been made on PFO/Ir(mppy)3 (0–12%) composites to rule out the possibility of singlet exciton energy transfer from the host polymer to the green dye. EL measurements have also been made to study the behavior of the composites in the presence of dc bias. The dominant mechanism for energy transfer from PFO to Ir(mppy)3 is found to be self-trapping of the charge carriers in the dye molecules, due to the extremely low LUMO and high HOMO levels as compared with PFO, thereby producing EL in the green region.