Inderpreet Singh

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

Improving the efficiency of a poly(3-hexylthiophene)-CuInS2 photovoltaic device by incorporating graphene nanopowder

Abstract. In the present work, the effect of incorporation of graphene on the poly(3-hexylthiophene) (P3HT):CuInS2 quantum dot (CIS QD)-based solar cell has been studied. For this purpose, the concentration of graphene is varied from 0 to 0.01% w/w in P3HT–CIS (1:0.5) film. It is found that graphene does not deteriorate the absorption of the composite film. It assists in dissociating the photogenerated excitons (both in P3HT and QDs) owing to its two-dimensional structure and high electron affinity as is evident by photoluminescence (PL) quenching. At 0.01% w/w concentration of graphene about ∼95% of PL is quenched. The electrical characteristics show that the incorporation of graphene enhances the efficiency of the device by establishing interconnected conducting pathways in the volume of polymer matrix. The maximum efficiency is observed to be 1.5% at 0.005% w/w content of graphene. However, at higher concentration, i.e., 0.01% w/w, the device starts deteriorating.

Optical absorption spectrum of single-walled carbon nanotubes dispersed in sodium cholate and sodium dodecyl sulfate

Commercially procured single-walled carbon nanotubes were dispersed in 2 wt% solution of sodium cholate and also in 1 wt% solution of sodium dodecyl sulfate. The absorption spectrum of the suspensions was studied in ultraviolet–visible–near-infrared (UV–vis–NIR) range. Two distinct bands, each containing three peaks, were observed in NIR range for both the suspensions. These peaks correspond to transitions between van Hove singularities E11 and E22 in the density of states of the semiconducting nanotubes. Comparing positions of the observed peaks with the empirical Kataura plot, the diameters and chiralities of the nanotubes were estimated. Using tight binding approximations, the diameter of the nanotubes was also estimated theoretically. Discrepancies between the theoretically calculated diameters and those obtained by empirical Kataura plots are found to be higher for E11 peaks. It has been suggested that the reason for this discrepancy is that the observed E11 peaks are blue-shifted due to Coulomb interactions and exciton formation.

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.

Study of photoluminescence quenching and DC conductivity measurements in polymer-SWNT composite films for various SWNT concentrations

Conducting polymer-SWNT composite films have a high potential in the area of photovoltaic devices and light emitting diodes (LEDs). SWNTs have high electron affinity and high mobility for electrons. As a result of this, the separation of photoinduced charges in the solar cell is very fast and efficient. For LEDs, the polymer-SWNT composite films act as efficient electron transporting layer (ETL). In the present work, composite solutions of P3HT and SWNTs were prepared in 1,2-dichlorobenzene with nanotube to polymer mass ratios varied from 0% to 30%. The films from this solution were spin coated on a glass substrate with the help of spinner keeping the film thickness ∼100 nm. The photoluminescence (PL) of the film was observed with the help of Shimadzu Spectrofluorophotometer. It was found that the PL intensity decreases by ∼90% as the concentration of the SWNT is increased from 0% to 30%. This decrease is due to the transfer of electrons from P3HT to SWNTs before the exciton in the polymer can decay radia...

Effect of lattice strain on structural and optical properties of ZnO nanorods grown by hydrothermal method

In this work, we have synthesized ZnO nanorods over ZnO seeds/ITO/glass substrate by the facile hydrothermal method. ZnO seeds are grown at different temperatures ranging from 150°C to 550°C in steps of 100°C. We have studied the effect of strain on the structural and optical properties of ZnOnanorods. It was observed that the growth temperature of seed layer has an influence over the lattice strain present in the nanorods. The as synthesized nanorods were characterized by scanning electron microscope (SEM), x-ray diffraction (XRD) and photoluminescence (PL). SEM images confirm the formation of dense arrays of vertically aligned nanorods on seeds which are grown at 350°C. In addition to this, XRD patterns reveal that these ZnO nanorods are preferentially oriented along (002) direction. The strain analysis based on the XRD results reveals that the minimum value of strain is obtained at 350°C which is attributed to the improved crystalline quality of the interface of seed layer and nanorods leading to their...

Effect of annealing on graphene incorporated poly-(3-hexylthiophene):CuInS2 photovoltaic device

The effect of thermal annealing on the power conversion efficiency (PCE) of poly(3-hexylthiophene) (P3HT):CuInS2 quantum dot:graphene photovoltaic device has been studied by analyzing optical characteristics of composite films and electrical characteristics of the device with structure indium tin oxide/poly[ethylene dioxythiophene]:poly[styrene sulfonate] (ITO/PEDOT:PSS)/P3HT:CIS:graphene/LiF/aluminum. It was observed that after annealing at 120°C for 15 min a typical device containing 0.005 % w/w of graphene shows the best performance with a PCE of 1.3%, an open-circuit voltage of 0.44V, a short-circuit current density of 7.6 mA/cm2, and a fill factor of 0.39. It is observed that the thermal annealing considerably enhances the efficiency of solar cells. However, an annealing at higher temperature such as at 140°C results in a decrease in the device efficiency.

Improving photoluminescence intensity of poly[2-methoxy-5-2’-ethylhexyloxy)-1, 4-phenylenevinylene] blended with graphene nanosheets

Poly[2-methoxy-5-2’-ethylhexyloxy)-1, 4-phenylenevinylene] (MEH-PPV) is an important material and has wide application in optical devices like light emitting diode. However, low mobility of carriers limits the performance of the devices. In the present work, we report improving the optical and electrical characteristics of MEH-PPV by blending graphene nonosheets with it. The carrier mobility increases with increasing blending content of graphene nanosheets, which in turn increases the Photoluminescence (PL) of the graphene/MEH-PPV blend. The PL intensity increases by 7 times at the blending content of 0.005 wt %.