Pramod Kumar Bhatnagar

Temperature and electric-field dependences of hole mobility in light-emitting diodes based on poly [2-methoxy-5-(2-ethylhexoxy)-1,4-phenylene vinylene]

The current-voltage characteristics of poly [2-methoxy-5-(2-ethylhexoxy)-1,4-phenylene vinylene] (MEH-PPV)-based hole-only light-emitting diodes are measured as a function of temperature. The hole current is found to be space-charge limited, providing a direct measure of the mobility as a function of temperature and electric field. A thermal activation energy of 0.2eV is obtained for the zero-field mobility, with a room-temperature low-field mobility value for holes of 3.3×10−7cm2∕Vs. The hole mobility exhibits field dependence in accordance with the Poole-Frenkel effect. The combination of space-charge effects and field-dependent mobility thus provides a consistent description of hole transport as a function of temperature and bias voltage in MEH-PPV films.

Ultraviolet electroluminescence from zinc oxide nanorods/deoxyribonucleic acid hybrid bio light-emitting diode

Ultraviolet (UV) light-emitting diode using salmon deoxyribonucleic acid (sDNA)- cetyltrimethylammonium complex as an electron blocking layer and zinc oxide (ZnO) nanorods as emissive material was fabricated. UV emission, which was blue shifted up to 335 nm with respect to the band edge emission of 390 nm, was observed. This blue shift was caused due to accumulation of electrons in the conduction band of ZnO because of a high potential barrier existing at the sDNA/ZnO interface. C 2011 Society of Photo-Optical Instrumentation Engineers (SPIE).

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.

An anomalous behavior in degraded bulk heterojunction organic solar cells

An anomalous behavior—a change in polarity with the passage of time in the bulk heterojunction poly(3-hexylthiophene) (P3HT):6,6-phenylC61 butyric acid methyl ester (PCBM) organic solar cells—is reported here. This work is a continuation of our previous work where the initial degradation of the organic solar cells, freshly prepared up to 4 h, was mainly due to domain formation in the active layer. With the passage of time, the activity at the interfaces starts becoming significant. A decrease of VOC and JSC, leading to a change in polarity, has been reported and explained up to 300 h after fabrication.

Fabrication and Characterization of Graphene Field Effect Transistor: Study of Interfacial Effects for Device Reliability

In the present work, we report fabrication and electrical characterization of a back gated graphene field effect transistor (GFET). We have focused our study on the interfacial effect (graphene/SiO2) on the performance of the device. Hysteresis was observed in the drain conductance when measured with respect to dual gate sweep voltage, which increases with increasing sweeping voltage range. The conductance was observed to increase with increase in temperature but there was no reduction in the hysteresis. This proved that temperature annealing could improve the channel conductivity but not the interfacial effects. Further, a metal oxide semiconductor (MOS) device was fabricated with SLG inserted in between the metal and oxide layer and its capacitance-voltage (C-V) characteristics were studied. A small series capacitance (2.1 nF) was observed to be existing in series with the oxide capacitance (4.5 nF) which was attributed to the trap states at the interface of graphene and SiO2 layer. Also, the flat band voltage was not affected by the incorporation of graphene layer in the MOS device indicating no change in the work function of the metal gate (Cr/Au). This is an advantageous situation where graphene does not alter its work function also being impermeable, restricts the diffusion of metal particles through the SiO2.