S. C. Jain

Electro‐optic response of polymer dispersed liquid‐crystal films

The electro‐optic response of polymer dispersed liquid‐crystal (PDLC) films is reported as a function of frequency and amplitude of the applied voltage and size of the LC droplets. We found that the threshold voltage is minimum and sharpest at frequencies near a few kHz. Visual and optical response studies show that there are two types of PDLC films; type I, which exhibit large partial optical memory, and type II, which quickly regain their original levels of transmittance after switching off relatively small driving voltage. It was observed that both types of PDLC films, in general, exhibit a two‐step decay involving fast and slow components. However, the order in which the two components appear as the voltage is increased is different for the two types of PDLC films. Observations under the polarizing microscope show that the LC droplets in the two types of PDLC films undergo different transformations as the applied voltage is changed.

A model for the J-V characteristics of P3HT:PCBM solar cells

Current-voltage (J-V) characteristics of an organic bulk heterojunction solar cell have been modeled and compared with the measured characteristics of solar cell based on the blend of poly(3-hexylethiophene) (P3HT) and phenyl [6,6] C61 butyric acid methyl ester (PCBM). In an undoped organic double Schottky junction diode, for V<Vbi the electric field remains constant and is given by (Vbi−V)/d, where Vbi is the built-in voltage, V is the applied voltage, and d is sample thickness. We considered the effect of this constant electric field on the charge carrier transport and solved the drift and diffusion equations to model the J-V characteristics. For V<Vbi the current is found to be dominated by diffusion. A comparison of the theoretical results with the experimental data measured in dark and under different illumination intensities shows good agreement.

Broad spectral sensitivity and improved efficiency in CuPc/Sub-Pc organic photovoltaic devices

We demonstrate organic photovoltaic devices incorporating two donors, namely, copper phthalocyanine (CuPc) and boron sub-phthalocyanine chloride (Sub-Pc) in association with single acceptor fullerene (C60) with sensitivity extending across the visible solar spectrum. It has been found that the absorption in different spectral regions in CuPc and Sub-Pc results in efficient harvesting of incident light photons which leads to enhanced power conversion efficiency (η). An enhancement in η from 0.64%, in the device architecture indium-tin-oxide (ITO)/CuPc(20 nm)/C60(40 nm)/bathophenanthroline (BPhen) (8 nm)/Al(150 nm), to ~1.3% in the optimized device having a 2 nm layer of Sub-Pc in the geometry ITO/CuPc(18 nm)/Sub-Pc(2 nm)/C60 (40 nm)/BPhen (8 nm)/Al(150 nm) has been observed. This enhancement in η is dominantly attributed to the increment in short circuit current density (Jsc) due to efficient photon harvesting by incorporation of dual donors.

A new method for the measurement of series resistance of solar cells

A method is proposed which uses the observed nonlinearity in the short-circuit current versus light intensity curve at high intensities, to determine the series resistance of solar cells.

Effect of illumination intensity and temperature on open circuit voltage in organic solar cells

The effect of illumination intensity and temperature on open circuit voltage (Voc) in organic photovoltaic devices has been investigated. Voc is observed to saturate at high illumination intensities. The illuminated J-V characteristics at different intensities intersect the dark characteristic at a single point. This intersection point is shown to be equal to the built-in voltage (Vbi) in the sample. A reduction in temperature shows increment in saturated Voc. This increment in saturated Voc is attributed to the variation of Vbi with temperature. A model has been presented that explains the observed behavior of Vbi at different temperatures.

Bulk‐induced alignment of nematic liquid crystals by photopolymerization

A novel technique to produce uniform alignment of nematic liquid crystals is presented. The technique is based on producing an anisotropic polymer network in the bulk nematic liquid crystal by polymerizing a photoresist material. Both uniform planar alignment and homeotropic alignment can be obtained depending on the curing conditions. In the case of planar alignment, the nematic director orientation is governed by the direction of the polarization vector of the linearly polarized ultraviolet light, thus making it possible to produce a defined azimuthal orientation of the nematic director. This technique obviates the need to give any previous treatment to the bounding substrates and as such should be independent of the nature of the substrate.

Electro-Optic Studies on Polymer Dispersed Liquid Crystal Films Prepared by Solvent- Induced Phase Separation Technique

Abstract The electro-optic performance characteristics of PDLC films using two different polymers, namely, poly (methyl methacrylate) (PMMA) and poly (vinyl chloride:vinyl acetate 17%) (PVC:VAC-17) and liquid crystal mixture E-8 (BDH, England) have been studied. It is found that the PMMA-based PDLC films have more superior mechanical and electro-optical properties than the PVC:VAC-17-based PDLC films. The performance characteristics of PDLC films, in general, improve significantly with increasing temperature. An optimum weight fraction of liquid crystal mixture is to be added in the polymer for good performance of the PDLC films. The finite solubility of liquid crystal mixture in the polymer affects the properties of the polymer matrix and hence the electro-optic performance characteristics significantly.

A model for the current–voltage characteristics of organic bulk heterojunction solar cells

In an organic bulk-heterojunction (BHJ) solar cell there is a constant electric field (Vbi − V)/d (where Vbi is the built-in voltage, V is the applied voltage and d is the sample thickness) in the active layer. Current–voltage (J–V) characteristics of an organic BHJ solar cell based on a blend of poly(3-hexylethiophene) (P3HT) and phenyl [6,6] C60 butyric acid methyl ester (PCBM) have been modelled by solving drift and diffusion equations taking into account the constant electric field. We fabricated the BHJ solar cells and measured their J–V characteristics. A comparison of the observed characteristics with theory shows excellent agreement. The diffusion component of the current dominates for V < Vbi and the current is not space charge limited.

Thermo‐electro‐optic switch based on polymer dispersed liquid crystal composite

A new type of thermo‐electro‐optic switch based on polymer dispersed liquid crystal (PDLC) composite is reported below. The PDLC composite has been made by dispersing a dual frequency addressable liquid crystal (LC) mixture in an isotropic polymer matrix in the form of very fine droplets of micron and submicron sizes. The PDLC cell in the presence of an electric field can be made to switch from transparent to scattering state or vice‐versa by varying its temperature by ±≥1 °C, respectively. Moreover, the transition temperature of the PDLC switch can be set by varying the frequency of the electric biasing field.

Effect of Active Layer Thickness on Open Circuit Voltage in Organic Photovoltaic Devices

The effect of active layer thickness has been investigated on open circuit voltage (Voc) in bilayer organic photovoltaic (OPV) devices. Voc has been found to vary nonlinearly with the active layer thickness even with the same donor/acceptor materials. We present a model which gives a nonlinear variation of built-in voltage (Vbi) with the active layer thickness. The nonlinear variation of Voc with active layer thickness has been attributed to the variation of Vbi in the samples. The investigations reported here suggest that Vbi controls the Voc in OPV devices.