R. B. Lal

Perovskites: transforming photovoltaics, a mini-review

Abstract. The recent power-packed advent of perovskite solar cells is transforming photovoltaics (PV) with their superior efficiencies, ease of fabrication, and cost. This perovskite solar cell further boasts of many unexplored features that can further enhance its PV properties and lead to it being branded as a successful commercial product. This article provides a detailed insight of the organometal halide based perovskite structure, its unique stoichiometric design, and its underlying principles for PV applications. The compatibility of various PV layers and its fabrication methods is also discussed.

Crystal growth and optical properties of 4-aminobenzophenone crystals for NLO applications

Single crystals of 4-aminobenzophenone, a nonlinear optical (NLO) material have been grown successfully by a novel solution growth as well as modified Bridgman-Stockbarger techniques. The transmission spectrum (420-1400 nm) and SHG efficiencies were determined. 4-Aminobenzophenone crystals gave a lower limit on the second harmonic generation (SHG) efficiency of 10%. Laser damage threshold measurements conducted at the indicated wavelengths gave values of 1-2 GW/cm 2 .

Pyroelectric Properties of PVDF:MWCNT Nanocomposite Film for Uncooled Infrared Detectors and Medical Applications

The biocompatible and flexible polyvinylidene fluoride (PVDF) has a number of interesting pyroelectric and piezoelectric properties with fast, dynamic response for use in touch/tactile sensors, infrared detectors and thermal vidicon/imaging devices. Pyroelectric multi-walled carbon nano-tubes:polyvinylidene fluoride nano-composite films were fabricated via the solution casting technique. The dielectric, pyroelectric and piezoelectric characteristics of composite films were measured. Using foregoing parameters, figures-of-merit for infrared detectors, and thermal-vidicons were calculated. The results indicated figures-of-merit of the composite film that were higher than pristine polyvinylidene fluoride film. The potential applications of composite films in medical devices are also discussed.

Etch pit study of different crystallographic faces of L-arginine phosphate (LAP)

Abstract Etch pit studies have been made on different crystallographic faces of LAP single crystals. A new etchant has been developed which produces reproducible and good quality pits. The observed etch pits reveal the symmetry of different faces and the etching action somewhat depends upon the relative abundance of the various faces in the crystals grown from aqueous solution.

Growth and characteristics of organic and semiorganic nonlinear optical crystals

The growth, synthesis and characteristics of a new organic nonlinear optical crystal, a derivative of Schiff base compounds, 4-nitrobenzylidene-4-chloroaniline are reported. Good-quality crystals have been successfully grown using Bridgman-Stockbarger (BS) Technique. The organic crystal, 4-nitrobenzylidene-4-chloroaniline, shows good second harmonic generation of 1.064 micron wavelength. A recent work done on the semiorganic nonlinear crystals are reviewed along with results of growth of a few L-arginine phosphate type crystals is presented.

Bulk crystal growth, thermophysical, and optical properties of lead magnesium niobate-lead titanate (PMN-PT) piezoelectric single crystals

Piezoelectric single crystals of lead magnesium niobate-lead titanate Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) show superior properties as compared to piezoelectric ceramics and piezoelectric films in device applications. However, the applications of PMN-PT single crystals are limited due to the lack of a simple and reproducible fabrication technique. By using the flux method, we have successfully obtained PMN-PT single crystals. The size of the obtained crystals varied from 2 to 5 mm3, mostly showing regular cubic shape. The microstructure and the growth mechanism of the as-grown single crystals are investigated by scanning electron microscopy. From simultaneous differential calorimetry and thermogravimetric analysis (SDT) measurements, we have found that PMN-PT melts at 1264.12°C. The long wave-length optical modes in PMN-PT single crystals have been investigated using Raman scattering measurements.

Improvement of organic solar cell efficiency by solution-processed doping of pentacene in PEDOT:PSS layer

In the current research, organic solar cells (OSCs) with various concentrations of pentacene in Poly(ethylenedioxythiopene):Poly(styrenesulfonate) (PEDOT:PSS) interface layer were investigated for better hole extraction. The ITO/Pentacene + PEDOT:PSS/P3HT:PCBM/Al-fabricated solar cell fabricated via brush coating provides superior photovoltaic, electrical and optical characteristics when compared with the ITO/PEDOT:PSS/P3HT:PCBM/Al solar cell. The ITO/Pentacene + PEDOT:PSS/P3HT:PCBM/Al solar cells deliver a VOC ~350 mV and 2.57% efficiency. It is observed that the optimized concentration of pentacene doping in PEDOT:PSS layer, along with an active layer of P3HT and PC60BM, doubles the efficiency of the device, when compared with pristine PEDOT:PSS layer. The degradation studies of the fabricated bulk heterojunction OSCs reveal that the degrading abilities of ITO/Pentacene + PEDOT:PSS/P3HT:PCBM/Al solar cells are 60% more better than those of ITO/PEDOT:PSS/P3HT:PCBM/Al devices. Thus, this work will ultimately contribute toward fully solution processed painted device, which will provide low-cost manufacturing and improved stability of pentacene-based organic photovoltaics.

Computation of current responsivity of a bimorph pyroelectric infrared detector

Pyroelectric detectors for infrared radiation are thermal sensors operating at ambient temperature, unlike semiconductor detectors, which require cooling. They have a uniform spectral response in a wide range of wavelengths, including main band of infrared transmission of the earths atmosphere. The effective sensitivity and performance depend not only on the sensor material characteristics but also on the thermal performance of the complete structure of a detector. Recently, it has been proposed that bimorph (two pyroelectric materials stacked together) detector structure shall show better pyroelectric performance than the monomorph. Thus, a one-dimension thermal diffusion equation has been solved for nlayered structure for pyroelectric bimorph films. In such a system, performance of any number of layers of a detector structure can be derived, predicted, and optimized using these computations. Using viable bimorph element sensor configurations and materials parameter, the calculated and predicted current responsivity and other parameters are presented. It is predicted that greater enhancement of the bimorph current responsivity relative to that of monomorph using well-known pyroelectric materials, require higher modulating frequency or thicker bottom (lower) pyroelectric layer of the bimorph detector structure.

Present status of TGS-based pyroelectric infrared detectors and computational simulation of their integration with silicon technology

Pyroelectric detectors of infrared radiation are fast-response thermal sensors operating at ambient temperature unlike semiconductor detectors, which require cooling. Their spectral response is uniform in a large range of wavelengths, including the main band of IR transmission within the earths atmosphere. A further increase in pyroelectric response is possible by integrating pyroelectric sensors with silicon technology. Triglycine sulfate (TGS) based pyroelectric detectors are the most sensitive among available ferroelectric materials. Efforts made so far in improving their growth yield, mechanical properties and figures of merit for their use, as infrared detectors will be presented. Effective sensitivity and performance depend not only on the pyroelectric sensor element material characteristics, but also on the thermal performance of the complete structure of a detector, such as substrate material (Si), absorbing layer, and isolation layers including associated electronics. Thus, we have calculated the thermal transfer function by solving a one-dimensional thermal diffusion equation for a single element n-layer structure. From which the performance of any number of layers detector structure can be derived, predicted, and optimized. Using various single sensor configurations and pyroelectric parameters of modified TGS crystals grown in our laboratory; the calculated and predicted repsonsivity and other parameters of integrated detector system will be presented. The results obtained are encouraging for the development of TGS thin film based detectors.

Computation of Frequency Response of Integrated Pyroelectric Infrared Detectors

Pyroelectric detectors of infrared radiation are fast-response thermal sensors operating at ambient temperature unlike semiconductor detectors, which require cooling. Their spectral response is uniform in a large range of wavelengths, including main band of IR transmission of the earths atmosphere. Triglycine Sulfate (TGS) based pyroelectric detectors are the most sensitive among available ferroelectric materials. The effective sensitivity and performance depend not only on the sensor element material characteristics but also on the thermal performance of the complete structure of a detector such as substrate material, absorbing layer, and isolation layers. Thus we have solved one-dimension thermal diffusion equation for an n-layered structure. From which the performance of any number of layers of a detector structure can be derived, predicted, and optimized. Using viable single element sensor configurations and pyroelectric parameters of deuterated triglycine sulfate (DTGS) crystals grown in our laboratory; the calculated and predicted current responsivity and other parameters are presented. A one dimensional heat conduction equation was solved analytically and the detector parameters were calculated using MatLab 6.0. The results obtained are encouraging for the development of DTGS based thin film-based detectors.