Orientation of liquid-crystal molecules on substrates exposed to argon-ion bombardment

The possibility for homogeneous orientation of liquid-crystal (LC) molecules was studied after argon-ion bombardment of substrates (Si, Ge) transparent in the infrared (IR) spectral region. The orientation of the LC molecules was monitored by IR spectroscopy. Nematic LC 4-methoxybenzylidene-4′-butylaniline (MBBA) was used to study the molecular orientation. Absorption spectra were analyzed near the band with maximum at 1630 cm⁻¹ corresponding to − CH=N-group vibrations along the long axis of the MBBA molecule. The type and degree of initial orientation of the LC molecules were determined from the correlation of the integrated absorptions of this band without and with an applied electric field (above the threshold voltage for the Fredericksz effect). It has been established that an increased ion fluence results in a planar orientation of the LC molecules and in a gradual transformation of the planar orientation of the molecules to a homeotropic one with preliminary argon-ion bombardment of substrates at energies of 250 eV and 1.25 keV, respectively. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 6, pp. 763–767, November–December, 2008.     

Electrochromic blueshift in polymer-dispersed liquid-crystal cells

Electrochromic blueshift in the absorption band of polymer-dispersed liquid-crystal cells is reported as a function of applied electric field. The changes in the peak absorption wavelength, absorption broadening, and their possible relationships with the nonlinear optical properties of polymer-dispersed liquid-crystal cells are discussed.     

Self-activated luminescence of zinc sulfide

The nature of the luminescence of zinc sulfide in the spectral range (360), 380–420 nm (SAL) at 80K is analyzed. It is shown that the appearance of SAL radiation is accompanied by additional absorption in the region (350) 365–370 nm at 80K. The low-temperature spectra of sphalerite, exposed to different radiation and subjected to different treatment, are studied. The multiband luminescence of the isoelectronic sulfur impurity in ZnO deposits in the region 383–640 nm is discussed. It is concluded that SAL luminescence is attributable to the localization of excitons in ZnS on clusters, whose formation precedes the precipitation of the ZnO·S phase. The formation of oxygen clusters and deposits on dislocations in ZnS is studied.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 60–66, February, 1987.     

Self-activated luminescence in zinc-sulfo-selenide crystals

The orange luminescence at 2.15eV of As-grown ZnS_0.52Se_0.48crystals has been studied. The peak energy, halfwidth and Gaussian band shape exhibit a temperature dependence which is similar to that of the self-activated (SA) luminescence of ZnS at 2.66 eVand to that of ZnSe at 2.03 eV, and which can be described satisfactorily by a configurational coordinate model. However, optically detected magnetic resonance experiments at 1.8 K show that the luminescence is due to the recombination of an electron thermally released from a donor state with a hole trapped at an A-center acceptor state. The temperature dependence of the thermoluminescence spectra above 77 K and their spectral shift under additional IR exposure are also consistent with radiative recombination involving distant donor-acceptor pairs.     

Si-Ge-GaAs nanoheterostructures for photovoltaic cells

Synthesis from molecular beams in an ultrahigh vacuum is a promising method for producing multilayer semiconducting thin-film structures for high-efficiency conversion of heat and solar energies into electricity, where cascade converters with complex optimized chemical compositions and alloying profiles are necessary. Until recently, nanotechnologies of heterostructures, such as quantum wells, superlattices, and quantum dots, were not applied for photovoltaic conversion. The state of the art of technologies in this field is analyzed.     

Visualization of the langmuir-blodgett film structure in liquid-crystal cells

The use of liquid-crystal (LC) cells for the visualization of the Langmuir-Blodgett (LB) film structure is described. Images appearing in LC cells upon LB film application are presented. The images are analyzed in relation to the LB film structure (application conditions) and to the relief formed by indium tin oxide (ITO) electrodes.     

Polymer heterostructures with embedded carbon nanotubes for efficient photovoltaic cells

Polymer photovoltaic cells (PVC) are intensely investigated because of their potential advantages over Si-based PVCs. Their present drawbacks are low conversion efficiency, limited exciton diffusion length, poor hole carriers transport and short lifetime. The highest conversion efficiency achieved so far in spin-coated polymer blends is close to 5%. Recently, efficiency growing has been demonstrated in multilayer architectures involving a donor/acceptor bulk heterojunction. Alternatively, a nanomaterial has been added to the polymer active layer to facilitate excitons dissociation and carriers transport through the polymer matrix. In this work we investigate both these approaches, first embedding single wall Carbon Nanotubes (SWCNT) in the polymeric matrix to improve the electrical transport and second studying the optical absorption of different polymer thin films to optimize the spectral response of the donor/acceptor heterojunction.     

Voltage sensitivity of the optical nonlinearity in nematic liquid-crystal cells

The influence of an external quasi-static electric field on the third-order nonlinear optical properties of a nematic liquid crystal has been studied. We demonstrate both in theory and experiment that the cell behaves like a Kerr-Medium. The sensitivity of the cell can be controlled by the applied voltage. If optical feedback is provided by the use of a dielectric mirror, optical bistability can be achieved. The switching behaviour of the cell may also be controlled by the applied voltage.     

Rotational reorientation of the director in twisted nematic liquid-crystal cells

The orientational relaxation of the director to its equilibrium orientation under electric, elastic, and viscous torques arising in twisted nematic liquid-crystal cells is investigated. It is shown that the relaxation time of the director depends strongly on the external electric field strength and weakly on the energy of anchoring of liquid-crystal molecules to the surfaces of the cell. The relaxation time of the director anomalously increases in electric fields close to the Fréedericksz threshold. It is established that, at specific strengths of the external electric field, the relaxation can occur in the form of traveling waves propagating from one edge to the other edge of the twisted nematic liquid-crystal cell. The calculations of the relaxation processes in the vicinity of the nematic-smectic A phase transition temperature demonstrate that the distortion of the director field is uniform over the entire cross section of the liquid-crystal cell and does not depend on the strength of anchoring of the liquid-crystal molecules to the surfaces of the cell.     

Impurity photovoltaic effect in silicon solar cells doped with two impurities

In this work, a numerical study has been carried out to investigate the impurity photovoltaic (IPV) effect for silicon solar cells doped with two impurities (indium and thallium). It is found that the conversion efficiency \(\eta \) of the IPV solar cell doped with two impurities can improve by 2.21 % absolute, which is greater than that of the IPV solar cell doped with indium ( \(\Delta \eta =1.63\,\%\) ), but less than that of the one doped with thallium ( \(\Delta \eta =2.69\,\%\) ). It is concluded that introducing two IPV impurities may not be a good selection for implementing the IPV effect since one impurity with poorer IPV effect can absorb some sub-bandgap photons while contributing fewer currents. The location of impurity energy level is critical to the IPV cell performance. For an acceptor-type IPV impurity, the optimized location of the IPV impurity energy level locates at 0.20–0.26 eV above the valence band edge. Our results may help to make better use of the IPV effect for improving solar cell efficiency.     

Anomalous reorientation processes in liquid-crystal cells with hybrid orientation in response to a temperature gradient

The orientational relaxation of the director field, the velocity field, the temperature field, and the stress tensor components in a liquid-crystal cell with a hybrid orientation in response to a temperature gradient is investigated. The anomalies of the orientational relaxation of these fields are numerically analyzed in the case where the temperature of one of the bonding surfaces is close to the nematic-smectic A phase transition temperature.     

A fullerene-based dyad for organic photovoltaic cells

We describe the synthesis and the photophysical properties of a fullerene–azothiophene dyad, as well as the photovoltaic performance of cells incorporating the dyad and the surface morphology of the device active layer. The results have been compared with those obtained on the blend, in equimolar ratio, between the azothiophene dye and a fulleropyrrolidine. This revealed a pivotal role played by the morphology on both the photophysical behaviour and the device performance. While scanning force microscopy studies for the dyad exhibited fairly smooth surfaces, in the case of the blend they displayed a micrometre-scale phase segregation between the two components. We suggest that, in the latter case, the lack of photo-induced electron transfer evidenced by the photophysical study, and the relevant reduction of the cell performance (up to more than one order of magnitude with respect to the dyad), could be ascribed to the different morphology. Because of the strong optical absorption in the visible region, the dyad-based solar cells gave notable results if compared with those reported in the literature for other donor–acceptor linked systems. A power-conversion efficiency of 0.37% under 80-mWcm^-2 white-light illumination has been achieved by tuning the thickness of the dyad film, though the fullerene–azothiophene is not yet optimised in terms of photo-induced electron transfer. PACS 81.05.Tp; 73.50.Pz; 71.35.Gg; 72.20.Jv     

Advanced solar materials for thin-film photovoltaic cells

As one of the most promising solutions for the green energy, thin-film photovoltaic cell technology is still immature and far from large-scale industrialization. The major issue is getting low cost and stable module efficiency. To solve these problems, a large amount of advanced solar materials have been developed to improve all parts of solar cell modules. Here, some new solar material developments applied in different critical parts of chalcogenide thin-film photovoltaic cells are reviewed. The main efforts are focused on improving light trapping and antireflection, internal quantum efficiency and collection of photo-generated carriers.     

Hermaphroditic liquid-crystal microlens

We demonstrate a flat microlens that exhibits hermaphroditic focusing properties. When the input polarization is parallel (perpendicular) to the liquid-crystal directors, the lens exhibits a positive (negative) focal length. To select the proper polarization, we could rotate the polarizer (or the lens) mechanically or by use of an electrically controlled twisted nematic liquid-crystal cell. Details of the lens structure and of the device's fabrication and performance are described.     

Fabry Perot etalon with polymer cholesteric liquid-crystal mirrors

We present what we believe to be the first implementation of a Fabry-Perot (FP) etalon using polymer cholesteric liquid-crystal mirrors. These polymer mirrors have each been fabricated onto a single substrate, which allows the FP cavity spacing to be only a few micrometers wide. For the experimental results presented, cavity lengths of 13.8 and 7.6 mum yield near-infrared free spectral ranges of 24.8 and 45.6 nm, respectively. The measured finesse of 14.31 is approaching the limitation imposed by the reflectivity of the mirrors.     

Broadband polarization correction with programmable liquid-crystal modulator arrays

We demonstrate a novel method of parallel, multiwavelength state-of-polarization (SOP) correction. Using a new liquid-crystal modulator array design, we are able to rotate the distorted input SOP spectrum to a fixed linear state on a wavelength-by-wavelength basis. We report experimental correction of up to 25.5-dB polarization-dependent loss over a 13-nm bandwidth around 1550 nm.