A. K. Thakur

Bistability in deformed helix ferroelectric liquid crystal

Bistability in deformed helix ferroelectric liquid crystal (DHFLC) has been investigated by electro-optical and dielectric methods and compared with the bistability phenomenon in surface stabilized ferroelectric liquid crystal (SSFLC). Bistability in DHFLC as well as SSFLC is based on different physical phenomena. In SSFLC, it occurs due to surface effect and in DHFLC, by the critical deformation of the helix, under the application of time delayed square voltage pulse of known magnitude and frequency. Memory behavior of DHFLC and SSFLC cells has been compared by optical, electrical, dielectric, and hysteresis methods and the difference has been analyzed. It has been observed that memory effect in DHFLC is independent of surface effect but critically depends on the applied voltage and frequency of time delayed square wave pulse.

Relation between charge carrier density and lifetime in polymer-fullerene solar cells

The recombination kinetics of photo-generated charge carriers in polymer-fullerene solar cells has been studied. The transition between bimolecular to monomolecular (trap-assisted) recombination has been investigated by transient open-circuit voltage decay and intensity dependence of open-circuit voltage at different temperatures to quantify the trap delimitation energy. It has been observed that the critical parameter underpinning the transition between bimolecular to monomolecular recombination is the density of photo-generated charge carriers and their lifetime. The correspondence between carrier lifetime and density of photo-carriers has been experimentally observed for the first time.

Is Curie–Weiss law valid in every ferro-to-para transition?

The Curie–Weiss law is obeyed where there is a change in the order parameter and the symmetry of molecules at the transition temperature (Tc) of ferro-to-para phase. The ferroelectric transition is not strictly governed by the Curie–Weiss law. In fact, a number of solid ferroelectrics show deviation from the Curie–Weiss law. It has been shown in the present letter that the de-Vries electroclinic liquid crystals, with a wide smectic A (Sm-A) phase and a high electroclinic effect, do not obey the Curie–Weiss law. The nonlayer shrinkage leads to symmetry conservation that makes the observance of soft mode impossible and hence the deviation.

Optical memory effect in a deformed helix ferroelectric liquid crystal.

Optical memory in a deformed-helix ferroelectric liquid crystal is proposed by deforming the helix under the application of a square-voltage pulse of known magnitude and frequency. This effect is based on the electromechanical effect of helix deformation due to the electric field. When the interaction between the electric field and the dipole is sufficiently strong, all of the dipoles align along the electric field. In such a situation the interlayer dipole-dipole interaction is strong enough to balance the elastic deformation energy. When the electric field is switched off, the molecules remain in a static, balanced state owing to the dipole-dipole interaction and hence the memory effect.

Thickness-independent memory effect in ferroelectric liquid crystals

Memory in ferroelectric liquid crystals (FLCs) is well known in the literature where thickness of the cell is less than the pitch value of the material. Here, we report a thickness independent memory in a class of FLCs called the de Vries electroclinic liquid crystals. Thickness independency of memory effect is observed by dielectric spectroscopy and texture observation. The memory observed in SmC* phase of de Vries material is entirely different from conventional FLCs. In the former case, it is the inherent property of the material but in the latter it is dependent on the cell geometry. In de Vries material, it is probably the randomization that is playing a major role.

Dielectric Relaxation Near the Transition Temperature of Sm C*-Sm A Phase in Electroclinic Liquid Crystal

The dielectric relaxation processes of electroclinic liquid crystal material have been investigated in a thin (2.5 w m) planarly well aligned cells in the frequency range of 50 Hz to 10 MHz. The behaviour of Goldstone and Soft mode which comes due to phase fluctuation and tilt fluctuation of the molecules has been studied at different temperatures and different bias fields in Sm C* and Sm A phases. It has been observed that Goldstone mode appears in Sm C* phase and soft mode in Sm A phase. However, near the transition temperature of Sm C*-Sm A, the dielectric permittivity due to Goldstone mode and soft mode could not be separated. It is concluded and discussed that there is no sharp transition temperature of Sm C*-Sm A phase in electroclinic liquid crystal, due to the continuous decrease in the dielectric permittivity and increase in the relaxation frequency near the transition of Sm C* to Sm A phase.

A dielectric mode in electroclinic liquid crystals

The dielectric properties of electroclinic and ferroelectric liquid crystal materials have been investigated in the frequency range of 100Hz–10MHz. A dielectric mode has been predicted in electroclinic liquid crystals near the transition temperature of Sm-C*–Sm-A phase. It has been observed that the investigated material has nonlayer shrinkage near the transition temperature of Sm-C*–Sm-A phase and shows anomalous behavior of dielectric spectra, tilt, and texture which is entirely different from the behavior of ferroelectric liquid crystals (FLCs). The dielectric relaxation frequency and tilt angle are almost constant with respect to temperature near the transition temperature under high bias field, which is characteristically different from FLCs. The high dielectric permittivity near Tc owing to the presence of mode contribution leads to the fact that there is an intralayer phason variation few degrees before transition. The mode has been named random mode due to its origin from randomization of tilt nea...

Memory effect in smectic-A phase of ferroelectric liquid crystal

Memory effect has been observed in smectic-A (Sm-A) phase of ferroelectric liquid crystals (FLCs) using dielectric spectroscopy and texture observation. The dielectric relaxation processes in FLC materials with wide range of Sm-A phase and a high electroclinic coefficient have been investigated near the phase transition of Sm-C*-Sm-A phase in thin and planarly aligned cells in the frequency range 10Hz–10MHz. Here phason mode (Goldstone mode) and soft mode contribution in dielectric permittivity could not be clearly separated, near and slightly above the transition temperature Tc. If there is a contribution of phason mode after Tc in Sm-A phase then there is a possibility of getting memory even in Sm-A phase where the molecules get clamped in one direction and stay there even when the field has been removed. A comparative analysis of dielectric permittivity as well as texture in Sm-C* phase has been done on a virgin cell and on a cell in which bias has been applied in Sm-A phase just 1–2°C above Tc for 1mi...

Memory effect in Sm C* phase of electroclinic liquid crystals

Dielectric, textural, and electro-optical investigations have been carried out on a material exhibiting de Vries smectic A phase transition in sample cells of various thicknesses. Thickness independent memory effect has been observed in this special class of ferroelectric liquid crystals (FLCs) known as de Vries electroclinic liquid crystals. Unlike the FLC materials, surface anchoring energy does not play a role in the memory effect due to the fact that pitch is around 2 micron and basic assumption of SSFLC effect is not valid. The conversion of short-range azimuthal (Φ) ordering of molecules in Sm C* phase into a long-range azimuthal ordering under the influence of field could be one of the possible explanations. The transformation of molecular ordering in Sm C* phase under field may be mediated by dipolar interaction and viscoelastic properties.

Detection of Continuous Sm C*–Sm A Phase Transition in Electroclinic Liquid Crystals by Dielectric Method

The dielectric relaxation processes near the transition temperature (Tc) of Sm C*–Sm A phase transition in an electroclinic liquid crystal have been investigated in a homogeneously aligned cell of 2.5 µm thickness in the frequency range of 50 Hz to 10 MHz. No abrupt change in phase during the Sm C*–Sm A phase transition has been detected by the dielectic method; therefore no soft mode contribution to the dielectric permittivity near Tc in the Sm C* phase has been found. It has also been observed that the dielectric strength (Δe) near the transition temperature is very high. These two unusual behaviours of electroclinic liquid crystal material have been analyzed by means of theoretical calculations, taking into account the effect of the electric field in generalized Landau-type free energy densities (ΔF). The experimental results in this study agree well with theoretical predictions.