Time‐dependent charge transport in operating poly(3‐hexylthiophene):[6,6]‐phenyl‐C61‐butyric acid methyl ester (P3HT:PCBM) bulk heterojunction organic photovoltaic (OPV) devices has been characterized with impedance spectroscopy. Devices with varied composition and morphology were measured over a range of illumination intensities ranging from dark conditions to 1 sun and applied bias voltages ranging from 0.0 V to 0.75 V. Using an equivalent circuit model, materials properties such as dielectric constant and conductivity were determined and found to be in agreement with values measured by other methods. Average carrier lifetimes were also extracted from the model and found to correlate with measured power conversion efficiencies. At the short circuit condition and ∼1 sun illumination, the average electron lifetime was found to vary from 7.8 to 22 μs for devices with power conversion efficiencies ranging from 2.0 to 2.5%. These results suggest that impedance spectroscopy is an effective tool for predicting how processing parameters can impact device performance in organic bulk heterojunction photovoltaic devices.

Articles you may be interested inThe effect of geometric and electric constraints on the performance of polymer-stabilized cholesteric liquid crystals with a double-handed circularly polarized light reflection band J. Appl. Phys.Reflective reversed-mode polymer stabilized cholesteric texture light switchesThe reflection notch bandwidth of a cholesteric liquid crystal (CLC), equal to the product of the liquid crystal (LC) birefringence (Dn), and the pitch length (p o ), is typically on the order of 50-100 nm in the visible portion of the electromagnetic spectrum. Static bandwidths greater than 100 nm can be observed in CLCs that possess a pitch gradient throughout the thickness of the cell. In this work, we report on polymer stabilized CLC (PSCLC) systems that exhibit electrically controllable, dynamic bandwidths governed by the strength of a direct current (DC) electric field applied across the sample. Symmetric notch broadening which increases linearly with field and reaches a maximum value at 4 V/lm is observed. Removal of the field returns the PSCLC cell to its original optical properties. A seven fold increase in bandwidth was observed for 22 lm thick cells which contained LCs with a small birefringence ($0.04). A variety of CLC mixtures with small positive or negative dielectric anisotropies are shown to exhibit this reversible dynamic bandwidth broadening. The magnitude of the effect was dependent on the amount of polymer stabilization controlled by initial monomer content. The underlying mechanism is partially elucidated by examining cells simultaneously in transmission and reflection and observing differences when modulating the DC polarity across the cell. Different mechanisms for the observed effects are discussed in terms of consistency with our experimental results.

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In this work, we have studied the temperature dependence of a cholesteric liquid-crystal laser coupled to an optical fiber, with a view towards optical fiber sensor applications. To stabilize the laser emission, we developed a procedure to align the liquid crystal placed in the fiber. Unexpected oscillations in the laser emission were observed as the temperature was varied, which can be understood in terms of the competition between bulk and surface anchoring torques.

We show that bent-core liquid crystalline materials exhibit non-Newtonian flow in their optically isotropic liquid phase. We conjecture that this behavior is due to the existence of nanostructured, fluctuating clusters composed of a few smectic-like layers. Shear alignment of these clusters explains the shear thinning observed in bent-core liquid crystals having either a nematic phase or nonmodulated smectic phase. By contrast, smectogens having a modulated smectic phase do not shear thin at low shear rates, but even show a slight shear thickening which may be due to entanglements of wormlike and/or helical clusters.

Recently it was found that fluid smectic phases of bent core liquid crystals formed freestanding fibers of extremely high slenderness ratios. Studies of these fibers showed that their structure was composed of concentric cylindrical smectic layers. For this configuration to be stable there must be an energy term that desires bending of the smectic layers. We show that an energy term that deals with the divergence of the dipolar direction can encourage layer bending if the layer chirality value is allowed to vary. The energy term associated with holding the layer chirality is closely related to layer compressions and electrical self-interactions. For our model, we assumed a simple smectic-C geometry with constant molecular tilt and cone angle defined by the director with respect to the layer normal, but allowed a constant variation of the polar direction about the director. Applying this simplified model to a free energy which accounts for director distortions, divergence of the polar direction, biaxial layer strain, surface tension, and electrical self-interactions, we were able to show consistency between the stable fiber radius and other properties predicted in our model to results from experimental studies.

We show how frustrations in molecular packing generated by molecular substitutions and electrostatic potentials necessitate a local C1 symmetry and lead to spontaneous polarization splay and layer bending in bent-core liquid crystals. By estimation of the entropic changes due to curvature, we calculated the elastic constants that drive the spontaneous layer bend and polarization splay and obtained length scales of these deformations. Finally, we propose a structural phase diagram which describes how the experimentally observed helical superstructures, layer undulation, polarization splay, and spontaneous fiber formation depend on the direction and magnitude of the spontaneous polarization.

Seminal findings in the liquid-crystal properties of bent-core molecules, such as the observation of ferroelectricity and spontaneous breaking of chiral symmetry in smectic phases composed of achiral molecules, [1][2][3] have broad implications for the general field of soft condensed matter. However, their practical applications [4][5][6][7][8][9] are limited because they appear only at high temperatures (>70 8C) owing to the bends in the molecules, which lead to locking into layered smectic structures. To overcome this difficulty, one needs to prohibit the locking mechanism either by molecular design or creating mixtures, or by combining both. Mixtures of bent-core and rod-shaped molecules have shown interesting properties, such as enhancement of the chirality in cholesterics, [10,11] induction of antiferroelectric order in smectics, [12,13] or a complete miscibility of smectic bent-core and nematic rod-shape substances. [14,15] Although there are few examples in which bent-core materials do not crystallize at room temperature, [16,17] they are glassy and could not be switched at room temperature. Here we show that by mixing suitable bent and rod-shape molecules one can form fluid liquid crystals at room temperature that change birefringence color at electric fields, thus opening up a path towards possible practical applications.We have studied binary mixtures of several rod-shape and bent-core molecules, but will focus only on those that showed complete miscibility, and provided an electrically switchable smectic phase at convenient temperature ranges. The rodshape compound 4-n-octyloxyphenyl 4-n-hexyloxybenzoate (6OO8) [18] is one of the simplest liquid-crystal materials exhibiting nematic and tilted smectic (SmC) mesophases. The bent-core components 4-chloro-1,3-phenylene bis[4-(10-decenyloxy)benzoyloxy] benzoate (ClPbis10BB) [19] and 4,6-dichloro-1,3-phenylene-bis[4 0 -(9-decen-1-yloxy)-1,1 0 -biphenyl]4-carboxylate (10DClPBBC), [20] contain one and two chlorine atoms on their central rings in the 4 and 4,6 positions, respectively. This enhances their ability to form a nematic phase at relatively low, although still elevated temperatures. [19][20][21] In addition, they have double bonds on their terminal groups, which further decreased the nematic phase range compared to those with unsaturated chains. [22] In ClPbis10BB, which has been the subject of several studies, [23][24][25] the arms are relatively flexible because the outer benzene rings are separated by ester groups, whereas in 10DClPBBC the aromatic rings of the arms are directly linked, making them much more rigid. Differential scanning calorimetry (DSC) and textural observations show complete miscibility with 6OO8 for both bent-core compounds. All mixtures have a nematic (N) phase below the isotropic (I), and in the intermediate concentration range a smectic (Sm) phase below the nematic. The I-N transition enthalpies linearly decrease from about 3 J g À1 to 0.7 J g À1 from 100% to 0% 6OO8 content, whereas the enthalpy at the transition to the induc...