Dipolar Films Research Articles

Construction of Interface Dipoles by Surface Doping and Their Role in the Open Circuit Voltage in Polymer Solar Cells

A kind of dipolar interface is realized by surface doping of poly-(3,4-ethylenedioxythiophene) (PEDOT) with tetrafluoro-tetracyano-quinodimethane (F4TCNQ). PEDOT is in situ synthesized by electrochemical polymerization of 3,4-ethylenedioxythiophene (EDOT) on an indium tin oxide (ITO) electrode, and then F4TCNQ is spin-coated atop the PEDOT layer. Because the LUMO of F4TCNQ is lower than the HOMO of PEDOT, the spontaneous electron transfer from PEDOT to F4TCNQ results in a bilayered structure of PEDOT cations and F4TCNQ anions. Thus, a permanent interfacial dipole is formed in the surface-doping system. The surface doping not only enhances the conductivity of PEDOT, but also increases the surface work function of the electrode. The dipolar film is applied as the anode interface in polymer solar cells (PSCs), and the results show that such an interface dipole plays a very important role in the open circuit voltage (V oc) of the PSCs.

Dielectric Modulation of Two-Dimensional Dipolar Materials

Spontaneous pattern formation plays an important role in a wide variety of natural phenomena and materials systems. A key ingredient for the occurrence of modulated phases is the presence of competing interactions, generally of different physical origins. We demonstrate that in dipolar films, a prototypical system for pattern formation, patterns can be induced by dielectric effects alone. A rich phase diagram arises, where striped and circular morphologies emerge with geometric properties that can be controlled through variation of particle shape and substrate permittivity or permeability. These effects are particularly enhanced by metamaterial substrates.

Magnetic patterns in dipolar films with close packed structures

It is described and explained the dynamics of the transition from in-plane to perpendicular magnetization for dipolar films with close packed structures when the c/ a ratio, with c the interlayer separation and a the two-dimensional lattice constant, is varied. Several arrangements of distinct number of stacked layers are considered. Magnetization switching follows a mechanism based on the role of the interlayer coupling, which is understood as the contribution of the interaction between dipoles of different layers to the studied effects. Calculations for sufficiently low temperatures show that each magnetic configuration transition leaves a fingerprint as an entropy maximum when the latter is plotted as a function of c/ a. Thus the entropy allows to determine the c/ a range of stability of the different magnetic patterns.

Structure−Performance Correlations in Vapor Phase Deposited Self-Assembled Nanodielectrics for Organic Field-Effect Transistors

Organic field-effect transistor (OFETs) are fabricated using thin, vapor-deposited films of both the gate dielectric (vapor-deposited self-assembled nanodielectric, v-SAND) and the organic semiconductor. The nanoscopic self-assembled gate dielectrics are structurally organized via molecular precursor hydrogen-bonding interactions, followed by planarization with a vapor-deposited inorganic SiO(x) film. It is shown here that the metal-insulator-semiconductor (MIS) and OFET device electrical properties are sensitive to the v-SAND molecular dipolar orientation. In addition, alternating (organic/inorganic/organic/...) and nonalternating (1 organic layer + 1 inorganic layer) v-SAND microstructural arrangements are investigated, and the microstructures are correlated with MIS and OFET device characteristics. Films with alternating microstructures have larger capacitances than nonalternating films of the same thickness. However, they also have larger leakage currents, associated with the enhanced polarization of well-ordered dipolar films. For pentacene OFETs, the largest mobilities (approximately 3 cm(2)/(V s)) are associated with the high-capacitance nonalternating microstructure, and the lowest mobilities (approximately 0.5 cm(2)/(V s)) are associated with the alternating microstructure. v-SAND gated ambient-stable, n-type organic semiconductors show the opposite trends, where slightly greater OFET performance is observed with the lower-capacitance gate dielectric. For the p-type and one of the n-type v-SAND-based OFETs, the performance (under vacuum and ambient) is comparable to, or surpasses, that of previously reported devices using conventional SiO(2) as the gate dielectric. More importantly, the devices fabricated here operate at far lower voltages. These results indicate that v-SAND dielectrics are promising for future flexible organic electronics requiring low-temperature, solvent-free deposition conditions.

Relaxation processes in dipolar liquid crystal film on metals surfaces

The influence of electron injection from the aluminium electrode into organic liquid crystalline film, comprising the polar molecules, such as 4- n-pentyl-4′-cyanobiphenyl, on the orientational relaxation process is investigated. The numerical study of the system of hydrodynamical equations that includes both director reorientation and fluid flow, excited by that reorientation, for number of dynamic regimes has been carried out. It is shown that the relaxation process of the director n ˆ to its equilibrium position n ˆ eq is sensitive to electron injection from the metal electrode into organic dipolar film, whereas, the effect of backflow on that process is vanishingly weak.

Effective anchoring energy in dipolar organic film on metals surfaces

The influence of electron injection from the metal electrode into organic liquid crystal dipolar film on the effective anchoring energy (EAE) of the polar organic film is discussed from the energy point of view. It is shown that the accounting for the injected carrier in organic film results in a polynomial function for the EAE expanded up to the fourth order in cos θ s, where θ s is the polar angle of the director n ˆ at the film/metal interface. It is also shown that in a certain range of the location of centroid of the injected carrier z ¯ the destabilizing surface polarization mechanism may lead to destruction of the linear anchoring strength coefficient w 1. The strong influence of z ¯ on the quadratic term w 2 also has been demonstrated.

Electrostatic energies stored in dipolar films and analysis of decaying process of a large surface potential of Alq3 films

The decaying process of a large surface potential of as-deposited Alq3 film was analyzed with consideration of electrostatic energies stored in the film. The analysis includes the cooperative molecular field effect and it shows that the derived equilibrium condition of dipolar films with electron traps supports the concept of the alignment of Fermi-level of metal and surface Fermi-level of dipolar films. The potential decay of Alq3 films on Al electrodes by photo-exposure is explained due to detrapping of trapped electrons that are immediately transported to the injected electrodes along with the disordering of polar structure of Alq3 films.

Domain wall roughening in dipolar films in the presence of disorder.

We derive a low-energy Hamiltonian for the elastic energy of a Néel domain wall in a thin film with in-plane magnetization, where we consider the contribution of the long-range dipolar interaction beyond the quadratic approximation. We show that such a Hamiltonian is analogous to the Hamiltonian of a one-dimensional polaron in an external random potential. We use a replica variational method to compute the roughening exponent of the domain wall for the case of two-dimensional dipolar interactions.

AN INTEGRAL EQUATION APPROACH TO ORIENTATIONAL PHASE TRANSITIONS IN TWO AND THREE DIMENSIONAL DISORDERED SYSTEMS

The use of inhomogeneous Ornstein-Zernike equations to analyze phase transitions and ordered phases in magnetic systems is explored both in bulk three dimensional disordered Heisenberg systems and in a simple model for a two dimensional ferrofluid monolayer. In addition to closures like the Mean Spherical Approximation, Hypernetted Chain and Zerah-Hansen approximation, the inhomogeneous Ornstein-Zernike equation must be complemented by a one-body closure, for which the Born-Green equation has been used in this paper. The results obtained prove that the proposed approach can furnish accurate estimates for the paramagneticferromagnetic transition in the three dimensional Heisenberg spin fluid, reproducing reliably the structure of the isotropic and ordered phases. In two dimensions, the results are fairly accurate as well, both for the dipolar film alone and in the presence of external perpendicular fields. At high densities/dipole moments the equation seems to predict a transition to a phase in which the dipoles lie mostly in the plane and are aligned into vortex-like structures. Evidence of this new phase is found in the simulation at somewhat higher couplings.