Interfacial layer dipole formation

The presence of an electrical potential drop, i.e., interfacial potential, across the boundary between two dissimilar phases, as well as at their surfaces exposed to a neutral gas phase, is the most characteristic feature of every interface and surface electrified due to the ion separation and dipole orientation. This charge separation is usually described as the formation of the ionic and dipolar double layers. The main interfacial potential is the Galvani potential (termed also by Trasatti the operative potential), which is the difference of inner potentials (p and of both phases. It is a function only of the chemical... 

The magnitude of the injection barrier is open to conjecture. Meanwhile there is consensus that energy barriers can deviate significantly from the values estimated from vacuum values of the work-function of the electrode and from the center of the hole and electron transporting states, respectively. The reason is related to the possible formation of interfacial dipole layers that are specific for the kind of material. Photoelectron spectroscopy indicates that injection barriers can differ by more than 1 eV from values that assume vacuum level alignment [176, 177]. Photoemission studies can also delineate band bending close to the interface [178]. 

Two major effects were proposed as causes for improved electron extraction (a) upon sublimation of the subsequent aluminum layer the liF dissociates, whereby metallic Li atoms may be formed that consequently n-dope the organic semiconductor (fullerene or polymer) under formation of li" and, e.g., AIF3 [86,94,96] or (b) the liF layer could result in an interfacial dipole layer shifting the work function of the electrode [82,90,91]. Both of these viewpoints have been shown to hold merit, as it was demonstrated by photoelectron spectroscopy that for very thin (sub-nanometer) layers of LiF dissociation and consequent n-type doping occurred, whereas for thicker layers (a few nanometers) the formation of a dipole was evidenced [97]. 

We introduce a simple theoretical approach to analyze the factors which determine the formation of the interfacial dipole layer (IDL) between electron-donating and accepting molecular materials. IDL between strong donors and acceptors is formed mainly due to a charge transfer process. However, the IDL appears from polarization interactions in the case of complexes of weak donors and acceptors. In addition to the IDL formation, our calculations also predict the modification of the redox properties of both materials at the interface. 

Second, the interfacial dipole layer may be formed by the polarization of an electronic cloud within the molecules. The vacuum level shift at the interface is proportional to the component of the dipole moment perpendicular to the interface. Since the interaction is much stronger between nearest donor/acceptor molecules, it seems possible to elucidate the factors influencing the formation of the IDL by performing quantum-chemical calculations on just a dimer. 

When the free electrostatic charge in phase a turns to zero, = 0 and = X . The surface potential of a liquid phase is dictated by a certain interfacial orientation of solvent dipoles and other molecules with inherent and induced dipole moments, and also of ions and surface-active solute molecules. For solid phases, it is associated with the electronic gas, which expands beyond the lattice (and also causes the formation of a dipolar layer) other reasons are also possible. 

The region of contact of two different materials, generally of differing chemical potentials or work functions will give rise to an interfacial potential and therefore a dipolar surface layer. In the event that one or both of the materials has a high dielectric constant or is perhaps easily dissociated, then the formation of ionic species at this surface dipole layer can occur. 

Whereas the work function of the electrodes is measured by photoelectron spectroscopy, the organic materials are usually characterized by cyclic voltammetry [51]. The values can be extrapolated to the gas phase by choosing an appropriate reference and neglecting the influence of the polarity of the solvent in which the measurements are taken. In addition, the formation of interfacial potentials due to the formation of dipole layers in the solid-state device is neglected [52]. Making these assumptions, the energy levels obtained by cyclic voltammetry can be compared with the electrode work... 

The observed increase in the solar cell power conversion efficiency from 2.2 to 4.0% was attributed to the formation of interfacial dipoles between aluminum cathode and PEG layer which improved electron extraction from the active layer. We note also that PEG cannot participate as true HBETL material because of its insulating nature. 

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