Rami Cohen

MOLECULAR ELECTRONIC TUNING OF SI SURFACES

Abstract Assembling quinolinium-based chromophores on silicon surfaces provides a new route to electronic control over such semiconducting surfaces. The two-step process by which the molecules are grafted on to the surface involves first coupling the organic functionality to silicon, followed by chromophore anchoring. These synthetic steps are monitored by XPS, UV-Vis and FTIR spectroscopies. Using contact potential difference measurements we found that the electron affinity of the modified silicon is a function of the molecules dipole moment. The same technique shows a pronounced effect of the sub-nanometer siloxane-based, coupling-agent, layer by itself on the band bending and band-bending modification as function of chromophore adsorption.

Controlling surfaces and interfaces of semiconductors using organic molecules

Abstract Control over semiconductor surface energetics can be achieved using different chemisorbed organic molecules with diverse electronic properties. We find evidence of such control over CdTe upon adsorption of dicarboxylic acid derivatives with different substituted phenyl rings. FT-IR measurements show that the dicarboxylic acid derivatives bind as carboxylates to form approximately one monolayer. Such chemisorption modifies both the band bending and the electron affinity (up to 500 and 700 mV, respectively), as measured by contact potential difference WPM Changes in band bending result from a coupling between molecular orbitals and surface states close to the valence band and depend on the withdrawing character of the phenyl substituent. A model is presented to interpret and explain the data.

Structural influence on the excimer emission from a dimorphic crystalline stilbene

Abstract The compound 2,4-dichloro-3′-methyl- trans -stilbene is dimorphic. The two forms are assigned “pair” and “stack” crystal structures on the basis of their cell dimensions and of the structures of the photodimers derived from them. Excimer emission is observed from both forms, with that from the stack structure being of higher energy. This is in keeping with an earlier theoretical analysis and argues for the validity and generality of this analysis.

ENGINEERING THE INTERFACE ENERGETICS OF SOLAR CELLS BY GRAFTING MOLECULAR PROPERTIES ONTO SEMICONDUCTORS

The electronic properties of semiconductor surfaces can be controlled by binding tailor-made ligands to them. Here we demonstrate that deposition of a conducting phase on the treated surface enables control of the performance of the resulting device. We describe the characteristics of the free surface of single crystals and of polycrystalline thin films of semiconductors that serve as absorbers in thin film polycrystalline, heterojunction solar cells, and report first data for actual cell structures obtained by chemical bath deposition of CdS as the window semiconductor. The trend of the characteristics observed by systematically varying the ligands suggests changes in work function rather than in band bending at the free surface, and implies that changes in band line-up, which appear to cause changes in band bending, rather than direct, ligand-induced band bending changes, dominate.

Creation of second-order nonlinearity in polymers by light-induced asymmetric charge injection.

A new method for creating and enhancing second-order optical nonlinearity in polymer-dye films by asymmetric photoinjection of electric charges is introduced. The samples are composed of two separate layers, an organic photoconductor that generates the electric charges and a hyperpolarizable organic film that traps them. The nonlinearity caused by asymmetric charge injection does not arise from dipolar alignment; consequently, it can be observed either in the absence of, or to enhance, the conventional dipolar alignment nonlinearity.

Charge Injection Induced Optical Nonlinearity of ‘In-Plane’ Poled Polymer Films : Effects of Host Polymer, Substrate and Electrodes

Abstract ‘In plane’ poling of polymer films containing hyperpolarizable dye molecules can lead to large optical nonlinearities along a direction perpendicular to that of the applied field. The magnitude of this nonlinearity depends on the polymer host, substrate and electrode metal. After removal of the field, decay of the nonlinearity also varies considerably from host to host. Measurement of the current flow during poling is consistent with a model for formation of the perpendicular nonlinearity via charge injection from the electrodes, diffusion into the bulk polymer and charging of the dye.

Second Order Optical Nonlinearity of Polymers Induced by Charge Injection Asymmetry

Abstract A new mechanism for inducing bulk asymmetry and second order optical nonlinearity in polymer-dye systems is described. An anisotropic distribution of charges is established inside the medium resulting from the application of high voltage via an asymmetric electrode structure. The nonlinearity induced via this mechanism is perpendicular to the externally applied electric field. It is proven that the nonlinearity induced by charge injection cannot arise from dipolar alignment of molecules, but rather results from the nonlinearity of charged dye dimers in a bulk charge gradient.

Charge-injection-induced optical nonlinearity in in-plane poled polymer films: enhanced temporal stability and effect of different polymeric matrices and substrates

Significant asymmetry and optical nonlinearity (second harmonic generation) can be produced by injection of electric charge into dye/polymer blends. The strength of the nonlinearity and its temporal stability depend on the electrical properties of the polymer and the polymer interface. It is shown conclusively that the mechanism for producing the nonlinearity does not arise from dipolar alignment of the dye in an electrostatic field; we even demonstrate production of nonlinearity under conditions of alternating current injection i.e. zero electrostatic field.