Application of electroreflectance analysis for organic semiconductor thin films

Abstract

Many organic semiconductors with conjugate bond structure possess photoconductivity. Conduction mechanism of organic materials exhibits 'dualism' since both intramolecular as well as inter-molecular aspects are involved in the excitation, absorption and transport of charge carriers. Modulation spectroscopy promises to be the most accurate method for analysis of organic photoconductors, especially of thin films. In this technique a periodic perturbation is applied to the material under study and the effect of the perturbation is separated from reflection or absorption while scanning through a given wavelength range by use of lock-in phase sensitive detection method. In electromodulation, particularly in electrolyte electromodulation, the applied field on the material produces changes in the dielectric function which corresponds to the change in reflectance. When the applied field is low the line-shape of spectrum is third-derivative like in comparison with the unmodulated reflectance spectrum. Using Aspnes three-point method the transitions corresponding to critical points can be determined. When the field is intermediate Franz-Keldysh oscillations, which are dc bias dependent, appear on the higher energy side of the transition energy from which the role of intra-molecular as well as intermolecular aspects in conduction mechanism can be understood and the carrier concentration could be determined. Though the electroreflectance method has been developed for inorganic semiconductors, it could be effectively applied for organic/molecular semiconductors as well if the constituent molecules are assumed to be the lattices. The study of organic photoconductors is very important since they are more and more promising especially in photocopying, photovoltaic and solar cells.

Electrical and electroreflectance studies on ortho-chloranil-doped polyaniline

“…3) is according to Xavier [10]. It consists of a quartz-halogen lamp focused through a pair of quartz-lenses onto the entrance slit of a monochromator.…”

Optical differential reflectance spectroscopy on thin molecular films

“…Among them are, for example, reflection absorption infrared spectroscopy (RAIRS), 26,27,43,44 also called infrared reflection absorption spectroscopy (IRAS), 45,46 polarisation modulation infrared reflection absorption spectroscopy (PMIRRAS), [47][48][49] and electroreflectance (ER). 50 Due to the very high number of publications on these and similar methods it is beyond the scope of this contribution to give a complete literature overview of the field.…”