Band-To-Band Transitions in Poly (Phenyl Methyl Silane)
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BAND-TO-BAND TRANSITIONS IN POLY (PHENYL METHYL SILANE)* R. G. KEPLER AND J. M.ZEIGLER Sandia National Laboratories, Albuquerque, NM 87185
ABSTRACT We have measured the quantum efficiency for carrier generation as a function of photon energy and the electroabsorption spectrum in poly (phenyl methyl silane). The experimental data are consistent with a band gap of about 4.6 eV. A strong absorption peak which is observed at 3.7 eV is shown to be an exciton band. INTRODUCTION Polysilanes are silicon backbone polymers with two side groups, typically carbon based, attached to each silicon atom [1]. It has recently been found that these materials exhibit a wide variety of interesting electronic properties, many of which are very similar to those of =r- conjugated polymers and molecular crystals. They typically absorb strongly in the UV and many exhibit a high quantum efficiency for fluorescence [2]. In the solid state they tend to be excellent photoconductors [3,4] and have exhibited unusually high nonlinear optical coefficients for materials which are transparent in the visible [5]. It is now well established that these interesting properties result from the a electrons which are delocalized on the silicon backbone~ but our understanding of the states of these electrons is still rudimentary. In this paper we report measurements of the quantum efficiency for charge carrier generation in poly (phenyl methyl silane) for photon energies from 3.65 eV to 5.65 eV and of the electroabsorption spectrum from 3.25 eV to 5.2 eV. The experimental results indicate that the free carrier band gap is about 4.6 eV. The electroabsorption spectrum exhibits a feature consistent with a band gap of 4.6 eV and shows that the strong absorption peak at 3.7 eV results from the formation of an exciton in an electronic transition with a change in polarizability of about 3x10"23 cm 3 .
EXPERIMENTS Poly (phenyl methyl silane) is a silicon chain with a phenyl and a methyl group attached to each silicon atom in stereochemically random fashion. The material *This work performed at Sandia National Laboratories supported by the U. S. Department of Energy under contract number DE-AC04-76DP00789.
Mat. Res. Soc. Symp. Proc. Vol. 173. ©1990 Materials Research Society
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used in these experiments was prepared by the method of Zeigler [6] and the molecular weight was such that a molecule consisted of a chain of about 3800 silicon atoms. In a previous publication [3], we reported on many of the photoconductive properties of this material. For the charge carrier generation experiments reported on inthis paper, samples a few micrometers thick were cast from toluene solution on a quartz substrate which had been coated with a vacuum-evaporated, semitransparent aluminum film. An aluminum electrode about 10 mm in diameter was then evaporated on top of the polymer film. The light source for these experiments was a pulsed dye laser with a pulse width of about 5 ns and the light was incident on the sample through the quartz plate and the semi-transparent aluminum e
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