Electronic excitations of poly(methylphenylsilane) films

Abstract

We report electroabsorption (EA), two-photon absorption (TPA), and the spectrum for charge-carrier generation in thin films of poly(methylphenylsilane) (PMPS). The one-photon absorption to a singlet exciton, which begins at ${\mathit{E}}_{\mathit{g}}$=3.5 eV, appears in EA and indicates a polarizability change of 1.8×${10}^{\mathrm{-}22}$ ${\mathrm{cm}}^3$. The onset around 4.6 eV of direct generation of charge carriers by photons defines the interband gap ${\mathit{E}}_{\mathit{b}}$ and correlates with an EA feature at 4.65 eV that we associate with charge transfer between polymer segments. The TPA around 4.4 eV is to the second ${}_{}{}^1{}_{}{}^{}$ ${\mathit{A}}_{\mathit{g}}$ state and gives the alternation gap ${\mathit{E}}_{\mathit{a}}$, while the broad TPA above 5 eV is assigned to an even-parity state derived from exciting two electrons across ${\mathit{E}}_{\mathit{g}}$. A Pariser-Parr-Pople model for σ conjugation in polysilanes accounts for ${\mathit{E}}_{\mathit{g}}$, for polarizability changes, for TPA, and for the singlet-triplet gap ${\mathit{E}}_{\mathit{t}}$ in terms of correlated states. We contrast the spectra of amorphous PMPS with films of poly(di-n-hexylsilane), whose side groups are ordered at low temperature, and compare EA spectra in polysilanes with EA in crystalline organic solids and wideband semiconductors.