Electronic excitations of poly(methylphenylsilane) films

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 Eg=3.5 eV, appears in EA and indicates a polarizability change of 1.8×{}10^-22 cm³. The onset around 4.6 eV of direct generation of charge carriers by photons defines the interband gap Eb 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{A}g$ state and gives the alternation gap ${E}ₐ$, while the broad TPA above 5 eV is assigned to an even-parity state derived from exciting two electrons across ${E}g. A Pariser-Parr-Pople model for σ conjugation in polysilanes accounts for{E}g$, for polarizability changes, for TPA, and for the singlet-triplet gap ${E}ₜ$ 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.