Electroabsorption and photoconductivity studies of the Frenkel excitons and negative polarons in poly(2,5-pyridinediyl)

Abstract

We present electroabsorption and photocurrent measurements on disordered and oriented films of the conjugated polymer poly(2,5-pyridinediyl) (PPY). The electroabsorption spectrum is dominated by a Stark-red-shift of the first allowed optical transition at 3.2 eV and by the emergence of a normally one-photon forbidden state at 3.7 eV. The spectrum was successfully fitted to a linear combination of the linear absorption spectrum and its first and second derivative revealing the transition at 3.2 eV to be due to Frenkel excitons. Polarized EA spectroscopy on oriented films allow us to separate the parallel and perpendicular components (with respect to the polymer chain axis) of the first allowed optical transition. Analysis of the spectra provide an indication for the existence of a small number of delocalized Wannier-like excitons photogenerated by light polarized perpendicular to the polymer chains. Photoconductivity measurements have been carried out on ITO/PPY/Au (semitransparent) sandwich cells using the four possible directions of the applied electric field and illumination directions. Analysis of the results shows that the photocurrent originates from the dissociation of excitons, which are photogenerated inside the polymer near the negative electrode and evidence is given for the electron transporting properties of PPY.