Spectroscopic investigations of a luminescent conjugated polymer

Abstract

This work presents experimental results and comprehensive analysis of the first electroabsorption and photocurrent measurements on the conjugated polymer poly(2,5-pyridinediyl) (PPY). In the electroabsorption experiment the change of the UV/vis absorption spectrum of disordered (spin-cast) and oriented (stretched), 100 nm thin PPY films due to the presence of a strong electric field was measured. The spectra were successfully fitted to a linear combination of the linear absorption spectrum and its first and second derivative. From the analysis of the obtained linear coefficients it is concluded that 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 2A(_g) state at 3.7 eV. In addition polarised electroabsorption spectroscopy on oriented films and on fihns in sandwich configuration have been used to investigate the alignment of the polymer chains and the directions of the relevant transition dipole moments in PPY. The photoconductivity measurements have been carried out on thin films of PPY m the sandwich configuration of the structure ITO/PPY/semitransparent metal. Photocurrent spectra have been recorded using the four possible directions of the applied electric field and illumination directions and the dependence on temperature, film thickness and applied electric field has been investigated. The combined analysis of the experimental results shows that the photocurrent originates from the dissociation of excitons, which are photogenerated inside the polymer near the negative electrode. Finally evidence is given for the electron fransporting, semiconducting properties of PPY and the photocurrent-voltage-characteristic have been successfully modelled with an Onsager-theory yielding an apparent exciton binding energy of 0.14 eV. All results of the work are compared and contrasted with those reported for other conjugated polymers.