Photogeneration and recombination processes of neutral and charged excitations in films of a ladder-type poly(para-phenylene)

Abstract

We introduce a version of the cw photomodulation technique, measured far from the steady state, for obtaining the quantum efficiency, $\eta ,$ of long-lived photoexcitations in $\pi$-conjugated polymers. We apply this technique to films of a ladder-type poly(para-phenylene) [mLPPP] for studying the photogeneration action spectra, $\eta \mathrm{}\left(E\right),$ and recombination kinetics of photogenerated neutral and charged excitations such as singlet and triplet excitons and charged polarons. Whereas the $\eta \mathrm{}\left(E\right)\mathrm{}$ spectrum for singlet excitons shows a step function increase at a photon energy, E, close to the optical gap $\left(\simeq 2.6\hspace{0.5em}\mathrm{eV}\right),$ both triplet and polaron $\eta \mathrm{}\left(E\right)\mathrm{}$ spectra show, in addition, a monotonous rise at higher E. The rise for triplets is explained by singlet exciton fission into triplet pairs, and from a model fit we get the triplet exciton energy $\left(\simeq 1.6\hspace{0.5em}\mathrm{eV}\right).\mathrm{}$ For polarons this rise is modeled by an electron intersegment tunneling process. The electroabsorption spectrum is also measured and analyzed in terms of Stark shift of the lowest lying exciton, ${1B}_u,$ and enhanced oscillator strength of the important ${\mathrm{mA}}_g$ exciton. A consistent picture for the lowest excited state energy levels and optical transitions in the neutral (singlet and triplet) and charged manifolds is presented. From both the exciton binding energy of $\simeq 0.6\hspace{0.5em}\mathrm{eV}$ and the singlet-triplet energy splitting of $\simeq 1\hspace{0.5em}\mathrm{eV},$ we conclude that the $e-e$ interaction in mLPPP is relatively strong. Our results are in good agreement with recent ab initio band structure calculations for several $\pi$-conjugated polymers.