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

Abstract

We introduce a version of the cw photomodulation technique, measured far from the steady state, for obtaining the quantum efficiency, $\ensuremath{\eta},$ of long-lived photoexcitations in $\ensuremath{\pi}$-conjugated polymers. We apply this technique to films of a ladder-type poly(para-phenylene) [mLPPP] for studying the photogeneration action spectra, $\ensuremath{\eta}(E),$ and recombination kinetics of photogenerated neutral and charged excitations such as singlet and triplet excitons and charged polarons. Whereas the $\ensuremath{\eta}(E)$ spectrum for singlet excitons shows a step function increase at a photon energy, E, close to the optical gap $(\ensuremath{\simeq}2.6 \mathrm{eV}),$ both triplet and polaron $\ensuremath{\eta}(E)$ 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 $(\ensuremath{\simeq}1.6 \mathrm{eV}).$ 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 $\ensuremath{\simeq}0.6 \mathrm{eV}$ and the singlet-triplet energy splitting of $\ensuremath{\simeq}1 \mathrm{eV},$ we conclude that the $e\ensuremath{-}e$ interaction in mLPPP is relatively strong. Our results are in good agreement with recent ab initio band structure calculations for several $\ensuremath{\pi}$-conjugated polymers.