Photogeneration and recombination processes of neutral and charged excitations in films of a ladder-type poly(para-phenylene)
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(E), and recombination kinetics of photogenerated neutral and charged excitations such as singlet and triplet excitons and charged polarons. Whereas the eta(E) spectrum for singlet excitons shows a step function increase at a photon energy, E, close to the optical gap (similar or equal to 2.6 eV), both triplet and polaron 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 (similar or equal to 1.6 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, 1 B-u, and enhanced oscillator strength of the important 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 similar or equal to 0.6 eV and the singlet-triplet energy splitting of similar or equal to 1 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. [S0163-1829(99)13531-8].