Multiphoton excited photoconductivity in polyfluorene

Abstract

We investigate exciton dissociation from even- and odd-parity excited states in poly(9,9 dioctylfluorene) using multiphoton excitation of a photodiode device by femtosecond pulses with tunable photon energies. A transition from cubic to quadratic dependence of the photocurrent on the excitation intensity is observed with increasing photon energy. The cubic dependence is due to a three-photon process consisting of a two-photon absorption followed by further excitation to higher-lying states of weakly bound excitons through absorbing an extra photon. The quadratic dependence corresponds to a direct excitation to higher two-photon states of excitons that are more likely to dissociate into charge carriers. The transition from cubic to quadratic dependence between 2.05 and $2.2\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ establishes the threshold for exciton dissociation from the even-parity states between 4.1 and $4.4\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. The efficient direct dissociation of odd-parity excitons due to single-photon excitation is observed from $4.1\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}5.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$.