One-photon intrinsic photogeneration in anthracene crystals

Abstract

Steady state one-photon intrinsic photogeneration in anthracene crystals has been measured using 1M Na2SO3 aqueous solutions as electrodes, as a function of photon energy and electric field (up to 6×107 V m−1). Anomalies in previous measurements with aqueous electrodes have been resolved. The values of absolute photoconduction yield now agree for aqueous, blocking, and metal electrodes, and for pulsed and steady state measurements. The Onsager theory of geminate recombination explains the shape of the current-field dependence better than does a theory of Knights and Davis. Interpretation of the current-field curves in terms of Onsager’s theory yields values of r0 (the initial separation of the charges in the ion pair) as a function of photon energy and dielectric constant, assuming a delta function distribution for r0. Two ion pair states appear to be formed: at hν=3.9 to 4.2 eV, r0A=1.8 nm (for εr=3.8); at 4.5 to 5.0 eV, r0B = 2.5 nm. For hν=4.2 to 4.5 eV, r0 undergoes a smooth transition. The maximum value (0.1) of φ1, the one-photon ion pair production efficiency, occurs at 4.2±0.15 eV, near the maximum in η, the two-photon ion pair yield. The agreement of peaks in φ1 and η with a peak at 4.1 eV in σ2F, the two-photon absorption cross section, is discussed. From the energy dependence of φ1 and the magnetic field effect on prompt fluorescence, it is concluded that, in the process of singlet exciton fission into two triplet excitons, the lowest ion pair state is not the fissionable singlet. Only a very small [0.07(±0.03) %], direction independent, magnetic field effect on the intrinsic photocurrent was observed (at a photon energy of 4.5 eV).