Temperature dependence of intrinsic carrier generation in anthracene single crystals

Abstract

The pulse technique is used to measure free carrier yields in anthracene single crystals as a function of temperature, photon energy, and applied field. Field dependence plots, measured over the temperature range 200–300 K, exhibit slope-to-intercept ratios described by S/I=3.2×10−5 (298/T)2 cm/V. This result is in quantitative agreement with that predicted by Onsager’s theory of geminate recombination. Activation energy (Ea) results are reported over a wide range of photon energies. We find that Ea is essentially constant (Ea=0.090 eV) over the photon energy range 4.4–5.2 eV and again constant (Ea=0.068 eV) over the range 5.4–6.2 eV. This observation is consistent with autoionization from two spectroscopically observed states: system II (1B2u) in the lower energy range, and system III (1B1u) in the higher energy range. The average thermalization lengths that would be associated with these Ea values are in the range 10–70 Å depending on the form chosen for the distribution of charge pair configurations.