Generation and Recombination of Holes and Electrons in Anthracene

Abstract

The generation of electrons and holes in anthracene crystals by x rays from a 600-kV pulsed x-ray source was studied. The average energy deposited per electron-hole pair created $W$ was found to be 3\ifmmode\times\else\texttimes\fi{}${10}^{3}$ eV. The number of carriers generated was independent of temperature from ---25 to 25\ifmmode^\circ\else\textdegree\fi{}C and depended linearly upon the x-ray intensity. At high intensity, recombination effects become important and the electron-hole recombination coefficient was found to be (3\ifmmode\pm\else\textpm\fi{}1)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}6}$ ${\mathrm{cm}}^{3}$ ${\mathrm{sec}}^{\ensuremath{-}1}$. The value of the recombination coefficient is shown to be in good agreement with a theoretical value calculated on the basis of a very small mean free path for electrons and holes, and the large value of $W$ is explained in terms of this model. These results imply that only a very small fraction of electron-hole pairs produced by a low-energy process such as a band-to-band transition would appear as free carriers in anthracene, and provide a possible explanation for the commonly observed exponential temperature dependence of photoconductivity.