Generation and Recombination of Holes and Electrons in Anthracene

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×${10}^3$ eV. The number of carriers generated was independent of temperature from —25 to 25°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±1)×${10}^{-6\mathrm{}}$ ${\mathrm{cm}}^3$ ${\sec }^{-1\mathrm{}}$. 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.