Knowledge Bank

Photoconductivity produced in anthracene by irradiation with 1 to 2 microsecond light pulses was studied as a function of applied voltage, temperature, light intensity and wave length of excitation. It has been found that due to the existence of very long tails of the induced charge pulses, the Hecht theory cannot be applied directly. However, by assuming that the initial linear rise is due to drift of a fraction of charge carriers produced, a reasonable and reproducible value of $1cm2/v$ sec. for mobility of holes which varies as $T-n$ (where $n=1.7$ on an average) with temperature is obtained. The pulse height shows an anomalous superlinear variation with applied field. Hence neither the lifetime nor the range could be computed. The variation of photoconductivity with temperature, intensity of illumination and wave length of excitation is consistent with results reported in the literature. Using Brown and Wilds' approximation the quantum efficiency comes to about $1.6\times 10-4$. This low value together with the pulse height behavior suggests that there may exist a thin region at the illuminated surface where considerable recombination takes place. The results are such that no definite conclusion could be reached as to the mechanism of generation of charge carriers. However, the zeroth-order analysis yields reasonable and consistent values of the transport parameters.