Photoconductivity of α-Methylnaphthalene Alone and with an Incorporated Solute

Abstract

Exposure of α-methylnaphthalene to strongly absorbed uv light and externally applied voltage produced photocurrent in the range of 10−9 to 10−7 A/cm2; their magnitude depended on the polarity of the illuminated electrode. The illumination through the positive electrode gave a photocurrent 15 times greater than when the negative electrode was illuminated. The mobilities of the positive and negative carriers were determined to be 1.2×10−4 and 5×10−5 cm2/V·sec, respectively. The former value is of the same order of magnitude as the theoretical value calculated for a positively charged α-methylnaphthalene ion. The temperature dependence of the mobility of the positive carrier indicated that the carrier may be either an ion or a hole with a long hopping time. Current—voltage measurements show that saturation is not reached at the highest field strength employed, 40 000 V/cm, and most likely would not be reached below fields of 106 V/cm. On this basis a theoretical efficiency of 1% for charge production was estimated. A non-Ohmic behavior of the photocurrent was observed, which suggested the presence of either a potential well at the electrode or the influence of space charge effects. The incorporation of a solute, 9,10-diphenylanthracene, reduced the current to 25% of that for the solvent alone, indicating extensive energy transfer to the solute and not simply to differences in mobility. The mechanism of charge production was interpreted either as an exciton interacting with the electrode to produce holes or as ionization of the solvent near the electrode with differences in recombination rates of the charges with the electrode.