Charge carrier generation and mobilities in single crystal benzophenone

Abstract

Electron drift mobilities in the principle crystal directions of single crystal benzophenone have been measured for 170K<or=T<or=300K. The magnitude of the room temperature drift mobilities in the a, b and c directions (0.16 cm2 V-1 s-1, 0.049 cm2 V-1 s-1 and 0.055 cm2 V-1 s-1 respectively) are consistent with small polaron charge transport. This is further supported by the thermally activated mobility for T<230K giving a polaron binding energy of 0.07 eV. The generated charge as a function of light intensity has also been studied. The results show that the formation of free carriers occurs through the interaction of a free singlet exciton and a singlet exciton trapped at the surface of the crystal. This results in the formation of a free electron and trapped hole. A collected charge strongly dependent on applied field has also been observed and is accounted for on the basis of a combined Warter-Weiz Cobas model. This gives a minimum charge generation depth of 500 AA and a charge generation efficiency of 10-3.