Space Charge Controlled Currents in Insulators: Detection of Virtual Anode

Using tetracene crystals provided with hole injecting contacts (CuI(s) and Ce(SO4)2(aq)) we have located the position of the virtual anode xm (potential maximum) within the crystal and have studied its voltage dependence. For this purpose, we used the triplet and singlet excitons as tracers, making particular use of the characteristic response of these excitons to an external magnetic field. The voltage dependence of xm follows that predicted by the simplified form of the exact theory of Sinharay and Meltzer. It appears that xm lies within the crystal for all voltages up to the saturation voltage. It is shown that a photoenhanced current will be produced not only by detrapping deep in the interior of the crystal, but by detrapping in the vicinity of the virtual anode xm.