Rectification, space-charge-limited current, photovoltaic and photoconductive properties of Al/tetracene/Au sandwich cell

Abstract

Tetracene thin films sandwiched between two evaporated metal electrodes show rectification and photovoltaic effects. The rectification does not follow the J-V equation for Schottky barriers or p-n junctions. A theory is presented to explain the rectification and photovoltaic response. The forward current of the diode (Au positive) is space-charge-limited current (SCLC) and is attributed to hole injection from the gold electrode. The forward current varies as Vl+1/d2l+1, where V is the voltage, d the thickness and 1=(Tc/T)∼3. The SCLC is related to an exponential distribution of traps ∼7×1019/cm3. Other results related with trapping are also presented. No variation of capacitance with voltage is found, indicating that the tetracene film is completely depleted. In the photovoltaic mode, the Au electrode is positive and the Al negative. The photovoltaic current and voltage are related by the relation V=V0(1−e−aJ). The short-circuit photocurrent varies as Fn where n varies between 0.6 and 1 and F is the incident light intensity. This again indicates a possible exponential trap distribution. The open-circuit photovoltage is attributed to the difference in work function of the two electrode materials. Electron microscopy pictures indicate that the tetracene films are possibly polycrystalline. The spectral responses of the short-circuit photocurrent or photocurrent with an applied voltage are different for light incident on the two electrodes indicating the importance of metal/tetracene interface on the carrier generation process.