Chapter 398 - Dark Electrical Measurements and Photovoltaic Effects of α-ZnPc Thin Films in Schottky — Barrier Cells

Abstract

The electrical and photovoltaic properties of α-ZnPc thin films in Schottky junction cells have been carried out in air ambient. These cells showed rectification when α-ZnPc material was flanked between ohmic gold and blocking indium electrodes. The current density–voltage characteristics under forward bias are found to exhibit two distinct regions of ohmic and space-charge-limited conduction (SCLC). The square power dependence in SCLC indicated that current conduction is limited by a by a discrete trapping level above the valence band edge. Optimum values of power conversion efficiency ŋ of 5.5% and quantum efficiency φ of 42.5% for input light at low power density of 0.6 mWcm–2 have been obtained for monochromatic irradiation at 633 nm. The decrease in ŋ with increasing the intensity is attributed to a space charge limitation due to nonlinear resistance. Schottky-barrier parameters of the cell in the dark and under different monochromatic light intensities have been determined from the capacitance–voltagc measurements. Light intensity increased significantly the space charge density, which is a major concern as a possible limiting factor for the performance of organic photovoltaic cells.