Phthalocyanine organic solar cells: Indium/x-metal free phthalocyanine Schottky barriers

Abstract

A thin organic film of polycrystalline particles of x-metal free phthalocyanine (x-H2Pc) dispersed in a polymer binder, when sandwiched between tin oxide (NESA) and indium electrodes, is shown to exhibit a strong photovoltaic effect. The photovoltaic and rectification properties of In/x-H2Pc/NESA sandwich cells are reported. From the photovoltaic action spectra, the active region responsible for electric power generation was found to be confined to the metal/semiconductor interface. A Schottky barrier width of 300 Å was determined, which allows the capture of 30% of solar irradiance. An electron trap density of 3×1017/cm3 and a Schottky barrier built-in potential of 0.63 V are estimated from C-V measurements. At low voltage, the dark current in the forward direction varies exponentially with voltage: from this dependence values of 2×10−9 A/cm2 and 1.3–2.6 for the saturation current J0 and diode quality factor n are determined. At higher voltage, a super quadratic dependence of forward current on voltage indicated that current conduction is limited by an exponentially decreasing distribution of traps. At peak solar power (135 mW/cm2), a power conversion efficiency (η′) of 1.2% to transmitted light has been obtained. The transmittance of the indium electrode approached 2%. The devices exhibit open circuit voltages Voc of 0.45 V and short-circuit currents of 0.2 mA/cm2 at Air Mass Zero (AMO) sunlight. Therefore, the engineering efficiency of our device approached 0.03%. The monochromatic quantum efficiency of free-carrier generation approached 75% at low light level; this diminished to 30% at solar intensities, characteristic of a cell with large series resistance. The effect of pigment loading, cell thickness, light intensity, binder material, dye sensitization, and the nature of barrier electrode has been studied and optimized.