Stable Organic Solar Cells with Mg:Ag Contacts

Abstract

Stable and efficient organic solar cells with Mg (20 at. %): Ag alloy cathodes and bulk-heterojunction absorber layers from metal (e.g., Cu, Zn)-phthalocyanine and C60 small molecules are demonstrated. Device efficiencies of 4.0% under an illumination of 100 mW/cm2 at 25̊C were achieved as a result of the fine adjustment of the cathode work function as well as of the absorber design. By combining low and high work function materials, the work function in both Mg/Ag bilayer and Mg:Ag alloy layer cathodes was adjusted for optimum photovoltaic parameters. The electric and photovoltaic properties of the devices are discussed with respect to the cathode layer structure. The formation of the absorber/cathode interface was investigated by x-ray photoelectron spectroscopy measurements (XPS). The work function of the absorber and cathode layers were determined from the XPS high binding energy cutoff (HBEC) spectra. For optimized devices, the work function of the cathode at the side adjacent to the absorber layer equals 4.0…4.1 eV. While devices with Mg/Ag bilayer contacts exhibit a 65% efficiency drop in the first month after the preparation, devices with Mg:Ag alloy contacts demonstrate stable photovoltaic parameters within the time of the study of ∼ 1 year.