Coevaporated calcium-silver metal alloys as contact for highly transparent organic solar cells

Abstract

We present 10-nm thick metal alloys created by coevaporation of calcium and silver as transparent top electrode for inverted bulk heterojunction small-molecule organic solar cells (SMOSC). It is shown that such alloys exhibit sufficiently high conductivity to replace previously used thicker (>14 nm) pure Ag contacts. At the same time, Ca:Ag alloys have higher transmittance compared to pure Ag, which is desirable for semitransparent SMOSC where the light passes through the device and the metal contact. We find that a metal contact consisting of 1 nm Ag underlayer and 10 nm Ca:Ag of 1:1 mixing ratio (total Ag content by volume of 6 nm) on glass has an average transmittance in the visible range of 76% with a sheet resistance of 27.1 Ω/□, approaching the performance of ITO while being applied by a much gentler deposition method. Application of 10-nm thick alloys in semitransparent SMOSC lead to transmittance in the visible range Tvis of up to 49% and efficiency η of up to 2.3%, showing superior performance compared to devices with pure Ag top contacts.