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

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 T vis of up to 49% and efficiency η of up to 2.3%, showing superior performance compared to devices with pure Ag top contacts.