Self-assembled monolayer modified ZnO/metal bilayer cathodes for polymer/fullerene bulk-heterojunction solar cells

A simple method was developed to tune the interface of cathode in polymer solar cells by inserting a layer of ZnO/self-assembled monolayer (SAM) between a poly (3-hexylthiophene): [6,6]-phenyl-C-61 butyric acid methyl ester bulk-heterojunction film and a metal cathode. We found that the device performance could be significantly altered depending on the dipole direction and chemical bonding between the SAM and metals. With appropriate choice of SAMs, devices show dramatically improved efficiencies and even high work-function metals such as Ag and Au could be used as cathodes. This finding provides an efficient method for interface engineering in organic-based optoelectronic devices. (C) 2008 American Institute of Physics.