  The performance of organic solar cells (OSCs) can be greatly improved by
incorporating silica-coated gold nanorods (Au@SiO2 NRs) at the interface
between the hole transporting layer and the active layer due to the plasmonic
effect. The silica shell impedes the aggregation effect of the Au NRs in
ethanol solution as well as the server charge recombination on the surface of
the Au NRs otherwise they would bring forward serious reduction in open circuit
voltage when incorporating the Au NRs at the positions in contact with the
active materials. As a result, while the high open circuit voltage being
maintained, the optimized plasmonic OSCs possess an increased short circuit
current, and correspondingly an elevated power conversion efficiency with the
enhancement factor of ~11%. The origin of performance improvement in OSCs with
the Au@SiO2 NRs was analyzed systematically using morphological, electrical,
optical characterizations along with theoretical simulation. It is found that
the broadband enhancement in absorption, which yields the broadband enhancement
in exciton generation in the active layer, is the major factor contributing to
the increase in the short circuit current density. Simulation results suggest
that the excitation of the transverse and longitudinal surface plasmon
resonances of individual NRs as well as their mutual coupling can generate
strong electric field near the vicinity of the NRs, thereby an improved exciton
generation profile in the active layer. The incorporation of Au@SiO2 NRs at the
interface between the hole transporting layer and the active layer also
improves hole extraction in the OSCs.
