Block Copolymer Templating of Hydrothermally-Grown ZnO Nanorod Arrays with Controlled Density for Photovoltaic Devices

The ability to control the diameter and density of ZnO nanowires in hydrothermally-prepared forests is critical for customization of ZnO arrays for targeted applications. Accomplishing this in a facile and scalable manner remains a significant challenge. Self-assembly of block copolymers is well-suited to ordering nanoscopic domains over macroscopic areas, and block copolymer micelles in particular offer advantages of regular, long-range periodicities when cast in a monolayer without any post-processing. We present a fully solution-based method for high-fidelity templated synthesis of ZnO nanorod arrays directed by a confluent monolayer film of self-assembled micelles of a block copolymer (BCP), poly(styrene-b-4-vinyl pyridine) (PS-b-P4VP). Growth of the ZnO nanorods proceeds from an underlying seed layer by diffusion of Zn salts through the micelles. The location and diameter of the resulting nanorods are controlled by the location and size of the P4VP micelle cores with growth in intervening spaces inhibited by the PS corona. Using simple geometrical relationships, density and inter-particle spacing of nanorods can be rationally designed based on block copolymer molecular weight and volume fraction. To demonstrate the significant benefits of this ability, we prepare ZnO:P3HT ordered bulk heterojunction solar cells using templated arrays and show improved PV efficiency proportional to our ability to match charge-transport limiting length scales.