Advanced nanostructuring of metal phthalocyanines for organic photovoltaic devices

The field of organic photovoltaics (OPV) has benefited greatly from improved structuring of materials at the nanoscale. The ideal active layer morphology in these devices is an interpenetrating network of donor and acceptor materials with feature dimensions that match their short exciton diffusion lengths. Here we report on the optimization of metal phthalocyanine (MPc) nanorod arrays through the use of glancing angle deposition (GLAD) and block copolymer (BCP) seeding. GLAD is a technique which uses oblique substrate angles and substrate motion during physical vapor deposition to produce structured columnar thin films with a variety of morphologies. It has recently garnered interest in the OPV community for structuring MPc's and has been shown to nearly double the photoconversion efficiency of CuPc/PCBM devices. So far, only elementary substrate motion has been used and the films are very thin (60 nm or less). As posts are grown higher, they tend to broaden significantly and irregularly, which prevents further photocurrent gains. Growing these structured films thicker while maintaining ideal dimensions would enable greater light capture and higher photocurrent. To mitigate post broadening, we employed the fsweep method which involves periodically rotating the substrate back and forth throughout film deposition to maintain 3D shadowing while at glancing flux incidence. In addition, we used substrate seeding via block copolymers to predefine growth sites in order to achieve periodic post arrangement. The result from these techniques is more uniform film growth with post diameters of 40-50 nm being maintained to lengths in excess of 400 nm. Post spacing was as low as 60 nm. These dimensions are well suited to the short exciton diffusion lengths in these materials. Improved device results are shown.