Organic nanocolumn arrays and organic solar cells

In the past decade, the power conversion efficiency of organic solar cells had been prominently improvemed. These increases were achieved in both polymer and small-molecule organic solar cells through the use of the bulk heterojunction. Ideally, the two phases of donor and acceptor materials should alternate on the length scale of the exciton diffusion length, and the two phases should be interdigitated and crystalline, to ensure high charge carrier mobility and reduced recombination. Crystalline nanocolumn arrays of two widely studied organic semiconductors, i.e., Fullerene (C60) and pentacene, were fabricated by glancing angle deposition (GLAD), and characterized by scanning electron microscopy and X-ray diffraction (Fig. 1) [1,2]. For both materials, column diameters of typically 100 nm are found on the transparent conductive oxide ITO, essentially independent of column height (up to 360 nm for pentacene). The molecular surface diffusion length on the substrate is identified as a critical parameter for the formation of columnar nanostructures by GLAD. Then, organic solar cells were fabricated as small area sandwich devices. The pentacene nanocolumn array was deposited by GLAD on a pre-cleaned indium tin oxide (ITO)/glass substrate. The PCBM was purchased from NanoC and used as received. The PCBM solution was spin-coated on top of the pentacene nanocolumn array. The devices were finalized by the deposition of a ~100 nm samarium through a shadow mask. Device preparation was performed under ambient condictions, except for the evaporation of the top electrod. Devices were characterized in a dry-argon atmosphere. Current-voltage characteristics were measured using a Keithley 2400 SMU. Devices clearly exhibited the rectifying behaviour in dark and photovoltaic effect under illumination