Nano-Scale Investigation of Structural and Electrical Properties of Self-Organized Thin Films of Phthalocyanines: A Progress towards New Photovoltaic Material

Abstract

Ongoing efforts to improve the efficiency of organic photovoltaic cells emphasize the significance of the architecture of molecular assemblies in thin films, at nanometer and micron length scales, to enhance both exciton diffusion and charge transport, in donor and acceptor layers. Controlled growth of molecules via self-assembly techniques presents new opportunities to develop nano-structured organic thin films for electronic devices. This thesis is focused on controlling the orientation of phthalocyanine molecular assemblies in thin films in order to demonstrate the impact of microscopic control of molecular order on electrical properties and organic solar cell device performance.The studies performed here provide insights into the self-assembling behavior, film morphology, nanoscale electrical conductivity, and photovoltaic properties of a disk-shaped peripherally substituted phthalocyanine (Pc) molecule possessing amide functional groups in the side chains. Amide functionality was integrated in the side chains of this phthalocyanine molecule with the purpose of increasing the intra-columnar interaction through formation of a hydrogen bonding network between molecules, and to guide columnar orientation in a preferred direction via specific surface-molecule interactions. It is realized that molecule-substrate interactions must dominate over molecule-molecule interactions to achieve control over the deposition of molecules in a preferred direction for organic solar cell applications. Microscopic imaging and spectroscopic studies confirm the formation of flat-lying, well ordered, layered phthalocyanine films as anticipated.The remarkable electrical conductivity of the flat-lying phthalocyanine molecules, as studied by Conducting tip Atomic Force Microscopy (C-AFM) provide the impetus for the formation of organic solar cells based on layers of these hydrogen bonding phthalocyanine molecules. The photocurrent from devices that are made with the ordered Pc molecules and disordered Pc molecules as the primary photoactive donor layer, and vacuum deposited C60 as the acceptor material, were evaluated. The results presented here demonstrate the feasibility of increasing the photogenerated current by controlling the molecular organization in the photo active layer.