Understanding exciton diffusion is a crucial step in the engineering and fabrication of organic photovoltaic (OPV) devices. We studied a variety of organic dyes in our research lab including various derivatives of carboalkoxyphenylporphyrins in solution-cast nanometer thick films. We have established basic guidelines on how the exciton diffusion is affected by the arrangement and packing of peripheral alkyl groups in carboalkoxyphenylporphyrin solution-cast nanoscale thin films. The overall goal of this work is to link the molecular organization to singlet exciton diffusion parameters. Photoluminescent (PL) lifetime decays (s) and quenching efficiencies (Q) of solution-cast thin porphyrin thin films lightly doped with [6,6]-phenyl-C61-butryic acid methyl ester (PCBM) have been measured. These data have been used with a 3D exciton eDiffusion Monte Carlo simulation model to generate the exciton diffusion parameters and nanocomposition in the thin films. The length and branching of the peripheral alkyl groups in carboalkoxyphenylporphyrins influence the excited-state dynamics in a thin film. Using longer and linear peripheral alkyl chains increases the PL decay lifetimes and exciton diffusion lengths (LD). Structural studies such as Grazing Incidence Wide Angle X-Ray Scattering (GIWAXS) and X-ray diffraction (XRD) have indicated that molecular arrangement from longer and linear peripheral alkyl chains leads to nematic packing on the surface of thin films which is favorable for exciton diffusion in carboalkoxyphenylporphyrin thin films. Preliminary data collected for current-voltage curves measured from ternary OPV devices fabricated using these carboalkoxyphenylporphyrins along with poly-3-hexylthiophene (P3HT) and PCBM also supports our previous exciton diffusion studies. Porphyrin derivatives with longer PL decay lifetimes (s) and exciton diffusion lengths (LD) have shown higher photocurrents and improved power conversion efficiencies. Our findings are an important step towards a deeper understanding of the exciton diffusivity and molecular packing relationship in order to improve the power conversion efficiency of thin film organic solar cells.