Minor changes in processing conditions and composition of blended conjugated polymer materials have led to sometimes unpredictable properties of resulting plastic optoelectronic devices due to the complex morphology of conjugated polymer materials. We report a Single Particle Spectroscopy study of composite poly-(3-hexylthiophene) (P3HT)/1-(3-Methoxycarbonylpropyl)-1-phenyl-[6.6]C-61 (PCBM) nanoparticles fabricated with varying doping levels of PCBM to investigate changes in morphology of P3HT with respect to blend composition. We find that this material system consists of crystalline nanodomains with two distinct morphologies that have characteristic optical signatures, and that exhibit a change in their relative abundance with changing PCBM dopant concentration in favor of the more densely packed crystal structure with higher PCBM concentrations. (C) 2009 Elsevier B.V. All rights reserved.