Nano-domain behaviour in P3HT:PCBM nanoparticles, relating material properties to morphological changes

In this article we report a morphological study of the poly(3-hexylthiophene) (P3HT): phenyl-C61-butyric acid methyl ester (PCBM) material system in nanoparticle organic thin films. The morphology and chemical composition of unannealed and annealed P3HT:PCBM nanoparticles has been investigated using scanning transmission X-ray microscopy (STXM) for a range of P3HT molecular weights (Mw=5–72 kg mol−1). Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) have been used to support the STXM data. We find that unannealed P3HT:PCBM nanoparticles (NPs) exhibit a common core–shell morphology, with a PCBM-rich core and P3HT-rich shell. The morphology of the thermally annealed NP films is highly dependent upon the molecular weight of the P3HT and is determined by PCBM diffusion through the P3HT matrix. Two PCBM diffusion mechanisms operate within this system: (1) at high molecular weights diffusion of molecular PCBM dominates whilst, (2) at low molecular weights diffusion of the PCBM cores is significant. The Stokes–Einstein continuum model for diffusion has been used to determine a threshold molecular weight at which the diffusion of PCBM cores is activated in these films. The calculated value (Mw~38–25 kg mol−1) is shown to agree very well with experimental observations. Finally, a model for the morphological evolution of annealed P3HT:PCBM NP films is developed.