Imaging of Photoinduced Interfacial Charge Separation in Conjugated Polymer/Semiconductor Nanocomposites

Though having attracted extensive interests as the substitution for dye-sensitized TiO2 solar cell, hybrid nanocomposites of conducting polymer, and inorganic semiconductors for solar cells have still gave much less conversion efficiency. The photovoltaic processes are expected to take place mainly near the heterogeneous interfaces but directly visualizing those processes and correlating them with local nanostructures have not been established yet. Such a relationship is very important for the improvement of structural designs of the hybrid systems with better conversion efficiencies. In this article, two advanced microscopic techniques, near-field. scanning optical microscopy, and scanning electrostatic potential microscopy are employed to explore the photoinduced charge separation processes and kinetics in nanocomposites of poly(1,4-phenylenevinylene) and TiO2 nanoparticles, as well as their association with the local nanostructures. The results suggest that charge separation can be the dominant pathway for excitons near the interfaces as competing with fluorescence. Also, maps of charge redistribution upon illumination directly show the limited migration distance (similar to 20 nm) of holes in PPV films in consequence of electron injection into TiO2 nanoparticles. The migration distance of holes is of the same magnitude as the exciton diffusion length, and both of them are critical factors in photovoltaic applications of the hybrid composite and can potentially limit the light conversion efficiency. Thus, the results provide beneficial guidelines for nanostructure designs of hybrid solar cells.