Nanoscale Imaging of Exciton Transport in Organic Photovoltaic Semiconductors by Tip-Enhanced Tunneling Luminescence

In organic solar cells, the efficiency of the exciton transport and dissociation across donor-acceptor (D/A) interfaces Is controlled by the nanoscale distribution of the donor and acceptor phases. The observation of photoluminescence quenching Is often used as confirmation for efficient exciton dissociation but provides no information on the nanoscoplc nature of the exciton transport. Here we demonstrate nanoscale imaging of the exciton transport In films consisting of the conjugated polymer poly(3-hexylthiophene) (P3HT, electron donor) blended with the C60 derivative 1-(3-methoxyearbonyl)-propyl-1- phenyl-(6,6)C61 (PCBM, electron acceptor) by a tunneling luminescence spectroscopy based on atomic force microscopy. The excitonic luminescence Is significantly enhanced when the conjugated polymer is coupled to the plasmon excitation at the tip (tip-enhanced luminescence). This effect allows one to dramatically improve the detection efficiency of the excitonic luminescence and, consequently, resolve individual domains of the conjugated polymer in which the exciton will recombine before dissociation at the D/A interface. Under thermal annealing conditions promoting the segregation of the donor and acceptor phases, a clear increase of the luminescence Is seen from polymer-rich regions, consistent with domains of dimensions much larger than the exciton diffusion length. The described scanning luminescence microscopy can thus be applied to the optimization of the blends used in solar cells.