Local probing of photocurrent and photoluminescence in a phase-separated conjugated-polymer blend by means of near-field excitation

In this paper scanning near-field microscopy is used to characterize polymer blends for photovoltaic applications, and fluorescence imaging and photoconductivity are combined to elucidate the spatial distribution and relative efficiency of current generation and photoluminescence in different domains of compositionally heterogeneous films. Focus is placed oil a binary system consisting of poly [(9,9-dioctylfluorene) -alt-benzothiadiazole] (F8BT) and poly[(9,9-dioctylfluorene)-alt-(bis(NN-(4-butylphenyl))-bis(N,N'-phenyl-1,4-phenylenediamine))] (PFB), spun from xylene solutions, so as to obtain phase separation on micrometer and nanometer length scales. Protruding regions with diameters of about 5 mu m in the topography image coincide with regions of high photocurrent (PC) and luminescence; these regions are identified as being F8BT rich. A general method to estimate the photoluminescence efficiency in the different domains of phase-separated blends is proposed. As expected, lack of enhancement of the PC signal at the boundaries between protruding and lower-lying phases indicate that these microscale boundaries play a small role in the charge generation by exciton splitting. This is consistent with the domains compositional inhomogeneity, and thus with finer phase separation within the domains. We also provide all analysis of the extent to which the metallized probe perturbs the near-field photocurrent signal by integrating Poisson's equation. Finally, by using a Bethe-Bouwkamp, model, the energy absorbed by the polymer film in the different regions is estimated.