Photoluminescence of conjugated polymer blends at the nanoscale

Here we report on a combined photoluminescence and morphological study of a polymer-polymer blend composed of a copolymer of derivatives of polyspirobifluorene and polyfluorene (PBFF) and a derivative of polyphenylene vinylene (MDMO-PPV). Evidence of partial Forster energy transfer from PBFF to MDMO-PPV is revealed by steady-state and time-resolved photoluminescence. Atomic force microscopy (AFM) on blends with different weight ratios of host (PBFF) and guest (MDMO-PPV) polymers shows that in blends with a MDMO-PPV concentration higher than 10% two phases are present. The nature of the phases is identified by means of scanning near-field optical microscopy (SNOM). Using this technique the topography and the local photoluminescence of the thin film are simultaneously mapped with a spatial resolution of similar to 100 nm. The local measurements reveal that the PL signal of MDMO-PPV is present in the whole surface, but with variations of the PL intensity and spectral shape. The correlation of the SNOM, time-resolved and steady-state photoluminescence measurements allows us to identify the two phases, one rich in PBFF and the other rich in MDMO-PPV.