Laser light scattering as a probe of phase separation in conjugated polymer films of donor-acceptor blends

Light scattering and absorption in the visible and near-infrared ranges were studied in films of donor-acceptor blends of poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) with 2,4,7-trinitrofluorenone (TNF) or 1,5-dinitroantraquinone (DNAQ). Charge-transfer interaction in the electronic ground state of MEH-PPV/TNF and MEHPPV/DNAQ blends results in optical absorption in the bandgap of MEH-PPV. At the same time, because of donor-acceptor phase separation, light scattering can grow with increasing the acceptor content in the blends contributing to their optical transmission. To evaluate the amount of absorbed and scattered light separately, we have elaborated a simple method in which the optical transmission of the films is measured at several laser wavelengths using a wide aperture photodetector with or without a dump blocking the straight laser beam. Making some assumptions about the scattering indicatrix, the scattering and absorption coefficients of the blends can be evaluated. We have found that the amount of scattered light in MEH-PPV/TNF blends does not depend smoothly on the acceptor content. At the molar acceptor concentration MEH-PPV:TNF below 1:0.33, the light scattering is low and depends on the TNF content only weakly, whereas at higher ratios, the scattered light begins to grow dramatically. We assign this growth to phase separation in the blend owing to excess acceptor molecules that are not involved in charge-transfer interaction with the donor. In MEH-PPV/DNAQ blends, the phase separated DNAQ forms large species, which weakly contribute to the observed light scattering.