Optical probes of ∏-conjugated polymer blends with strong acceptor molecules

We used a variety of optical probes to study the primary and long-lived photoexcitations in blends of polyphenylene-vinylene derivative poly(2-methoxy-5(2'-ethyl)hexoxy-phenylenevinylene) (MEH-PPV) with various concentrations of a strong electron-acceptor molecule 2,4,7-trinitro-9-fluorenone (TNF) for photovoltaic applications. The optical probes include fs transient photomodulation in a broad spectral range from 0.25 to 2.5 eV and a variety of cw spectroscopies such as steady-state photomodulation and its excitation spectrum, photoluminescence, and electroabsorption. With excitation above the polymer optical gap we found that the fs transient midinfrared-photoinduced absorption band and cw photoluminescence, which are both due to photogenerated excitons, are dramatically quenched in MEH/TNF blends with increasing TNF concentration.