Optical probes of π -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. In addition significant charge-transfer (CT) species are also instantaneously photogenerated but they undergo fast geminate recombination within ~10 ps; this explains a major inefficiency of this blend for photovoltaic applications. Indeed, I-V measurement of a photovoltaic cell made from (1:1) MEH-PPV/TNF blend under illumination yields photocurrent density in the μA/cm² range, three orders smaller than a device made from MEH-PPV/C₆₀ blend. Nevertheless the few photogenerated CT species that escape geminate recombination are subsequently captured in traps forming long-lived polarons. When using ``below-gap'' excitation in the near-infrared spectral range we found that short-lived CT species and long-lived polarons are also photogenerated with high quantum efficiency. This shows that a CT complex state is formed below the polymer optical gap in these blends, as verified by electroabsorption spectroscopy but has low dissociation efficiency; this is in contrast to polymer blends with fullerene molecules, where apparently the CT complex state below the gap has much higher dissociation efficiency.