Abstract :
Summary form only given. We report detailed photoinduced absorption spectra of poly[2-methoxy, 5-(2´ethyl-hexyloxy)-p-phenylenevinylene], MEH-PPV, and its composite with C/sub 60/, from 0.01 eV (100 cm/sup -1/) to 1.3 eV (11,000 cm/sup -1/). The photoinduced absorption spectra of MEH-PPV are characterized by subgap electronic absorptions with a lower energy feature superposed with two peaks at 0.25 eV and 0.4 eV and a higher energy feature with onset at 1.1 eV together with infrared-active vibrational modes below the lower energy feature. The two features at 0.4 eV and 1.1 eV are assigned to the lower and upper energy bipolaron transitions, respectively. Another lower energy peak at 0.25 eV, which is absent in spectra of PPV, can be assigned to lower polaron transition. The magnitude of spectral response is smaller than that of PPV by a factor of 10. These results arise from the weak interchain coupling in MEH-PPV due to the large side group. As a result, interchain transport is inhibited by large interchain separation leading to the suppression of bipolaron formation, and the early time recombination is enhanced, resulting in weak spectral response compared with PPV. Upon addition of five percent of C/sub 60/ to MEH-PPV, the photoinduced absorption spectra are significantly enhanced in nearly two orders of magnitude. The photoinduced charge transfer from MEH-PPV onto C/sub 60/ enhances the quantum efficiency for photogeneration of charge carriers. As a result, the lifetime of the excited state is extended, thereby leading to the enhanced photoinduced respomm of MEH-PPV.
