Spectral properties and photophysics of conjugated polymers in electric fields

Aggregation is perhaps the most ubiquitous challenge to be overcome if semiconducting polymers are to supplant their more popular---and better understood---inorganic semiconducting counterparts. Important functional features that exhibit sensitivity to polymer morphology such as photo-emission and charge migration are essential to applications such as electroluminescence and photocurrent generation. Because conjugated polymers are amorphous chemical systems, a large degree of random behavior is observed in their spectral and photophysical properties that arise from extrinsic and intrinsic morphological defects. Using a combination of Stark spectroscopy (both absorption and fluorescence) and fluorescence microscopy, the optoelectronic properties of the electroluminescent polymer poly[2-methoxy-5-(2'-ethylhexoxy)-1,4-phenylene vinylene] (MEH-PPV) and several model oligomers are explored as isolated molecules in solvent glass matrices, as dopants in host-guest polymer systems, and as micro-aggregates. Stark spectroscopy has been used to measure the trace of the change in polarizability (trDelta a ↔ ) and the absolute value of the change in dipole moment (|Delta m&ar; |). From electrofluorescence, the measured values of trDelta a ↔ increase from 500+/-60A3 in OPPV-5 to 2000+/-200A 3 in MEH-PPV. Good agreement is found between these values and those measured by electroabsorption suggesting these properties do not differ strongly between absorption and emission. Evidence of electric-field-induced fluorescence quenching of MEH-PPV in dilute solvent glasses was also found. The magnitude of quench is found to increase from 0.012+/-0.014% in O2PPV-5 to 0.50+/-0.02% in MEH-PPV. When normalized to the square of the applied electric field, the magnitude of quench is comparable to that reported in the literature for thin films of MEH-PPV. In addition, fluorescence quenching was also observed in the oligomers with a magnitude that increases with increasing chain length. Using the values of trDelta a ↔ measured by electrofluorescence, a model is developed to qualitatively explain the chain length dependence to the fluorescence quench observed in the oligomers as a function of exciton delocalization along the oligomer backbone. Various interpretations are offered to explain the origin of this quenching behavior and its chain length dependence including the effect of electric fields on the nonradiative relaxation modes in the molecule. Two analytical expressions are derived that predict the magnitude of quench in terms of field effects on intersystem crossing or internal conversion. Of these two, only the latter expression accurately predicts the magnitude of quench observed in OPPVs and MEH-PPV.