Optical study of pi-conjugated polymers and pi-conjugated polymers/fullerene blends.

摘要

In this research, we studied the optical properties of a variety of pi-conjugated polymers and pi-conjugated polymers/fullerene blends, using various continuous wave optical spectroscopies.;We found an illumination-induced metastable polaron-supporting phase in films of a soluble derivative of poly-p-phenylene vinylene (MEH-PPV). Pristine, MEH-PPV polymer films in the dark do not show long-lived photogenerated polarons. Prolonged UV illumination, however, is found to induce a reversible, metastable phase characterized by its ability to support abundant long-lived photogenerated polarons. We also discovered a photobleaching band in our photomodulation measurement around 0.9eV that scales with and thus is related to the observed polaron band. In the dark, the illumination-induced metastable phase reverts back to the phase of the original MEH-PPV within about 30 min at room temperature.;We also applied our experimental techniques in polymer/fullerene blends for studying the photophysics of bulk heterostructures with below-gap excitation. In contrast to the traditional view, we found that below-gap excitation, which is incapable of generating intrachain excitons, nevertheless efficiently generates polarons on the polymer chains and fullerene molecules. Using frequency dependence photomodulation, we distinguished between the two mechanisms of photoinduced charge transfer using above-gap and below-gap excitations, and found a distinguishable long polaron lifetime when photogenerated with below-gap excitation. The polaron action spectrum extends deep inside the gap as a result of a charge-transfer complex state formed between the polymer chain and fullerene molecule. Using the electroabsorption technique, we were able to detect the optical transition of the charge transfer complex state that lies below the gap of the polymer and the fullerene. With appropriate design engineering the long-lived polarons might be harvested in solar cell devices.;Another system studied was platinum-containing conjugated polymers in which the intrachain Pt atom was incorporated into the polymer either in each (Pt-1) or in every three (Pt-3) monomer units. The heavy metal Pt atom that is incorporated in the polymer chain dramatically increases the spin-orbit coupling, and this influences both the intersystem crossing rate, and the phosphorescence emission intensity. We discuss an interesting effect for the photoexcited triplets, which dramatically affects the phosphorescence spectrum and intensity at high temperature.