Nonlinear optical spectroscopy of photoexcited states in pi-conjugated polymers

In this research we used a variety of nonlinear optical (NLO) spectroscopies to study the photoexcitations and nonlinear optical properties of pi-conjugated polymers. The polarized NLO spectroscopies include ultrafast pump-probe photomodulation (PM), two-photon absorption, electroabsorption, and laser action. In order to cover a broad spectral range in transient PM we used two ultrafast laser systems: a low-intensity high-repetition rate laser system in the spectral range from 0.13 to 1.05 eV and high-intensity low-repletion rate laser system in the spectral range from 1.2 to 2.6 eV.; The transient PM spectra of pi-conjugated polymers show that the primary photoexcitations in chain-like polymer films and solutions are singlet intrachain excitons that have a characteristic PM spectrum comprising of two photomduced absorption (PA) bands, PA, at mid-IR and PA2 at visible-near IR range, respectively, and a stimulated emission band at the PL spectral range. For polymers with more ordered structure such as regioregular poly (3-hexylthiophene) (RR-P3HT) the interchain interaction increases substantially; on consequently interchain excitons become important photoexcitation species. This leads to photogeneration of polarons pairs, and ultimately free polarons may also be generated in a later time, which depends on the nano-morphology of the polymer chains in the films.; Laser action has been measured in poly (phenylene-ethynylene)/poly (phenylene-vinylene) [PPE-PPV] copolymer. We found laser action in dilute PPE-PPV solutions but not in films; this represents the first laser action reported in PPE-type polymers. A study of laser action in solutions of various copolymer concentrations confirms the lack of lasing in concentrated solutions and explains its absence in films.; In addition to transient PM spectroscopy we used also two-photon absorption and electroabsorption to study the singlet excited states order and primary photoexcitations in polyfluorene, an important blue emitting pi-conjugated polymer. A polarization memory of ∼2.2 typical to pi-conjugated polymer chains characterizes all three NLO spectra reflecting the highly anisotropic third-order NLO coefficient chi(3) of the polymer chains. The four essential excited states, namely, the 1Bu, mAg, nBu and kAg, were used to satisfactorily fit all three nonlinear optical spectra using the summation over states model. The combination of the three NLO spectra and the model fit conclusively show that the 'band model' typical to inorganic semiconductors cannot describe the PFO polymer. On the contrary, a strongly bound exciton with intrachain binding energy of ∼0.9 eV dominates the linear and NLO spectra of this polymer.; Organic photovoltaic devices using pi-conjugated polymers have been intensely investigated due to the potential of cheap solar energy harvesting. Blends of conjugated polymers with fullerene derivatives have been demonstrated as one of the most promising materials for photovoltaic devices, in which the ultrafast charge transfer from polymers to fullerene derivative and the associated high quantum efficiency for charge separation increases the photoconductivity efficiently. In our studies we used transient PM spectroscopy to study the photoinduced charge transfer in fullerene-doped polymers, in comparison with the transient PM spectra of the pristine polymer counterparts.