Fabrication and Spectroscopic Study of Nanostructured Materials Containing Poly(3-hexylthiophene)

Abstract

Excited-state dynamics of nanostructured materials containing regioregular poly(3-hexylthiophene) (P3HT) has been investigated by using static and time-resolved optical spectroscopy. A brief overview on synthetic methods and optical properties of P3HT and device applications of hybrid nanostructured materials containing P3HT is presented in Chapter 1. Chapter 2 describes the effect of gold nanoparticles (NPs) on excited-state dynamics of P3HT. P3HT-coated gold (Au@P3HT) hybrid NPs have been prepared via a grafting-to approach, and compared with pristine P3HT to investigate the dynamics of exciton relaxation using time-resolved transient-absorption and fluorescence spectroscopy. In efforts to facilitate the efficient dissociation of photo-generated excitons, we have incorporated gold nanoparticles having surface-plasmon resonances into thiol-terminated P3HT to fabricate Au@P3HT nanocomposites. The first-excited singlet state of Au@P3HT decays faster than that of pristine P3HT due to interactions with surface-plasmon resonances

excitons undergo dissociation via energy transfer from P3HT to the surface-plasmon state of a gold nanoparticle in Au@P3HT nanocomposites. The lowest triplet state of P3HT is also less populated due to the energy transfer while its lifetime is slightly reduced by the presence of gold nanoparticles. Thus, it is suggested that our incorporation of gold nanoparticles into P3HT reduces the recombination of geminate excitons and, thereby, increases the probability of exciton dissociation. Chapter 3 describes effects of gold nanorods (Au NRs) on excited-state dynamics and photovoltaic performances of hybrid nanocomposites containing P3HT. P3HT-stabilized Au NRs (Au-NR@P3HT nanocomposites) have been facilely fabricated by incubating Au NRs with thiol-terminated P3HT. The S1 decay of Au-NR@P3HT nanocomposites is found to be slower than that of pristine P3HT, suggesting that the stretched-strand conformation of P3HT chains attached to Au NRs makes structural relaxation more difficult. The amplitude of T1 absorption is much smaller in Au-NR@P3HT nanocomposites than in pristine P3HT, indicating that the intersystem crossing of S1 excitons into T1 excitons does not occur efficiently in Au-NR@P3HT nanocomposites due to the nonflexible character of aggregated P3HT chains. From the comparison of the performances of organic photovoltaic devices, we have found that the device with the 3% embedding of Au-NR@P3HT nanocomposites into the P3HT matrix of the active layer shows significantly improved photovoltaic performances (27% enhancement in the power conversion efficiency), suggesting that the surface-plasmon resonances of Au NRs enhance the dissociation as well as the generation of excitons highly. Chapter 4 describes the coupling-type dependent excited-state of P3HT NFs, such as J-aggregates and H-aggregates. Thin nanofibers (NFs) of J-dominant aggregates and thick NFs of H-dominant aggregates have been fabricated by self-assembling P3HT-coated Au NPs. With increasing excitation energy, the fraction of the fast emission decay component has been found to decrease, suggesting that the fast formation of polaon pairs, localized, and delocalized polarons results from higher singlet exciton states. In both NFs, whereas triplet (T1) excitons have been hardly observed with excitation at 532 nm, they have been observed with excitation at 355 nm, revealing that T1 excitons within NFs are mainly generated through the singlet fission from a higher singlet exciton state rather than through intersystem crossing. Chapter 5 describes excited-state dynamics of amphiphilic diblock-copolymer self-assembled in mixed selective solvents. An amphiphilic diblock-copolymer consisting of P3HT and poly(4-vinylpyridine) has been synthesized and self-assembled from mixed selective solvents of tetrahydrofuran and methanol to produce nanostructures having photoluminescence spanning from the blue to the red. The emission decay times of the nanostructures have been found to increase with the fraction of methanol, suggesting that the probability of the nongeminate recombination of relaxed S1 excitons decreases with the increase of medium polarity. The emission decay times are shorter at 690 nm than at 650 nm, indicating that two-dimensional interchain effect is more important for the 0-1 vibronic transition than for the 0-0 transition. The initial intensity percentage of the fast component of biphasic emission decay is much larger with excitation at 532 than with excitation at 355 nm, suggesting that other charge carriers such as polarons are generated rapidly from S1 excitons in competition with vibrational relaxation.