| This PhD thesis studied the optical and electronic properties of pi-conjugated polymers with nondegenerate ground state, and their potential optoelectronic applications for organic solar cells. The main theme of the investigations has been the material structure, ranging from the conventional one dimensional pi-conjugated polymers to polymers having two-dimensional lamellae structure. The principle experimental technique used in the studies has been the continuous wave photo-modulation technique.; Two polymers have been the focus of the interest: poly(3-alkyl substituted) thiophene(P3AT) with different regioregularities, and substituted poly (p-phenylene-ethynylene)(PPE). Films of both polymers have morphology variations, which give rise to different photophysics. It is found out that the optical properties are very sensitive to the film morphology.; The cw photomodulation spectrum of regiorandom poly (3-alkyl substituted) thiophene (RRa-P3AT) contains both charged excitations (polarons) and neutral triplet excitons; whereas the primary excitation measured with picosecond (ps) techniques is the intrachain excitons.; In regioregular P3ATs, long-lived interchain excitons are found. Further study by femtosecond (fs)/picosecond (ps) spectroscopy reveals that the primary excitations in this type of P3HT are excitons with a larger interchain component.; Delocalized polaron (DP) excitations were found in regioregular poly(3-alkyl substituted) thiophene(RR-P3AT) film, where the polymer chains self-assemble to form quasi two-dimensional structures with increased interchain interaction. These novel photoexcitations were thoroughly studied and summarized in this thesis.; It is found out that the DP low energy band, DP 1 (peaks below 0.1eV), has associated Infrared (IR) vibrations with reverse absorption bands (anti-resonances (AR)) caused by photoinduced quantum interference. These AR features are well explained by extending the amplitude mode model to include nonadiabatic terms.; The nature of the photoexcitation recombination dynamics was investigated in films of PPEs. Both monomolecular and bimolecular recombination kinetics for different excited species were observed. Their morphology-dependent photophysics were also investigated.; One of the major applications of pi-conjugated polymers is organic solar cells. However, the underlying mechanisms to improve their quantum efficiency are still unclear. One major concern is to reduce current losses due to geminate recombi nation. In this work, various efforts to increase the energy conversion efficiency of organic solar cells are presented. |
