| Description |
In this work, we used the pump-probe photomodulation (PM) spectroscopy technique to measure the transient PM spectrum and decay kinetics in various n - conjugated polymers (PCPs) films and blends. Using two ultrafast laser systems, we covered a broad spectral range from 0.25 - 2.5 eV in the time domain from 200 fs to 1 ns with 120 fs time resolution. We also used continuous wave (CW) photomodulation spectroscopy, photoluminescence (PL), electro-absorption, and doping-induced absorption to study the photoexcitations and other optical properties of PCPs and guest/host blends. In particular, we studied two different types of Poly(thienylenevinylene) polymer derivatives. One polymer type is the ordered region-regular (RR) and regio-random (RRa) - PTV in which the dark exciton, 2Ag, is the lowest excited state. In these polymers, the photoexcited exciton shows very fast decay kinetics due to the internal conversion to the dark exciton, which results in weak PL emission; thus, these two polymers are nonluminescent. The other PTV derivative is the imide - PTV which is more luminescent due to the proximity of 1Bu and 2Ag states, that results in longer decay kinetics and a difference between the calculated value of the QEPL (9%) and the measured one (1%). We also demonstrate transient strain spectroscopy in RR - PTV thin films, where the ultrafast energy release associated with the exciton decay gives rise to substantial static and dynamic strains in the film that dramatically influences the film's transient PM response. We also study the photophysics of poly(dioctyloxy) phenylenevinylene polymer with different isotopes, where we substituted hydrogen (H - polymer) by deuterium (D -polymer), and 12 C by 13C isotopes. From the transient decay kinetics measurements, we found that the exciton recombination in DOO - PPV consists of two processes. These are: intrinsic monomolecular, and exciton-exciton annihilation (bimolecular). In the D - polymer, different probe frequencies of the main exciton photoinduced absorption band (PA1) show a variety of decay kinetics that result from various photoexcitations that contribute to the spectrum. Comparing the transient PM spectrum at 1 ns time delay to the CW PM shows the formation of triplet excitons, which is possible due to singlet fission of mAg (at 2.9 eV) into two triplets (2 X 1.4 eV). In the last part of this thesis, we summarize our studies of organic light emitting diodes (OLED) devices based on a host/guest blend of polyfluorene polymer that is mixed with various percentages of Ir(btp)2acac molecules. In this mixture, the PFO (host) shows blue fluorescence, whereas the Ir-complex (guest) has red phosphorescence emission; thus, OLED based on this mixture can serve as a "white OLED." Since the PFO emission spectrum perfectly matches the absorption band of the Ir-complex, it induces an efficient energy transfer from the PFO host to the Ir-complex guest molecules, which we tried to time resolve by the transient PM method. |
