Ultrafast Spectroscopic Study On Optoelectronic Properties Of Organic Molecules And Conjugated Polymers

Organic light-emitting diode usually works at solid state and requires the light-emitting molecules to have high solid state luminescence; so many new molecules with the property of aggregation induced emission (AIE) have been synthesized. However the mechanism of AIE is still far from fully understood. The understanding of AIE mechanism is the basis to develop more AIE materials with better quality.As a prominent polymer solar cell material. poly(3-hexyl thiophene) (P3HT) has been extensively studied. However, having intermediate standing between traditional semiconductor and molecules, maintaining heterogeneous nanostructures, P3HT is highly complex with its electronic properties. The dynamics and morphological dependence of various photo-excitations in P3HT film have attracted extended scientific interest, but there are still many unclear mechanisms. For P3HT:PCBM blend film, it still under debate how the excitons so efficiently reach the donor/acceptor interface to undergo charge separation.In this thesis, we applied both pump-probe and fluorescence up-conversion techniques to study these problems and the results are listed below.1. System built-up:We have built up the pump-probe and fluorescence up-conversion systems. We have write he control and data collection programs using visual basic language. Good signal to noise ratio have been achieved and can detect weak signals.2. The mechanism of aggregation induced emission (AIE):It was believed that the mechanism for AIE is the restriction of the torsional/vibrational non-radiative relaxation pathways in aggregate form, which results in the emission enhancement. However, when we applied time resolved fluorescent measurements of CNDPASDB, a molecule with electron donor and acceptor group, we firstly found that its AIE machnism is the aggregation induced restriction of the transition from the local excited (LE) state to the intramolecular charge transfer (ICT) state. In high polar solvent, LE state decays rapidly, accompanied by the emergence of an ICT state. Because the ICT state is a relatively dark state, the emission in the high polar solution is low. Aggregation of the molecules restricts the transition from the LE state to the ICT state, resulting in a enhancement of the emission. We have also performed pump-probe experiments for CNDPASDB. It shows that the initial decay of the fluorescence is not accompanied by the recovery of the ground state bleaching, but an emergence of a new intermediate excited state-ICT state. Then, for comparison, we performed pump-probe measurements for CNDPDSB, a molecule without donor acceptor group. It shows that the decay of the fluorescence is accompanied by the recovery of the ground state bleaching, without going through any intermediate state. So the reason for its low emission in solution state is the efficient torsional/vibrational non-radiative relaxation, Aggregation of the molecules restricts the free motion of the molecules and the non-radiative relaxation becomes inefficient, resulting in the enhancement of the emission. In this thesis, we have found two different AIE mechanisms for two different kinds of molecules, which is of great importance to the development of AIE materials. This work is introduced by the American Chemistry Society in its weekly web feature "Noteworthy Chemistry"3. The polaron pair dynamics in P3HT film:It was reported that the polaron pair decay to the singlet excited state with a life time of 0.7 ps. However through comparison of the dynamics of the peak emission, ground state bleaching and polaron pair absorption, we found that the ground state bleaching recovery has an additional fast decay of 0.7 ps compared to the fluorescence decay, and this additional decay is also found in the polaron pair absorption dynamics. Therefore we conclude that the polaron pairs actually decay back to the ground state. Furthermore, through comparison of the transient absorption spectra shape of the exciton (4 ps) and the polaron pair (the differential spectral of 0.5 ps and 4 ps), it is found that the polaron pairs decay back to a certain domain with specific absorption feature. This work gives a clear picture of the polaron pair dynamics, and is important to understand the photophysics in P3HT thin film.4. The morphology and function of P3HT film:Different stacking modes of the long polymer chains will result in different degree ofπ-conjugation, and so different morphologies and electronic structures. But until now, research on distinguishing different morphologies in one P3HT film and the relation of the morphology and function are still lacking. Through decomposing the transient spectra for different lifetimes, we, for the first time, found that there are at least four different morphologic domains, each with its own specific function to generate different photoexcitations. (1) Polaron is possibly generated in weakly coupled J aggregate domain with the 0-0 transition being pronounced. It has the most red-shifted absorption spectra and there is almost no absorption below the 510 nm. (2) Polaron pairs are generated from the higher energy places such as the kink, torsion or bents. The spectra is blue-shifted to the polaron spectra, and the 0-0 transition is allowed. The absorption under 510 nm is weak. (3) Excitons of the aggregate domain are mostly formed in the weakly coupled H-aggregate region, with a blue shifted absorption spectra, which has more absorption below 510 nm. The 0-0 transition is relatively restricted. Excitons can also be formed in the polaron region and polaron pair region and the exciton annihilation rate is different for excitons generated in different regions. (4) excitons from the unaggregated domain are formed in the totally unaggregated region, with a P3HT solution absorption feature. It has higher energy then the excitons in the aggregate region, and can rapidly transfer to the aggregate region through energy transfer. So there is no ground state bleaching signal for these excitons. This is the first time that different spectra signatures for different morphological places are revealed, and the relationship between the morphology and photo-excitations are built, which is of great importance to understand the photophysics in P3HT film.5. The exciton diffusion and charge transfer dynamics in P3HT:PCBM film:It was found that the phase separation of 13-15 nm in P3HT:PCBM blend film is the most favorable state to achieve high conversion efficiency. It was commonly believed that the excitons reach the donor/acceptor interface by diffusion to undergo charge separation. However the excion diffusion in polymer is slow, then how can the excitons reach the interface so efficient? So the diffusion model are in doubt. We have measured the excitation intensity dependent fluorescence dynamics and found that the exciton diffusion can be well described by a three dimensional diffusion model. We have got the charge transfer radius Rct of 4.8 to 9 nm, and the diffusion length of 0.5 nm in 8 ps, much smaller than Rct. It implies that the excitons reach the interface not by diffusion but possibly by the delocalization of the excitons in a large P3HT domain. This work is important to understand the charge separation process in polymer solar cell.