Ultrafast Charge Photogeneration Dynamics Of Donor-Acceptor Copolymers Pbdttt And Pf(S)Dcn

One of the main structural characteristics of the donor-acceptor copolymer is that the donor and acceptor monomers are arranged alternately on a conjugate unit. Organic solar cell based on the bulk heterojunction (BHJ) structure of fullerene-blended donor-acceptor copolymer has achieved a light-to-electric power conversion efficiency (PCE) of～10%, yet the PCE still needs to be improved for practical applications. Understanding of the fundamental photophysical properties of the donor-acceptotr copolymer and the light conversion mechanisms of the BHJ blend are indispensable for overcoming the bottleneck of improving PCE. For deepen the understanding of the relationship of macromolecular structures and the light conversion mechanisms, this thesis made use of time-resolved absorption/fluorescence spectroscopies, spectroelectrochemistry and atomic force microscopy, etc., to investigate the primary charge photogeneration processes of one-dimensional (1D) PBDTTT and two-dimensional (2D) PF(S)DCN copolymers in solutions, as well as in neat and fullerene-blended BHJ layers. Emphases were put on the impacts of side chain on the electronic structure, the solution-phase conformation and the solid-phase nanostructure of semiconducting copolymer. The structure-activity relationship revealed in this work will be helpful in deepening the understandings in the light conversion mechanisms, as well as in advancing the PCE via molecular engineering. Main contents are summarized below.1. Solution-phase conformations and the charge photogeneration dynamics of one dimensional D-A copolymers PBDTTT. We performed steady-state and time-resolved spectroscopies of alkoxy (-E) substituted PBDTTT-E and alkyl (-C) substituted PBDTTT-C in diluted chlorobenzene solutions at25℃and120℃, respectively. The comparative investigation on PBDTTTs reveals their thermally robust self-aggregated conformations. Considerably higher packing order was found for the PBDTTT-C self-aggregation with respect to PBDTTT-E. It was shown that PBDTTT-C on photoexcitation gave rise to a branching ratio of charge separated state (CS) over charge transfer state (CT) more than20%higher than PBDTTT-E did. The side chain effects on the macromolecular conformation and on the charge photogeneration dynamics were rationalized by quantum chemical analysis. The PBDTTT self-aggregation in solution were suggested to be morphological precursors of the solid-state photoactive layers, which were suggested to be rather crucial for the photovoltaic performance of semiconducting copolymer.2. Charge photogeneration dynamics of PBDTTT neat films and PBDTTT-fullerene blend films. The primary excitation/charge dynamics of neat PBDTTT films show that the excess excitation energy significantly enhances the P·-yield, and that PBDTTT-C gives rise to>50%higher P·+yield than PBDTTT-E does irrespective to the excitation photon energy. For the BHJ blends, it was found that PBDTTT-C:PC6iBM exhibited a4.5ps and a～120ps processes of CT-to-CS conversion, which were followed by a nanosecond process of CS-to-CT recombination. In addition, the excess energy was suggested to play an important role in promoting the CT dissociation as well as in suppressing the CS recombination. Furthermore, a-80ps process of P·+migrating away from the BHJ interface was observed for the PBDTTT-C:PC61BM blend. However, the CT-CS interconversion and the P·+migration were indistinctive for the PBDTTT-E:PC61BM blend. These results confirm that the solution-phase macromolecular conformation intimately affected the film morphology, indicating that side chain optimization can effectively improve the polymer-fullerene biphasic crystallinity and thereby the photovoltaic performance.3. Excited-state dynamics of two dimensional D-A copolymers PF(S)DCN in diluted solution. The two-dimensional D-A copolymers PFDCN and PFSDCN differ each other by an additional conjugated "-C=C-" bond in the side chain of the former. They were both found to poss prominent intra-molecular charge transfer (ICT) characters. Intriguingly, photoexcitation of PF(S)DCN solution generates polaron pair (PP) rather than neutral exciton (Ex). Investigation of the side chain effect on the photogeneration dynamics of PP and Ex showed that, owing to the stronger ICT characters, PFDCN gives rise to significantly higher PP yield and faster PPâ†'Ex conversion (7ps) than PFSDCN does. The results will be helpful for designing high-performance two-dimensional photovoltaic copolymer.4. Ultrafast charge photogeneration dynamics of neat and fullerene-blended PF(S)DCN films. Impacts of side chain on the charge photogeneration dynamics of neat PF(S)DCN films and PF(S)DCN:PC71BM blends were investigated. The results indicated that, with respect to the solution phase, the lowest excited states of PF(S)DCN in neat film maintained higher coplanarity between the conjugated backbone and the side chain and hence lower excited state energy. In addition, photoexcitation of PF(S)DCN films directly generate Ex and P·+in paralleling. The subtle side-chain variation significantly affects the morphology of blend film, the P·+/Ex production ratio, as well as the charge recombination dynamics of the PF(S)DCN:PC71BM blend. The underlying light-conversion mechanisms were discussed in terms of the polymer-polymer and the polymer-PC71BM interaction.