Photoinduced Charge Transfer Dynamics in Blue-Coloured Diketopyrrolopyrrole (DPP) Dye-Sensitized Solar Cells

The inception of push-pull organic dyes utilizing the donor-pi conjugated bridge-acceptor (D-pi-A) motif has created a paradigm shift in the design of efficient nanocrystalline dye-sensitized solar cells (DSSCs), offering control over aesthetic and performance properties. Especially, sensitizer embedding diketopyrrolopyrrole (DPP) core for solar cell application has attracted great attention because of its promising performance and aesthetic blue coloration. The core of this thesis is devoted to the understanding of the exact mechanism and dynamics of the charge separation in DPP-sensitized solar cells, where a number of photoinduced processes coexist and are kinetically entangled. To understand this complex combination of processes, fundamental backgrounds are introduced in the first chapter. Next, detailed descriptions of the instruments employed in this study are provided in the second chapter. In Chapter 3, detailed intermolecular interactions of D-pi€-A DPP-based dye molecules on the redox-inactive Al2O3 is investigated at the exciton level. The results evidence that charge transfer (CT) excitons formed between neighboring dyes are involved in the lateral interaction. And this arises from the ordered molecular configuration on the surface due to the interaction between electron-rich donor and relatively electron-poor DPP core in the neighboring dye. On the other hand, when the donor is deprived of DPP dye, excimer-like excited states are formed upon photoexcita-tion and involved in the interaction due to pi-pi interaction. This suggests that the donor moiety plays a critical role in intermolecular interactions. In Chapter 4, the investigation is performed on how the lateral interaction studied in Chapter 3 affects the electron injection process of DPP-sensitized TiO2 films, by comparing the dynamics of donor-installed and donor-deprived DPP dye sensitizers. It is found that lateral intermolecular interactions exist even on TiO2, and takes place during several picoseconds. It is also found that this causes the charge injection process to become inefficient, especially in donor-deprived dye, because the delayed injection rate is slower than that of the lateral interaction. From these observations, it is suggested that the intermolecular interaction is another factor that can have a decisive influence on interfacial CT processes in DSSC. In Chapter 5, based on previous observations, we explore the reason why asymmetric DPP dye sensitizer offers better photovoltaic performances compared to that of the symmetric DPP one. An enhanced J-aggregates formation of symmetric thienyl-DPP (ThDPP) dye on TiO2 is observed com-pared to the case of asymmetric phenyl-DPP-thienyl (PhDPPTh) dye sensitizer. We find that these J-aggregates also inject electron upon excitation, and their oxidized form interacts with surrounding charge species. Moreover, their injection becomes inefficient as light intensity enhances. This result is in line with the nonlinear photocurrent dynamics of symmetric DPP dye sensitizer, which is be-lieved to the cause of device inefficiency. Through the studies carried out within this thesis using DPP dye sensitizers, a step-by-step understanding of the charge transfer mechanisms and in particular their dependence upon lateral in-termolecular electronic interactions is constructed. We hope that they will open new paths for further optimizations of the efficiency of DSSCs.