Singlet Fission In Planar Aromatic Molecules

Dye-sensitized solar cells (DSSCs) have attracted growing interest among chemical researchers due to its low-cost, low-pollution and high power conversion efficiency. However, the part of the energy of the absorbed photons which are higher than the bandgap of the semiconductor would convert into heat and photons with energy below the bandgap which will not contribute to power conversion are wasted. These place a limit to the efficiency of the solar cells. Several ways have been proposed to avoid this energy loss. Singlet fission (SF), a process in which a high-energy singlet exciton splits into two lower-energy triplet excitons, has the potential to increase the efficiency of the solar cells to45%. In addition, much attention surrounding DSSCs has been concentrated on the study of sensitization of n-type semiconductors, n-DSSCs, over the past20years. However, theoretically the simultaneous sensitization of both the n-type semiconductor and the p-type semiconductor in a tandem cell can lead to42%of the power conversion efficiency. As a result, increasing research activity has been focused on p-DSSCs.Tetracene has been extensively studied in the region of SF and its SF quantum yield has reached200%. But its instability towards light and oxygen in solutions results in great challenge for the investigation of SF in solutions. The introduction of substituents to the backbone of tetracene can tune the energy levels of the frontier molecular orbital and the energy levels of the excited states, and then affect their stabilities and SF efficiency. The effects of substituents on stabilities and SF efficiency have been systematically discussed in chapter2and3. Perylenetetracarboxylic Diimide (PDI) derivatives have been widely used as photoelectric materials for their excellent photochemical and thermal stability. A new p-DSSCs sensitizer was preliminarily studied in chapter4. The research work of this thesis includes:Chapter1:The concept and the mechanism of SF have been thoroughly introduced first, and then the recent research progress, including the development on new SF organic molecules, new SF theories, new techniques for SF studies, have been reviewed. The recent development on the sensitization of p-type semiconductors by organic dyes have also been reviewed in this chapter..Chapter2:A series of tetracene derivatives with different substituents at different positions of the tetracene backbone were designed. The energy levels of frontier molecular orbital and the relevant excited states were calculated with density functional theory (DFT) and time-dependent density functional theory (TDDFT), respectively. Introduction of multiple electron-withdrawing groups at a position is an efficient way to improve the stabilities of tetracene derivatives and reduce the activation energy for SF.Chapter3:The frontier molecular orbital energies and the activation energy of SF of a series of norbomadiene-linked tetracene dimer derivatives were calculated with DFT and TDDFT. The Davydov splitting and the electron-transfer integrals for measuring the coupling strength were obtained. Substitution on the tetracene backbone does not make great impact on the coupling strength between the tetracene subunits and the driving force for SF in the "face to face" stacked dimers. However, the relative orientations between the two tetracene rings can affect the coupling strength and the driving force of SF significantly. Enforced coupling strength can accelerate the rate of SF.Chapter4:We have designed and synthesized a p-type DSSCs sensitizer, which composed of a PDI acceptor and triphenylamine donor. The frontier molecular orbital energies were calculated and the absorption spectra were simulated. The simulated spectrum has closely resembled that of the experimental recorded one, which suggested that our calculated results are reliable. By comparing the calculated HOMO energy with that of NiO, we can conclude that the sensitization of NiO with this new dye is theoretically possible.