The Theoretical Study On Singlet Fission Of Tetracene And Pentacene Derivatives

Solar cell research attracted a lot of attention due to the global warming problem. However, the high cost resulted from the low photoelectric conversion efficiency restricts the widely use of photo-voltaic cells. Hence, the key is to find a way to promote the photoelectric conversion efficiency. Among the various factors, which influence the photoelectric conversion efficiency, the most important one is the materials applied to the cells have no ability to fully convert the high-energy photons into electrical energy. When the photon energy is higher than the band gap of the semiconductors, the excess energy of the photon will dissipate in the form of heat. Meanwhile, the low energy photons, such as infrared light, cannot be absorbed by the cells. If using singlet splitting (Singlet Fission, SF), making a absorbed high-energy photons splitting into two low-energy photons, and injecting the produced two electron/hole pairs into the battery electrodes, the high-energy photons will be fully used and the photoelectric conversion efficiency could breakthrough the Shockley-Queisser limit and promote to as high as 46%.Both tetracene and pentacene are recognized as the most promising compounds for carried out SF due to the small molecular weight, simple molecular structure and the high yields of triplet states. However, the annoying problem is that these acene-based materials degrade easily in the presence of light and air due to the photo-oxidation. Functionalization of organic molecules is used to be a strategy to change the solubility and photostability, and the excited-state energy levels can also be tuned at the same time. Considering the fact that N-heteroacenes and thiophene-based materials exhibit unique and excellent electronic and optical properties, we designed a series of sp2-hybridized nitrogen-containing tetracene and pentacene derivatives, as well as a group of thiophene-containing tetracene and pentacene compounds in this thesis, and discussed the effects of nitrogen and thiophene on the stability and the properties related to SF of tetracene and pentacene in detail. Specific work is as follows:Chapter 1:The research background, mechanism, thermodynamic energy condition and current situation of the development of SF have been outlined, and the above aspects in tetracene and pentacene have been discussed in detail. Then, the influence of substituent, nitrogen heterocyclic and thiophene ring on stability and the parameters related to the photoelectric properties of tetracene and pentacene have been introduced.Chapter 2:Using the density functional theory (DFT) and time-dependent density functional theory (TDDFT), we investigated the effects of substituted position, number and the substituted pattern (pyridine ring, ring, pyridazine pyrazine ring) of sp2-hybridized nitrogen atom (=N-) on the frontier molecular orbital energies and excited state energies related to singlet fission (SF). The introduction of nitrogen atom significantly decreases the energies of frontier molecular orbitals and hence improves their stabilities. For the molecules with an exothermic SF, introducing nitrogen atom will reduce the energy loss of singlet fission and improve the SF efficiency. On the contrary, for the molecules with an endothermic SF, introduction of nitrogen atom has the opposite effect.Chapter 3:The effects of the introduction of thiophene ring on structure geometries, frontier molecular orbital energies and ability of SF for some tetracene and pentacene derivatives were theoretically investigated by quantum chemical methods. For fused benzene-thiophene structures, the (β,β) connection pattern can reduce the HOMO-LUMO gap and enhance the diradical character, hence will promote the thermodynamic driving force of singlet fission. But the (α,β) connection pattern results in a increased HOMO-LUMO gap, and the thermodynamic driving force of SF will decrease or even disappear.