Theoretical Study On The Photophysical And Charge Transport Properties Of Fluorene-and Carbazole-Based Derivatives

Due to the excellent light-emitting and charge transport properties, organic^-conjugated molecules hold considerable attention in organic light-emitting diodes (OLEDs), organic field effect transistors (OFET), and organic photovoltaic cells (OPVs), and they have become very important research contents on the optoelectronic applications. Recently, with many advantages, such as wide variety of sources, low production costs, flexible and large displays,π-conjugated organic semiconductors, especially for organic compounds containing fluorene and carbazole unit, have been focused on. Chemical modifications, could be used not only to restrict the nonradiative decay process and thus to improve the luminescence quantum efficiency, but also to tailore the molecular packings form and intermolecular interactions and thus to improve the charge transfer rate, leading to improve the efficiency of optoelectronic devices.In this paper, the electronic structures, optoelectronic properties, and charge transfer properties of a series of organic compounds containing fluorene and carbazole unit were investigated by quantum theoretical studies. The results suggest new theoretical basis and direction for design of novel organic materials. Our work mainly includes three aspects as follows.1. As an important building block for optoelectronic applications, various chemical modifications at C9-position of fluorene have been proposed to enhance its performance by suppressing the well-known keto effect. In order to identify different substitution effects on the photophysical and charge transport properties of fluorene, we systematically studies the electronic structures and photophysical behaviors of fluorene (FR) and its three dimerized counterparts, namely,9,9’-spirobifluorene (SBF),9,9’-bifluorenylidene (BFD), and bis(biphenyl-2-2-diyl)allene (BDA), by employing density functional theory (DFT) calculations. The changes in bond length alternation indicate that the geometrical relaxations of the fluorene unit in its dimerized derivatives are smaller than FR compound. This fact was further proved by the nonradiative decay rate estimated of the first excited singlet state for each compound. In addition, the injection abilities were evaluated in terms of the ionization potentials and electron affinities, and the carrier transport properties were discussed in the framework of Marcus theory. We find BFD could be a better ambipolar transport material, and BDA can be used as a high-efficient luminescent building unit with excellent hole transport property.2. To maximize the efficiency of heterojunction organic photovoltaics (HJOPVs), it is imperative to increase not only the open-circuit voltage (VOC) but also the short-circuit current (ISC). Therefore, it is desirable to find an organic acceptor material that possesses a higher LUMO level for higher VOC and can absorb photons in the solar spectrum efficiently for larger ISC.In this paper, in comparison to the typical donor poly(3-hexylthiophene)(P3HT) and acceptor [6,6]-phenyl-C61-butyric acid ester ([60]PCBM), the geometries, electronic structures, absorption spectra and transport properties of a series of organic compounds containing9,9’-bifluorenylidene (9,9’BF) were systematically investigated using density functional and the semiclassical Marcus charge transfer theory calculation to evaluate their potential severing as acceptor. Our results indicate that the absorption spectra of99’BF derivatives have better overlap with the solar spectrum than that of [60]PCBM, and higher LUMOs result in a significant enhancement of VOC when they are used in HJOPVs with P3HT as donor material. On the other hand, these compounds own higher electron carrier mobilities comparing with [60]PCBM. The study also demonstrates that the H-shaped compounds based on the99’BF backbone possess the good photophysical and charge transport properties, can be promising organic semiconductor for heteroj unction photovoltaics.3. Organic π-conjugated semiconductor materials have been extensively used in organic light-emitting diodes (OLEDs). Among them, carbazole derivatives are commonly used as hole-transporting materials. Here, the electronic structure and transport properties of Dibenzo[2,3:5,6]pyrrolizino[1,7-bc]-indolo[1,2,3-lm]carbazole (DiPICz) were investigated systematically by means of semiclassical Marcus charge transfer theory at the density functional theory level. The results indicate that the mobility of electron, is5.81×10-2cm-·V-1·s-1, which is higher two orders of magnitude than that of hole (6.02×10-4cm2·V·-1·S-1), indicative of abnormal electron-transporting materials. We further proved that both the small electron internal reorganization energies and the character of higher-order molecular packing in its single crystals are crucial to good electron transport properties. In the’herringbone’packing of DiPICz crystal, the presence of typical slipped face-to-face packing and the formation of weak hydrogen bonds such as C-H…N and C-H…π interactions cause two dimensional electron transport networks, which play an important role in electron transport.