Theoretical Studies On The Optical Properties Of The Sensitizers And Molecular Design

The organic sensitizers, with high molar extinction coefficients, low cost and flexibility in terms of molecular design, have been widely applied in the fields of dye-sensitized solar cells (DSSC) and organic bulk-heterojunction solar cell (BHJ). As an important component of DSSC, the sensitizer and the interactions between sensitizer and semiconductor electrode play an important role in the photoelectric conversion efficiency of DSSC. In this thesis, density functional theory (DFT) and time-dependent DFT (TDDFT) are employed to investigate the ground-and excited-state properties of isolate sensitizer and sensitizer-TiO2 clusters adsorption systems. Besides, a series of sensitizers are designed and theoretically studied. This thesis aims at deeply understanding the photoelectric conversion of DSSC and seeking approaches to improve the performance of DSSC. The thesis is organized as follows:First, the structure-property relationship of the three indole-based organic dyes (WS-2, WS-6 and WS-11) is theoretically investigated. From the partial density of states (PDOS), polarization, frontier molecular orbital and charge difference density distribution, the influences of different group to the photochemistry property of the sensitizers are demonstrated. It is found that the introduced alkyl chain of WS-6 will not change the ability of the light absorption and charge transfer of the sensitizer, but only spatially suppresses the interaction between the neighboring dyes. The insertion of the hexylthiophene near the donor of WS-11 breaks the coplanar structure between intramolecular acceptor unit and conjugated bridge, which is unfavorable for the charge transfer process of the organic sensitizers.Second, spectral response and light absorption strength of photosensitizer determine the efficiency of DSSC, and developing more efficient dyes has become an important means for the improvement of DSSC performance. XS54, XS55 and XS56 are three high-efficiency organic sensitizers, and DSSC based on XS54 possesses the highest photoelectric conversion efficiency. However, these sensitizers have weak response in red region of the UV-Visible absorption spectrum. In order to extend the spectral response of the organic dyes, we design a series of new sensitizers based on XS54 molecule. Porphyrins and benzothiadiazole (BTD) unit were introduced into the conjugated unit of XS54, and the gained sensitizers P5 and P6 possess broad absorption spectrum and strong ability of charge transfer, which make up for the shortage in the light absorption in red region for XS54.Third, the ground-and excited-state properties of the triphenylamine-based organic dyes TC1, TC2, TC3 and TC4 are theoretically studied, and TC4 possess the widest spectral response and strongest absorption. Since the photoelectric conversion of DSSC is the electron transfer from the excited sensitizer to the semiconductor electrode surface, the sensitizer/TiO2 cluster adsorption systems TC4-(TiO2)n (n=1～8) are established. The calculations indicate that the adsorption system TC4-(TiO2)6, with the dye molecule bonding to three-fold coordinated Ti atom of the cluster, possesses the narrowest HOMO-LUMO energy gap and strong interaction between the sensitizer and TiO2 cluster, benefiting for the light absorption.At last, TP and TT are two donor molecules for small organic solar cells, and they respectively self-assemble into centrosymmetric dimers in the films. This kind of supramolecular arrangement promotes the electron transfer in the solar cell, and then influences the performance of solar cell. Besides, the complicated electronic field in the solar cell may be another influence factor to the charge transfer. Therefore, we theoretically investigated the charge transfer process of TP and TT dimers in the electric field with different directions and strengths. It is found that the electric field parallel to the conjugated unit of sensitizer promotes the intermolecular charge transfer between the two molecules of dimer system. Under the effect of electric field, intermolecular charge transfer happens more easily on TP dimer than TT dimer, which might be caused by the larger overlap area between the conjugated units of the two TP molecules.