Theoretical Study And Rational Design Of Some Novel Organic Dye Sensitizers

Dyes-sensitized solar cell (DSSC), as a promising renewable energy conversion device for solving "Terawatt Challenge", is receiving an ever-increasing amount of attention due to their flexibility, easy preparation, environmental-friendly, low cost and high efficiency. Currently, the power conversion efficiency (PCE) of DSSC still lags behind that of the traditional silicon-based solar cells, which will retard their large-scale commercial application. As a key component of DSSC, sensitizers play an important role in affecting the lighting harvesting efficiency, the interfacial charge transfer process and ultimately the overall PCE. However, the experimental design and optimization of sensitizers for the improvement of PCE is still a grand challenge because of the complicated and intricate interfacial charge transfer process. Importantly, quantum chemical calculations could provide insight into the influences of DSSC components on the PCE at a molecular level, which is crucial for us to better understanding the operational mechanism of DSSC. Thus, quantum chemical calculations manifest themselves as a reliable and suitable avenue to boost the performance of DSSC.In comparison with metal complex dyes, metal-free organic dyes with Donor-π-linker-Acceptor (D-π-A) structural motif have the desirable advantages such as strong molar absorptivity, low cost, synthetic flexibility in molecular design. Herein, density functional theory (DFT) and time-dependent DFT (TDDFT) calculations on Chlorophyll a, INI and fused porphyrin sensitizers were carried out to demonstrate the effects of sensitizers on light-harvesting efficiency in an attempt to shed light on the structure-property relationships and thus to provide theoretical guidelines for the further optimization and design of organic sensitizers. The details are described in the following:1. Nine new D-π-A metal-free sensitizers, INI1-INI9, based on indolizino[3,4, 5-ab]isoindole(INI) as an electron donor, were investigated using density functional theory (DFT) and time-dependent DFT calculations. Compared with D5 and D9, some major factors affecting the performance of the cell, including light harvesting, electron injection, dye regeneration, and charge recombination, are taken into consideration. Our calculations show that these novel INI-based sensitizers have an absorption maximum at 440-500 nm when their π conjugated bridge is attached at different positions on the aromatic ring, and an excellent charge separation characteristic. INI2 shows better performance than D9 owing to a theoretical maximum short-circuit current density of 13.26 mA·cm-2. Fortunately, condensed Fukui function calculations suggest that INI2 is the easiest of the proposed sensitizers to obtain owing to its largest nucleophilic index at 2 position of the INI aromatic ring. Based on calculations of the adsorption of the dyes on a TiO2 cluster, indirect electron injection may be the main path from dye to TiO2 for INI2 and D5. Our calculations indicate that the proposed INI dyes will be promising candidates for high-performance dye-sensitized solar cells.2. The electronic spectral properties of Chlorophyll a and its seven meso-aza. substituted derivatives as an photosensitizer for dye-sensitized solar cell (DSSC) have been studied using time-dependent density functional theory(TDDFT) with tuned range-separated hybrid density functional theory(RSH-DFT). Aza-substitution effect on Qy band are discussed by analyzing frontier molecular orbital and electronic absorption spectrum. The results show that the tuning range-separated hybrid density functional LC-coPBE* (range separation parameters ω=0.65) with 6-31G(d) basis set performs best for the Qy transition band of Chlorophyll a. Aza derivative have the red-shifted and more intense Qy absorption bands in the NIR region as compared to that of Chlorophyll a because of the lower energy gap and increased transition dipole moment along y axis of molecules. Based on the charge density difference analysis for the lowest transition of dyes adsorption on (TiO2)48 cluster, its suggests that indirect electron injection be the main path from dye to TiO2 conduction band, which cause to the lower photoelectric conversion efficiency for present Chlorophyll dyes.3. Density functional theory (DFT) methods were employed to calculate series dyes of D-π-A type perylene dianhydride fused porphyrins with different donor groups. The geometry, frontier molecular orbitals, electronic absorption spectra and charge-transfer properties were explored systematically. The results show that by increasing the electron-donating ability to conjugate and increase the strength of the group, can effectively regulate the LUMO level and then change the size of the energy gap, electron absorption spectra and charge transfer characteristics. The results show that changes in the spectral properties of the central metal porphyrin derivatives greater impact; through enhanced donor electron donating groups, to increase the strength of the body conjugate groups, the compounds can effectively enhance the efficiency in the near infrared absorption at significantly improved photovoltaic properties of these compounds. Such molecules to improve the efficiency of absorption of solar energy has important potential value.