Theoretical Studies On Photovoltaic Properties Of Organic Materials Containing Nitrogen, Oxygen，and Sulfur Atoms

Compared to inorganic materials, the organic materials in solar cell applicationsdisplay the advantages of the low cost, easy modification on the molecular level andbeing1easily made into a flexible film of large area. Therefore, more and moreattentions have been paid to the research of the organic photovoltaic materials. But thephotoelectric conversion efficiency of organic photovoltaic cells is much lower thanthat of the inorganic ones. In2011, Yella etc. reported that they had raised thephotoelectric conversion efficiency of organic photovoltaic cells to12.3%, which isstill far away from the large-scale production and application. Therefore, the mosturgent for the researcher is to try to improve the photoelectric conversion efficiency oforganic photovoltaic cells. Usually, there are two resolutions on improving the energyconversion efficiency of organic solar cells: One method is to look for new materialswith good photovoltaic performance; the other one is to optimize the structure of thephotovoltaic cells. For searching the better organic photovoltaic materials, manyexperiment chemists have synthesized large amount of new organic compound. Onthe other hand, theoretical chemists also perform the theoretical research on designingand properties. In deed, the theoretical research results can provide the reasonableguide and forecasting to experiment synthesis.In this thesis, we adopted theoretical calculation methods to design the new kindsof organic photovoltaic material containing N, O and S atoms, the geometrical andelectronic structures and the photographic properties of the materials were explored.The photovoltaic performance of the material was evaluated theoretically. Thepurpose of the work is to find the organic photovoltaic material with better propertiesand to explore the tuning law to the photovoltaic material on the molecular level. In the first part of the work, we designed five organic compound containing thethiophene, quinoxaline and pyridine group as the main block by the introduction ofthienopyrazine （TP） and benzothiadiazole （BT） groups. Introduction of TP and BTgroup resulted in the increase of the HOMO energy level and the decrease of theLUMO energy level, then a much narrow bandgap and the red-shifted absorptionspectrum. On the other hand, due to the incorporation of TP and BT group, theionization potentials decreased while the ion affinities increased, the reorganizationenergy of both electrons and holes turn smaller and the difference between the twowas significantly reduced. Therefore, the decorated to the molecules improved thecharge transport property. The theoretical investigation revealed that the introductionof the modifier group could effectively regulate the electronic structure of themolecule, thereby improving the photovoltaic performance of the material.In the second part of the work, nine donor-acceptor-donor （D-A-D） typephotovoltaic molecular systems with the building block of thiophene and benzotriazole group were designed and the geometrical and electronic structures and thephotographic properties of the materials were explored. Introducing an electronwithdrawing group in the electron-deficient benzotriazole group can play the role ofadjusting the bandgap, absorption spectra and the charge-transporting properties. Theinvestigation announced that the introduction of a phenyl or substituted phenyl groupon benzotriazole led to the absorption spectra of the systems to extend to600-700nm,the strong sunlight irradiation region, in the meanwhile the charge transport abilitywere improved, especially in the respect of the equilibrium properties of electron andhole transport. These findings pointed the direction for the further regulation of themolecular photovoltaic properties with other electron-withdrawing groups.In the third part of the work, we designed nine kinds of D-Ï€-A type of dyemolecules on DSSC （dye sensitized solar cell）, wherein the donor unit was2,6-dimethyl-4-ylidene pyran, while the Ï€ conjugated bridge portion was selected assingle-thiophene, fused thiophene and dule-thiophene ring respectively, and the2-cyano-acrylic acid, the propynoic acid and2-nitro-acrylic acid was arranged as the acceptor unit respectively. By the theoretical calculation, the frontier molecularorbitals, absorption spectra, and the adsorption of TiO₂particles as well as the excitedstate lifetime of the dye molecules were explored. The investigation results displayedthat the dyes containing dual-thiophene Ï€ bridge unit showed better photovoltaicproperties than the ones with single-thiophene and fused thiophene Ï€ bridge units,meaning the positive significance of the growth of theÏ€conjugated bridge on theimprovement of their sensitization performance. On the other hand, CN-3, NO-3andthree dyes with the propynoic acid acceptor unit were able to effectively absorb thesunlight with wavelength longer than400nm, possessed the nanosecond scalelifetime in the excited state to ensure that the electrons successfully transferred to theconduction band of the semiconductor, and could generate stable adsorption with TiO₂,all of which are the premise for the dyes to act the sensitization. The results indicatedthat the propynoic acid and2-nitro-acrylic acid can be the promising selection as theacceptor unit on constructing the DSSC dye besides the classic2-cyano-acrylic acidunit, and it can be predicted that by selecting the appropriate donor and Ï€conjugated bridge unit can make the dye performance being further improved.