Synthesis And Application Of The Large D-A-D Conjugation π-bridge-based Metal-free Organic Dyes For Dye-Sensitized Solar Cells

Dye-sensitized solar cells(DSSCs) are considered to be one of the most promising solar cells owing to their unique advantages of low materials cost, simple fabrication process, easy colour adjustment and high power conversion efficiency. Sensitizer, as one of the key components in the DSSCs, plays a critical function in light harvesting and electron injection. That is to say, the ability of sensitizer directly determines the quality of the DSSCs. Among many photosensitizers, metal-free organic dyes have currently attracted worldwide scientific research attentions due to their high molar extinction coefficients, ease of structure tuning and environmental friendliness.In this thesis, a series of novel donor-acceptor-donor(D-A-D) conjugation π-bridge-based organic dyes are designed and synthesized, which have been successfully applied in DSSCs. The structure-performance relationships of the dyes and soalr cells are systematically studied through investigating their photophysical, electrochemical and photovoltaic properties. The specific research works are mainly based on the following parts.In the second chapter, four new metal-free organic dyes DTP1-4 are synthesized, where the D-A-D large conjugation structure dithienopyrrolobenzothiadiazole(DTPBT) is employed as a π-spacer. The effect of the dyes’ absorption spectra, energy levels and photovoltaic performances are studied with comparing the different electron donors(triphenylamine and phenothiazine) and disparate positions of inserted thiophene unit. The results indicates that the four dyes exhibit high molar extinction coefficients, which may be attributed to the incorporation of the large planar structure of DTPBT. In addition, due to the similar structure to the propeller of triphenylamine and the larger HOMO energy value, triphenylamine-based dye obtains much better performance than phenothiazine-based dye. Moreover, introduction of the thiophene unit into the π-spacer can extend the absorption spectra but leads to serious intermolecular aggregation and the decreased electron lifetime. Consequently, triphenylamine-based organic dye DTP4 obtains the highest efficiency of 7.55% with 1 mM CDCA as the co-adsorbent.In the third chapter, on the basis of previous work, four novel metal-free organic dyes DT1-4 with D-A-D featured dithienopyrrolobenzotriazole(DTBT) unit as π-spacer are designed and synthesized. Simliar to DTP1-4, DT1-4 exhibit good light harvesting ability with high molar extinction coefficients(5.9–7.0 × 104 M-1 cm-1) and broad spectral response. It is found that the different alkyl chain groups on the DTBT unit greatly influence the photovoltaic performance of the dyes. The branched alkyl chain(2-ethylhexyl chain) is superior to the liner alkyl chain(hexyl chain) in suppressing charge recombination and restricting intermolecular aggregation. In the end, under standard AM 1.5G solar light station, DT4 sensitized solar cell acheives the best power conversion efficiency of 8.05%.In the previous sections, most of the dyes obtaining high efficiencies are greatly depended on the co-adsorption of CDCA. Thus, in the fourth chapter, we hope to reduce the intermolcular π-π stacking of dyes through shorten the large planar conjugation structure. A series of new dyes DQ1-5 containing a dipentyldithienoquinoxaline(DPQ) moiety are synthesized, where DPQ with a D-A-D structure is applied as a π-spacer. When the electron donating ability of the dye is stronger, the HOMO value will drop and the absorption spectrum will red-shifted which affect the performance of the solar cell. In addition, it is found that the inserting of electron-rich group 3,4-ethylenedioxythiophene(EDOT) and electron-withdrawing unit benzothiadiazole(BTD) into the π-spacer can significantly narrow the HOMO-LUMO energy gap, broaden the IPCE response. The incorporation of EDOT will greatly upshift the HOMO energy level and decrease the electron lifetime and overall conversion efficiency. On the other hand, the introduction of BTD causes larger IPCE value, better electron transition properties and more effective suppression of charge recombination. Finally, all the five dyes show no aggregation in the TiO2 films, and dye DQ5 with indoline as the donor and BTD inserted in the π-spacer obtains the best efficiency of 7.12%.In the five chapter, two new double D-π-A branched organic dyes(DB-D and DB-B) with the hexylene linkage linked in the donor and the π-bridge position, respectively, are synthesized. For comparison, we also synthesize the corresponding single D-π-A branched organic dye SB-B. The results show that the hexylene connector location has a small influence on the photo-physical and electrochemical properties, but a significant impact on the dye aggregation degree, dye loading amount and light harvesting ability, thus leading to a disparate photovoltaic performance. DB-D with the linkage linked in the donor part exhibits better suppression of intermolecular aggregation in TiO2 surface than DB-B with linkage connected in the π-bridge position. In comparison with the efficiency(4.32%) of the single D-π-A branched organic dye SB-B, the efficiency(6.13%) of DB-D is enhanced to about 42%. However, the efficiency(3.65%) of DB-B is dropped about 16%.