Design And Photoelectric Properties Of Porphyrin Sensitizers For Increasing Of Long Wavelength Absorption

In this paper, three strategies have been employed to improve the longwavelength absorption of porphyrin-based sensitizer systems in DSSC, i.e. reductionof the porphyrin to dihydroporphyrin, co-sensitized system between Chlorin-typesensitizers and BODIPY sensitizers, and construction of bis-porphyrin system.Chlorin-type framework with long wavelength absorption has been prepared bythe1,3-dipolar cycloaddition between A3B-porphyrin P0and nitrile oxides(2,6-dichlorophenyl nitrile oxide and2,4,6-trimethylphenyl nitrile oxide). Then afterthe metallization of the monoadducts by Zn2+and the following hydrolysis ofCOOMe group, Chorin-type sensitizers ChD1a-4a with2,6-dichlorophenyl group andChD1b-4b with2,4,6-trimethylphenyl group were obtained and characterized by1H-NMR, MS, UV-Vis, fluorescence, cyclic voltammetry, I-V and IPCE spectra, aswell as the TD-DFT calculation. By analysis of the above results, it was found that thephotovoltaic performance increased with growing absorption intensity of Q band onTiO2film. This result indicated the contribution of intrinsic enhanced absorptionproperties of chlorins in Q-band regions on the improvement of the overallphotovoltaic performance in DSSCs. On the other hand, their capability of solarenergy conversion also exhibited close relationship with steric structures of thesensitizers. For ChD1a-4a, the lower steric hindrance group would aggregate in thesolution when the dichlorophenyl group locate at10-position of Chlorin framework.Thus the orientation of steric hindrance group to the5-or15-position (the anchorgroup is at20-position) gave higher solar energy-to-electricity conversion efficiency.By contrast, ChD1b-4b with the higher steric hindrance group would preventaggregation, and the orientation of steric hindrance group to the10-position (theanchor group is at20-position) afforded higher solar energy-to-electricity conversionefficiency.BODIPY Bod0has been synthesized by using2,4-dimethylpyrrole andmethyl-4-formylbenzoate as the starting materials via condensation-,oxidation-reaction and the following coordination reaction with BF3OEt2.Subsequently, BODIPY sensitizers BodD1-4with strong absorption in longwavelength area were obtained by the Knoevenagel reaction between the Bod0and electron-rich aromatic aldehyde (e.g.4-(N,N-diphenyl)aminobenzene aldehyde,4-(N,N-dimethyl)aminobenzene aldehyde) followed by hydrolysis of COOMe group,and their structures have been characterized by1H-NMR and MS spectra. On the basisof co-sensitization of ChD1a and BodD2, the optimized molar ratio between ChD1a:BodD2was found to be8:1by comparing the spectra of UV-Vis in solution, UV-Vison TiO2films, surface coverage, I-V and IPCE in different molar ratios. According tothe optimal molar ratio, the photovoltaic performance of two co-sensitization systemChD1a/BodD1and ChD1a/BodD2have been evaluated. The system ofChD1a/BodD1gave better photo to electricity efficiency. This could be due to thetwo aromatic group in BodD2would undergo π-π stacking in gaining of aggregationwhich will decrease the total photo to electricity efficiency.Porphyrin intermediates P1and P2have been prepared via [2+2] method with4-methyldipyrromethane and methyl-4-formylbenzoate as the starting matrials. Aftera serials of reaction, i.e. reduction, oxidation and Wittig reaction, four porphyrindimers were obtained, where two of them contain COOMe group at meso-postion offree base porphyrin moiety and the other two contain CHO group at meso-postion offree base porphyrin moiety. Finally, two bisporphyrin sensitizers DPD1-2wereprepared by hydrolysis of the COOMe group and characterized by1H-NMR and MSspectra. Furthermore, their optical properties, chemical properties and photovoltaicperformance have been measured to compare with those of the monomers MPD1-2.In the fluorescence emission spectra of DPD1-2, the fluorescence intensity of Zincporphyrin moiety decreased and the intensity of free base porphyrin increased. Thisresult indicated that the electron/energy transfer from Zinc porphyrin unit to free baseporphyrin unit in the presence of light and free base porphyrin unit could act as a“electron-hub” between the electron from Zinc porphyrin to TiO2, which wouldprevent the electron back and increase the injection efficiency to TiO2. Moreover, inthe EIS measurement, the large moleculars of bisporphyrin decreased the combinationbetween photoelectron and I3-, and so the resistance of bisporphyrins in dark increasedwhich demonstrated the decrease of dark current. By analysis the photovoltaicperformance of the four sensitizers, DPD2gave the best result among them. In thecase of DPD1, its poor solubility lead to the lower adsorption on TiO2film, so itdidn t give better photo electricity conversion efficiency.