Study On Photovoltaic Materials For Dye-sensitized Solar Cells

Dye-sensitized solar cells （DSSCs） have attracted extensive attention due to theirsimple preparation, low costs, and environmental friendly. They are the research hotspotin the field of solar cell in recent years. The organic dye plays an important role inDSSCs. The photophysical and electrochemical property of the organic dye hassignificant influence on the photovoltaic performance of DSSCs. Based on simplerod-like D-π-A organic dye, a series of starburst2D-D-π-A organic dyes have beenengineered and synthesized for studying the effects of molecular structure of the dye onthe photophysical, electrochemical and photovoltaic properties in this dissertation. Thestructures of the dyes have been characterized by mass spectra （MS） and nuclearmagnetic resonance （NMR） technologies. The photophysical and electrochemicalproperties of the dyes were studied. DSSCs based on these dyes were constructed anddetailed relationship between dye structure and solar cell performance has beeninvestigated.For the rod-like D-π-A organic dyes with different electron donors （carbazole forTC301, phenothiazine for WD1and diphenylamine for L0）, the dye L0with thestronger electron donor diphenylamine has the greater absorption property. The groundstate redox potential of L0is more positive than the I/I3redox potential relativetoTC301and WD1, indicative of the more significant driving force for the reduction ofthe oxidized dye. In comparison with TC301and WD1, the photovoltaic performanceof L0with the diphenylamine group as eletron donor is significantly improved.Electrochemical impedance spectroscopy was performed to elucidate the difference ofthe photovoltaic performance of the three dyes. The DSSC based on L0gives the bsetoverall conversion efficiency2.03%.The dithiafulvenyl （DTF） units as additional electron donors were introduced intorod-like D-π-A organic dye L0to form D-D-π-A organic dye WD2and starburst2D-D-π-A organic dye WD3. The introduction of DTF unit has a good influence onimproving the absorption property and preventing the molecular π-π aggregation. TheHOMO and LUMO energy levels of the two dyes agreed well with the requirement for an efficient photosensitizer of DSSCs. The research results indicate the short circuitphotocurrent as well as the open circuit voltage increase significantly with theincreasing of DTF unit, leading to a linear increment in device efficiency with DTF unit.Electrochemical impedance spectroscopy was performed to elucidate the difference ofthe photovoltaic performance of the dyes. The best performance was found in starburst2D-D-π-A organic dye WD3, in which two DTF units were introduced into atriphenylamine unit. It displayed an overall conversion efficiency of4.41%.The phenothiazine and carbazole units as additional electron donors wereintroduced into rod-like D-π-A organic dye L0to form starburst2D-D-π-A organic dyesWD4-7, in which the cyanoacetic acid and rhodanine-3-acetic acid as the differentelectron acceptor groups. The absorption bands of WD5and WD7withrhodanine-3-acetic acid as acceptor exhibits obvious red-shift compared to that of WD4and WD6with cyanoacetic acid as acceptor. This shift is owing to the strongelectron-withdrawing ability of rhodanine-3-acetic acid. In addition, the introduction ofphenothiazine and carbazole with a non-planar conformation in triphenylamine donorpart has a good influence on preventing the molecular π-π aggregation. The researchresults indicate that the cyanoacetic acid acceptor favors better properties of DSSCsthan that of rhodanine-3-acetic acid acceptor in these dyes. The lower efficiency of solarcell based on dye with rhodanine-3-acetic acid acceptor is mainly because thedelocalization of the excited state is broken between the4-oxo^-2-thioxothiazolidine ringand the acetic acid, which affects the electron injection from the dye to the conductionband of TiO2. The best performance was found in dye WD4, in which phenothiazineunit as additional electron donor and cyanoacetic acid as electron acceptor. It displayedan overall conversion efficiency of4.54%.The introduction of benzene, thiophene, and furan as conjugated linker into theWD4with better photovoltaic performance to from starburst2D-D-π-A organic dyesWD8-10. The effects of the different conjugated linkers on the photophysical,electrochemical and photovoltaic properties of these organic dyes were investigated.The absorption bands of WD9（thiophene linker） and WD10（furan linker） wasred-shifted compared with that of WD8with benzene linker. This was due to betterelectron donating abilities of WD8and WD10. The performances of the DSSCs basedon the WD8-10in the presence of CDCA were studied. CDCA did not improve the device’s performance, however, it decreased the power conversion efficiency. Apossible explanation was that the amount of dye adsorbed on the TiO2surface wasreduced by the coadsorption of CDCA, resulting in a loss of active light harvesting. Thebest performance was found in WD10, in which furan unit as conjugated linker. Itdisplayed an overall conversion efficiency of6.79%. Under the same experimentalconditions, the reference dye N3generated an efficiency of8.0%.