Insight To The Structural Modifications And Photovoltaic Performances Of Organic Sensitizers

The research and development of renewable energy have got much more attentions due to the increasing risk of energy crisis.The dye-sensitized solar cells(DSSC)has become the most promising photovoltaic technology because of its low cost,high photoelectric conversion efficiency(PCE),easy to make flexible devices and easy building integration.The DSSC have been the star of photovoltaic technology since the Gratzel group has reported the breakthrough work in 1991.Now many groups all over the world are devoting themselves to improving the PCE and durability of DSSC.At present,the PCE based on ruthenium and porphyrin dyes is 11.9%and 13%,respectively,while the PCE based on metal-free organic dyes has reached 14%.Therefore,metal-free organic dyes provide a new way to further improve the efficiency of DSSC.Metal-free organic dyes have obtained more and more attentions due to simply synthesis,flexible structural modulation and high molar extinction coefficients,but their structural and performance relationship is still not very clear.In general,the effective sensitizers usually adopt D-π-A configuration.Therefore,a series of new D-π-A dyes are designed to get insight to the effects of structural adjustment on the performance of DSSC.The main contents include the following three parts:(1)Herein four novel D-π-A dyes based on triphenylamine(ZHG1,ZHG2,ZHG3 and ZHG4)by modifying the π-bridges have been prepared.Compared with ZHG1,the power conversion efficiency(PCE)of ZHG2 is improved to 6.1%after the introduction of ethynyl.But further extending the conjugation of π-bridges by introducing chromophore 4,8-bis(n-octyloxy)-benzo[1,2-b:4,5-b’]dithiophene(BDT)in ZHG3 conversely decreases the PCE to 4.6%.Improving the coplanarity by replacing the cyclobenzene with thiophene in ZHG4 after introducing BDT further decreases the PCE of ZHG4 to 4.3%.The theoretical calculations indicate that the LUMOs of ZHG3 and ZHG4 mainly delocalize over benzothiadiazole which is far from anchoring groups.Cyclic voltammetry experiments indicate that LUMOs energy level of ZHG3 and ZHG4 are lower than that of ZHG1 and ZHG2.These results both affect the ability of injecting electrons into the TiO2 conduction band.The X-ray photoelectron spectroscopy(XPS)shows that the mean thickness of dye coverage for ZHG1,ZHG2,ZHG3 and ZHG4 is 16 A,18 A,27 A and 24 A,respectively.So tilt angle of dye backbone anchored on TiO2 film is in the order of ZHG1>ZHG2>ZHG4>ZHG3,which is consistent with the dye coverage of outmost TiO2 surfaces.This result indicates that the intermolecular π-π aggregation in ZHG3 and ZHG4 with overlong π-bridges is more serious compared with ZHG1 and ZHG2.Perhaps the above two factors are the reason that PCEs of ZHG3 and ZHG4 are lower than ZHG1 and ZHG2.So it is very important to find a balance point between electron injection ability,intermolecular π-π aggregation and the expansion of the light absorption range.(2)Three new sensitizers ZHG5,ZHG6 and ZHG7 have been prepared by gradually improving the steric hindrance of auxiliary donors and the power conversion efficiencies(PCE)are 5.64%,5.32%and 2.74%.UV-Vis absorption indicates that the molar extinction coefficients decrease with the increased steric hindrance of auxiliary donors.X-ray photoelectron spectroscopy(XPS)indicates that the tilt angles of ZHG5 and ZHG6 anchored on the TiO2 film are similar and ZHG7 is almost standing rather vertical on the TiO2 film with the smallest tilt angle.Dye desorption experiment and XPS indicate that the dye loading amount of ZHG6 with larger steric hindrance is lower than that of ZHG5,ZHG7 with largest auxiliary donor has the maximum loading amount due to its smallest tilt angle.Larger auxiliary donor of ZHG6 leads to higher open circuit voltage(Voc=734 mV)but lower shorter circuit current(Jsc=12.06 mA cm-2)compared with that of ZHG5(Voc=730 mV,Jsc=12.63 mA cm-2).However,dense packing of dye ZHG7 anchored on the TiO2 leads to more serious intermolecular π-π aggregation effects.Perhaps this effect and lowest molar extinction coefficient are the reason that DSSC based on ZHG7 have the lowest PCE.So above results indicate that auxiliary donor with overlarge steric hindrance will have smaller tilt angle of dye anchored on TiO2 and lead to more dye loading amount but serious intermolecular π-π aggregation effects.(3)The anchor groups are the key factor for improving the PCE and durability of DSSC because they are the electron acceptors.So it is an efficient way to improve the PCE and durability of DSSC by developing new anchor groups.Three sensitizers ZHG7,ZHG8 and ZHG9 based on the phenothiazine with different anchoring groups have been prepared.ZHG7 adopts cyanocrylic acid as acceptor.The cyano of cyanocrylic acid has been replaced by acetyl amino and pyridine in ZHG8 and ZHG9,respectively.Dye desorption experiment and photovoltaic performance test have proved that ZHG8 have much stronger dye adsorption stability,regardless of acid or base solution,and the power conversion efficiency(PCE)of ZHG8(2.33%)is just a little smaller than that of ZHG7(2.74%).ZHG9 exhibits strongest adsorption stability but with lower PCE(0.37%).Analysis of FTIR spectroscopy indicates that the acetyl amino in ZHG8 adopts bidentate binding mode adsorbed on TiO2 film and pyrindine in ZHG9 can form coordination bond between anchoring group and Lewis acid sites of the TiO2 surface.The above two aspects can be considered as the key factor of stronger dye adsorption ability.We think these results can provide some enlightenment for the future design of the novel dyes with stronger adsorption ability.