Investigating The Effect Of Polyiodine Ions On The Performance Of Dye-sensitized Solar Cells Using In Situ Raman Spectroscopy

Dye-sensitized solar cells（DSSCs）are cost-effective photovoltaic devices that are expected to be commercialized due to their low cost and high efficiency.However,how to avoid and solve the performance mismatch that often occurs when DSSCs operate in natural environments,especially the problems caused by electrolytes,is still one of the biggest problems faced in the existing commercialization process.Therefore,in situ characterization methods and effective strategies are urgently needed to solve these problems.However,the existing electrochemical characterization methods cannot realize the in situ monitoring of DSSCs under working conditions,which greatly limits the commercialization process of DSSCs.In this thesis,the new species I₅^-generated by DSSCs under working conditions was monitored by in situ Raman spectroscopy,and the mechanism and distribution of the production were clarified by auxiliary electrochemical characterization,and evidence of the influence of I₅^-on the short circuit current(I_SC)of DSSCs was found,which provided a new idea for the commercial design of DSSCs.The following were mainly studied:（1）The production and distribution of polyiodide ions in DSSCs were studied by Raman spectroscopy.Five electrolytes C1～C5 with different I₂contents were prepared,and they were assembled into fiber-type dye-sensitized solar cells（FDSSCs）with good sealing,and the generation of I₅^-in the five FDSSCs was monitored by in situ Raman spectroscopy.Studies have shown that I₅^-is not produced even if the I₂content of the electrolyte alone is increased;I₅^-begins to be produced when assembled into complete FDSSCs;However,I₅^-cannot be produced in FDSSCs that do not contain dye N719.This indicates that I₅^-is a product of the work of FDSSCs and that its production is closely related to dye molecules.This conclusion has also been verified in the real-time monitoring experiment of Raman and electrochemical combination of FDSSC-C1～C5,that is,it is found that the Raman band intensity of I₅^-and dye N719 shows a trend of trade-off under different bias voltages.Specifically,after the Raman intensity of I₅^-near open circuit voltage(V_OC)decreased to 0in FDSSC-C1～C3,the Raman band intensity of dye N719 gradually increased.In FDSSC-C4～C5,the Raman band intensity of I₅^-did not disappear or disappeared very late after the bias pressure was greater than V_OC,and correspondingly,the Raman band of dye N719 did not appear or appeared very late.In addition,in situ Raman imaging was used to locate the distribution of species in intact FDSSCs.The results show that the I₅^-production position is consistent with the attachment position of the dye N719 molecule,that is,it is mainly concentrated on the photoanode,while the distribution position of I₃^-is closer to the Pt direction.To verify the accuracy of the results,we also performed in situ Raman monitoring of the photoanode and counter electrode of planar DSSCs with self-evaporating electrolytes,respectively.The results show that there is only a weak I₃^-(110 cm^-1)Raman band on the surface of the counter electrode（Pt）.For the surface of the photoanode,no obvious Raman band of polyiodine ions was detected on the surface of Ti O₂nanoarrays（TNA）near the electrolyte.High intensity Raman band of I₃^-(110 cm^-1)and I₅^-(168 cm^-1)were detected on the surface of TNA near conductive glass（FTO）.This further proves that I₅^-is concentrated on the side of TNA close to FTO（and this side is above,which can exclude the influence of gravity）.Since the proximity of TNA to the FTO side is the only way to transport the effectively collected electrons to the external circuit,the phenomenon that I₅^-generated during the operation of DSSCs still accumulates in large quantities near the FTO side of the photoanode after the electrolyte evaporates,which proves that I₅^-participates in the transport of charge inside DSSCs.（2）The effect of the accumulation of polyiodine ions on DSSCs were studied by Raman spectroscopy.The in situ Raman spectroscopy was used to explore what factors affected the production of I₅^-in DSSCs,and further explore whether the accumulation of I₅^-would affect the performance of DSSCs.In this part of the experiment,the commonly used electrolyte concentration C3 was used to assemble planar DSSCs.The relationship between the content of I₅^-and the performance of DSSCs under forward and reverse bias,room temperature and low temperature preservation was studied.The analysis found that I₅^-intensified under reverse bias and began to dissociate I₅^-under forward bias in DSSCs.This result shows that the generation and dissociation of I₅^-are related to the direction of the internal current of the DSSCs.In addition,it is found that cryopreservation leads to a large accumulation of I₅^-in DSSCs,and a large accumulation of I₅^-affects charge transport,that is,the limit current(I_lim)and I_SCof DSSCs develop in the direction conducive to the performance improvement of DSSCs,which is conducive to further optimizing the in situ monitoring scheme of DSSCs in extreme environments（low temperature）.In addition,based on the different intensity changes of I₅^-when DSSCs are in short circuit and normal working conditions,we innovatively invented a method of in situ Raman spectroscopy to inspect and repair a battery in a non-normal working state in series connected large DSSCs module,which is expected to realize efficient and rapid on-site maintenance of DSSCs modules.