In Situ Investigation Of The Interaction Between Dye And Photoanode In DSSC Using Raman Spectroscopy

Dye-sensitized solar cells（DSSCs）are considered as a high cost-effective photovoltaic device because of their low manufacturing cost and simple process,and have attracted much attention.In recent years,research on DSSCs has mainly focused on achieving better device performance through the material development of dyes,semiconductor photoanode,counter electrode,and electrolyte in DSSCs.The photoelectric conversion efficiency mainly depends on the carrier transport path at the interfaces of a DSSC,and interface modulation is an effective strategy to improve the overall performance of DSSCs.In this thesis,a strategy to selectively design the surface structure of Ti O₂photoanode is proposed,and the photoanode operation mechanism of DSSCs is studied by in situ Raman spectroscopy.And the real-time dynamic monitoring of micro-interfaces of DSSCs under working conditions are also realized by using micro-Raman spectroscopy technology,which provided an effective strategy for precisely adjusting the photoanode interface to improve the photovoltaic conversion rate of DSSCs.The main research contents are as follows:1.The effect of composite photoanode on the performance of DSSCs revealed by Raman spectroscopyA high-efficiency Ti O₂NP@TNAs photoanode with Ti O₂nanoparticles（Ti O₂NPs）filled on the surface of Ti O₂nanotube arrays（TNAs）was prepared by the hydrolysis of titanium tetrachloride droplets.The dye-sensitized specific surface area was thus increased,thereby the open circuit voltage(V_OC)of DSSCs was also increased too.The evolution of the photoanode/electrolyte interface of two DSSCs（TNA-DSSCs and Ti O₂NP@TNA-DSSCs）under working conditions was measured by nondestructive Raman spectroscopy,the results confirm that the formation of polyiodide（I₅^-）formed at the photoanode/electrolyte interface during the photosensitization process.In addition,the change trend of V_OCof DSSCs and Raman spectral intensity of polyiodide are consistent.At 168 cm^-1,the band intensity attributed to I₅^-vibration gradually decreases with the increasing bias voltage,when the bias value is greater than V_OC,the I₅^-band disappears,and when the bias value is restored to below V_OC,the I₅^-strength can reappear.This phenomenon confirms the correlation between V_OC（one of the photovoltaic indicators of DSSCs）and spontaneous polyiodides in DSSCs.In situ Raman spectroscopy revealed that I₅^-was spontaneously generated and reversibly regenerated in DSSCs under operating conditions.Therefore,the V_OCof DSSCs can be semi quantitatively evaluated by the Raman band intensity of I₅^-(located at 168 cm^-1),and this result confirms that in situ Raman technique provides a molecular-level theoretical basis for the mechanism of DSSCs.2.The charge transfer at N719/Ti O₂interface revealed by Raman spectroscopyThe N719 powder,N719 solution,N719 dye adsorbed by Ti O₂,and the materials assembled into complete DSSCs were characterized by Raman spectroscopy,and the characteristic bands of N719 dye were assigned in detail by Raman spectroscopy.At the same time,the theoretical Raman spectrum of N719 adsorbed on the surface of Ti O₂was simulated by density functional theory（DFT）,and the experimental results and theoretical calculation results were compared and analyzed in detail.When the complete DSSCs were assembled,the Raman band at 1545 cm^-1was assigned to theνs（C=C）vibration in the bipyridine ring,and its band position and intensity changed significantly during the operation of DSSCs,which is related to the interaction of Ti O₂and N719 and the enhanced charge transfer of photogenerated electrons in the electrolyte.In addition,the Raman bands intensity ofνs（COO^-）at 1377 cm^-1andνs（C=O）at 1727 cm^-1,which are assigned to carboxyl groups,are very weak or even disappear.These results proves that the chemisorption between the dyes and the Ti O₂surface,the bipyridine moiety of the N719dye molecule is bound to the Ti O₂surface through a carboxylic acid bond,and this is also the only Raman spectral evidence for the existence of chemisorption between N719 and the Ti O₂surface.3.The effect of electrolyte concentration on the performance of DSSCs revealed by Raman spectroscopyFour electrolytes with the same I₂/I^-molar ratio and different electrolyte concentrations（0.01 M I₂/0.1 M Li I,0.025 M I₂/0.25 M Li I,0.05 M I₂/0.5 M Li I,and 0.1M I₂/1.0 M Li I）were prepared.The complete DSSCs were assembled with these four different concentrations of electrolyte,and the Raman spectra and the performance of DSSCs were compared and analyzed.The results show that too low or too high electrolyte concentration will adversely affect the performance of DSSCs.When the bias voltage is lower than V_OC,the intensity of the Raman band at 168 cm^-1assigned to I₅^-decreases with the increase of the bias voltage;when the bias voltage exceeds V_OC,DSSCs is no longer under the spontaneous working condition,the intensity of 168 cm^-1gradually approaches zero,indicating that I₅^-in the electrolyte gradually disappears.This result once again confirms that the Raman band intensity of I₅^-(located at 168 cm^-1)can be used to semi-quantitatively evaluate the V_OCindex of DSSCs.