Monitor Charge Transfer Process In Dye Sensitized Solar Cells With The Structure Of Semiconductor-dye-metal By SERs

Following the first report by Oregan and Gratzel in 1991,dye-sensitized solar cells?DSSCs?have gained increasing attention as a new type solar cells because of their high efficiency,low cost,environmental benignity,and ease of manufacture.In this kind of cell,dyes adsorb the energy of photons to generate excited electrons,which are transferred efficiently to the conduction band?CB?of the semiconductor.Thus,the amount of dye loading and the degree of charge-transfer?CT?properties of DSSCs are key criteria for a high incident photon-to-electron conversion efficiency.At present,the research about DSSCs are focusing on how to optimize the structure of the cell for higher efficiency.However,the interfacial CT process is not yet clear,which can influence the performance of DSSCs.Only understanding how the electrons are transferred in the DSSCs,it is possible to improve their performance and efficiency.Therefore,people are devoted to find an effective way to investigate the CT process in the DSSCs.Surface enhanced Raman scattering?SERS?spectra,a kind of vibration spectrum with highly sensitive,can be affected by CT process easily,which can act as a kind of CT probe.Based on the above considerations,in this thesis,we prepare a series of semiconductor-dye-metal systems to simulate the structure of DSSCs.And set up an analytical method of CT process in DSSCs monitored by SERS,which is improtant for the furthure improvement of the performance of cell.In this thesis,we utilized SERS spectra to investigate the CT process in DSSCs with the structure of semiconductor-dye-metal.The main research results are shown as follows:?1?Interfacial CT process in the system of Ag/N719/TiO2 monitored by SERS.The simulated structure of DSSCs with metal-dye-semiconductor,Ag/N719/n-TiO₂?n=0-3?with high order,was constrated by the conbination method of surface sol-gel and layer-by-layer self-assembly.The binding type in the system of Ag/N719/n-TiO₂ was obtained by monitored their SERS spectra at excitation wavelengths of 532,633 and 785 nm.Meanwhile,the CT process can be adjusted by changing the number of TiO₂ layer,which can result in TiO₂-layer-dependent-SERS spectra: additioning the layer of TiO₂ can enhance the band at 1266 cm-1 selectively due to the CT process.Herein,utiliz another band at 1024 cm-1,which can not be affected by CT effect.The degree of CT process??CT?was calculated based on these two bands with different sensitive to CT effect,which can estimate the contribution from CT process in the whole system cotaning SPR and CT effects at the same time.It is the first time to estimate CT process in a system with N719,a kind of non-C2 v symmetric molecular,quantitatively based on the concetion of degree of CT process.Finally,the CT model of Ag/N719/n-TiO₂ system was proposed according to the layer dependent-SERS spectra,laser energy dependent-SERS spectra and componental energy levels.In this work,we proposed a new idea by using the well-known conception “?CT” in SERS to estimate the CT process in DSSCs,which provides guidance for the investigation of CT process in DSSCs based on SERS sepctra.?2?The influence on CT process in TiO2/N3/Ag induced by Ag.In order to approach the actual structure of DSSCs,anatase and rutile TiO₂ nanoparticals?NPs?were utilized as the base to construct TiO₂/N3/Ag semiconductor-dye-metal system to investigate how Ag NPs affect CT process in this system.It can be found that introuducing Ag NPs in TiO₂/N3 can improve the efficiency of DSSCs via the following two aspects:?1?The SPR effect from Ag NPs can enhance the optical absorption and broaden the range of light response;?2?The Fermi level of Ag can offer a new CT pathway for the electrons injection,which can enhance the ?CT andreduce the CT threshold from 2.071 e V?TiO₂/N3?to 1.877 e V?TiO₂/N3/Ag?.At last,compare SERS spectra of TiO₂/N3/Ag based on TiO₂ calcined with 400,450 and500 °C,respectively.As the TiO₂ calcined under 450 °C present higher surface area?contributing to assemble more N3 molecules for higher light harvesting efficiency?and more surface defects?promoting lager degree of CT process?.Therefore,TiO₂?450 °C?/N3/Ag with the largest ?CT and the lowest CT threshold is more suitable as the basic structure of DSSCs.?3?The influence on CT process in ZnO-TiO₂/N3/Ag induced by ZnO-TiO₂ heterojunction.In this part,we prepared dendritic crystal ZnO-TiO₂ heterojunction semiconductor via two-step hydrothermal synthesis method.And utilize it as the base to construct DSSCs system with the structure of ZnO-TiO₂/N3/Ag.In order to explore how the composition of ZnO and TiO₂ affect the CT process in such DSSC system,SERS spectra and ?CT of ZnO/N3/Ag,TiO₂/N3/Ag,and ZnO-TiO₂/N3/Ag were compared.It can be found that as comapred with utilization of the single semiconductor materials,utilizing dendritic crystal ZnO-TiO₂ heterojunction semiconductor can increase the adsorption capacity of dye,which leads to higher light harvesting.Meanwhile,the synergistic effect between ZnO and TiO₂ induced by CT can promote the separation of electronic-hole pairs,which can improve the CT efficiency of DSSCs.Furthermore,after introducing the Ag NPs into ZnO-TiO₂/N3 system,Ag can participate into the CT process between ZnO and TiO₂,which reduces the CT threshold of synergistic effect.Thus,semiconductor/N3/Ag systems based on the dendritic crystal ZnO-TiO₂ heterojunction semiconductor are an excellent potential structure for DSSCs.?4?The influence on CT process and thresholed in Au@Ag/N3/n-TiO₂ induced by Au core.In this part,we employed the seeding growth method to fabricate Au@Ag core-shell NPs exhibiting Ag-like optical properties.And utilize it as the base to constract high orderd Au@Ag/N3/n-TiO₂ system to explore the effect caused by introducing Au core into Ag NPs.Compare TiO₂-layer-dependent-SERS spectra of Au@Ag/N3/n-TiO₂ and Ag/N3/n-TiO₂ system,it can be found that introducing Au as core into Ag NPs can lower the energy level of the Au@Ag/TiO₂ complex owing to energy level equilibration,thus reducing the CT threshold involving TiO₂.Finally,five bimetal with different composition ratios?Au@Ag1-5?were prepared for the fabrication of bimetal/N3/n-TiO₂ systems.It was found that introducing Au as the core in Ag NPs appropriately reduces the CT threshold involving TiO₂.This decreased CT threshold arises from energy level equilibration in the Au@Ag1/N3/n-TiO₂ and Au@Ag2/N3/n-TiO₂ systems.And it is due to dual effects of energy level equilibration and activation of Ag at the core-shell surface induced by the inner Au in the Au@Ag4/N3/n-TiO₂ system.