Design And Study Of Electrodes For Ruthenium(Ⅱ)-Bipyridine-Dye Co-Sensitized Solar Cells

Solar cells can continuously convert solar energy into electric ones,which makes them great potential to complement global energy consumption demand.As one of the representatives of third-generation solar cells,dye-sensitized solar cells（DSSCs）have received extensive attention from many researchers because of their simple manufacturing process,good environmental compatibility of electrode materials,easy structure modification,cost-effectiveness,and abundant raw materials.The DSSCs typically consist of a photoanode（semiconductor oxide and dyes）,a counter electrode,and an electrolyte.The dye-loaded photoanode plays the function of capturing photons and transferring electrons in DSSCs,which are one of the most important sources of photoelectric conversion efficiency（PCE）of the cells.And the counter electrode plays a role in collecting electrons and catalyzing the rapid reduction of I₃^-to I^-in DSSCs,which requires excellent conductivity and catalytic activity of counter electrode materials.Although the highest reported PCE of DSSCs reached 15%,there still exists a long gap from its theoretical value（30%）.Therefore,aiming at the problems of insufficient solar energy utilization of traditional dyes（N719 and N749）in DSSCs and low cost-effectiveness of Pt electrode materials,this dissertation has proposed the preparation of co-sensitized photoanodes（N719/Ru¹ and N749/Ru¹）and the excellent performance MoTe₂ counter electrode(（micron sheets and nanosheet array）,and investigated the effects of N719/Ru¹ and N749/Ru¹ in the photoanodes on light capture and electron transfer,and MoTe₂ counter electrode on I₃^-ions’catalytic reduction and electrons’collection.The main work of this dissertation was summarized as following:1.Preparation and performance study of ruthenium（Ⅱ）-bipyridine-dye co-sensitized photoanodesIn a traditional TiO₂ photoanode,N719 and N749 sensitizers have become the most commonly used high-efficiency photosensitizers because of their good stability and high efficiency.However,their light absorption around 450 nm decreases sharply,often making their DSSCs underutilize sunlight and thereby limiting the improvement of the cell’s PCE.The co-sensitization technology can effectively broaden the absorption spectrum,to improve the light capture efficiency and photocurrent of the photoanode,and then increase the PCE of the device.Thus,in this chapter,we have chosen ruthenium complex[Ru（2,2’-bipyridine）₂（4,4’-dicarboxy-2,2’-bipyridine）]（PF₆）₂（short for Ru¹）as co-sensitizer,due to its strongest adsorption center at 450 nm.（1）Preparation and performances of N719/Ru¹ co-sensitized photoanodesFirst,the photosensitizer Ru¹ was prepared in the work of this dissertation.Then,N719 and Ru¹ photosensitizers were successively loaded onto the TiO₂ film by a step-by-step sensitization,obtaining an N719/Ru¹ co-sensitized photoanode.And the adsorption capacities of the two photosensitive dyes on the TiO₂ films were adjusted by changing the concentrations of their corresponding sensitizer solutions.Electrochemical test results showed that with the concentration ratio N719 to Ru¹ of 1:1（0.4 mM/0.4 mM）,the N719/Ru¹ co-sensitized DSSCs output the maximum PCE of6.33%,which was 19%higher than that of the pure N719 sensitized ones（5.34%）.Furthermore,a series of data analysis indicated that the main reason for the enhancement in PCE should be due to that the absorption spectra of the two photosensitive dyes complement each other,which provides a better light capture ability,thereby increasing the utilization rate of the solar spectrum by the cells.（2）Preparation and performances of N749/Ru¹ co-sensitized photoanodesN749 is a derivative of the ruthenium-based dye N719 and has an even wider spectral response range than that of N719,especially in the long wavelength region（570～750 nm）.To obtain a higher-performance DSSC device,N749,and Ru¹photosensitizers were also successively coated on the TiO₂ films,obtaining N749/Ru¹co-sensitized photoanodes.Similarly,the adsorption capacities of the two dyes on the TiO₂ films were optimized by changing the concentrations of the sensitizer solution.Electrochemical test results showed that when the concentration ratio of N749 to Ru¹is 4:3（0.4 mM/0.3 mM）,the N749/Ru¹ co-sensitized DSSCs show the best PCE of6.71%,which is 16%higher than that of the pure N749 sensitized ones.2.Design of MoTe₂ micro-/nano-structures and their catalysis effectOften,the expensive raw material price of conventional Pt electrodes hinders the practical commercial application of DSSCs.Thus,it is crucial to find a counter electrode material with a high catalytic activity but a low cost.Metal telluride has low cost,and unique electrocatalytic properties similar to Pt,making it an excellent candidate to replace Pt electrodes.Here in this chapter,we replaced the Pt counter electrode with MoTe₂,a two-dimensional transition metal chalcogenide compound with excellent catalytic performance,and chose the N719 sensitized TiO₂ as photoanodes.（1）Preparation and performances of disordered MoTe₂ micro-sheet based counter electrodesFirst,we prepared an irregularly-shaped and randomly-oriented MoTe₂ micro-sheet（MS）powder by the chemical vapor deposition method,in which Te vapor reacted directly with Mo powder under a protective atmosphere.And then the obtained powder was transferred to fluorine-doped tin oxide（FTO）conductive glass to obtain MoTe₂-MS counter electrodes.Electrochemical performance results showed that MoTe₂-MS-based DSSCs obtained a low PCE of 4.88%.This unsatisfactory result might be due to the base surface of 2H-MoTe₂ being almost inert to electrochemical reactions,which makes the catalytic performances of the counter electrode mainly depend on the limited electrocatalytic active site near the MoTe₂ MSs’edge.Moreover,the random orientation of MoTe₂ MSs greatly increases the internal electron transport resistance within this electrode structure,thereby leading to a great reduction in catalytic performance.（2）Preparation and performances of vertical MoTe₂ nanosheet array based counter electrodesBecause of the above problems,we in-situ grew a vertically-oriented MoTe₂nanosheet array（NSA）film on the Mo substrate using a chemical vapor deposition method,method,and directly served it as the counter electrode for DSSCs.This design of MoTe₂ NSAs can not only increases the density of exposed active edge sites in the counter electrode but also improve the conductivity of the electrode,which is more conducive to electron transport.As a result,the PCE of MoTe₂-NSA-based DSSCs reached 7.31%,which was 37%higher than that of the Pt-based ones（5.34%）.Furthermore,a series of characterizations and analysis showed that the growth temperature often imposes a great influence on the micromorphology of MoTe₂ NSA,which thereby greatly influences its catalytic properties.When the growth temperature reaches 710°C,the whole MoTe₂-NSA electrode possesses the largest density of the catalytic active site,which is most conducive to the reduction of I₃^-ions,leading to the best electrocatalytic performance.（3）Preparation and performances of N719/Ru¹-MoTe₂-and N749/Ru¹-MoTe₂-based DSSCsTo further improve the electrochemical performances of the co-sensitized DSSCs,we direct assembled the DSSCs with the MoTe₂-NSA counter electrode and the optimized N719/Ru¹ and N749/Ru¹ sensitizers,which can make the DSSCs improved by the structure of both the counter electrode and the photoanode.After electrochemical tests,the N719/Ru¹-MoTe₂-based and the N749/Ru¹-MoTe₂-based DSSCs exhibited PCEs of 7.80%and 8.07%,which are 46%and 51%higher than that of the N719-Pt-based ones,respectively.