Optimization Of TiO₂/Long Persistence Phosphor Composite Photoanodes For Dye-sensitized Solar Cells

Dye-sensitized Solar Cells?DSSCs?have been considered as one of the most promising third-generation solar cells due to their many advantages,such as friendly environment,extensive and abundant raw materials,low cost,simple fabrication process,and etc.Up till now,titanium dioxide?TiO₂?nanomaterials have been the most widely studied photoanode materials in DSSCs due to their stable physical/chemical properties,non-toxic,high economic efficiency,and environmental friendliness.Nevertheless,TiO₂ possesses a wide band-gap?3.2 eV?and a low carrier mobility(0.1-4 cm² V^-1 s^-1),which make it have bad behaviors in the absorption of incident light and the collections of photo-generated carriers.These insufficiencies will greatly limit the photoelectric conversion efficiency?PCE?of the assembled DSSCs,seriously hampering the research in field of DSSCs in recent years.In view of the insufficient light absorption capacity of TiO₂ materials,this dissertation has introduced long persistence phosphors?LPPs?to modify TiO₂ nanoparticles?NPs?and constructed a new TiO₂/LPP photoanode for DSSCs,and optimized the structure of TiO₂/LPP photoanodes to enhance the absorption efficiency of incident solar light and separation and transport of photo-generated carriers and to finally greatly improve the PCE of the devices.At the same time,this dissertation has also explored and verified the main enhancement reasons of LPP layers on the photoelectric performances of DSSCs,which will provide a technical guidance for the design of high performance LPP-enhanced DSSCs in the future.The basic design and main content of this dissertation was summarized as follows:?1?Fabrication and optimization of LPP/P25-TiO₂ NP composite photoanodesBecause TiO₂ often possesses a low absorption on visible light,it is beneficial to introduce LPP backscatter layers?including downconversion effect and long afterglow effect?to enhance DSSCs'absorption of incident sunlight and accordingly increase their PCEs.In this work,the traditional commercial P25 TiO₂ NPs?short for P25?were used as the raw materials,and a P25 thin layer with a certain thickness was coated on the FTO glass substrates via the simple doctor blade method.After that,another layer of commercial green LPPs?SrAl₂O₄:Eu?was coated on the P25 layer,and a type of P25/LPP composite photoanodes were finally obtained via a post annealing treatment.After assembling into DSSCs,results showed that the P25/LPP composite photoanodes achieved photoelectric conversion efficiency?PCE?of 7.16%,which is 24.3%higher than that of the pure P25 one?5.76%?.Moreover,from the optimization of the thicknesses of LPP and P25 in the composite photoanodes,it was found that when the P25 layer is of 12?m and the LPP layer is of 25?m,the resultant P25/LPP composite photoanodes output the highest PCE.Furthermore,combining the analyses of EIS and PL results,the main enhancement reasons in photoelectric properties of DSSCs in this case was found to be the improved light absorption efficiency of photoanode via the backscattering effect?including down-conversion effects?and afterglow effect of LPP materials.And the formed P25/LPP heterojunctions can also enhance the separation of photo-generated carriers at the interfaces between photoanode materials and electrolyte.In addition,the P25/LPP based DSSCs can still work in dark with the maximum PCE value up to 46.94%.?2?Fabrication and optimization of carbon-doped mesoporous TiO₂ NP/LPP composite photoanodesMesoporous TiO₂?m-TiO₂?has higher specific surface area and more adsorption sites for dye molecule than the common dense commercial P25 materials.Thus,it is of great importance to replace P25 with m-TiO₂ to enhance the PCEs of DSSCs.In view of this point,carbon-doped m-TiO₂ raw materials for photoanodes were prepared by hydrothermal method with a pore-forming agent of PVP to tune their morphologies and microstructures and to introduce carbon doping.After assembly,the pure m-TiO₂�based DSSCs showed a PCE of 6.24%,which is much higher than that of the pure P25ones?5.76%?.And combining the analyses of TEM and EIS data,it can be found that due to the introduction of PVP,many pores or interconnected channels were formed in and between m-TiO₂ NPs,which can enhance the transport and diffusion of their electrolyte ions;and at the sametime,a small amount of carbon converted from PVP during the annealing treatment on photoanode materials improved the conductivity of m-TiO₂,which improves the photogenerated carriers'transport.Subsequently,m-TiO₂/LPP composite photoanodes were prepared using the same procedure as the above work.And the assembled DSSCs showed a PCE up to 8.05%,which was respectively improved by 39.8%and 29.0%compared to those of the DSSCs based the pure P25and pure m-TiO₂ NPs.Furthermore,we have systematically explored the effect of PVP doses on the photoelectric properties of m-TiO₂/LPP-based DSSCs,and found that the assembled DSSCs showed the highest PCE at the PVP content of 8 wt.%.