Design Of Photoanodes For Long Persistence Phosphor Enhanced Dye-Sensitized Solar Cells

Photovoltaic power has been considered as one of the most effective way to fundamentally solve our country’s energy problems.And during the last thirty years,great progesses have been made in the development and applications of solar cells.As one of the typical representatives of the third-generation solar cells,dye-sensitized solar cells（DSSCs）have attracted wide attention due to their unique advantages,such as low production costs,no environmental pollution,good compatibility,and etc.However,compared to the first two generations of traditional solar cells,DSSCs often show much lower photoelectric conversion efficiencies（PCEs）.Up till now,the reported PCE record value is only 13.6%.It was often demonstrated that the main factors affecting the DSSCs’performances are the generation and transport of photo-generated electrons in the photoanode,which are respectively dependent on the adsorbed dye molecules and the used semiconductors.Thus,the researches on improving the PCE of DSSCs have been mainly focused on the design of photoanodes.With the introduction of various new semiconductor nanostructures,the PCE of DSSCs increases year by year,but with a slow growth rate.And there still exists a big deficiency in PCE compared to the first two generations of solar cells.This big deficiency has often influenced with many foctors,among which the mismatch between sunlight energy and absorption spectrum of dye molecules is another one of the key factors restricting the improvement of DSSCs’PCEs.Traditional N719 dye molecules mainly show a light absorption within green wave band（ca.520 nm）.Thus,the ultraviolet and near-infrared light in sunlight cannot be effectively absorbed,which restricts the improvement in PCE of DSSCs.In view of this point,green commercial phosphor powders were first employed to enhance the photoelectric performances of DSSCs.Unfortunately,the particle size of commercial phosphor powders usually vary from several to tens of micrometers.And the too big difference between the particles sizes of commercial phosphor powders and the semiconductor nanostructure used in phtoanodes often enhances the light reflection at the interface of semiconductor/phosphor powder,thereby greatly limits the entry of sunlight（especially ultraviolet light）into the LPP backscattering layer.Therefore,aiming at this problem,in this dissertation,long persistence phosphor（LPP）nanomateirals were successfully introduced into photoanodes,and effect of LPP which has average size of particle size distribution on the modification enhancement of DSSCs was systematically studied.After series of optimization and treatments on LPP composited photoanodes,a high PCE of 9.1%was obtained.At the same time,the response of LPP-enhanced DSSCs to the dark environment and UV irradiation was further investigated.Finally,the main work and research ideas involved in this dissertation are summarized as follows:（1）Preparation and optimization of TiO₂@Nano LPP composite photoanodesIn view of too big size of commercial phosphor particles,in this dissertation,a type of nanoscale LPP（Nano LPP）materials of Sr₂Mg Si₂O₇:Eu²⁺,Dy³⁺with different average size of particle size distributions were synthesized via a liquid phase method,and first incorporated into TiO₂@Nano LPP composite nanofibers via self-designed"V"-shaped groove electrospinning technology.Then,a series of TiO₂@Nano LPP composite photoanodes were prepared using TiO₂@Nano LPP composite nanofibers with different doping contents and average size of particle size distributions of Nano LPP via doctor blade method combining with a post annealing process.Finally,cell tests indicate that the assembled DSSCs using TiO₂@Nano LPP photoanodes with doping content of 10 wt.%LPP and average size of particle size distribution of 320nm show the maximum PCE of 6.56%,which is improved by 55.82%compared to that of the ones based on the pure TiO₂photoanodes.Furthermore,via analysis on the results of EIS andOCVD tests,the above enhancements in PCE could be mainly due to the the enhanced down-conversion effect and afterglow performances from nano LPP.Besides,it is worh noting here that despite a large enhancement obtained in PCE,the total PCE value still differs significantly to the highest record reported in the literatures,which is mainly due to the insufficient phosphorescence intensity of our self-synthesized Nano LPP materials.（2）Preparation and optimization of TiO₂@RC-LPP composite photoanodes and TiCl₄treatmentsIn view of insufficient phosphorescence intensity of self-synthesized Nano LPP materials,commercial green phosphor powder of Sr Al₂O₄:Eu²⁺,Dy³⁺was chosen as phosphor enhancement material.And a series of refined commercial long persistence phosphor（RC-LPP）powders were obtained via mechanical disintegration processes.After optimization on the mechanical disintegration processes and TiCl₄treatments,the assembled DSSCs using TiO₂@RC-LPP composite photoanodes show the maximum PCE 9.06%.This PCE value was further improved by 38.11%compared to that in the former design.This enhamcenent could be mainly because of the high phosphorescence intensity of the commercial LPP material that significantly enhances the down-conversion effect and afterglow performances.Moreover,the dense layer by TiCl₄pre-treatment plays a protective role in photoanodes,effectively inhibiting recombination of photo-generated carriers while enhancing the electron transport between LPP and TiO₂particles.Besides,we further tested the output response of the LPP-enhanced DSSCs to the dark state and UV irradiation.This dissertation can also provide a reference for the preparation and design of high-performance all-weather solar cell devices and multi-function light sensors.