Study Of The Photovoltaic Performance Of Perovskite Solar Cells Based On Ho、Yb、Mg/Li Doped TiO₂

Organic-inorganic hybrid methylamine/methylether lead iodide perovskites solar cells（PSCs）due to their perfect conditions of light absorption materials:suitable direct bandgap,large absorption coefficient,excellent carrier transport performance and high defect tolerance have attracted extensive considerable attention.In particular,the solid-state perovskite solar cells in 2009,with a power conversion efficiency（PCE）of 3.8%,rapid growth to a remarkably high value of 22.1%was obtained in 2017.In just a few years,the PCE of PSCs has improved rapidly that the silicon solar cells,organic solar cells,thin-solar cells have not been achieved.In addition,PSCs also have the advantages of easy preparation,low material cost,and well-stocked supply of raw materials.Therefore,PSCs promise to become the next generation of solar cells!However,the energy band gap（Eg）of the perovskite material is between 1.5 and 2.04 eV,the solar energy spectrum cannot be used by PSCs that the lower energy than energy band gap.Therefore,it is very important that the near-infrared light convert into visible light for the high-efficiency perovskite solar cells.Generally speaking,the structure of PSCs is FTO/dense layerTiO₂/mesoporous layer TiO₂/perovskite layer/hole transport layer/gold electrode.The sunlight through the glass substrate and the TiO₂ layer irradiates the perovskite layer.The perovskite absorbs light to generate electron-hole exciton pairs.Due to the binding energy of excitons is very poor that the electron holes have been separated at the room temperature,which the electron transport layer and hole transport layer conducts to the two poles of the PSCs to form difference of potential.Therefore,the light absorption,electron injection efficiency and the electron transport speed of the PSCs can be improved by optimizing the electron transport layer.The main research contents of this dissertation are as follows:Firstly,the optimal TiO₂ up-conversion luminescent material is prepared and optimized by using the sol-gel method,and then the mp-TiO₂ is modified by the up-conversion TiO₂.The perovskite layer uses a methylamine methylether lead iodide as the light absorbing layer of the PSCs,and absorbs the energy of sunlight to generate electron-hole pairs.At the same time,the content of optimal UC-TiO₂ and pure TiO₂ in the modified layer was optimized to obtain the best PSCs.Its performance and reasons were further studied.The first part:By adding the rare earth element Ho³⁺and Yb³⁺in up-conversion TiO₂ that can absorb more energy in the near infrared region,which the up-conversion luminescence intensity of the TiO₂ is further increased by adding Mg²⁺.The optimal up-conversion TiO₂（UC-Mg-TiO₂）was obtained by optimizing the amount of Ho³⁺,Yb³⁺,Mg²⁺.The mp-TiO₂ was modified by different content UC-Mg-TiO₂ that the up-converting luminescent material can convert near-infrared light into visible light when sunlight passes through the electron transport layer（TiO₂ layer）so that can increase the performance of PSCs.The optimal PSCs was obtained by optimizing UC-Mg-TiO₂ and pure TiO₂ content to modify mp-TiO₂,in which the PCE of PSCs reached 16.3%.An increase by1.1%compared to the PCE of 15.2%compared with the without modified PSCs.In order to explain this perovskites solar cells efficiency,in which made a series of tests and characterization performance.The causes were analyzed using X-ray diffraction（XRD）,scanning electron microscopy（SEM）,energy dispersive spectroscopy（EDS）,transient and steady-state fluorescence spectroscopy（PL,TRPL）.Integrated these test results can be concluded that 60%of UC-Mg-TiO₂ modified mesoporous layer not only can improve the transmission performance of TiO₂ on the carrier can enlarge the utilization of sunlight that can’t be used by PSCs convert the near-infrared light into can be absorbed visible light,and eventually got 16.3%of the photoelectric conversion efficiency.The second part:On the basis of the first part,we changed the doping elements of up-conversion TiO₂.In the first part the TiO₂ doped with Ho³⁺,Yb³⁺,and Mg²⁺to obtain upconversion luminescent materials.In this section we use Li to replace Mg to get better upconversion luminescence.The experiment result shows that the upconversion fluorescence of UC-Li-TiO₂ is stronger than UC-Mg-TiO₂.When the PCE of PSCs were optimized,the conductivity of mp-TiO₂ modified with 50%UC-Li-TiO₂ was higher than that of the mp-TiO₂ modified with 60%UC-Mg-TiO₂.From the ultraviolet photoelectron spectroscopy（XPS）and ultraviolet-visible near-infrared absorption spectroscopy can be seen that Ho³⁺,Yb³⁺,Li⁺doped TiO₂ can effectively reduce the oxygen vacancy and reduce the band gap of TiO₂.From PL,TRPL and electrochemical impedance spectroscopy（EIS）can be seen that 50%UC-Li-TiO₂ modified mp-TiO₂ can expedite electron transfer and decrease recombination.So,the modification of ETL by UC-Li-TiO₂ with conductive characteristic can increase performance of PSCs.Finally,a high photoelectric PCE reached about 16.9%.