Synthesis And Modification Of Organic-inorganic Hybrid Perovskite Solar Cells

The organic-inorganic hybrid perovskite solar cells are new type of solar cells with perovskite materials as the light harvesting layer.From the rise of the perovskite solar cells to today,its power conversion efficiency?PCE?has exceeded22%.As the light harvesting layer,organic-inorganic hybrid perovskite materials obtain high carrier mobility,micron-scale carrier diffusion length,adjustable light absorption range,large absorption coefficient,high defect tolerance,bipolar charge transport,and low recombination rate.These good performances make it as a new star in semiconductor materials.There are many factors that will affect the performance of perovskite solar cells,such as the grain size,coverage and composition of perovskite materials,the interface of each layer.In this paper,We investigate the titanium dioxide?TiO₂?nanorod arrays based perovskite solar cells through studying the effects of perovskite film quality,interface of devices and content of perovskite materials on perovskite solar cell.Tetragonal CH₃NH₃PbI₃?MAPbI₃?films are prepared as light harvesting layer through interdiffusion process.Then,Acetic Acid vapor assistant thermal annealing?HAc VATA?based on the interdiffusion process is utilized to optimize the MAPbI₃ film,which improves the performance of corresponding perovskite solar cells.Finally,a new CsBr treatment based on the HAc VATA is introduced to facilitate further improvement of the photovoltaic performance and stability of resulting devices.The main contents of this paper are as follows:?1?Perovskite light harvesting layer is prepared by an interdiffusion process.PbI₂ and CH₃NH₃I?MAI?were sequentially spin-coated on TiO₂ nanorod arrays substrate to prepare MAPbI₃ film.The crystal structure and morphology of the prepared MAPbI₃,and the photoelectric properties and output stability of the corresponding device were characterized.Various MAPbI₃ light harvesting layers were prepared through adjusting the concentration of MAI.The MAPbI₃ film obtains high pore filling fraction and coverage when the MAI concentration is 30 mg/ml.And almost no PbI₂ exists in MAPb I₃ films prepared by 30 mg/ml MAI.It can be concluded from the Photocurrent density-photovoltage characteristics?J-V curves?that the device fabricated by 30 mg/ml MAI shows good photovoltaic performance,which obtains a PCE of 7.38%with short-circuit current density(J_SC)in 17.24mA/cm²,open circuit voltage(V_OC)in 0.79.V,and fill factor?FF?in 0.54.?2?Optimizing perovskite light harvesting layer through acetic acid?HAc?vapor-assisted annealing method to improve the PCE of corresponding devices.On the basis of interdiffusion method,the HAc vapor was firstly introduced during the annealing process to prepare high performance MAPbI₃ film.The prepared MAPbI₃light harvesting layers and the photovoltaic properties of corresponding devices were characterized.HAc vapor can provide a moist environment for the reaction of MAI and PbI₂,which is beneficial to the interdiffusion of MAI and Pb I₂ and promote the reaction.In addition,in the HAc VATA process,MAPb I₃ slightly dissolve under the existence of HAc vapor,and then recrystallized.This process can make the MAPbI₃crystal grains fill the pores more effectively.On the other hand,HAc can improve the hydrophilicity of TiO₂ nanorods,resulting in enhancing the contact between TiO₂and Pb I₂.This enhancement can inherit to the contact between TiO₂ and MAPb I₃after PbI₂ changing into MAPb I₃.Compared with the conventional MAPb I₃ prepared by the interdiffusion method,the MAPb I₃ light harvesting layers prepared by HAc VATA method obtain higher pore filling fraction,which can effectively reduce the electron-hole recombination.Furthermore,MAPbI₃ films prepared by HAc VATA possess better crystallinity.The steady-state photoluminescence?PL?spectra show that MAPb I₃ prepared by HAc VATA has better charge transport property between TiO₂ and MAPb I₃,demonstrating that HAc VATA can significantly improve the interface between TiO₂ and MAPbI₃.Finally,by optimizing the HAc VATA time,a high performance perovskite solar cell with an average PCE of 12.20%was fabricated for 45 min HAc VATA,which is 63.54%higher than the conventional interdiffusion method.?3?Improve the PCE and stability of perovskite solar cells by Cs Br anhydrous methanol solution treatment for perovskite light harvesting layer.On the basis of HAc VATA,the MAPbI₃ film was treated by a solution of CsBr in anhydrous methanol.Trough this method,Cs⁺and Br^-are doped into MAPbI₃ simultaneously to form Cs_xMA_1-xPbI_3-yBr_y.The doped Cs⁺and Br^-will improve the stability and V_OCC of resulting devices,respectively.In addition,MAPbI₃ partly decomposed under the anhydrous methanol and then recrystallize due to the existence of CsBr,which can promote the growth of perovskite crystal effectively.It is worth noting that this Cs Br treatment can effectively improve the interface between the perovskite layer and the hole transport layer?HTL?.The conduction band of formed Cs_xMA_1-xPbI_3-yBr_y is raised,forming a type-II band with MAPbI₃.The Cs_xMA_1-xPb I_3-yBr_y can act as a buffer layer between MAPb I₃ and the HTL,hindering the recombination between the electrons on the conduction band of MAPb I₃ and the holes on the LUMO of HTL.SEM results show that the grain size of prepared perovskite film will augment with the increase of Cs Br concentration.However,the photovoltaic performance does not improve with the increase of grain size.This is mainly due to the fact that the amount of doped Cs⁺and Br^-rise with the increasing CsBr concentration,resulting in a larger bandgap of the perovskite materials,which will decrease the light absorption.Finally,by optimizing the concentration of CsBr solution,a perovskite solar cell with the best PCE of 16.02%was prepared by 2 mg/ml CsBr treatment.Furthermore,the PCE still remains at around 14%after 3500 h while the sample was stored at humidity within 15%,room temperature and darkness atmosphere.