【作者】 刘莉；

【导师】 杨海滨；

【作者基本信息】 吉林大学 ， 凝聚态物理， 2018， 博士

【摘要】 有机-无机杂化钙钛矿太阳电池是一种以有机-无机杂化钙钛矿材料作为光吸收层的新型太阳电池,从钙钛矿太阳电池的崛起发展到今天,其光电转化效率已超过22%。作为钙钛矿太阳电池的光吸收层,有机-无机杂化钙钛矿材料由于其载流子迁移率高、载流子扩散长度能达到微米级、光吸收范围可调节、吸收系数大、缺陷容忍度高、双极性电荷传输和复合速率低等特性而成为半导体材料中的一颗新星。影响钙钛矿太阳电池性能的因素有很多,例如钙钛矿膜层的晶粒尺寸、钙钛矿膜层的覆盖率、钙钛矿材料的成分、界面情况等。本文以二氧化钛（TiO₂）纳米棒阵列为基底,主要从钙钛矿光吸收层的成膜质量、界面情况以及钙钛矿成分方面对钙钛矿太阳电池的性能影响进行了研究。首先利用互扩散法制备了四方相的有机-无机杂化钙钛矿CH₃NH₃PbI₃（MAPb I₃）光吸收层,然后在此基础上利用冰醋酸（HAc）溶剂辅助退火对MAPb I₃膜进行了优化,提高了其光电性能。最后,在HAc溶剂辅助退火的基础上,利用溴化铯（CsBr）无水甲醇溶液处理MAPbI₃膜,使得相应钙钛矿太阳电池的光电转化效率和稳定性有了进一步提高。该论文的主要内容如下:（1）利用互扩散法制备钙钛矿光吸收层。将碘化铅（PbI₂）和甲基碘化胺（CH₃NH₃I,MAI）依次旋涂在TiO₂纳米棒基底上,以制备MAPb I₃膜。对所制备MAPb I₃的晶体结构、形貌以及相应器件的光电性能及输出稳定性进行了表征。通过调节MAI的浓度,制备出了不同的MAPbI₃光吸收层。当MAI浓度为30 mg/ml时,MAPbI₃的孔隙填充度和覆盖率均较好,所制备出的MAPb I₃几乎不含Pb I₂,所对应的器件表现出了良好的光电性能,光电转化效率达7.38%,短路电流密度为17.24 mA/cm²,开路电压为0.79 V,填充因子为0.54。（2）通过HAc溶剂辅助退火优化钙钛矿光吸收层进一步提高相应器件的光电转化效率。在互扩散法制备MAPbI₃膜的基础上,首次在制备MAPbI₃的退火过程中引入HAc溶剂,并对所制备的MAPb I₃光吸收层和相应器件的光伏性能进行了表征。在HAc溶剂辅助退火过程中,HAc蒸汽能为MAI与PbI₂的反应提供一个润湿的环境,有利于MAI和PbI₂的互扩散,促进反应的进行。此外,MAPb I₃会在HAc蒸汽的作用下轻微溶解,然后重结晶,这能有效促进MAPb I₃晶粒在Ti O₂纳米棒阵列中的渗透。另一方面,HAc能提高Ti O₂的亲水性,有利于PbI₂在TiO₂上很好地附着,最后使MAPbI₃与TiO₂的结合质量提高。与常规互扩散法所制备的MAPb I₃相比,HAc溶剂辅助退火方法制备的MAPbI₃光吸收层具有更好的孔隙填充度,这能有效减少电子-空穴对的复合。此外,HAc溶剂辅助退火制备的MAPb I₃具有更好的结晶性。稳态荧光光谱（PL）结果显示,HAc溶剂辅助退火制备的样品在Ti O₂与MAPbI₃之间具有更好的电荷传输,证明HAc溶剂辅助退火能明显改善Ti O₂与MAPbI₃的界面。最后,通过优化HAc溶剂辅助退火时间,在退火45 min时制备出了平均光电转化效率高达12.20%的钙钛矿太阳电池,这比常规互扩散法提高了63.54%。（3）通过CsBr无水甲醇溶液处理钙钛矿光吸收层提高相应钙钛矿太阳电池的光电转化效率和稳定性。在HAc溶剂辅助退火制备MAPb I₃的基础上,利用CsBr的无水甲醇溶液处理MAPbI₃膜。通过这种方法,能够将Cs⁺和Br^-同时掺杂进MAPb I₃里,形成Cs_xMA_1-xPb I_3-yBr_y。掺杂的Cs⁺能提高相应器件的稳定性,而Br^-掺杂有利于提高电池的开路电压。另外,MAPb I₃晶粒会在无水甲醇的作用下先部分分解,然后在CsBr的作用下重结晶,有效促进了钙钛矿晶粒的长大。值得注意的是,这种CsBr处理能有效改善钙钛矿层与空穴传输层的界面。形成的Cs_xMA_1-xPb I_3-yBr_y导带位置有所提高,能与MAPbI₃形成type-Ⅱ型异质结,有效阻碍MAPb I₃导带上的电子与空穴传输层最低未占轨道（LUMO）上的空穴复合。SEM结果显示,随着CsBr浓度的增加,钙钛矿的晶粒尺寸逐渐增大。但是其光电性能并未随晶粒尺寸的增加而提高,这主要是因为掺杂的Cs⁺和Br^-的量会随着CsBr浓度的增加而增大,导致相应钙钛矿的禁带宽度变大,从而降低光吸收。通过优化CsBr的浓度,在2 mg/ml时制备出了性能最好的钙钛矿太阳电池,其光电转化效率高达16.02%,并且在湿度低于15%、室温、黑暗条件下,经过3500小时以后仍保持在14%左右,具有良好的稳定性。更多还原

【Abstract】 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.更多还原