【作者】 李金城；

【导师】 于涛；

【作者基本信息】 南京大学 ， 凝聚态物理， 2021， 硕士

【摘要】 金属卤化物钙钛矿太阳电池是一种新兴的光伏器件,目前其实验室的认证光电转换效率高达25.5%,成为硅基太阳电池的强有力的挑战者和竞争者。回顾其发展历程,大量的研究都集中在优化经济有效的低温溶液法以制备出高纯相、均匀致密、缺陷态更少的钙钛矿多晶薄膜,并由此发展出组分工程、溶剂工程和添加剂工程等富有成效的策略。此外,吸附在薄膜表面的物质,例如氧气、水,同样会对钙钛矿薄膜的光电性质和稳定性造成极大的影响。作为应用最为广泛的制备方法,低温溶液法并没有类似真空沉积法的超洁净环境,其自身就存在着不可避免但又很容易被忽视的弊端:会引入有机物质,这些有机物质可能会以杂质或缺陷的形式存在并破坏最终光电器件的性能,但是这个问题并没有得到太多的关注。研究表明,即使完全隔绝大气环境,在手套箱中制备的杂化钙钛矿多晶薄膜其表面和内部区域仍存在着大量的包含碳、氧等元素的杂质,钙钛矿和杂质之间会发生自发的电荷转移,导致功函数变化和能带弯曲,并且阻碍载流子的输运。在广受关注的全无机金属卤化物钙钛矿材料中,CsPbBr₃拥有相对最理想的湿热稳定性,CsPbBr₃钙钛矿太阳电池的整个制备流程可以在空气环境下进行,既具有潜力的商业化前景,又可作为开展新型太阳电池探索性实验的理想平台。当制备环境从手套箱转移到空气环境,钙钛矿薄膜的化学组成成分和光电性质都会发生显著变化。因此上述的杂质问题对于在空气环境下用两步溶液法制备的CsPbBr₃薄膜会更加显著。测试结果表明CsPbBr₃薄膜的表面和内部确实残留着大量来源于有机溶剂和大气环境的固有杂质,而且固有杂质的存在会影响CsPbBr₃晶体中原子的化学环境。紫外臭氧处理技术可以同时产生高能量的光子以及强氧化性的气体,是一种强大且使用广泛的去除有机污染物残留的清洁手段。在钙钛矿太阳电池领域里,紫外臭氧处理不仅可以清洁基底表面,例如透明导电氧化物和电荷传输层,还可以增强基底的浸润性以更好地耦合随后沉积的功能层,因此紫外臭氧处理已经成为制备高效的钙钛矿太阳电池必不可少的步骤。然而,对钙钛矿吸光层直接进行紫外臭氧处理并提升太阳能电池效率还未见诸报导,可能的原因在于紫外光被认为会对杂化钙钛矿太阳电池的稳定性造成不利影响,杂化钙钛矿中的有机成分也很容易在紫外臭氧处理的过程中被氧化。鉴于CsPbBr₃出色的稳定性及其全无机组成成分,紫外臭氧处理可能是去除无意中引入的固有杂质的潜在方法,从而获得基于CsPbBr₃薄膜的高质量光电器件。实验结果表明:紫外臭氧处理可以有效地去除附着在CsPbBr₃晶体界面不期望的固有杂质而不会对晶体结构和形貌造成破坏。经过适当的紫外臭氧处理后,由薄膜组装而成的太阳电池的光电转换效率由7.37%提升到7.82%。光致发光光谱和空间电荷限制电流测试表明经过紫外臭氧处理后:对薄膜而言,电子缺陷态密度从4.05×10¹⁶cm^-3减少至3.45×10¹⁶cm^-3,载流子平均寿命从1.30ns增加到5.81ns,表明非辐射复合得到抑制;同时,下层的CsPbBr₃薄膜和上层的碳电极界面间的孔洞大大减少,接触更为紧密,有助于提升界面间的电荷收集效率;对整个器件而言,太阳电池展现出更大的内建电场,更少的缺陷复合以及更平滑的电荷输运,电荷载流子得以更高效地分离、传输和抽取。本工作指出了根源于全环境溶液制备法的固有杂质对CsPbBr₃薄膜的光电性质及其光伏器件的性能的影响,并提出了一种去除固有杂质的简单有效的策略。希望借此能够引起研究人员对同样是通过全环境溶液法制备的光电材料中固有杂质的关注。此外,本文提出的紫外臭氧后处理策略不仅能为清洁其它全无机卤化钙钛矿提供一些的思路和借鉴,而且很容易集成到现有的制备流程中去,这将有利于加快钙钛矿太阳电池的商业化进程。更多还原

【Abstract】 Metal-halide perovskite solar cells（PSCs）are a kind of emerging photovoltaic devices.At present,their champion cell’s power conversion efficiency in the laboratory is up to 25.5%,making them strong challengers and competitors of silicon solar cells.In the development of perovskite solar cells,a lot of studies have focused on optimizing the cost-effective and low-temperature solution process to prepare polycrystalline perovskite thin films with high phase purity,good morphology and fewer defects,and thereby fruitful strategies,such as composition engineering,solvent engineering and additive engineering have been developed.In addition,the substances adsorbed on the perovskite surface,such as oxygen and water,will have a great impact on the optoelectronic properties and stability.As the most widely-used preparation method,the solution process,without the ultra-clean environment similar to the vacuum deposition,has unavoidable drawback that can easily be ignored:organic substances may be introduced.These organic substances behave as impurities or defects that deteriorate the performance of final optoelectronic devices.However,the influence of these impurities in the solution-processed perovskite films is less noticed.It has been demonstrated that polycrystalline hybrid perovskite films fabricated in glovebox had considerable inherent impurities containing oxygen and hydrocarbons in both surface and bulk regions despite careful avoidance of exposure to ambient atmosphere.Spontaneous electron transfer occurs between the films and the inherent impurities adsorbed at the interface of its crystal surface,which causes band bending and work function changes,and impedes carrier transport.Among the all-inorganic halide perovskite that have attracted increasing attention,CsPbBr₃ possesses the best moisture and thermal stability.The whole fabrication process of CsPbBr₃ PSCs can be conducted in ambient air,making itself a potential application and an excellent platform for various exploration.It is noteworthy that the chemical composition and optoelectronic properties of the perovskite films will exhibit a significant change when the fabrication process transfer from glovebox to the ambient environment.So,the problem of the above inherent impurities would be much more striking for CsPbBr₃ films fabricated by two-step method in ambient air.Our results show that there are considerable inherent impurities in the surface and bulk region of CsPbBr₃ films,which origin from organic solvents and ambient air.The inherent impurities will affect the chemical environment of cesium,lead and bromin in the CsPbBr₃ crystal.UVO treatment,which can produce high energy photon and strong oxidizing gas simultaneously,is a powerful and widely-used cleaning means to remove organic residual.In the field of PSCs,the UVO treatment can not only clean the substrates,such as transparent conducting oxides and charge transport layer,but also increase surface wettability for better coupling with the subsequently deposited layer,making it an essential step in the process of fabricating PSCs.However,UVO treatment directly on perovskite films to increase their solar cell performance has never been reported.The associated reasons may lie in that the ultraviolet light is generally considered to be detrimental to the stability of the hybrid perovskite solar cells.Meanwhile,the organic components in hybrid perovskites are easily oxidized during UVO treatment.However,given the excellent stability of CsPbBr₃ as well as its all-inorganic components,UVO treatment may be a potential method for removing the inadvertently introduced organic impurities,thereby obtaining high-quality CsPbBr₃-based optoelectronic devices.Our results show that the undesirable inherent impurities attached to the surface and grain boundary can be eliminated effectively without damaging the crystal structure and morphology of the CsPbBr₃ films.After proper UVO post treatment,the device exhibits an improved power conversion efficiency of 7.82%from 7.37%.PL and space charge limited current measurements further confirmed that:for the thin films,the electron trap state density is reduced from 4.05×10¹⁶ cm^-3 to 3.45×10¹⁶ cm^-3,and the average lifetime is prolonged from 1.30 ns to 5.81 ns dramatically,indicating the non-radiative recombination is suppressed.Meanwhile,the voids between the underlying CsPbBr₃films and upper carbon electrode are reduced,exhibiting a much more intimate contact,thereby the charge collection at the interface will be effectively promoted.For the whole device,the assembled PSCs exhibits higher built-in potential,fewer trap-assisted recombination and smoother charge transport.Therefore,the charge carriers can be separated,transported and extracted more effectivelyThis work not only points out the influence of inherent impurities stemming from all-ambient solution process on the optoelectronic properties of CsPbBr₃ films and the performance of the assembled photoelectric device,but also proposes a simple and effective strategy to remove these impurities.Therefore,we hoped that this work will draw researchers’attention to the inherent impurities in optoelectronic materials that are also prepared by solution method in ambient air.In addition,the proposed UVO post-treatment can not only provide ideas and references for cleaning other all-inorganic halide perovskites,but also can be easily integrated into the fabrication process,which will help to advance the commercial process of PSCs.更多还原