SnO2应用于钙钛矿太阳电池电子传输层的研究进展

Progress of SnO2 as Electron Transport Layer for Perovskite Solar Cells

钙钛矿太阳电池(PSCs)自2009年问世以来,其最高光电转换效率(PCE)已达到25.2%,其中电子传输层(ETLs)对电池的性能和稳定性至关重要。目前使用最多的ETLs材料为TiO2,但它仍存在一些缺点:(1) TiO2/钙钛矿界面之间存在电荷势垒,导致界面间存在较多的电荷累积,从而致使界面电荷转移效率低;(2) TiO2具有较高的紫外光催化活性,长期在紫外光照射下会导致钙钛矿分解;(3) TiO2常需要500℃左右的高温烧结以提高其结晶性,因此不利于在有机柔性衬底上制备。SnO2作为一种n型无机半导体材料,具有宽禁带、高透光性、高导电率、高电子迁移率、低温制备等优点,近年来已被广泛地研究替代传统的TiO2作为PSCs的ETLs。SnO2的制备方法很多,但目前应用于PSCs中的主要方法包括一般溶液法、溶胶-凝胶法、化学浴沉积法等,这些制备工艺较为简单,且能够在低温下合成。采用传统制备方法合成的单一ETLs,例如TiO2、ZnO、SnO2等,虽然应用在PSCs中取得了不错的效果,但也存在薄膜覆盖性差,有明显针孔、界面缺陷以及不稳定等问题。因此,很多研究者尝试制备复合ETLs,例如TiO2/SnO2、ZnO/SnO2、SnO2/SnO2等,以达到互补效应。虽然SnO2本身具有十分优异的光电特性,但界面能级匹配与物理接触可能并非最优,不适当的制备工艺会导致薄膜出现明显的缺陷而成为载流子复合中心,从而引起PCE下降。通过提高电子的抽取与传输能力,向SnO2中掺入一些金属离子,可达到让SnO2与钙钛矿吸光层能级更加匹配的目的,常用的金属离子包括Li+、Mg2+、Al3+、Y3+、Sb3+、Nb5+等。为了改善SnO2薄膜的质量与表面态,减少表面缺陷,优化界面物理接触,常采用一些材料对SnO2薄膜表面进行钝化,从而减少界面与表面的复合中心,增强电子的传输能力,改善电池性能。本文首先介绍了SnO2的晶格结构和特性,然后从SnO2的单一电子层、复合电子层、元素掺杂、界面钝化等角度,总结了SnO2作为PSCs电子传输层的制备方法、策略、机理以及近年来的研究进展,指出低温制备高质量SnO2-ETLs为高效PSCs器件实现柔性化提供了可能。

Perovskite solar cells(PSCs)have achieved the highest photovoltaic conversion efficiency(PCE)of 25.2%since their introduction in 2009,in which the electron transport layers(ETLs)are crucial for the performance and stability of the cells.Currently,titanium dioxide is the most widely used ETL material,but it still has some disadvantages:(1)there is a charge barrier between titanium dioxide/perovskite interfaces,which leads to a large amount of charge accumulation between the interfaces,resulting in low efficiency of charge transfer at the interfaces;(2)titanium dioxide has a high UV photo-catalytic activity,which will lead to the decomposition of perovskite under UV irradiation for a long time;(3)titanium dioxide usually needs high-temperature sintering at around 500℃to improve its crystallinity,so it is not conducive to preparation on organic flexible substrates.As an n-type inorganic semiconductor material,tin dioxide has the advantages of wide bandgap,high light transmittance,high conductivity,high electron mobility,low temperature preparation,etc.In recent years,it has been widely studied to replace commonly used titanium dioxide as the ETLs of PSCs.There are various preparation methods for tin dioxide applied in PSCs.The commonly used methods include general solution method,sol-gel method,chemical bath deposition method,etc.These processes of preparing tin dioxide are relatively simple and can be conducted at relatively low temperatures.Single ETLs prepared by traditional methods,such as titanium dioxide,zinc oxide,tin oxide,etc.,have achieved good results in PSCs,but there are also problems such as poor coverage,obvious pinholes,interface defects,and instability,which lead to decreased device performance.Therefore,many researchers tried to prepare double ETLs,such as titanium dioxide/tin oxide,zinc oxide/tin oxide,tin oxide/tin oxide,etc.,so as to achieve complementary effect.Although tin oxide itself has very excellent photoelectric characteristics,the interface level matching and phy-sical contact may not be optimal.Improper preparation process leads to obvious defects in the film and becomes carrier recombination centers,which leads to the decrease of PCE.In order to better match the energy levels of tin oxide and perovskite and improve the extraction and transfer ability of electrons,some metal ions are added to tin oxide for this purpose.Commonly used metal ions include Li+,Mg 2+,Al 3+,Y 3+,Sb 3+,Nb 5+,etc.In order to improve the quality and surface state of tin oxide films,reduce surface defects and optimize interface physical contact,some materials are often used to passivate the surface of tin oxide grains,so as to reduce the recombination centers of interfaces and surfaces and enhance electron transfer ability,promoting the device performance.In this paper,the lattice structure and properties of tin oxide are introduced,and the preparation methods,strategies and mechanism of tin oxide as PSCs ETLs are summarized from the perspective of single electron layer,double electron layer,element-doping,and interface passivation.It is pointed out that the high quality tin oxide ETLs prepared at low temperatures provide the possibility for high efficiency flexible PSCs.