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+, Mg2+, Al3+, Y3+, Sb3+, Nb5+, 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. © 2020, Materials Review Magazine. All right reserved.
