Table 1 Performance comparison of typical perovskite solar cells with TiO2 as the electron transport layer [6] The preparation methods of dense layer mainly include spin coating, spray pyrolysis, atomic layer deposition, microwave sintering, magnetron sputtering, etc. Generally, spin coating and spray pyrolysis are simple and easy to operate. Dense layers of other metal oxides can be prepared in essentially the same way. However,titanium exhaust tubing, both spin-coating and spray pyrolysis require high temperature annealing at 500 ° C to transform anatase TiO2 into anatase phase and improve its ability to transport electrons, which limits the application of anatase TiO2 on flexible substrates. Moreover, the thermal contraction during the phase transformation process will leave holes on the surface of the film, making the connectivity between particles worse [2]. Therefore, the preparation of dense anatase TiO2 at low temperature has become one of the important research directions of perovskite solar cells. The reported low-temperature fabrication methods of TiO2 dense layers include atomic layer deposition (ALD, 200 ° C), spin-coating of anatase TiO2 particles (< 150 ° C), low temperature plasma enhanced atomic layer deposition (PEALD, 80 ° C) and low temperature chemical bath deposition (70 ℃). 2.2 Interface optimization of the dense layer On the basis of perovskite thin film materials with regular morphology, the device performance mainly depends on the reasonable design of device structure and the matching of interface energy levels. In addition, the behavior of carrier migration and recombination at the interface between layers is not only related to the aggregation morphology of the active layer, but also depends on the size of the interface barrier between the electron transport layer or the hole transport layer and the electrode. In order to obtain more efficient and stable solar cells, the contact interface is usually optimized,ti6al4v, such as passivation of the surface of titanium dioxide. The photoelectric conversion efficiency of the solar cell can be improved by depositing a thin layer of Sb2S3, Cs2CO3 (2 nm) and other materials on the TiO2 dense layer as a common dense layer. The TiO2 dense layer was modified by C60-SAM, TiCl4 and UV (O3) treatment, which can improve the contact between the dense layer and the perovskite layer, promote charge transport and reduce electron recombination, and improve the conversion efficiency. A dense layer that is too thin also affects the coverage of the perovskite sensitizing layer; if the dense layer is too thick, electrons are recombined before being transported from the perovskite layer to the conductive substrate. At present, titanium filler rod ,Titanium 6Al4V wire, the optimized thickness of the dense layer is generally 30 to 100 nm. III. TiO2 porous layer At present, most PSCs utilize submicron-thick porous metal oxide films to adsorb perovskite, which are called porous layers. Similar to the dense layer materials, semiconductors with matching energy level structure and high carrier mobility can be used as electron (or hole) transport layer materials with mesoscopic structure. The electron transport layer represented by TiO2 mesoporous nanoparticles has been widely used in perovskite batteries. Because perovskite materials also have good electron transport properties, high band gap oxides such as Al2O3 and ZrO2 can also be used to make porous layers of perovskite solar cells. Mesoporous TiO2 has a large specific surface area, which is convenient for the maximum adsorption of perovskite materials and provides a space for the oriented growth of perovskite films. In addition, the mesoporous TiO2 can be fully contacted with the perovskite material to ensure maximum photogenerated charge separation and charge injection. 3.1 Particle Size, Pore Size, and Film Thickness The thickness of the porous layer has a crucial effect on the perovskite film, and its presence contributes to the complete conversion of PbI2 to perovskite. It is reported that the size of TiO2 particles not only affects the implantation of precursors and the contact between perovskite crystals and TiO2, but also affects the charge transport kinetics at the perovskite/TiO2 interface. With the increase of the thickness of the porous layer, the dark current in the TiO2 porous material will also increase linearly, resulting in the decrease of electron concentration and voltage. When the perovskite completely fills the pores of TiO2, the direct contact between TiO2 and the hole transport layer can be effectively avoided, and the electron recombination is reduced. The large pore size in porous TiO2 is also more conducive to the filling of perovskite particles. In fact, TiO2 particle size, pore size and film thickness do not have a linear relationship with the photoelectric performance of the cell, and these parameters are mutually influenced and interacted. This factor is also one of the reasons for the unstable efficiency of perovskite solar cells, and only by exploring their optimal conditions can the whole photovoltaic device be further optimized. 3.2 Crystal form and morphology Due to the better electron transport properties of anatase phase titanium dioxide, it is often used as electron transport material in photovoltaic devices, and a few researchers use rutile phase titanium dioxide. In addition to the crystal form of titanium dioxide, the morphology has an important impact on the light absorption, electron transport and electron capture of the cell. Titanium dioxide nanosheets can improve the contact between perovskite and porous layer, and titanium dioxide nanotubes with less grain boundaries can significantly improve the light absorption and electron collection efficiency. The porous nano-TiO2 fibers with different diameters and lengths were prepared by electrospinning. The results showed that the fibers with too small diameter were discontinuously distributed, and the fibers with too large diameter were arranged too closely, which hindered the adsorption of perovskite. Fig. 3 Structure diagram of different nanorod lengths [2] 3.3 Modification of porous TiO2 Surface treatment and doping are effective means to modify titanium dioxide materials, and the properties of materials can be significantly improved by reasonable control of conditions. The Nb-doped rutile-type titanium dioxide nanorods are used as photoanodes, so that the photoelectric conversion efficiency of the solar cell is remarkably improved; MgO is used as a compact layer, and the porous TiO2 with a small part of MgO adsorbed is used as a skeleton layer, so that the effective injection of electrons is facilitated, and the recombination of carriers is reduced. The interfacial contact between TiO _ 2 and CH _ 3NH _ 3PbX _ 3 plays an important role in determining the growth of perovskite crystal and the charge separation. Although TiO2 has a suitable energy level and generally acts as an electron transport layer to block holes, it has poor conductivity, which results in additional ohmic losses and an undesirable space charge distribution. The Y-doped titanium dioxide is used as a porous layer, which can not only improve the morphology of the perovskite layer, but also enhance the adsorption of the perovskite layer and the electron transport performance in the battery. Al2O3, ZnO and ZnSO4 are less used in perovskite solar cells because their comprehensive properties are far inferior to those of TiO2. 3.4 Stability In terms of stability, the device performance of perovskite solar cells based on mesoporous TiO2 structure decays rapidly under UV irradiation due to the desorption of oxygen on the surface of TiO2 itself. There are many oxygen vacancies or defects on the surface of TiO2, and these deep level defects will adsorb oxygen radicals in the air, and this adsorption is unstable. TiO2 generates electron-hole pairs under the excitation of ultraviolet light. The holes in the valence band react with oxygen radicals and release oxygen molecules, thus forming a free electron and a positively charged oxygen vacancy in the conduction band. The free electron quickly recombines with the holes in HTM. However, the energy level of the defect state caused by the oxygen vacancy is relatively deep, and when the photogenerated electrons are transferred to it, it is difficult to jump to the conduction band again, so they can only recombine with the internal holes, resulting in a decrease in short-circuit current and a decline in cell performance. IV. Outlook Currently, TiO2 is the most widely used electron transport layer material in perovskite solar cells. In order to further improve the photoelectric conversion efficiency of solar cells, the preparation of nano-TiO2 with high specific surface area, low defects and appropriate pore size is helpful to adsorb more photosensitizers, thus producing greater photocurrent and reducing defects. Doping and surface modification of TiO2 are helpful to improve its performance. References [1] Tan, H., et al., Efficient and stable solution-processed planar perovskite solar cells via contact passivation. Science, 2017. [2] Que Yaping, Weng Jian, Hu Linhua et al. Application of TiO 2 in Perovskite Solar Cell [J]. Progress in chemistry , 2016, 28(1): 40-50. [3] Jung H S, Park N G. Perovskite solar cells: from materials to devices[J]. small, 2015, 11(1): 10-25. [4] Wang Weiqi, Zheng Huifeng, Lu Guanhong, et al. Research Progress of Nanometer Metal Oxides in Perovskite Battery [J]. Journal of Inorganic Materials , 2016, 31(9)：897-907. [5] Bai Yubing, Wang Qiuying, Lv Ruitao, et al. Progress in Perovskite Solar Cell [J]. Science Bulletin , 2016, 61: 489-500. [6] Yang Ying, Gao Jing, Cui Jiarui, et al. Progress in Perovskite Solar Cell [J]. Journal of Inorganic Materials , 2015, 30(11): 1131-1138. 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