42 Characterization and Application of Nanomaterials (2021) Volume 4 Issue 2 doi:10.24294/can.v4i2.1324 ORIGINAL RESEARCH ARTICLE Research progress of nanoarray structure transport layers in per- ovskite solar cells Fangxin Tan1, Shan Cong1*, Qinghua Yi1, Zhida Han1,2, Yushen Liu1 1 School of Electronic and Information Engineering, Changshu Institute of Technology, Changshu 215500, China. E-mail: congshan@cslg.edu.cn 2 Jiangsu Laboratory of Advanced Functional Materials, Changshu Institute of Technology, Changshu 215500, China ABSTRACT The electron/hole transport layer can promote charge transfer and improve device performance, which is used in perovskite solar cells. The nanoarray structure transport layers can not only further promote carrier transport but also re- duce recombination. It also has a great potential in enhancing perovskite light absorption, improving device stability and inhibiting the crack nucleation of different structure layers in perovskite solar cells. This paper reviewed the research progress of perovskite solar cells with different nanoarray structure transport layers. The challenges and development directions of perovskite solar cells based on nanoarray structure transport layers are also summarized and prospected. Keywords: Perovskite Solar Cells; Nanoarray; Transport Layers ARTICLE INFO Received: 5 July 2021 Accepted: 29 August 2021 Available online: 5 Septemper 2021 COPYRIGHT Copyright © 2021 Fangxin Tan, et al. EnPress Publisher LLC. This work is licensed under the Creative Commons Attribution- NonCommercial 4.0 International License (CC BY-NC 4.0). https://creativecommons.org/licenses/by- nc/4.0/ 1. Introduction Organic-inorganic hybrid perovskite materials have attracted much attention because of their adjustable bandgap, long carrier diffusion length and high light absorption coefficient. They are the preferred ma- terials for solar cells prepared by the low-temperature solution meth- od[1,2]. At present, the certified conversion efficiency of perovskite solar cells (PSC) has exceeded 25%[3]. Conventional perovskite solar cells mainly include planar perovskite solar cells and mesoporous perovskite solar cells. The electron/hole transport layer can accelerate the electron/ hole transport in the device and reduce the interface recombination, which plays an important role in improving the efficiency and stability of the device. At present, various organic and inorganic materials are wide- ly used in perovskite solar cells, such as 2,2’, 7,7’ -Tetra [N, N-bis (4-methoxyphenyl) amino]-9,9’-Spiro-OMeTAD, poly [bis (4-phenyl) (2,4,6-trimethylphenyl) amine] (PTAA), titanium oxide (TiO2), zinc oxide (ZnO), tin oxide (SnO2), nickel oxide NiO, and so on[4–7]. the researchers found that the transport layer of nanoarray structure ,such as titanium oxide nanorodarray, zinc oxide nanoarray, not only could promote charge transmission but also reduce interface recombination. It also could enhance perovskite light absorption, inhibit crack nucle- ation of different structural layers in perovskite solar cells, and improve the stability of flexible devices[8–10]. This paper introduces the research progress at home and abroad, which includes the design and prepara- 43 tion of nanoarray structures of different electron and hole transport layer materials. 2. Research progress of nanoarray electron transport layer 2.1 Metal oxide nanoarrays The electron transport layer materials of a met- al oxide represented by TiO2, ZnO, and SnO2, are widely used in perovskite solar cells, photodetectors and other optoelectronic devices with their excellent electron transport capacity, relatively good envi- ronmental stability and convenient preparation pro- cess[11–14]. 2.1.1 Titanium oxide TiO2 TiO2 is easy to prepare, low-cost, with appropri- ate energy level and electron transport capacity. PSC use TiO2 as an electron transport layer material wide- ly. At present, the research on TiO2 nanoarray mainly focuses on the preparation method and structural composition to optimize its performance and struc- ture. It improve electron transport efficiency using TiO2 nanoarray, reduces interfacial recombination, enhances the light absorption of photoactive layer, playing a good role in improving the stability of the device. Kim et al. reported an efficient perovskite solar cell using submicron (~0.6 μm) rutile TiO2 [15]. TiO2 nanorods were grown by hydrothermal method, and the reaction time adjusted the length of nanorods. Experiments showed that the current density would decrease due to the increase of nanorods. It is main- ly due to the tilt of the nanorods during the growth process, which affects the filling of perovskite and ultimately affects the device efficiency (Figure 1). Huh et al. enhanced the performance of PSC by selectively growing TiO2 nanorods[16]. They selec- tively grew TiO2 nanorod substrate by nanoimprint lithography and hydrothermal growth method. Ex- periments show that TiO2 nanorods, as an electron transmission channel, can effectively improve the electron-hole separation energy, reduce interface recombination, and obtain an open circuit voltage of 1.12 V. In addition, TiO2 nanorods can increase the optical path of the incident light, enhance the light absorption of the active layer and improve the device performance. Hu et al. prepared a TiO2 nano column array by grazing angle deposition[17]. Scanning electron microscope studied The TiO2 nano column array, scanning near-field optical microscope, and UV- Vis absorption. It found that the TiO2 nano column transport layer can enhance the optical absorption of perovskite through a large number of precursor penetration paths, near-field light concentration, and partial UV shielding. Thus it improves the short cir- cuit current and stability of the device. Liu et al. deposited the optimized CdS shell on TiO2 nanoarray at room temperature by a simple chemical bath method, which significantly improved the efficiency and stability of PSCs[18]. Experiments show that, on the one hand, the CdS shell can pas- sivate oxygen vacancies on the TiO2 surface, prevent electron-hole recombination and protect the calcium titanium layer. On the other hand, the insertion of Cds shell on the surface of TiO2 is helpful to form a type-II structure, which can further accelerate electron transfer and the Cds@TiO2 coaxial nanoar- ray structure provides enough space for perovskite implantation. Xiao et al. successfully doped Nb in titanium oxide array by hydrothermal method[19]. The research shows that in the interface between TiO2 and perovskite, TiO2 doped with Nb is more efficient for electron transport and charge separation, which (a) ~ (c) effect of different TiO2 nano array height on device performance; (d) ~ (f) scanning electron microscope images of different TiO2 nano array heights[15]. Figure 1. Application of TiO2 nanoarray. 44 provides an effective way to prepare PSCs with high performance and high stability. 2.1.2 Zinc oxide ZnO Similar to TiO2, ZnO is also a common elec- tron transport material in perovskite solar cells. Its electron mobility is higher than TiO2, and it is more controllable jn form and has a lower synthesis tem- perature. These advantages make it widely used in PSC[20,21]. At present, the research on ZnO nanoar- rays mainly focuses on many ways, such as how to improve preparation methods,interface modification, increase perovskite filling, reduce carrier recombina- tion as well as improve device efficiency. Tulus et al. Deposited gold nanoparticles on the surface of ZnO nanorod array using vacuum deposi- tion technology to increase the hole Schottky barrier at the interface of ZnO perovskite active layer, as shown in Figure 2(a). Further block holes, passiv- ate carrier composite defects on the surface of ZnO nanorods, reduce the loss of open-circuit voltage of devices, and improve the filling factor, thus, the con- version efficiency of the battery improved[22]. Dong et al. doped the ZnO nanorod array with Al and formed an Al-Zn-O layer on the surface of the ZnO nanorod. While passivating the surface defects of the ZnO nanorod, they also adjusted the energy level to make the energy level of the active layer better match that of the electron transport layer, improve the electron transport efficiency, and finally improve the device[23]. Mahmood et al. combined with the dual char- acteristics of polyethyleneimine (PEI) as dipole layer and selective polymer cover, prepared N: ZnO nanoarrays with high aspect ratio and electron-rich nitrogen doping through low-temperature solu- tion-phase hydrothermal growth[24]. Then he applied them in PSC to effectively improve the penetration of calcium titanium ore in the array, inhibit carrier recombination and improve device efficiency (Figure 2). At the same time, they successfully applied PEI coating on the N: ZnO array adjusted the work func- tion, and obtained stable and hysteresis-free devices with device efficiency > 16%. More interestingly, Zhao et al. prepared highly ordered ZnO nanorod arrays at low temperatures (90°C)[25]. This nanoarray structure has excellent mechanical stability when combined with a flexible substrate, and its performance remains 90% after 1,000 bending cycles with a radius of curvature of 4 mm. It has excellent bending resistance and dura- bility and has great application potential in foldable photovoltaic devices. 2.1.3 Stannic oxide SnO2 In recent years, planar PSCs with SnO2 ETLs have developed rapidly due to the excellent charac- teristics of SnO2 materials, such as appropriate ener- gy level, high electron mobility, good transmittance, excellent chemical stability, and inactive UV catal- ysis. Similar to zinc oxide, stannic oxide also faces many problems of morphology and defect states. Optimize the stannic oxide nanorod arrays using the same passivation strategy[26]. Song et al. doped SnO2 nanocrystals with Y3+ by typical solvothermal method[27]. The results show that Y dopant has a more appropriate energy level structure and better carrier dynamics perfor- mance, and obtains devices with a total efficiency of 20.71%. Gao et al. proposed an effective in-situ template self-etching method on the basis of a series (a) device structure diagram of conventional low aspect ratio (LAR) ZnO nanoarrays and (b) high aspect ratio (HAR) ZnO nanoarrays with PEI coating (illustration: PEI as a capping agent for controlling nanotube growth); based on the scanning electron microscope cross-sectional images of 1,070 nm intact cells of LAR N: ZnO NRs (c) and HAR N: ZnO NRs (d) without PEI coating, the results show that HARZnO nanoarrays have better perovskite wetting properties[24]. Figure 2. Application of ZnO nanoarrays. 45 of controllable experiments, stannic oxide nanotube arrays were prepared with zinc oxide nanorods as sacrificial templates, and the growth mechanism of nanotubes was studied[28]. By comparing stannic ox- ide nanotube array PSC devices with similar SnO2 or ZnO PSCs, PSC with stannic oxide nanotube array as transport layer has better long-term stability and UV stability (Figure 3). This work emphasizes the importance of the material selection of an electron transport layer and provides an idea for realizing ideal electron transport layer/substrate homogeneous junction to promote electron transport. (a) schematic diagram based on SnO2 nanoarray and its devices; (b) the corresponding scanning electron microscope cross-sec- tional image, with a scale of 500 nm; (c) comparison diagram of long-term stability based on SnO2, TiO2, and ZnO nanoarrays; (d) based on the UV stability comparison diagram of SnO2 and TiO2 nanoarray perovskite solar cells, the results show that SnO2 nanoarray perovskite solar cells have better stability[28]. Figure 3. Application of SnO2 nanoarray. 2.2 Other nanoarray electron transport mate- rials Through previous work, it finds that zinc com- pounds usually have excellent electrical properties. so it is also an important topic to develop the elec- tron transport layer materials of zinc compound. Tavakoli et al. introduced Zn2SnO4 nanorod array as electron transport layer into perovskite solar cells, although the short-circuit current of the obtained devices increased and the hysteresis significantly re- duced. However, the open-circuit voltage loss caused by energy level mismatch still needs to solve[29]. 3. Research progress of hole trans- port layer of nanoarray The hole transport materials commonly used in PSC mainly include organic materials and inorganic hole transport materials. These holes materials need to meet the conditions of good hole transport capac- ity, high stability, and matching with perovskite en- ergy level. Organic hole transport materials, such as polyethylene dioxythiophene-poly(styrene sulfonate) (PEDOT: PSS), poly[bis (4-phenyl) (2,4,6-trimeth- ylphenyl) amine] (PTAA), are of relatively complex synthesis, and it is not easy to prepare nanorod array structure. In addition, the acidity of PEDOT: PSS and the instability of unsaturated carbon bonds in organic materials are not conducive to the long-term stable operation of the device. The preparation pro- cess of inorganic hole transport materials, such as nickel oxide NiO, cuprous iodide CuI, cuprous thio- cyanate CuSCN, is relatively simple and convenient structural regulation[30–32]. Gan et al. successfully synthesized CuSCN nanorod arrays with good crystallinity and elec- trical properties at room temperature by adjusting the synthesis temperature and deposition potential, which opened an opportunity for their application in the field of optoelectronic devices[33]. Xi et al. ap- plied CuSCN nanowire array and its similar derived microstructure to perovskite solar cells to achieve a photoelectric conversion efficiency of more than 7.5%. At the same time, the research results show that the introduction of CuSCN nanowire array and its derived microstructure is conducive to obtaining a more regular perovskite active layer interface, improving the crystal orientation of perovskite and reducing crystal defects at the active layer and inter- face, reducing carrier recombination and improving device performance[34]. 46 Anandan et al. introduced the wet prepared CuO nanorod array into the dye-sensitized solar cell and obtained a cell with 0.29% conversion efficiency, which proved that the CuO nanorod array could be used as the photoanode (hole transport layer) of the dye-sensitized solar cell. But the CuO band gap was small (about 1.2 eV), such natural defect makes it impossible to completely separate photogenerated electron-hole pairs, resulting in the recombination of a large number of carriers at the CuO interface, affecting the efficiency of the device and greatly lim- iting its application[35]. Cong et al. prepared well-crystallized nickel ox- ide nanorod arrays at room temperature by grazing angle deposition. It proved that the introduction of nickel oxide arrays reduced the reflection of the light incident surface, increased the light capture of the device, passivated the interface defects between the hole layer and the active layer, induced perovskite crystal growth, obtained high-quality perovskite films, and finally obtained more than 20% photo- electric conversion efficiency. At the same time, the advantages of low-temperature preparation make this structure also have a conversion efficiency of more than 17% on the flexible substrate. Moreover, due to the mesoporous structure of the array, the device stress is relieved and the stable operation of the de- vices on the flexible substrate is guaranteed, which proves the application potential of nanorod array in flexible devices[36] (Figure 4). Zheng et al. introduced copper phthalocya- nine (CuPc) nanorod arrays into organic solar cells. Through close engagement with PCBM molecules in the active layer, the dark current intensity of the device reduces, and the device performance is dou- bled compared with the device based on planar CuPc structure. It also shows that this structure can effec- tively passivate the defects between the active layer and the electrode[37]. Zhang shows that it can effec- tively replace the current mainstream Spiro-OMe- TAD hole transport materials doped with a lithium salt, which integrates the undoped CuPc nanorod array with the electrode from the perspective of chemical stability. And obtain a battery with high re- peatability with an efficiency of 16.1%[38]. 4. Conclusion This paper describes the application and re- search progress of nanoarray transport layer materi- als in PSC. Generally, these devices mainly focus on the preparation and optimization of oxide materials. The preparation methods are a popular such as hy- drothermal method, vacuum deposition, template method. Generally, the devices with nanoarray trans- port layer materials can improve the carrier transport efficiency, and enhance the absorption of the photo- active layer. At present, most of the devices prepared by these materials are still small-area devices based on a glass substrate. There is still great potential in the preparation of flexible devices and large-area de- (a) grazing angle deposition of NiO nanoarrays; (b) the finite element analysis of the electric field distribution diagram with/without NiO nanoarray substrate. The results show that NiO nanoarray structure is conducive to increasing light transmission and reducing light reflection; (c) the finite element analysis stress distribution diagramof the flexible substrate with/without NiO nanoarray.The results show that the NiO nanoarray structure has better bending resistance[36]. Figure 4. Application of NiO nanoarrays. 47 vices. In the future, it is necessary to select appropri- ate flexible substrates for the high-quality growth of nanoarrays through interface modification or doping to improve the carrier mobility, reduce the carrier recombination, improve the device stability, and realize the preparation and application of flexible wearable devices. Conflict of interest The authors declare that they have no conflict of interest. Acknowledgements Project: The National Natural Science Founda- tion of China Project “Study on stability mechanism of perovskite solar cells based on NiO nano arrays” (62005027); Jiangsu Natural Science Foundation Project “Growth of efficient and stable TiO2 array @(rGO/Cu2O) heterojunction electrode by polymer assisted deposition and its photoelectrochemical hy- drolysis mechanism” (BK20181037). References 1. Kim HS, Lee CR, Im JH, et al. 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