SnO2-based electron transporting layer materials for perovskite solar cells: A review of recent progress

Journal of Energy Chemistry 35 (2019) 144–167

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Review

SnO2 -based electron transporting layer materials for perovskite solar cells: A review of recent progress Yichuan Chen a,b, Qi Meng a, Linrui Zhang a, Changbao Han a, Hongli Gao a, Yongzhe Zhang a,∗, Hui Yan a,∗ a b

College of Material Sciences and Engineering, Beijing University of Technology, Beijing 100124, China School of Mechanical and Electrical Engineering, Jingdezhen Ceramic Institute, Jingdezhen 333403, Jiangxi, China

a r t i c l e

i n f o

Article history: Received 4 July 2018 Revised 19 November 2018 Accepted 19 November 2018 Available online 23 November 2018 Keywords: Perovskite solar cells Electron transport materials Tin oxide Nanostructures

a b s t r a c t In recent years, due to their high photo-to-electric power conversion efficiency (PCE) (up to 23% (certified)) and low cost, perovskite solar cells (PSCs) have attracted a great deal of attention in photovoltaics field. The high PCE can be attributed to the excellent physical properties of organic–inorganic hybrid perovskite materials, such as a long charge diffusion length and a high absorption coefficient in the visible range. There are different diffusion lengths of holes in electrons in a PSC device, and thus the electron transporting layer (ETL) plays a critical role in the performance of PSCs. An alternative for TiO2 , to the most common ETL material is SnO2 , which has similar physical properties to TiO2 but with much higher electron mobility, which is beneficial for electron extraction. In addition, there are many facile methods to fabricate SnO2 nanomaterials with low cost and low energy consumption. In this review paper, we focus on recent developments in SnO2 as the ETL of PSCs. The fabrication methods of SnO2 materials are briefly introduced. The influence of multiple SnO2 types in the ETL on the performance of PSCs is then reviewed. Different methods for improving the PCE and long-term stability of PSCs based on SnO2 ETL are also summarized. The review provides a systematic and comprehensive understanding of the influence of different SnO2 ETL types on PSC performance and potentially motivates further development of PSCs with an extension to SnO2 -based PSCs. © 2018 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. and Science Press. All rights reserved.

Yichuan Chen is currently a Ph.D. student under the supervision of Prof. Hui Yan at the Key Lab of Thin Films Faculty of Materials Science and Engineering, Beijing University of Technology. His current research focuses on high-efficiency and stable perovskite solar cells.

∗

Qi Meng is currently a master student under the supervision of Prof. Hui Yan at the Beijing University of Technology. Her current research focuses on stable perovskite solar cells.

Corresponding authors. E-mail addresses: [email protected] (Y. Zhang), [email protected] (H. Yan).

https://doi.org/10.1016/j.jechem.2018.11.011 2095-4956/© 2018 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. and Science Press. All rights reserved.

Y. Chen et al. / Journal of Energy Chemistry 35 (2019) 144–167 Linrui Zhang is currently a Ph.D. student under the supervision of Prof. Hui Yan at the Key Lab of Thin Films Faculty of Materials Sci@Engin. Beijing University of Technology. His current research focuses on thin solar cells.

Changbao Han is currently a professor of College of Materials Science and Engineering and a core faculty member of Semiconductor Film Devices Centre at the Beijing University of Technology. His current research focuses on semiconductor photoelectric devices and energy harvesting systems, such as solar cells, Light-emitting diodes (LED), triboelectric nanogenerator and the application in industry. More than 50 journal articles have been published, including Advanced Materials, Nano Energy, Optics Express, etc. The average IF is larger than 10 and his entire publications have been cited for over 800 times with an h-index of 21. He held 14 authorized patents and four of these patents were performed to Beijing Nairteng Co. Hongli Gao is a full lecturer at the BeiJing University of Technology (BJUT). She received her Ph.D. degree in Material Sciences from the Institute of Semiconductors, Chinese Academy of Sciences (ISCAS) in 2014. And later she did his postdoc at the BJUT from 2014 to 2016, mainly working in organic solar cells and perovskite solar cells. After that she joined the College of Applied Science of BJUT, as a lecturer. Her present research interests are photovoltaic materials and devices.

Yongzhe Zhang is currently a full professor in Beijing University of Technology (BJUT). He obtained the “Program for Overseas Talents Aggregation” award and “Beijing Nova program” award in 2014, and he is also the “specially-invited expert” of Beijing. He received his Ph.D. degree in 2009 from Lanzhou University majored in Condensed Matter Physics. Then he moved to Yonsei University in South Korea as a postdoc and Nanyang Technological University in Singapore as a research scientist from 2010 to 2013. His present research interests are optoelectronic materials and devices such as photodetector and solar cells. Until now, he has published more than 60 peer-review research papers on Nature Communications, ACS Nano, Advanced Functional Materials, Small, Journal of Power Source etc. Hui Yan is currently a full professor in Beijing University of Technology (BJUT). He received his Ph.D. degree from Kanazawa University in 1993, and later he did his postdoc at the Chinese University of Hong Kong from 1993 to 1996. Since 1996, he joined BJUT as a full professor. His present research interests are semiconductor materials and their optoelectronic devices such as solar cells and photodetector.

1. Introduction Perovskite was discovered in 1839 and its general chemical formula was determined to be ABX3 , which originally referred to a kind of ceramic oxide. Organic–inorganic hybrid perovskite (OIHP) material is one of the most promising materials for high PCE and low-cost solar cells. The PCE has rapidly jumped from 3.8% to 23.3% in less than 10 years [1–9]. OIHP materials have a high lightabsorption coefficient [10], long carrier diffusion [11–13] and high defect tolerance [14], which makes them ideal absorption materials for solar cells [3,15–26], photodetectors [27–29], light-emitting diodes [30], etc. The general chemical formula of OIHP materials

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is ABX3 (Fig. 1a), where A is an organic cation group or inorganic cation (like CH3 NH3 + , NH2 CH = NH2 + , CH3 CH2 NH3 + , or Cs+ ), B is metal cation (i.e. Pb2+ , Sn2+ , Ge2+ ) and X is halogen anion (such as I− , Br− , Cl− , F− ) [31]. In 2009, CH3 NH3 PbBr3 (MAPbBr3 ) and CH3 NH3 PbI3 (MAPbI3 ) were creatively introduced into dye-sensitized solar cells (DSSCs) by Miyasaka and co-workers, and the PCE of the perovskite DSSCs were 3.13% and 3.81%, respectively [1]. In 2012, Kim et al. used a solid hole transporting layer (HTL, Spiro-OMeTAD) to replace the liquid electrolyte in DSSCs and used TiO2 as the electron transporting layer (ETL); the PCE of this material was measured to be 9.7% [2]. Since then, PSCs have garnered much more attention from researchers. In 2013, Liu et al. [3] used vapor deposition to replace the traditional one step solution method and used TiO2 as the ETL, and obtained a PCE of up to 15.4% [3]. Zhou et al. used polyethyleneimine ethoxylated to modify the ITO electrode and reduce its surface work function [4]. In addition, they used yttrium (Y)-doped TiO2 to increase its carrier concentration and to improve the electron transporting channel. With these modifications they were able to achieve a PCE of 19.3% [4]. In 2015, Yang et al. [5] deposited high-quality formamidinium lead iodide (FAPbI3 ) films on the TiO2 ETL to fabricate FAPbI3 -based PSCs, obtaining a maximum PCE of 20.2%. In 2017, Seok and co-workers reduced the deep-level defect density by introducing additional iodide ions into the organic solution, which was used to form the PSC active layer through an intramolecular exchanging process [7]. The certified PCE of PSCs was up to 22.1% [7]. Jiang et al. [32] used tin dioxide (SnO2 ) as the ETL and achieved PSCs with a PCE of 21.6% [32]. Jeon et al. [8] synthesized a fluorene-terminated hole-transporting material, which was used to fabricate the PSCs, and a PCE of 23.2% was obtained (certified efficiency of 22.6%). The brief history of PSCs is displayed in Fig. 1(b). In PSCs, three typical structures are usually used: a mesoporous structure, a regular planar heterojunction structure, and an inverted planar heterojunction structure (Fig. 1c–e). Excellent electron transporting layer (ETL) materials have a high electron injection efficiency, high electron affinity and ionic potential (i.e. n type semiconductor), meanwhile, it must also prevent holes from recombining at the indium-doped tin oxide (ITO) or fluorine-doped tin oxide (FTO) electrode interface. For high-performance PSCs, the ETL material should meet the following criteria: (a) good optical transmittance in the visible range, which reduces the optical energy loss, (b) the energy levels of ETL materials should match that of perovskite materials, which improve the electron extraction efficiency and block holes, (c) have good electron mobility, and (d) be mass producible and cost efficient to fabricate. As a result, the design and material properties of the ETL are crucial for solar cell performance [31,33,34]. Fig. 2 shows energy levels of commonly used ETL materials in PSCs, such as TiO2 , SnO2 , ZnO, C60 , PCBM, WOX . In PSC devices, TiO2 is the most widely used ETL material [26,37–55]. The proper band gap and high transmittance of TiO2 guarantee the high performance of PSCs. However, to obtain a high quality compact or mesoporous TiO2 film, high temperature (>450 °C) annealing is necessary under most conditions, which restricts its application in flexible devices and increases the production cost. Additionally, electron mobility of perovskite materials is about 7.5 cm2 V−1 s−1 , while that of TiO2 is in the range of 0.1– 4 cm2 V−1 s−1 [56]. The difference in electron mobility could cause unbalanced charge transport in PSCs [56]. Besides the TiO2 as the ETL material, organic conducting materials have also been widely used as the ETL material in PSC devices, such as the fullerene and its derivatives [57–79]. Jeng et al. used C60 as the acceptor resulting in a poor PCE of 3.0% at 0.55 V in VOC [57]. Torrientes et al. used a new isoxazolino [60] fullerene derivative as the ETL to fabricate PSCs. The best PCE of 14.5% was

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Fig. 1. (a) Crystal structure of organic–inorganic hybrid perovskites. (b) The brief development history of PSCs (2009 [1], 2011 [35], 2012 [36], 2013 [3], 2014 [4], 2015 [5], 2016 [6], 2017 [7], 2018 [8]). (c) PSCs device structure of mesoporous structure, (d) PSCs device structure of planar heterojunction structure, (e) PSCs device structure of inverted planar heterojunction structure.

obtained [80]. The other fullerene derivative, [6,6]-phenyl C61 butyric acid methyl ester (PCBM), inherits the high electron mobility of fullerenes and shows good solubility due to the introduction of organic groups. As a result, poly(3,4-ethylenedioxythiophene): polystyrene sulfonate (PEDOT:PSS) and PCBM are also widely used

as the HTL material and the ETL material in PSC devices, respectively. In 2014, You et al. fabricated PSC devices under 120 °C, and a PCE of 11.5% was obtained on glass substrates and a PCE of 9.2% was achieved for a flexible substrate [81]. In 2017, Zhang et al. reported another fullerene derivative (α -bis-PCBM), which was

Fig. 2. Energy levels of usually used materials in PSCs.

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Table 1. Comparison of the electrical properties of SnO2 , ZnO and TiO2 [31].

Crystal structure Energy band gap (eV) Surface work function (eV) Electron mobility (cm2 V s−1 ) Refractive index Electron effective mass (m∗ ) Relative dielectric constant Electron diffusion coefficient (cm2 s−1 )

SnO2

TiO2

ZnO

Rutile 3.50–4.0 4.71–5.33 240 2.0 0.3 9.6 Nanoparticle film: 6.22 × 10−6

Rutile, anatase, brookite 3.0–3.2 4.5–5.0 0.1–4 2.5 9 170 Bulk TiO2 : 0.5; Nanoparticles: 10−8 –10−4

Rocksalt, zinc blende, wutzite 3.2–3.3 4.45–5.3 Bulk ZnO: 205–300; Nanowire: 1000 2.0 0.26 8.5 Bulk ZnO: 5.2; nanoparticle film: 1.7 × 10−4

used to fill the vacancies and grain boundaries of the perovskite films, leading to a PCE of 20.8% [76]. Zhou et al. used one-step formation processes to form a perovskite/PCBM heterojunction, resulting in a PCE of 17.8% [78]. Zinc oxide (ZnO) is another widely used ETL material. In 2016, Tseng et al. developed high quality, fully covered Al-doped ZnO thin films as the ETL material [82]. The best PCE was 17.6%. Recently, Azmi et al. fabricated ZnO-based PSCs device via C60 -SAM modification, their PSCs achieved a high PCE of 18.82% [83]. In comparison with TiO2 , the PCE of ZnO-based PSC device is lower than that of TiO2 -based PSC devices. Furthermore, the chemical stability of ZnO is poor, which prevents outdoor applications of the ZnO-based PSCs device. Both TiO2 and SnO2 have similar energy level positions (Fig. 2), crystal structures, and physical properties (Table 1). The PCE of SnO2 -based PSC device exceeds 20% [32,84–90], and thus has attracted many researchers’ attention. SnO2 is considered as a promising alternative to TiO2 due to the following reasons: (1) SnO2 has high electron mobility (240 cm2 V−1 s−1 , Table 1 [91]), which can potentially improve the electron transport efficiency and reduce the recombination losses of PSCs versus other ETL materials [19,91–93]. (2) SnO2 has a deep conduction band and good energy levels as shown in Fig. 2. The excellent band energy level alignment at the ETL/perovskite interface will enhance electron extraction and hole blocking [94]. (3) SnO2 has a lower crystallization temperature than that of TiO2 , and therefore SnO2 is easier to crystallize and dope, which opens the potential for use in flexible solar cells, tandem solar cells, and large-scale commercialization at a lower cost [92,93,95–103]. (4) SnO2 has exhibited excellent chemical stability, UV-resistance, good antireflection, and less photocatalytic activity in comparison with TiO2 or other ETL materials, which is helpful for overall device stability and lifespan [86]. Although the physical properties of SnO2 and TiO2 are mainly similar, there are also some distinct properties each material processes. As a result, studies on SnO2 based PSCs enrich the family of PSCs, which will in turn help to improve the performance of PSCs. In this review, we discuss recent developments in the use of SnO2 as ETL materials for PSCs, expose the origin of performance differences in SnO2 -based PSCs and TiO2 -based PSCs, summarize common fabrication methods for SnO2 materials and systematically review the application of various types of SnO2 as the ETL in PSCs. This review presents a timely update on recent developments of SnO2 -based PSCs and provides guidelines for further optimization and design of PSCs based on SnO2 ETL and beyond. 2. Planar PSCs with a compact SnO2 film as ETL materials PSCs employ a typical n–i–p heterojunction (Fig. 3a). The heterojunction contacts rely primarily on local accumulation/inversion (change in carrier density) instead of impurity doping level inside the semiconductor. These local accumulations/inversions are caused by a difference in the Fermi level at the interface to establish carrier-selective contacts (Fig. 3) [104]. In PSC devices, the difference between the Fermi level of the HTL and the ETL es-

tablishes the built-in potential. Under illumination, the perovskite layer produces the photon-generated electron and hole. The role of the built-in potential is to separate the photon-generated electron and hole, which facilitates them to drift to the ETL and HTL, respectively. Once a suitable ETL material is found, it often has to be thin to reduce the resistance and absorption losses [104]. To achieve high-performance PSC devices, a uniform and thin SnO2 film with low density of defects and pinhole-free is essential. SnO2 compact films are usually deposited by radio frequency (RF) magnetron sputtering, atomic layer deposition, chemical solution deposition, and spray pyrolysis [102]. 2.1. RF sputtering fabrication compact SnO2 film as ETL material The RF sputtering is a vacuum coating technology, and is typically used to deposit high quality transparent conductive oxide (TCO) thin films, such as SnO2 , TiO2 , ZnO, ITO, FTO. A gas, usually argon (Ar), is injected into a vacuum chamber. Under a high frequency and high voltage, the gas forms a high energy ion flow. The ionized gas particles bombard a target and materials sputtered off the target are deposited on a substrate and form a film made of the target material. The film properties are defined by important parameters including RF power, work gas pressure, sputtering time (film thickness), ratio of different work gases, bias voltage, composition and purity of target. For RF sputtering, it is an efficient and repeatable method to deposit SnO2 films. Tao et al. studied the influence of sputtering time (thickness) of SnO2 films on PSC performance [105]. In their research, roomtemperature compact SnO2 films were fabricated by RF sputtering with different sputtering times (0 min, 20 min, 30 min, 40 min, marked as FTO, RFMS-20 (12.6 nm), RFMS-30 (18.9 nm), RFMS-40 (25.2 nm), respectively). As shown in Fig. 4(a, b), the roughness of sample RFMS-30 (12.9 nm, Fig. 4b) is lower than the bare FTO (14.1 nm, Fig. 4a). From Fig. 4(b), the sputtered SnO2 film has a smoother surface and more uniform grain sizes. For PSC devices, the average PCE of FTO, RFMS-20, RFMS-30, RFMS-40 is 5.11%, 10.09%, 12.42% and 8.97%, respectively (Fig. 4c). The best PCE of sample RFMS-30 is shown in Fig. 4(d) with a value of 13.68% [105]. 2.2. Spin-coating deposited compact SnO2 film as ETL material Spin-coating is the most commonly used to deposit SnO2 compact films. The main advantage of spin-coating is that it is easy to define the composition of chemical elements and control the film thicknesses of the depositing thin films. The spin-coating process starts with a SnO2 precursor solution, which is made from a sol–gel solution via aging. The sol–gel solution is then spin-coated onto ITO or FTO substrate and then annealed, creating SnO2 compact films. For spin-coating, important parameters include precursor concentration, solvents, annealing temperature, methodology and technique. The SnO2 precursors include SnCl2 •2H2 O, SnO2 nanoparticles, SnO2 colloids, and tin isopropoxide, to name a few examples. Ke et al. reported creating SnO2 films by utilizing a facile solution approach, where a SnCl2 •2H2 O precursor was spin-coated onto a FTO

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Fig. 3. Schematic diagram of (a) heterojunction solar cells, (b) energy level and Fermi level (dark) of heterojunction solar cells [104].

Fig. 4. (a) AFM image of FTO substrate [105]. (b) AFM image of sputtered SnO2 film on FTO substrate [105]. (c) J–V curves of different planar PSCs [105]. (d) J–V curves of the best performing PSCs using SnO2 ETL [19]. (e) J–V curves of PSCs device [106]. (f) J–V curves of PSCs device [107]. (g) J–V curves of PSCs device [86]. (h) Schematic illustration of Fermi level of EDTA, SnO2 and EDTA-SnO2 relative to conduction band of the perovskite layer [108]. (i) EDTA-SnO2 measured under both reverse- and forwardscan directions [108].

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Fig. 5. Cross-sectional and top-view SEM images of SnO2 films on FTO substrate annealed at: (a), (b) 250 °C, (c), (d) 500 °C. Photovoltaic parameters with different annealing temperatures: (e) current density, (f) voltage, (g) fill factor, and (h) PCE obtained from 12 cells. (i) TRPL of perovskite. (j) J–V curves of PSCs device. (k) Cross-sectional SEM image of PSCs device [109].

substrate and then annealed in ambient air at 180 °C for 1 h [19]. The best solar cells from this approached achieved a PCE of 17.21% (Fig. 4d) [19]. In 2016, Murugadoss et al. used water and ethanol as solvents and tin chloride (SnCl2 ) as a precursor [106]. The compact SnO2 was spin-coated onto an FTO substrate and then annealed at 200 °C for 1 h, creating a full cover and smooth SnO2 film in ethanol [106]. Their PSC devices achieved a maximum PCE of 8.38% with ethanol solvent (Fig. 4e). Compared to water, ethanol is a better dispersant for the SnCl2 precursor. Song et al. used SnO2 nanoparticles as a precursor, and then baked three times at 150 °C for 5 min before a finally baking at 200 °C for 1 h. This technique created a SnO2 compact film, which produced a PCE of 13.0% (Fig. 4f) [107]. A SnO2 colloid is also often used as precursor to form compact SnO2 films. Jiang et al. used this technique to obtain a uniform and pinhole-free SnO2 film for using as ETL material in PSCs [86]. The maximum PCE of their SnO2 -based PSC devices was 20.54%, and was almost free of hysteresis (Fig. 4g) [86]. Because of the enhanced charge extraction and reduced recombination at the interface between perovskite and SnO2 , the hysteresis can be eliminated, as shown in Fig. 4(g) [86]. In 2017, Jiang et al. obtained PSCs with a PCE of 21.6% for small sizes (0.0737 cm2 ) and 20.1% for large sizes (1 cm2 ) with a moderate residual PbI2 in perovskite layer [32]. Yang et al. reported success in suppressing hysteresis

and recorded an efficiency for planar-type devices using EDTASnO2 as the ETL [108]. Ethylene diamine tetraacetic acid (EDTA) provided excellent modification of ETLs in organic solar cells owing to its strong chelation function. The Fermi level of EDTA-SnO2 is very close to the conduction band of perovskite, which is beneficial for enhancing VOC (Fig. 4h). A record 21.60% (certified at 21.52% by Newport) PCE was obtained for planar-type PSCs using EDTA-SnO2 with negligible hysteresis (Fig. 4i) [108]. For SnO2 thin films, the annealing temperature is an important parameter, as it has a decisive effect on the crystal quality and surface morphology of the films. Jung et al. used a tin (IV) isopropoxide (Sn(OCH(CH3 )2 )4 ) precursor to grow the SnO2 films [109]. The pre-dried SnO2 films were heated further in ambient air with temperatures of 100, 150, 200, 250, 300, 350, 400 and 500 °C for 30 min. From the cross-sectional and top-view SEM images, the FTO is fully covered by SnO2 at 250 °C (Fig. 5a, b), while the FTO is not fully covered at 500 °C (shown in Fig. 5c, d) [109]. The difference is mainly due to volume shrinkage from forming nanocrystallites at the higher annealing temperature of 500 °C. The maximum PCE of their PSCs was 16.8% at 250 °C (Fig. 5e–h). From biexponential decay calculations, the time of 0.1 M-based SnO2 is only 1.08 ns (Fig. 5i) [109]. After doping a potassium ion in the perovskite lattice, a PCE of 19.17% was obtained with negligible hysteresis (Fig. 5j, k) [109].

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Fig. 6. (a) Cross-sectional SEM image of PSCs device, (b) J–V curves of TiO2 and SnO2 -based PSCs [114]. (c) SEM image of PEALD SnO2 , (d) J–V curves, (e) Cross-sectional SEM image of PSCs, (f) Normalized performance parameters of a flexible PVSC versus bending cycles [98]. (g) Layer sequence of the device, (h) J–V characteristics of the solar cell [112].

However, traditional processing of SnO2 necessitates a hightemperature and/or long-duration sintering step that limits substrate choice and introduces manufacturing challenges. To overcome this limitation, many researchers reported success with novel annealing methods, such as millisecond-pulsed photonicallyannealed [110] and UV-sintered [111] methods. Zhu et al. used SnCl4 -based precursor and a new annealing method (i.e. millisecond-pulsed photonically-annealed) to obtain high quality SnO2 films [110]. The PCEs of this SnO2 -based PSCs were up to 15.3%. Low temperature (about 70 °C) UV/ozone treatment was used to fabricate SnO2 films by Huang et al. [111]. The PSCs exhibited an impressive PCE of 16.21% [111]. 2.3. ALD deposition of compact SnO2 film as ETL material Besides RF sputtering and spin-coating methods, atomic layer deposition (ALD) technology is another common method to deposit SnO2 compact films. ALD is a technology derived from chemical vapor deposition, which is widely used in the semiconductor industry to produce high quality semiconductor films. The ALD deposition cycle is divided into four steps: (1) precursor A is pulse injected into a reaction chamber and forms a single molecule film on substrate by chemical absorption. (2) The unreacted precursor is swept by an inert gas flow. (3) Another precursor B is injected into the reaction chamber and then reacts with the precursor A absorbed on the substrate, and a thin product film is formed. (4) The excess precursor and the by-products of the reaction are swept by the inert gas flow [31]. Due to the limitation of chemical absorption, one reaction cycle deposits only a single molecular layer. As a result, the whole process is accurately controlled and has a high coverage scale for the produced film. Moreover, in contrast to RF sputtering, the ALD coating process is utilizes at much lower energies, which prevents the substrate from being damaged by high energy ions [31]. Thus, ALD has been widely used to fabricate

ultrathin and high quality, compact SnO2 films for ETL materials [98,112–117]. Baena et al. obtained 15 nm thick SnO2 compact film by ALD (Fig. 6a), which had a high coverage scale [114]. This SnO2 -based PSC device yielded a stabilized PCE of over 18% without hysteresis (Fig. 6b) [114]. In PSC devices, the thickness of the ETL is an important factor affecting device performance. Wang et al. controlled the SnO2 film thickness with varied reaction cycles (Fig. 6c) and studied the effect of SnO2 film thickness on PSC performance [98]. In their research, they fabricated SnO2 films with thicknesses ranging from 0.5 to 25 nm using ALD for ordinary and flexible PSC devices with the structure of glass or PET/FTO/SnO2 /C60 -SAMs/MAPbI3 /spiro-OMeTAD/Au (Fig. 6e) [98]. They first researched the performance dependence on reaction cycles of 40, 70, 100, 130, 160, and 190; all with a deposition temperature of 100 °C. On the glass/FTO substrate, the maximum PCE was 19.03% with C60 SAM modifiers on the SnO2 film surface (Fig. 6d). For the PET/FTO substrate, a PCE of 16.80% was achieved with the same PSC structure (Fig. 6d). Their flexible PSC devices exhibited excellent mechanical stability even after bending the material 200 times. The PCE of these flexible PSCs was 14.00%, which is approximately 85% of its initial PCE value (Fig. 6f) [98]. In 2017, Wang et al. refined and optimized the ALD deposition process to synthesize SnO2 thin films for planar PSCs with stabilized output powers of up to 20.3% [116]. Traditional ITO and FTO substrates are expensive and are about 43% of the total raw material cost for PSC devices [118,119]. Using a Glass/SnOx /Ag/SnOx /MAPbI3 /spiro-OMeTAD/MoO3 /Ag structure, Hu et al. fabricated an ITO-free PSC as shown in Fig. 6(g) [112]. For the SnOx /Ag/SnOx bottom electrode, the SnOx was prepared at 80 °C using ALD. Ag thin film was deposited by RF magnetron sputtering [112]. A maximum PCE of 11.0% was achieved from their PSCs (Fig. 6h) [112]. This study provides new ideas into the research of ITOfree and FTO-free PSCs.

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3. SnO2 nanoparticles as ETL materials for PSCs device Recently, low temperature solution processed planar-structure PSCs have attracted great attention, despite their power conversions being lower than that of their high temperature mesoporous counterparts. The inclusion of thin mesoporous (mp) layers in PSCs acts as an energy bridge, facilitating electron transfer from perovskite to the ETL, and also enhances the contact area between the ETL and the perovskite absorbant layer. Mesoporous TiO2 is the most widely used in PSC devices, but it needs high temperature annealing. Furthermore, under the ultraviolet light, Ti4+ easily reduces into Ti3+ , and thus increases instability and charge recombination [120]. This UV instability can be improved by using SnO2 to replace TiO2 ; thus, developing mesoporous SnO2 based PSCs is a promising approach to improve PCE and stability. Many researchers introduce SnO2 nanoparticles to replace the mesoporous TiO2 . Typical synthetic methods of SnO2 nanoparticles include the hydrothermal method [121,122], microwave-synthesis [123], and chemical bath deposition [124]. In 2015, Zhu et al. reported using a simple silica-templated hydrothermal method to synthesize mesoporous SnO2 single crystals (Fig. 7a), and to fabricate PSC devices (Fig. 7b) [121]. On the SnO2 film surface, upon spin-coating a thin TiO2 layer, the PCE of PSCs increased to 8.54% from the initial 3.76% (Fig. 7c) [121]. In the following year, they had also successfully synthesized SnO2 nanocrystals (NCs) with an average size of approximately 10 nm via the hydrothermal method (Fig. 7d). The SnO2 NCs were used as the ETL (the surface morphology shown in Fig. 7e) in the inverted p–i–n planer PSC device [122]. With a structure of FTO/NiO/MAPbI3 /SnO2 NCs (or C60 /SnO2 NCs)/Ag, they obtained an average PCE of 11.65% for SnO2 NCs ETL-based devices [122]. After inserting a C60 layer at the perovskite/SnO2 interface, the device could effectively suppress the charge recombination at the perovskite/SnO2 interfaces, facilitating charge extraction in the device, wherein the highest PCE of 18.8% was obtained (Fig. 7f) [122]. For microwave-synthesized SnO2 nanoparticles, heating by microwave irradiation has a shorter reaction time than the traditional non-aqueous sol–gel synthesis, allowing for more accurate control of the reaction parameters such as the pressure, temperature and reaction time. These characteristics all favor higher yield and reproducibility [123]. In 2017, Abulikemu et al. reported that SnO2 nanoparticles were synthesized by a microwave-assisted nonaqueous sol–gel route in an organic medium [123]. The nanoparticles grew for 1.5 h with an average oblong irregular nanoparticle size of about 3 nm (Fig. 7g inset) [123]. The best PCE of 14.2% was achieved with a 3 nm SnO2 –NC ETL (Fig. 7h) [123]. Among numerous techniques, chemical bath deposition (CBD) has been proven to be a highly efficient, low-cost, and lowtemperature method for fabrication of various semiconductor films with high quality and high volume, which can be successfully used to deposit CdS buffer layers in the mass production of CuInGaSe2 thin-film solar cells [125]. In 2017, Barbe et al. used CBD to fabricate amorphous SnO2 (a-SnO2 , Fig. 7i). The optical gap of aSnO2 (4.4 eV) was higher than that of crystalline SnO2 (3.6 eV) [124]. Thus, the deeper valence band of a-SnO2 guarantees a strong hole-blocking property. It was used as the ETL in their PSCs, obtaining a PCE of 14.8% (Fig. 7j) [124]. Xiong et al. used a new technology with fully high temperature process to synthesize Mgdoped SnO2 QDs as the blocking layer (bl) and very thin SnO2 nanoparticles as the mesoporous layer [91]. PSC device structure is shown in Fig. 7(k). It is clear that the perovskite material completely penetrated the pores of mp-SnO2 , increasing the contact area of SnO2 with perovskite absorbant layer, and forms a better contact between the bl and mp layer. Their bl/mp SnO2 -based PSC devices exhibited a high PCE of 19.21% and was hysteresis-free (Fig. 7l) [91].

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Lee et al. introduced a SnO2 self-passivating layer to replace the mp-TiO2 structure [126]. As seen in Fig. 8(a), they produced a ∼20 nm thick passivated tin oxide (PTO) layer with excellent surface coverage over the FTO substrate [126]. They used ultrathin TiO2 (∼5 nm) and PTO (∼20 nm) as the ETL and PTAA as the HTL in their PSCs (Fig. 8b). The mesoporous PTO enhanced electron injection and reduced interface defect density at the TiO2 /PTO/perovskite interface (electron transporting as illustrated in Fig. 8b inset). A maximum PCE of 19.80% was obtained (Fig. 8c) [126]. In 2018, Roose et al. reported that mesostructured SnO2 and gallium-doped SnO2 (Ga:SnO2 ) electrodes were synthesized using a structure directing block-copolymer on compact SnO2 thin film method [102]. From the SEM images in Fig. 8(d) and (e), similar pinhole sizes are observed for SnO2 (54 ± 13 nm) and Ga:SnO2 (57 ± 12 nm) films. They used aluminum doped zinc oxide (AZO) for the front electrode and Ga:SnO2 as the ETL, effectively fabricating the ITO-free electrode PSC device (Fig. 8f) [102]. The best PCE for m-Ga:SnO2 based PSCs was 17.0%, with a stabilized PCE of 16.4%, and the best PCE for undoped m-SnO2 was only12.7% (Fig. 8g) [102].

4. Doped SnO2 as the ETL material for PSCs Undoped SnO2 is a wide band gap and high transmittance ntype semiconductor material, whose electrical properties critically depend on its intrinsic defects (Sn interstitials (Snin ) or O vacancies (VO )) [127]. However, for solar cells, conductivity should be maximized without affecting the transmission, all while minimizing energy loss [127]. Many elements have been tested as dopants, such as: Nb5+ [92], Sb5+ [100], Li+ [97], Mg2+ [91], Al3+ [101], Ga3+ [102], and Y3+ [103]. In PSC devices, the doping of SnO2 can improve conductivity induce an upward shift of the Fermi level of SnO2 , which facilitates injection and transfer of electrons from the conduction band of the perovskite material to the SnO2 ETL, leading to reduced charge recombination and improved PCE of the PSCs. For doping of SnO2 , the doping density is an important factor affecting PSCs performance. Xiong et al. have researched the effect of Mg doping density for SnO2 on PSC performance [93]. In their research, they deposited Mg-doped SnO2 films with Mg doping densities of 0, 2.5, 5, 7.5, 10 and 20 mol%. Mg doping could decrease the carrier density and increase the carrier mobility. In comparison to undoped SnO2 , for the 7.5% Mg-doped SnO2 film, the carrier density reduced to 1.2 × 1015 cm−3 form 1.2 × 1017 cm−3 , and the mobility increases over 5 times (undoped SnO2 : 9.865 cm2 V−1 s−1 , 7.5% Mg-doped SnO2 : 55.19 cm2 V−1 s−1 ) [93]. They obtained an average PCE of 7.186% for undoped SnO2 -based PSCs. After Mg doping, the average PCE of the devices was significantly improved to 10.91%, 11.98%, 14.55%, 10.39% and 9.925% for 2.5%, 5%, 7.5%, 10% and 20% PSCs, respectively. The best PCE of 14.55% was achieved with 7.5% Mg:SnO2 based PSCs [93]. Antimony (Sb) is one of the most common n-type dopants for SnO2 , which was investigated by Bai et al. [100]. They synthesized high quality Sb-doped SnO2 nanocrystals, used as the ETL in their PSC devices. After doping, the carrier concentration of SnO2 increased to 8.2 × 1022 cm3 from 6.7 × 1021 cm3 , and the upward shift of the Fermi level was calculated to be around 60 meV [100]. The increased Fermi level matched closely with the electron quasi-Fermi level of MAPbI3 under illumination without dragging down the quasi-Fermi level splitting in the PSCs, which enhanced the VOC of the PSCs [100]. The charge recombination lifetime (0.87 μs) of the Sb:SnO2 PSC device was substantially longer than that (0.34 μs) of the undoped SnO2 PSCs. The longer charge recombination lifetime could suppress interfacial charge recombination. The PCE of PSCs was improved to 17.7% from 15.7%, and the VOC increased to 1.06 V from 1.01 V [100].

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Fig. 7. (a) TEM image of mesoporous SnO2 single crystals, (b) cross-sectional image of a typical TiO2 –SnO2 MSC-based PSCs, (c) J–V curves of PSCs [121]. (d) TEM image of hydrothermal SnO2 NCs, (e) SEM image of hydrothermal SnO2 layer, (f) J–V curve of the top-performing PSCs [122]. (g) SEM image of the SnO2 –NCs, (h) J–V curves of PSCs [123]. (i) SEM image of a-SnO2 , (j) J–V curves of PSCs [124]. (k) Cross-sectional SEM image of 100 nm thick mp SnO2 PSCs, (l) J–V curves [91].

Lithium (Li) is univalent element and using it as a dopant in SnO2 can improve conductivity as well as induce a downward shift of the conduction band minimum of SnO2 , which facilitates injection and transfer of electrons from the conduction band (CB) of perovskite to the SnO2 ETL. In 2016, Park et al. used a lowtemperature solution-processed method to synthesize Li-doped SnO2 (Li:SnO2 ) nanocrystals, which were then spin-coated on a FTO/glass substrate and used as the ETL in the PSC devices (Fig. 9b)

[97]. As seen in Fig. 9(a), the grain size of Li:SnO2 was about 5 nm (EELS-mapping is shown in Fig. 9 inset). After Li doping, the conductivity improved to 9.1 × 10−5 S cm−1 from 4.14 × 10−5 S cm−1 [97]. Substitution of Sn4+ with Li+ ion could induce a decrease in the CB. From UPS results (Fig. 9c, d), the Fermi level (EF ) of Li:SnO2 and SnO2 was calculated as −4.85 eV and −4.7 eV, respectively. The Eg of Li:SnO2 was reduced from −4.22 eV (SnO2 ) to −4.35 eV. The deeper ECB of Li:SnO2 increased the driving

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Fig. 8. (a) SEM images of bare FTO and the FTO/PTO, respectively, (b) cross-sectional SEM image of PSCs device, (c) J–V curves of PSCs [126]. (d) SEM image of undoped m-SnO2 , (e) SEM image of Ga:SnO2 (2.5%), (f) cross-section SEM image of a PSCs device, (g) J–V curves of PSCs using undoped and 2.5% Ga-doped SnO2 [102].

force of electron injection from the CB of perovskite to SnO2 [97]. The best PCE for both backward and forward sweeps in the Li:SnO2 -devices were 18.2% (backward, 15.05% of SnO2 -device) and 17.14% (forward, 13.55% of SnO2 -device), respectively (Fig. 9e) [97]. Meanwhile, Li:SnO2 was used as the ETL for flexible PSCs (Fig. 9f), and a maximum PCE of 14.78% was obtained (Fig. 9f) [97]. Al-doped in SnO2 also enhanced the charge transport ability in planar PSCs. Chen et al. reported a low-temperature synthesis of Al-doped SnO2 (Al:SnO2 ) as the ETL in PSCs [101]. The champion cell based on Al:SnO2 exhibited a higher efficiency of 12.10% than that of SnO2 (9.02%) [101]. Yang et al. reported a yttrium-doped tin oxide (Y:SnO2 ) material for electron transport in PSCs [103]. The Y:SnO2 conductivity of 1.05 × 10−5 S cm−1 was higher than SnO2 (7.5 × 10−6 S cm−1 ); the higher conductivity could reduce the contact resistance and facilitate the charge extraction, thus improving JSC [103]. The band gaps of the SnO2 film and Y:SnO2 film are 3.65 and 3.70 eV, respectively. From their calculations, the conduction band minimum (CBM) of Y:SnO2 was ≈ 0.13 eV higher than that of SnO2 . Hence, the devices using Y:SnO2 ETL had higher VOC than those using an undoped SnO2 ETL (Fig. 9g) [103]. They found that Y doping could promote the formation of well-aligned and homogeneously distributed SnO2 nanosheet arrays (NSAs), as shown in Fig. 9(j) and (k). The PSCs based on a Y:SnO2 ETL showed a maximum

PCE of 17.29%, compared to 13.38% for PSCs with undoped-SnO2 as the ETL (Fig. 9h) [103]. The active area of 1 cm2 for Y:SnO2 -based PSCs had a maximum PCE of 12.60% (Fig. 9i) [103]. Niobium (Nb) doped SnO2 (Nb:SnO2 ) could enhance the electron mobility, and effectively passivate electron traps for SnO2 . In 2017, Ren et al. synthesized SnO2 and Nb:SnO2 ETLs using lowtemperature solution-processes. The surfaces of SnO2 and Nb:SnO2 films were both uniform, flat, and pinhole-free (Fig. 10a, b) [92]. After Nb doping, the electron mobility of Nb:SnO2 increased to 2.16 × 10−4 cm2 V−1 s−1 from 1.02 × 10−4 cm2 V−1 s−1 (SnO2 ), and the electron density of Nb:SnO2 decreased to 1.74 × 1015 cm−3 from 2.39 × 1015 cm−3 (SnO2 ) [92]. As a result, the PCE was rapidly increased to 17.57% (Nb:SnO2 -based PSCs) from 15.13% (SnO2 -based PSCs) (Fig. 10c) [92]. Recently, Roose et al. synthesized mesostructured Ga:SnO2 and they found that Ga-doping severely decreased the trap state density in SnO2 , reducing charge recombination. A maximum PCE of 17.0% and a stabilized PCE of 16.4% were obtained (Fig. 8g) [102]. For ETL materials, the lower free electron density prevents carrier recombination, and the higher electron mobility facilitates fast extraction of electrons from perovskite to ETLs, contributing to improve JSC and PCE for planar SnO2 -based PSCs [91,93]. Xiong et al. investigated Mg-doped SnO2 with a 500 °C annealing with varied Mg doping density (Fig. 10d). A significant reduction in carrier density (to 7.1 × 1014 cm−3

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Fig. 9. (a) TEM image of Li:SnO2 thin films, (b) Cross-sectional SEM image of PSCs, (c) and (d) UPS spectra for SnO2 and Li:SnO2 , (e) J–V curves measured of Li:SnO2 based PSCs, (f) J–V curves of the flexible PSCs [97]. (g) Cross-sectional SEM image of a PSCs using a Y-SnO2 ETL, (h) J–V curves of the PSCs, (i) J–V curves of PSCs of active area 1 cm2 , (j) cross-sectional SEM image of perovskite films, (k) SEM image of Y-SnO2 nanosheets [103].

Fig. 10. (a) SEM image of SnO2 , (b) SEM image Nb:SnO2 films, (c) J–V curves of SnO2 and Nb:SnO2 ETL based PSCs [92]. (d) SEM image of 5% Mg-incorporated QD SnO2 annealed at 500 °C, (e) J–V curves of planar SnO2 -based PSCs, (f) standard deviations on PCEs to evaluate reproducibility [91].

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(500-Mg:SnO2 ) from 2.1 × 1015 cm−3 (500-SnO2 )) and a higher carrier mobility of 171.31 cm2 V−1 s−1 was observed [91]. The JSC increased to 21.67 mA cm−2 from 20.53 mA cm−2 (Fig. 10e) [91]. Their Mg:SnO2 -based planar PSCs device exhibited a high PCE of 17.86% (Fig. 10e), and the highest average PCE of 16.42% (Fig. 10f) [91]. 5. SnO2 /composite structure as ETL materials for PSCs SnO2 has been reported as an alternative ETL for high efficiency PSCs. It has been shown that at low-temperature, compact SnO2 films exhibit good band edge alignment with the perovskite active layer, high electron mobility and good antireflection behavior. In PSC devices, the low-temperature solution method (LTSM) is most commonly used to prepare the perovskite active layer and the SnO2 ETL. OIHP materials have higher carrier mobility and lower internal recombination; therefore, surface recombination plays a crucial role in determining the performance of PSCs. For SnO2 ETLs, the recombination mainly comes from intrinsic and bulk defects. Similar to other semiconductor materials, point defects affect the electrical and optical properties of SnO2 . Oxygen vacancies (VO ) and tin interstitials (Sni ) are two main intrinsic defects in SnO2 films because their formation energy is very low thus these defects form readily [128]. The bulk defects are mainly pinhole and thus crack in SnO2 films, which lead to current leakage in PSCs and reduce the photovoltaic performance. Reducing the current leakage energy loss and retarding the carrier recombination at the interface are both crucial to enhance the performance of PSCs. These SnO2 films still need to be covered by a thin layer of metal oxide (i.e. TiO2 , ZnO, MgO.) or organic materials (i.e. C60 , PCBM, benzoic acid (BA), 3-aminopropanoic acid (C3), 4-pyridinecarboxylic acid (PA), 4-cyanobenzoic acid (CBA) and 4-aminobenzoic acid (ABA)) to reduce the surface and bulk trap density, and to suppress carrier recombination. 5.1. SnO2 and metal oxide composite structure as ETL material for PSCs In PSC devices, double layers of metal oxide composite structures are commonly used in compact/mesoporous films as ETLs. For example, for TiO2 , the SnO2 –TiO2 double-layer brings better bandgap matching at the perovskite/FTO interface, which promotes charge extraction and reduces surface recombination. Huang et al. used a low-temperature technique to prepare the SnO2 –TiO2 layer [129]. The SEM images of SnO2 and SnO2 with TiCl4 treatment are shown in Fig. 11(a) and (b) (AFM images in Fig. 11a, b inset). After TiCl4 treating, the surface roughness of SnO2 layer was reduced from 7.69 to 6.26 nm [129]. The SnO2 based PSC exhibited a low efficiency of 6.2%, but the PCE of SnO2 –TiO2 based PSC was up to 14.8% (Fig. 11c) [129]. The PCE improvement can be mainly attributed to the SnO2 –TiO2 double-layer improving the electron transport efficiency, resulting in reduced electron recombination (Fig. 11d) [129]. Duan et al. used cl-SnO2 /mp-TiO2 as ETL in PSC devices and the surface of cl-SnO2 was modified and treated with SnCl4 aqueous solution [130]. After the SnCl4 treatment, the recombination behavior in the cell interior is greatly retarded, achieving a peak PCE of 15.07% [130]. In flexible PSC devices, SnO2 is also widely used as the ETL because the crystallization temperature of SnO2 is lower than that of TiO2 . Meanwhile, SnO2 exhibits better chemical stability than ZnO, and less photocatalytic activity than either TiO2 or ZnO, both of which are helpful for improving flexible device stability. Dagar et al. studied and reported efficient flexible PSCs (FPSCs) and modules based on SnO2 /mp-TiO2 [95]. Their PSC structure is shown in Fig. 11(e) and the J–V curves are shown in Fig. 11(f). A maximum PCE of 14.8% was obtained with SnO2 /mp-TiO2 ETL, which was 30%

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higher than the PCE of cells with only SnO2 as the ETL (11.4%) [95]. In order to evaluate the mechanical properties of FPSCs, they carried out bending tests with varied curvature radii (34, 28, 25, 21, 12.5, 7.2, and 4 mm). Starting from the largest radius (Fig. 11g) and enduring 100 bending cycles, the PCE maintained 60% of its initial efficiency value (14.8%), even with the smallest curvature radius (4 mm). From the above results, the FPSCs device demonstrated excellent mechanical properties [95]. For SnO2 –TiO2 mesoporous nanocomposite layer, a high PCE PSCS can be achieved using interfacial synergies of the discontinuous spot between SnO2 and TiO2 particles. Using this strategy, SnO2 ETL-based PSCs can better suppress charge recombination, which effectively reduces energy loss [131]. Dong et al. used this strategy and obtained the peak PCE of 17.58% [131]. Lee et al. revealed a low-temperature processed SnO2 with self-passivating nature used in a SnO2 scaffold PSC device [126]. They combined a compact TiO2 underlayer (∼5 nm) with the SnO2 scaffold layer (∼20 nm) (Fig. 8b inset). The best planar structure PSC device demonstrated photovoltaic values in with JSC of 22.58 mA cm−2 , VOC of 1.13 V, FF of 0.78, and a PCE of 19.80% under 1 sunlight illumination (Fig. 8c) [126]. Modifying the transparent conductive electrode surface can also reduce the energy loss and prohibit the carrier recombination at the interface, which improves the performance of the PSCs. In 2017, Ma et al. incorporated an ultrathin wide bandgap dielectric MgO nanolayer between the FTO electrode and SnO2 ETL of planar PSCs (Fig. 12a, b), which enhanced the electron transport and hole blocking [132]. As illustrated in Fig. 12(d), spin-coated perovskite on top of the monolayer SnO2 ETL could penetrate through the pinholes and cracks among the SnO2 grains. Thus, this might create a direct contact between perovskite layer and FTO electrode, leading to electron–hole recombination and strong current leakage at the interface. A MgO interlayer was incorporated to hinder this disadvantageous contact between the perovskite and FTO [132]. Evidently in Fig. 12(e), the MgO layer prevents the perovskite from reaching to the FTO surface, decreasing the possibility of electron– hole recombination and current leakage, thus improving the performance of the device. By using MgO/SnO2 ETL, they achieved a PCE of 18.23% (Fig. 12c), an 11% improvement compared to that without a MgO modifier (a PCE of 16.43%) [132]. Meanwhile, after using ITO instead of FTO, a higher PCE of 18.82% was obtained [132]. This work demonstrates a new direction to enhance the performance of PSCs with TCO electrode surface modification [132]. In 2018, Jiang et al. reported a HTL-free PSC device structure (Fig. 12f, g and the schematic in Fig. 12g inset). Their ETL was a SnO2 compact layer (c-SnO2 ), laid under a meso–TiO2 nanocrystalline layer, a meso–ZrO2 insulating layer and a porous carbon electrode, printed in sequence [133]. A peak PCE of 13.77% was obtained (Fig. 12h) [133]. Electrochemical impedance spectroscopy (EIS) measurements were conducted to investigate the chargetransport and interfacial charge-transfer processes of MPSCs. [133]. 5.2. SnO2 and organic materials composite structure as ETL material for PSCs Fullerene and its derivatives, such as C60 , phenyl-C61 -butyric acid methyl ester (PCBM), are excellent acceptors for ETL materials in PSCs [81,134–141]. Their charge extraction is highly efficient, and the fullerene can passivate the surface defects of perovskite and the SnO2 layer. Using SnO2 solely as the ETL still has some drawbacks such as inefficient charge extraction and interface recombination, but when combined with fullerenes or graphene, these issues could be reduced or eliminated. Many researchers have tried to use C60 -self-assembled or PCBM to modify SnO2 surface, resulting in a moderate improvement to the performance of PSCs. In 2016, Wang et al. used a C60 -self-

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Fig. 11. (a) SEM image of SnO2 , (b) SEM image of SnO2 with TiCl4 treatment, (c) J–V curves of PSCs, (d) schematic diagrams illustrate the band alignment and charge recombination in the cells with SnO2 –TiO2 [129]. (e) Cross-section SEM image of a FPSCs, (f) J–V curves of the best performing FPSCs, (g) Normalized PCE of FPSCs [95].

Fig. 12. (a) Schematic of the PSCs device structure, (b) cross-section SEM image of a device, (c) J–V curves of the PSCs, (d) schematic of the perovskite can directly contact the FTO surface, (e) The MgO HBL moderate the penetration of perovskite reaching the FTO surface [132]. (f) Cross-sectional SEM image of device without perovskite, (g) cross-sectional SEM image of device with perovskite, (h) J–V curves of the champion MPSCs [133].

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Fig. 13. (a) Schematic device structure, (b) cross-sectional SEM image of PSCs, (c) schematic energy band diagram of PSCs, (d) SEM image of perovskite film, (e) photoluminescence decay of perovskite films, (f) J–V curves of PSCs [98]. (g) Illustration of the PSCs, (h) cross-sectional SEM image of PSCs device, (i) J–V curves of the PSCs [122].

assembled (C60 -SAMs) monolayer to passivate the SnO2 film surface, forming SnO2 /C60 composite film structure as the ETL in PSCs (Fig. 13a, b) [98]. C60 -SAMs can passivate the SnO2 surface and optimize band edge alignment (Fig. 13c), which can promote electron transfer from the perovskite active layer to the FTO electrode, reducing charge recombination in the interface of SnO2 /perovskite. For SnO2 /C60 -SAMs spin-coated perovskite films, grain sizes greater than 1 μm can be obtained (Fig. 13d). In comparison to SnO2 /FTO, Fig. 13(e) shows that the perovskite film deposited on C60 -SAMs/SnO2 /FTO has lower charge carrier lifetimes (perovskite/SAMs/SnO2 /FTO: 22 ns; perovskite/SnO2 /FTO: 57 ns). This demonstrates that electron extraction can be enhanced by modifying perovskite/SnO2 interface with the C60 SAMs [98]. The PCE of PSCs based on C60 -SAM/SnO2 /FTO was 19.02% (Fig. 13f) which was higher than that of 17.16% PSCs based on SnO2 /FTO [98]. In inverted planar structure PSC devices, a thin C60 layer inserted in the perovskite/SnO2 interface could also promote electron transfer. Zhu et al. fabricated invert planar structure PSC devices with C60 /SnO2 NCs as the ETL. Their schematic structure and crosssectional SEM image is shown in Fig. 13(g) and (h), respectively. The perovskite/C60 /SnO2 enhanced the PL quenching efficiency, and in turn their PCEs reached 18.8% and 18.4% for reverse and forward scans, respectively, with negligible J–V hysteresis (Fig. 13i) [122].

C60 -SAMs modifies the SnO2 surface to improve the photovoltaic performance parameters, improve the quality of p–n junction, and reduce the J–V hysteresis for SnO2 -based PSC devices. Wang et al. investigated the effects of thermal annealing for SnO2 /C60 SAMs ETLs for PSC device performance [116]. The device structure and the electric-field difference are shown in Fig. 14(a). It is clear that there were only strong peaks at the SnO2 /perovskite interfaces, while the peaks at the Spiro-OMeTAD/perovskite interfaces were relatively weak. This suggests that the thermal annealing in ambient air reduces the imbalance of charge transportation at the ETL/perovskite and HTL/perovskite interfaces, leading to a significantly reduced J–V hysteresis [116]. Annealing at 150 °C resulted in a PCE of 20.16% (reverse) and 19.47% (forward) with a slightly reduced J–V hysteresis (Fig. 14b) [116]. The trap density of states (tDOS) is shown in Fig. 14(c) for all PSCs, indicating that the SnO2 /C60 SAMs ETL interface recombination rates did not significantly change for low temperature annealings, but that the electrical conductivity of the SnO2 increased with increasing thermal annealing temperatures (Fig. 14d). Xiao et al. used ALD to deposit SnO2 and spin-coated C60 -SAM on the SnO2 surface (Fig. 14e). From the Kelvin probe force microscopy (KPFM) results, the main potential drop was observed at the p–n junction of SnO2 /C60 -SAM/perovskite [142]. Meanwhile, the electric-field difference peak intensity indicated a better interface

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Fig. 14. (a) Cross-sectional KPFM of PSCs, (b) J–V curves of PSCs [116]. (c) Cross-sectional KPFM of PSCs, (d) J–V curves of PSCs [142]. (e) TRPL decay transient spectra of perovskite films deposited on SnO2 and SnO2 /PCBM ETLs, (f) J–V curves of PSCs [143].

quality for SnO2 /C60 -SAM/perovskite than for SnO2 /perovskite. They achieved a max PCE of 19.28% (reverse) and 19.25% (forward) for SnO2 /C60 -SAM, and hardly any J–V hysteresis was observed (Fig. 14f) [142]. Whether it is an ordinary PSC device or a flexible device, a double-layer ETL (TiO2 /PCBM or SnO2 /PCBM) contact design can reduce charge recombination and improve the performance of PSCs. Ke et al. used ultrathin PCBM to passivate both the perovskite grains boundaries and SnO2 /perovskite interface, reducing cracks and pinholes found in the SnO2 and the perovskite layers (Fig. 14g, h) [143]. In this study, they fabricated PSC devices with a regular cell structure (Fig. 14i), using either SnO2 or SnO2 /PCBM as the ETL. In comparison to the perovskite film deposited on the SnO2 ETL, the film on the SnO2 /PCBM ETL had

lower trap state density and a faster electron transfer velocity (Fig. 14j) [143]. They obtained a peak PCE of 19.12% and a steady-state PCE of 17.75% for planar PSCs using an optimized SnO2 /PCBM ETL (Fig. 14k) with a steady-state current density of 19.48 mA cm−2 [143]. For flexible PSC devices, Lam et al. achieved a PCE of 14.3% using SnO2 /PCBM as the ETL [144]. The interfacial chemical interactions are a critical factor in determining the optoelectronic properties of PSCs (Fig. 15b, c) [145]. Zuo et al. introduced different functional groups (i. e. PA, BA, CBA, ABA, and C3 ) onto the SnO2 surface, forming various chemical interactions with the perovskite layer (Fig. 15a) [145]. Surface modifications to the PA, BA, CBA, ABA, and C3 were performed to reduce trap states on the SnO2 surface and to influence photocarrier dynamics (Fig. 15d–g). Meanwhile, different functional

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159

Fig. 15. (a) Schematic diagram of PSCs device structure (left) and SAMs between the SnO2 and perovskite film. (b) SEM cross-sectional image of PSC device. (c) SEM image of perovskite films on SnO2 substrate. (d) Steady state and (e) transient PL spectra of perovskite film on SnO2 with different SAMs. (f) Transient photovoltage of PSCs. (g) Schematic diagram for the charge dynamics at perovskite/SnO2 interface in the PSCs device, (h) ultraviolet photoelectron spectroscopy of SnO2 , (i) J–V curves of PSCs device, (j) EQE spectra and integrated JSC of champion PSC, (k) simulated EQE and IQE spectra of PSCs with PA modification. (l) Hysteresis I–V characteristic curves of PSCs. (m) Steady-state photocurrent and output power of PSCs [145].

SAM groups modified the work function of the SnO2 surface (Fig. 15h) [145]. After SAMs modified the SnO2 surface, the corresponding PSC device parameters are shown in (Fig. 15i) and are summarized in Table 2 [145]. PSCs with PA modified SnO2 ETLs achieved the best PCE of 18.77%, but a weak hysteresis phenomenon was also observed (Fig. 15l). The maximum EQE was ∼94% and the integrated JSC was 21.76 mA cm−2 (Fig. 15j). Across nearly whole absorption region, the calculated IQE spectra were over 90% (Fig. 15k) [145]. For the PSCs with PA modified SnO2 ETL, the steady state output PCE of 17.7%, with the bias of 0.88 V and the photocurrent of 20.1 mA cm−2 , was achieved (Fig. 15m) [145].

Table 2. Device performance of PSCs with different SAMs modification. SAMs

Ef (eV)

JSC (mA cm−2 )

VOC (V)

FF

PCEa (%)

None CBA PA BA C3 ABA

4.21 4.29 4.17 4.16 4.07 4.05

21.65 21.66 22.03 21.88 21.48 22.00

1.06 1.08 1.10 1.11 1.08 1.04

0.749 0.781 0.774 0.746 0.738 0.721

17.16 18.27 18.77 18.11 17.11 16.50

(16.21 (17.24 (18.01 (17.14 (16.30 (15.21

± ± ± ± ± ±

0.89) 0.68) 0.57) 0.64) 0.49) 0.89)

a The PCE values outside of the parentheses are the best, and those in parentheses are the averaged values with deviations.

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Fig. 16. (a) Cross-sectional FE-SEM image of synthesized SnO2 NWs arrays on FTO substrate. (b) TEM image of a typical SnO2 NW. (c) High-resolution TEM (HRTEM) lattice image of the SnO2 NW with [−101] zone axis, (d) transmittance spectrum of bare FTO and SnO2 NW/FTO. (e) TEM image of a typical TiO2 nanoshell-coated SnO2 NW, (f) HRTEM lattice image showing the SnO2 NW/TiO2 interface. (g)–(j) STEM image of the TiO2 nanoshell-coated SnO2 NW, and elemental maps for Ti, O, and Sn, respectively. (k) Illustration of epitaxial TiO2 on SnO2 . (l) Raman shift spectrum TiO2 . (m) J–V curve of bare SnO2 NW based ETL PSCs. (n) J–V curves of TiO2 NP-based mp-ETL and TiO2 /SnO2 NW-based 1D-ETLPSCs. (o) J–V curve of the best PSC among the surface treated TiO2 /SnO2 NW-based 1D-ETL PSCs [149].

6. PSCs based on other SnO2 nanostructures

to treat TiO2 /SnO2 NWs surface, they achieved a maximum PCE of 14.2% (Fig. 16o) [149].

6.1. SnO2 nanowires SnO2 nanowires (NWs) are widely used one-dimensional (1D) structures, which has high aspect ratios that can provide highly efficient transport channels for the injected electrons. In addition, 1D nanostructures have less grain boundaries, which reduces the number of dead-ends [146]. SnO2 NWs have been widely used and studied in DSSCs [146–148]. Han et al. reported high-performance PSCs with excellent electron transport properties using SnO2 1D ETL [149]. In their research, the 1D array-based ETL was grown on a FTO substrate via vapor–liquid–solid (VLS) reaction (Fig. 16a–c). In the range of 400– 10 0 0 nm, the SnO2 NWs/FTO and the bare FTO substrate exhibited similar transmittance of 80% (Fig. 16d) [149]. When they used the bare SnO2 NWs as ETL in PSCs, a very low VOC (0.77 V) and PCE (5.91%) was observed (Fig. 16m). By using TiO2 nanoshell layers to wrap around the SnO2 , the performance of cells can be improved. In their research, they used the ALD method to deposit TiO2 nanoshell layers onto the SnO2 NWs with the help of O2 plasma (shown in Fig. 16e–l) [149]. For TiO2 /SnO2 NWs-based PSCs, an average PCE of 11.9% was obtained (Fig. 16n). After using TiCl4

6.2. SnO2 nanotube Nanostructured SnO2 is known to have a higher electron mobility and a more negative conduction band minimum (CBM) than nanostructured TiO2 [150]. Gao et al. used ZnO NRs as templates and a uniform nanostructured porous SnO2 shell was spincoated onto the ZnO NRs surface, creating a SnO2 nanotube (SnO2 – NTs, Fig. 17a, b). They fabricated PSC devices with a structure of glass/FTO/SnO2 –NTs/perovskite/P3HT/Au (Fig. 17a, b). A maximum PCE of 12.26% (reverse) and 11.3% (forward) was obtained (Fig. 17c) [151]. From the corresponding EQE curve (Fig. 17d), the value of integrated current density was only 17.99 mA cm−2 . The steady-state current was 15.9 mA cm−2 with a stable PCE of 12.1% for over 10 0 0 s (Fig. 17e) [151]. The device properties confirmed that such SnO2 –NTs possessed excellent charge transporting performance and a proper energy level matching with perovskite. However, PSC devices based on SnO2 NTs are deficient due to lower VOC (0.76 V) and JSC (15.9 mA cm−2 ) [151].

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Fig. 17. (a) Scheme of fabrication process of PSC, (b) cross-sectional SEM images of SnO2 -DNTs and PSC devices. (c) J–V curves of the best PSC device, (d) EQE spectrum and integrated current density, (e) stable photocurrent and PCE output performance [151].

6.3. SnO2 nanosheets The advantages of SnO2 are numerous, including low chemical reactivity, low photocatalytic activity [152], a wider band gap (wider than TiO2 and ZnO), higher electron mobility, and low processing temperatures. The ETL contains a thin compact SnO2 layer under a mp-SnO2 nanosheets layer. The SnO2 nanosheet layer plays multiple roles such as improving photon collection, preventing moisture infiltration, and enhancing long-term stability for the PSC device. Furthermore, SnO2 nanosheets: (1) promote the formation of well aligned and more homogeneous distribution of SnO2 nanosheets, which allows for better perovskite permeation, better contacts of perovskite with SnO2 nanosheets, and improves electron transfer from perovskite to ETL. (2) SnO2 nanosheets amplify the band gap and upshifts the band energy level, resulting in better energy level alignment with perovskite and reduced charge recombination at nanosheets/perovskite interfaces [103]. Hydrothermal growth is a common way to synthesize SnO2 nanosheets. Zhou et al. synthesized SnO2 nanosheets via the hydrothermal method and used these nanosheets as ETLs to replace TiO2 in PSCs. They fabricated the PSC device with a structure of FTO/SnO2 /MAPbI3 /spiro-OMeTAD/Au, and observed a peak PCE of 6.86% (SnO2 nanosheets), which is higher than the 5.10% of SnO2 nanoparticle-based PSCs [153], because the SnO2 nanosheets have better electron transportation properties than SnO2 nanoparticles. Liu et al. reported developing a layered nano-SnO2 (cpSnO2 /SnO2 nanosheets) ETL, grown using a low-temperature hydrothermal method [154]. After growing the SnO2 nanosheets, PbI2 and CH3 NH3 I were deposited on SnO2 nanosheets, and then annealed at 70 °C for 40 min in ambient air. After annealing, spiroOMeTAD was spin-coated on the top of MAPbI3 [154]. The PSCs based on cp-SnO2 /SnO2 nanosheets (4 h grown) revealed a maximum PCE of 16.17% [154]. With increased growth time, the number of SnO2 nanosheets increased, which contributed to quicker extraction and transfer of the electrons. The use of yttrium-doped tin dioxide (Y:SnO2 ) nanosheet arrays as the ETL material was reported by Yang et al. [103]. They found that Y doping can promote the formation of well-aligned and homogeneously distributed SnO2 nanosheet arrays (Fig. 9j, k). The Y:SnO2 was used as the ETL in the PSC devices; the device struc-

tures are shown in Fig. 9(g). The best PCE obtained was 17.29%, compared to that of only SnO2 -based of 13.38% (Fig. 9h) [103]. A Y:SnO2 -based PSC with an active area of 1 cm2 demonstrated a maximum PCE of 12.60% (Fig. 9i) [103]. 7. Improving the PCE and long-term stability for SnO2 -based PSCs 7.1. Methods of improving PCE for SnO2 -based PSCs The requirements for highly efficient solar cells include material with high minority carrier lifetime, high absorption coefficient, wide absorption spectrum, low layered, complete photocurrent extraction, and reduced electrical loss. OIHP materials have diffusion lengths which exceed 1 μm for carrier and high carrier diffusion velocity [11–13], and a high optical absorption coefficient (>1.0 × 105 ) [10]. Thus, OIHP materials are an ideal absorber material for solar cells [6–19]. The performance of SnO2 ETL based PSCs has made great progress, with PCEs now exceeding 20% [32,84–90] and closing in on TiO2 based PSCs. OIHP materials have high carrier mobility and low internal recombination; therefore, surface recombination plays an important role in determining the performance of PSCs. For SnO2 ETL, the recombination mainly comes from the intrinsic defects and bulk defects of the SnO2 . Similar to other semiconductor materials, point defects affect the electrical and optical properties of SnO2 . The oxygen vacancies (VO ) and tin interstitials (Sni ) are the two main intrinsic defects in SnO2 , because their formation energy is very low, and thus these defects form readily [128]. 7.1.1. Improving SnO2 preparation technology and physical properties MAPbI3 -based PSC devices are the most widely studied. Dong et al. reported PSC devices using a nanocrystalline SnO2 thin film, prepared by a sol–gel method for the ETL in PSC device [155]. An average PCE of 6.87% was achieved and the VOC and FF were only 0.84 V and 0.41, respectively [155]. The low VOC and FF were mainly attributed to severe charge recombination that occurred at the interface between SnO2 and MAPbI3 [155]. Ke et al. used SnCl2 •2H2 O as a precursor and spin-coated it on to the FTO substrate and then annealed at 180 °C for 1 h in ambient air,

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Fig. 18. (a) Cross-sectional SEM image of the PSC device. (b) Energy levels diagram of the device base on the SnO2 :GQDs before and after illumination. (c) Histogram of the PCE values based on SnO2 and SnO2 :GQDs PSCs. (d) J–V curves of the best PSCs. (e) Stabilized PCEs measured at their respective Vmp 0.947 V and 0.902 V, respectively. (f) EQE spectra of the best-performing cells [89].

creating the SnO2 ETL material (Fig. 4e) [19]. An average PCE of 16.02% was achieved for their best-performing planar cell using SnO2 as the ETL [19]. Dong et al. also used SnCl2 •2H2 O as a precursor and spin-coated it onto a FTO substrate. Their PSCs achieved a steady-state PCE of 19.20% and 18.48%, respectively [96]. Wang et al. obtained ∼20 nm SnO2 thin film by ALD, fabricating the PSC structure of FTO/SnO2 /MAPbI3 /spiro-OMeTAD/Au. This PSC device achieved a PCE of 17.16% [98]. In PSC devices, the doping in SnO2 enhances conductivity and induces an upward shift of the Fermi level of SnO2 , which facilitates injection and transfer of electrons from the conduction band of perovskite. This simultaneously leads to reduced charge recombination and improved photovoltaic properties of PSCs. Bai et al. synthesized high-quality Sb-doped SnO2 nanocrystals as ETL materials for PSC devices. After Sb doping, the carrier concentration of the SnO2 increased from 6.7 × 1021 to 8.2 × 1022 cm3 [100]. The PCE of these PSCs improved from 15.7% to 17.7%, and the VOC increased from 1.01 to 1.06 V [100]. Yang et al. reported that yttriumdoped tin dioxide (Y:SnO2 ) ETL material [103] improved conductivity to 1.05 × 10−5 S cm−1 compared to just 7.5 × 10−6 S cm−1 for undoped SnO2 . The enhanced conductivity could reduce the contact resistance and facilitate the charge extraction, benefitting to improve JSC . The PSCs based on Y:SnO2 ETL displayed a maximum PCE of 17.29% (VOC = 1.08 V, JSC = 22.55 mA cm−2 , FF = 71%), while those based only on SnO2 was 13.38% (VOC = 1.05 V, JSC = 19.31 mA cm−2 , FF = 66%, Fig. 9h) [103]. For ETL materials, a low free electron density can prohibit carrier recombination and a high electron mobility can facilitate electron extraction from perovskite to the ETL, which both improves JSC and PCE for PSCs [91,93]. Xiong et al. created a crack-free Mg-doped SnO2 film with an annealing temperature of 50 0 °C (50 0-Mg:SnO2 ) and observed that the carrier density reduced to 7.1 × 1014 cm−3 (500-Mg:SnO2 ) from 2.1 × 1015 (500SnO2 ), and also observed that the carrier mobility was higher at 171.31 cm2 V−1 s−1 [91]. From the results in Fig. 10(e), it is seen that JSC increased to 21.67 mA cm−2 from 20.53 mA cm−2 [91].

Their Mg:SnO2 -based planar PSC devices exhibited a high PCE of 17.86% (Fig. 10e), and the highest average PCE of 16.42% (Fig. 10f) [91]. 7.1.2. Interface modification SnO2 has been certified as an effective ETL for achieving highperformance PSCs, but the numerous trap states in the SnO2 film from intrinsic defects and bulk defects increases the interface charge recombination and thereby decreases the PSC performance. Interface modification can optimize interface contact, mitigate electron–hole recombination and hysteresis, improve electron transfer, reduce surface trap densities and increase carrier collection, which are all very important to obtain a high-performance and stable PSC device. Fullerene and its derivatives (i.e. C60 , PCBM) are usually used to modify the SnO2 /perovskite interface, which can reduce the interface charge recombination and improve the photovoltaic performance of the PSC device. Ke et al. used ultrathin PCBM to passivate the interface of SnO2 /perovskite and fabricated PSC devices with a structure of FTO/SnO2 /PCBM/MAPbI3 /Spiro/Au. The best PCE they achieved was 19.12% [143]. Wang et al. used C60 SAMs to modify SnO2 surface, obtaining a maximum PCE of 19.03% [98]. Graphene quantum dots (GQDs) have a non-zero bandgap and luminesce on excitation [156,157]. This bandgap is adjustable by changing the surface functional group and the size of the GQDs [157,158]. GQDs are an ideal modification material for DSSCs and PSCs, because it has very unique properties, such as high PL quantum productions, desirable optical properties, efficient multiform carrier generation, adjustable band gaps, high carrier mobility, and high specific surface area [89,159,160]. By using GQDs to modify SnO2 surface, the electrons from the GQDs will effectively fill the electron trap sites as well as improve the conductivity of SnO2 , which is beneficial for enhancing the electron extraction efficiency, reduce the recombination at the ETL/perovskite interface, and mitigate hysteresis [89]. Xie et al. used GQDs to modify the SnO2 surface which enables quick transfer of the photogenerated

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Table 3. The overview of PSCs based on SnO2 ETL material. Device structure

JSC (mA cm−2 )

VOC (V)

FF

PCE (%)

Ref.

FTO/SnO2 /(FAPbI3 )0.85 (MAPbBr3 )0.15 /spiro-OMeTAD/Au FTO/TiO2 -SnO2 NWs/MAPbI3 /spiro-OMeTAD/Au FTO/SnO2 /MAPbI3 /spiro-OMeTAD/Au FTO/SnO2 /CsFAMAPbI3- x Brx /spiro-OMeTAD/Au ITO/Sb:SnO2 /MAPbI3 /spiro-OMeTAD/Au ITO/SnO2 /(FAPbI3 )0.97 (MAPbBr3 )0.03 /spiro-OMeTAD/Au FTO/NiO/MAPbI3 /C60 /SnO2 NCs/Ag ITO/SnO2 /MAPbI3 /spiro-OMeTAD/Au ITO/a-SnO2 /MAPbI3 /spiro-OMeTAD/Au FTO/Al:SnO2 /MAPbI3 /spiro-OMeTAD/Au ITO/SnO2 /MAPbI3 /spiro-OMeTAD/Au PET/ITO/SnO2 /meso-TiO2 /MAPbI3 /spiro-OMeTAD/Au FTO/SnO2 /MAPbI3 /spiro-OMeTAD/Au PET/FTO/SnO2 /MAPbI3 /spiro-OMeTAD/Au FTO/SnO2 / mp-TiO2 /MAPbI3 /spiro-OMeTAD/Au FTO/cl-SnO2 /mp-TiO2 /MAPbI3 /spiro-OMeTAD/Au Glass/SnOx /Ag/SnOx /MAPbI3 /spiro–OMeTAD/MoO3 /Ag FTO/SnO2 /MAPbI3 /spiro–OMeTAD/Ag ITO/SnO2 /(FAPbI3 )x (MAPbBr3 )1– x /spiro–OMeTAD/Au ITO/SnO2 /(FA2 PbI3 )0.875 (CsPbBr3 )0.125 /Spiro–OMeTAD/Au FTO/TiO2 –SnO2 /MAPbI3 /spiro–OMeTAD/Au ITO/SnO2 /MAPbI3 /spiro-OMeTAD/Au FTO/MgO/ SnO2 / MAPbI3 /spiro-OMeTAD/Au FTO/SnO2 /Cs5 (MA0.17 FA0.83 )95 Pb(I0.83 Br0.17 )3 /spiro-OMeTAD/Au FTO/Nb:SnO2 /(FAPbI3 )0.85 (MAPbBr3 )0.15 /spiro-OMeTAD/Au FTO/SnO2 @a-TiO2 / (FAPbI3 )0.85 (MAPbBr3 )0.15 /spiro-OMeTAD/Au PET/ITO/SnO2 /C60 -SAM/MA0.7 FA0.3 PbI3 /spiro-OMeTAD/Au FTO/SnO2 /PCBM/BAx (FA0.83 Cs0.17 )1- x Pb(I0.6 Br0.4 )3 /spiro-OMeTAD/Au FTO/SnO2 /C60 -SAM/MA0.7 FA0.3 PbI3 /spiro-OMeTAD/Au ITO/SnO2 :GQDs/MAPbI3 /spiro-OMeTAD/Au FTO/Y:SnO2 /MAPbI3 /spiro-OMeTAD/Au FTO/SnO2 /MAPbI3 /PTAA/Au ITO/SnO2 /SAMs/MAPbI3- x Clx /spiro-OMeTAD/Au ITO/SnO2 /MAPbI3 -PAA/spiro-OMeTAD/Au ITO/SnO2 /MAPbI3 -PVP/spiro-OMeTAD/Au ITO/SnO2 /MAPbI3 -b-PEI/spiro-OMeTAD/Au AZO/Ga:SnO2 /Cs0.05 FAx MA2.95- x PbIx Br3- x /spiro-OMeTAD/Au FTO/SnO2 /PCBM/MAPbI3 /spiro-OMeTAD/Au FTO/Li:SnO2 /MAPbI3 /spiro-OMeTAD/Au PET/FTO/SnO2 /MAPbI3 /spiro-OMeTAD/Au FTO/SnO2 /C60 -SAM/MAPbI3 /spiro-OMeTAD/Au PET/FTO/SnO2 /C60 -SAM/MAPbI3 /spiro-OMeTAD/Au FTO/SnO2 /MAPbI3 /spiro-OMeTAD/Au FTO/c-SnO2 /MAPBI3 /ZrO2 /Carbon FTO/SnO2 /(FAPbI3 )0.97 (MAPbBr3 )0.03 /Carbon FTO/SnO2 /mp-SnO2 /MAPbI3 /spiro-OMeTAD/Au ITO/ZnO-SnO2 /MAPbI3 /spiro-OMeTAD/Ag FTO/Mg:SnO2 /MAPbI3 /spiro-OMeTAD/Au FTO/SnO2 /MAPbI3- x (SCN)x /spiro-OMeTAD/Au Ag/Si/ITO/SnO2 /MA0.37 FA0.48 Cs0.15 PbI2.01 Br0.99 /spiro-OMeTAD/MoOx /ITO FTO/SnO2 /MAPbI3 /spiro-OMeTAD/Ag FTO/Mg:SnO2 /mp-SnO2 /MAPbI3 /spiro-OMeTAD/Ag ITO/PEDOT:PASS/MAPbI3 /PCBM/Al:ZnOx NPs/SnO2 /spiro-OMeTAD/Ag FTO/SnO2 /C60 -SAM/MA0.7 FA0.3 PbI3 /spiro-OMeTAD/Au

21.30 21.20 23.27 22.59 22.50 24.88 21.80 21.24 21.30 19.40 22.32 20.70 22.20 20.56 20.86 22.63 16.10 21.95 23.86 22.64 22.58 22.98 22.70 22.75 22.36 22.90 22.11 22.70 23.20 23.05 22.55 21.40 22.03 21.53 21.74 21.94 22.80 22.61 23.27 21.98 21.56 20.50 22.31 22.75 19.50 22.76 19.60 20.92 22.82 13.26 22.80 21.67 18.60 22.61

1.14 1.02 1.11 1.17 1.06 1.09 1.12 1.10 1.05 1.03 1.06 1.04 1.17 1.14 1.08 1.02 1.10 1.07 1.12 1.14 1.13 1.13 1.10 1.10 1.08 1.20 1.10 1.14 1.09 1.13 1.08 1.06 1.10 1.16 1.15 1.07 1.07 1.12 1.11 1.08 1.13 1.11 1.03 0.92 1.07 1.05 1.06 0.99 0.94 1.68 1.11 1.11 0.88 1.13

0.74 0.655 0.67 0.75 0.742 0.757 0.77 0.659 0.663 0.58 0.77 0.66 0.74 0.69 0.78 0.658 0.606 0.69 0.806 0.74 0.78 0.772 0.73 0.73 0.727 0.764 0.754 0.80 0.764 0.778 0.71 0.67 0.774 0.794 0.809 0.71 0.7 0.758 0.707 0.642 0.781 0.738 0.79 0.67 0.66 0.68 0.688 0.688 0.754 0.734 0.758 0.742 0.777 0.798

18.40 14.20 17.21 20.73 17.70 20.54 18.80 14.20 14.80 12.10 18.10 14.07 19.20 16.11 17.58 15.07 11.00 16.21 21.60 19.17 19.80 20.02 18.23 18.20 17.57 21.10 18.36 20.60 19.28 20.31 17.29 15.30 18.77 19.65 20.23 16.67 17.00 19.12 18.20 15.29 19.03 16.80 18.16 13.98 14.50 16.17 14.30 13.56 16.23 16.37 19.21 17.86 12.70 20.30

[114] [149] [19] [85] [100] [86] [122] [123] [124] [101] [161] [95] [96] [96] [131] [130] [112] [111] [32] [109] [126] [87] [132] [162] [92] [84] [117] [88] [142] [89] [103] [110] [145] [90] [90] [90] [102] [143] [97] [97] [98] [98] [163] [133] [164] [154] [165] [93] [166] [99] [91] [91] [113] [116]

electrons to the SnO2 interface from perovskite [89]. After GQDs modify the SnO2 surface, the Fermi level (EF ) of SnO2 :GQDs decreased from 4.35 to 4.01 eV (Fig. 18b), which was beneficial to improve the VOC of the device. They used this SnO2 :GQDs ETL to fabricate the PSC devices with a structure of ITO/SnO2 :GQDs/MAPbI3 /Spiro-OMeTAD/Au (Fig. 18a) [89]. These devices exhibit a higher average VOC and PCE (1.12 ± 0.02 V, 19.2% ± 1.0%) than those using bare SnO2 (1.09 ± 0.02, 16.6% ± 0.9%) (Fig. 18c). In their research, a peak PCE of 20.31% for reverse scan and a peak PCE of 19.68% for forward scan were observed for the SnO2 :GQDs ETL PSC (Fig. 18d) [89]. For PSCs with only SnO2 ETL, they obtained a PCE of 17.91% for reverse scan and a PCE of 15.84% for forward scan (Fig. 18d) [89]. The steady-state PCEs of the devices with the SnO2 :GQDs, SnO2 ETLs was 20.23% and 17.47%, respectively (Fig. 18e). The integrated photocurrent densities from the EQEs were 21.36 mA cm−2 and 22.47 mA cm−2 , respectively (Fig. 18f) [89].

7.1.3. Chemical molecular engineering For the perovskite material, its band gap can be adjusted by exchanging the chemical molecule or element. Through intramolecular exchange, some organic molecule (formamidinium (FA) molecular) and inorganic atoms (such as caesium (Cs), bromine (Br), chlorine (Cl)) are usually used to swap methylamine (MA) or iodine (I) in MAPbI3 to form Csx FAy MA1- x - y PbI3- x - y Brx Cly , thus adjusting the band gap of the PSC. Baena et al. synthesized (FAPbI3 )0.85 (MAPbBr3 )0.15 (bandgap of 1.56 eV) as the absorber layer for PSCs and an almost hysteresis-free PCE of 18.4% was obtained even at a high voltage of 1.19 V [114]. Adding inorganic Cs atoms to form triple-cation perovskite compositions, the CsFAMAPbI3- x Brx (1.62 eV) was used as the active layer and SnO2 as the ETL in the PSC device by Anaraki et al. [6,85]. They prepared SnO2 using ALD, spin-coating, and spin-coating chemical bath deposition (SC-CBD). A peak PCE of 20.8% was obtained with SC-CBD. In addition, a high VOC of

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Fig. 19. (a) Schematic illustration of the proposed self-assembled 2D–3D perovskite film structure. (b) Enlarged SEM image of x = 0.03 films, (c) enlarged SEM image of x = 0.09 films, (d) enlarged SEM image of x = 0.16 films. (e) Stability of non-encapsulated, (f) stability of encapsulated (solar cell devices using FA0.83 Cs0.17 Pb(I0.6 Br0.4 )3 (blue lines; labeled as pristine) and BA0.09 (FA0.83 Cs0.17 )0.91 Pb(I0.6 Br0.4 )3 (red lines; labeled as BA/FA/Cs) perovskite active layers) [88].

1.214 V was also obtained, which was higher than that of only SnO2 prepared using ALD (PCE = 18.34% and VOC = 1.13 V) [85]. Jiang et al. used (FAPbI3 )0.97 (MAPbBr3 )0.03 for the absorbent layer to fabricate PSC devices with the following structure: ITO/SnO2 /(FAPbI3 )0.97 (MAPbBr3 )0.03 /Spiro-OMeTAD/Au. Their PSCs achieved a PCE of 20.54% [86]. In 2017, Jiang et al. obtained PSCs with the PCE of 21.6% in small size (0.0737 cm2 ) and 20.1% in large size (1 cm2 ) PSCs with a moderate residual PbI2 in perovskite layer [32]. Song et al. used (FAPbI3 )0.85 (MAPbBr3 )0.15 (bandgap of 1.62 eV) as the absorbent layer in their PSCs and observed a PCE of 21.1% [84]. For the perovskite layer, adding some polymer (i.e. BA: nbutylammonium, C4 H9 NH3 ; PVP: poly(4-vinylpyridine); PAA: polyacrylic acid; b-PEI: branched polyethyleneimine) to modify the perovskite films, can improve the efficiency and stabil-

ity of the PSC devices. Wang et al. introduced polymer BA to form highly crystalline 2D-3D heterostructure perovskite ((BA)2 (FA0.83 Cs0.17 )n -1 Pb(I0.6 Br0.4 )3 n +1 ) films to suppress nonradiative charge recombination in PSC devices [88]. In complete PSCs, they obtained a peak PCE of 20.6% with a 1.61 eV-bandgap perovskite and 17.2% with a 1.72 eV-bandgap perovskite [88]. In the MAPbI3 perovskite precursor solution, Zuo et al. introduced a polymer (PVP, PAA, b-PEI) to fabricate MAPbI3 -polymer perovskite films (MAPbI3 -PVP, MAPbI3 -PAA, MAPbI3 -b-PEI) [90]. Their PSC devices achieved a peak PCE of 20.2% with MAPbI3 -PVP on SnO2 ETL [90]. In 2017, Wang et al. added lead thiocyanate (Pb(SCN)2 ) into their perovskite solution to form MA0.7 FA0.3 PbI3- x (SCN)x a perovskite film [116]. Their planar PSCs had a stabilized PCE of up to 20.3% [116]. To summarize, Table 3 presents the structure and performance of various PSCs based on SnO2 ETL materials.

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7.2. Methods of improving stability for SnO2 -based PSCs Perovskite materials are easily susceptible to thermal decomposition and hydrodecomposition, leading to poor stability for PSC devices. The poor stability of the perovskite materials and devices is a big challenge and greatly hinders the PSC devices from leaving the laboratory and into industry and outdoor applications [167]. For the stability of these perovskite materials and devices, it is necessary to consider the effects of temperature, illumination and atmospheric exposure (oxygen, moisture). For SnO2 -based PSCs device, device stability is an essential topic and is being researched by many researchers [85,88,90,91,102,107,113,122,133,144,151,154,162,164]. Due to the poor stability of MAPI3 , the first step is to change the chemical constituents or the structure of the perovskite. In comparison to 3D structure perovskites, 2D structure perovskites have a higher carrier mobility and better ambient stability [88,168,169]. A 2D-3D heterostructured (BA)2 (FA0.83 Cs0.17 )n -1 Pb(I0.6 Br0.4 )3 n +1 has been studied by Wang et al. (Fig. 19a) [88]. They formed 2D perovskite platelets and interspersed them among the grains of highly orientated 3D perovskite (Fig. 19b–d). In complete PSCs, they obtained a peak PCE of 20.6% with a 1.61 eV-bandgap perovskite and 17.2% when employing a 1.72 eV-bandgap perovskite [88]. For nonencapsulated PSCs device, the BA/FA/Cs improved long-term stability because after 1005 h, the BA/FA/Cs-based PSCs device maintained 80% of its initial PCE (t80 lifetime), while the FA/Cs-based PSC device lost nearly 50% of its initial PCE (Fig. 19e) [88]. They then proceeded to encapsulate the devices with a hot-melt polymer foil and a glass coverslip. For the BA/FA/Cs-based PSCs device, the t80 lifetime improved dramatically to 3880 h, meanwhile, the t80 lifetime of FA/Cs-based PSCs device still lagged behind at 3350 h (Fig. 19f) [88]. Liu et al. used a low-temperature hydrothermal method to deposit the SnO2 ETL onto the PSC device and obtained a cell with a peak PCE of 16.17% [154]. The PCE was maintained above 90% of its initial value after 30 0 0 h, even without being encapsulated [154]. Zhu et al. used NiOx as HTL material to replace spiro-OMeTAD, fabricating inverted p–i–n PSC devices [122]. After storing for 30 days in an environment with a relative humidity of 70–80%, their SnO2 -based PSCs maintained 80% of its initial PCE [122]. When alkali metal cations are introduced into the perovskite material, the stability of the PSC device can be enhanced. Ma et al. fabricated a triple-cation perovskite film of Cs5 (MA0.17 FA0.83 )95 Pb(I0.83 Br0.17 )3 as the PSC active layer, and used electron-beam evaporation to deposit SnO2 as the ETL [162]. Their PSC device reached a PCE up to 18.2% without any interface modification, and retained 97% of its initial PCE after being stored for over 30 days [162]. HTL-free PSCs are also able to maintain longterm stability [133,164,170]. Lin et al. fabricated a HTL-free device using SnO2 as the ETL and a PCE of 14.5% was achieved [164]. After 3600 h, the PCE of their PSCs device was almost undiminished [164]. In summary, the best performing PSC based on a SnO2 ETL exhibit a PCE of 21.6% (JSC = 23.86 mA cm−2 , VOC = 1.12 V, FF = 0.806) [32], compared to the best PCE of PSC based on TiO2 ETL of 23.2% (JSC = 24.91 mA cm−2 , VOC = 1.144 V, FF = 0.813) [8]. The main reason for this difference is the higher density of surface defects in SnO2 compared to TiO2 . Surface defects (intrinsic and extrinsic) cause acceptor or donor states within the band gap, which mediates the electron recombination at the perovskite/SnO2 interface, resulting in decreased VOC and FF. The higher density of surface defects also causes charge accumulation at the perovskite/SnO2 interface and is one of the main causes for the hysteresis. Weber et al. demonstrate that current-voltage hysteresis in PSCs is dominated by the dynamic formation and release of ionic charges at the interfaces [171]. Many researchers have

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reported that methods such as interface modification [91,122], doping in SnO2 or perovskite [97,172], and mesoporous structure [91] can be used to eliminate the hysteresis. In the future for PSC devices, more work is needed to reduce non-radiative recombination and improve charge transport, which will further improve VOC values and fill factors. Many approaches have been used to improve the performance of PSC devices and eliminate the hysteresis, such as interface modification, doping of the ETL and perovskite layer, and increasing the grain size of perovskite, among others. Future research on SnO2 ETL based PSCs will continue to compete with TiO2 . Further research to optimize the quality of the SnO2 ETL will result in more efficient charge transfer by improving charge collections and reducing the interface recombination. Meanwhile, one key issue of SnO2 ETL based PSCs is the poor long-term stability and it must be addressed. To improve the stability of PSCs, interdisciplinary research is required to find new stable materials, better choices for electrodes, barrier layers, charge transport layers and encapsulation strategies. 8. Summary In recent years, the performance of PSCs employing a SnO2 ETL has made great progress, as the PCE has been increased from 3.76% to over 21.6%. In this article, we briefly summarized the research progress of PSCs based on a SnO2 ETL in recent years. Fabrication techniques of SnO2 ETL materials, such as RF sputtering, spin-coating, and ALD, are described. For flexible PSC devices, lowtemperature methodologies are required, thus SnO2 nanomaterials have emerged as a potential candidate for these applications, due to their low temperature and rich variety of fabrication methods, and they present a distinct advantage compared to TiO2 ETL materials. In the future, more work is needed to finely control the properties of the SnO2 ETL, including film quality, defects, and surface work function, which will improve charge transfer efficiency by reducing the interface recombination and energy loss. The goal is to build SnO2 -based PSC devices with the efficiency and the stability comparable to mesoporous structure devices. Acknowledgments This project was supported by the National Natural Science Foundation of China (NSFC 61574009 and 11574014). Conflict of interest The authors declare no conflict of interest. References [1] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, J. Am. Chem. Soc. 131 (17) (2009) 6050–6051. [2] H.S. Kim, C.R. Lee, J.H. Im, K.B. Lee, T. Moehl, A. Marchioro, S.J. Moon, R. Humphry-Baker, J.H. Yum, J.E. Moser, M. Grätzel, N.G. Park, Sci. Rep. 2 (2012) 591–597. [3] M. Liu, M.B. Johnston, H.J. Snaith, Nature 501 (7467) (2013) 395–398. [4] H. Zhou, Q. Chen, G. Li, S. Luo, T.B. Song, H.S. Duan, Z. Hong, J. You, Y. Liu, Y. Yang, Science 345 (6196) (2014) 542–546. [5] W.S. Yang, J.H. Noh, N.J. Jeon, Y.C. Kim, S. Ryu, J. Seo, S.I. Seok, Science 348 (6240) (2015) 1234–1237. [6] M. Saliba, T. Matsui, J.Y. Seo, K. Domanski, J.P. Correa–Baena, M.K. Nazeeruddin, S.M. Zakeeruddin, W. Tress, A. Abate, A. Hagfeldt, M. Grätzel, Energy Environ. Sci. 9 (6) (2016) 1989–1997. [7] W.S. Yang, B.W. Park, E.H. Jung, N.J. Jeon, Y.C. Kim, D.U. Lee, S.S. Shin, J. Seo, E.K. Kim, J.H. Noh, S.I. Seok, Science 356 (6345) (2017) 1376–1379. [8] N.J. Jeon, H. Na, E.H. Jung, T.Y. Yang, Y.G. Lee, G. Kim, H.W. Shin, S. Il Seok, J. Lee, J. Seo, Nat. Energy 3 (2018) 682–689. [9] NREL, Best Research-Cell Efficiencies NREL. https://www.nrel.gov/pv/assets/ images/efficiency- chart- 20180716.jpg, 07-17-2018. [10] S. De Wolf, J. Holovsky, S.J. Moon, P. Loper, B. Niesen, M. Ledinsky, F.J. Haug, J.H. Yum, C. Ballif, J. Phys. Chem. Lett. 5 (6) (2014) 1035–1039. [11] S.D. Stranks, G.E. Eperon, G. Grancini, C. Menelaou, M.J. Alcocer, T. Leijtens, L.M. Herz, A. Petrozza, H.J. Snaith, Science 342 (6156) (2013) 341–344.

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