2D-3D heterostructure enables scalable coating of efficient low-bandgap Sn–Pb mixed perovskite solar cells

Nano Energy 66 (2019) 104099

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2D-3D heterostructure enables scalable coating of efficient low-bandgap Sn–Pb mixed perovskite solar cells

T

Linxiang Zenga,b,1, Zongao Chena,1, Shudi Qiua,1, Jinlong Hua, Chaohui Lia,c, Xianhu Liuc, Guangxing Liangd, Christoph J. Brabece,f, Yaohua Maia,∗∗, Fei Guoa,∗ a

Institute of New Energy Technology, College of Information Science and Technology, Jinan University, Guangzhou, 510632, China Department of Materials Science and Engineering, Zhejiang University, Hangzhou, 310027, China c National Engineering Research Center for Advanced Polymer Processing Technology, Zhengzhou University, Zhengzhou, 450002, China d Shenzhen Key Laboratory of Advanced Thin Films and Applications, Shenzhen University, 518060, Shenzhen, China e Institute of Materials for Electronics and Energy Technology (i-MEET), Friedrich-Alexander University Erlangen-Nürnberg, Martensstrasse 7, 91058, Erlangen, Germany f Helmoltz Institute Erlangen Nuremberg (HI-ErN), Forschungszentrum Juelich, Immerwahrstrasse 2, 91058 Erlangen, Germany b

ARTICLE INFO

ABSTRACT

Keywords: 2D-3D heterostructure Low bandgap Perovskite solar cells Blade coating Sn–Pb mixed perovskite Vacuum-assisted crystallization

Low-bandgap photovoltaic absorbers based on mixed tin-lead (Sn–Pb) halide perovskites offer promising opportunities to fabricate efficient multi-junction solar cells. However, the current Sn–Pb mixed perovskite solar cells (PSCs) were mainly prepared using lab-scale spin-coating, greatly hindering their application for large-area device fabrication. Here, we report a simple and robust methodology for scalable deposition of dense and uniform Sn–Pb mixed perovskite films by one-step blade coating. High quality perovskite films with different Sn–Pb ratios are readily prepared by vacuuming the freshly coated precursor films followed by an anneal process. Solar cells based on these bladed Sn–Pb mixed perovskite absorbers showed decent photovoltaic behaviors. Further enhancement of device performance was realized via surface defects passivation using phenethylammonium bromide (PEABr). It was found that the formation of a thin layer of 2D Ruddlesden-Popper perovskite on top of 3D bulk perovskite significantly suppressed charge recombination. As a consequence, the opencircuit voltage (VOC) of the solar cells (Eg = 1.35 eV) was dramatically lifted from 0.71 V to 0.78 V, yielding high efficiencies of over 15%. Moreover, notable improvement in shelf and moisture stability was observed due to the protection barrier of the 2D perovskite capping layer.

1. Introduction In recent years, solution-processable organic-inorganic metal halide perovskite absorbers have received considerable interest mainly due to their superb optoelectronic properties and defect tolerance in delivering highly efficient photovoltaic devices [1–3]. Within a decade of development, Pb-based halide perovskite solar cells have demonstrated certified power conversion efficiencies (PCE) exceeding 25%. However, the energy bandgaps of these Pb-based perovskites (typically higher than 1.45 eV) are much larger than the ideal bandgap (~1.25 eV) for single junction solar cells, as predicted by the Shockley-Queisser (SQ) radiative limit [4]. Alternatively, the tandem concept is well known for addressing the two deficits of the single-junction photovoltaic devices, namely thermalization and below bandgap losses, providing an

effective approach to overcome the SQ efficiency limits [5–9]. The construction of highly efficient tandem solar cells needs to broaden the light harvest to near-infrared spectrum [10,11]. In this context, it is highly demanded to develop low-bandgap perovskite absorbers that can be served as bottom sub-cells of tandem devices. Motivated by these considerations, mixed Sn–Pb binary halide perovskites have recently drawn growing attention [12–16], primarily because their bandgap can be tuned to as small as 1.2 eV by simply varying the ratio of Sn and Pb. Past few years have witnessed a prosperous progress in PCE growth of low-bandgap mixed Sn–Pb PSCs. Early in 2014, Hayase and co-workers reported that the Sn–Pb mixed perovskite MASn0.5Pb0·5I3 shows a narrow bandgap with an absorption onset at 1060 nm [12]. After that, Kanatzidis's group and Jen's group reported low-bandgap MASnXPb1-XI3 PSCs in normal and inverted

Corresponding author. Corresponding author. E-mail addresses: [email protected] (Y. Mai), [email protected] (F. Guo). 1 These authors contributed equally to this work. ∗

∗∗

https://doi.org/10.1016/j.nanoen.2019.104099 Received 22 July 2019; Received in revised form 31 August 2019; Accepted 4 September 2019 Available online 07 September 2019 2211-2855/ © 2019 Elsevier Ltd. All rights reserved.

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architectures, respectively [13,14]. Using a mixed precursor solution consisting of formamidinium tin iodide (FASnI3) and methylammonium lead iodide (MAPbI3), Yan's group reported low-bandgap (FASnI3)0.6(MAPbI3)0.4 PSCs with PCEs up to 15% [17]. Further increasing the thickness of the perovskite absorber layer to 620 nm, the efficiency was pushed to a certified 17% [7]. Very recently, Yan's group reported mixed Sn–Pb PSCs with the highest PCE exceeding 19% by reducing the saturation current via bromine doping [18]. Despite these remarkable progress in achieving high device performance, it is noticed that all these reported low-bandgap Sn–Pb binary PSCs were fabricated using lab-scale spin-coating. The distinct advantage of the commonly used spin-coating is that uniform perovskite films can be facilely prepared using small amount of solution, although most of the material is wasted during substrate rotation [19,20]. Unfortunately, the major deficit of the spin-coating is that it is neither scalable nor translatable to scalable process, which precludes its application for high-throughput manufacture [21]. Furthermore, the typically used anti-solvent crystallization strategy coupled with the spincoating process poses a notable irreproducibility concern for large-area coating [22]. This is mainly because the anti-solvent step is extremely sensitive to the processing parameters such as dipping time, volume, distance to the surface, and solvent atmosphere of the local environment [23]. In this regard, the anti-solvent crystallization method is also unlikely to be transferred to large-area printing lines due to the excess and spilling of washing solvents. In this context, it has become apparently urgent to re-design a novel crystallization strategy that is compatible with the large-scale printing techniques. In this work, we report a generic crystallization protocol that allows for scalable coating of high-quality thin films of low-bandgap Sn–Pb mixed perovskites via one-step blade coating. In contrast to previous works on the blade coating of Pb-based perovskites where the precursor deposition is commonly carried out at temperatures higher than the perovskite crystallization required [24–28], in our technology the precursor films is deposited at room temperature. This is innovatively enabled by the deployment of a vacuum extraction process, which allows to prevent the migration and aggregation of the perovskite ingredients by fast removing the excess solvent of the freshly printed precursor liquid films. Dense and uniform large-area perovskite thin films with different Sn–Pb ratios are readily obtained with assistance of vacuum extraction followed by a low-temperature annealing step. Solar cells fabricated based on these scalable coated Sn–Pb mixed perovskite films show efficiencies of ~12% with excellent reproducibility. Further upon a surface passivation using PEABr, the VOC of the devices was improved markedly from 0.71 V to 0.78 V, yielding a high efficiency of 15.15%. The notable enhancement in VOC of the solar cells is attributed to the suppression of non-radiative recombination as a result of the formation of a thin layer of Ruddlesden-Popper (2D-RP) perovskite.

solvent to quickly wash off the excess solvent during high speed substrate rotation [19]. Inspired by the anti-solvent crystallization approach, we expect that the removal of excess solvent can be realized equally effectively and more robustly by using a vacuum extraction process. It is further anticipated that the obtained intermediate films could decouple the wet precursor film deposition with the subsequent thermal annealing, thus facilitating the growth of perovskite films in a controllable manner. We note that although vacuum assisted crystallization has been reported by few groups for the fabrication of Pb-based perovskite films using spin-coating [29–33], the deposition of Sn–Pb mixed perovskite films via scalable printing methods has not been reported. Fig. 1a shows the schematic steps of the vacuum-assisted crystallization for the preparation of Sn–Pb mixed perovskite films by blade coating. As the first step, the Sn–Pb mixed perovskite precursor film is blade deposited which was carried out in inert atmosphere to prevent the aggression of water molecules and oxidation of Sn2+ to Sn4+. The freshly coated precursor film contains large amount of solvent which shows a transparent yellow appearance (Fig. 1a). Once the precursor wet film is subjected to a vacuum chamber and pumped to 1000 Pa, a light brown thin film was obtained after staying inside the chamber for 90 s (Fig. 1a). We designate the vacuum treated precursor film as “intermediate phase”. We note that the resulting intermediate film looks quite similar to these obtained by the anti-solvent approach, suggesting the effectiveness of the vacuum induced pre-crystallization. Eventually, a highly crystalline perovskite film of dark brown in color was obtained by annealing the intermediate film at 100 °C for 10 min (Fig. 1a). To demonstrate the generic application of the vacuum-assisted crystallization strategy, we prepared a series of low-bandgap Sn–Pb mixed perovskite films with different Sn/Pb ratios by blade coating. The Sn–Pb mixed perovskite precursor solutions were prepared following the procedure from the Yan's group, where the 1 M MAPbI3 and 1 M FASnI3 stock solutions were firstly prepared [7,17,18]. Into the two stock solutions, a small amount of tin fluoride (SnF2, 10 mol%) and methylammonium chloride (MACl, 10 mol%) was added to retard the oxidation of Sn2+ and increase the crystal size of the FASnI3 and MAPbI3, respectively. The Sn–Pb binary perovskite solutions consisting of (FASnI3)X(MAPbI3)Y with different mole ratios of X:Y = 1:0, 9:1, 3:1, 2:1 1:1, 1:2, 1:3, and 0:1 were prepared by mixing the stock solutions of the two with desired volume ratio. To unveil the impact of the vacuum process on the crystallization dynamics of the Sn–Pb mixed perovskite film, we chose perovskite composed of (FASnI3)0.25(MAPbI3)0.75 as an example to illustrate the morphology and structure evolution of the films prepared at different processing stages. It is found that the vacuum process represents an ultimate important step for the scalable deposition of high-quality Sn–Pb mixed perovskite films by blade coating, which is otherwise not achievable by naturally drying of the printed precursor films. SEM images shown in Fig. S1 evidence that, without vacuum extraction, the naturally dried perovskite films independent of a subsequent thermal annealing yield course surfaces with simultaneously presence of ultralarge (up to 20 μm) and very small (down to tens of nanometer) crystal grains. The reason for the coarse morphology with large grains can be ascribed to the low nucleus density as a result of slow solvent evaporation rate. XRD spectra reveal the presence of perovskite characteristic diffraction peaks with unfavored crystal orientation towards (112) plane (Fig. S1). In distinct contrast, when the freshly coated precursor film was subjected to vacuum extraction process, a dark brown intermediate film with high reflectance was obtained. XRD pattern evidences the formation of perovskite with preferred orientation of (110) plane (Fig. 1b). The SEM image shows that the intermediate film consists of uniformly sized crystal grains with complete surface coverage. These results validate that the mild vacuum process is capable of producing dense and uniform perovskite film by creating high nucleus density. Nevertheless, we would like to point out that the purity and crystallinity of the

2. Results and discussion 2.1. Preparation of Sn–Pb mixed perovskite films by blade coating A distinct difference between the spin-coating and scalable blade coating is that the perovskite precursor films deposited by blade deposition contains plenty of excess solvent, other than resulting in the precipitation of solids in spin-coating process where a rapid evaporation of precursor solvent induces supersaturation in a very short time. In the course of scalable coating, the presence of the excess solvent in the freshly deposited precursor wet film requires to be judiciously removed which would otherwise result in unpredicted crystal phases due to the uncontrolled crystallization dynamics. In addition, the migration and aggregation of the precursor ingredients during the slow drying process would result in poor film morphology. The classic anti-solvent crystallization strategy provides a sound hint which underlines the importance of creating an intermediate film. The “frozen” intermediate stage is enabled by pouring an orthogonal 2

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Fig. 1. (a) Schematic illustration of the vacuum-assisted crystallization for the preparation of Sn–Pb mixed perovskite films via blade coating. (b) XRD patterns, (c) top-view SEM images, and (d) FTIR spectra of the intermediate phase (obtained only by vacuum extraction) and the annealed perovskite film (obtained by vacuum extraction followed by a thermal annealing) with Sn–Pb ratio of 1:3 ((FASnI3)0.25(MAPbI3)0.75).

intermediate perovskites film should be further enhanced via thermal annealing. It is indeed found that with subjection of the intermediate film to a thermal annealing, the resulting perovskite film showed much improved crystallinity as evidenced by the intensity and the half width at half maximum (HWHM) of the (110) diffraction plane (Fig. 1b). Furthermore, the SEM images in Fig. 1c reveal that the crystal grains of the intermediate film were enlarged from ~200 nm to ~280 nm after thermal annealing (Fig. S2), indicating enhanced crystallinity. Fig. 1d shows the FTIR spectra which indicate that the solvent DMSO is presented in the intermediate film and, is completely removed upon a thermal annealing. Overall, these results demonstrate that a vacuum process is capable of producing dense and uniform Sn–Pb mixed perovskite films deposited by scalable blade coating. Meanwhile, a subsequent thermal annealing is highly demanded to further enhance the crystallinity of the film by a second crystal growth with the removal of the residual solvent DMSO.

orthorhombic (Amm2) crystal structure [34,35], whereas MAPbI3 has a tetragonal (I4 cm) crystal structure (β-phase) [13]. It is observed that all the mixed Sn–Pb perovskites shows only one peak (~24.5°) within the 2θ range between 22° and 25° which could be indexed to (113) plane in the Amm2 space group, suggesting the orthorhombic crystal structure of the Sn–Pb mixed perovskites (Fig. 2b). In comparison, two peaks (23.5° and 24.5°) within the 2θ range of 22°–25° was observed for the pure MAPbI3 film, which could be indexed to (211) and (202) planes in the tetragonal I4 cm space group. These XRD results suggest that all the mixed Sn–Pb perovskites adopt the orthorhombic crystal structure with the Sn and Pb atoms randomly occupy the metal sites of corner-sharing octahedra. UV–vis absorbance spectra shown in Fig. 2c evidence that the absorption onsets of the Sn–Pb mixed perovskites extends to wavelength > 1000 nm. With increase of FASnI3 component, the absorption onset first shifts to longer wavelength, reaches a maximum value, and then moves back to shorter wavelength. This nonlinear relationship of the bandgap of the Sn–Pb mixed perovskites stems from the anomalous band gap behavior which was first observed by Kanatzidis's group [13]. The optical bandgaps of the prepared perovskite films extracted from the UV–vis absorbance were estimated to be 1.60, 1.44, 1.33, 1.30, 1.25, 1.21, 1.24, and 1.38 eV for the Sn–Pb ratios of 0:1, 1:9, 1:3, 1:2, 1:1, 2:1, 3:1, and 1:0 perovskites (inset of Fig. 2c), respectively. The surface morphology of the blade-coated perovskite films is also apparently correlated with the ratio of Sn–Pb. As displayed in Fig. 1d, without Sn incorporation the pure MAPbI3 film shows uniform and compact morphology with relatively large grain size of up to 1 μm. With addition of moderate content of Sn, the film gets rougher but still shows high surface coverage with dense and compact crystal grains. When the Sn–Pb ratio increases to 3:1, the film gets much rougher with noticeable

2.2. Thin film characterization of the bladed Sn–Pb perovskites Having elucidated the functionality of the vacuum-assisted crystallization protocol, we now continue to examine the structural, optical and morphological properties of the blade-coated (FASnI3)X(MAPbI3)Y films with different Sn–Pb ratios by X-ray diffraction (XRD), Ultraviolet–visible (UV–vis) spectroscopy and scanning electron microscopy (SEM), respectively. As shown in Fig. 2a, The XRD patterns of the prepared (FASnI3)X(MAPbI3)Y films all show intense characteristic peaks of the perovskite crystal structure. However, the diffraction peak intensities of the (110) and (220) planes decreases as the FASnI3 component increases, indicating the Sn incorporation in the perovskite film reduces film crystallinity. It has been reported that FASnI3 has an 3

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Fig. 2. (a) XRD patterns and (b) magnified XRD patterns with 2θ range between 22° and 25°, (c) UV–vis spectra, and (d) SEM images of the blade-coated (FASnI3)X(MAPbI3)Y perovskite films with different mole ratio of FASnI3:MAPbI3. The inset in (c) shows the bandgaps as a function of Sn:Pb ratios. The scale bar in (d) is 1 μm.

pinholes presented in the film. The pure FASnI3 perovskite film shows rather low surface coverage, which is similar to previous observations of the spin-coated films [36]. The inferior film morphology of the Sn–Pb mixed perovskite films with high content of Sn, particularly for the pure Sn-based perovskite, can be ascribed to the rapid crystallization kinetics of the Sn-based perovskites [36]. It has been reported that additives have to be incorporated to modulate the crystallization dynamics to achieve all Sn-based perovskite films with good surface coverage [35–38].

properties of the surfaces. In fact, the energy levels of Sn-based perovskites reported in literature have shown rather inconsistent values [12,13]. Fig. 3c shows the representative photocurrent density-voltage (J-V) characteristics of the blade-coated Sn–Pb mixed perovskite solar cells, where the thickness of the perovskite absorber layers is around 400 nm. The corresponding photovoltaic parameters are summarized in Table 1. The pure MAPbI3 solar cells showed a decent PCE of 14.87% with an open-circuit voltage (VOC) of 1.00 V, a short-circuit current density (JSC) of 20.10 mA cm−2, and a fill factor (FF) of 74%. For the Sn–Pb mixed perovskite solar cells, the VOC of the devices decreases with increase of Sn content, which can be due to the decrease in the bandgaps. Consequently, the PCE of the Sn–Pb mixed perovskite solar cells deliver relatively lower PCEs compared to the Pb-based reference. Noteworthy, most of the Sn–Pb mixed perovskite cells show high FFs of > 70%, indicating efficient charge extraction and collection within the device. The external quantum efficiency (EQE) spectra of the prepared perovskite devices are shown in Fig. 3d, where a clear extension of the photon response down to > 1000 nm is observed. The integrated photocurrent density over the AM 1.5G solar spectrum is within 5% error of the values obtained from J-V measurement (Table 1). In Fig. S3, we show the statistic photovoltaic parameters collected from more than 20 devices of each Sn–Pb composition. The narrow distribution of the photovoltaic parameters suggests good reproducibility of the perovskite films prepared by blade coating. We also evaluated the influence of vacuum time and vacuum pressure on film quality of the printed perovskites and their device performance. The results suggest that the vacuum time does not

2.3. Photovoltaic performance To evaluate the photovoltaic performance of the blade-coated lowbandgap Sn–Pb mixed perovskite films, we fabricated inverted planar solar cells with a structure of ITO/PEDOT:PSS/(FASnI3)X(MAPbI3)Y/ PCBM/BCP/Ag (Fig. 3a). Due to the presence of pinholes and the relatively low crystallinity of the perovskite films with higher content of Sn, we have therefore fabricated Sn–Pb mixed perovskite solar cells with Sn–Pb ratios of 0:1, 1:9, 1:3, 1:2 and 1:1. The energy diagram of the device stack is schematically presented in Fig. 3b. In the device stack, the PEDOT:PSS serves as a hole selective layer to facilitate hole transfer from the perovskite to the ITO electrode. The PCBM serves as an electron selective layer, which blocks holes and promotes electron transfer. It should be noted that the precise band edge positions of (FASnI3)X(MAPbI3)Y perovskites is not given, because it is difficult to obtain consistent results using UV photoemission spectroscopy measurements. This is mainly due to the facile oxidation of Sn2+ to Sn4+ when the film is exposed in air, which could alter the electronic 4

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Fig. 3. (a) Schematic architecture of the inverted planar p-i-n solar cells using Sn–Pb mixed perovskites as light absorber. (b) Simplified energy diagram of the Sn–Pb mixed perovskites solar devices. (c) The representative J-V curves of the blade-deposited (FASnI3)X(MAPbI3)Y solar cells measured under AM 1.5G solar spectrum with light intensity of 100 mW cm−2 from reverse scan. (d) EQE spectra of the blade-deposited (FASnI3)X(MAPbI3)Y solar cells with the integrated current density indicated.

significantly impact the film morphology, crystallinity as well as the device performance (Fig. S4). However, we observed that the vacuum pressure plays an essential role in determining the quality of perovskite films (Fig. S5). Exacting the solvent of precursor films at low vacuums yields perovskite films with large crystal grains and unfavored crystal orientation. As a result, the solar cells delivered rather low device performance. We note that perovskite thin films with decent morphology and crystallinity were readily obtained with only a vacuum extraction step (Fig. 1c and d), it would be curious to examine the photovoltaic performance of the vacuum processed films without thermal annealing. It is found that, compared with the annealed perovskite-based device (Fig. 3c), the solar cell prepared from unannealed perovskite showed an inferior photovoltaic performance (Fig. S6), which is probably due to the presence of DMSO residual and relatively low crystallinity. These observations underline the importance of a subsequent thermal annealing in delivering desired photovoltaic performance.

contributed to the inferior device efficiencies. For instance, the Sn–Pb (1:3) device yielded a low VOC of 0.71 V which is apparently lower compared with the spin-coated devices with the same Sn–Pb ratios [39–41]. Generally, the loss of VOC can be ascribed to the considerable nonradiative recombination due to the presence of prominent defects either at the bulk or the surfaces of the grains [42–44]. The XRD spectra and SEM images have revealed high quality of the perovskite films in terms of phase purity and micro morphology. We therefore suspect that the VOC losses mainly arises from the defects existing on the surface of the perovskite films. On the other hand, it has been showed that the Sn–Pb mixed perovskite exhibits a much lower absorption coefficient at long wavelength range [7,40], which is reflected by the low EQE values of the devices between 800 and 1000 nm (Fig. 3d). This encourages us to simultaneously increase the thickness of the Sn–Pb mixed perovskite absorber film to enhance the light harvest. Here, we show that the VOC of Sn–Pb mixed perovskite devices can be significantly enhanced by up to 70 mV, from 0.71 V to 0.78 V, by using PEABr as the surface passivator. Fig. 4a compares the typical J-V curves of Sn–Pb (1:3) solar cells, where the perovskite absorber films were blade-deposited with and without PEABr surface treatment. With simultaneous increasing the thickness of the perovskite absorber layer to more than 600 nm, the efficiency of the 1:3 Sn–Pb mixed perovskite

2.4. Surface passivation via 2D/3D heterojunction Taking a look at the device performance, it is observed that there is a large Eg-VOC deficit for the prepared Sn–Pb mixed PSCs which greatly Table 1 Photovoltaic parameters of the representative Sn–Pb mixed perovskite solar cells. Composition ratio (Sn:Pb)

Bandgap (eV)

JSC (mA/cm2)

VOC (V)

FF (%)

PCE (%)

0:1 1:9 1:3 1:2 1:1

1.60 1.44 1.33 1.30 1.25

20.10 21.23 21.98 21.33 21.62

1.00 0.77 0.71 0.70 0.64

74 75 75 71 70

14.87 12.10 11.68 10.60 9.69

(19.15)* (20.17)* (21.30)* (20.75)* (20.93)*

Note: The JSC values in brackets are calculated from the integration of EQE with the AM 1.5 G solar spectrum. 5

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Fig. 4. (a) J-V curves of the low-bandgap (FASnI3)0.25(MAPbI3)0.75 perovskite solar cells without and with PEABr (1 mg mL−1) surface treatment. (b) Cross-sectional SEM image of a typical low-bandgap perovskite solar cell with active layer thickness of ~600 nm. (c) EQE spectra of the devices without and with PEABr treatment. (d) Statistical comparison of the VOC and PCE values from 5 bathes of devices. (e) TRPL curves of the (FASnI3)0.25(MAPbI3)0.75 perovskite films passivated without and with PEABr. (f) TPV characteristics of the two solar cells.

device with PEABr treatment is dramatically enhanced from 12.18% to 15.15%, giving a JSC of 24.90 mA cm−2, a VOC of 0.78 V, and an FF of 78% (forward scan). By contrast, the control device without PEABr treatment shows a much lower VOC of 0.71 V and FF of 70% (Table 2). We note that the efficiency over 15% of our blade-coated solar cells reaches the state-of-the-art efficiencies of spin-coated Sn–Pb perovskite devices with similar Sn:Pb ratios [39–41]. Additionally, compared to the pristine device, the solar cell with PEABr surface passivation exhibits smaller negligible hysteresis. In Fig. 4b, we show a typical crosssectional SEM image of a completed perovskite solar cell, where the perovskite absorber layer consists of vertical aligned crystals throughout the device. Such vertically stacked grains could benefit charge carriers transport to the respective charge extraction layers with minimal recombination at grain boundaries. Fig. 4c presents the EQE spectra of the two devices with and without PEABr surface passivation. Both EQEs show similar broad photon response plateau with onset at around 950 nm, which is consistent with the UV–vis absorption results (Fig. S7). The integration of the EQE spectra with the AM 1.5 G solar spectrum yielding JSC values of 24.14 and 24.34 mA cm−2 for the device without and with PEABr treatment, which are close to the values obtained from the J-V measurements (Fig. 4a). Note that all our devices were unencapsulated during J-V measurements which was performed in air atmosphere and the EQE measurements were carried out after the J-V measurements. To illustrate the robustness of the surface

passivation, more than 25 devices in five bathes were fabricated. Compared to the control devices without PEABr passivation, all the PEABr-treated PSCs showed a significant VOC and PCE enhancement, demonstrating excellent reliability of the surface passivation (Fig. 4d). To unveil the origin of the VOC enhancement with PEABr surface passivation, we measured time-resolved photoluminescence (TRPL) of the prepared perovskite films (Fig. 4e). The PL life time of the PEABr treated perovskite film is calculated to 5.8 ns which is ~6 times higher than the films without PEABr treatment, suggesting suppressed surface recombination of the perovskite film with PEABr treatment. We further measured the transient photovoltage (TPV) characteristic of the two solar cells operated at open circuit conditions (Fig. 4f). The charge recombination constants (τr) are extracted as the time interval during which the photovoltage decays to 1/e of the its initial value immediately after decay [45]. The τr of the PEABr treated device is as high as 140 μs, which is more than twofold higher than the 65 μs of the device without PEABr treatment. The longer carrier decay time indicates slower charge recombination rates which should be the main factor responsible for the enhanced VOC of the PEABr passivated devices. To gain further insight into the origin of the passivation effect by PEABr surface treatment, we examined the structure and morphology of the prepared perovskite films with particular focus on the surface properties. We fabricated a perovskite films by spin coating of PEABr solutions with a high concentration of 5 mg mL−1. XRD spectra show that in addition to the typical perovskite characteristic peaks, several new periodic diffraction peaks at 2θ ≈ 5.4°, 10.8°, 16.2°, 21.6°, 27.2°, and 32.7° appear (denoted by asterisks in Fig. 5a and Fig. S8 where the Y-axis is in logarithmic scale). From Fig. S8, we also notice that excess PEABr remains in the perovskite films when the PEABr concentration is too high (5 mg mL−1). It is obvious that these new periodic diffraction peaks can be indexed neither to the XRD pattern of the Sn–Pb mixed perovskite nor the pure precursor PEABr. Indeed, these diffraction peaks at low angles are indicative of a material with a larger unit cell,

Table 2 Photovoltaic parameters of the (FASnI3)0.25(MAPbI3)0.75 perovskite solar cells with and without PEABr (1 mg mL−1) surface treatment. Surface treatment

Scan direction

JSC (mA/cm2)

VOC (V)

FF (%)

PCE (%)

W/O PEABr

Reverse Forward Reverse Forward

24.5 24.34 24.90 24.90

0.71 0.71 0.78 0.78

70 73 78 77

12.18 12.61 15.15 14.96

With PEABr

6

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Fig. 5. (a) XRD spectra of the lowbandgap (FASnI3)0.25(MAPbI3)0.75 perovskite films without and with spincoating of PEABr solution with concentrations of 1, 5 mg mL−1. For comparison, the XRD pattern of the PEABr is presented. Several new diffraction peaks appear upon the PEABr passivation (denoted by stars). (b) A zoom-in XRD spectra between 2θ of 2.5°–12° shown in (a); the peak at 2θ = 4.7° denoted by pounds is indexed to PEABr residual. The SEM image of the (FASnI3)0.25(MAPbI3)0.75 perovskite film without (c) and with PEABr (1 mg mL−1) treatment (d).

which can be ascribed to the layered 2D-RP perovskite of PEA2PbBr2I2 with a periodic pattern of Δ2θ ~ 5.4° (Fig. S9) [46–48]. To further validate the formation of the 2D capping layer, we carried out grazing incident X-ray diffraction (GI-XRD) spectroscopy to detect the structural of the perovskite surface (Fig. S10). It is observed that, at low X-ray incident angle of 0.1°, the PEABr passivated film shows only low angle diffraction peaks at 2θ of 5.4° and 10.8°. With increase the X-ray incident angle to 0.2°, the characteristic diffraction peaks of the 3D perovskite appear. These observations confirm the as-formed 2D perovskite is capping on top of 3D perovskite bulk. For comparison, the control perovskite film without PEABr treatment exhibits only characteristic diffraction peaks of 3D perovskite. The as-formed wide bandgap 2D-RP perovskite capping layer can passivate the surface defects of the 3D Sn–Pb perovskite layer, which plays an essential role in suppressing the nonradiative recombination. Fig. 5c and d shows the top-view SEM images of the Sn–Pb (1:3) perovskite films without and with 1 mg mL−1 PEABr treatment. Compared to the pristine perovskite film, the PEABr treatment does not significantly change the crystal size. However, the PEABr passivated film shows a smoother surface where the grain boundaries are not as sharp as the control film, probably due to the growth of 2D-RP perovskites. The atomic force microscope (AFM) images reveal a root mean square surface roughness of 12.9 nm and 27.3 nm for the perovskite film without and with PEABr treatment, respectively (Fig. S11).

Fig. 6. J-V curves of a low-bandgap (FASnI3)0.25 (MAPbI3)0.75 perovskite solar cells with active area of 1 cm2 measured from both reverse and forward scan.

negligible. Compared with small-area cells (0.09 cm2), the large-area device preserved full VOC values, while a ~18% reduction in PCE for the large-area cells is mainly caused by the large sheet resistance of the ITO [49,50]. Nevertheless, to the best of our knowledge, this is the first demonstration of the large-area Sn–Pb mixed perovskite solar cells. These results validate the generality and robustness of our technology for the large-area deposition of high-quality Sn–Pb mixed perovskite films by printing methods.

2.5. Fabrication of large-area solar cells To demonstrate the possibility for scale-up of the Sn–Pb mixed perovskite films prepared by blade coating, we fabricated solar cells with active areas of 1 cm2. Fig. 6 shows the J-V curves of the prepared large-area (FASnI3)0.25(MAPbI3)0.75 solar cell, which also exhibit excellent performance, with JSC, VOC, and FF reaching values of 23.46 mA cm−2, 0.78 V, and 68%, respectively, yielding to a PCE of 12.44% from reverse scan. Hysteresis of the large-area device is also

2.6. Stability of the Sn–Pb mixed perovskite solar cells Lastly, we evaluated the stability of our blade-deposited Sn–Pb mixed perovskite solar cells with and without PEABr passivation. The 7

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Fig. 7. Shelf stability of the lowbandgap (FASnI3)0.25(MAPbI3)0.75 perovskite solar cells with (a) and without (b) PEABr passivation. The devices were stored in both N2-filled glove box and ambient air with relative humidity of 50% ± 10%. (c) XRD spectra of the (FASnI3)0.25(MAPbI3)0.75 perovskite films without (bottom) and with (top) PEABr passivation during the 14 h' storage in ambient condition (relative humidity of 100%).

solar cells were separately stored in N2-filled glove box and ambient air with relative humidity of 50% ± 10%. As shown in Fig. 7a and b, the devices independent of PEABr passivation maintained more than 80% of their original PCEs after 432 h’ storage in N2-filled glove box. However, when the solar cells were kept in ambient air, the PEABr passivated devices preserved more than 50% of its initial PCE, whereas the solar cell without PEABr passivation dropped to 25% of its initial efficiency. The enhanced stability of the PEABr passivated devices can be ascribed to the formation of the 2D perovskite capping layer, which has been widely demonstrated to possess an enhanced moisture and oxygen resistivity [46,47,51,52]. Interestingly, we observe that the performance decline of the solar cells mainly come from the JSC reduction whereas all the devices showed almost constant or even slightly increased VOC values with well-preserved FFs (Fig. S12). These results indicate that the charge extraction in the devices were fairly good during the stability measurement. To verify the enhanced moisture resistivity of the PEABr passivated perovskite films, we tracked the XRD spectrum evolution of the (FASnI3)0.25(MAPbI3)0.75 perovskite films which were stored at 100% relative humidity (Fig. 7c). As can be seen, the pristine film decomposed totally after 14 h, whereas the film with PEABr surface passivation retained the perovskite characteristic peaks with slightly reduced intensity. In addition, a close comparison between the degraded films with their original films evidences the formation of δ-phase FAPbI3 in the non-passivated perovskite film, whereas several new diffraction peaks with Δ2θ ~4° emerged which likely resulted from the insertion of water molecules into the layered perovskites (Fig. S13). Nevertheless, these results suggest that the enhanced stability of the PEABr passivated (FASnI3)0.25(MAPbI3)0.75 thin films and devices can be due to the protection of the 2D-RP layered perovskites.

devices demonstrated much enhanced shelf stability in air atmosphere due to the protection barrier of 2D layered perovskites. Our results demonstrate important progress in the preparation of high-quality Sn–Pb perovskite films for high-performance single- and multi-junction solar cells by scalable methods. 4. Experimental section 4.1. Materials and solution preparation Lead iodide (PbI2, 99.9985%) was purchased from Alfa Aesar. Methylammonium iodide (MAI), phenylethyl ammonium bromide (PEABr) and PEDOT:PSS (Heraeus-Clevios 4083) was purchased from Xi'an p-OLED Co. Tin(II) iodide (SnI2, 99.99%), Tin(II) fluorid (SnF2), methylammonium chloride (MACl, 98%), and all the solvents were purchased from Sigma-Aldrich. Bathocuproine (BCP), formamidinium iodide (FAI), PC61BM was purchased from Lumtec. All the chemicals were used as received without further purification. The 1 M FASnI3 stock solution was prepared by dissolving 372 mg SnI2 and 172 mg FAI with 10 mol% (15.6 mg) SnF2 in mixed solvent of 800 μL DMF and 200 μL DMSO. The 1 M MAPbI3 precursor solution was prepared by dissolving 461 mg PbI2 and 159 mg MAI with 10 mol% (6.8 mg) MACl in mixed solvent of 800 μL DMF and 200 μL DMSO. The Sn–Pb mixed perovskite precursor solutions with different ratio of the Sn/Pb were obtained by mixing FASnI3 and MAPbI3 stock solutions with desired volume ratio. 4.2. Sn–Pb mixed perovskite films deposition by blade-coating In this study, all the perovskite films including MAPbI3, FASnI3 and Sn–Pb mixed perovskites were prepared by blade coating. The blade deposition of perovskite films was carried out on a commercial blade coater (ZAA2300. H from ZEHNTNER) using a ZUA 2000.100 blade (from ZEHNTNER) at room temperature in nitrogen-filled glovebox. For the substrates with a dimension of 25 ✕ 25 mm, a solution of 20 μL was used for blade coating. The gap for solution load between the blade and substrate was fixed at 200 μm, and the coating speed was fixed at 8 mm/s. Once the perovskite precursor solution spread onto the substrate by blade-coating, the obtained precursor wet film was transferred into a vacuum chamber. To achieve an intermediate film by extraction the excess solvent of the precursor film, the vacuum chamber was pumped to 1000 Pa and kept at the vacuum for 1 min 30 s. The obtained intermediate film was subsequently brought out of the vacuum chamber and annealed at 100 °C for 5 min inside the glovebox to further

3. Conclusion In summary, we have developed, for the first time, a robust crystallization protocol that allows for scalable fabrication of high-quality Sn–Pb mixed perovskite films via one-step blade coating. Central to this technology is the deployment of a vacuum process which enables to produce a stable intermediate phase by removing excess solvent of the freshly deposited precursor film. Solar cells based on the prepared Sn–Pb mixed perovskites showed decent performances which were dramatically enhanced by surface defects passivation due to the formation of a 2D-RP perovskite layer. As a result, the VOC values of the 1:3 Sn–Pb perovskite solar cells were improved significantly from 0.71 V to 0.78 V due to the suppressed nonradiative recombination, leading to high efficiencies of over 15%. Moreover, the 2D-3D based 8

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crystallize the film. The obtained perovskite layer has a thickness of ~400 nm. Perovskite films with thickness of ~600 nm were obtained by blade coating precursor solutions of 30 μL with a speed of 15 mm/s.

Appendix A. Supplementary data

4.3. Solar cell fabrication

References

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.nanoen.2019.104099.

The prepatterned ITO-coated glass substrates (25*25 mm2 in dimension with sheet resistance of 17 ohm/sq) were cleaned by ultrasonication in detergent, acetone, and isopropanol for 10 min each. PEDOT:PSS films were coated on the cleaned ITO substrate at 4000 rpm for 50 s and then dried at 175 °C for 30 min. The perovskite absorber layer was subsequently deposited using the vacuum-assisted bladecoating as described above. PEABr passivated perovskite films were achieved by spin-coating a layer of PEABr (4000 rpm for 30 s) with different concentrations of 1 and 5 mg mL−1 on top of the bladed perovskite layers. To grow the 2D layered perovskite, the PEABr covered 3D perovskite film was further subjected to an annealing at 100 °C for 30 min inside the glovebox. Subsequently, electro-transporting layer PC61BM (20 mg mL−1 in chlorobenzene) and interfacial layer BCP (2.5 mg mL−1 in isopropanol) were successively deposited on top of perovskite films by spin coating at 2000 rpm for 30 s and 5000 rpm for 30 s, respectively. Finally, 100-nm-thick Ag layer was deposited by thermal evaporation. The active areas of the solar cells are 0.09 cm2 and 1 cm2 for the small-size and large-area devices, respectively, which are defined by the overlapping between the top Ag and bottom ITO electrodes.

[1] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, J. Am. Chem. Soc. 131 (2009) 6050–6051. [2] M.A. Green, A. Ho-Baillie, H.J. Snaith, Nat. Photonics 8 (2014) 506–514. [3] M. Gratzel, Nat. Mater. 13 (2014) 838–842. [4] W. Shockley, H.J. Queisser, J. Appl. Phys. 32 (1961) 510–519. [5] F. Guo, N. Li, F.W. Fecher, N. Gasparini, C.O.R. Quiroz, C. Bronnbauer, Y. Hou, V.V. Radmilovic, V.R. Radmilovic, E. Spiecker, K. Forberich, C.J. Brabec, Nat. Commun. 6 (2015) 7730. [6] G.E. Eperon, T. Leijtens, K.A. Bush, R. Prasanna, T. Green, J.T.W. Wang, D.P. McMeekin, G. Volonakis, R.L. Milot, R. May, A. Palmstrom, D.J. Slotcavage, R.A. Belisle, J.B. Patel, E.S. Parrott, R.J. Sutton, W. Ma, F. Moghadam, B. Conings, A. Babayigit, H.G. Boyen, S. Bent, F. Giustino, L.M. Herz, M.B. Johnston, M.D. McGehee, H.J. Snaith, Science 354 (2016) 861–865. [7] D.W. Zhao, Y. Yu, C.L. Wang, W.Q. Liao, N. Shrestha, C.R. Grice, A.J. Cimaroli, L. Guan, R.J. Ellingson, K. Zhu, X.Z. Zhao, R.G. Xiong, Y.F. Yan, Nat Energy 2 (2017) 17018. [8] D. Forgacs, L. Gil-Escrig, D. Perez-Del-Rey, C. Momblona, J. Werner, B. Niesen, C. Ballif, M. Sessolo, H.J. Bolink, Adv Energy Mater 7 (2017) 1602121. [9] T. Leijtens, K.A. Bush, R. Prasanna, M.D. McGehee, Nat Energy 3 (2018) 828–838. [10] C.U. Kim, J.C. Yu, E.D. Jung, I.Y. Choi, W. Park, H. Lee, I. Kim, D.K. Lee, K.K. Hong, M.H. Song, K.J. Choi, Nano Energy 60 (2019) 213–221. [11] S.J. Zhu, X. Yao, Q.S. Ren, C.C. Zheng, S.Z. Li, Y.P. Tong, B. Shi, S. Guo, L. Fan, H.Z. Ren, C.C. Wei, B.Z. Li, Y. Ding, Q. Huang, Y.L. Li, Y. Zhao, X.D. Zhang, Nano Energy 45 (2018) 280–286. [12] Y. Ogomi, A. Morita, S. Tsukamoto, T. Saitho, N. Fujikawa, Q. Shen, T. Toyoda, K. Yoshino, S.S. Pandey, T.L. Ma, S. Hayase, J. Phys. Chem. Lett. 5 (2014) 1004–1011. [13] F. Hao, C.C. Stoumpos, R.P.H. Chang, M.G. Kanatzidis, J. Am. Chem. Soc. 136 (2014) 8094–8099. [14] F. Zuo, S.T. Williams, P.W. Liang, C.C. Chueh, C.Y. Liao, A.K.Y. Jen, Adv. Mater. 26 (2014) 6454–6460. [15] X.M. Lian, J.H. Chen, Y.Z. Zhang, M.C. Qin, J. Li, S.X. Tian, W.T. Yang, X.H. Lu, G. Wu, H.Z. Chen, Adv. Funct. Mater. 29 (2019) 1807024. [16] M.Y. Liu, Z.M. Chen, Q.F. Xue, S.H. Cheung, S.K. So, H.L. Yip, Y. Cao, J. Mater. Chem. 6 (2018) 16347–16354. [17] W.Q. Liao, D.W. Zhao, Y. Yu, N. Shrestha, K. Ghimire, C.R. Grice, C.L. Wang, Y.Q. Xiao, A.J. Cimaroli, R.J. Ellingson, N.J. Podraza, K. Zhu, R.G. Xiong, Y.F. Yan, J. Am. Chem. Soc. 138 (2016) 12360–12363. [18] C.W. Li, Z.N. Song, D.W. Zhao, C.X. Xiao, B. Subedi, N. Shrestha, M.M. Junda, C.L. Wang, C.S. Jiang, M. Al-Jassim, R.J. Ellingson, N.J. Podraza, K. Zhu, Y.F. Yan, Adv Energy Mater 9 (2019) 1803135. [19] N.J. Jeon, J.H. Noh, Y.C. Kim, W.S. Yang, S. Ryu, S.I. Seok, Nat. Mater. 13 (2014) 897–903. [20] F. Ye, H. Chen, F.X. Xie, W.T. Tang, M.S. Yin, J.J. He, E.B. Bi, Y.B. Wang, X.D. Yang, L.Y. Han, Energy Environ. Sci. 9 (2016) 2295–2301. [21] Z. Li, T.R. Klein, D.H. Kim, M.J. Yang, J.J. Berry, M.F.A.M. van Hest, K. Zhu, Nat Rev Mater 3 (2018) 18017. [22] F. Guo, W. He, S. Qiu, C. Wang, X. Liu, K. Forberich, C.J. Brabec, Y. Mai, Adv. Funct. Mater. 29 (2019) 1900964. [23] M. Saliba, J.P. Correa-Baena, C.M. Wolff, M. Stolterfoht, N. Phung, S. Albrecht, D. Neher, A. Abate, Chem. Mater. 30 (2018) 4193–4201. [24] Y.F. Zhong, R. Munir, J.B. Li, M.C. Tang, M.R. Niazi, D.M. Smilgies, K. Zhao, A. Arnassian, Acs Energy Lett 3 (2018) 1078–1085. [25] Z.B. Yang, C.C. Chueh, F. Zuo, J.H. Kim, P.W. Liang, A.K.Y. Jen, Adv Energy Mater 5 (2015) 1500328. [26] S. Tang, Y.H. Deng, X.P. Zheng, Y. Bai, Y.J. Fang, Q.F. Dong, H.T. Wei, J.S. Huang, Adv Energy Mater 7 (2017) 1700302. [27] Y.H. Deng, E. Peng, Y.C. Shao, Z.G. Xiao, Q.F. Dong, J.S. Huang, Energy Environ. Sci. 8 (2015) 1544–1550. [28] J.B. Li, R. Munir, Y.Y. Fan, T.Q. Niu, Y.C. Liu, Y.F. Zhong, Z. Yang, Y.S. Tian, B. Liu, J. Sun, D.M. Smilgies, S. Thoroddsen, A. Amassian, K. Zhao, S.Z. Liu, Joule 2 (2018) 1313–1330. [29] F.X. Xie, D. Zhang, H.M. Su, X.G. Ren, K.S. Wong, M. Gratzel, W.C.H. Choy, ACS Nano 9 (2015) 639–646. [30] X. Li, D.Q. Bi, C.Y. Yi, J.D. Decoppet, J.S. Luo, S.M. Zakeeruddin, A. Hagfeldt, M. Gratzel, Science 353 (2016) 58–62. [31] B. Ding, Y. Li, S.Y. Huang, Q.Q. Chu, C.X. Li, C.J. Li, G.J. Yang, J. Mater. Chem. 5 (2017) 6840–6848. [32] Q.W. Han, Y.S. Bai, J. Liu, K.Z. Du, T.Y. Li, D. Ji, Y.H. Zhou, C.Y. Cao, D. Shin, J. Ding, A.D. Franklin, J.T. Glass, J.S. Hu, M.J. Therien, J. Liu, D.B. Mitzi, Energy Environ. Sci. 10 (2017) 2365–2371. [33] F. Guo, S. Qiu, J. Hu, H. Wang, B. Cai, J. Li, X. Yuan, X. Liu, K. Forberich, C.J. Brabec, Y. Mai, Adv. Sci. 6 (2019) 1901067. [34] T.M. Koh, T. Krishnamoorthy, N. Yantara, C. Shi, W.L. Leong, P.P. Boix, A.C. Grimsdale, S.G. Mhaisalkar, N. Mathews, J. Mater. Chem. 3 (2015) 14996–15000. [35] S.J. Lee, S.S. Shin, Y.C. Kim, D. Kim, T.K. Ahn, J.H. Noh, J. Seo, S.I. Seok, J. Am.

4.4. Characterizations The crystal structure was characterized by Bruker D8 Advance X-ray diffractometer (XRD) with CuKα radiation operated at 40 kV and 40 mA. The surface microstructure and cross-sectional morphology of the thin films and devices were imaged by a Field-emission scanning electron microscope (SEM, FEI Apreo LoVac). A double-beam spectrophotometer (Lambda 950, PerkinElmer) equipped with an integrated sphere was used for the UV–vis absorption measurement. Transmittance Infrared (IR) measurements were carried out employing a Fourier Transform Infrared (FTIR) spectrometer (Thermo Scientific Nicolet 380). The time-resolved photoluminescence was carried out at the Time-Resolved and Spectroscopy (Fluo Time 300, PicoQuant GmbH). Current density-voltage (J-V) characteristics of the solar cells were measured using a Keithley 2400 source meter. The illumination was provided by a Newport Oriel 92192 solar simulator with an AM1.5G filter, operating at 100 mW cm−2, which was calibrated by a standard silicon solar cell from Newport. Both forward and backward scans were performed, and the scan speed was fixed at 0.15 V s−1. The external quantum efficiency (EQE) of the perovskite solar cell device was measured by using a QE-R instrument from Enlitech. All measurements of the films and devices were performed in ambient conditions and the devices were not encapsulated. Conflicts of interest The authors declare no conflict of interest. Acknowledgements The work was supported by the National Natural Science Foundation of China (Grant No. 61705090). C.J.B. acknowledges funding through the Bavarian Ministry of Economic Affairs (ZAE -FZJ Cooperation) and the Bavarian Ministry of Science and Culture (Soltech Initiative). F.G. acknowledges funding from the Opening Project of Key Laboratory of Materials Processing and Mold, Zhengzhou University. 9

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Jinlong Hu received his Ph.D degree in chemical engineering and technology from Hunan University in 2018. Currently he is working as a postdoc in the Institute of New Energy Technology at Jinan University. His current research interests focus on the design and fabrication of highly efficient and stable perovskite solar cells.

Chem. Soc. 138 (2016) 3974–3977. [36] N.K. Noel, S.D. Stranks, A. Abate, C. Wehrenfennig, S. Guarnera, A.A. Haghighirad, A. Sadhanala, G.E. Eperon, S.K. Pathak, M.B. Johnston, A. Petrozza, L.M. Herz, H.J. Snaith, Energy Environ. Sci. 7 (2014) 3061–3068. [37] M.E. Kayesh, K. Matsuishi, R. Kaneko, S. Kazaoui, J.J. Lee, T. Noda, A. Islam, Acs Energy Lett 4 (2019) 278–284. [38] C.M. Tsai, N. Mohanta, C.Y. Wang, Y.P. Lin, Y.W. Yang, C.L. Wang, C.H. Hung, E.W.G. Diau, Angew. Chem. Int. Ed. 56 (2017) 13819–13823. [39] X. Liu, Z.B. Yang, C.C. Chueh, A. Rajagopal, S.T. Williams, Y. Sun, A.K.Y. Jen, J. Mater. Chem. 4 (2016) 17939–17945. [40] Z.B. Yang, A. Rajagopal, C.C. Chueh, S.B. Jo, B. Liu, T. Zhao, A.K.Y. Jen, Adv. Mater. 28 (2016) 8990–8997. [41] D. Chi, S.H. Huang, M.Y. Zhang, S.Q. Mu, Y. Zhao, Y. Chen, J.B. You, Adv. Funct. Mater. 28 (2018) 1804603. [42] T.S. Sherkar, C. Momblona, L. Gil-Escrig, J. Avila, M. Sessolo, H.J. Bolink, L.J.A. Koster, Acs Energy Lett 2 (2017) 1214–1222. [43] P. Schulz, D. Cahen, A. Kahn, Chem. Rev. 119 (2019) 3349–3417. [44] X. Shi, R. Chen, T. Jiang, S. Ma, X. Liu, Y. Ding, M. Cai, J. Wu, S. Dai, Sol RRL (2019) 1900198. [45] W. Chen, Y.Z. Wu, Y.F. Yue, J. Liu, W.J. Zhang, X.D. Yang, H. Chen, E.B. Bi, I. Ashraful, M. Gratzel, L.Y. Han, Science 350 (2015) 944–948. [46] C.M.M. Soe, W.Y. Nie, C.C. Stoumpos, H. Tsai, J.C. Blancon, F.Z. Liu, J. Even, T.J. Marks, A.D. Mohite, M.G. Kanatzidis, Adv Energy Mater 8 (2018) 1700979. [47] K.T. Cho, G. Grancini, Y. Lee, E. Oveisi, J. Ryu, O. Almora, M. Tschumi, P.A. Schouwink, G. Seo, S. Heo, J. Park, J. Jang, S. Paek, G. Garcia-Belmonte, M.K. Nazeeruddin, Energy Environ. Sci. 11 (2018) 952–959. [48] Q. Jiang, Y. Zhao, X.W. Zhang, X.L. Yang, Y. Chen, Z.M. Chu, Q.F. Ye, X.X. Li, Z.G. Yin, J.B. You, Nat. Photonics 13 (2019) 500-500. [49] W. Chen, L.M. Xu, X.Y. Feng, J.S. Jie, Z.B. He, Adv. Mater. 29 (2017) 1603923. [50] Y.Z. Wu, F.X. Xie, H. Chen, X.D. Yang, H.M. Su, M.L. Cai, Z.M. Zhou, T. Noda, L.Y. Han, Adv. Mater. 29 (2017) 1701073. [51] P. Chen, Y. Bai, S.C. Wang, M.Q. Lyu, J.H. Yun, L.Z. Wang, Adv. Funct. Mater. 28 (2018) 1706923. [52] G. Liu, H. Zheng, X. Xu, S. Xu, X. Zhang, X. Pan, S. Dai, Adv. Funct. Mater. (2019) 1807565.

Chaohui Li is currently a Master student majored in Materials Science and Engineering at Zhengzhou University. He is engaged in the bulk passivation of scalable printed lead-tin mixed perovskite solar cells.

Xianhu Liu is currently an associate professor at National Engineering Research Center for Advanced Polymer Processing Technology, Zhengzhou University, China. He received his PhD degrees from Zhengzhou University in 2015 and Friedrich-Alexander-University ErlangenNuremberg in 2016, respectively. His research interests focus on advanced polymer processing technology, bioinspired polymer composites, polymer rheology, flow-induced crystallization, oil/water separation materials and so on.

Linxiang Zeng is currently a joint Ph.D. candidate in Zhejiang University and Jinan University. She received her Master degree in organic chemistry from Hunan Normal University in 2012. Her research interest is focused on solution prepared perovskite nanocrystals and their optoelectronic applications.

Guangxing Liang is an associate researcher in College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen, China. He obtained his PhD degree from University of Rennes 1, Rennes, France. He has extensive experience in solar energy materials and solar cells, including Sb2Se3, CZTS and perovskite, etc. He published over 100 science citation index (SCI) journal papers. He is regular journal paper reviewers for more than 10 journals. He is the academic editors of Advances in Materials Science and Engineering.

Zongao Chen is a Master candidate in the College of Information Science and Technology, Jian University, Guangzhou. His is currently working on scalable coating of perovskite thin-films for photovoltaic applications.

Christoph J. Brabec received his Ph.D. in Physical Chemistry from Linz University, Austria, and then joined the group of Alan Heeger at UC Santa Barbara (USA) for a sabbatical. He joined the SIEMENS research labs (project leader) in 2001, Konarka in 2004 (CTO), Erlangen University (Professor for Material Science) in 2009, ZAE Bayern e.V. (Scientific director and board member) in 2010. In 2018 he was appointed as director of the Helmholtz Institute Erlangen Nürnberg and became honorary professor at the University of Groningen. His research interests include all aspects of solution processing organic, hybrid and inorganics semiconductor devices with a focus on photovoltaics and renewable energy systems.

Shudi Qiu received his Bachelor degree in 2018 from Guangdong University of Petrochemical Technology. Currently, he is pursuing his Master degree at Jinan University and focuses on the defects passivation and the fabrication of large-area perovskite modules.

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L. Zeng, et al. Yaohua Mai received his Ph.D. degree from Nankai University and Forschungszentrum Jülich in 2006. After that he became a post-doctoral researcher in Utrecht University. In 2008, he co-found Baoding Tianwei Solarfilms Co. Ltd, a thin film silicon PV module manufacturer with annual production capacity of 75 MW, and served as chief technology officer. He joined Hebei University in 2013 and became full professor. He is now professor and director of Institute of New Energy Technology (iNET) at Jinan University. He is mainly working on thin film and c-Si based photovoltaic materials and devices.

Fei Guo received his Ph.D. in material science from Friedrich-Alexander University Erlangen-Nuremberg in 2015. After a year's postdoctoral training at the group of Prof. Christoph J. Brabec, he joined the Institute of New Energy Technology (iNET) at Jinan University since 2017. His current research interests focus on printed optoelectronic devices based on perovskites and organic semiconductors.

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