The low temperature solution-processable SnO2 modified by Bi2O2S as an efficient electron transport layer for perovskite solar cells

Journal Pre-proof The low temperature solution-processable SnO2 modified by Bi2O2S as an efficient electron transport layer for perovskite solar cells Jinyun Chen, Zhuoneng Bi, Xueqing Xu, Huangzhong Yu PII:

S0013-4686(19)32068-7

DOI:

https://doi.org/10.1016/j.electacta.2019.135197

Reference:

EA 135197

To appear in:

Electrochimica Acta

Received Date: 21 August 2019 Revised Date:

17 October 2019

Accepted Date: 3 November 2019

Please cite this article as: J. Chen, Z. Bi, X. Xu, H. Yu, The low temperature solution-processable SnO2 modified by Bi2O2S as an efficient electron transport layer for perovskite solar cells, Electrochimica Acta (2019), doi: https://doi.org/10.1016/j.electacta.2019.135197. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

The low temperature solution-processable SnO2 modified by Bi2O2S as an efficient electron transport layer for perovskite solar cells Jinyun Chena,b, Zhuoneng Bic, Xueqing Xuc*, Huangzhong Yua,c* a

School of Physics and Optoelectronics, South China University of Technology, 510640 Guangzhou, China

b

School of Materials Science & Engineering, South China University of Technology, 510640 Guangzhou, China

c

Key Laboratory of Renewable Energy, Guangdong Key Laboratory of New and Renewable Energy Research and Development, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, 510640 Guangzhou China

Abstract Electron transport layer (ETL), promoting electron transportation and electron extraction, is an essential component of high efficiency perovskite solar cells (PVSCs). SnO2 has been proved to be an excellent ETL for efficient PVSCs due to its good optical transparency, high electrical conductivity, and suitable band position. In this work, we develop low temperature solution-processable SnO2 thin film modified by Bi2O2S nanoparticles as effective ETL of PVSCs for the first time. The results show the modification of the Bi2O2S nanoparticles can passivate oxygen vacancies of SnO2 thin film, resulting in less charge recombination and improved charge transport. Furthermore, SnO2 thin film modified by Bi2O2S nanoparticles enhances film morphology of overlying perovskite active layer, including larger grain size, better crystallinity. These eventually result in a remarkable improvement of performance of

*

Corresponding author. E-mail address: [email protected] (H.Z. Yu), [email protected] (X.Q.Xu) 1

PVSCs. Compared to PVSCs with pristine SnO2 as ETL, the power conversion efficiency (PCE) of PVSCs with SnO2 modified by optimized Bi2O2S nanoparticles as ETL is raised to 17.13% from 14.61% with suppressed hysteresis. In addition, the modification of Bi2O2S can slightly enhance the stability of PVSCs due to reduced oxygen vacancies of SnO2 and better crystallinity of perovskite film. This work not only provides an effective mean of surface modification of SnO2, but also shows the Bi2O2S material has potential for applications for PVSCs. Key words: Tin oxide, Bismuth oxysulfide, Perovskite solar cells, Electron transporting layer, Oxygen vacancies, Morphology. 1. Introduction Since the first report in 2009, power conversion efficiency (PCE) of perovskite solar cells (PVSCs) has been continuously recorded at an unprecedented rate, and has entered the ranks of solar cells with high PCE. In less than a decade, the PCE of PVSCs has been rapidly increased from 3.8% to a certified 23.7% [1-6]. Most high efficiency PVSCs usually adopt a mesoscopic structure, which requires complex and high temperature processing [7, 8]. In this regard, planar structure PVSCs have attracted much attention because of its simple structure, easy preparation process and low temperature processing [9, 10]. The structure of planar PVSCs usually consists of perovskite light absorption layer and two selective compact layer, which forms a sandwich configuration [11]. Selective compact layer, including electron transport layer (ETL) and hole transport layer (HTL), are essential for charge transport and extraction in the PVSCs, which are essential component for high efficient PVSCs [12, 2

13]. Metal oxides, such as titanium oxide (TiOx) and zinc oxide (ZnO) have been developed as ETLs due to their high optical transmittance, environmental stability and superior conductivity [14, 15]. More recently, tin oxide (SnO2) has become as a most promising metal oxide for ETL in PVSCs because of its superior optical and electrical properties. SnO2 combines high electrical conductivity with high optical transmittance in the visible range of the electromagnetic spectrum [16, 17]. SnO2 prepared by low-temperature processing has been considered as excellent ETL for flexible PVSC. Tian et al. achieved 13% PCE by spin-coating SnO2 nanoparticles on an ITO substrate and then annealing at 200 °C [18]. Fang et al. achieved 14.82% PCE in forward scan and 17.21% PCE in reverse scan by thermal decomposition of SnCl2·2H2O solution at 180 °C [13]. You et al. achieved 19.9% certified PCE with minor hysteresis by using high quality SnO2 nanocrystal colloidal [5]. However, SnO2 ETL prepared by low-temperature processing exhibits a mixture of amorphous and crystal structure, leading to the existence of large amount of oxygen vacancies which can trap electrons and deteriorate electronic properties of SnO2 as the same situation in ZnO [19, 20]. Interfacial modification of ETLs is the intentional introduction of functional compound for the purpose of suppressing trap-assisted charge recombination and improving film morphology of perovskite thin film. It has been proven to be an effective method to decrease surface defects of SnO2 and improve the poor electrical coupling of SnO2/ perovskite interface. Zhan et al. have reported that fullerene derivative modified low-temperature processed SnO2 surface could passivate oxygen 3

vacancy related defect, which not only improved charge extraction photo-generated charge carriers, but also improved the quality of the perovskite film with enlarged grain size [19]. Antonio Abate et al. reported that small amount of Ga doped SnO2 could reduce the number of trap states, which was likely to be the result of the elimination of oxygen vacancies in the SnO2 lattice by incorporation with Ga3+ ions, but further increased doping concentration could lower the PCE of the device due to an increasing trap state density caused by Ga dopant [20]. Mahmud et al. also reported that UVO treatment passivated oxygen vacancy in ZnO film, which ensures lower trap-assisted recombination and enhanced charge transfer property at perovskite/ETL interface [21]. In recent years, layered bismuth compounds (Bi2O2S, Bi2O2Se, Bi2O2Te et al) have attracted enormous attentions in pollutant purification, photocatalysis and photodetector due to their nontoxicity, ultrahigh hall mobility, layered structure and narrow band gap [22-24]. In our previous research work, our group has found that Bi2O2S nanoparticles can effectively passivate the oxygen vacancies related surface traps of the ZnO nanorod arrays surface, reduce the series resistance, improve the electrical coupling of ZnO NRAs/active layer [25]. Bi2O2S has a looser structure with a packing factor (PF) of 0.66, which is close to some well-known photoelectric material, such as anatase TiO2 and MAPbI3 with a PF of 0.65. Looser structure with low PF afford vast space for atom vibration, leading to a lager exiton Bohr radius, which can result in a longer carrier lifetime. Furthermore, the open structure is more deformable, which can increase mobility by lowering hopping barrier [22]. Especially, 4

Bi2O2S and MAPbI3 have similar looser structure, which might be helpful for MAPbI3 grain growth on Bi2O2S. Nevertheless, investigation is missing for the application of Bi2O2S in PVSCs. Here, we synthesize a bismuth oxysulfide (Bi2O2S) nanomaterial with a simple hydrothermal method and apply it to in-situ modify SnO2 nanomaterial synthesized by sol-gel method, and PVSCs with SnO2 modified by Bi2O2S as ETL are fabricated for the first time. Compared to PVSCs using pristine SnO2 as ETL, the PCE of PVSCs with SnO2 modified by optimized Bi2O2S as ETL is raised to 17.13% from 14.61% with decreased hysteresis. Bi2O2S modified SnO2 exhibits reduced oxygen vacancies, leading to less recombination, which facilitates charge transportation and charge extraction. Furthermore, Bi2O2S modified ETL enhances film morphology of overlying perovskite layer, including larger grain size, better crystallinity, leading to better photovoltaic performance. 2. Experimental section 2.1 Materials The patterned ITO glass (~10 Ω/square) was purchased from China Southern Glass Holding Corp. CsI (99.5%), FAI (99.5%), MAI (99.5%), PbI2 (99.99%) , PbBr2 (99.99%), MABr (99.5%), 2,2`,7,7`-tetrakis(N,N`-di-p-methoxyphenylamine)-9,9`spirobifuorene) (Spiro-OMeTAD) (99.8%) was purchased from Xi'an Polymer Light Technology

Corp.

without

further

purification.

SnCl2.2H2O

(99.995%),

chlorobenzene (99.9%), DMF (99%), Lithium bis(trifluoromethanesulfonyl) imide (Li-TFSI) (99.95%) was purchased from Sigma-Aldrich. DMSO (99%) was 5

purchased from Alfa Aesar. Acetonitrile (99.9%) was purchased from Acros, 4-tert-butylpyridine (96%) was purchased from TCI. 2.2 Synthesis of Bi2O2S nanoparticles 1 mmol CH4N2S was added into 20 ml NaOH solution (0.1mol) with stirring until it became a homogeneous solution. Then 0.1 mmol Bi2O3 powder was added into the solution with vigorous stirring, the color of the solution turn into white. Subsequently,

the

white

mixture

solution

was

transferred

into

a

polytetrafluoroethylene (PTFE) lined pressure autoclave with a thermal annealing at 180° for 40 hours. The products were treated by centrifugation and washing with water and anhydrous ethanol step by step. Bi2O2S powder was obtained after all the above process, which is proved to be pure phase by powder X-ray diffraction (XRD). Finally, prepared Bi2O2S powder was dispersed into ethanol with vigorously ultrasonic concussion. The stock solution was ready for use after filtered with 0.45 µm PTFE filter. 2.2 Preparation of precursor solution To prepare ETL precursor solution with sol-gel method, 90 mg SnCl2.2H2O was dissolved in 4 mL anhydrous ethyl alcohol with stirring and aged for 48 hours to form 22.5 mg/mL SnCl2.2H2O solution stock. Then Bi2O2S ethanol solution (0 %, 2%, 5%, 10% volume concentration) were added into SnCl2.2H2O stock to form ETL precursor solution. To prepare MAPbI3 perovskite precursor solution, 580.9 mg PbI2 was mixed with 190.8 mg CH3NH3I at 1.05:1 molar ratio in 900 µL anhydrous DMF and 100 µL 6

anhydrous DMSO solution. To prepare (CsFAMA)Pb(IBr)3 mixed perovskite precursor solution, 1 mol FAI, 1.1 mol PbI2, 0.2 mol MABr, 0.2 mol PbBr2 was dissolved in a mixture solvent of DMF/DMSO (4:1, by volume). Next, 50 µL CsI solution (pre-dissolved as a 1.5 mol stock solution in DMSO) was added to the mixed solution to achieve the desired perovskite precursor solution with proper excess lead halide. To prepare HTL solution, 72 mg Spiro-OMeTAD was dissolved into 1mL chlorobenzene, with addition of 29 µl of 4-tert-butylpyridine and 18 µl of Li-TFSI solution. Li-TFSI solution was prepared by dissolving 520 mg Li-TFSI into 1 ml acetonitrile in advance. 2.3 Fabrication of the photovoltaic devices ITO-coated substrates were cleaned by sonication with detergent, deionized water, acetone and isopropanol sequentially for 15 min. After blowing with N2, ITO was treated with oxygen plasma for 10 min. 35 µL ETL precursor solution was cast onto as treated ITO at 4000 rpm for 30 s. Subsequently, the substrate was placed on a hot plate at 190 ℃ for 1 hour (thin film with Bi2O2S modification was named Bi2O2S:SnO2). The as deposited ETLs were then treated with oxygen plasma for 10 min. Perovskite precursor solution was spin-coated onto ETL sequentially at 1000 rpm for 5 s, 3500 rpm for 40 s, followed by quickly drop-casting 200 µL chlorobenzene as anti-solvent within 10 s. The sample was quickly transferred to 100 hot substrate for 15 min. The as prepared Spiro-OMeTAD solution was spin-coated onto the perovskite layer at 4000 rpm for 30 s. The sample was put into a desiccator 7

overnight to ensure adequate oxidation of HTL. Then 2 nm molybdenum trioxide (MoO3) was evaporated onto HTL with a module defining active area 0.15 cm2 at a vacuum of 2.8×10−4 Pa, followed by the final Ag cathode deposition (80 nm). 2.4 Measurements The UV-vis spectra of ETL were measured with a HP 8453 spectrophotometer. Absorbance spectra of the perovskite films were recorded with a PerkinElmer Lambda 750 UV/VIS/NIR spectrophotometer. Photoluminescence spectra (PL) were measured on a SPEX 1681 automated spectrofluorometer. The Raman spectra were recorded on an inVia-58P056 Raman spectrometer, using 532 nm focused excitation laser. Scanning electron microscopy (SEM) that visualizes film morphology was carried out on ZEISS Merlin. Structural analysis of the thin films was carried out via high-resolution XRD Bruker Discovery 08 with a Hi-Star area detector. The chemical analysis of SnO2 and SnO2:Bi2O2S was recorded on Kratos Axis Ulra DLD X-ray photoelectron spectroscopy (XPS) under conditions of Al Kα monochromatic X-ray source. Curve fitting and background subtraction of XPS data was processed using CASA XPS software. The work function (WF) of SnO2 and SnO2:Bi2O2S thin film was measured with Kelvin Probe using KP Technology SKP5050. The current density-voltage (J-V) characteristics of PVSCs were recorded using a computer controlled Keithley 2400 source unit under a calibrated AM 1.5 G solar simulator (100 mW cm-2) at room temperature. 3. Results and discussion

8

(212) (261)

(101)

(141) (002) (151) (112) (221) (161) (132) (170) (250)

(130)

(b)

(111)

(020)

(120) (040)

(110)

Intensity ( a.u.)

(a)

PDF#34-1493 10

20

30

40

50

60

70

80

2θ (degree)

(d)

(c)

Fig. 1 (a) X-ray diffraction pattern of the Bi2O2S powder. (b) TEM of synthesized Bi2O2S powder. (c) HRTEM of (110) spacing plane, (d) selected-area electron diffraction (SAED) pattern of the Bi2O2S powder.

Powder X-ray diffraction pattern of Bi2O2S nanoparticles is shown in Fig. 1(a), all the diffraction peak can be assigned to Bi2O2S (PDF#34-1493), the diffraction peaks of Bi2O2S nanoparticles at 2θ = 14.9, 24.2, 27.4, 30.0, 32.3, 32.7, 33.7, 45.0, 45.6, 47.4, 50.9, 53.8, 55.3, 57.3, 58.3, 59.2, 61.6, 62.3, 69.2, and 72.4° match (020), (110), (120), (040), (130), (101), (111), (141), (060), (002), (151), (112), (221), (161), (132), (170), (250), (080), (212), and (261) planes of the orthorhombic structure of Bi2O2S, respectively. No other impurity peaks appear, which shows that we synthesizes pure Bi2O2S compound. Fig. 1(b) shows that Bi2O2S is the nanoparticles with diameter of 9

more than 10 nanometers, whose size is smaller than the previous reported one [22]. Clear lattice fringe of an individual Bi2O2S nanoparticle is observed in Fig. 1(c), which indicates that the prepared Bi2O2S nanoparticle has good crystallinity. The interplanar spacing of 0.38 nm is observed, which corresponds to the (110) planes of orthorhombic Bi2O2S. Fig. 1(d) shows selected area electron diffraction (SAED) patterns of the Bi2O2S powder, which indicates that it is monocrystalline. Thus the pure Bi2O2S nanoparticles with orthorhombic structure have been synthesized. Fig. S1 (a) shows the X-ray diffraction (XRD) pattern of different concentrations (0%, 2%, 5%, 10%) Bi2O2S modified SnO2 thin film at 450

thermal annealing.

Curve bread diffraction peak around 20° belongs to amorphous glass substrate which supports our sample. Other diffraction peaks can be assigned to rutile tetragonal structure of SnO2 (PDF#41-1445). The XRD diffraction peaks of pure SnO2 thin film treated at 450℃ are not obvious (Fig. S1 (a)), which indicates the synthesized SnO2 materials have poor crystallization, there may be a large number of mixture of crystal SnO2 and amorphous SnOx in the SnO2 materials. If there is not 450℃ thermal annealing, The SnO2 thin films synthesized at low temperature cannot show diffraction peaks at all (Fig. S1 (b)), which also further shows there exists vast amorphous SnOx in the SnO2 materials synthesized by sol-gel method. The presence of the crystal SnO2 and large amount of amorphous SnOx leads to the existence of oxygen vacancy related defects in the SnO2 materials. These trap states can serve as recombination center, leading to poor charge transfer and serious interface recombination [19]. 10

Fig. S2 (a) shows TEM image of SnO2 film decorated Bi2O2S nanoparticles, which suggests that Bi2O2S nanoparticles are well distributed in SnO2. Fig. S2 (b) shows HRTEM of SnO2 film decorated Bi2O2S nanoparticles, the interplanar spacing of 0.38 nm corresponds to the (110) planes of orthorhombic Bi2O2S and the interplanar spacing of 0.32 nm corresponds to the (110) planes of SnO2. Clear lattice fringes of Bi2O2S and SnO2 suggest that Bi2O2S nanoparticles are well decorated on SnO2. Fig. S2 (c) shows selected area electron diffraction (SAED) patterns of the Bi2O2S and SnO2, which indicates that orthorhombic monocrystalline structure of Bi2O2S and polycrystalline structure of SnO2. Fig. S2 (d) gives the distribution diagram of particle

Normalized Intensity (a.u.)

size according to Fig. S2 (a), the average size of Bi2O2S is 17.6 nm. 1.0

0% 2% 5% 10%

(a)

(b)

0.8 0.6 0.4 0.2 0.0 300

350

400

450

500

550

600

Normolized intensity (a.u.)

Wavelength (nm)

1.0

0% 2% 5% 10%

(c)

0.8

Fig. 2 (a) steady state PL spectra of

0.6

various ETLs measured on quartz glass. (b)

0.4

Proposed schematic model for PL spectrum.

0.2 0.0 200

(c) Raman spectrum of SnO2 and SnO2: 300

400

500

600

700 -1

800

900

Raman shift (cm )

Bi2O2S on quartz glass. 11

The room temperature photoluminescence spectra (PL) were implemented to investigate the trap states of different ETLs. As shown in Fig. 2 (a), SnO2 thin film has three emission bands, which respectively correspond to near band edge emission at 362 nm, 381 nm and visible emission at 429 nm. Recombination of oxygen vacancies with electrons in the conduction band (CB) gives rise to trap emission in visible blue light (429 nm) [26, 27]. As compared with pristine SnO2, the emission in visible blue light of SnO2:Bi2O2S decreases with the increase of Bi2O2S concentration (2%, 5%) at first, suggesting the reduction of oxygen vacancies with the modification of Bi2O2S. This phenomenon can be explained by the reduction of oxygen vacancies with introduction of n-type Bi2O2S. But when a further increase of Bi2O2S (10%) can result in an increase of oxygen vacancies. Therefore, optimized modification concentration of Bi2O2S should be 5%. Generally speaking, oxygen vacancies have been shown to be the most common defects in metal oxides and typically act as radiative centers in the process of luminescence [27]. We propose a scheme for the in Fig. 2 (b) to explain PL emission mechanism. Oxygen vacancies are present with three different configurations, neutral (Vo), singly charged (Vo•) and doubly charged (Vo••). The excitation of SnO2 starts with creation of holes in the VB and electrons in the CB, then the holes can be trapped at Vo• center to form Vo••.

When negative charges are introduced into SnO2 lattice,

they can be compensated by Vo•, thus less holes at Vo• can be subsequently transferred to Vo••. Therefore, n-type Bi2O2S can introduce negative charges, which can lead to compensation of Vo• and Vo••. Trap emission in visible blue light (429 nm) is 12

attributed to recombination of Vo•• center with electrons in the CB, which is reduced with reduction of Vo•• by Bi2O2S modification [28]. Raman spectroscopy is a powerful tool to distinguish surface-related defects of nanoparticles. Here the SnO2 nanoparticles are typical of rutile, the normal lattice vibrational modes of the SnO2 at the Brillouin zone is given by the following equation: Γ= A1g + A2g + A2u + B1g + B2g + 2B1u + Eg + 3Eu [29]. Fig. 2 (c) is Raman spectra of SnO2 and SnO2:Bi2O2S on quartz glass. Raman peaks at 487 cm-1, 606 cm-1, 795 cm-1 correspond to Eg, A1g, B2g Raman active vibration modes, respectively [30]. The peak at 559 cm-1 results from phonon confinement effect arisen from oxygen vacancies [31, 32]. It is obvious that peak at 559 cm-1 is reduced with the modification of Bi2O2S (2%, 5%). Especially, 5% Bi2O2S modified SnO2 shows the minimum oxygen vacancies. While further increase of modified Bi2O2S concentration (10%) will

0% 5%

200

400

600

800

Sn 3d 5/2

0% 5%

(b) Sn 3d 3/2

Intensity (a.u.)

Sn MNN

O KLL

Sn 3p 3/2 Sn 3p 1/2

C 1S

Sn 4d 0

486.9eV 486.6eV

O 1s

Intensity (a.u.)

(a)

Sn 3d 3/2

Sn 3d 5/2

introduce extra oxygen vacancies, which agrees well with results of PL spectra.

495.3eV 495.0eV

485

1000

490

495

Binding energy (eV)

Binding energy (eV)

13

500

(c)

(d) Intensity (a.u.)

Intensity (a.u.)

0%

5%

Bi 4f

S 2p

528

530

532

156

534

Binding energy (eV)

Bi 4f

7/2

5/2

S 2p

3/2

1/2

160

164

Binding energy (eV)

Fig. 3 Typical XPS of (a) survey. (b) O, (c) Sn, (d) Bi2O2S modified SnO2 was confirmed by the existence of Bi and S elements as shown in peak deconvolution of Bi 4f and S 2p.

As a sensitive spectroscopy technique for measuring the composition of the surface elements, XPS is considered as a powerful tool for confirming the presence of surface oxygen vacancies and detecting the chemical environment in which the elements are located. XPS was implemented on SnO2 ETL and optimized Bi2O2S: SnO2 (5%) thin film to further investigate surface states of SnO2. The binding energy obtained in the XPS analysis is corrected for specimen charge by referencing the C 1s peak to 284.6 eV. Fig. 3 (a) shows the full scanned spectra with the attribution of corresponding peak. Fig. 3 (b, c, d) shows the high-resolution scans spectra of Sn, O, and S atoms [33]. It is depicted in Fig. 3 (b) that Sn 3d 5/2 and Sn 3d 3/2 peaks of the pristine SnO2 film are located at 486.9 eV and 495.3 eV, respectively, indicating the formation of SnO2 [34]. A negative shift in the binding energy by about 0.3 eV is observed for the Sn 3d 5/2 and Sn 3d 3/2 peaks when Bi2O2S is introduced into SnO2 layer. The O1s core level spectrum divided into two peaks is shown in Fig. 3 (c). Here the two peaks at about 530.3 eV and 531.5 eV, which can be attributed to lattice 14

oxygen atom bond to Sn4+ and oxygen vacancies [35, 36]. It is obvious that peak at 531.5 eV is dramatically decreased, suggesting that oxygen vacancies are reduced with the small amount modification of Bi2O2S. These results are in accordance with the results of PL and Raman spectra. Generally, oxygen vacancies are acting as native electron acceptors due to positively charged. Thus, due to electron drawing effect of oxygen vacancies, the electron density of nearby Sn can decrease. The decrease of electron density causes the increase of binding energy. The negative energy shift can be explained based on the fact that the electron density of the Sn orbital increases with the passivation of oxygen vacancies [30, 31]. Fig. 3 (d) gives the Bi 4f7/2 peak at 158.3 eV, Bi 4f5/2 peak at 163.3 eV, and S 2p1/2 peak at 156.8 eV, S 2p3/2 peak at 162.3 eV [23, 24], respectively. It is clear from above that Bi2O2S modified SnO2 is successfully attained. (a)

(b)

(c)

(d)

200nm 200nm

Fig. 4 The top morphology of (a) pristine SnO2 film; (b) 5% Bi2O2S modified SnO2 film; Typical 15

top view SEM images of perovskite thin films deposited on (c) pristine SnO2 film; (d) 5% Bi2O2S modified SnO2 film.

Apart from charge extraction and transportation, ETLs serve as growth substrate for perovskite absorber layer to influence the crystallinity of active layer, thus morphology of ETLs should be taken into consideration. In general, a dense, smooth, pinhole-free morphology of ETLs is required for highly efficient PVSCs [37]. Top view SEM was used to visualize surface morphology of ETLs. The SEM of pristine SnO2 film in Fig. 4 (a) shows many tiny convex parts on the surface, leading to the existence of cracks and pin-holes, which can result in direct contact of ITO substrate and perovskite absorber layer. The SEM of 5% Bi2O2S modified SnO2 film in Fig. 4 (b) shows a dense, smooth surface of Bi2O2S modified SnO2, which can prevent carrier recombination through cracks and pinholes. Fig. 4 (c, d) both show dense perovskite film deposited on SnO2 and 5% Bi2O2S modified SnO2 film, while the perovskite crystals deposited on Bi2O2S modified SnO2 substrate exhibit larger grain size. The small particles in perovskite grain boundary may be ascribed to excess PbI2, which can passivate grain boundary, leading to an enhanced performance [38]. Fig. S3 shows the XRD patterns of perovskite films on SnO2 and SnO2:Bi2O2S substrates, where the diffraction peaks at 14.25°, 28.33° and 31.68° are attributed to the <110>, <220> and <310> faces of the perovskite crystalline [39]. The minor peak at 12.39° results from a 5% excess of PbI2, the presence of which can passivate grain boundary [38]. It is noteworthy that the perovskite thin films exhibit similar XRD peak positions, while there exist difference in diffraction intensity. The perovskite thin film 16

deposited on 5% Bi2O2S modified SnO2 substrate exhibits the strongest diffraction intensity. This phenomenon suggests Bi2O2S modified SnO2 substrate enhances perovskite crystallization, which is in accordance with the SEM result. It has been reported that morphology and crystallization of perovskite thin film are of great significance to photovoltaic performance of PVSCs. The enhanced crystallization together with larger grain size, less grain boundaries favors charge transport and reduces recombination, which is beneficial for the photovoltaic performance of PVSCs. Morphology of perovskite thin film, especially processed from one step spinning coating method, can be easily influenced by substrate properties due to surface induction effect [40]. Affinity between substrates and the perovskite layer would affect the quality of the perovskite film [41]. In our case, MAI and PbI2 are uniformly mixed and cast onto a substrate. Crystallization is controlled by the nucleation of the perovskite thin film on the substrate, which can be affected by the substrate properties. Such enhancement in the crystalline quality of MAPbI3 on 5% Bi2O2S modified SnO2 surface is likely due to the different surface nature as revealed by SEM and contact angle measurement. In general, a thin dense ETL with less pin hole (Fig. 4 (b)) will suppress heterogeneous nucleation, leading to the formation of perovskite thin film with large grain and flat surface [5, 42, 43]. Fig. 5 shows the water contact angle (WCA) increases from 80.8° to 88.2° as modification concentration change from 0% to 5%. Surface with higher WCA can increase nucleation spacing between solution droplet and substrate due to lower 17

surface tension dragging force, which can increase grain boundary mobility and allow the crystalline film to grow in larger degree of freedom [19, 44, 45]. Based on the report and contact angle results (Fig. 5), it clearly indicates that SnO2 film modified by the Bi2O2S with higher WCA allows larger degree of freedom for the crystal grain growth and thus leading to highly crystalline thin film [44-46]. Moreover, Bi2O2S and MAPbI3 have similar looser structure, which might be helpful for MAPbI3 grain growth on Bi2O2S. The respective packing factor (PF) values of Bi2O2S and MAPbI3 are 0.66 and 0.65, both of them possess similar loose structure. The PF of a unit cell is relevant to the study of materials science, where it explains many properties of materials [22, 24]. A lower PF value of material possesses wider space for atom vibration and lower elastic stiffness, leading to higher carrier mobility and longer carrier lifetime, better photoelectric related properties [24]. The similar looser structure might be beneficial to perovskite crystal growth. According to the previous report, surface S atom on ETL could bond to Pb2+ in perovskite film, which is favorable for perovskite crystal growth [47, 48]. It also creates a novel pathway of electron transport to accelerate electron transfer and reduce interfacial charge recombination [48]. The S atom of orthorhombic Bi2O2S located on the surface of SnO2 might bond to Pb2+ in perovskite film, which can serve as nucleus center, promoting the perovskite crystals growth.

18

(a)

(b) 0%

5%

80.8°

88.2°

Fig. 5 The water contact angle of 0% and 5% Bi2O2S modified SnO2 substrate.

(b)

0% 2% 5% 10%

20

(c)

10

0 0.0

0.2

0.4

0.6

0.8

1.0

Current density (mA/cm2)

Current density (mA/cm2)

(a)

Voltage (V)

(e)

Current density (mA/cm2)

EQE (%)

25 20

60

10

0.2

0.4

0.6

0.8

1.0

1.2

Fig. 6 (a) device structure shown in cross-section SEM form; (b) the energy level diagram of IPSCs

15 0% 2%

40

10%

20

5

0

0

400

500

600

700

used in this work; (c) J–V curves of devices under

10

5%

300

(d)

Voltage (V)

100 80

20

0 0.0

1.2

0% 2% 5% 10%

reverse scan. (d) J–V curves of devices under forward scan. (e) EQE of corresponding devices.

800

Wavelength (nm) 19

Table 1 Performance of PVSCs with various Bi2O2S modified SnO2 ETLs under reverse and forward scan a

type

A

B

C

D

ETLs

Jsc

Voc

FF

PCE

Hysteresis

(mA cm-2)

(V)

(%)

(%)

index (%)

Reverse

SnO2

22.11±0.15

1.02±0.01

64.8±0.4

14.61±0.12

Forward

SnO2

21.68±0.09

1.02±0.01

60.9±0.5

13.46±0.11

Reverse

SnO2/Bi2O2S (2%)

22.24±0.14

1.06±0.01

66.3±0.4

15.63±0.16

Forward

SnO2/Bi2O2S (2%)

21.75±0.10

1.05±0.02

63.8±0.4

14.57±0.21

Reverse

SnO2/Bi2O2S (5%)

22.35±0.16

1.09±0.01

70.3±0.5

17.13±0.17

Forward

SnO2/Bi2O2S (5%)

21.94±0.12

1.09±0.01

67.4±0.5

16.11±0.15

Reverse

SnO2/Bi2O2S (10%)

21.79±0.10

1.04±0.02

58.1±0.5

13.17±0.11

Forward

SnO2/Bi2O2S (10%)

20.88±0.08

1.04±0.01

56.2±0.5

12.20±0.10

a) The average PCE were based on twenty devices. Fig. 6 (a) shows the conventional device structure, where Spiro-OMeTAD and MoO3 was deposited on top of the active layer as the hole transport layer (HTL), and various Bi2O2S modified SnO2 layers as the ETLs. According to Fig. S4 (a), (b) and (c), highest occupied molecular orbital (HOMO) level and lowest unoccupied molecular orbital (LUMO) level of Bi2O2S is calculated at -4.36 eV and -5.74 eV. As is shown in Fig. 6 (b), suitable band position of Bi2O2S can form graded band alignment, which facilitates charge transfer from perovskite layer to the respective electrodes, and lower HOMO level can prevent hole transferring to SnO2 leading to a less recombination [16]. Furthermore, Fig. S4 (d) shows the work function (WF) value 20

7.9

6.8

5.9

7.4

of pristine SnO2 is 4.68 eV, and the WF of SnO2:Bi2O2S decreases to 4.59, 4.56, 4.55 eV with 2%, 5%, 10% Bi2O2S modification. This decreased WF value will contribute to a better energy level alignment and facilitate electron extraction to the ETL [37, 39]. Fig. S5 shows the transmittance property of various ETLs, which suggests that thin films of SnO2 and SnO2:Bi2O2S possess good light transmission, and are qualified for ETLs. Fig. 6 (c) shows the current density-voltage curves (J-V) of planar MAPbI3 solar cells based on pristine SnO2 and 2%, 5%, 10% Bi2O2S modified SnO2 ETLs (named A, B, C, D for convenience) measured under reverse voltage scan direction beneath simulated one sun illumination. The device A based on the pristine SnO2 ETL achieves a PCE of 14.61%, a Voc of 1.02 V, a Jsc of 22.11mA cm−2, a FF of 0.65 when measured under a reverse scan. The device performance is considerably improved when Bi2O2S is introduced into SnO2. The optimum device performance is achieved when using SnO2:5% Bi2O2S (device C) as ETL, which displays a significantly increased PCE of 17.13% under reverse scan, with a Voc of 1.09 V, a Jsc of 22.35 mA cm−2, and a FF of 0.70. However, as the concentration of Bi2O2S further increases to 10%, the device performance starts to come down, and the device D shows a PCE of 13.17% with smaller Voc of 1.04 V, a lower Jsc of 21.79 mA/cm2, and a reduced FF of 58.1%. The high PCE of device C mainly comes from increased Voc and FF. The suppressed charge recombination and decreased WF of SnO2:Bi2O2S due to Bi2O2S modification improve the performance of PVSCs. Performance of PVSCs with various Bi2O2S modified SnO2 ETLs under reverse scan is listed in Table 1. Fig. 6 (d) shows the current density-voltage curves (J-V) of planar MAPbI3 solar cells (A, B, C, 21

D) measured under forward voltage scan direction beneath simulated one sun illumination, and corresponding device performance values are also listed in Table 1. The device C achieves a better PCE (16.11%) measured under a forward scan compared to device A (13.46%) due to the modification of Bi2O2S. The hysteresis index (h = (PCEreverse - PCEforward)/PCEreverse) was used to investigate hysteresis behavior in the PVSCs, the SnO2:(5%) Bi2O2S exhibits a smaller hysteresis index of 5.9% compared to the index of 7.9% for SnO2 device. The results demonstrate that the hysteresis behavior of the device can be decreased markedly by the SnO2:Bi2O2S ETLs. The reduced hysteresis of the device modified by Bi2O2S is due to the reduced defect states, which facilitates charge transport. Fig. 6 (e) shows the EQE spectra of devices modified by 0%, 2%, 5%, 10% Bi2O2S, the integrated Jsc from EQE of corresponding devices is 21.76 mA/cm2, 21.96 mA/cm2, 22.28 mA/cm2, 21.19 mA/cm2, respectively, which is very close to Jsc from J-V curves. 5 0% 2% 5% 10%

4

ITO/MAPbI3 ITO/SnO2/MAPbI3 ITO/SnO2:Bi2O2S/MAPbI3

(b) Intensity (a.u.)

Absorbance (a.u.)

(a)

3 2 1 0 400

500

600

700

650

800

700

12

100000

800

850

900

Rrec

0% 5%

(d)

10

-Z`` (Ω)

2

Rtr

-dV/dJ(Ω.cm )

Rs

750

Wavelength (nm)

Wavelength (nm)

(c) 50000

8

Rs=2.96 Ω

6 4

Rs=0.64 Ω

2 0% 5%

22

0 0

50000

Z` (Ω)

100000

0 0.00

0.02

0.04

0.06 -1

0.08 -1

2

(Jsc-J) (mA .cm )

0.10

0.12

Fig. 7 (a) UV-vis absorbance of perovskite

(e)

1.08

film; (b) Room temperature PL of perovskite

Voc (V)

1.46KBT/q 1.04

film; (c) EIS and fitting parameters for SnO2 1.58KBT/q

1.00

and 5% Bi2O2S modified SnO2 PVSCs. (d) the 0.96

linear curve of the relationship of –dV/dJ vs 10

2

100

Light Intensity (mW/cm )

(JSC-J)−1. (e) Voc dependence on light intensity.

The ultraviolet-visible (UV-vis) absorption spectra of perovskite thin films deposited on 0%, 2%, 5%, 10% Bi2O2S modified SnO2 ETLs are shown in Fig. 7 (a). The enhanced absorption of the MAPbI3 film deposited on (2%, 5%) SnO2:Bi2O2S is mainly attributed to the better crystallinity of perovskite thin films [45]. We took a further study on the charge extraction with the introduction of Bi2O2S. Room temperature PL in Fig. 7 (b) was carried out on the ITO/perovskite, ITO/SnO2/perovskite, ITO/SnO2:Bi2O2S (5%)/perovskite, which displays a reduced intensity upon the using of SnO2 and SnO2: Bi2O2S ETLs. Our result of more decreased PL intensity of SnO2: Bi2O2S ETL than SnO2 ETL suggests a more efficient charge separation from MAPbI3 to ETL, which can be attributed to suitable band alignment and reduced oxygen vacancies related defect, leading to more efficient electrons extraction. Electrical impedance spectra (EIS) of devices were performed to further investigate the charge transport and recombination. EIS is particularly useful characterization technique to reveal interface transport and recombination. Fig. 7 (c) shows the EIS of devices based on SnO2 and 5% Bi2O2S modified SnO2 at frequency 23

ranging from 1 MHz to 0.1 Hz under dark environment. The circuit contains series resistance (Rs), two constituents of transfer resistance (Rtr) at MAPbI3/HTL interfaces and Rrec between photo-generated electrons and holes [49-51]. The Rs can be assigned to the value of the high-frequency intercept on the real axis and the Rrec data can be calculated based on the low-frequency range [50, 51]. The Rs and Rrec estimated by fitting the Nyquist plots of the perovskite solar cells using SnO2 ETL (5% Bi2O2S modified SnO2 ETL) are 45.34 (21.68) Ω cm2 and 40251.57 (72620.54) Ω cm2, respectively. Apparently, the Rs value of the 5% Bi2O2S modified SnO2 ETL based device is lower than that of SnO2 ETL based device, while the Rrec value of the 5% Bi2O2S modified SnO2 ETL based device is significantly higher than that of SnO2 ETL based device, suggesting more efficient electrons transport and much lower interface recombination. The J–V curves show good rectification characteristics, the RS of the cell can be calculated according to the diode equation: [12, 52] –dV / dJ =εKBT (JSC-J)−1qe-1+Rs

(2)

Where KB is Boltzmann constant, T is the absolute temperature, qe is electronic charge quantity, ε is ideality factor. Fig. 7 (c) shows the linear fit plot of –dV / dJ vs. (JSC-J)−1 , the value of RS is equal to the intercept on the y axis. The RS of the 0% and 5% Bi2O2S modified PVSC device is 2.91 Ω cm2 and 0.86 Ω cm2, respectively. The low Rs of 5% Bi2O2S modified PVSC device suggest more efficient charge transport. There are two main recombination processes in PVSCs, including monomolecular recombination and bimolecular recombination. Molecular recombination refers to 24

bound electron hole pair recombination and trap sites recombination of electron and hole, and bimolecular recombination means the free electron and hole recombination [40 53]. Recombination process is analyzed by Voc dependence on light intensity with the following equation: [54, 55] Voc =

( ) 

+ constant

(3)

Where KB is Boltzmann constant, T is absolute temperature, q is elementary charge, ε is the ideality factor. In open circuit situation, there is no photo-generated current, which suggests photo-generated charge carriers recombine within the device. Therefore, recombination investigation near Voc can reveal recombination mechanism [40]. In addition, the device performance is determined by trap-dominated monomolecular recombination when ε approaches to 2. While the slope approaches to 1 if bimolecular recombination is dominated [40, 55]. It is apparent from Fig. 7 (d) that monomolecular recombination in the solar cell with Bi2O2S modification is significantly reduced as the ideality factor dropped from 1.59 to 1.46. The suppressed recombination may come from reduced oxygen vacancies in Bi2O2S modified SnO2 interface as well as less grain boundary in perovskite film. The enhanced performance is mainly attributed to the increase of Voc and FF, which results from the reduced charge recombination and improved electron transport [42]. The enhanced Voc parameters is partially due to suppressed trap-related recombination which results from reduced oxygen vacancies with the modification of Bi2O2S [21, 25]. Furthermore, charge recombination mainly takes place at grain boundaries, which is definitely detrimental for Voc. Generally, larger grain size means 25

less grain boundaries. Therefore, larger perovskite grain size with the modification of Bi2O2S explains the enhanced Voc. The suppressed recombination with Bi2O2S modification is further proved by EIS result and Voc dependence on light intensity. The enhancement of FF parameter is probably due to better morphology and reduced oxygen vacancies related defects, resulting in low Rs. Reduced Rs contributes to charge transport, leading to an enhanced FF.

Normalized PCE

1.0

0.8

0.6

0.4 0% 5%

0.2 0

5

10

15

20

Time (days) Fig. 8 Normalized PCE as a function of time measured directly during storage of planar SnO2 and 5% Bi2O2S modified SnO2 PVSCs monitored every day.

The stability of the planar SnO2 PVSCs and Bi2O2S:SnO2 PVSCs was also comparatively investigated based on ten devices. The devices were stored in a desiccator in the air (~25 % humidity) at room temperature for long-term stability test. As shown in Fig. 8, 57.9 % and 77.2% of initial PCE were retained for planar SnO2 devices (dark line) and Bi2O2S:SnO2 devices (blue line) after 20 days storage, respectively. The enhanced stability can be attributed to better crystallinity of perovskite film as well as reduced oxygen vacancies related defects [19, 25]. 26

Table 2 Performance of PVSCs based on (CsFAMA)Pb(IBr)3 mixed perovskite active layer with 5% Bi2O2S modified SnO2 ETLs under forward scanb

ETLs

Jsc (mA cm-2)

Voc (V)

FF (%)

PCE (%)

Forward

SnO2

22.37±0.12

1.10±0.02

70.9±0.4

17.44±0.11

Reverse

SnO2

22.79±0.09

1.10±0.01

72.2±0.5

18.10±0.11

Forward

SnO2/Bi2O2S (5 %)

22.81±0.12

1.15±0.01

75.3±0.5

19.75±0.15

Reverse

SnO2/Bi2O2S (5 %)

22.98±0.12

1.15±0.01

76.2±0.5

20.14±0.15

type

E

F

b)

Hysteresis index

The average PCE were based on ten devices.

It is worth mentioning that this approach can be also successfully applied in other perovskite active layer system. Fig. S6 (a) shows the current density-voltage curves (J-V) of (CsFAMA)Pb(IBr)3 mixed perovskite planar solar cells based on pristine SnO2, 5% Bi2O2S modified SnO2 ETLs measured under reverse voltage scan direction beneath simulated one sun illumination. Performance of (CsFAMA)Pb(IBr)3 mixed PVSCs is listed in Table 2. Compared with device E with pristine SnO2 as ETL, the PCE of device F based on optimized Bi2O2S:SnO2 ETL is raised to 20.14% from 18.10%. This indicates that Bi2O2S modified SnO2 ETLs is a universal method for improving the performance of PVSCs. 3. Conclusion We synthesize a bismuth oxysulfide (Bi2O2S) compound and apply it in PVSCs for the first time. Compared with PVSCs using pristine SnO2 as ETL, the PCE of PVSCs based on optimized Bi2O2S:SnO2 ETL is raised to 17.13% from 14.61%. The results show the modification of the Bi2O2S nanoparticles can passivate oxygen 27

4.1%

1.9%

vacancies of SnO2 thin film, resulting in less charge recombination and improved charge transport. Furthermore, SnO2 thin film modified by Bi2O2S nanoparticles enhances film morphology of overlying perovskite active layer, including larger grain size, better crystallization. These eventually result in a remarkable improvement of Voc and FF parameters of PSCs. Notably, this work unlocks the utility of Bi2O2S in PVSC and provide a modification method for developing low temperature flexible PVSC. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant No. 61974045), the Natural Science Foundation of Guangdong Province (Grant No. 2017A030313), Dongguan Core Technology Research Project Funding (Grant No. 2019622163008), Project on the Collaborative Innovation and Environmental Construction Platform of Guangdong Province (Grant No. 2018A050506067) and Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development (Grant No. Y909kp1001). Conflict of Interest The authors declare no conflict of interest. Reference [1] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, Organometal halide perovskites as visible-light sensitizers for photovoltaic cells, J. Am. Chem. Soc. 131 (2009) 6050-6051. [2] M.M. Lee, J. Teuscher, T. Miyasaka, T.N. Murakami, H.J. Snaith, Efficient hybrid solar cells based on meso-superstructured organometal halide perovskites, Science 338 (2012) 643-647. [3] J. Burschka, N. Pellet, S.J. Moon, R. Humphrybaker, P. Gao, M.K. Nazeeruddin, M. Grätzel, Sequential deposition as a route to high-performance perovskite-sensitized solar cells, Nature 499 28

(2013) 316-319. [4] N.J. Jeon, J.H. Noh, Y.C. Kim, W.S. Yang, S. Ryu, S.I. Seok, Solvent engineering for high-performance inorganic-organic hybrid perovskite solar cells, Nat. Mater. 13 (2014) 897-903. [5] Q. Jiang, L. Zhang, H. Wang, X.Yang, J. Meng, H. Liu, Z. Yin, J. Wu, X. Zhang, J. You, Enhanced electron extraction using SnO2 for high-efficiency planar-structure HC(NH2)2PbI3-based perovskite solar cells, Nat. Energy 2 (2017) 1-7. [6] L. Wang, H. Zhou, J. Hu, B. Huang, M. Sun, B. Dong, G. Zheng, Y. Huang, Y. Chen, L. Li, A Eu3+-Eu2+ ion redox shuttle imparts operational durability to Pb-I perovskite solar cells, Science 363 (2019) 265-270. [7] L. Etgar, P. Gao, Z. Xue, Q. Peng, A.K. Chandiran, B. Liu, M.K. Nazeeruddin, M. Grätzel, Mesoscopic CH3NH3PbI3/TiO2 heterojunction solar cells, J. Am. Chem. Soc. 134 (2012) 17396-17399. [8] A. Mei, X. Li, L. Liu, Z. Ku, T. Liu, Y. Rong, Grätzel, M. A hole-conductor–free, Fully printable mesoscopic perovskite solar cell with high stability, Science 345 (2014) 295-298. [9] S. Sun, T. Salim, N. Mathews, M. Duchamp, C. Boothroyd, G. Xing, T.C. Sum, Y.M. Lam, The origin of high efficiency in low-temperature solution-processable bilayer organometal halide hybrid solar cells, Energy Environ. Sci. 7 (2014) 399-407. [10] W.N. Zhang, X.Z. Zhang, T.Y. Wu, W.H. Sun, J.H. Wu, Z. Lan, Interface engineering with NiO nanocrystals for highly efficient and stable planar perovskite solar cells, Electrochim. Acta 293 (2019) 211-219. [11] J. Zhang, H. Luo, W. Xie, X. Lin, X. Hou, J. Zhou, X. Chen . Efficient and ultraviolet durable planar perovskite solar cells via a ferrocenecarboxylic acid modified nickel oxide hole transport layer, Nanoscale 10 (2018) 5617-5625. [12] X. Zeng, T. Zhou, C. Leng, Z. Zang, M. Wang, W. Hu, M. Zhou. Performance improvement of perovskite solar cells by employing a CdSe quantum dot/PCBM composite as an electron transport layer, J. Mater. Chem. A 5 (2017) 17499-17505. [13] W. Ke, G. Fang, Q. Liu, L. Xiong, P. Qin, H. Tao, J. Wang, H. Lei, B. Li, J. Wan, G. Yang, Y. Yan, Low-temperature solution-processed tin oxide as an alternative electron transporting layer for efficient perovskite solar cells, J. Am. Chem. Soc. 137 (2015) 6730-6733. [14] Q. Jiang, X. Zhang, J. You, SnO2: A Wonderful Electron Transport Layer for Perovskite Solar 29

Cells, Small 14 (2018) 1801154. [15] C. Huang, H. Yu, J. Chen, J. Zhang , Z. Wu, C.Hou. Improved performance of polymer solar cells by doping with Bi2O2S nanocrystals, Sol. Energy Mater. Sol. Cells 200 (2019) 110030. [16] L. Xiong, Y. Guo, J. Wen, H. Liu, G. Yang, P. Qin, G. Fang. Review on the application of SnO2 in perovskite solar cells, Adv. Func. Mater. 28 (2018) 1802757. [17] X.X. Jiang, Y.L .Xiong, Z.H. Zhang, Y.G. Rong, A.Y. Mei, C.B. Tian, J. Zhang, Y.M. Zhang, Y.X. Jin, H.W. Han, Q.J. Liu, Efficient hole-conductor-free printable mesoscopic perovskite solar cells based on SnO2 compact layer, Electrochim. Acta 263 (2018) 134-139. [18] J. Song, E. Zheng, J. Bian, X.F. Wang, W. Tian, Y. Sanehira, T. Miyasaka, Low-temperature SnO2-based electron selective contact for efficient and stable perovskite solar cells, J. Mater. Chem. A 3 (2015) 10837-10844. [19] K. Liu, S. Chen, J. Wu, H. Zhang, M. Qin, X. Lu, Y. Tu, Q. Meng, X. Zhan, Fullerene derivative anchored SnO2 for high-performance perovskite solar cells, Energy Environ. Sci.11 (2018) 34633471. [20] B. Roose, C.M. Johansen, K. Dupraz, T. Jaouen, P. Aebi, U. Steiner, U.A. Abate, A Ga-doped SnO2 mesoporous contact for UV stable highly efficient perovskite solar cells, J Mater. Chem. A 6 (2018) 1850-1857. [21] M.A. Mahmud, N.K. Elumalai, M.B. Upama, D. Wang, V.R. Gonçales, M. Wright, A. Uddin, Passivation of interstitial and vacancy mediated trap-states for efficient and stable triple-cation perovskite solar cells, J. Power Sources 383 (2018) 59-71. [22] X. Zhang, Y. Liu, G. Zhang, Y. Wang, H. Zhang, F. Huang, Thermal decomposition of bismuth oxysulfide from photoelectric Bi2O2S to superconducting Bi4O4S3, ACS Appl. Mater. Inter. 7 (2015) 4442-44428. [23] E.A. Bondarenko, E.A. Streltsov, M.V. Malashchonak, A.V. Mazanik, A.I. Kulak, E.V. Skorb, Giant Incident Photon-to-Current Conversion with Photoconductivity Gain on Nanostructured Bismuth Oxysulfide Photoelectrodes under Visible-Light Illumination, Adv. Mater. 29 (2017) 1702387. [24] S. Meng, X. Zhang, G. Zhang, Y. Wang, H. Zhang, F. Huang, Synthesis, Crystal Structure, and Photoelectric Properties of a New Layered Bismuth Oxysulfide, Inorg. Chem. 54 (2015) 57685773. 30

[25] Z.P. Wu, H.Z. Yu, S.W. Shi, Y.P. Li, Bismuth oxysulfide modified ZnO nanorod arrays as an efficient electron transport layer for inverted polymer solar cells, J. Mater. Chem. A 7 (2019) 14776-14789. [26] A.D. Pramata, K. Suematsu, A.T. Quitain, M. Sasaki, T. Kida, Synthesis of Highly Luminescent SnO2 Nanocrystals: Analysis of their Defect-Related Photoluminescence Using Polyoxometalates as Quenchers, Adv. Funct. Mater. 28 (2018) 1704620. [27] F. Gu, S. F. Wang, M.K. Lü, G.J. Zhou, D. Xu, D.R. Yuan, Photoluminescence properties of SnO2 nanoparticles synthesized by sol− gel method, J. Phy. Chem. B 108 (2004) 8119-8123. [28] A. Kar, S. Kundu, A. Patra. Surface defect-related luminescence properties of SnO2 nanorods and nanoparticles, J. Phy. Chem. C 115 (2010) 118-124. [29] S. Roy, A.G. Joshi, S. Chatterjee, A.K. Ghosh, Local symmetry breaking in SnO2 nanocrystals with cobalt doping and its effect on optical properties, Nanoscale 10 (2018) 10664-10682. [30] Z. K. He, Q. Sun, Z. Shi, K. Xie, A.R. Kamali, Molten salt synthesis of oxygen-deficient SnO2 crystals with enhanced electrical conductivity, Appl. Surf. Sci. 465 (2019) 397-404. [31] Y. Yang, Y. Wang, S. Yin, Oxygen vacancies confined in SnO2 nanoparticles for desirable electronic structure and enhanced visible light photocatalytic activity, Appl. Surf. Sci. 420 (2017) 399-406. [32] Y. Liu, M. Liu, Growth of aligned square-shaped SnO2 tube arrays, Adv. Func. Mater. 15 (2005) 57-62. [33] L. Xiong, M. Qin, G.Yang, Y. Guo, H. Lei, Q. Liu, W. Ke, H. Tao, P. Qin, S. Li, H. Yu, G. Fang, Performance enhancement of high temperature SnO2-based planar perovskite solar cells: electrical characterization and understanding of the mechanism, J. Mater. Chem. A 4 (2016) 8374-8383. [34] H. Luo, L.Y. Liang, H.T. Cao, Z.M. Liu, F. Zhuge, Structural, chemical, optical, and electrical evolution of SnO(x) films deposited by reactive rf magnetron sputtering, ACS Appl. Mater. Inter. 4 (2012) 5673-5677. [35] Y.M. He, L.H. Zhang, M.H. Fan, X.X. Wang, M.L. Walbridge, Q.Y. Nong, Y. Wu, L.H. Zhao, Z-scheme SnO2-x/g-C3N4 composite as an efficient photocatalyst for dye degradation and photocatalytic CO2 reduction, Sol. Energy Mater. Sol. Cells 137 (2015) 175-184. 31

[36] L. Liu, S. Liu, Oxygen Vacancies as an Efficient Strategy for Promotion of Low Concentration SO2 Gas Sensing: The Case of Au-Modified SnO2, ACS Sustainable Chem. Eng. 6 (2018) 13427-13434. [37] Z. Wang, M.A. Kamarudin, N.C. Huey, F. Yang, M. Pandey, G. Kapil, T.L. Ma, S. Hayase. Interfacial Sulfur Functionalization Anchoring SnO2 and CH3NH3PbI3 for Enhanced Stability and Trap Passivation in Perovskite Solar Cells, ChemSusChem 11 (2018) 3941-3948. [38] D.H. Cao, C.C. Stoumpos, C.D. Malliakas, M.J. Katz, O.K. Farha, J.T. Hupp, M.G. Kanatzidis, Remnant PbI2, an unforeseen necessity in high-efficiency hybrid perovskite-based solar cells?, APL Mater. 2 (2014) 091101. [39] L. Zuo, Q. Chen, N. De Marco, Y.T. Hsieh, H. Chen, P. Sun, S.Y. Chang, H. Zhao, S. Dong, Y. Yang, Tailoring the Interfacial Chemical Interaction for High-Efficiency Perovskite Solar Cells, Nano Lett. 17 (2016) 269-275. [40] Q. Xue, G. Chen, M. Liu, J. Xiao, Z. Chen, Z. Hu, X. F. Jiang, B. Zhang, F. Huang, W. Yang, H. L. Yip, Y. Cao, Improving Film Formation and Photovoltage of Highly Efficient Inverted-Type Perovskite Solar Cells through the Incorporation of New Polymeric Hole Selective Layers, Adv. Energy Mater. 6 (2016) 1502021. [41] X. Zhao, L. Tao, H. Li, W. Huang, P. Sun, J. Liu, Y. Shen, Efficient planar perovskite solar cells with improved fill factor via interface engineering with graphene. Nano letters 18 (2018), 2442-2449. [42] W. Q. Wu, D. Chen, R.A. Caruso, Y.B. Cheng, Recent progress in hybrid perovskite solar cells based on n-type materials, J. Mater. Chem. A 5 (2017) 10092-10109. [43] G. Yang, C. Chen, F. Yao, Z. Chen, Q. Zhang, X. Zheng, J. Ma, H. Lei, P. Qin, L. Xiong, W. Ke, G. Li, Y. Yan, G. Fang, Effective Carrier-Concentration Tuning of SnO2 Quantum Dot Electron-Selective Layers for High-Performance Planar Perovskite Solar Cells, Adv. Mater. 30 (2018) 1706023. [44] C. Bi, Q. Wang, Y. Shao, Y. Yuan, Z. Xiao, J. Huang, Non-wetting surface-driven high-aspect-ratio crystalline grain growth for efficient hybrid perovskite solar cells, Nat. Commun. 6 (2015) 7747. [45] W. Nie, H.H Tsai, J.C. Blancon, F.Z. Liu, C.C. Stoumpos, B. Traore, M. Kepenekian, O. Durand, C. Katan, S. Tretiak, J. Crochet, P.M. Ajayan, M.G. Kanatzidis, J. Even, A.D. Mohite, 32

Critical role of interface and crystallinity on the performance and photostability of perovskite solar cell on nickel oxide, Adv. Mater. 30 (2018) 1703879. [46] M. Noh, M. F, N. , A. Arzaee, J. Safaei, N. A.Mohamed, H. P. Kim, A. R. M. Yusoff, M. A. M.

Teridi. Eliminating oxygen vacancies in SnO2 films via aerosol-assisted chemical vapour deposition for perovskite solar cells and photoelectrochemical cells, J. Alloys. Compounds 773 (2019) 997-1008. [47] R. Chen, J. Cao, Y. Duan, Y. Hui, T. T. Chuong, D. Ou, N. Zheng. High-efficiency, hysteresis-less, UV-stable perovskite solar cells with cascade ZnO–ZnS electron transport layer, J Am. Chem. Soc 141 (2018) 541-547. [48] Z. Wang, M. A. Kamarudin, N. C. Huey, F. Yang, M. Pandey, G. Kapil, S. Hayase. Interfacial Sulfur Functionalization Anchoring SnO2 and CH3NH3PbI3 for Enhanced Stability and Trap Passivation in Perovskite Solar Cells, ChemSusChem 11 (2018) 3941-3948. [49] J. Zhang, W. Mao, J. Duan, S. Huang, Z. Zhang, W. Ou-Yang, X. Chen. Enhanced efficiency and thermal stability of perovskite solar cells using poly (9-vinylcarbazole) modified perovskite/PCBM interface, Electrochim. Acta 318 (2019) 384-391. [50] L. Xiong, M. Qin, C. Chen, J. Wen, G. Yang, Y. Guo, G. Fang, Fully high-temperature-processed SnO2 as blocking layer and scaffold for efficient, stable, and hysteresis-free mesoporous perovskite solar cells, Adv. Func. Mater. 28 (2018) 1706276. [51] W. Ke, D. Zhao, C. Xiao, C. Wang, A. J. Cimaroli, C. R. Grice, K. Zhu. Cooperative tin oxide fullerene electron selective layers for high-performance planar perovskite solar cells, J. Mater. Chem. A 4 (2016) 14276-14283. [52] Z. Bi, Z. Liang, X. Xu, Z. Chai, H. Jin, D. Xu, J. Li, M. Li, G. Xu, Fast preparation of uniform large grain size perovskite thin film in air condition via spray deposition method for high efficient planar solar cells, Sol. Energy Mater. Sol. Cells 162 (2017) 13-20. [53] D. Credgington, F.C. Jamieson, B. Walker, T.Q. Nguyen, J.R. Durrant, Quantification of geminate and non-geminate recombination losses within a solution-processed small-molecule bulk heterojunction solar cell, Adv. Mater. 24 (2012) 2135-2141. [54] F. Cai, J. Cai, L. Yang, W. Li, R.S. Gurney, H. Yi, A. Iraqi, D. Liu, T. Wang, Molecular engineering of conjugated polymers for efficient hole transport and defect passivation in perovskite solar cells, Nano Energy 45 (2018) 28-36. 33

[55] M. Zhu, W. Liu, W. Ke, L. Xie, P. Dong, F. Hao, Graphene Modified Tin Dioxide for Efficient Planar Perovskite Solar Cells with Enhanced Electron Extraction and Reduced Hysteresis, ACS Appl. Mater. Inter. 11 (2019) 666-673.

34

Conflict of interest statement We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled, “The low temperature solution-processable SnO2 modified by Bi2O2S as an efficient electron transport layer for perovskite solar cells”.