Ethyl acetate green antisolvent process for high-performance planar low-temperature SnO2-based perovskite solar cells made in ambient air

Accepted Manuscript Ethyl acetate green antisolvent process for high-performance planar low-temperature SnO2-based perovskite solar cells made in ambient air Wenyuan Zhang, Yuanchao Li, Xin Liu, Dongyan Tang, Xin Li, Xi Yuan PII: DOI: Article Number: Reference:

S1385-8947(19)31692-4 https://doi.org/10.1016/j.cej.2019.122298 122298 CEJ 122298

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

27 April 2019 5 July 2019 18 July 2019

Please cite this article as: W. Zhang, Y. Li, X. Liu, D. Tang, X. Li, X. Yuan, Ethyl acetate green antisolvent process for high-performance planar low-temperature SnO2-based perovskite solar cells made in ambient air, Chemical Engineering Journal (2019), doi: https://doi.org/10.1016/j.cej.2019.122298

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Ethyl acetate green antisolvent process for highperformance planar low-temperature SnO2-based perovskite solar cells made in ambient air Wenyuan Zhang1, Yuanchao Li1, Xin Liu2, Dongyan Tang1, Xin Li1, *, Xi Yuan3,* 1MIIT

Key Laboratory of Critical Materials Technology for New Energy Conversion

and Storage, School of Chemistry and Chemical Engineering, State Key Lab of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin 150090, China. 2School 3Key

of Environment, Harbin Institute of Technology, Harbin 150090, China.

Laboratory of Functional Materials Physics and Chemistry of the Ministry of

Education, Jilin Normal University, Siping 136000, China. *Corresponding author: Xin Li; Xi Yuan E-mail address: [email protected] (X. Li); [email protected] (X. Yuan)

ABSTRACT One-step antisolvent deposition has been considered as one of the most feasible methods to obtain high-performance perovskite solar cells (PSCs). However, most of the reported high-performance PSCs are based on the toxic anti-solvents, which is a major issue for the potential commercialization of PSCs. SnO2 has been successfully used as an efficient electron transport layer (ETL) material in PSCs, but the preparation of low-temperature processed crystallization SnO2 ETLs is still a challenge. In this work, ethyl acetate (EA) as a green antisolvent is introduced into the perovskite 1

crystallization process, resulting in uniform and compact perovskite films with large grain size, reduced grain boundaries, and low defect density. Low-temperature (100 ℃) processed SnO2 ETL provides good interface contact between ETL and perovskite layer, facilitating photoelectron extraction and transport. As a result, a champion power conversion efficiency (PCE) of 17.83% has been achieved. More importantly, unencapsulated PSC retains 84.80% of its original PCE value after storage in atmosphere for 80 days (>1900 h). Apart from great air stability, the final devices also show excellent thermal (100 ℃) stability. It is particularly noteworthy that all the preparation and measurement processes were performed under ambient conditions. These findings present a green path towards manufacturing efficient and stable airprocessed PSCs. Keywords: Perovskite solar cells, Ethyl acetate, SnO2, Environmental-friendly, Ambient conditions

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1. Introduction Organic-inorganic hybrid perovskite-based thin film solar cells have attracted increasing attentions owing to their excellent power conversion efficiency (PCE). The quality of perovskite films plays a key role in the device performance of perovskite solar cells (PSCs). To date, anti-solvent method has been considered as one of the most possible ways to gain highly uniform perovskite film and is widely used in PSC devices [1-3]. Unfortunately, most of the reported high-performance PSCs are based on the toxic anti-solvents, such as chlorobenzene, toluene, and diethyl ether, which is a major issue for the potential commercialization of PSCs [4-8]. Hence, in order to reduce environmental pollution and human health risk, the technology with green anti-solvents has been developed. For example, Zhang et al. fabricated planar PSC with efficiency beyond 19% through methoxybenzene anti-solvent [9]. Jung group introduced anisole as an anti-solvent for fabrication of large-area PSCs [10]. Bu and co-workers studied the synergic interface optimization with ethyl acetate (EA) in mixed PSCs [11]. It is expected that environmental-friendly solvents have the huge potential to realize future industrialization of PSCs. For low-temperature solution-processing, there will be inevitable grain boundaries (GBs) existed in polycrystalline perovskite films that are unstable and increase the coupling [12, 13]. GBs offer pathways for ionic migration and moisture penetration, which leads to perovskite film degradation [14, 15]. Like other soft ionic solids, halide perovskites have various ionic point defects [16-18]. These defects often act as nonradiative charge-recombination centers to affect performance and stability of PSCs [16, 3

19, 20]. Although various passivation techniques and grain-growth approaches have been developed to improve perovskite film quality, passivation or reduction of GBs and defects still are a challenge owing to their complexity and diversity [21-23]. Besides perovskite films, electron-transport layers (ETLs) are also crucial for PSCs performance. Recently, SnO2 as an n-type metal oxide has been explored as ETL [2427] due to its band alignment to perovskite, wide optical bandgap, superior chemical stability, and high electron mobility [28-30]. Furthermore, SnO2 ETL can be fabricated by lower-temperature methods, making it more suitable for flexible electronic devices and large-scale manufacturing [31-35]. Song et al. reported a PSC with PCE of 13% using SnO2 ETL synthesized at 200 °C. Fang group prepared planar PSCs with average PCE of 16.02%, in which the SnO2 ETLs were synthesized in air at 180 °C [25]. You and co-workers employed solution-processed SnO2 nanoparticles as ETL and annealed at 150 °C, a 19.9 ± 0.6% certified efficiency was achieved [36]. Liu et al. reported a 21.3% PCE using fullerene derivative anchored SnO2, in which SnO2 annealed at 150°C in ambient atmosphere [37]. Jen's group reported efficient SnO2 ETLs prepared by the dual-fuel combustion method at a low temperature of 140°C [38]. Ozone atomic-layer deposition was introduced to prepare SnO2 by Hagfeldt and co-workers, in which SnO2 was deposited at 118 °C [39]. Recently, a novel growth route of SnO2 ETLs was reported by Dong et al., they crystallized SnO2 below 80 °C and achieved the PCE above 19% [40]. Generally, the annealing temperature used in deposition process to form low-temperature SnO2 layer should be within the moderate range (<200 °C). However, much lower temperature led to difficulty in forming high-quality SnO2 ETLs 4

due to the incomplete reaction of metal precursors [41]. Thus, keeping the deposition process at reasonable low-temperature range while maintaining high device efficiency and avoiding hazardous or expensive processing route is still a challenge. Aiming for commercial applications, low-cost and easy manufacturing processes of PSCs become more significant. At present the reported top-performance PSCs are mainly fabricated inside glove box with highly controlled conditions [42, 43], which limited the applicability of PSCs. Although a certain effort has been devoted to obtain air-processed PSCs [44-48], the device performance is not ideal, exhibiting relatively low PCE and sever hysteresis. It is very necessary to accelerate device performance improvement of air-processed PSCs. Herein, the perovskite films were prepared by one-step spin-coating strategy using EA green anti-solvent, the process was shown in Fig. 1. SnO2 films were synthesized through solution-processing at a low temperature of 100 ℃. Using the structure of FTO/SnO2/CH3NH3PbI3(MAPbI3)/Spiro-OMeTAD/Au, a champion PCE of 17.83% can be achieved. Remarkably, the unencapsulated device exhibits superior air-stability, retaining 84.80% of its initial efficiency after storage in ambient atmospher for more than 1900 h. Meanwhile, the device also shows remarkable thermal stability, retaining 81.03% of original PCE value when exposed to 100℃ for 10 h. It is worth noting that all the preparation were performed under ambient air conditions. Moreover, the effects of EA and low-temperature-processed SnO2 on the device performance were thoroughly investigated using steady-state photoluminescence (PL) spectroscopy, timeresolved photoluminescence (TRPL) spectroscopy, transient photovoltage (TPV) 5

measurement, Fourier transform infrared (FTIR) spectroscopy, electrochemical impedance spectroscopy (EIS), space-charge-limited current (SCLC) analysis and density functional theory (DFT).

Fig. 1. Schematic diagram of the fabrication procedures of MAPbI3 film via one-step method.

2. Experimental

2.1 Reagents and materials The SnO2 aqueous colloidal dispersion (tin (IV) oxide, 15 wt % in H2O colloidal dispersion) was obtained from Alfa Aesar. Lead iodide (PbI2), Methylammonium iodide (CH3NH3I) and 2, 2′, 7, 7′-tetrakis-(N, N-di-4-methoxyphenylamino)-9, 9′ spirobifluorene

(Spiro-OMeTAD)

were

Xi'an Polymer Light Technology Corp. bis(trifluoromethanesulfonyl)imide

purchased

4-tert-butypyridine

(Li-TFSI)

were

obtained

and from

from lithium Kanto.

Dimethylformamide (DMF, purity > 99 %), and dimethyl sulfoxide (DMSO, purity > 99 %) were purchased from Youxuan Trade. All other anhydrous solvents were purchased from Aladdin. All chemicals and solvents were used without any further 6

purification. 2.2 Fabrication of SnO2 film The SnO2 aqueous colloidal dispersion is diluted with deionized water in a volume ratio of 1:5 and then was stirred overnight at room temperature as well as sonicated for 15 min. After that, the SnO2 solution was coated onto the substrates by spin-coating at 500 rpm. for 3 s, 5000 rpm. for 40 s, then, the compact SnO2 layer was annealed at 100 ° C for 20 min. This process was repeated there times and then the SnO2 film was annealed at 100 °C for 1 h in atmospheric environment without humidity control. 2.3 Solar cells fabrication Fluorine-doped tin oxide (FTO) substrates were first etched by zinc powder and 2M HCl, and then ultrasonically cleaned in detergent solution, deionized water, ethyl alcohol and acetone for 30 min respectively, followed by an UV-ozone treatment for 15 min. The CH3NH3PbI3 perovskite precursor solutions were prepared by dissolving 484 mg PbI2 and 159 mg CH3NH3I in 1 mL mixed solvent of DMF and DMSO (4:1 volume/volume). The perovskite precursor solution was deposited onto the prepared FTO/SnO2 substrate by a typical one-step anti-solvent method. Where the precursor solution was spin-coated on the substrates at a low speed of 500 rpm for 3 s followed by a high speed of 3000 rpm for 50 s, and 400 mL of anti-solvent (EA and ether) was dripped at a constant speed on the substrate. The film was heated at 70 °C for 5 min and 100 °C for 10 min to obtain a mirror-like brown-black perovskite film. After cooling to room temperature, the hole transport layer solution containing 80 mg of Spiro-OMeTAD, 28.5 μL 4-tert-butypyridine, 17.5 μL Li-TFSI (520 mg of Li-TFSI in 7

1 mL of acetonitrile) all dissolved in 1 mL chlorobenzene was deposited by spin-coating at 3000 rpm for 20 s. Finally, about 60 nm thick Au counter electrode was deposited via vacuum thermal evaporation at rate of 1.0 Å s-1. The 0.12 cm2 active area of the PSCs was determined through a non-reflective metal mask. All processes were operated in air (the average temperature was 25 °C, average relative humidity was 37.5 %). 2.4 Characterization and measurement X-ray diffraction (XRD) pattern was obtained by a Panalytical Empyrean X-ray diffractometer with a Cu Kα radiation (λ=1.540598 Å) at a scan rate of 10° min-1. The morphology and cross-section SEM image of the samples were observed by the Cold Field Emission scanning electron microscopy (SEM, Carl Zeiss, Supra55). Atomic force microscopy (AFM) was obtained by Bruker Dimension FastScan Scanning Probe Microscope (SPM) using the tapping mode. The UV-Visible spectrometer (TU1901, Beijing Purkinje General Instrument Co., Ltd) was used to measure the UV-vis absorption spectrum of different films. Steady-state photoluminescence (PL) and timeresolved photoluminescence (TRPL) spectroscopies measurements were conducted on V2.7 fluorescence spectrometer from HORIBA. Fourier transform infrared (FTIR) spectroscopies were surveyed with Fourier Transform Infrared Spectrometer (Nicolet is50). The water contact angle measurement was performed using a JC2000C1 contact angle goniometer. The transient photovoltage (TPV) decay measurement was carried out on a home-made instrument. The samples were excited by a laser radiation pulse, which from a PolarisII second-harmonic Nd:YAG laser. The TPV signal was registered by a 500 MHz TDS 5054 digital phosphor oscilloscope. Molecular electrostatic 8

potential (MEP) was simulated through density functional theory (DFT) at B3LYP function with 6-31G(d, p) basis set, which was performed with the Gaussian 09 program package [49]. Photocurrent densitye-voltage (J-V) curves and the electron trap-state density tests were measured with an electrochemical workstation (VersaSTAT 3, Ametek, USA) and a 150 W xenon lamp class ABB solar simulator (94021A, Newport, USA) as standardized by a standard Si solar cell (1218, Newport, USA), where PSC devices were illuminated under AM1.5 radiation (1 sun conditions, 100 mW cm-2). The sweep rate was 0.2 V s-1. 50 devices were fabricated and measured independently to obtain the statistical histograms of PCEs. The incident photon-to-electron conversion efficiency (IPCE) was measured using the Crowntech solar cell quantum efficiency measurement system (QTest Station 500AD, USA). Electrochemical impedance spectroscopy (EIS) was obtained by the electrochemical workstation with a bias of -1.10 V, the process was in the dark state and the frequency range was 0.1-100 KHz. All the measurements were performed in air condition and the devices were stored without any encapsulation.

3. Results and Discussion For high-performance PSCs, the formation of high-quality perovskite film is very important. Anti-solvent technique is widely used for film production. The effect of the EA on the morphology of perovskite films was investigate by SEM (Fig. 2a and 2b) and AFM (Fig. 2c-f). Compared to control sample (ether processed MAPbI3 films, defined as ether-MAPbI3), a homogeneous flat and pinhole-free perovskite film treated 9

with EA (EA-MAPbI3) can be achieved. The root-mean-square roughness (Rq) value of the EA-MAPbI3 film is 6.6 nm, which is smaller than that of ether-MAPbI3 film (11.1 nm). This indicates that EA as the anti-solvents gives a smoother surface. Low surface roughness can facilitate contact with the hole-transport layer (HTL), thereby reducing interface resistance for accelerating of charge transport [50]. Meanwhile, the SEM images also show that the EA-MAPbI3 grain size increase from ~200 nm to ~450 nm in comparison to the ether-MAPbI3, accompanying with the reduction of GBs. These are expected to reduce the density of defects, inhibit the ion motion, and minimize nonradiative charge-recombination, enhancing the device performance and stability [51].

Fig. 2. Top view SEM images of (a) EA-MAPbI3 film and (b) ether-MAPbI3 film. AFM height images of (c) EA-MAPbI3 film and (d) ether-MAPbI3 film, and corresponding Rq roughness are also displayed therein. 3D AFM height images of (e) EA-MAPbI3 film and (f) ether-MAPbI3 film. In order to deeply understand the effects of EA on the improved film quality at the 10

molecular level, we probed the interaction between EA and DMF/DMSO. The MEP of EA is calculated by using with DFT, as displayed in Fig. 3a. The MEP is an important parameter for analyzing the nature of the intermolecular interaction as well as predicting the reacting site [52, 53]. In principle, different MEP values are represented by different colors, which is in increasing order red < orange < yellow < green < blue. The red color area (negative potential) of the MEP implies electron-rich areas [54], which is a necessary condition for the formation of hydrogen bonds. From Fig. 3a, the maximum negative area is located on carbonyl oxygen atom, which provides the possibility of forming hydrogen-bonding with DMF/DMSO molecules. The schematic diagram of the hydrogen-bonding between EA and DMSO is displayed in Fig. 3b. The similar intermolecular interaction had been observed in the work by Zhao et al [10]. Furthermore, FT-IR measurements of EA, DMF, DMSO and mixed solvents systems were further carried out to verify the theoretical results. The FT-IR spectra of EA, DMSO as well as EA-DMSO are display in Fig. 3c. Obviously, the C=O stretching vibration of a single EA is at 1736.77 cm-1, while the stretching vibration of EA-DMSO is at 1732.64 cm-1, and the intensity is also significantly enhanced. In general, the chemical shift and increase of vibration peak indicate there are intermolecular forces (such as hydrogen-bonding) between two molecules [55, 56]. Therefore, we deem that the intermolecular hydrogen bonds are indeed formed between EA and DMSO. Nevertheless, there is no specific interaction between ether and DMSO due to the nonexistent chemical shift and change of vibration peak in FT-IR spectra (Fig. S1). The existence of hydrogen bonds between EA and DMSO can effectively prevent EA from 11

evaporating like ether after dripping, which avoid the irregular growth of the intermediate phase (MAI-PbI2-DMSO) under dry conditions [10], and might slow the perovskite crystallization process, while resulting in higher quality perovskite films with large grain size and smoother surface as well as improving the device performance and stability.

Fig. 3. (a) The molecular electrostatic potential (MEP) of EA. (b) The schematic diagram of the hydrogen-bonding between EA and DMSO. (c) The FT-IR spectra of EA, DMSO and EA-DMSO. The crystal structure of different perovskite films were investigated via XRD analysis, as displayed in Fig. 4a. The diffraction peaks emerged at 14.05°, 19.93°, 23.43°, 24.43°, 12

26.41°, 28.38°, 31.82°, 34.89°, 37.64°and 40.60° correspond to the (110), (112), (211), (202), (221), (220), (310), (204), (321), and (224) planes, which signifies both the MAPbI3 films are tetragonal [57, 58]. In addition, it can be found that two slight diffraction peak of PbI2 (12.59° and 33.60°) appeared in both samples. It is demonstrated that an appropriate amount of PbI2 can reduce the carrier recombination and improve the performance of the device by passivating the traps [59, 60]. Meanwhile, UV-vis absorption spectra were analyzed to study the optical properties of EA-MAPbI3 and ether-MAPbI3 films. As shown in Fig. 4b, the optical absorption of EA-MAPbI3 film is slightly stronger than that of ether-MAPbI3 film in the range of 300-500 nm. The absorption edges are located at around 800 nm, which matches the band gap of the tetragonal CH3NH3PbI3 (1.51 eV) [45]. Furthermore, in order to investigate the charge recombination dynamics, the steadystate PL and TRPL spectra of CH3NH3PbI3 films based on glass substrates are surveyed. As shown in Fig. 4c and d, the PL intensity of the EA-MAPbI3 film is enhanced, and the PL decay becomes slower, compared to the ether-MAPbI3 film, which implies that the nonradiative decay is effectively suppressed in EA-MAPbI3 films. The TRPL curves are fitted by a three-component exponential, given as Eq. (1): y = y0 + Ai exp [

- (x - x0) ] 𝑖

(1) Where Ai is the decay amplitude, τi is the PL decay time. Furthermore, the average PL decay lifetime (τave) is estimated via the Ai and τi values (Table S1) using Eq. (2).

13

ave =

 A𝑖𝑖2  A𝑖𝑖

(2)

The τave of the ether-perovskite film is 140.03 ns, while the τave of EA-perovskite film is significantly increased to 160.81 ns. In general, defects or traps in the crystal structure cause a quenching of PL, which lead to a reduced PL intensity and a shortened PL lifetime of photogenerated carriers in the absorber layer, because it is an additional nonradiative de-excitation path for carriers. Recently, some studies have shown that defects mainly exist in GBs [61, 62]. Here, the enhancement of PL intensity and growth of fluorescence lifetime implies that EA processed perovskite film can effectively reduce the densities of GBs and traps, thereby decreasing the nonradiative recombination of carriers [63]. To verify charge transfer enhancement resulting from EA as antisolvent, the charge recombination process under device operation was analyzed by TPV measurement. As a small-perturbation photoelectrochemical method, TPV measurement can provide accurate and valuable electrical information on the recombination kinetics [64]. In general, a laser pulse was adopted to produce a small disturbance on photovoltage, in which charge recombination in devices can be estimate from photovoltage loss [65-68]. Fig. 4e displays the TPV decay curves, and the charge carrier lifetime (τr) is defined as the time when the photovoltage decays to 1/e of the initial value. Obviously, the EAdevice shows a slower decay and its τr is 59.72 μs, which is longer than that of control device (25.90 μs). A long charge carrier lifetime could reduce charge recombination and facilitate charge transfer. Combined with the results of PL, TRPL and TPV, we consider that the EA as anti-solvent can yield a higher quality perovskite film, thus 14

effectively reducing nonradiative recombination due to the reduction of GBs.

Fig. 4. (a) XRD patterns and (b) UV-Vis spectra of EA-MAPbI3 film and ether-MAPbI3 film based on SnO2 ETLs, respectively. (c) PL and (d) TRPL spectra of EA-MAPbI3 film and ether-MAPbI3 film on glass substrates. (e) TPV of EA-device and ether-device. (f) Dark current-voltage curves of the electron-only devices showing VTFL kink point behavior, the inset displays the device structure. In order to further accurately explore the effect of EA on the quality of films, the trap 15

density within different prepared perovskite layers was calculated using the SCLC technique based on electron-only devices (FTO/SnO2/CH3NH3PbI3/PCBM/Au). Fig. 4f displays the dark current-voltage curves of EA and ether based devices, in the range of bias voltages, the linear relationship (pink and bright green line) of current and voltage represents the ohmic response of electron-only devices. While, when the bias voltage exceeds the kink point (trap-filled limit, TFL), the current rapidly increases nonlinearly (yellow and blue line), suggesting that the trap state is completely filled by the injected carriers [69, 70]. The trap density (Nt) can be calculated by the trap-filled limit voltage (VTFL) using Eq. (3).

eNtL2 VTFL = 2εε0

(3)

Where e is the elementary charge of the electron, Nt is the trap-state density, L is the thickness of perovskite film, ε is the relative dielectric constant of MAPbI3 (ε = 32), ε0 is the vacuum permittivity. The VTFL of EA-MAPbI3 film is 0.32 V, which is lower than that of ether-MAPbI3 film (0.54 V). The corresponding electron trap-state density is 7.08×1015 cm-3 and 1.19×1016 cm-3, respectively. The significantly lower trap-state density indicates that the quality of MAPbI3 film is much improved via EA as antisolvent, resulting for smoother surfaces and lower GBs in the MAPbI3 film. To demonstrate the photovoltaic performance of different devices, the PSCs with a FTO/SnO2/MAPbI3/Spiro-OMeTAD/Au architecture have been constructed in air. Fig. 5a shows the cross-sectional SEM image of the EA-PSCs device. The thickness of Spiro-OMeTAD layer and SnO2 layer are about 280nm and 40 nm correspondingly, among, the thickness of the SnO2 layer was further confirmed to be 38 nm by 16

performing AFM (Fig. S2). Comparing the light absorbing layer of EA-MAPbI3 PSC with ether-MAPbI3 PSC (Fig. S3), Both EA-MAPbI3 and ether-MAPbI3 have a thickness of 400 nm, however, it can be observed that the EA-MAPbI3 has more homogeneous grains with a larger size, which are consistent with the results shown in Fig. 2. Fig. 5b shows the J-V curves of the champion device based on EA-MAPbI3 with forward and reverse scan, respectively. The champion EA-PSC device yields excellent PCE of 17.83% with a Voc of 1.115 V, Jsc of 21.44 mA cm-2, and a FF of 74.58% under reverse scan. Nevertheless, the ether-PSC device PSC shows slightly inferior performance under reverse scan, obtaining PCE of 17.06% (Fig. S4 (a)), with a Jsc of 21.43 mA cm-2, a Voc of 1.099 V, and a FF of 73.07%. The PCE slight increase indicates that EA can be used as a green antisolvent to prepare more efficient MAPbI3-based PSCs in the atmosphere. It is worthwhile mentioning that the hysteresis index was decreased from 9.73% to 7.51% according to Eq. S1 (Supporting Information). Although the origin of J-V hysteresis remains under debate, some explanations are that ferroelectric effect, unbalanced charge carrier transport, ion migration, and trap-assisted charge recombination may be possible reasons for hysteresis [71]. Here, the smoother surface of EA-CH3NH3PbI3 film may promote tighter interface contact, thereby enhancing the charge transport and reducing the charge accumulation. In addition, the suppression of ion migration by formation of high-quality perovskite films is also important for reducing the hysteresis. Simultaneously, to guarantee the reproducibility of the experiment, the PCE statistic histogram of 50 devices based on EA-MAPbI3 is displayed in Fig. 5c. A majority of PCEs for the devices based on EA-MAPbI3 17

distributes in a range of 16-18%, which signifies the objectivity of the results.

Fig. 5. (a) Cross-sectional SEM image of a complete EA-PSC device. (b) J-V curves of the champion EA-device under forward and reverse scan. (c) IPCE spectrum and corresponding integrated Jsc. (d) histograms of photovoltaic PCEs from 50 devices. (e) The photo-current density and PCE with a given bias of -0.91 V. And (e) the EIS of the EA and ether processed devices. The IPCE for the champion EA-PSC is plotted in Fig. 5d, and the integrated Jsc value 18

(19.86 mA cm-2) from the IPCE spectrum is close to the JSC values from the J–V curve. The information of IPCE for ether-PSC can be found in Fig. S4 (b). The working stability of PSCs at the maximum power point is critical in practical applications. Therefore, the time-dependent stabilized photocurrents under a constant voltage bias at the maximum power point were monitored with time. After 300 s illumination, both two devices show a stabilized output photocurrent densities, stabilizing at 19.34 and 19.28mA cm-2 for EA-PSC (Fig. 4e) and ether-PSC (Fig. S4 (c), Supporting Information), yielding PCEs of 17.59 and 16.96% correspondingly. To get more insight on the dynamic process of carriers, we carried out EIS measurement under dark condition with the 0.1 Hz-100 kHz frequency range. Fig. 5f illustrates the Nyquist plots of different PSCs devices, which were fitted according to the equivalent circuit shown in the inset of Fig. 5f. The EA-device manifests smaller transfer resistance (Rct, 68.92 Ω) compared with ether-device (129.80 Ω), implying a more effective charge transfer and lower recombination rate. The effective electron extraction and transport performance may be attributed to the good interface contact between low-temperature processed SnO2 ETL and perovskite layer. Meanwhile, the low recombination rate is due to the smoother surface as well as the decrease of GBs for the EA-MAPbI3 film. This is consistent with TRPL and TPV results. The golden triangle constructed with efficiency, stability and cost should be given special consideration for commercial applications of PSCs [72]. Especially, long-term stability and thermal stability are very important for the device viability. As expected, the unencapsulated EA-based PSCs exhibit excellent humidity stability. As displayed 19

in Fig. 6a, the PCE of the EA-based device maintains 84.80% of original PCE after 80 days (> 1900 h), by contrast, the ether-based device only maintains 74.09%. Obviously, EA-based device shows better air long-term stability. On one hand, the improved environmental stability can be attributed to the enhanced hydrophobicity of perovskite layer. The contact-angle of water on EA-MAPbI3 and ether-MAPbI3 film are presented in Fig. S5. The contact angle of EA-MAPbI3 film (89.8°) is significantly increased compared with that of ether-MAPbI3 film (53.9°). It means that a more hydrophobic film surface can be obtained by using EA as antisolvent, illustrating that it can prevent the penetration of moisture and decomposition of perovskite film more effective. On the other hand, the EA-perovskite film with less GBs could limit path for water intrusion and obtain high humidity stability. Due to the ionic material nature, perovskite materials are sensitive to environmental factors, resulting in degradation of perovskite [73]. This process would be accelerated by heating. Unfortunately, the thermal-induced degradation cannot be circumvented by device encapsulation. Although 85℃ is the common testing temperature for photovoltaic panels [74], we measured PSCs at 100℃ to accelerate ageing within short time. The normalized PCE, Voc, Jsc and FF versus time is summarized in Fig. 6b. The EA-device still retains 81.03% of initial value after exposing to 100℃ for 10 h, nonetheless, the ether-devices give relatively limited thermal stability, only keeping 72.98% of initial PCE value. More noted, in the current study of thermal stability for MAPbI3-based PSCs, EA-device does show superior thermal resistance under conditions of heating at 100 °C in the atmosphere. In our case, EA-based perovskite film can effectively imped the ions migration and slow the 20

decomposition of perovskite through decreasing the GBs, thus greatly improving device thermal stability [75, 76].

Fig. 6. Photovoltaic parameters stability of EA-device and ether-device as a function of (a) storage time (days); (b) heat time (hous) at 100 ℃ in air without any encapsulation.

4. Conclusions In summary, with a device structure of FTO/SnO2/MAPbI3/Spiro-OMeTAD/Au, the high-efficiency and stable MAPbI3-based PSCs have been fabricated in ambient condition. Uniform and compact perovskite films with large grain size, reduced grain boundaries, and low defect density are obtained through introducing EA as antisolvent. We demonstrate that the high quality perovskites films may be attributed to the intermolecular forces between EA and DMSO. As a result, a champion PCE of 17.83% has been achieved. More importantly, unencapsulated PSC retains 84.80% of its original PCE value after storage in air for 80 days (>1900 h). Apart from excellent air stability, the final devices also show great thermal (100 ℃) stability. These findings 21

present a green path towards manufacturing efficient and stable air-processed PSCs.

Acknowledgement We are grateful for the financial support of this research from the National Natural Science Foundation of China (51779065, 51579057 and 11704152).

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Figure Captions Fig. 1. Schematic diagram of the fabrication procedures of MAPbI3 film via one-step method. Fig. 2. Top view SEM images of (a) EA-MAPbI3 film and (b) ether-MAPbI3 film. AFM height images of (c) EA-MAPbI3 film and (d) ether-MAPbI3 film, and corresponding Rq roughness are also displayed therein. 3D AFM height images of (e) EA-MAPbI3 film and (f) ether-MAPbI3 film. Fig. 3. (a) The molecular electrostatic potential (MEP) of EA. (b) The schematic diagram of the hydrogen-bonding between EA and DMSO. (c) The FT-IR spectra of EA, DMSO and EA-DMSO. Fig. 4. (a) XRD patterns and (b) UV-Vis spectra of EA-MAPbI3 film and ether-MAPbI3 film based on SnO2 ETLs, respectively. (c) PL and (d) TRPL spectra of EA-MAPbI3 film and ether-MAPbI3 film on glass substrates. (e) TPV of EA-device and ether-device. (f) Dark current-voltage curves of the electron-only devices showing VTFL kink point behavior, the inset displays the device structure. Fig. 5. (a) Cross-sectional SEM image of a complete EA-PSC device. (b) J-V curves of the champion EA-device under forward and reverse scan. (c) IPCE spectrum and corresponding integrated Jsc. (d) histograms of photovoltaic PCEs from 50 devices. (e) The photo-current density and PCE with a given bias of -0.91 V. And (e) the EIS of the EA and ether processed devices. Fig. 6. Photovoltaic parameters stability of EA-device and ether-device as a function of (a) storage time (days); (b) heat time (hous) at 100 ℃ in air without any encapsulation. 35

Fig. 1. Schematic diagram of the fabrication procedures of MAPbI3 film via one-step method.

36

Fig. 2. Top view SEM images of (a) EA-MAPbI3 film and (b) ether-MAPbI3 film. AFM height images of (c) EA-MAPbI3 film and (d) ether-MAPbI3 film, and corresponding Rq roughness are also displayed therein. 3D AFM height images of (e) EA-MAPbI3 film and (f) ether-MAPbI3 film.

37

Fig. 3. (a) The molecular electrostatic potential (MEP) of EA. (b) The schematic diagram of the hydrogen-bonding between EA and DMSO. (c) The FT-IR spectra of EA, DMSO and EA-DMSO.

38

Fig. 4. (a) XRD patterns and (b) UV-Vis spectra of EA-MAPbI3 film and ether-MAPbI3 film based on SnO2 ETLs, respectively. (c) PL and (d) TRPL spectra of EA-MAPbI3 film and ether-MAPbI3 film on glass substrates. (e) TPV of EA-device and ether-device. (f) Dark current-voltage curves of the electron-only devices showing VTFL kink point behavior, the inset displays the device structure.

39

Fig. 5. (a) Cross-sectional SEM image of a complete EA-PSC device. (b) J-V curves of the champion EA-device under forward and reverse scan. (c) IPCE spectrum and corresponding integrated Jsc. (d) histograms of photovoltaic PCEs from 50 devices. (e) The photo-current density and PCE with a given bias of -0.91 V. And (e) the EIS of the EA and ether processed devices.

40

Fig. 6. Photovoltaic parameters stability of EA-device and ether-device as a function of (a) storage time (days); (b) heat time (hous) at 100 ℃ in air without any encapsulation.

41

GRAPHICAL ABSTRACT

42

HIGHLIGHTS 

High quality perovskite films are obtained via ethyl acetate as green anti-solvent.



Long charge carrier lifetime and low carrier recombination rate are achieved.



A power conversion efficiency of 17.83% is obtained with a small J-V hysteresis in air.



The air, operational and thermal stability of SnO2 based perovskite solar cells are improved.

43