Improving the Performance and Reproducibility of Inverted Planar

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Improving the performance and reproducibility of inverted planar perovskite solar cells by using tetraethyl orthosilicate as the anti-solvent Mei Wang, Qiuyun Fu, Liang Yan, Pengju Guo, Ling Zhou, Geng Wang, Zhiping Zheng, and Wei Luo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b18402 • Publication Date (Web): 08 Jan 2019 Downloaded from http://pubs.acs.org on January 8, 2019

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ACS Applied Materials & Interfaces

Improving the performance and reproducibility of inverted planar perovskite solar cells by using tetraethyl orthosilicate as the anti-solvent Mei Wang, Qiuyun Fu*, Liang Yan, Pengju Guo, Ling Zhou, Geng Wang, Zhiping Zheng and Wei Luo School of Optical and Electronic Information, Engineering Research Center for Functional Ceramics of the Ministry of Education, Huazhong University of Science and Technology, Wuhan 430074, P. R. China *Corresponding author: [email protected]

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ABSTRACT Anti-solvent assisted crystallization has been extensively used for perovskite solar cells (PSCs). Whereas, this approach has a fatal drawback —— low reproducibility, originating from the extremely harsh operating conditions of the current anti-solvents. As a result, only skilled technicians are qualified to be scheduled to prepare perovskite thin films in order to fabricate high-efficiency devices, which lowers the pace of progress of PSCs. Besides, the most popular anti-solvents of toluene (TL) and chlorobenzene (CB) are highly toxic and carcinogenic. On account of these, we tried to develop a low hazardous anti-solvent that enable achieving high-efficiency and highly reproducible PSCs. Herein, tetraethyl orthosilicate (TEOS) was employed in the inverted NiOX-based planar PSC for engineering efficient perovskite layer, achieving a power conversion efficiency of 17.02% on glass substrates and 14.49% on flexible polymer substrates with negligible hysteresis, which was even outperforming TL and CB. More importantly, TEOS PSCs exhibited much higher reproducibility than their counterparts. These desirable features should be ascribed to the higher-quality perovskite films with larger grain size, reduced density of defects and thus smoother carrier transportation and slower carrier recombination. This work drives PSCs another step toward industrial scale commercialization and also pave the way for environmental rollable photovoltaic applications. KEYWORDS: Low hazardous anti-solvent, TEOS, Reproducibility, CH3NH3PbI3, Perovskite solar cell, Flexible

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INTRODUCTION Due to the superior properties of the hybrid organometallic halide perovskites, including strong light absorption, long carrier diffusion length and life time, facile solution-based processes, etc.,1-3 perovskite solar cells (PSCs) have drawn extensive attention from researchers in the last six years. The power conversion efficiency (PCE) of single junction PSCs was increasing rapidly to over 23%,4 indicating PSCs a promising candidate for low-cost and high efficient photovoltaic technology. As we all know, the quality of perovskite film significantly influences the performance of PSCs. A variety of different methods such as one-step deposition, sequential deposition, vapor deposition and anti-solvent assisted crystallization (ASAC) have been employed to deposit perovskite film.5-8ASAC is recognized as the simplest and quickest method to prepare perovskite film with superior quality and it is widely used currently. However, PSCs fabricated using these anti-solvents usually suffer low reproducibility, since the quality of perovskite films is significantly dependent on the spin coating condition (anti-solvent dripping amount and dripping time). The ultranarrow operating window makes it difficult to fabricate high-quality perovskite films for general experimenters who takes up a large proportion in a lab. This problem will surely retard the future development of PSCs. In addition, the most used anti-solvents of toluene (TL) 5 and chlorobenzene (CB)

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are cytotoxic and carcinogenic, hazardous to operators in

manufacturing and other people in the lab.10, 11 For these concerns, other anti-solvents have been developed to replace TL and CB, such as much higher reproducibility potential anti-solvent like diethyl ether 12 and less toxic anti-solvents like ethyl acetate or methoxybenzene.

13, 14

Although devices engineered with these anti-solvents have

obtained comparable performance to the common CB processed samples, however, they still show a certain toxic potential to man.15 What’s more, PSCs prepared using greener anti-solvent and possess high reproducibility have not been reported, which drive us to look for a new kind of anti-solvent. In this work, a new anti-solvent, tetraethyl orthosilicate (TEOS), who has much 3



lower toxicity (LD 50: 6270 mg/kg) compared with TL (LD 50: 5000 mg/kg) and CB (LD 50: 2290 mg/kg), was successfully employed in preparing NiOX-based planar inverted “p-i-n” PSCs. This device structure was selected because that it has many advantages such as low-temperature solution processability, lower hysteresis in comparision with planar “n-i-p” device. NiOX was selected as the hole transportation material instead of PEDOT: PSS to obtain respectable stability. As references, TL and CB were also used to prepare PSCs with the same structure. TEOS processed PSCs achieved a high PCE of 17.02% with no hysteresis, outperforming their counterparts. More importantly, PSCs processed with TEOS exhibited higher reproducibility, which significantly simplified the fabrication process. Furthermore, considering the low temperature processability, flexible PSCs fabricated on PEN/ITO substrates with a highest PCE of 14.49% were also demonstrated via this route.

RESULTS AND DISCUSSION To simplify, perovskites films processed by TEOS, TL, CB were denoted as TEOS-PSK, TL-PSK, CB-PSK, respectively. The corresponding PSCs, which were prepared from these perovskites films using the ITO/NiOX/MAPbI3/PC61BM/BCP/Ag structure, were denoted as TEOS PSC, TL PSC, CB PSC, respectively. The anti-solvents were introduced for preparing perovskite films during the spin coating of precursor solution. In our experiment, a N, N-dimethylformamide (DMF) solution containing equimolar CH3NH3I (MAI), PbI2, and dimethyl sulfoxide (DMSO) was prepared as the precursor solution. The relationship between the concentration of perovskite precursor solution and the spinning time was illustrated in Figure 1a. The concentration increased gradually with the evaporation of DMF and then remained unchanged after reached its maximum C0. We named the moment at C0 as t0, at which time the perovskite film started to be supersaturated and it was determined by the boiling point and vapor pressure of the solvents. As reported before, anti-solvents have a huge impact on the crystallization kinetics and final morphology of perovskite film by changing the nucleation and crystal growth rate.16-22 In the light of the classical 4


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nucleation theory, supersaturation plays a great role in the crystallization process, since the supersaturated state acts as the driving force in crystal formation. Once the antisolvent is dropped, which is usually a highly volatile and DMF-miscible solvent, the evaporation of DMF is accelerated during spin-coating, which leads to a considerably higher supersaturation compared with the conventional non anti-solvent assisted method. Therefore, highly homogeneous nucleation density can be achieved in a dramatically short time and the undesirable heterogeneous or secondary nucleation will be effectively suppressed. Besides, the anti-solvent retards perovskite crystallization by promoting the formation of intermediate phase MAI-PbI2-DMSO. Consequently, uniform and dense perovskite films can be obtained. Generally, anti-solvent should be dropped at a moment before t0. The moment to begin dropping the anti-solvent, which was called operating time, and the volume of anti-solvent are very difficult to control accurately. However, a proper procedure including the volume and the operating time of anti-solvent have to be adopted in order to achieve dense and uniform perovskite film. Here, PCE of PSCs processed by TL, CB and TEOS with different dripping volumes and operating times were presented in Figure1b, c and d respectively. It can be seen that the PCE of TL PSCs and CB PSCs had obvious peaks, which means that the optimal operating time and dripping volume for both of TL and CB were highly restricted. In contrast, the performance of TEOS PSCs showed less change at a wide operating time and dripping volume window. Therefore, we said that TEOS showed better reproducibility than TL and CB in preparing PSCs. Herein, the idea of “reproducibility” refers that the PSCs can be fabricated on a relatively relaxed operating condition while maintaining respectable efficiencies. In addition, relatively higher PCEs for TEOS were obtained.

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Figure 1. a) The relation between perovskite precursor solution concentration and spinning time when no antisolvent was dropped: t0= the moment of the film started to be turbid, C0= concentration at t0. The relation between PCE and dripping volume, operating time for b) TL PSCs, c) CB PSCs and d) TEOS PSCs respectively.

The current density-voltage (J-V) curves under forward and reverse bias scanning of the champion devices were presented in Figure 2a and the calculated photovoltaic parameters were listed in Table S1. Seen from the reverse scanning data, TEOS-PSC obtained a Voc of 1.028V, Jsc of 21.15 mA cm-2, FF of 78.3% and PCE of 17.02%, significantly higher than its counterparts with a Voc of 1.021V, Jsc of 19.65 mA cm-2, FF of 77.8%, PCE of 15.61% for CB-PSC and a Voc of 1.023V, Jsc of 19.95 mA cm-2, FF of 75.5%, PCE of 15.4% for TL-PSC. Compared to TL and CB, TEOS was able to improve Voc, Jsc and FF to a higher level, especially the Jsc. The same phenomenon could be seen in the forward scanning data. Furthermore, the negligible J-V hysteresis for these three kinds of device confirmed the PCE results effective.

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Figure 2. a) J-V curves of the best TL PSC, CB PSC and TEOS PSC tested under AM 1.5G simulated sunlight employing a 0.07 cm2 mask. b) EQE of the champion TL PSC, CB PSC and TEOS PSC. c) Stabilized output of the current density at the maximum power point with time for devices under AM 1.5G illumination. d) PCE distribution for 60 TL PSCs, 60 CB PSCs and 60 TEOS PSCs, respectively.

External quantum efficiency (EQE) spectra (Figure 2b) were tested to verify the measurements above. The spectra covered the whole visible range from 300 to 800 nm with an onset at about 780 nm. As expected, the EQE heights varied with three antisolvents, in the order of TL ≈ CB < TEOS, showing the same trend with the Jsc values. The integrated current density for the optimal TEOS PSC was 20.03 mA cm-2, which was much higher than its reference cells with a value of 18.61 mA cm-2 for TL PSC and 18.79 mA cm-2 for CB PSC. All values were within the reasonable error range and well matched with the values from J-V measurements.23,

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To identify the accuracy of

photovoltaic measurements, steady-state output current densities at maximum power point for different J-V curves were recorded with time under the humidity lower than 20% RH and without encapsulation (Figure 2c). Therefore, 0.86V external bias was 7



applied to both of TL and CB PSC, 0.88 V to TEOS PSC. The current density quickly reached a constant value of 18.15, 17.91 and 19.34 mA cm-2 for TL, CB and TEOS device, respectively, after the light was turned on. These values were comparable to the photocurrent profiles derived from J-V curves, justifying the accuracy of J-V measurement above. To study the reproducibility of this strategy, the statistical information of PCE for PSCs based on different anti-solvents were provided in Figure 2d. Herein, the statistic profiles were recorded from 60 individual cells for each antisolvent. Seen from the histograms, the results showed to be statistically significant and reproducible. Moreover, it also verified the practicability of TEOS as an anti-solvent. In order to clarify the excellent results induced by TEOS, we presented schematically the deposition procedure and formation mechanism of MAPbI3 films fabricated by different anti-solvents in Figure 3a. We detailed the perovskite film formation as follows: perovskite precursor solution was firstly dropped on the NiOX /ITO substrate then the substrate was accelerated to a certain rotation speed. Antisolvent of TL, CB or TEOS was dripped quickly onto the surface of the wet film at the moment of t (illustrated in Figure 1). At the end of spinning, precursor film was formed. After 100 ℃ annealing for 2 min, MAPbI3 film was obtained from the precursor film. Interestingly, it was found that the precursor films processed with different antisolvents exhibited an obvious difference in color, light brown for both of TL or CB samples and colorless for TEOS samples, suggesting the different composition of different precursor films. As reported, boiling point, dielectric constant and miscibility to DMSO or DMF of the anti-solvents (Table S2) were identified as the three critical parameters in determining the select of an-solvent.

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Considering that TEOS possess

similar boiling point, dielectric constant and miscibility to DMF with TL and CB, the difference inferred above should be attributed to the immiscibility of TEOS to DMSO 25

and it may also be the main reason for the better reproducibility for TEOS PSCs

showed in Figure 1d. When TL or CB was used as the anti-solvents, who were miscible to DMSO, a 1:1:1 adduct intermediate phase of MAI-PbI2-DMSO was formed firstly. Then a partial of DMSO was removed along with the volatilization of anti-solvent, 8


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leading to the crystallization of tiny amounts of MAPbI3 and therefore forming mixed phases (MAPbI3 and MAI-PbI2-DMSO) in the precursor films, which was consistent with the previous reports.26,27 The films changed from colorless to light brown during the spin coating process and appeared to be light brown. Interestingly, in the case of TEOS, who was immiscible to DMSO, a pure intermediate phase MAI-PbI2-DMSO was formed in the precursor film. The film was colorless throughout the whole spin coating process and appeared to be colorless. Different precursor films should have different effects on the final perovskite fims. Then we performed X-ray diffraction (XRD) and UV-vis absorption of TL-PSK, CBPSK and TEOS-PSK films on NiOX/ITO, as depicted in Figure 3b, c. All the diffraction peaks of the TEOS-PSK film were the characteristic peak of MAPbI3 except the small peak at 12.7° and they were completely consistent with those of the TL-PSK and CBPSK film. The small peak at 12.7° indicated that a small amount of PbI2 was present in the perovskite films, 28 who was always located at the grain boundary, playing a great role in passivation and thus was beneficial for the device performance.29 The difference was that the diffraction intensity of TEOS-PSK film was significantly stronger than those of the TL-PSK and CB-PSK film, especially for the (100) peak, indicating relatively higher crystallinity. The UV-vis absorption spectra as depicted in Figure 3c indicated that the three films present a similar absorption characteristic, covered the entire visible regions with an absorption edge of ~770nm. A slightly higher light absorption was obtained for TEOS-PSK film compared to those of TL-PSK and CBPSK film. This may be contributed from the improvement in the morphology of perovskite film.

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Figure 3. a) The schematic of perovskite film deposition based on each anti-solvent. b) XRD patterns of different perovskite films. c) UV-vis absorption spectra of perovskite films fabricated with different anti-solvents.

The microstructure of the device and perovskite films grown on NiOX/ITO substrates were detected by the scanning electron microscopy (SEM) and Atomic force microscope (AFM). Both the surface images (Figure 4a, b, c) and cross-sectional images (Figure 4d, e, f) showed the TEOS-PSK had greater film quality than that of the TL-PSK and CB-PSK, the former possessed much larger perovskite grains and less pinholes than the others. Besides, the cross-sectional images illustrated that the three devices have similar thickness of the perovskite layers. Aiming to detect the morphology of the perovskite film in a lager view, AFM was conducted and the topography images were presented in Figure 4g, h, i. They showed the same phenomenon with the surface SEM images. In order to demonstrate that the inhibition of TEOS on pinhole is reproducible rather than an accidental result. We prepared another three samples of TL-PSK, CB-PSK and TEOS-PSK, respectively. The topview images were shown in Figure S1. Pinholes always exist in TL-PSK and CB-PSK films, but they barely existed in TEOS-PSK films, when the experiments were 10


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performed under the same strict conditions. Taken together, the TEOS-PSK film had greater quality, with larger grains and less pinholes and thus less boundaries and electronic trap states, compared to the common TL-PSK and CB-PSK film. These superior properties were favor to improve the performance of photovoltaic devices.

Figure 4. Surface SEM image of a) TL-PSK, b) CB-PSK and c) TEOS-PSK film respectively. Cross-sectional SEM image of the typical d) TL PSC, e) CB PSC and f) TEOS PSC respectively. AFM topography image of g) TL-PSK, h) CB-PSK and i) TEOS-PSK film. The scale bare for SEM images were 200 nm. The selected scan area for AFM images were 5x5 μm. The layers from the bottom to the top in the cross-sectional images were: (ⅰ) ITO, (ⅱ) NiOX, (ⅲ) perovskite, (ⅳ) PC61BM, (ⅴ) BCP and (ⅵ) Ag.

Keeping this in mind, it is of great interest to explore further the electronic properties of these films, steady state photoluminescence (PL), Time resolved PL (TRPL) spectroscopy and the dark J-V curves were carried out. For steady PL and TRPL measurements, perovskite films were directly deposited on ITO substrate. As displayed in steady state PL spectra (Figure 5a), the PL intensity was much higher for TEOS-PSK film than TL-PSK and CB-PSK films. This indicated that the quality of 11



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perovskite film was enhanced when it was processed with TEOS. To quantify the charge recombination within perovskite films, we measured the TRPL decay for the three films and the results were displayed in Figure 5b. The average PL decay lifetimes (τave) could be obtained by fitting the decay curves using a cubic-exponential formula reported before 30 and the fitting data were recorded in Table S3. The τave of TEOS-PSK film was estimated to 70.69 ns, much longer than that of the TL-PSK film (44.23 ns) and CB-PSK film (44.98 ns). This suggested that there were fewer non-radiative recombination pathways, likely due to the reduced density of defects in the TEOS-PSK film. To shed light on the carrier recombination mechanism, dark J-V curves for TL, CB and TEOS PSCs were conducted (Figure S2). Voc could be estimated from the flowing equation:14 𝑉𝑂𝐶 =

𝑛𝑘𝑇 𝑞

ln

(

𝐽𝐿 𝐽0

)

+1

(1)

when the structure and operation temperature of the solar cells were fixed. In this equation, n, JL, J0, q, k and T respectively represent the ideality factor, photo current density, dark saturation current density, elementary charge, Boltzmann constant, and Kelvin temperature. The value of J0 is often acted as an effective means for characterizing the recombination in a device and consequently has an effect on the Voc values of a solar cell. The relationship between the current passing through the diode and the applied bias can be expressed by the following equation:

(

J = 𝐽0 𝑒

𝑞𝑉 𝑛𝑘𝑇

)

(2)

―1

The values of J0 were obtained by linear fitting the semi-logarithmic plots of the dark J-V curves in a range near Voc (Figure 5c). Both of the TL device and CB device had relatively high J0, with the value of 1.97x10-6 and 8.20x10-7 mA cm-2 respectively. Whereas, the J0 decreased to ≈ 1.35x10-8 mA cm-2 for the TEOS PSCs. The dramatically decreased J0 is an indicator of much lower recombination.31 Duing to the fact that Voc increases with the decrease of J0, TEOS PSCs would generate higher voltage.

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Figure 5. a) Steady state PL spectra of the TL-PSK, CB-PSK and TEOS-PSK films, respectively. b) TRPL spectra of the TL-PSK, CB-PSK and TEOS-PSK films, respectively. c) Semi-logarithmic plot of the dark J-V curves in a range near Voc.

Electrochemical impedance spectroscopy (EIS) is an effective tool to characterize the intrinsic interfacial charge transfer and transport kinetics. 32-35 To further investigate the charge transport and recombination, EIS was conducted for the solar cells prepared by each anti-solvent. It was measured at various applied bias (0.2, 0.4, 0.6 and 0.8V) from a frequency of 100 mHz to 100 kHz in dark. The AC amplitude was 5mV. As displayed in Figure 6 the first semicircle at high frequency mainly relates to charge transport, while the second semicircle at low frequency mainly represents bulk charge recombination of MAPbI3 or the interface charge recombination.36,

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The transport

resistances Rtr and recombination resistances Rrec were obtained by fitting the Nyquist plots with simplified equivalent circuit diagram (Inset of Figure 6a). The Rtr and Rrec values under different bias for the three samples were displayed in Figure 6d. The specific Rrec values were shown in Table S4. It was found that, the TEOS PSC showed a lower Rtr and higher Rrec than the TL or CB PSC regardless of the applied bias. These indicated that TEOS PSC exhibited smoother charge transportation and lower charge recombination rate. The difference of Rtr and Rrec among the three samples should be ascribed to the perovskite layers, given the fact that they adopted the same device structure and also the materials except for perovskites.

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Figure 6. Nyquist plots and the fitting curves of the a) TEOS PSC, b) TL PSC and c) CB PSC. (d) Rtr and Rrec deserved from the Nyquist plots. The EIS data were measured at different bias in dark. The equivalent circuit diagram used for fitting the Nyquist plots was given in the inset of Figure 6a.

These measurements collectively indicated that the TEOS-PSK possessed greater film quality with larger crystals size, reduced density of defects, resulting in longer charge recombination lifetime, lower dark saturation current density, smaller Rtr and higher Rrec. They were considered as the decisive factor for the improved performance of PSCs. Finally, inspired by the “all low-temperature” processability, flexible devices with a structure of PEN/ITO/NiOX/MAPbI3/PC61BM/BCP/Ag and processed with TEOS were fabricated accordingly, as depicted in Figure 7. The champion cell showed a high PCE of 14.49%, with a Voc of 0.97 V, a Jsc of 20.47 mA cm-2, and a FF of 73% for the reverse scan direction under AM 1.5G illumination and negligible J-V hysteresis was observed with scan direction, as demonstrated in Figure 7a. To adapt to the practical applications of flexible devices, we investigated their mechanical stability by bending them to different curvature. Desirably, they exhibited pretty stable performances with 14


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respect to the bending test at R= 4mm, 6mm, 8mm and 10mm after 200 flexing cycles respectively, as presented in the inset of Figure 7b. Only a 15% decrease in its initial value was observed even subjected to a curvature radius of 4mm. It indicated that the flexible devices possessed considerable mechanical stability which should contribute from the improved quality of perovskite films, due to the fact that the crack growth rate in uniform films is lower than that in films with pinholes when bending.

Figure 7. a) J–V curves of PEN/ITO/NiOX/MAPbI3(TEOS)/PC61BM/BCP/Ag flexible PSCs under illumination of 1 Sun and the corresponding photograph of the cell (inset). b) Normalized PCEs of the device with various bending radii (R): flat, 10, 8, 6, 4 mm.

CONCLUSIONS In conclusion, we have proposed a low hazardous anti-solvent of TEOS for fabricating high quality perovskite thin films in this report. This method enabled the formation of pinhole-free MAPbI3 films with larger grain size and reduced density of defects, reflected from the higher PL intensity and prolonged PL Decay lifetime, compared to the films processed from TL or CB. Thus, a high PCE of 17.02% with no hysteresis and a highly enhanced reproducibility of the NiOx-based planar inverted PSCs were obtained by the low hazardous solvent process, in contrast with the TL and CB PSCs (15.61% and 15.4% respectively). The dark J-V tests and EIS measurements based on various applied bias both demonstrated more efficient carrier extraction and slower carrier recombination occurred in TEOS PSCs. Considering the “all-lowtemperature” processability, we fabricated the flexible devices based on PEN/ITO 15



substrate. The champion flexible TEOS PSC achieved a PCE of 14.49% with considerable mechanical stability. Although both the perovskite material and the preparation technique need to be further optimized to improve the PCE of the PSCs to a higher level in our future work, results in this paper demonstrate the availability of TEOS as an environmental friendly anti-solvent for perovskite-based production and pay the way for wearable perovskite-based optoelectronic devices.

EXPERIMENTAL Device fabrication. Patterned ITO substrates (8Ω square-1) were cleaned ultrasonically with acetone, isopropanol and deionized water for 15 min, respectively and then soaked in ethyl alcohol for preparation. Then, NiOX films were deposited by spin coating (2000 rpm for 30s) the NiOX inks on ITO substrate at room temperature and then annealed at 130 ℃ for 20 min in air. The NiOx inks of 20mg/ml in deionized water were prepared via the method previous reported.38 The MAPbI3 perovskite precursor was prepared by mixing 0.159 g MAI, 0.461 g PbI2, 0.078 g DMSO and 0.6 g DMF. After ca. 1 h stirring, the perovskite precursor was filtered using 0.22 μm NYLON syringe filter and coated onto the ITO/NiOX substrates with speed of 4000 rpm for 20 s. During the spinning process, the substrate was treated by quickly drop-casting TL, CB or TEOS. Then the substrates were heated on a hot plate by 100 ℃ for 2 min. After that, a layer of PC61BM (20 mg/ml in CB) was deposited by spin coating at 2000 rpm for 45 s and the film was dried at 90 ℃ for 30 min. Then BCP in isopropanol (0.5 mg/ml) was dropwise on top of the PC61BM during 6000 rpm for 15 s spin-coating. The spin-coating processes of perovskite, PC61BM and BCP were conducted in a glove box filled with inert nitrogen gas. Finally, 100 nm thick silver electrodes were evaporated through a shadow mask by thermal evaporation. Patterned PEN/ITO substrates were adopted to fabricate flexible devices. Other experimental sections including the device characterization are included in the Supporting Information.

SUPPORTING INFORMATION 16


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The Supporting Information is available free of charge on the ACS Publications website. Experimental details, Photovoltaic parameters of the best devices, Summary table of physical properties of anti-solvents, parameters of TRPL spectra, the Rrec values, topview SEM images of PSK, dark J-V curves of PSCs, XRD patterns and SEM images of the mixed perovskite systems.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID: 0000-0001-6352-5144

NOTES The authors declare no competing financial interest.

ACKNOWLEDGMENTS Professor Hongwei Han was gratefully acknowledged for the completion of this work. This work was financially supported by the National Natural Science Foundations of China (No. 11575065), the National Nature Science Foundations of China (Grant No. 61571203), the National Key R & D Program of China (Grant No. 2017YFB0406301). The authors thank the characterization support from Wuhan National Laboratory for Optoelectronics and the Analytical and Testing Center of Huazhong University of Science and Technology (HUST). We are indebted to Zhihui Zhang and Sheng Li for EQE measurements, Deyi Zhang for EIS tests, Miao Duan and Yue Ming for TRPL tests.

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