In Situ Grain Boundary Modification via Two-Dimensional Nanoplates

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Energy, Environmental, and Catalysis Applications

In situ grain boundary modification via two-dimensional nanoplates to remarkably improve stability and efficiency of perovskite solar cells Xuejie Zhu, Shengnan Zuo, Zhou Yang, Jiangshan Feng, Ziyu Wang, Xiaorong Zhang, Shashank Priya, Shengzhong (Frank) Liu, and Dong Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b14954 • Publication Date (Web): 31 Oct 2018 Downloaded from http://pubs.acs.org on October 31, 2018

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ACS Applied Materials & Interfaces

In Situ Grain Boundary Modification via TwoDimensional Nanoplates to Remarkably Improve Stability and Efficiency of Perovskite Solar Cells Xuejie Zhu#a, Shengnan Zuo#a, Zhou Yanga, Jiangshan Fenga, Ziyu Wangc, Xiaorong Zhanga, Shashank Priyab, Shengzhong (Frank) Liu*ac, and Dong Yang*ab

aKey

Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education;

Shaanxi Key Laboratory for Advanced Energy Devices; Shaanxi Engineering Lab for Advanced Energy Technology, School of Materials Science and Engineering, Shaanxi Normal University, Xi’an 710119, China. bMaterials

Science and Engineering, Penn State, University Park, Pennsylvania 16802,

United States. cDalian

National Laboratory for Clean Energy, iChEM, Dalian Institute of Chemical

Physics, Chinese Academy of Sciences, Dalian, 116023, China.

ACS Paragon Plus Environment

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ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

#These

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authors contributed equally to this work.

*Corresponding author: Prof. Shengzhong (Frank) Liu, [email protected]; Prof. Dong Yang, [email protected]

KEY WORDS: perovskite solar cell, in situ, two-dimensional, grain boundary, stability

ABSTRACT

Even though record efficiency of 23.3% has been achieved in organic-inorganic hybrid perovskite solar cells, their stability remains a critical issue, which greatly depends on morphology of perovskite absorbers. Herein, we report a practical grain boundary modification to remarkably improve the humidity and thermal stability by in situ gradually growing twodimensional (2D) nanoplates between the grain boundaries of perovskite film using phenylethylammonium iodide (PEAI). The experimental results show that PEAI nanoplates play a critical role in stabilizing of perovskite thin films by reducing the moisture sensitivity and suppressing phase transition at the grain boundaries. In addition to the significant improved ambient stability, the grain boundary modification by PEAI can effectively suppress the nonradiative charge recombination at grain boundaries. As a result, the efficiency of perovskite solar cells is up to 20.34% with significant humidity and thermal stability. 1. INTRODUCTION


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The latest generation of organic-inorganic hybrid perovskite solar cells (PSCs) has become a superstar owing to the low-cost fabrication process and meteoric rise in power conversion efficiencies (PCEs) as high as 23.3% in last few years.1-12 In general, the light-absorber is methylammonium lead triiodide (MAPbI3) or formamidinium lead triiodide (FAPbI3),5,13 which exhibits excellent optical-electric properties, including lower exciton binding energies,14 longer electron and hole diffusion length,15 higher carrier mobilities,16,17 and more suitable bandgaps for optoelectronic devices.18 Even though high efficiencies of PSCs have achieved huge success, the thin-film perovskite absorbers demonstrate compositional degradation or phase transition under both heat and humidity. Therefore, one of the most important goals facing researchers is to overcome the long-term environmental stability issue.19,20 Ruddlesden-Popper two-dimensional (2D) perovskites have appeared as an actual path to solve the relevant photo- and chemical-stability.21-25 It can be defined as AnBn-1MnX3n+1, where A is a long chain alky cation, B is a MA cation, M is a metal ion, and X is a halide anion. The value of n determines the optical band gap. Although the 2D perovskites exhibit ultrastability due to the dielectric contrast effect, beneficial geometric effect and hydrophobic of large organic cations, their corresponding solar cells show low PCE owing to large exciton binding energy and the poor carrier transport caused by quantum conﬁnement effect.26,27 Recently, quasi-2D PSCs with high n values (n = 60) has been paying attention due to its higher PCE of 15.3%.24 However, the stability severely degraded because of reducing 2D perovskite ratio. Although the previous reported have been prepared in improving stability of 3D PSCs using 2D covering layer and 2D prompting crystal alignment, the PCE is still lower with complicated fabricating procedures.27-29 Therefore, it is urgency to exploit a facile technique to simultaneous improve stability and efficiency of 3D PSCs using 2D perovskite assistant strategy.


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Herein, we explore a simple route to modify the grain boundary by in situ growing long alkyl chain phenethylammonium iodide (PEAI). The 2D PEAI-based perovskite nanoplates would generate and vertically insert between the grain boundaries after reaction. It is found that the PEAI nanoplates can suppress the moisture permeation through the grain boundaries, resulting in high humidity durability. The FA0.95Cs0.05PbI3 film with PEAI nanoplates displays no phase transition when it placed in ~85% humidity for 500 hours, while the film without PEAI nanoplates shows serious phase transition under same condition for only 2 hours. Its thermal tolerance is also enhanced by PEAI nanoplates due to the good thermal stability for section of 2D perovskites. Meanwhile, the perovskite film with PEAI nanoplates displays the good crystal orientation, and the PEAI nanoplates can effectively suppress the non-radiative charge recombination at grain boundaries. As a result, the high quality of FA0.95Cs0.05PbI3 film is achieved, and its champion PCE is up to 20.34% with superior humidity and thermal stability. 2. EXPERIMENT DETAILS

2.1 Materials

Unless stated otherwise, all materials were acquired from Sigma-Aldrich and used as received without further purification. Hydroiodic acid (15 mL, 57 wt% in water) and FA acetate


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(99%, 7.5 g) were mixed into a 100 mL round-bottom flask under 0 °C, then the mixed solution was stirred for 2 h. After reaction, the solvents were removed at 60 °C using a rotary evaporator system, and the precipitate was dissolved by absolute ethanol. The powder was gained when added appropriate diethyl ether. Repeated above procedures until obtain white samples. Finally, FAI powder was put in a vacuum oven under 60 °C to dry for about 24 h. PEAI was synthesized using hydroiodic acid (57 wt% in water) and phenylethylamine (40 mmol). 15 mL hydroiodic acid was dropped into 5 mL phenylethylamine ethanol solution under 0 °C, and the solution was stirred until a colorless precipitate formed. The solid samples were filtered by Buchner funnel, and repeatedly washed with diethyl ether. Finally, the PEAI powders were obtained, and dried them in a vacuum oven at 60 °C for about 24 h. 2.2 Device fabrication

Fluorine-doped tin oxide (FTO) glasses were sequentially washed using acetone, isopropyl alcohol, and ethanol, then dried them by flowed nitrogen. All substrates were treated by UVozone for 15min. A compact TiO2 coaters were deposited on the treated FTO substrates according to a previous reported.30 The 1.3 mol/L FA0.95Cs0.05PbI3 precursor solution was obtained using a mixed solvent (gamma-butyrolactone (GBL):dimethyl sulfoxide (DMSO) = 7:3). The different contents of PEAI were added into FA0.95Cs0.05PbI3 precursor solution just before spin-coating. The FTO/TiO2 substrates were transferred into glove box at room temperature, and 80 μL of precursor solution was dropped onto the substrates. The spin-coating process immediately started with a speed of 1000 rpm and 4000 rpm for 10 s and 40 s, respectively. 200 μL anhydrous chlorobenzene was poured down to the substrate 20 s before the process end of 20 s. Different perovskite samples were annealed at 100 °C, 150 °C and 170 °C


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for

30

min.

Finally,

90

mg

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2,2',7,7'-tetrakis-(N,N-di-4-methoxyphenylamino)-9,9'-

spirobifluorene (spiro-OMeTAD) with 36 μL 4-tert-butylpyridine, 22 μL Li-TFSI solution (520 mg/mL in acetonitrile) was spin-coated on perovskite films at 5000 rpm for 30 s. Finally, the devices were covered with 80 nm-thick gold by using thermal evaporation system. 2.3 Characterization

X-ray diffraction (XRD) patterns were achieved by a SmartLab XRD system in an angle range from 5° to 50°. The scanning electron microscope (SEM) images for morphology of the perovskite films were gained by a HITACHI SU-8020 equipment. The absorption spectra were measured using a UV-Vis spectroscopy (UV-Lambda 950). The photoluminescence (PL) (excitation at 510 nm) and time-resolved PL (TRPL) spectra were measured with FLS980 spectrometer (Edinburgh instruments Ltd.). Photocurrent density-voltage (J-V) curves of devices without any encapsulation were measured in air by a solar simulator under AM 1.5G (100 mW/cm2), which adjusted using a reference silicon cell with KG5 filtered. The scan rate was 20 mV/s and no light soaking preconditioning was applied before measurement. was used to measure The external quantum efficiency (EQE) was accompanied by a QTest Station 2000ADI system (Crowntech Inc.) with the tungsten-halogen lamp for power of 150 W. 3. RESULTS AND DISCUSSION

The perovskite film with PEAI was fabricated by one-step deposition method, and the preparation process is shown in Figure S1. First, the precursor perovskite solution with PEAI was dropped on the substrate, and then spin-coated. Finally, the film was annealed at 170 °C for 30 min, and the details can be seen in experiment section. Figure 1a shows the XRD patterns of


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FA0.95Cs0.05PbI3 films without and with different contents of PEAI. Compared to pristine perovskite, both (110) and (220) characteristic peaks located at 14.03° and 28.40° are significantly improved after adding 0.1 mol/L PEAI. The peaks intensity of perovskite with PEAI increases about 100 times than that of perovskite without PEAI, as shown in Figure S2, suggesting an incredibly improved crystallinity. When the concentration of PEAI is up to 0.5 mol/L, the characteristic peaks of 2D perovskite appears in the small angle range, which would lead to relative inferior properties of carrier transport due to quantum well and large exciton binding energy in 2D perovskite.31,32 Therefore, the following discussions focus on perovskite with 0.1 mol/L PEAI additive.

Figure 1. XRD patterns of FA0.95Cs0.05PbI3 films (a) with different PEAI concentrations at 170 °C for 30 min, (b) 0.1 mol/L PEAI at various temperature for 30 min, and (c) without PEAI at different temperature for 30 min. Figure 1b and c give the XRD patterns of FA0.95Cs0.05PbI3 films with and without 0.1mol/L PEAI annealed at various temperatures for 30 min. It is found that a yellow δ-phase of


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FA0.95Cs0.05PbI3 was produced in both samples fabricated at room temperature, as indicated by the peak at 11.8°, which contributes to a classic P63mc for a hexagonal non-perovskite structure.33 However, compared to pristine perovskite, the α-phase (P3m1 trigonal perovskite phase)34 of FA0.95Cs0.05PbI3 film with 0.1 mol/L PEAI additive can be detected, indicating that the addition of PEAI is conducive to efficiently suppressing the phase transition. Figure 1c reveals that the δ-phase is still residual even the annealed temperature increased to 100 °C. Interestingly, the δ-phase in FA0.95Cs0.05PbI3 with PEAI disappears under same condition, further demonstrating that the PEAI can efficiently induce to form α-phase. Furthermore, the peak at 12.6° apportioned to PbI2 for the pristine FA0.95Cs0.05PbI3 appears after annealing at 170°C for 30 min, but the perovskite with 0.1 mol/L PEAI does not show any phase transition, indicating that PEAI can also enhance the thermal stability of FA0.95Cs0.05PbI3 film. The grazing incidence wide-angle X-ray scattering (GIWAXS) measurement was also performed by synchrotron radiation to gain insight into the orientation of the perovskite films. Figure S3a shows that the pristine FA0.95Cs0.05PbI3 film exhibits unbroken diffraction rings, demonstrating random orientation, in other words, the crystal orientation of perovskite film without PEAI is not only in in-plane (qxy) but also in out-of-plane (qz). In contrast, the FA0.95Cs0.05PbI3 film with PEAI displays discontinuous arcs accompanied by discrete Bragg spots (Figure S3b), demonstrating well-ordered crystalline orientation. Furthermore, the scattering pattern displays higher scattering intensity along qz, demonstrating that the crystal orientation is stacked in out-of-plane direction. The better orientation of perovskite film would be beneficial to improving device performance. From above results, it can be seen that the crystallinity, orientation and thermal stability of FA0.95Cs0.05PbI3 perovskite film significantly positive influence when introduce efficient PEAI additive, that may provide a promising way to


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fabricate perovskite devices with high efficiency and especially high stability for real application. Recent reported demonstrates that the humidity stability of perovskite materials has become one of the major hindrances for the commercialization of PSCs. The grain boundary in perovskite film is more defenseless to react with water molecules much than the grain surface, as shown in Figure 2a. Therefore, we plan to use the PEAI additive to modify the perovskite film by gradually forming nanoplate crystallites perovskite on grain boundary to prevent moisture permeation, resulting in high humidity stability. As shown in Figure 2b, the long chain PEAI based perovskite can function as a mutual between the adjacent grain margins, efficiently preventing the water permeate into the crack existing at the grain boundary.


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Figure 2. Schematic illustration of the interaction with moisture and the FA0.95Cs0.05PbI3 perovskite film (a) without and (b) with PEAI. Top-view SEM images of FA0.95Cs0.05PbI3 perovskite films (c) without and (d) with PEAI deposited on glass substrate. Cross-sectional SEM images of the completed devices using FA0.95Cs0.05PbI3 absorber layer (e) without and (f) with PEAI.


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Figure 2c and d show the top-view SEM images of perovskite films without and with PEAI deposited on the substrates, respectively. After PEAI was added in the precursor and the obtained perovskite films was treated at 170 °C for 30 min, we clearly observed the appearance of nanoplate crystallites between boundary grains, with the grain size approximate to that of the perovskite crystal grain. However, when fabricated at 150 °C for 30 min, there is no obvious flaky crystal, except something like mucosa at the grain boundary (seen in Figure S4). We hypothesized that the mucous membranes attributed to PEAI predecessor of the blocky flat crystals, and existed at the boundary. While the higher temperature finally facilitate the formation of the nanoplate crystallites to fill in the crack caused by the boundary, efficiently reducing the grain boundary. It also can be seen that the FA0.95Cs0.05PbI3 thin film without PEAI in the precursor exhibits mean grain size of ~ 0.62 µm (Figure 2c). However, the FA0.95Cs0.05PbI3 film with PEAI exhibits mean grain size of ~1.2 µm (Figure 2d), providing another powerful evidence that the PEAI can efficiently reduce the grain boundary. In addition, the cross-sectional SEM images are given in Figure 2e and 2f, it is clear that multiple PEAI-based perovskite plate crystals inserted among the original perovskite film, in accordance with the results shown in Figure 2d. The nanoplates preferentially orientated in the perpendicular direction to the substrate, which efficiently suppress non-radiative charge recombination, significantly beneficial to the device performance. For the FA0.95Cs0.05PbI3 perovskite films with PEAI, the PEAI cations expel from the perovskite crystalline grain as crystallization proceeds, but appear to act as a flux, accelerating crystal growth. More importantly, the nanoplate crystallites can prevent the intrusion of moisture into the grain boundaries, improving the humidity stability of perovskite films and therefore the PSCs.


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Figure 3. (a) UV-Vis absorption and steady-state PL spectra, and (b) TRPL spectra of FA0.95Cs0.05PbI3 films with and without PEAI. Figure 3a shows the UV-vis absorption and steady state PL spectra of FA0.95Cs0.05PbI3 films without and with PEAI. It can be seen that they have very similar absorption characteristic over the whole visible range, indicating that the PEAI modification does not change the bandgap and coefficient of the perovskite. Besides, the intensity of PL peak for FA0.95Cs0.05PbI3 with PEAI displays about one order of magnitude stronger than pristine perovskite film. We contribute the huge difference to the improved crystalline quality of the PEAI engineered perovskite films, leading to intrinsic higher PL yield. On the other hand, it can be also explained that the 2D


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perovskite based on PEAI exists a strong exciton binding energy, which leads to long life of free carriers, resulting in higher PL intensity.35 The quality of perovskite film can be reflected by the carrier recombination lifetime measured by TRPL.36 Figure 3b shows the TRPL spectra of FA0.95Cs0.05PbI3 without and with PEAI. By fitting the TRPL curves with bi-exponential equation, two decay lifetimes were observed for both perovskite films, and fitting parameters are summarized in Table S1. Generally, the fast decay lifetime (τ1) is qualified to the non-radiative recombination at grain boundary, while the long decay lifetime (τ2) is due to the recombination at surface. It is clear that the τ1 dominates the PL decay in perovskite film without PEAI, indicating that serious nonradiative recombination. However, the proportion of τ1 is rapidly reduced after adding PEAI, demonstrating that the non-radiative recombination is significantly decreased. In addition, the longer decay lifetime commonly means the better material quality and optoelectronic performance. The average carrier lifetime (τave) is calculated by eq (1):17

𝜏𝑎𝑣𝑒 =

∑𝐴𝑖𝜏2𝑖 ∑𝐴𝑖𝜏𝑖

(1)

where Ai and τi are a decay amplitude and a decay time, respectively. It is apparent that τave of perovskite film with PEAI obviously prolongs to 38.96 ns from 5.85 ns, further demonstrating the efficient reduced recombination in the perovskite film. The excellent crystallinity, orientation, optical properties and environmental stability of perovskite films inserted by PEAI based nanoplates at grain boundaries stimulate us to fabricate the photovoltaic devices. The J-V curves of the best PSCs based on FA0.95Cs0.05PbI3 films with PEAI are shown in Figure 4a, measured at a simulated AM 1.5G with reverse and forward


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scanning directions. The record a PCE of 20.34% is obtained with an open circuit voltage (Voc) of 1.07 V, short-circuit current density (Jsc) of 24.66 mA cm−2 and fill factor (FF) of 0.771 in reverse scan, and a PCE = 19.57% with Voc = 1.07 V, Jsc = 24.68 mA cm−2 and off = 0.741 in forward scan. For comparison, we fabricated devices using the conventional FA0.95Cs0.05PbI3 without PEAI and obtained a champion PCEs of 18.17% (reverse) and 16.62% (forward) with wide distribution (Figure S5).

Figure 4. (a) J-V characteristics for champion device based FA0.95Cs0.05PbI3 with PEAI scanned at both reverse and forward directions. (b) EQE and corresponding integrated current density, and (c) PCE distribution for PSCs with PEAI. (d) Steady state output at a maximum power point (0.90 V) for the champion device.


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From above results, it is clear that the hysteresis is significantly reduced when using PEAI. Generally, the hysteresis in PSCs is contributed to ion migration, trap density, imbalance charge transport between the hole transport layer and the electron transport layer.37,38 In present work, the trap density would be decreased, because PEAI-based nanoplates successfully insert between grain boundary to passivate the trap at grain boundary, and grain size also increases for perovskite film with PEAI. The hysteresis in PSCs based on FA0.95Cs0.05PbI3 films with PEAI is thereby suppressed. Furthermore, the smaller hysteresis presented in perovskite devices with PEAI is likely caused by imbalanced charge transport between spiro-OMeTAD (the hole mobility is about 10-3 cm2 V-1 s-1) hole transport layer and TiO2 (the electron mobility is about 10-4 cm2 V-1 s-1) electron transport layer.30 Figure 4b shows the EQE over the spectral range from 300 to 900 nm. The integrated current density from the EQE spectrum is calculated to be 22.95 mA cm−2 for FA0.95Cs0.05PbI3 film with PEAI, very close to the J-V measurements. Meanwhile, the EQE of PSCs without PEAI is shown in Figure S5c, and the integrated current density is 21.79%, lower than that of PSCs with PEAI. In fact, these devices demonstrate excellent reproducibility with an average efficiency of 17.82% over 50 devices, as illustrated through the statistical distribution presented in Figure 4c. With a fixed bias of 0.90 V on the device, a stable photocurrent density of 21.98 mA cm−2 is shown in Figure 4d, corresponding to a 19.70% of stabilized output PCE, which is consistent with the value from the J-V curve, indicating that the PCEs are near the real performance.


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Figure 5. Humidity stability of FA0.95Cs0.05PbI3 devices (a) without and (b) with PEAI under 85%-humidity for different times. Thermal stability of PSCs (c) without and (d) with PEAI annealed at 100 °C for different times. As we all known, the device stability is one of the most key factors to decide the real application. In order to detect the influence of the PEAI on the humidity and thermal stability, we investigated the performance of the devices stored in different moisture and temperature for different time intervals. Firstly, we stored the devices using different absorb layers with and without PEAI in a cabinet with normal indoors illumination and ~85% humidity to monitor the phase transition process. Figure 5a and 5b shows the J-V curves of pristine PSC and the device with PEAI exposed under ~85% humidity for different times, and the J-V key parameters list in


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Table S2. It can be found that the device without PEAI was severely destructed by the moisture, and the PCE dropped to 4.52% from 17.34% under ~85% humidity for only 2 hours. Amazingly, the PCE remain about 85% of its initial value for the device with PEAI even after stored in the same condition for a longer time of 200 hours. To further insight the reason for good humidity stability of PSCs with PEAI, corresponding XRD measurements of FA0.95Cs0.05PbI3 films with and without PEAI are carried out, as shown in Figure S6. It is clear that the α-phase of pristine perovskite almost transferred to yellow δ-phase under ~85% humidity for 2 hours, while it is no change in perovskite film with PEAI under same condition for 200 hours, in good coincidence with the humidity stability measurements mentioned above. In addition, we also measured the thermal stability of the PSCs based on various absorbers. Figure 5c and 5d demonstrate the J-V curves of the devices without and with PEAI stored in dark and different time period at 100 °C, and the parameters are shown in Table S3. Obviously, the efficiency of PSCs without PEAI sharply dropped and continually decreased to only 8.50% after 500 hours. In sharp contrast, PSCs with PEAI really show excellent performance that no obvious phase transition was observed with the increased store time at 100 °C. Surprisingly, PCE still remained 16.58% after stored for 500 hours. From their corresponding XRD (Figure S7), it can be seen that the perovskite without PEAI exhibits severe phase transition when the annealed time exceeded 200 hours, however, the perovskite with PEAI displays a small amount of decomposition even the annealed time up to 500 hours. The nanoplate crystallites with high crystalline quality between grain boundaries realized by the PEAI


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engineering strategy are considered to be the most important reason of the observed high device stabilities. 4. CONCLUSION

In conclusion, we have demonstrated the PEAI engineering strategy for conveniently fabricating perovskite films with high crystalline quality through a simple one-step solution process. The high crystalline, good electrical property and perfect morphology of perovskite film can simultaneously enhance the PCE and durability of PSCs. By using the simple and widely used FA0.95Cs0.05PbI3, the PCE of PSCs with PEAI is up to as high as 20.34%. More importantly, the PEAI engineering can significantly improve the humidity and thermal stability of perovskite devices. The unprecedented stability by the high-performing devices suggests a central role of the perovskite crystallization quality in photovoltaic applications. ASSOCIATED CONTENT

Supporting Information

The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Illustrate the fabrication process of FA0.95Cs0.05PbI3 film with PEAI, XRD patterns, GIWAXS images, SEM image, PCE distribution, J-V curves and EQE spectra.


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AUTHOR INFORMATION

Corresponding Author *Prof. Dong Yang, [email protected]

*Prof. Shengzhong (Frank) Liu, [email protected]

Notes The authors declare no competing interest.

ACKNOWLEDGMENT The authors acknowledge support from the National Natural Science Foundation of China (61604090/91733301), the National Key Research and Development Program of China (2016YFA0202403), the Shaanxi Technical In-novation Guidance Project (2018HJCG-17), the National University Re-search Fund (GK261001009), the Innovative Research Team (IRT_14R33), the 111 Project (B14041) and the Chinese-National-1000-Talent-Plan program. D. Yang acknowledges the financial support from NSFI/UCRC: Center for Energy Harvesting Materials and Systems (CEHMS). S. Priya acknowledges the financial support from Air Force office of Scientific Research under award number: FA9550-17-1-0341. REFERENCES


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In Situ Grain Boundary Modification via Two-Dimensional Nanoplates

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