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2D perovskite stabilized phase-pure formamidinium perovskite solar cells

2D perovskite stabilized phase-pure formamidinium perovskite solar cells ARTICLE DOI: 10.1038/s41467-018-05454-4 OPEN 2D perovskite stabilized phase-pure formamidinium perovskite solar cells 1 1 1 1 1 1 Jin-Wook Lee , Zhenghong Dai , Tae-Hee Han , Chungseok Choi , Sheng-Yung Chang , Sung-Joon Lee , 1 1 1 1 1 Nicholas De Marco , Hongxiang Zhao , Pengyu Sun , Yu Huang & Yang Yang Compositional engineering has been used to overcome difficulties in fabricating high-quality phase-pure formamidinium perovskite films together with its ambient instability. However, this comes alongside an undesirable increase in bandgap that sacrifices the device photo- current. Here we report the fabrication of phase-pure formamidinium-lead tri-iodide per- ovskite films with excellent optoelectronic quality and stability. Incorporation of 1.67 mol% of 2D phenylethylammonium lead iodide into the precursor solution enables the formation of phase-pure formamidinium perovskite with an order of magnitude enhanced photo- luminescence lifetime. The 2D perovskite spontaneously forms at grain boundaries to protect the formamidinium perovskite from moisture and suppress ion migration. A stabilized power conversion efficiency (PCE) of 20.64% (certified stabilized PCE of 19.77%) is achieved with a −2 short-circuit current density exceeding 24 mA cm and an open-circuit voltage of 1.130 V, corresponding to a loss-in-potential of 0.35 V, and significantly enhanced operational stability. Department of Materials Science and Engineering and California NanoSystems Institute, University of California, Los Angeles, CA 90095, USA. Correspondence and requests for materials should be addressed to Y.Y. (email: [email protected]) NATURE COMMUNICATIONS | (2018) 9:3021 | DOI: 10.1038/s41467-018-05454-4 | www.nature.com/naturecommunications 1 1234567890():,; ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-05454-4 remendous attention has been focused on hybrid per- temperature. However, the steady-state PCE was as low as 14.5%. ovskites (PVSK) since the first development of the solid- Several attempts have been made to utilize such approach, where 1–4 Tstate PVSK solar cell in 2012 . Rapid progress in power impressive improvements in performance and stability have been 21–25 conversion efficiency (PCE) has been achieved via compositional demonstrated . However, their performance and stability are and process engineering. As of 2017, the state-of-the-art PVSK still relatively poor comparing with those of MAPbI or mixed- solar cell achieved a certified PCE of 22.7%, which is on par with cation-halide PVSK solar cells. 5–12 well-established silicon solar cells . Here we report a method to fabricate high-quality stable Typical PVSK absorbers employ 3D ABX structures, where a FAPbI PVSK films using 2D PVSK. Incorporation of 2D phe- 3 3 monovalent ‘A-site’ cation in the cubo-octahedral site bonds with nylethylammonium lead iodide (PEA PbI ) PVSK into precursor 2 4 the BX octahedra. Compositional engineering has been con- solution enables the formation of phase-pure FAPbI films with a 6 3 sidered an important approach to enhance the stability and tenfold enhancement in photoluminescence (PL) lifetime. The 2D performance of PVSK solar cells. Important milestones have been PVSK is spontaneously formed at the grain boundaries of FAPbI achieved through compositional engineering. For example, to protects the FAPbI from moisture and assists in charge incorporation of the formamidinium (FA) cation into the ‘A-site’ separation/collection. Thanks to the superior optoelectronic has enabled the formation of a cubic FAPbI phase with a lower quality, we were able to fabricate a PVSK solar cell with a sta- bandgap (E ) of 1.48 eV, higher absorption coefficient and longer bilized efficiency of 20.64% (certified stabilized efficiency of carrier diffusion lengths than methylammonium (MA)-based 19.77%). Notably, the PVSK solar cell shows a peak V of 1.130 OC 6,7,13 tetragonal MAPbI (E = 1.57 eV) . However, FAPbI has V, corresponding to a loss-in-potential of 0.35 V considering the 3 g 3 poor ambient stability because its non-PVSK hexagonal phase is E of 1.48 eV versus 0.39 V for mixed-cation-halide perovskite thermodynamically more favorable than the cubic phase at room solar cells . Furthermore, the device demonstrates significantly temperature. Partial substitution of FA and I with MA and/or Br enhanced ambient and operational stability. has enabled fabrication of phase-pure FAPbI with improved performance and stability . Recently, incorporation of smaller inorganic ‘A’ cations, such as Cs and Rb, has further improved Results the stability and PCE of the PVSK solar cells with the lowest Effects of 2D perovskite on phase purity of FAPbI . FAPbI 3 3 14,15 open-circuit voltage (V )deficit of 0.39 V . As a result, films were prepared by the modified adduct method, in which N- OC 26,27 typical high efficiency devices nowadays incorporate PVSK with methyl-2-pyrrolidone (NMP) was used as a Lewis base .To FA, MA, Cs, Rb, and Br having relatively larger E than 1.60 g the PVSK precursor solution, 2D PVSK (PEA PbI ) precursors 2 4 15,16 eV . However, such compositional engineering has enhanced with different molar ratios ranging from 1.25 to 10 mol% were the V and stability at the expense of short-circuit current OC added. The steady-state PL spectra of the films were measured density (J ) due to increased E . Utilization of pure FAPbI is and are shown in Supplementary Fig. 1. As seen in Fig. 1a and SC g 3 desired in regards to its lower E , which is close to the optimum g Supplementary Fig. 1, we observed no obvious changes in PL value for a single junction solar cell suggested by the detailed peak position until the amount of 2D PVSK reached 10 mol%. balance limit . However, no efficient method has been developed With 10 mol% PVSK, the PL peak was blue-shifted by 6 nm. The so far to fabricate a high quality phase-pure FAPbI film and blueshift of the PL peak might be due to formation of a quasi-3D device. PVSK, where charge carriers are confined by large potential Recently, the manipulation of surface energy has been pro- barrier originated from the 2D PVSK . Based on this observa- posed as a means to stabilize metastable PVSK phases such as tion, we presume the added 2D PVSK does not result in the 18–20 CsPbI and FAPbI . Swarnkar et al. reported ambient stable formation of the quasi-3D PVSK if it remains below a certain 3 3 α-CsPbI in the form of a colloidal quantum dot, in which the threshold. This threshold was found to be lower than 10 mol%, contribution of surface energy significantly increases due to the where this quantity was then optimized based on photovoltaic high surface-to-volume ratio . Very recently, Fu et al. reported performance (Fig. 1a and Supplementary Fig. 2). A planar het- that the cubic FAPbI phase can be stabilized by functionalizing 3 erojunction architecture consisting of Indium doped SnO (ITO) the surface with large-sized organic molecules . They demon- glass/compact-SnO /PVSK/spiro-MeOTAD/Ag or Au was uti- strated that the functionalized surface contributes to lower for- lized for construction of PVSK solar cells in this study (cross- mation energy to stabilize the cubic FAPbI phase at room sectional scanning electron microscopic (SEM) images of the ab c 3.0 FAPbl w/ 1.67 mol% PEA Pbl 3 FAPbl 2 4 w/ 1.67 mol% PEA Pbl α 2.5 2 4 1.2 1.0 2.0 3D 0.8 1.5 α # # 1.5 1.0 0.4 0.5 quasi-3D * 700 800 900 0.0 Wavelength (nm) 750 0.0 1.0 02 46 8 10 10 20 30 40 400 500 600 700 800 900 2D perovskite (mol%) Two theta (degree) Wavelength (nm) Fig. 1 Crystallographic and absorption properties. a Peak position for steady-state photolumimescence (PL) spectrum and normalized power conversion efficiency (PCE) of the devices for FAPbI perovskite with different amount of added 2D PEA PbI perovskite. The error bar of the normalized PCE indicates 3 2 4 standard deviation of the PCEs. At least 10 devices were fabricated for each condition. b X-ray diffraction patterns, c absorption and normalized PL spectra of bare FAPbI and FAPbI with 1.67 mol% PEA PbI 2D perovskite. Inset of c shows onset region of the absorption spectra with linear approximation (solid 3 3 2 4 lines) 2 NATURE COMMUNICATIONS | (2018) 9:3021 | DOI: 10.1038/s41467-018-05454-4 | www.nature.com/naturecommunications PL peak position (nm) Normalized PCE (%) intensity (count) Absorbance (a.u.) Normalized PL intensity NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-05454-4 ARTICLE device are shown in Supplementary Fig. 3). The addition of 1.67 which the XRD peaks were slightly shifted toward higher angles mol% 2D PVSK was found to be optimal for the photovoltaic with the addition of relatively smaller amounts of 2D PVSK (1.25, performance (ca. 11% improvement in PCE). Notably, addition of 1.67, 2.50, and 5.0 mol%). This indicates that the lattice constant 10 mol% 2D PVSK significantly degraded the PCE to lower than of FAPbI is reduced, likely due to compressive strain associated 1%, which might result from formation of quasi-3D PVSK as the with the added 2D PVSK. We speculate that the reduction in large potential barrier originating from 2D PVSK could hinder lattice constant can be also related to the enhanced phase purity the charge transport. of cubic FAPbI as it will have equivalent effects with X-ray diffraction patterns (XRD) of bare FAPbI and FAPbI incorporation of smaller ‘A’ site cations on the tolerance 3 3 with 1.67 mol% PEA PbI are shown in Fig. 1b. As can be seen, factor and thus enthalpy of formation . Lower angle peaks at 2 4 the bare FAPbI film contains hexagonal non-PVSK phase (δ- around 12° appear upon addition of 10 mol% 2D PVSK phase) while the PVSK film prepared with 1.67 mol% PEA PbI corresponding to the formation of quasi-3D PVSK (inset of 2 4 29 32 shows pure PVSK phase . Even smaller amount of 2D PVSK Supplementary Fig. 4f) . The pure phase PVSK film with (1.25 mol% PEA PbI ) effectively suppresses the formation of δ- 1.67 mol% PEA PbI shows enhanced absorption over all wave- 2 4 2 4 phase (Supplementary Fig. 4). Furthermore, the overall signal lengths (Fig. 1c) compared to the bare FAPbI film where the intensity and full-width-half-maximum (FWHM) were enhanced absorption onset is hardly changed (Insent of Fig. 1c). The with the addition of the 2D PVSK, indicating improved absorption onset is complemented by almost identical normalized crystallinity. We speculate that the added large phenylethylam- PL spectra, which indicates that the E was maintained. The monium molecules from 2D PVSK precursors interact with enhanced absorption as seen when the 2D PVSK was added is FAPbI crystals to facilitate formation of the cubic PVSK phase probably due to an enhanced phase purity of the FAPbI , with 3 3 during crystallization . Such a speculation is correlated with the partial contribution from an enhanced light scattering owing to observation in the XRD measurements in Supplementary Fig. 4, the improved crystallinity . The absorption spectra with in which the signal intensity and FWHM of XRD peaks are different amounts of 2D PVSK are demonstrated in Supplemen- systematically enhanced with increased amounts of the added 2D tary Fig. 7. While all the PVSK films with 2D PVSK showed PVSK (Supplementary Figs. 4, 5). The enhancement of preferred enhanced absorption compared to bare FAPbI films, a slight orientation along the (001) plane with increased 2D PVSK also blueshift of the absorption onset with decreases in absorption indicates the added precursors of the 2D PVSK functionalize the over the whole-wavelength region was identified with the specific crystal facet to change the surface energy during the addition of 10 mol% of 2D PVSK, which is correlated with the crystal growth . A closer inspection on the normalized X-ray blueshift of the steady-state PL spectrum that can be associated diffraction (XRD) patterns of the PVSK films with different with the formation of quasi-3D PVSK. amounts of added 2D PVSK (Supplementary Fig. 6) was taken to find any correlations between the added 2D PVSK and crystal structure of FAPbI . Interestingly, a systematic change in peak 3 Photoluminescence properties and photovoltaic performance. position was observed with different amounts of 2D PVSK for Steady-state and time-resolved PL profiles were investigated in ab c 5×10 16 Bare FAPbl (control) Bare FAPbl Bare FAPbl 3 3 w/ 2D PVSK w/ 2D PVSK w/ 2D PVSK 4×10 12 w/ 2D PVSK and Cs (target) w/ 2D PVSK and Cs w/ 2D PVSK and Cs 3×10 6 10 2×10 6 4 1×10 10 0 700 750 800 850 900 0 1 23 12 14 16 18 20 22 Time (μs) Wavelength (nm) PCE (%) d e f 30 25 100 25 20 80 20 20 RS / FS Control Control 15 60 15 Target 15 Target Control, V =0.78 V 10 40 10 J VOC PC Target, V =0.91 V SC 10 –2 FF (mA cm ) (V) (%) 24.23 1.048 0.646 16.41 (23.70) (1.044) (0.603) (14.91) 5 20 5 24.44 1.126 0.765 21.06 (24.46) (1.125) (0.740) (20.37) 0 0 0 0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 020 40 60 300 400 500 600 700 800 900 Voltage (V) Time (s) Wavelength (nm) Fig. 2 Photoluminescence properties and photovoltaic performance. a Steady-state and b time resolved PL spectra of the perovskite films incorporating bare FAPbI , FAPbI with 2D perovskite and FA Cs PbI with 2D perovskite. Gray solid lines in b are fitted lines for each curve. c Power conversion 3 3 0.98 0.02 3 efficiency (PCE) distribution of the devices incorporating the perovskites. All the devices were fabricated in same batch. d Current density–voltage (J–V) curves, e steady-state PCE measurement and f external quantum efficiency (EQE) spectra of perovskite solar cells incorporating bare FAPbI (control) and FA Cs PbI with 2D perovskite (target). Photovoltaic parameters of the highest performing devices are summarized in the table in d, in which the 0.98 0.02 3 values with and without parenthesis are from reverse (from V to J ) and forward scan (from J to V ), respectively OC SC SC OC NATURE COMMUNICATIONS | (2018) 9:3021 | DOI: 10.1038/s41467-018-05454-4 | www.nature.com/naturecommunications 3 PL intensity (count) –2 Current density (mA cm ) –2 PL intensity (count) Current density (mA cm ) PCE (%) EQE (%) Count –2 Integrated J (mA cm ) SC ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-05454-4 Fig. 2a, b. The steady-state PL intensity was largely enhanced process parameters, the average PCE of 20.05 ± 0.45% was 5 6 more than five times from 4.3 ×10 to 2.3 × 10 with addition of demonstrated over 74 devices (Supplementary Fig. 15). We 1.67 mol% PEA PbI into FAPbI film (Fig. 2a). The large obtained the peak V of 1.130 V with the target device 2 4 3 OC enhancement of PL intensity was attributed to a significantly (Supplementary Fig. 16) corresponding to a loss-in-potential of elongated PL lifetime as seen in Fig. 2b. The time resolved PL 0.35 V considering a E of 1.48 eV, which is the lowest V deficit g OC profiles were fitted to exponential decay, in which bi- and tri- reported to date for PVSK solar cells. One of the target devices exponential decay models were used for the bare and 2D PVSK was sent out for measurement in an independent laboratory and incorporated PVSK films, respectively (Supplementary Table 1). achieved a certified stabilized PCE of 19.77% (Supplementary The relatively fast decay component (τ around 3 ns) was Fig. 17). The current–voltage curve and EQE spectra matched assigned to charge carrier trapping induced by trap states formed well with those measured by our group (Supplementary Fig. 18). due to the structural disorder such as vacancy or interstitial The enhanced device performance with 2D PVSK is mainly due defects while much slower components (τ , τ ) were assigned to to improved FF and V , which can be attributed to improved 2 3 OC 34–37 free carrier radiative recombination . With addition of 1.67 phase purity and elongated carrier lifetime with reduced defect mol% 2D PVSK, proportion of the fast decay component (τ ) was density, facilitating carrier transport and reducing the charge decreased (from 51.8% to 46.5%) while τ significantly elongated recombination . The reduced non-radiative recombination loss from 78.5 ns to 148.7 ns, which indicates reduced defect density with 2D PVSK was also confirmed in devices by electrolumines- and enhanced charge carrier lifetime. We attributed such cence (EL) measurements in Supplementary Fig. 19, in which −1 −2 improvements to enhanced phase purity and crystallinity of maximum radiance (40.4 Wsr cm ) and EL EQE (0.49%) of FAPbI as observed from XRD measurements (Fig. 1b), which the target devices were significantly enhanced compared to those −1 −2 15 decreases the structural disorders at grain interiors and/or of the control devices (2.87 Wsr cm , 0.06%) . boundaries . Moreover, a new decay component (τ ) with a significantly long lifetime (>1 μs) appeared after addition of the 2D PVSK, which is likely related to the added 2D PVSK. As a Moisture stability and TEM analysis. Under ambient conditions, result, the average PL lifetime was enhanced by almost one order a cubic FAPbI PVSK phase is subject to undergo conversion to a of magnitude from 39.4 to 376.9 ns with addition of 2D PVSK. hexagonal non-PVSK phase, resulting in serious degradation in 8,14 During the optimization of the device, incorporation of 2 mol% photovoltaic performance . The phase transformation is even Cs was found to further enhance the performance and reprodu- accelerated under high-relative humidity . To evaluate the effects cibility of the devices without a noticeable change in E (Sup- of 2D PVSK incorporation on phase stability, we investigated plementary Figs. 8–12, see Supplementary Notes 1, 2 and 3 for changes in absorbance of the film under relative humidity (RH) of additional discussion on the optimization process and impacts of 80 ± 5%. Figure 3a shows photos of the PVSK film stored for 2 mol% Cs). With additional 2 mol% Cs, the fraction of τ was different time. Bare FAPbI film was almost completely bleached 1 3 further decreased, indicating a further decreased defect density, within 24 h whereas no obvious change in color was observed which was also observed in previous studies . Consequently, the from the films containing 2D PVSK both with and without Cs. steady-state PL intensity and average PL lifetime was further Figure 3b demonstrates the absorbance (at 600 nm) of the FAPbI enhanced, rationalizing the improved PCE with 2 mol% Cs films with and without 2D PVSK as a function of exposure time (Supplementary Table 1). It is worth noting that the PL lifetime (individual absorption spectra can be found in Supplementary was significantly reduced with 10 mol% of 2D PVSK due to for- Fig. 20). The absorbance of the bare FAPbI rapidly degraded mation of quasi-3D PVSK (Supplementary Fig. 13). during 24 h, while FAPbI films with 2D PVSK did not show The PCE distribution of the devices incorporating correspond- noticeable degradation within 24 h. With addition of 2 mol% Cs, ing the PVSKs is compared in Fig. 2c (distribution of photovoltaic the film also remained stable after 24 h. The color change of the parameters can be found in Supplementary Fig. 14). The average bare FAPbI film under high RH is due to its transformation to photovoltaic parameters are summarized in Supplementary the δ-phase as can be seen in XRD spectra in Supplementary Table 2. The average PCE of the bare FAPbI PVSK solar cells Fig. 21a whereas no detectable change in color for the films with was significantly enhanced by 13% from 15.95 ± 0.36% to 18.08 ± 2D PVSK is correlated with their neat XRD spectra without the δ- 0.52% with addition of 1.67 mol% PEA PbI . The average PCE phase (Supplementary Figs. 21b, c). The enhanced phase stability 2 4 was further enhanced to 19.16 ± 0.37% with 2 mol% of Cs under high RH implies that the possible ingression pathway of (Hereafter, the devices based on bare FAPbI are denoted as moisture in the PVSK film is passivated. Previously, we demon- control while the devices based on FA Cs PbI with 1.67 mol strated grain boundary engineering techniques using the adduct 0.98 0.02 3 % PEA PbI are denoted as target for convenience). Current approach, in which the additives had precipitated at grain 2 4 34,38 density and voltage (J–V) curves of the optimized control and boundaries if not incorporated into the lattice of PVSK .We target devices are demonstrated in Fig. 1d, in which the highest supposed that grain boundaries within the film, which have been −2 PCE of the target device reached 21.06% (J : 24.44 mA cm , reported to be ingression pathways for moisture, might be pas- SC V : 1.126V, FF: 0.765) while a PCE of 16.41% was achieved with sivated by the added 2D PVSK . OC −2 the control device (J : 24.23 mA cm , V : 1.048V, FF: 0.646). Indeed, the vertically aligned 2D PVSK was sparsely observed SC OC A stabilized PCE of 20.64% was achieved with the target device from SEM images in Supplementary Fig. 10b, c with addition of while that of control device was 15.80% (Fig. 2e). External 2D PVSK (see also Supplementary Fig. 22 and Supplementary quantum efficiency (EQE) spectra of the devices were compared Note 4 for additional discussion). However, the enhanced −2 in Fig. 2f. An integrated J of 23.9 mA cm from the target moisture stability throughout the whole-film implies that the SC device was well-matched with the value measured from the J–V 2D PVSK probably exist along the grain boundaries. To confirm scan (<5% discrepancy), while control device shows that of 21.2 our assumption, transmission electron microscopic (TEM) −2 mA cm with a relatively large discrepancy of 14%. The images of the FAPbI film with 2D PVSK was analyzed in relatively large discrepancy from the control FAPbI device is Fig. 3c–e. The inset of Fig. 3c shows a chunk of the polycrystalline probably due to a more pronounced hysteresis, as seen in Fig. 1d, film scratched off from the substrate. Several hundreds of which also results in a large discrepancy between the stabilized nanometer sized grains and their boundaries are clearly visible PCE and the PCE measured from the J–V scan. The performance from the image, and from which one of the grains is magnified in of control device was highly reproducible. With optimized Fig. 3c. The region (1) in Fig. 3c, which is the grain interior, was 4 NATURE COMMUNICATIONS | (2018) 9:3021 | DOI: 10.1038/s41467-018-05454-4 | www.nature.com/naturecommunications NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-05454-4 ARTICLE a b 2.0 FAPbl 1.5 Bare FAPbl w/ 2D PVSK w/ w/ 2D PVSK and Cs 1.0 2D PVSK 0.5 w/ RH80% 2D PVSK T=20 °C and Cs 0.0 0 10 20 0 h 4 h 8 h 12 h 16 h 20 h 24 h Time (h) cd e FFT 3.2 Å 500 nm 8.0 Å FFT 8.0 Å (2) 3.2 Å 100 nm 5 nm 10 nm (1) Fig. 3 Improved moisture stability with 2D perovskite at grain boundaries. a Photos of the perovskite films incorporating bare FAPbI , FAPbI with 2D 3 3 perovskite and FA Cs PbI with 2D perovskite exposed to relative humidity (RH) of 80 ± 5% at 20 ± 2 °C for different time. b Evolution of absorption 0.98 0.02 3 of the films at 600 nm under RH 80 ± 5% at 20 ± 2 °C. The error bar indicates standard deviation of the absorbance measured from three films for each condition. c–e Transmission electron microscopic (TEM) images of the FA Cs PbI film with 1.67 mol% PEA PbI . Inset of c demonstrates the lower 0.98 0.02 3 2 4 magnification image showing the polycrystalline nature with grain boundaries. The highlighted area (1) and (2) were investigated in d and e, respectively. Inset of d and e show Fast Fourier transform (FFT) analysis of the area within boxes, respectively magnified and analyzed using Fast Fourier transform (FFT) in and PEA PbI PVSK shows type I band alignment. Such band 2 4 Fig. 3d, in which an inter-planar spacing of 3.2 Å is well-matched alignment resembles the alignment between PVSK and PbI with the (002) reflection of cubic FAPbI (Supplementary formed at grain boundaries, which was found to reduce charge 40,41 Table 3). At region (2), which is grain boundary, the FFT recombination and assist in charge separation/collection . analysis revealed an inter-planar distance of 8.0 Å (Fig. 3e), Thus, analogous advantages of 2D PVSK at grain boundaries can correlating to a characteristic (002) reflection of 2D PEA PbI be expected. Conductive atomic force microscopy (c-AFM) was 2 4 (Supplementary Fig. 23 and Supplementary Table 4). This performed in Fig. 4c–f to see spatially resolved electrical prop- supports the presence of 2D PVSK at grain boundaries, which erties of the films. Under ambient light conditions (Fig. 4c, d), was further confirmed by elemental distribution (EDS) analysis current flow in the PVSK film with 2D PVSK was higher at/near (Supplementary Fig. 24). At grain boundary regions, relatively the grain boundaries while relatively uniform current flow was larger amounts of carbon and nitrogen were detected, which observed in the bare FAPbI film. With light illumination (Fig. 4e, could be due to presence of phenylethylammonium cation in the f), the current flow was further enhanced at/near the grain 2D PVSK. boundaries with 2D PVSK whereas current flow in bare FAPbI film was uniformly increased, which indicates charge separation and collection of photo-generated electrons is facilitated more so Band structure and electrical properties. A schematic in Fig. 4a at grain boundaries with 2D PVSK. As suggested for PbI , thin shows 2D PVSK formation at the grain boundaries of the 3D 2D PVSK regions at grain boundaries might suffer downward PVSK film. Since the 2D PEA PbI PVSK with aromatic rings and 2 4 band bending under illumination (dashed line in Fig. 4b) where longer alkyl chains is expected to be more resistant to moisture, it photo-generated electrons are transferred from grain interiors. protects the defective grain boundaries of 3D PVSK, resulting in Due to the high-potential barrier to the holes, charge recombi- significantly enhanced moisture stability of the film. Regardless of nation will be reduced, which might be the origin of the superior the improved stability, however, one can expect degraded elec- PL lifetime and photovoltaic performance with 2D PVSK. tronic properties of the film due to the poor charge carrier mobility of the 2D PVSK. We investigated the band structure of FA Cs PbI (with 1.67 mol% of 2D PVSK) and PEA PbI Ambient and operational stability. Finally, the stability of the 0.98 0.02 3 2 4 PVSK, which is illustrated in Fig. 4b. The valence band maximum control and target devices was compared. Figure 5a demonstrates was measured using ultraviolet photoelectron spectroscopy (UPS, the changes in PCE of the unencapsulated devices stored in a Supplementary Fig. 25), while the E was determined from Tauc desiccator (relative humidity lower than 30%, evolution of an plots (Supplementary Fig. 26). As seen in Fig. 4b, FA Cs PbI individual photovoltaic parameter can be found in 0.98 0.02 3 NATURE COMMUNICATIONS | (2018) 9:3021 | DOI: 10.1038/s41467-018-05454-4 | www.nature.com/naturecommunications 5 Absorbance (a.u.) ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-05454-4 a b Grain Grain Grain boundary FACsPbl PEA Pbl FACsPbl Au 3 2 4 3 3.51 Spiro- 4.22 MeOTAD PEA Pbl FACsPbl 2 4 SnO 5.70 5.87 2D PVSK at grain boundary Substrate Under illumination cd e f 1 μm 1 μm 1 μm 1 μm fA fA 1 μm 1 μm1 μm1 μm –100 –50 Fig. 4 Band alignment and local conductivity with 2D perovskite. a Schematics of the device incorporating polycrystalline 3D perovskite film with 2D perovskite at grain boundaries and b band structure of each layer in device analyzed by ultraviolet photoelectron spectroscopy (UPS) and Tauc plots. Conductive atomic force microscopic (c-AFM) images of (c, e) bare FAPbI and (b, d) with 2D perovskite films on SnO coated ITO glass. The 3 2 measurement was carried out with bias voltage of 100 mV under (c, d) room light or (d, f) low intensity light illumination provided by the AFM setup. Inset of each image shows corresponding topology of the films. Scale bar at left side is for (c) and (d) while at right side is for e and f 42,44 Supplementary Fig. 27). While the control device degraded by trapped charge carriers associated with it . We suppose that 29% for 1392 h, the target device maintained 98% of its initial migration of the charged defects or ions is possibly suppressed by efficiency during this time. The operational stability of the devices 2D PVSK at grain boundaries. The temperature-dependent con- was also compared by maximum power point (MPP) tracking ductivity (σ) measurement of the lateral devices was performed to under 1 sun illumination in Fig. 5b. Without encapsulation, the evaluate the activation energy for the ion migration (Fig. 6). The PCE of the control device rapidly degraded by 68% during 450 activation energy (E ) for the migration can be determined min whereas that of target device was relatively less (20%) during according to the Nernst-Einstein relation , the time. We performed 500 h of light exposure test with the encapsulated control (bare FAPbI device) and target devices (w/ 3 σ E 0 a σðÞ T ¼ expð Þ; 1.67 mol% 2D PVSK). The encapsulated devices were exposed to T kT −2 ca. 0.9 sun (90 ± 5 mW cm ) under open-circuit condition, of which the steady-state PCE was periodically measured for dif- ferent exposure time. As seen in Fig. 5c, both of the devices where k is Boltzmann constant, σ is a constant. Inset of Fig. 6a showed a rapid initial decay in PCE followed by slower decay with describes the structure of the lateral devices. With bare FAPbI an almost linear profile, which is in agreement with previous PVSK, exponential enhancement in conductivity was clearly reports . After 500 h of exposure, the control device degraded to identified at around 130 K (Fig. 6a), which is attributed to con- ca. 52.3% of its initial PCE whereas the target device maintained tribution of ions. The E for bare FAPbI film was calculated to be a 3 72.3% of the initial PCE, indicating enhanced stability with 0.16 eV, indicating significant contribution of activated ions at addition of 2D PVSK. We could extract tentative T80 (time at room temperature, which might cause degradation of the material which PCE of the device decays to 80% of initial PCE) for the and device under operational condition with built-in electric field. devices by fitting of the post-burn-in region in which the PCE of The pronounced current–voltage hysteresis behavior was the device shows an almost linear decay profile (after 48 h). The observed even at very low temperature (180 K, Fig. 6c). In case of T80 for control and target devices were calculated to be 592 h and the PVSK film with 2D PVSK, the film did not show noticeable 1362 h, respectively. This indicates the stability of the device was enhancement in conductivity with increased temperature; significantly improved with addition of 2D PVSK. We also per- although, the overall conductivity was relativity lower than the formed MPP tracking of the encapsulated target device under bare FAPbI film (Fig. 6c). Even with moderate light illumination, −2 1 sun (100 mW cm ) illumination in Supplementary Fig. 28.A it does not show the indicative of activated ions. As a result, the total of 18.7% of initial PCE was degraded for 130 h of operation, current–voltage curve did not show any hysteresis behavior which is relatively slower compared to the device maintained at (Fig. 6d). As the grain boundaries of 3D perovskite were reported open-circuit condition. This is correlated with previous studies to be a major pathway for the migration of ions , passivating the that attributed the faster degradation under open-circuit condi- grain boundaries by incorporation of the ion-migration-immune tion to larger number of photo-generated charge carriers 2D PVSK likely suppressed overall ion migration in the target 43 45 recombining within the device . Under operational condition device . In addition, the improved phase purity of the film might with abundant photo-generated charges and built-in electric field, also partially contribute to the suppressed ion migration because the major factors causing the degradation of the devices might be the secondary phase can generate defect sites that can act as an the highly mobile and reactive charged defects (ions) and/or additional pathway for ion migration. We believe the suppressed 6 NATURE COMMUNICATIONS | (2018) 9:3021 | DOI: 10.1038/s41467-018-05454-4 | www.nature.com/naturecommunications Sprio-MeOTAD 3D PVSK ITO/SnO 2 NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-05454-4 ARTICLE ab 20 1.0 0.8 0.6 10 Control Control Target Target 0.4 0.2 Under dark, <RH 30%, T =25 °C Under 1 sun, RH 50% T =40 °C 0.0 0 500 1000 1500 0 100 200 300 400 Time (h) Time (min) Control 1.0 Target 0.8 0.6 –2 Under light (90 mW cm ), RH 50%, T = 40 °C 0.4 0 100 200 300 400 500 Time (h) Fig. 5 Improved stability with 2D perovskite. a Evolution of power conversion efficiency (PCE) of control and target devices. The devices were stored under dark with controlled humidity. b Maximum power point tracking of the devices under 1 sun illumination in ambient condition without encapsulation. −2 c Evolution of the PCEs measured from the encapsulated control and target devices exposed to continuous light (90 ± 5 mW cm ) under open-circuit condition. The stabilized PCEs were measured at each time. Initial stabilized PCEs for control and target devices were 14.5% and 17.5%, respectively. The broken lines are linear fit of the post-burn-in region (after 48 h). Relative humidity (RH) and temperature (T) are indicated in the graphs for each measurement ion migration contributes to enhanced operational stability of the Methods Synthesis of phenylethylammonium iodide. In a typical synthesis, 4.8 g of phe- target device. nethylamine (39.6 mmol, Aldrich, >99%) was dissolved in 15 mL of ethanol and placed in iced bath. Under vigorous stirring, 10.8 g of hydroiodic acid (57 wt% in H O, 48.1 mmol, Sigma-Aldrich, 99.99%, contains no stabilizer) was slowly added to the solution. The solution was stirred overnight to ensure complete reaction, Discussion which was followed by removal of the solvent by a rotary evaporator. The resulting We demonstrated a reproducible way to fabricate phase-pure solid was washed with diethyl ether several times until the color is changed to formamidinium tri-iodide PVSK with high-optoelectronic quality white. The white solid was further purified by recrystallization in mixed solvent of and stability by incorporating 2D PVSK. The large phenylethy- methanol and diethyl ether. Finally, white plate-like solid was filtered and dried lammonium molecules from 2D PVSK precursors interact with under vacuum (yield around 90%). FAPbI crystals to facilitate formation of the cubic PVSK phase during crystallization, which subsequently functionalize the grain Device fabrication. Indium doped tin oxide (ITO) glass was cleaned with suc- boundaries after completion of the crystallization. The resulting cessive sonication in detergent, deionized (DI) water, acetone and 2-propanol for phase-pure PVSK film has an identical E (1.48 eV) to that of 15 min, respectively. The cleaned substrates were further treated with UV-ozone to pure FAPbI with an order of magnitude enhanced PL lifetime. remove the organic residual and enhance the wettability. A total of 30 mM SnCl ∙2H O (Aldrich, >99.995%) solution was prepared in ethanol (anhydrous, Average PCE of 20.05 ± 0.45% over 74 devices and the highest 2 2 Decon Laboratories Inc.), which was filtered by 0.2 μm syringe filter before use. To stabilized PCE of 20.64% (certified stabilized PCE of 19.77%) was form a SnO layer, the solution was spin-coated on the cleaned substrate at 3000 achieved. Regardless of its low E , the PVSK solar cell showed the rpm for 30 s, which was heat-treated at 150 °C for 30 min. After cooling down to peak V of 1.130 V, corresponding to the lowest loss-in- room temperature, the spin-coating process was repeated one more time, which OC was followed by annealing at 150 °C for 5 min and 180 °C for 1 h. The SnO coated potential of 0.35 V among all the reported PVSK solar cells. Owed 2 ITO glass was further treated with UV-ozone before spin-coating of PVSK solu- to the functionalized grain boundaries by the 2D PVSK, the phase tion. The PVSK layer was prepared by the modified adduct method . The bare stability of the film under high RH significantly improved and FAPbI layer was formed from the PVSK solution containing equimolar amount of migration of ions (or charged defects) was suppressed, resulting HC(NH ) I (FAI, Dyesol), PbI (TCI, 99.99%) and N-Methyl-2-pyrrolidone (NMP, 2 2 2 in significantly improved ambient and operational stability of the Sigma-Aldrich, anhydrous, 99.5%) in N,N-Dimethylformamide (DMF, Sigma- Aldrich, anhydrous, 99.8%). Typically, 172 mg of FAI, 461 mg of PbI and 99 mg of device. We believe our approach to utilize spontaneously formed NMP were added to 600 mg of DMF. For the 2D PVSK (PEA PbI ) and Cs 2 4 grain boundary 2D PVSK will provide important insights for the incorporated PVSK, corresponding amount of FAI was replaced with PEAI and research community to design PVSK materials to achieve record CsI. For example, FAPbI with 1.67 mol% PEA PbI PVSK was formed from the 3 2 4 PCEs accompanied by high stability and longevity. precursor solution containing 166.4 mg of FAI, 8.2 mg of phenylethylammonium NATURE COMMUNICATIONS | (2018) 9:3021 | DOI: 10.1038/s41467-018-05454-4 | www.nature.com/naturecommunications 7 PCE (%) Normalized PCE Normalized PCE ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-05454-4 ab 4 –4 –5 Au Au 0 –6 Dark 10 10 Light –7 –2 E =0.16 eV 10 a –8 –4 56 78 9 10 11 12 3 4 5 6 7 8 9 10 11 12 –1 –1 1000/T (K ) 1000/T (K ) cd –7 –4 180 K 180 K –6 –9 –8 –11 –10 –13 –12 –6 –4 –2 0 24 6 –6 –4 –2 0 2 4 6 Applied voltage (V) Applied voltage (V) Fig. 6 Suppressed ion migration with 2D perovskite. Temperature-dependent conductivity of a bare FAPbI film and b with 1.67 mol% 2D PEA PbI 3 2 4 −2 perovskite. Red circles in b indicate the data measured under moderate light illumination (intensity lower than 10 mW cm ). Current–voltage curves measured from the devices at 180 K. c Bare FAPbI film and d with 1.67 mol% 2D PEA PbI perovskite 3 2 4 iodide (PEAI), 453.4 mg of PbI and 97.4 mg of NMP in 600 mg of DMF. With 2 spectroscopic (UPS) analysis was carried out using Kratos Ultraviolet photoelec- mol% of Cs, the precursor solution was prepared by mixing 163.0 mg of FAI, 8.2 tron spectrometer. He I (21.22 eV) source was used as an excitation source. The mg of PEAI, 5.0 mg of CsI (Alfa Aesar, 99.999%), 453.4 mg of PbI and 97.4 mg of PVSK films were coated on ITO substrate and grounded using silver paste to avoid NMP in 600 mg. For the best performing devices in Fig. 2d, the amount of DMF the charging during the measurement. Conductive atomic force microscopic was adjusted to 550 mg. Spin-coating of PVSK and hole transporting layer was (AFM) measurement was performed by Bruker Dimension Icon Scanning Probe performed in a glove box filled with dry air. The PVSK solution was spin-coated at Microscope equipped with TUNA application module. The TUNA module pro- 4000 rpm for 20 s where 0.15 mL of diethyl ether (anhydrous, >99.0%, contains vides ultra-high tunneling current sensitivity (<1 pA) with high-lateral resolution. BHT as stabilizer, Sigma-Aldrich) was dropped after 10 s on the spinning substrate. Antomony doped Si tip (0.01–0.025 Ohm-cm) coated with 20 nm Pt-Ir was used as The resulting transparent adduct film was heat-treated at 100 °C for 1 min followed a probe. To avoid the electrically driven degradation during the measurement, low by 150 °C for 10 min. (for the best performing target device, the annealing con- bias voltage (100 mV) was applied. The measurement was carried out under either dition was adjusted to 80 °C 1 min followed by 150 °C for 20 min) The spiro- room right or low intensity light illumination provided by AFM setup. The MeOTAD solution was prepared by dissolving 85.8 mg of spiro-MeOTAD temperature-dependent conductivity measurement was carried out using a com- (Lumtec) in 1 mL of chlorobenzene (anhydrous, 99.8%, Sigma-Aldrich) which was mercial probe station (Lakeshore, TTP4) in which temperature of the device was doped by 33.8 μl of 4-tert-butylpyridine (96%, Aldrich) and 19.3 μl of Li-TFSI controlled by thermoelectric plate and flow of liquid nitrogen. The electrical −1 (99.95%, Aldrich, 520 mg mL in acetonitrile) solution. The spiro-MeOTAD measurement was conducted with a source/measurement unit (Agilent, B2902A). solution was spin-coated on the PVSK layer at 3000 rpm for 20 s by dropping 17 μl of the solution on the spinning substrate. On top of the spiro-MeOTAD layer, ca. Device characterization. Current density–voltage (J–V) curves of the devices were −1 100 nm-thick silver or gold layer was thermally evaporated at 0.5 Ås to be used measured using Keithley 2401 source meter under simulated one sun illumination as an electrode. −2 (AM 1.5G, 100 mW cm ) in ambient atmosphere. The one sun illumination was generated from Oriel Sol3A with class AAA solar simulator (Newport), in which light intensity was calibrated by NREL-certified Si photodiode equipped with KG-5 Material characterization. The PVSK layer was coated on a SnO coated ITO −1 filter. Typically, the J–V curves were recorded at 0.1 Vs (between 1.2 V and −0.1 substrate for the measurements. UV-vis absorption spectra were recorded by U- V with 65 data points and 0.2 s of delay time per point). During the measurement, 4100 spectrophotometer (Hitachi) equipped with integrating sphere. The mono- the device was covered with metal aperture (0.100 cm )todefine the active area. All chromatic light was incident to the substrate side. X-ray diffraction (XRD) patterns the devices were measured without pre-conditioning such as light-soaking and were obtained by X-ray diffractometer (PANalytical) with Cu kα radiation at a scan applied bias voltage. Steady-state power conversion efficiency was calculated by −1 rate of 4° min . Surface and cross-sectional morphology of the films and devices measuring stabilized photocurrent density under constant bias voltage. The were investigated by scanning electron microscopy (SEM, Nova Nano 230). For the external quantum efficiency (EQE) was measured using specially designed system cross-sectional image, cross-sectional surface of the sample was coated with ca. (Enli tech) under AC mode (frequency = 133 Hz) without bias light. For electro- 1 nm-thick gold using sputter to enhance the conductivity. Transmission electron luminescence measurement, a Keithley 2400 source meter and silicon photodiode microscopic (TEM) analysis was performed by Titan Krios (FEI). The PVSK film (Hamamatsu S1133-14, Japan) were used to measure Current–voltage–luminance was scratched off from the substrate and dispersed in toluene by sonication for characteristics of PVSK solar cells. Electroluminescence spectra were recorded by 10 min, which was dropped on an aluminium grid. Accelerating voltage of 300 kV Horiba Jobin Yvon system, and used to calculate radiance and external quantum was used for the measurement. Steady-state photoluminescence (PL) signal was efficiency of PVSK solar cells. All the devices were assumed as Lambertian emitter analyzed by a Horiba Jobin Yvon system. A 640 nm monochromatic laser was used in the calculation. as an excitation fluorescence source. Time resolved PL decay profiles were obtained using a Picoharp 300 with time-correlated single-photon counting capabilities. The films were excited by a 640 nm pulse laser with a repetition frequency of 100 kHz Stability test. Moisture stability of the films was tested by exposing the PVSK films provided by a picosecond laser diode head (PLD 800B, PicoQuant). The energy under relative humidity of 80 ± 5% and room light. Absorbance of the films was −2 density of the excitation light was ca. 1.4 nJ cm , in which carrier annihilation and measured every 2 h while XRD of the films were recorded every 12 h. For the non-geminate recombination are negligible . Ultraviolet photoelectron devices, ex-situ test was conducted by storing the devices in desiccator (relative 8 NATURE COMMUNICATIONS | (2018) 9:3021 | DOI: 10.1038/s41467-018-05454-4 | www.nature.com/naturecommunications PVSK Current (A) T (a.u.) Current (A) T (a.u.) NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-05454-4 ARTICLE humidity, RH <30%) under dark condition. The device was taken out and mea- 24. Zhou, Y. et al. 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Quantum dot-induced phase stabilization of α-CsPbI perovskite for high-efficiency photovoltaics. Science 354,92–95 (2016). 19. Fu, Y. et al. Stabilization of the metastable lead iodide perovskite phase via Acknowledgements surface functionalization. Nano Lett. 17, 4405–4414 (2017). This work was supported by the Air Force Office of Scientific Research (AFOSR, Grant 20. Zhang, T. et al. Bication lead iodide 2D perovskite component to stabilize No. FA9550-15-1-0333), Office of Naval Research (ONR, Grant No. N00014-17-1- inorganic α-CsPbI perovskite phase for high-efficiency solar cells. Sci. Adv. 3, 2484), National Science Foundation (NSF, Grant No. ECCS-EPMD-1509955), and e1700841 (2017). Horizon PV. 21. Li, N. et al. Mixed cation FA PEA PbI with enhanced phase and ambient x 1-x 3 stability toward high-performance perovskite solar cells. Adv. Energy Mater. 7, 1601307 (2017). 22. Wang, F. et al. Phenylalkylamine passivation of organolead halide perovskites Author contributions enabling high-efficiency and air-stable photovoltaic cells. Adv. Mater. 28, J.-W.L. conceived an idea and led overall project under supervision of Y.Y. J.-W.L. and 9986–9992 (2016). Z.D. fabricated devices and characterized the materials. T.-H.H. assisted the device 23. Wang, Z. et al. Efficient ambient-air-stable solar cells with 2D-3D fabrication and performed EL measurement. C.C. and Y.H. carried out TEM char- heterostructured butylammonium-caesium-formamidinium lead halide acterization. S.-Y.C. helped UPS measurement. S.L. performed temperature-dependent perovskites. Nat. Energy 2, 17135 (2017). conductivity measurement. H.Z. synthesized the PEAI. N.D.M. and P.S. NATURE COMMUNICATIONS | (2018) 9:3021 | DOI: 10.1038/s41467-018-05454-4 | www.nature.com/naturecommunications 9 ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-05454-4 commented on the manuscript. 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2D perovskite stabilized phase-pure formamidinium perovskite solar cells

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ARTICLE DOI: 10.1038/s41467-018-05454-4 OPEN 2D perovskite stabilized phase-pure formamidinium perovskite solar cells 1 1 1 1 1 1 Jin-Wook Lee , Zhenghong Dai , Tae-Hee Han , Chungseok Choi , Sheng-Yung Chang , Sung-Joon Lee , 1 1 1 1 1 Nicholas De Marco , Hongxiang Zhao , Pengyu Sun , Yu Huang & Yang Yang Compositional engineering has been used to overcome difficulties in fabricating high-quality phase-pure formamidinium perovskite films together with its ambient instability. However, this comes alongside an undesirable increase in bandgap that sacrifices the device photo- current. Here we report the fabrication of phase-pure formamidinium-lead tri-iodide per- ovskite films with excellent optoelectronic quality and stability. Incorporation of 1.67 mol% of 2D phenylethylammonium lead iodide into the precursor solution enables the formation of phase-pure formamidinium perovskite with an order of magnitude enhanced photo- luminescence lifetime. The 2D perovskite spontaneously forms at grain boundaries to protect the formamidinium perovskite from moisture and suppress ion migration. A stabilized power conversion efficiency (PCE) of 20.64% (certified stabilized PCE of 19.77%) is achieved with a −2 short-circuit current density exceeding 24 mA cm and an open-circuit voltage of 1.130 V, corresponding to a loss-in-potential of 0.35 V, and significantly enhanced operational stability. Department of Materials Science and Engineering and California NanoSystems Institute, University of California, Los Angeles, CA 90095, USA. Correspondence and requests for materials should be addressed to Y.Y. (email: [email protected]) NATURE COMMUNICATIONS | (2018) 9:3021 | DOI: 10.1038/s41467-018-05454-4 | www.nature.com/naturecommunications 1 1234567890():,; ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-05454-4 remendous attention has been focused on hybrid per- temperature. However, the steady-state PCE was as low as 14.5%. ovskites (PVSK) since the first development of the solid- Several attempts have been made to utilize such approach, where 1–4 Tstate PVSK solar cell in 2012 . Rapid progress in power impressive improvements in performance and stability have been 21–25 conversion efficiency (PCE) has been achieved via compositional demonstrated . However, their performance and stability are and process engineering. As of 2017, the state-of-the-art PVSK still relatively poor comparing with those of MAPbI or mixed- solar cell achieved a certified PCE of 22.7%, which is on par with cation-halide PVSK solar cells. 5–12 well-established silicon solar cells . Here we report a method to fabricate high-quality stable Typical PVSK absorbers employ 3D ABX structures, where a FAPbI PVSK films using 2D PVSK. Incorporation of 2D phe- 3 3 monovalent ‘A-site’ cation in the cubo-octahedral site bonds with nylethylammonium lead iodide (PEA PbI ) PVSK into precursor 2 4 the BX octahedra. Compositional engineering has been con- solution enables the formation of phase-pure FAPbI films with a 6 3 sidered an important approach to enhance the stability and tenfold enhancement in photoluminescence (PL) lifetime. The 2D performance of PVSK solar cells. Important milestones have been PVSK is spontaneously formed at the grain boundaries of FAPbI achieved through compositional engineering. For example, to protects the FAPbI from moisture and assists in charge incorporation of the formamidinium (FA) cation into the ‘A-site’ separation/collection. Thanks to the superior optoelectronic has enabled the formation of a cubic FAPbI phase with a lower quality, we were able to fabricate a PVSK solar cell with a sta- bandgap (E ) of 1.48 eV, higher absorption coefficient and longer bilized efficiency of 20.64% (certified stabilized efficiency of carrier diffusion lengths than methylammonium (MA)-based 19.77%). Notably, the PVSK solar cell shows a peak V of 1.130 OC 6,7,13 tetragonal MAPbI (E = 1.57 eV) . However, FAPbI has V, corresponding to a loss-in-potential of 0.35 V considering the 3 g 3 poor ambient stability because its non-PVSK hexagonal phase is E of 1.48 eV versus 0.39 V for mixed-cation-halide perovskite thermodynamically more favorable than the cubic phase at room solar cells . Furthermore, the device demonstrates significantly temperature. Partial substitution of FA and I with MA and/or Br enhanced ambient and operational stability. has enabled fabrication of phase-pure FAPbI with improved performance and stability . Recently, incorporation of smaller inorganic ‘A’ cations, such as Cs and Rb, has further improved Results the stability and PCE of the PVSK solar cells with the lowest Effects of 2D perovskite on phase purity of FAPbI . FAPbI 3 3 14,15 open-circuit voltage (V )deficit of 0.39 V . As a result, films were prepared by the modified adduct method, in which N- OC 26,27 typical high efficiency devices nowadays incorporate PVSK with methyl-2-pyrrolidone (NMP) was used as a Lewis base .To FA, MA, Cs, Rb, and Br having relatively larger E than 1.60 g the PVSK precursor solution, 2D PVSK (PEA PbI ) precursors 2 4 15,16 eV . However, such compositional engineering has enhanced with different molar ratios ranging from 1.25 to 10 mol% were the V and stability at the expense of short-circuit current OC added. The steady-state PL spectra of the films were measured density (J ) due to increased E . Utilization of pure FAPbI is and are shown in Supplementary Fig. 1. As seen in Fig. 1a and SC g 3 desired in regards to its lower E , which is close to the optimum g Supplementary Fig. 1, we observed no obvious changes in PL value for a single junction solar cell suggested by the detailed peak position until the amount of 2D PVSK reached 10 mol%. balance limit . However, no efficient method has been developed With 10 mol% PVSK, the PL peak was blue-shifted by 6 nm. The so far to fabricate a high quality phase-pure FAPbI film and blueshift of the PL peak might be due to formation of a quasi-3D device. PVSK, where charge carriers are confined by large potential Recently, the manipulation of surface energy has been pro- barrier originated from the 2D PVSK . Based on this observa- posed as a means to stabilize metastable PVSK phases such as tion, we presume the added 2D PVSK does not result in the 18–20 CsPbI and FAPbI . Swarnkar et al. reported ambient stable formation of the quasi-3D PVSK if it remains below a certain 3 3 α-CsPbI in the form of a colloidal quantum dot, in which the threshold. This threshold was found to be lower than 10 mol%, contribution of surface energy significantly increases due to the where this quantity was then optimized based on photovoltaic high surface-to-volume ratio . Very recently, Fu et al. reported performance (Fig. 1a and Supplementary Fig. 2). A planar het- that the cubic FAPbI phase can be stabilized by functionalizing 3 erojunction architecture consisting of Indium doped SnO (ITO) the surface with large-sized organic molecules . They demon- glass/compact-SnO /PVSK/spiro-MeOTAD/Ag or Au was uti- strated that the functionalized surface contributes to lower for- lized for construction of PVSK solar cells in this study (cross- mation energy to stabilize the cubic FAPbI phase at room sectional scanning electron microscopic (SEM) images of the ab c 3.0 FAPbl w/ 1.67 mol% PEA Pbl 3 FAPbl 2 4 w/ 1.67 mol% PEA Pbl α 2.5 2 4 1.2 1.0 2.0 3D 0.8 1.5 α # # 1.5 1.0 0.4 0.5 quasi-3D * 700 800 900 0.0 Wavelength (nm) 750 0.0 1.0 02 46 8 10 10 20 30 40 400 500 600 700 800 900 2D perovskite (mol%) Two theta (degree) Wavelength (nm) Fig. 1 Crystallographic and absorption properties. a Peak position for steady-state photolumimescence (PL) spectrum and normalized power conversion efficiency (PCE) of the devices for FAPbI perovskite with different amount of added 2D PEA PbI perovskite. The error bar of the normalized PCE indicates 3 2 4 standard deviation of the PCEs. At least 10 devices were fabricated for each condition. b X-ray diffraction patterns, c absorption and normalized PL spectra of bare FAPbI and FAPbI with 1.67 mol% PEA PbI 2D perovskite. Inset of c shows onset region of the absorption spectra with linear approximation (solid 3 3 2 4 lines) 2 NATURE COMMUNICATIONS | (2018) 9:3021 | DOI: 10.1038/s41467-018-05454-4 | www.nature.com/naturecommunications PL peak position (nm) Normalized PCE (%) intensity (count) Absorbance (a.u.) Normalized PL intensity NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-05454-4 ARTICLE device are shown in Supplementary Fig. 3). The addition of 1.67 which the XRD peaks were slightly shifted toward higher angles mol% 2D PVSK was found to be optimal for the photovoltaic with the addition of relatively smaller amounts of 2D PVSK (1.25, performance (ca. 11% improvement in PCE). Notably, addition of 1.67, 2.50, and 5.0 mol%). This indicates that the lattice constant 10 mol% 2D PVSK significantly degraded the PCE to lower than of FAPbI is reduced, likely due to compressive strain associated 1%, which might result from formation of quasi-3D PVSK as the with the added 2D PVSK. We speculate that the reduction in large potential barrier originating from 2D PVSK could hinder lattice constant can be also related to the enhanced phase purity the charge transport. of cubic FAPbI as it will have equivalent effects with X-ray diffraction patterns (XRD) of bare FAPbI and FAPbI incorporation of smaller ‘A’ site cations on the tolerance 3 3 with 1.67 mol% PEA PbI are shown in Fig. 1b. As can be seen, factor and thus enthalpy of formation . Lower angle peaks at 2 4 the bare FAPbI film contains hexagonal non-PVSK phase (δ- around 12° appear upon addition of 10 mol% 2D PVSK phase) while the PVSK film prepared with 1.67 mol% PEA PbI corresponding to the formation of quasi-3D PVSK (inset of 2 4 29 32 shows pure PVSK phase . Even smaller amount of 2D PVSK Supplementary Fig. 4f) . The pure phase PVSK film with (1.25 mol% PEA PbI ) effectively suppresses the formation of δ- 1.67 mol% PEA PbI shows enhanced absorption over all wave- 2 4 2 4 phase (Supplementary Fig. 4). Furthermore, the overall signal lengths (Fig. 1c) compared to the bare FAPbI film where the intensity and full-width-half-maximum (FWHM) were enhanced absorption onset is hardly changed (Insent of Fig. 1c). The with the addition of the 2D PVSK, indicating improved absorption onset is complemented by almost identical normalized crystallinity. We speculate that the added large phenylethylam- PL spectra, which indicates that the E was maintained. The monium molecules from 2D PVSK precursors interact with enhanced absorption as seen when the 2D PVSK was added is FAPbI crystals to facilitate formation of the cubic PVSK phase probably due to an enhanced phase purity of the FAPbI , with 3 3 during crystallization . Such a speculation is correlated with the partial contribution from an enhanced light scattering owing to observation in the XRD measurements in Supplementary Fig. 4, the improved crystallinity . The absorption spectra with in which the signal intensity and FWHM of XRD peaks are different amounts of 2D PVSK are demonstrated in Supplemen- systematically enhanced with increased amounts of the added 2D tary Fig. 7. While all the PVSK films with 2D PVSK showed PVSK (Supplementary Figs. 4, 5). The enhancement of preferred enhanced absorption compared to bare FAPbI films, a slight orientation along the (001) plane with increased 2D PVSK also blueshift of the absorption onset with decreases in absorption indicates the added precursors of the 2D PVSK functionalize the over the whole-wavelength region was identified with the specific crystal facet to change the surface energy during the addition of 10 mol% of 2D PVSK, which is correlated with the crystal growth . A closer inspection on the normalized X-ray blueshift of the steady-state PL spectrum that can be associated diffraction (XRD) patterns of the PVSK films with different with the formation of quasi-3D PVSK. amounts of added 2D PVSK (Supplementary Fig. 6) was taken to find any correlations between the added 2D PVSK and crystal structure of FAPbI . Interestingly, a systematic change in peak 3 Photoluminescence properties and photovoltaic performance. position was observed with different amounts of 2D PVSK for Steady-state and time-resolved PL profiles were investigated in ab c 5×10 16 Bare FAPbl (control) Bare FAPbl Bare FAPbl 3 3 w/ 2D PVSK w/ 2D PVSK w/ 2D PVSK 4×10 12 w/ 2D PVSK and Cs (target) w/ 2D PVSK and Cs w/ 2D PVSK and Cs 3×10 6 10 2×10 6 4 1×10 10 0 700 750 800 850 900 0 1 23 12 14 16 18 20 22 Time (μs) Wavelength (nm) PCE (%) d e f 30 25 100 25 20 80 20 20 RS / FS Control Control 15 60 15 Target 15 Target Control, V =0.78 V 10 40 10 J VOC PC Target, V =0.91 V SC 10 –2 FF (mA cm ) (V) (%) 24.23 1.048 0.646 16.41 (23.70) (1.044) (0.603) (14.91) 5 20 5 24.44 1.126 0.765 21.06 (24.46) (1.125) (0.740) (20.37) 0 0 0 0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 020 40 60 300 400 500 600 700 800 900 Voltage (V) Time (s) Wavelength (nm) Fig. 2 Photoluminescence properties and photovoltaic performance. a Steady-state and b time resolved PL spectra of the perovskite films incorporating bare FAPbI , FAPbI with 2D perovskite and FA Cs PbI with 2D perovskite. Gray solid lines in b are fitted lines for each curve. c Power conversion 3 3 0.98 0.02 3 efficiency (PCE) distribution of the devices incorporating the perovskites. All the devices were fabricated in same batch. d Current density–voltage (J–V) curves, e steady-state PCE measurement and f external quantum efficiency (EQE) spectra of perovskite solar cells incorporating bare FAPbI (control) and FA Cs PbI with 2D perovskite (target). Photovoltaic parameters of the highest performing devices are summarized in the table in d, in which the 0.98 0.02 3 values with and without parenthesis are from reverse (from V to J ) and forward scan (from J to V ), respectively OC SC SC OC NATURE COMMUNICATIONS | (2018) 9:3021 | DOI: 10.1038/s41467-018-05454-4 | www.nature.com/naturecommunications 3 PL intensity (count) –2 Current density (mA cm ) –2 PL intensity (count) Current density (mA cm ) PCE (%) EQE (%) Count –2 Integrated J (mA cm ) SC ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-05454-4 Fig. 2a, b. The steady-state PL intensity was largely enhanced process parameters, the average PCE of 20.05 ± 0.45% was 5 6 more than five times from 4.3 ×10 to 2.3 × 10 with addition of demonstrated over 74 devices (Supplementary Fig. 15). We 1.67 mol% PEA PbI into FAPbI film (Fig. 2a). The large obtained the peak V of 1.130 V with the target device 2 4 3 OC enhancement of PL intensity was attributed to a significantly (Supplementary Fig. 16) corresponding to a loss-in-potential of elongated PL lifetime as seen in Fig. 2b. The time resolved PL 0.35 V considering a E of 1.48 eV, which is the lowest V deficit g OC profiles were fitted to exponential decay, in which bi- and tri- reported to date for PVSK solar cells. One of the target devices exponential decay models were used for the bare and 2D PVSK was sent out for measurement in an independent laboratory and incorporated PVSK films, respectively (Supplementary Table 1). achieved a certified stabilized PCE of 19.77% (Supplementary The relatively fast decay component (τ around 3 ns) was Fig. 17). The current–voltage curve and EQE spectra matched assigned to charge carrier trapping induced by trap states formed well with those measured by our group (Supplementary Fig. 18). due to the structural disorder such as vacancy or interstitial The enhanced device performance with 2D PVSK is mainly due defects while much slower components (τ , τ ) were assigned to to improved FF and V , which can be attributed to improved 2 3 OC 34–37 free carrier radiative recombination . With addition of 1.67 phase purity and elongated carrier lifetime with reduced defect mol% 2D PVSK, proportion of the fast decay component (τ ) was density, facilitating carrier transport and reducing the charge decreased (from 51.8% to 46.5%) while τ significantly elongated recombination . The reduced non-radiative recombination loss from 78.5 ns to 148.7 ns, which indicates reduced defect density with 2D PVSK was also confirmed in devices by electrolumines- and enhanced charge carrier lifetime. We attributed such cence (EL) measurements in Supplementary Fig. 19, in which −1 −2 improvements to enhanced phase purity and crystallinity of maximum radiance (40.4 Wsr cm ) and EL EQE (0.49%) of FAPbI as observed from XRD measurements (Fig. 1b), which the target devices were significantly enhanced compared to those −1 −2 15 decreases the structural disorders at grain interiors and/or of the control devices (2.87 Wsr cm , 0.06%) . boundaries . Moreover, a new decay component (τ ) with a significantly long lifetime (>1 μs) appeared after addition of the 2D PVSK, which is likely related to the added 2D PVSK. As a Moisture stability and TEM analysis. Under ambient conditions, result, the average PL lifetime was enhanced by almost one order a cubic FAPbI PVSK phase is subject to undergo conversion to a of magnitude from 39.4 to 376.9 ns with addition of 2D PVSK. hexagonal non-PVSK phase, resulting in serious degradation in 8,14 During the optimization of the device, incorporation of 2 mol% photovoltaic performance . The phase transformation is even Cs was found to further enhance the performance and reprodu- accelerated under high-relative humidity . To evaluate the effects cibility of the devices without a noticeable change in E (Sup- of 2D PVSK incorporation on phase stability, we investigated plementary Figs. 8–12, see Supplementary Notes 1, 2 and 3 for changes in absorbance of the film under relative humidity (RH) of additional discussion on the optimization process and impacts of 80 ± 5%. Figure 3a shows photos of the PVSK film stored for 2 mol% Cs). With additional 2 mol% Cs, the fraction of τ was different time. Bare FAPbI film was almost completely bleached 1 3 further decreased, indicating a further decreased defect density, within 24 h whereas no obvious change in color was observed which was also observed in previous studies . Consequently, the from the films containing 2D PVSK both with and without Cs. steady-state PL intensity and average PL lifetime was further Figure 3b demonstrates the absorbance (at 600 nm) of the FAPbI enhanced, rationalizing the improved PCE with 2 mol% Cs films with and without 2D PVSK as a function of exposure time (Supplementary Table 1). It is worth noting that the PL lifetime (individual absorption spectra can be found in Supplementary was significantly reduced with 10 mol% of 2D PVSK due to for- Fig. 20). The absorbance of the bare FAPbI rapidly degraded mation of quasi-3D PVSK (Supplementary Fig. 13). during 24 h, while FAPbI films with 2D PVSK did not show The PCE distribution of the devices incorporating correspond- noticeable degradation within 24 h. With addition of 2 mol% Cs, ing the PVSKs is compared in Fig. 2c (distribution of photovoltaic the film also remained stable after 24 h. The color change of the parameters can be found in Supplementary Fig. 14). The average bare FAPbI film under high RH is due to its transformation to photovoltaic parameters are summarized in Supplementary the δ-phase as can be seen in XRD spectra in Supplementary Table 2. The average PCE of the bare FAPbI PVSK solar cells Fig. 21a whereas no detectable change in color for the films with was significantly enhanced by 13% from 15.95 ± 0.36% to 18.08 ± 2D PVSK is correlated with their neat XRD spectra without the δ- 0.52% with addition of 1.67 mol% PEA PbI . The average PCE phase (Supplementary Figs. 21b, c). The enhanced phase stability 2 4 was further enhanced to 19.16 ± 0.37% with 2 mol% of Cs under high RH implies that the possible ingression pathway of (Hereafter, the devices based on bare FAPbI are denoted as moisture in the PVSK film is passivated. Previously, we demon- control while the devices based on FA Cs PbI with 1.67 mol strated grain boundary engineering techniques using the adduct 0.98 0.02 3 % PEA PbI are denoted as target for convenience). Current approach, in which the additives had precipitated at grain 2 4 34,38 density and voltage (J–V) curves of the optimized control and boundaries if not incorporated into the lattice of PVSK .We target devices are demonstrated in Fig. 1d, in which the highest supposed that grain boundaries within the film, which have been −2 PCE of the target device reached 21.06% (J : 24.44 mA cm , reported to be ingression pathways for moisture, might be pas- SC V : 1.126V, FF: 0.765) while a PCE of 16.41% was achieved with sivated by the added 2D PVSK . OC −2 the control device (J : 24.23 mA cm , V : 1.048V, FF: 0.646). Indeed, the vertically aligned 2D PVSK was sparsely observed SC OC A stabilized PCE of 20.64% was achieved with the target device from SEM images in Supplementary Fig. 10b, c with addition of while that of control device was 15.80% (Fig. 2e). External 2D PVSK (see also Supplementary Fig. 22 and Supplementary quantum efficiency (EQE) spectra of the devices were compared Note 4 for additional discussion). However, the enhanced −2 in Fig. 2f. An integrated J of 23.9 mA cm from the target moisture stability throughout the whole-film implies that the SC device was well-matched with the value measured from the J–V 2D PVSK probably exist along the grain boundaries. To confirm scan (<5% discrepancy), while control device shows that of 21.2 our assumption, transmission electron microscopic (TEM) −2 mA cm with a relatively large discrepancy of 14%. The images of the FAPbI film with 2D PVSK was analyzed in relatively large discrepancy from the control FAPbI device is Fig. 3c–e. The inset of Fig. 3c shows a chunk of the polycrystalline probably due to a more pronounced hysteresis, as seen in Fig. 1d, film scratched off from the substrate. Several hundreds of which also results in a large discrepancy between the stabilized nanometer sized grains and their boundaries are clearly visible PCE and the PCE measured from the J–V scan. The performance from the image, and from which one of the grains is magnified in of control device was highly reproducible. With optimized Fig. 3c. The region (1) in Fig. 3c, which is the grain interior, was 4 NATURE COMMUNICATIONS | (2018) 9:3021 | DOI: 10.1038/s41467-018-05454-4 | www.nature.com/naturecommunications NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-05454-4 ARTICLE a b 2.0 FAPbl 1.5 Bare FAPbl w/ 2D PVSK w/ w/ 2D PVSK and Cs 1.0 2D PVSK 0.5 w/ RH80% 2D PVSK T=20 °C and Cs 0.0 0 10 20 0 h 4 h 8 h 12 h 16 h 20 h 24 h Time (h) cd e FFT 3.2 Å 500 nm 8.0 Å FFT 8.0 Å (2) 3.2 Å 100 nm 5 nm 10 nm (1) Fig. 3 Improved moisture stability with 2D perovskite at grain boundaries. a Photos of the perovskite films incorporating bare FAPbI , FAPbI with 2D 3 3 perovskite and FA Cs PbI with 2D perovskite exposed to relative humidity (RH) of 80 ± 5% at 20 ± 2 °C for different time. b Evolution of absorption 0.98 0.02 3 of the films at 600 nm under RH 80 ± 5% at 20 ± 2 °C. The error bar indicates standard deviation of the absorbance measured from three films for each condition. c–e Transmission electron microscopic (TEM) images of the FA Cs PbI film with 1.67 mol% PEA PbI . Inset of c demonstrates the lower 0.98 0.02 3 2 4 magnification image showing the polycrystalline nature with grain boundaries. The highlighted area (1) and (2) were investigated in d and e, respectively. Inset of d and e show Fast Fourier transform (FFT) analysis of the area within boxes, respectively magnified and analyzed using Fast Fourier transform (FFT) in and PEA PbI PVSK shows type I band alignment. Such band 2 4 Fig. 3d, in which an inter-planar spacing of 3.2 Å is well-matched alignment resembles the alignment between PVSK and PbI with the (002) reflection of cubic FAPbI (Supplementary formed at grain boundaries, which was found to reduce charge 40,41 Table 3). At region (2), which is grain boundary, the FFT recombination and assist in charge separation/collection . analysis revealed an inter-planar distance of 8.0 Å (Fig. 3e), Thus, analogous advantages of 2D PVSK at grain boundaries can correlating to a characteristic (002) reflection of 2D PEA PbI be expected. Conductive atomic force microscopy (c-AFM) was 2 4 (Supplementary Fig. 23 and Supplementary Table 4). This performed in Fig. 4c–f to see spatially resolved electrical prop- supports the presence of 2D PVSK at grain boundaries, which erties of the films. Under ambient light conditions (Fig. 4c, d), was further confirmed by elemental distribution (EDS) analysis current flow in the PVSK film with 2D PVSK was higher at/near (Supplementary Fig. 24). At grain boundary regions, relatively the grain boundaries while relatively uniform current flow was larger amounts of carbon and nitrogen were detected, which observed in the bare FAPbI film. With light illumination (Fig. 4e, could be due to presence of phenylethylammonium cation in the f), the current flow was further enhanced at/near the grain 2D PVSK. boundaries with 2D PVSK whereas current flow in bare FAPbI film was uniformly increased, which indicates charge separation and collection of photo-generated electrons is facilitated more so Band structure and electrical properties. A schematic in Fig. 4a at grain boundaries with 2D PVSK. As suggested for PbI , thin shows 2D PVSK formation at the grain boundaries of the 3D 2D PVSK regions at grain boundaries might suffer downward PVSK film. Since the 2D PEA PbI PVSK with aromatic rings and 2 4 band bending under illumination (dashed line in Fig. 4b) where longer alkyl chains is expected to be more resistant to moisture, it photo-generated electrons are transferred from grain interiors. protects the defective grain boundaries of 3D PVSK, resulting in Due to the high-potential barrier to the holes, charge recombi- significantly enhanced moisture stability of the film. Regardless of nation will be reduced, which might be the origin of the superior the improved stability, however, one can expect degraded elec- PL lifetime and photovoltaic performance with 2D PVSK. tronic properties of the film due to the poor charge carrier mobility of the 2D PVSK. We investigated the band structure of FA Cs PbI (with 1.67 mol% of 2D PVSK) and PEA PbI Ambient and operational stability. Finally, the stability of the 0.98 0.02 3 2 4 PVSK, which is illustrated in Fig. 4b. The valence band maximum control and target devices was compared. Figure 5a demonstrates was measured using ultraviolet photoelectron spectroscopy (UPS, the changes in PCE of the unencapsulated devices stored in a Supplementary Fig. 25), while the E was determined from Tauc desiccator (relative humidity lower than 30%, evolution of an plots (Supplementary Fig. 26). As seen in Fig. 4b, FA Cs PbI individual photovoltaic parameter can be found in 0.98 0.02 3 NATURE COMMUNICATIONS | (2018) 9:3021 | DOI: 10.1038/s41467-018-05454-4 | www.nature.com/naturecommunications 5 Absorbance (a.u.) ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-05454-4 a b Grain Grain Grain boundary FACsPbl PEA Pbl FACsPbl Au 3 2 4 3 3.51 Spiro- 4.22 MeOTAD PEA Pbl FACsPbl 2 4 SnO 5.70 5.87 2D PVSK at grain boundary Substrate Under illumination cd e f 1 μm 1 μm 1 μm 1 μm fA fA 1 μm 1 μm1 μm1 μm –100 –50 Fig. 4 Band alignment and local conductivity with 2D perovskite. a Schematics of the device incorporating polycrystalline 3D perovskite film with 2D perovskite at grain boundaries and b band structure of each layer in device analyzed by ultraviolet photoelectron spectroscopy (UPS) and Tauc plots. Conductive atomic force microscopic (c-AFM) images of (c, e) bare FAPbI and (b, d) with 2D perovskite films on SnO coated ITO glass. The 3 2 measurement was carried out with bias voltage of 100 mV under (c, d) room light or (d, f) low intensity light illumination provided by the AFM setup. Inset of each image shows corresponding topology of the films. Scale bar at left side is for (c) and (d) while at right side is for e and f 42,44 Supplementary Fig. 27). While the control device degraded by trapped charge carriers associated with it . We suppose that 29% for 1392 h, the target device maintained 98% of its initial migration of the charged defects or ions is possibly suppressed by efficiency during this time. The operational stability of the devices 2D PVSK at grain boundaries. The temperature-dependent con- was also compared by maximum power point (MPP) tracking ductivity (σ) measurement of the lateral devices was performed to under 1 sun illumination in Fig. 5b. Without encapsulation, the evaluate the activation energy for the ion migration (Fig. 6). The PCE of the control device rapidly degraded by 68% during 450 activation energy (E ) for the migration can be determined min whereas that of target device was relatively less (20%) during according to the Nernst-Einstein relation , the time. We performed 500 h of light exposure test with the encapsulated control (bare FAPbI device) and target devices (w/ 3 σ E 0 a σðÞ T ¼ expð Þ; 1.67 mol% 2D PVSK). The encapsulated devices were exposed to T kT −2 ca. 0.9 sun (90 ± 5 mW cm ) under open-circuit condition, of which the steady-state PCE was periodically measured for dif- ferent exposure time. As seen in Fig. 5c, both of the devices where k is Boltzmann constant, σ is a constant. Inset of Fig. 6a showed a rapid initial decay in PCE followed by slower decay with describes the structure of the lateral devices. With bare FAPbI an almost linear profile, which is in agreement with previous PVSK, exponential enhancement in conductivity was clearly reports . After 500 h of exposure, the control device degraded to identified at around 130 K (Fig. 6a), which is attributed to con- ca. 52.3% of its initial PCE whereas the target device maintained tribution of ions. The E for bare FAPbI film was calculated to be a 3 72.3% of the initial PCE, indicating enhanced stability with 0.16 eV, indicating significant contribution of activated ions at addition of 2D PVSK. We could extract tentative T80 (time at room temperature, which might cause degradation of the material which PCE of the device decays to 80% of initial PCE) for the and device under operational condition with built-in electric field. devices by fitting of the post-burn-in region in which the PCE of The pronounced current–voltage hysteresis behavior was the device shows an almost linear decay profile (after 48 h). The observed even at very low temperature (180 K, Fig. 6c). In case of T80 for control and target devices were calculated to be 592 h and the PVSK film with 2D PVSK, the film did not show noticeable 1362 h, respectively. This indicates the stability of the device was enhancement in conductivity with increased temperature; significantly improved with addition of 2D PVSK. We also per- although, the overall conductivity was relativity lower than the formed MPP tracking of the encapsulated target device under bare FAPbI film (Fig. 6c). Even with moderate light illumination, −2 1 sun (100 mW cm ) illumination in Supplementary Fig. 28.A it does not show the indicative of activated ions. As a result, the total of 18.7% of initial PCE was degraded for 130 h of operation, current–voltage curve did not show any hysteresis behavior which is relatively slower compared to the device maintained at (Fig. 6d). As the grain boundaries of 3D perovskite were reported open-circuit condition. This is correlated with previous studies to be a major pathway for the migration of ions , passivating the that attributed the faster degradation under open-circuit condi- grain boundaries by incorporation of the ion-migration-immune tion to larger number of photo-generated charge carriers 2D PVSK likely suppressed overall ion migration in the target 43 45 recombining within the device . Under operational condition device . In addition, the improved phase purity of the film might with abundant photo-generated charges and built-in electric field, also partially contribute to the suppressed ion migration because the major factors causing the degradation of the devices might be the secondary phase can generate defect sites that can act as an the highly mobile and reactive charged defects (ions) and/or additional pathway for ion migration. We believe the suppressed 6 NATURE COMMUNICATIONS | (2018) 9:3021 | DOI: 10.1038/s41467-018-05454-4 | www.nature.com/naturecommunications Sprio-MeOTAD 3D PVSK ITO/SnO 2 NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-05454-4 ARTICLE ab 20 1.0 0.8 0.6 10 Control Control Target Target 0.4 0.2 Under dark, <RH 30%, T =25 °C Under 1 sun, RH 50% T =40 °C 0.0 0 500 1000 1500 0 100 200 300 400 Time (h) Time (min) Control 1.0 Target 0.8 0.6 –2 Under light (90 mW cm ), RH 50%, T = 40 °C 0.4 0 100 200 300 400 500 Time (h) Fig. 5 Improved stability with 2D perovskite. a Evolution of power conversion efficiency (PCE) of control and target devices. The devices were stored under dark with controlled humidity. b Maximum power point tracking of the devices under 1 sun illumination in ambient condition without encapsulation. −2 c Evolution of the PCEs measured from the encapsulated control and target devices exposed to continuous light (90 ± 5 mW cm ) under open-circuit condition. The stabilized PCEs were measured at each time. Initial stabilized PCEs for control and target devices were 14.5% and 17.5%, respectively. The broken lines are linear fit of the post-burn-in region (after 48 h). Relative humidity (RH) and temperature (T) are indicated in the graphs for each measurement ion migration contributes to enhanced operational stability of the Methods Synthesis of phenylethylammonium iodide. In a typical synthesis, 4.8 g of phe- target device. nethylamine (39.6 mmol, Aldrich, >99%) was dissolved in 15 mL of ethanol and placed in iced bath. Under vigorous stirring, 10.8 g of hydroiodic acid (57 wt% in H O, 48.1 mmol, Sigma-Aldrich, 99.99%, contains no stabilizer) was slowly added to the solution. The solution was stirred overnight to ensure complete reaction, Discussion which was followed by removal of the solvent by a rotary evaporator. The resulting We demonstrated a reproducible way to fabricate phase-pure solid was washed with diethyl ether several times until the color is changed to formamidinium tri-iodide PVSK with high-optoelectronic quality white. The white solid was further purified by recrystallization in mixed solvent of and stability by incorporating 2D PVSK. The large phenylethy- methanol and diethyl ether. Finally, white plate-like solid was filtered and dried lammonium molecules from 2D PVSK precursors interact with under vacuum (yield around 90%). FAPbI crystals to facilitate formation of the cubic PVSK phase during crystallization, which subsequently functionalize the grain Device fabrication. Indium doped tin oxide (ITO) glass was cleaned with suc- boundaries after completion of the crystallization. The resulting cessive sonication in detergent, deionized (DI) water, acetone and 2-propanol for phase-pure PVSK film has an identical E (1.48 eV) to that of 15 min, respectively. The cleaned substrates were further treated with UV-ozone to pure FAPbI with an order of magnitude enhanced PL lifetime. remove the organic residual and enhance the wettability. A total of 30 mM SnCl ∙2H O (Aldrich, >99.995%) solution was prepared in ethanol (anhydrous, Average PCE of 20.05 ± 0.45% over 74 devices and the highest 2 2 Decon Laboratories Inc.), which was filtered by 0.2 μm syringe filter before use. To stabilized PCE of 20.64% (certified stabilized PCE of 19.77%) was form a SnO layer, the solution was spin-coated on the cleaned substrate at 3000 achieved. Regardless of its low E , the PVSK solar cell showed the rpm for 30 s, which was heat-treated at 150 °C for 30 min. After cooling down to peak V of 1.130 V, corresponding to the lowest loss-in- room temperature, the spin-coating process was repeated one more time, which OC was followed by annealing at 150 °C for 5 min and 180 °C for 1 h. The SnO coated potential of 0.35 V among all the reported PVSK solar cells. Owed 2 ITO glass was further treated with UV-ozone before spin-coating of PVSK solu- to the functionalized grain boundaries by the 2D PVSK, the phase tion. The PVSK layer was prepared by the modified adduct method . The bare stability of the film under high RH significantly improved and FAPbI layer was formed from the PVSK solution containing equimolar amount of migration of ions (or charged defects) was suppressed, resulting HC(NH ) I (FAI, Dyesol), PbI (TCI, 99.99%) and N-Methyl-2-pyrrolidone (NMP, 2 2 2 in significantly improved ambient and operational stability of the Sigma-Aldrich, anhydrous, 99.5%) in N,N-Dimethylformamide (DMF, Sigma- Aldrich, anhydrous, 99.8%). Typically, 172 mg of FAI, 461 mg of PbI and 99 mg of device. We believe our approach to utilize spontaneously formed NMP were added to 600 mg of DMF. For the 2D PVSK (PEA PbI ) and Cs 2 4 grain boundary 2D PVSK will provide important insights for the incorporated PVSK, corresponding amount of FAI was replaced with PEAI and research community to design PVSK materials to achieve record CsI. For example, FAPbI with 1.67 mol% PEA PbI PVSK was formed from the 3 2 4 PCEs accompanied by high stability and longevity. precursor solution containing 166.4 mg of FAI, 8.2 mg of phenylethylammonium NATURE COMMUNICATIONS | (2018) 9:3021 | DOI: 10.1038/s41467-018-05454-4 | www.nature.com/naturecommunications 7 PCE (%) Normalized PCE Normalized PCE ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-05454-4 ab 4 –4 –5 Au Au 0 –6 Dark 10 10 Light –7 –2 E =0.16 eV 10 a –8 –4 56 78 9 10 11 12 3 4 5 6 7 8 9 10 11 12 –1 –1 1000/T (K ) 1000/T (K ) cd –7 –4 180 K 180 K –6 –9 –8 –11 –10 –13 –12 –6 –4 –2 0 24 6 –6 –4 –2 0 2 4 6 Applied voltage (V) Applied voltage (V) Fig. 6 Suppressed ion migration with 2D perovskite. Temperature-dependent conductivity of a bare FAPbI film and b with 1.67 mol% 2D PEA PbI 3 2 4 −2 perovskite. Red circles in b indicate the data measured under moderate light illumination (intensity lower than 10 mW cm ). Current–voltage curves measured from the devices at 180 K. c Bare FAPbI film and d with 1.67 mol% 2D PEA PbI perovskite 3 2 4 iodide (PEAI), 453.4 mg of PbI and 97.4 mg of NMP in 600 mg of DMF. With 2 spectroscopic (UPS) analysis was carried out using Kratos Ultraviolet photoelec- mol% of Cs, the precursor solution was prepared by mixing 163.0 mg of FAI, 8.2 tron spectrometer. He I (21.22 eV) source was used as an excitation source. The mg of PEAI, 5.0 mg of CsI (Alfa Aesar, 99.999%), 453.4 mg of PbI and 97.4 mg of PVSK films were coated on ITO substrate and grounded using silver paste to avoid NMP in 600 mg. For the best performing devices in Fig. 2d, the amount of DMF the charging during the measurement. Conductive atomic force microscopic was adjusted to 550 mg. Spin-coating of PVSK and hole transporting layer was (AFM) measurement was performed by Bruker Dimension Icon Scanning Probe performed in a glove box filled with dry air. The PVSK solution was spin-coated at Microscope equipped with TUNA application module. The TUNA module pro- 4000 rpm for 20 s where 0.15 mL of diethyl ether (anhydrous, >99.0%, contains vides ultra-high tunneling current sensitivity (<1 pA) with high-lateral resolution. BHT as stabilizer, Sigma-Aldrich) was dropped after 10 s on the spinning substrate. Antomony doped Si tip (0.01–0.025 Ohm-cm) coated with 20 nm Pt-Ir was used as The resulting transparent adduct film was heat-treated at 100 °C for 1 min followed a probe. To avoid the electrically driven degradation during the measurement, low by 150 °C for 10 min. (for the best performing target device, the annealing con- bias voltage (100 mV) was applied. The measurement was carried out under either dition was adjusted to 80 °C 1 min followed by 150 °C for 20 min) The spiro- room right or low intensity light illumination provided by AFM setup. The MeOTAD solution was prepared by dissolving 85.8 mg of spiro-MeOTAD temperature-dependent conductivity measurement was carried out using a com- (Lumtec) in 1 mL of chlorobenzene (anhydrous, 99.8%, Sigma-Aldrich) which was mercial probe station (Lakeshore, TTP4) in which temperature of the device was doped by 33.8 μl of 4-tert-butylpyridine (96%, Aldrich) and 19.3 μl of Li-TFSI controlled by thermoelectric plate and flow of liquid nitrogen. The electrical −1 (99.95%, Aldrich, 520 mg mL in acetonitrile) solution. The spiro-MeOTAD measurement was conducted with a source/measurement unit (Agilent, B2902A). solution was spin-coated on the PVSK layer at 3000 rpm for 20 s by dropping 17 μl of the solution on the spinning substrate. On top of the spiro-MeOTAD layer, ca. Device characterization. Current density–voltage (J–V) curves of the devices were −1 100 nm-thick silver or gold layer was thermally evaporated at 0.5 Ås to be used measured using Keithley 2401 source meter under simulated one sun illumination as an electrode. −2 (AM 1.5G, 100 mW cm ) in ambient atmosphere. The one sun illumination was generated from Oriel Sol3A with class AAA solar simulator (Newport), in which light intensity was calibrated by NREL-certified Si photodiode equipped with KG-5 Material characterization. The PVSK layer was coated on a SnO coated ITO −1 filter. Typically, the J–V curves were recorded at 0.1 Vs (between 1.2 V and −0.1 substrate for the measurements. UV-vis absorption spectra were recorded by U- V with 65 data points and 0.2 s of delay time per point). During the measurement, 4100 spectrophotometer (Hitachi) equipped with integrating sphere. The mono- the device was covered with metal aperture (0.100 cm )todefine the active area. All chromatic light was incident to the substrate side. X-ray diffraction (XRD) patterns the devices were measured without pre-conditioning such as light-soaking and were obtained by X-ray diffractometer (PANalytical) with Cu kα radiation at a scan applied bias voltage. Steady-state power conversion efficiency was calculated by −1 rate of 4° min . Surface and cross-sectional morphology of the films and devices measuring stabilized photocurrent density under constant bias voltage. The were investigated by scanning electron microscopy (SEM, Nova Nano 230). For the external quantum efficiency (EQE) was measured using specially designed system cross-sectional image, cross-sectional surface of the sample was coated with ca. (Enli tech) under AC mode (frequency = 133 Hz) without bias light. For electro- 1 nm-thick gold using sputter to enhance the conductivity. Transmission electron luminescence measurement, a Keithley 2400 source meter and silicon photodiode microscopic (TEM) analysis was performed by Titan Krios (FEI). The PVSK film (Hamamatsu S1133-14, Japan) were used to measure Current–voltage–luminance was scratched off from the substrate and dispersed in toluene by sonication for characteristics of PVSK solar cells. Electroluminescence spectra were recorded by 10 min, which was dropped on an aluminium grid. Accelerating voltage of 300 kV Horiba Jobin Yvon system, and used to calculate radiance and external quantum was used for the measurement. Steady-state photoluminescence (PL) signal was efficiency of PVSK solar cells. All the devices were assumed as Lambertian emitter analyzed by a Horiba Jobin Yvon system. A 640 nm monochromatic laser was used in the calculation. as an excitation fluorescence source. Time resolved PL decay profiles were obtained using a Picoharp 300 with time-correlated single-photon counting capabilities. The films were excited by a 640 nm pulse laser with a repetition frequency of 100 kHz Stability test. Moisture stability of the films was tested by exposing the PVSK films provided by a picosecond laser diode head (PLD 800B, PicoQuant). The energy under relative humidity of 80 ± 5% and room light. Absorbance of the films was −2 density of the excitation light was ca. 1.4 nJ cm , in which carrier annihilation and measured every 2 h while XRD of the films were recorded every 12 h. For the non-geminate recombination are negligible . Ultraviolet photoelectron devices, ex-situ test was conducted by storing the devices in desiccator (relative 8 NATURE COMMUNICATIONS | (2018) 9:3021 | DOI: 10.1038/s41467-018-05454-4 | www.nature.com/naturecommunications PVSK Current (A) T (a.u.) Current (A) T (a.u.) NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-05454-4 ARTICLE humidity, RH <30%) under dark condition. The device was taken out and mea- 24. Zhou, Y. et al. 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Phenylalkylamine passivation of organolead halide perovskites Author contributions enabling high-efficiency and air-stable photovoltaic cells. Adv. Mater. 28, J.-W.L. conceived an idea and led overall project under supervision of Y.Y. J.-W.L. and 9986–9992 (2016). Z.D. fabricated devices and characterized the materials. T.-H.H. assisted the device 23. Wang, Z. et al. Efficient ambient-air-stable solar cells with 2D-3D fabrication and performed EL measurement. C.C. and Y.H. carried out TEM char- heterostructured butylammonium-caesium-formamidinium lead halide acterization. S.-Y.C. helped UPS measurement. S.L. performed temperature-dependent perovskites. Nat. Energy 2, 17135 (2017). conductivity measurement. H.Z. synthesized the PEAI. N.D.M. and P.S. NATURE COMMUNICATIONS | (2018) 9:3021 | DOI: 10.1038/s41467-018-05454-4 | www.nature.com/naturecommunications 9 ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-05454-4 commented on the manuscript. 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