3D Perovskite and the

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Letter

Unraveling the Growth of Hierarchical Quasi 2D/3D Perovskite and the Carrier Dynamics Liang Li, Ning Zhou, Qi Chen, Qiu yu Shang, Qing Zhang, Xindong Wang, and Huanping Zhou J. Phys. Chem. Lett., Just Accepted Manuscript • Publication Date (Web): 12 Feb 2018 Downloaded from http://pubs.acs.org on February 12, 2018

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The Journal of Physical Chemistry Letters

Unraveling the Growth of Hierarchical Quasi 2D/3D Perovskite and the Carrier Dynamics Liang Liab, Ning Zhoub, Qi Chenc, Qiuyu Shangb, Qing Zhangb, Xindong Wang*a, and Huanping Zhou*b a. Department of Physical Chemistry, University of Science and Technology Beijing, Beijing 100083, P.R.China. E-mail: [email protected] b. Beijing Key Laboratory for Theory and Technology of Advanced Battery Materials, Department of Materials Science and Engineering& Department of Energy and Resources Engineering, College of Engineering, Peking University, Beijing 100871, P.R.China. E-mail: [email protected] c. School of Materials Science and Engineering, 5 Zhongguancun South Street, Beijing Institute of Technology, Beijing 100081, P.R.China

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ABSTRACT The construction of hybrid perovskites (both 2 dimension (2D) and 3 dimension (3D)) has attracted intensive research interest recently. Here, a facile, two-step consecutive deposition method was developed for the first time to grow a hierarchical quasi-2D/3D perovskite superstructure, with oriented quasi-2D ((BA)2(MA)n−1PbnI3n+1) perovskite nanosheet (NS) perpendicular aligned on 3D perovskites. The superstructure are found to be the mixture of multiple perovskite phases, with n = 2, 3, 4 and 3D perovskite, however, the n value was naturally increased from top to the bottom that is distinct from many other work. We found that the concentration gradient, namely, the initial ratio and amount of BAI/MAI, collectively contributing the spatially confined nucleation and growth of oriented quasi-2D superstructure perovskite on 3D perovskites. An efficient charge carrier transfer was demonstrated from small-n to large-n phases in this perovskite superstructure, indicating a different type of energy funnel from top to the bottom.

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Recently, both three-dimensional (3D) and quasi two-dimensional (quasi-2D) halide perovskite materials have been emerged as promising candidates for applications in photovoltaic1-10 and other optoelectronic devices.11-16 The 3D hybrid perovskites (namely CH3NH3PbI3) with various appealing properties, including high absorption coefficient,17 high mobility,18 low trap-state density,19-21 and low exciton binding energy22 have been intensively explored. Quasi-2D organic–inorganic hybrid perovskites is generally in the form of A2(CH3NH3)n-1PbnI3n+1, where A is a large alkyl ammonium cation, and n is the layer number of inorganic slabs between two A spacer. This class of materials, not only inherent some fundamental properties that similar to 3D hybrid perovskites,23,24 but also possess additional characteristics, e.g. dramatically improved luminescence resulting from quantum confinement25,26 and moisture resistance associated with the insulating cation spacer.2,27-29 Therefore, the quasi-2D hybrid materials are extremely attractive for long term optoelectronic application,2,13,29-31 and it is crucial to understand the corresponding materials composition, crystal structure, growth thermodynamics and optoelectronic properties. In the pursuit of the optoelectronic application, research efforts on Q-2D hybrid perovskite materials follows two major aspects: 1) the theoretical and experimental understanding in the materials growth,32 structure,32,33 and optical properties;34-36 2) continuous improvement in the design of the relevant solar cells,2,37,38 light emitting diodes13,15 and other types of devices.30,39 The A cations differ in size determines the formation and the structure of quasi-2D A2(CH3NH3)n-1PbnI3n+1 perovskite structure,40 as well as the n value,41 which ultimately influences the optoelectronic properties of the resulting quasi-2D materials.41 Very recently, the quasi-2D perovskite films are found to be the mixture of multiple perovskite phases, with n = 2, 3, 4 and ≈ ∞, aligned in the order of n (n value decreased from top to the bottom) perpendicular to the substrate.33 This particular structure can promote both the electron and hole transfer, due to the perfect band-alignment naturally existed in the materials, which 3



may further benefit the efficient emission and photovoltaic properties.33 Meanwhile, the quasi-2D perovskite have been successfully proven to be efficient in photovoltaic devices, based on the incorporation of n-butylamine (BA) or phenylethylammonium (PEA) as the organic motif.41 While impressive long term stability achieved in this type of perovskite solar cells, it still suffered from relatively poor efficiency,2,41 which is attributed to the inhibition of out-of-plane charge transport induced by the organic cations.2,42 To overcome the insulating feature imposed by larger spacing layers between the conducting inorganic slabs, the quasi-2D perovskite thin film are then produced with near-single-crystalline quality, in which the crystallographic planes of the inorganic perovskite component have a strongly preferential out-of-plane alignment with respect to the contacts in planar solar cells to facilitate a photovoltaic efficiency of 12.52%.2 Although significant progress have been achieved on the quasi-2D perovskite materials and devices,2 great challenges are still posed on the synthesis of high quality quasi-2D perovskite structures with tunable charge transfer capability, as well as the profound understanding on the underlying thermodynamic mechanism.

Here, we presented a facile, two-step consecutive deposition method to grow a hierarchical quasi-2D/3D perovskite ((BA)2(MA)n−1PbnI3n+1) (MA: methylammonium) superstructure, with oriented smaller n value perovskite nanosheet (NS) perpendicular aligned on the 3D perovskite film. The hierarchical perovskite superstructure was also found to be the mixture of multiple perovskite phases with n = 2, 3, 4 and 3D perovskite, while n value naturally increased from top to the bottom, which is different from previous study33 with n value aligning along the opposite way. The growth thermodynamics of the perovskite superstructures are thoroughly revealed based on the time or temperature dependent X-ray diffraction (XRD), transient absorption (TA), scanning electron microscopy (SEM) and photoluminescence (PL) measurement. It was identified that the ratio of BAI/MAI and the corresponding concentration, collectively contributing the spatially confined nucleation and growth of smaller n phase (n=2,3,4) nanosheet on 3D perovskite film. This unique perovskite 4


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hierarchical structure can further tune the internal charge transfer, benefiting from the vertically grown quasi-2D NS on the 3D film. This work represents a significant step to exquisite control the perovskite hierarchical structure, which may benefit the fundamental understanding of perovskite growth and its application in photovoltaics and other optoelectronics devices.

We synthesized a quasi-2D/3D perovskite hierarchical superstructure by employing a consecutive two-step process, whereas the as-deposited PbI2 film was dipped into BAI/MAI isopropanol solutions with desired stoichiometry for a period of time. The detailed description is shown in the experimental section, and the control results are shown in Figure S1. Figure 1a-f and Figure S2a-f shows scanning electron microscope (SEM) images of the quasi-2D/3D perovskites, by systematically varying the initial mass ratio of BAI/MAI from 1:4 to 3:1 at room temperature. It was found that quasi-2D/3D perovskite NSs began to grow on the underlying perovskite film until the initial BAI/MAI ratio is equal to or higher than 2:1. Surprisingly, the growth direction of the NS aligned approximately perpendicular to the substrate. In order to confirm the formation of quasi-2D/3D superstructure, we continue to optimize the ratio of BAI/MAI from 1.25:1 to 3:1 with 0.25 as the interval. Figure S3 indicated that perovskite NSs were still absent on perovskite substrate even when BAI/MAI was up to 1.75:1. This suggests that a BAI: MAI ratio of over 2:1 is necessary for the formation of quasi-2D perovskite NSs. It can be also seen from Figure 1e and Figure 1f that, the density of the NSs on perovskite film increases with the increasing BAI/MAI ratio in the growth solution. In order to identify the phase of the as-prepared perovskites, we carried out XRD measurement as shown in Figure S4a. All the film preserved dominate 3D perovskite phase, with the main characteristic peaks located at 14.08ο, 24.48ο, and 28.41ο (Figure S4a).43 One small peak was observed at diffraction angle (2θ) of 9.3ο, representing the (040) crystallographic planes of (BA)2(MA)Pb2I7 perovskite (Figure S4b).44 We also provided 2-D GIWAXS images of superstructure perovskite with different initial mass ratio of BAI/MAI by fixing MAI concentration to 10 mg/mL, as shown in Figure S5. It was observed that 5



the predominant phase of both structures is the 3D perovskite. Accordingly, the crystal plane stacking pattern in the perovskites superstructure changed along with BAI doping. The diffraction mottling (the blue section ring at qxy≈10 nm−1) indicates a strong diffraction intensity for (110) plane with preferred stacking orientation, in which q is the scattering vector in reciprocal space. Upon the introduction of BAI, the peak position and relative intensities for (110) plane almost kept unchanged, but the diffraction mottling at high azimuth angle disappeared. To be clearer, we integrated the GIWAXS pattern azimuthally along the ring at qxy≈10 nm−1. As depicted in Figure S5c and d, the absence of diffraction signal of (110) plane at azimuth angle of about 90° in the superstructure, indicated that the BAI incorporation suppresses the perovskite (110) plane stacking along the out-of-plane direction. This result also facilitates further understanding on the growth mechanism and orientation of quasi two-dimensional perovskites.

Figure 1 SEM images of oriented quasi-2D/3D perovskite structure grown in IPA solution at room temperature with different initial mass ratio of BAI/MAI by fixing MAI concentration to 10 mg/mL; a) 1:4, b) 1:3, c) 1:2, d) 1:1, e) 2:1, f) 3:1.

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Figure 2 The PL emission spectra of the quasi-2D/3D perovskite prepared with different initial ratio of BAI/MAI, illuminated from a) the back side of the film; b) the front side of the film; the enlarged emission spectra (illuminated from the front and back sides) of the quasi-2D/3D perovskite superstructure prepared with BAI/MAI ratio of c) 2:1 and d) 3:1. To identify the specific structure of the as-prepared perovskites, we conducted PL measurement. Figure 2a depicts the PL emission spectra of the hierarchical perovskite structure prepared with different initial ratio of BAI/MAI illuminated from the back side of the film (glass side). We can find that the emission peak of the 3D perovskite gradually increases with the raise of the BAI/MAI ratio, probably due to the stronger quantum confinement. Besides, the emission peak of the 3D perovskite gradually shifts from 766 nm to 742 nm with the increase of the BAI/MAI ratio, which indicates that the 3D perovskite is gradually doped by BA. In addition, when the initial ratio of BAI/MAI is reaching 3:1, two weak peaks at 608 nm and 645 nm appears in the emission spectra which can be assigned to (BA)2(MA)n−1PbnI3n+1 perovskites with n = 7



3 and 4, respectively.33 The residual PbI2 is considered as the origin of the peaks at around 525 nm (Figure S6). Figure 2b demonstrates the emission spectra of the obtained structures illuminated from the front side of the film. Similarly, the intensity of the emission peak of the 3D perovskite gradually increases with the raise of the initial ratio of BAI to MAI, along with the blue shift of the emission peak of the 3D perovskite. However, when the BAI/MAI ratio is 3:1, the intensity of the 3D perovskite emission peak is reduced. It is speculated that large amount of BA consumed too much PbI2 on the surface, and as a result less 3D perovskite was formed due to the insufficient PbI2 amount. More importantly, we can observe three strong peaks at 575 nm, 608 nm, 645 nm (Figure 2b), which could be assigned to the (BA)2(MA)n−1PbnI3n+1 perovskites with n = 2, 3 and 4, respectively, suggested by recent reports.33 The substantially improved intensity of the (BA)2(MA)n−1PbnI3n+1 (n = 2, 3 and 4) perovskites illuminated from the front side, confirmed that the NSs on the surface are composed by quasi-2D superstructure. Figure 2c is the enlarged emission spectra of the oriented quasi-2D/3D perovskite superstructure prepared with initial BAI: MAI ratio of 2:1 illuminated from the front and back sides of the film. Accordingly, we can find that the intensity of the emission peak from the front sides of the film is nearly 15 times stronger than that from back sides. This phenomenon shows that quantum confinement effect of surface quasi-2D perovskite NSs is quite strong25. This result is consistent with the obtained carrier lifetimes fitted from time-resolved photoluminescence (TRPL) (Figure S8 & Table S1). Similarly, Figure 2d is the enlarged emission spectra of the oriented quasi-2D/3D superstructure prepared with BAI/MAI ratio of 3:1 illuminated from both side. It was observed that the intensity of the emission peak at 750 nm from the back side is much stronger than that of front side. Thus, it is speculated that the quasi-2D perovskite NSs are mostly distributed on the surface of 3D perovskite substrate, which is different from the latest work33. Notably, the n value was naturally increased from top to the bottom within the superstructure, which is distinct from most of recent reports regarding quasi-2D film obtained by one-step process. 8


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Figure 3 Confocal fluorescence intensity images for (a) 515 nm and (b) 585 nm upon 488

nm

excitation;

c)

Comparative

bandgap

energy

alignment

of

(BA)2(MA)n−1PbnI3n+1 perovskites with different n values. The possible carrier transfer mechanisms, hole transfer from large-n to small-n perovskites and electron transfers from small-n to large-n perovskites, are all energetically allowed. The confocal fluorescence emission measurement is further employed to detect the homogeneity of the superstructures. The corresponding samples were prepared on a glass substrate by the mentioned two-step procedure with 2:1 BAI/MAI ratio. Darker regions are clearly distinguishable in the case of the 515 nm emission mapping (Figure 3a). When detecting the 585 nm (Figure 3b) emission, we observed a reduced quantity of the prominent dark regions. In correlation to our SEM images, we presume that the regions with different contrast represent morphological differences, as shown in the 515 nm emission mapping. Figure 3c describes the bandgap energy alignment of (BA)2(MA)n−1PbnI3n+1 perovskites with different n values. The possible carrier transfer mechanisms including, hole transfer from large-n to small-n 9



perovskites and electron transfers from small-n to large-n perovskites that are all energetically favored.33 However, as the n value is aligned in an opposite direction that is different from the conventional quasi-2D film, this superstructure will enable a different energy funnel which will be discussed in the following sections. In addition, quasi-2D phase on the surface is facilitating to improve the water proof capability and long term stability of the perovskite superstructure (Figure S9).

Figure 4 a) UV−vis absorption spectra and b) light harvesting efficiency (LHE, i.e., percentage of incident light absorbed)45 of the oriented quasi-2D/3D perovskite NSs film on 3D perovskite substrate prepared with different initial ratio of BAI/MAI. TA spectra at different delay times of a typical quasi-2D/3D perovskite superstructure with BAI/MAI ratio of 2:1 under c) back- and e) front-excitations. TA kinetics probed at n = 2, 3, 4 and n = ∞ bands under d) back and f) front-excitations. 10


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To elucidate the interaction mechanism between different-n perovskite components of quasi-2D/3D perovskite superstructure, we conducted UV−vis absorption and femtosecond TA measurement. Figure 4a describes UV−vis absorption spectra of the quasi-2D/3D perovskite structure prepared with different initial ratio of BAI/MAI. The absorption peaks located in 575 nm, 608 nm, 645 nm and 750 nm, which are identified as n = 2 (2.17 eV), n = 3 (2.04 eV), n = 4 (1.93eV) and 3D perovskite phases, in agreement with recent report33. We can find that when the initial ratio of BAI/MAI is at 2: 1 and 3: 1, the n = 2, n = 3 and n = 4 perovskite phases are clearly observed with enhanced absorption at the region of 550-700 nm. However, the 3D perovskite phase in the superstructure obtained from BAI/MAI ratio at 2:1 and 3:1 become obviously weaker. This is also consistent with the SEM result, confirming the NS on the top of 3D perovskite film is in the form of quasi-2D structure with a mixture of n=2, 3, 4 phases. Light harvesting efficiency of the oriented quasi-2D/3D perovskite superstructure prepared with different initial ratio of BAI/MAI was shown in Figure 4b. It can be seen that the presence of the NS structure enhances the light harvesting efficiency of the perovskite structure from 550 nm to 700 nm. We conducted femtosecond TA to study the charge carrier dynamics in the hierarchical quasi-2D/3D perovskite superstructure. The conventional quasi-2D perovskite film with n=4 obtained by one-step process was used for comparison. Figure 4c shows the TA spectra at the indicated delay times under back-excitation (at 400 nm, 3.3 µJ/cm2/pulse), in which the TA spectra are dominated by the bleach signals from the 3D phase. However, the TA spectra of a conventional quasi-2D perovskite film with n=4

under back-excitation were dominated by the bleach

signals from small-n phase (Figure S11a) For comparison, the TA spectra of the hierarchical quasi-2D/3D film with the front-excitation are shown in Figure 4e. The TA spectra exhibit two bleach peaks at n = 3 and 3D perovskite absorption bands. A small bleach peak between 645 nm and 735 nm is identified as the n>3 perovskite phase. This difference in TA spectra between back and front-excitations shows the small-n perovskite phase grown on the surface of 3D perovskite phase in our 11



superstructure, which is consistent with the result observed in PL spectra. We investigated the TA kinetics (probed at the n = ∞ band, 750 nm) under back-excitation at 400 nm. (Figure 4d and Figure S11b). By fitting the TA kinetics with multiexponential functions, we obtained the exciton lifetime of ∼22 ps and 160 ps (Table S2), for the hierarchical quasi-2D/3D perovskite superstructure and conventional quasi-2D film, respectively. It should be noted that although both quasi-2D/3D perovskite superstructure and conventional quasi-2D film were mixture of multiple phases, the n alignment in these structure is in an opposite way. Therefore, the shorter exciton lifetime from the hierarchical quasi-2D/3D perovskite indicated that the photogenerated carriers are mainly recombined in large n perovskite phase, and less energy transfer from small-n to large-n likely due to the insufficient absorption of small-n on the top of perovskite. We further investigated the TA kinetics (probed at n=3 and n = ∞ band) for the present superstructure under front-excitation at 750 nm. (Figure 4e). The exciton lifetime (probed at n = ∞ band) (42 ps) is longer than the exciton lifetime (probed at n = 3 band) (9 ps) (Figure 4f & Table S2), resulting from the efficient electron transfer from small-n to large-n and the presence of small-n phase on the top. These TA results confirmed that the energy transfer from small n to large n was efficient when illuminated from the small n side, and the present superstructure serves a distinct energy funnel that photoexciton will transfer from top to the bottom.

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Figure 5 SEM images of perovskite ﬁlms grown at room temperature for 1 h with different BAI/MAI concentration (by fixing BAI/MAI mass ratio at 2:1); a) 7.5 mg, b) 10 mg, c) 12.5 mg, d) 15 mg, e) 17.5 mg, f) 20 mg, g) 30 mg and h) 40 mg. To understand the film growth mechanism, we carried out SEM measurement for the hierarchical structure upon different growth condition. The concentration of the BAI and MAI in the growth solution was first considered. Figure 5 describes SEM images of quasi-2D/3D perovskite ﬁlms grown at room temperature for 1 h with different organic cation concentration by fixing the same BAI/MAI mass ratio at 2:1. There are only sparse NSs on the surface when the film is prepared under the concentration of 7.5 mg/mL MAI. (Figure 5a) As the concentration increased to 10 mg/mL, denser NSs are grown on the surface (Figure 5b), indicating that the density of NSs will increase with the raise of the MAI concentration. However, this trend is reversed when the MAI concentration is in the range of 10 mg/mL-15 mg/mL. As shown in Figure 5c and Figure 5d, the increase of MAI concentration leads to sparser NSs on the surface. Furthermore, when the MAI concentration exceeds 15 mg/mL, the vertical aligned NSs were absence from the 3D perovskite film. (Figure 5e-h) Based on the above observations, we believe that a 10 mg/mL-15 mg/mL MAI concentration is critical for the formation of dense hierarchical quasi-2D/3D perovskite superstructure. Interestingly, we found that the hierarchical quasi-2D/3D perovskite both appeared when the concentration of BAI exceeded 20 mg/mL, as shown in Figure 1 and Figure 13



5. In general, we should consider whether the absolute BAI concentration is the key to obtain quasi-2D/3D perovskite superstructure rather than the initial ratio of BAI/MAI and their concentrations. Thus, we fabricated the perovskite structure with fixed BAI absolute concentration (20 mg/mL or 30 mg/mL) and changed the initial ratio of BAI/MAI from 1:2 to 2:1(Figure S12). We find the NSs only appeared in the initial ratio of 2:1. Therefore, we concluded that the absolute BAI concentration couldn’t serve as the sole parameter that determines the growth of NSs. We also studied the temperature effect that influences the growth of quasi-2D/3D superstructure. Figure S15 shows SEM images of PbI2 thin films dipping into a MAI/BAI solution, by varying the substrate temperature or the MAI/BAI solution temperature from 25 °C to 75 °C. As shown in Figure S15, a decrease in temperature for the PbI2 film (Figure S15a-c) or solution (Figure S15d-f) will both lead to the formation of NSs with slightly lower density, which could be attributed to the decreased nucleation sites. Furthermore, by comparing Figure S15a-c and Figure S15d-e, we found that the temperature effect from solution was similar to that of the PbI2 film. Notably, the variation on the PbI2 film temperature, although only impacts the crystal growth in a very short period of time, still leads to the formation of NSs with different density. This suggested that the quasi-2D NSs were kinetically favorable and mainly formed in the initial stage, despite the higher Gibbs formation energy of quasi-2D perovskite as suggested by recent report. Figure S16 shows the emission spectra of hierarchical quasi-2D/3D perovskite superstructure prepared with different PbI2 film temperature illuminated from the front side of the film. The amount of the small n value perovskite was increased with the increase of PbI2 film temperature, which is in agreement with the SEM images. In addition, Figure S17 shows top view SEM images of PbI2 thin films and the corresponding quasi-2D/3D perovskite structure obtained by spin-coating the precursor solution with or without acetonitrile. Remarkably, we can observe that the orientation of the NSs could be readily tuned by controlling the morphology of the surface of the PbI2 film via additive involvement.

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Figure 6 Schematic of the dendritic growth mechanism of the oriented quasi-2D perovskite NSs. Unraveling the growth mechanisms of quasi-2D/3D perovskite superstructures is an essential prerequisite to understand the mechanisms regarding the orientation of qusi-2D perovskite tuned by BAI. On the basis of the above analysis, we proposed a growth mechanism of this hierarchical quasi-2D/3D perovskite structure, as shown in Figure 6. In the initial stage, the quasi-2D/3D perovskite (n=2, 3, 4) is likely to grow first on the PbI2 film surface when compared to the three-dimensional perovskite. This is also suggested by recent reports, that the multiple perovskite phases naturally align in the order of n along the direction perpendicular to the substrate33, whereas the smaller n value was formed first in the bottom when the PbI2 and BAI/MAI were mixed homogeneously in one-step coating.33 It should be mentioned that the formation of the present quasi-2D/3D perovskite superstructure followed a two-step process, in which the transformation of PbI2 to perovskite phase was occurred from the top to the bottom. Along with the formation of quasi-2D/3D perovskite on the surface, the concentration of BAI near the PbI2 surface is dramatically decreased, which is not sufficient for its subsequent lateral growth. However, the BAI concentration in the bulk solution remains rather high that is favorable for the vertical growth of quasi-2D NSs. In contrast, the 3D perovskite phase was more likely grown under the quasi-2D NS perovskite, as a result of the much smaller size of MA+. We 15



further found that the BAI concentration gradient in the precursor solution is centrally relevant to the vertical growth of perovskite NS on the film. Too higher or lower BAI concentration is not suitable for the formation of hierarchical quasi-2D perovskite structure. This is likely due to that too much nucleation center will promote the lateral growth of thin film without forming NS. On the contrary, the quasi-2D NS couldn’t exhibit a preferred growth without continuous supply of BAI with certain concentration. It is possible to have epitaxial growth from a polycrystalline film with majority of the perovskite NSs aligned nearly perpendicular to the 3D perovskite substrate, as PbI2 films on ITO are highly textured with grains aligned preferentially along certain crystallographic direction. Nevertheless, not all grains are aligned. Still there is a subtle amount of quasi-2D NSs grow parallel to the substrate, which is probably due to some newly formed NSs may hamper the growth of the nearby NSs. Thus, only those NSs with growth axis nearly aligned perpendicular to the substrate could continue to grow, resulting the array-like appearance of the NSs. The growth mechanism as described here of oriented quasi-2D/3D perovskite superstructures is similar to the mechanism of dendritic growth.

Figure 7 a) Schematic illustration of the photodetector; b) Photocurrent responses of the photodetectors on light illumination showing time-dependent photosensitivity with a time interval of 10s at 0V; High sensitivity to visible light is one of the most interesting feature of perovskites, which is the basic parameter required for photodetector. Recently, perovskite photodetectors using a wide range of materials (e.g. thin film,46-48 nanowire,49-51 NS,30,52-54 single crystal55,56) have been reported. Our hierarchical structure 16


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quasi-2D/3D perovskite superstructure based photodetector (Figure 7a) exhibits substantially high light current (2.26 mA, corresponding responsivity 0.184 A/W, corresponding detectivity 8.13×1010 Jones), which is the due to the improved light harvesting efficiency at the region of 550-700 nm in the present hierarchical structure quasi-2D/3D perovskite. In a photo-switching measurement (Figure 7b), the device displayed nearly 103 on/off current ratio under no bias (0 V), which is one of high responsivity

among

perovskite

photodetectors.

The

hierarchical

perovskite

superstructures with small-n phase perovskite on the surface of 3D film could simultaneously improve the long-term moisture stability of the photodetector.

Conclusion In summary, a facile, two-step continuous deposition method was developed for the first time to grow oriented quasi-2D perovskite NS structure on 3D perovskites film. The n value, density and thickness of the quasi-2D NS could be varied by changing the growth parameters, such as time, temperature, initial reactant concentration, ratio, and PbI2 film. Both the initial ratio of BAI/MAI and the corresponding concentration play a key role in driving the nucleation and growth of oriented quasi-2D perovskite NS on 3D perovskites, which is similar to the dendritic growth mechanism. The carrier transfer within the unique superstructure was found to be efficient from the small n to the large n phase, indicating a distinct energy funnel that photoexciton will transfer from top to the bottom. We also demonstrated a 103 on/off ratio of the photodetector based on the hierarchical quasi-2D/3D perovskite superstructure. This work provides an exquisite control on the perovskite hierarchical structure, which may enrich the fundamental understanding of perovskite growth and its application in photovoltaics and other optoelectronics devices.

Acknowledgements H. Zhou acknowledges funding support from the National Natural Science Foundation of China (51722201) (U1508202) (51672008) (51673025), National Key 17



Research and Development Program of China Grant No. 2017YFA0206701, 2016YFB0700700, Young Talent Thousand Program and ENN Group. The authors thank support from Beijing Key Laboratory for Magneto-photoelectrical Composites and Interface Science.

Supporting Information Experimental Section; FESEM images; optical images; figures of XRD patterns; contact angle images and tables of fitting parameters.

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