In Situ Fabrication of Highly Luminescent Bifunctional

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In Situ Fabrication of Highly Luminescent Bifunctional Amino Acid Crosslinked 2D/3D NH3C4H9COO(CH3NH3PbBr3)n Perovskite Films Taiyang Zhang, Liqiang Xie, Liang Chen, Nanjie Guo, Ge Li, Zhongqun Tian, Bingwei Mao,* and Yixin Zhao* photoluminescence (PL) yield for potential optoelectronic application such as in LED and laser devices.[10–18] Luminescent MAPbBr3 films have been deposited onto mesoporous Al2O3 substrate.[19,20] Beside regular bulk lead halide perovskite, 2D or nanostructured lead halide perovskites had been previously synthesized for use as LEDs or other optoelectronic materials.[21–36] These progresses have demonstrated the great success of classic solution chemistry method in the preparation of high quality lead halide nanocrystals with excellent photoluminescent properties for optoelectronic applications. It should be pointed out that although high quality nanocrystals can be successfully fabricated into devices via follow-up processes, a high quality film is more easy-to-process than nanocrystals suspensions for optoelectronic applications. For example, smooth lead halide perovskite films have been directly fabricated via simple one-step or two-step method in solar cell applications.[37] However, we found the high quality smooth planar MAPbBr3 films for solar cells usually do not exhibit high photoluminescent properties as the nanocrystals suspension or the film deposited in the mesoporous Al2O3 matrix. This is because the MAPbBr3 films usually consist of large perovskite nanocrystals with size up to hundreds nanometers to micrometers, while the mesoporous support can be used to prevent crystal growth and control the particle size. As recently reported, lead halide perovskite nanoplate with quantum confine without the mesoporous support could grow in large crystals and thus lose the quantum confine effect.[38] It would be urgent to develop a facile method to fabricate highly luminescent perovskite-based film with good controllability and high stability. Metal-organic frameworks (MOFs) are periodic networks formed by coordination of metal ions with organic molecules. This configuration bears the similar structure as the organic– inorganic perovskite from the viewpoints that both consist of the metal ions and organic molecules in the framework or lattice. Inspired by the ease preparation and various advantages of the MOFs configuration,[39–41] here, we report a strategy for fabricating highly luminescent and stable lead bromide

The perovskite quantum dots are usually synthesized by solution chemistry and then fabricated into film for device application with some extra process. Here it is reported for the first time to in situ formation of a crosslinked 2D/3D NH3C4H9COO(CH3NH3)nPbnBr3n perovskite planar films with controllable quantum confine via bifunctional amino acid crosslinkage, which is comparable to the solution chemistry synthesized CH3NH3PbBr3 quantum dots. These atomic layer controllable perovskite films are facilely fabricated and tuned by addition of bi-functional 5-aminovaleric acid (Ava) of NH2C4H9COOH into regular (CH3NH3)PbBr3 (MAPbBr3) perovskite precursor solutions. Both the NH3+ and the COO− groups of the zwitterionic amino acid are proposed to crosslink the atomic layer MAPbBr3 units via PbCOO bond and ion bond between NH3+ and [PbX6] unit. The characterizations by atomic force microscopy, scanning electron microscopy, Raman, and photoluminescence spectroscopy confirm a successful fabrication of ultrasmooth and stable film with tunable optical properties. The bifunctional crosslinked 2D/3D Ava(MAPbBr3)n perovskite films with controllable quantum confine would serve as distinct and promising materials for optical and optoelectronic applications.

1. Introduction The exceptional and unparalleled progress in perovskite halides (e.g., CH3NH3PbI3 or MAPbI3) solar cells in the past years have attracted tremendous research attentions, which has been even described as “perovskite fever.”[1–9] Beside the high photovoltaic performance, the lead halide perovskites also demonstrate their superior optical properties such as tunable band gap and high T. Zhang, N. Guo, G. Li, Prof. Y. Zhao School of Environmental Science and Engineering Shanghai Jiao Tong University 800 Dongchuan Road, Shanghai 200240, China E-mail: [email protected] L. Xie, L. Chen,Prof. Z. Tian, Prof. B. Mao State Key Laboratory of Physical Chemistry of Solid Surfaces Department of Chemistry College of Chemistry and Chemical Engineering, iChEM Xiamen University Xiamen 361005, China E-mail: [email protected]

DOI: 10.1002/adfm.201603568

Adv. Funct. Mater. 2016, DOI: 10.1002/adfm.201603568

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perovskite planar film with crosslinked 2D/3D configuration with tunable quantum confine as shown in Figure 1. In this novel configuration, we adopted a bifunctional molecule of NH2C4H9COOH (denoted as Ava) to crosslink the [PbX6] metal halide units in the segments of perovskite structure to form a crosslinked framework structure. As illustrated in the Figure 1, the Ava molecules in the form of +NH3C4H9COO− crosslink these framework units via its protonated NH3+ groups to replace the surface MA and deprotonated COO− group to coordinate with unoccupied surface Pb, respectively. This configuration is denoted as Ava(MAPbBr3)n, where n is molar ratio of MAPbBr3/Ava that is also consistent with the layer number of the octahedral [PbX6] unit. Figure 1 shows the schematic illustration of Ava(MAPbBr3)n of crosslinked structure with two layers of [PbX6] units. This novel Ava(MAPbBr3)n configuration helps not only the formation of smooth planar film but also help control of the quantum confine via adjusting the n values.

2. Results and Discussion Ava(MAPbBr3)n (n = 2,4,8) films are formed by varying amount of Ava amino acid added into stoichiomertic MABr+PbBr2 precursor solutions. However, the films still exhibit standard MAPbBr3 perovskite XRD pattern.[42] This confirms that the

additional of Ava does not affect the structure of the MAPbBr3 unit as shown in Figure 1. Nevertheless, the Ava(MAPbBr3)n perovskite films show peak width broadening with decrease of MAPbBr3 unit number, which reveals that the addition of Ava in precursor solution may reduce the grain size of MAPbBr3 perovskite crystals in certain orientation, which is consistent with the proposed configuration for Ava(MAPbBr3)n with limited number of unit. Furthermore, small-angle XRD patterns suggest the presence of periodic structure of the Ava(MAPbBr3)n (n = 2,4,8) samples, but diffraction peaks are shifted and the periodic unit sizes calculated from these 2θ values are 2.2, 3.6 and 6 nm, which leads to our proposal of unit cells for Ava(MAPbBr3)n with n equals to 2, 4, and 8. Of course, the layer numbers of the 2D/3D Ava(MAPbBr3)n perovskite should be a distribution with average value of n rather than a single homogenous n values, which has been previous observed in the quasi 2D perovskites by Sargent group.[43] The regular MAPbBr3 perovskite film prepared via solvent engineering is typically rough films full of varied sized MAPbBr3 nanocrystals as shown in SEM image of Figure 2, the AFM image show the clear crystal grain boundary with up to 100 nm roughness.[44,45] In contrast, the Ava(MAPbBr3)n films are ultrasmooth and the crystal grain became less observable in the SEM with the reduced number of n, which is consistent with their AFM images. The roughness of these Ava(MAPbBr3)n films characterized by AFM is

Figure 1.  A) Schematic chemical and B) crystal structure of Ava crosslinked Ava(MAPbBr3)n (n = 2). C) XRD and D) small-angle XRD pattern of Ava(MAPbBr3)n films.

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full paper Figure 2.  SEM images of A) regular MAPbBr3 and Ava(MAPbBr3)n [n = 8 B), 4 C), 2 D) perovskite films; AFM images of a) regular MAPbBr3 and Ava(MAPbBr3)n [n = 8 b), 4 c), 2 d) perovskite films. The scale bar is 500 nm.

decrease with the n value, the Ava(MAPbBr3)2 sample exhibited less than 10 nm roughness as compared to MAPbBr3. Figure 3 gives the UV–vis and fluorescence spectra of Ava(MAPbBr3)n (n = 2, 4, 8) perovskite films. The absorbance peaks of these Ava(MAPbBr3)n perovskite films show blue shift with increase of Ava amount. The band gap of Ava(MAPbBr3)n is widened to 2.58 eV, which is about 0.23 eV higher than that of regular MAPbBr3 perovskite. The UV–vis spectroscopic results clearly demonstrate that the bandgaps of the Ava(MAPbBr3)n perovskites can be tuned by adjusting the n value of Ava(MAPbBr3)n. Furthermore, the defect-related absorption edge tailing becomes less severer and even totally disappears in the Ava(MAPbBr3)n perovskite films. The florescence spectra of the Ava(MAPbBr3)n perovskite films, including wavelength, intensity, and emission peak width, also exhibit an obvious Ava dependence. Figure 3b shows that the regular MAPbBr3 film has a very weak fluorescence peak at ≈550 nm with a half bandwidth of 40 nm, which

is consisted with its UV–vis spectra showing absorbance edge tailing and their varied large crystal sizes. The emission wavelength of these Ava(MAPbBr3)n perovskite films can be tuned from 550 to 500 nm. Their emission peaks become narrower with increase of Ava amount, and the half bandwidth decreases to less than 20 nm in Ava(MAPbBr3)2 perovskite film. More impressively, the fluorescence intensity of the Ava(MAPbBr3)n films shows a significant dependence on the Ava/MAPbBr3 ratio: The photoluminescence intensity of the Ava(MAPbBr3)2 perovskite film is almost three orders of magnitude higher than that of the regular smooth MAPbBr3 perovskite film. And the photoluminescence quantum yield of the Ava(MAPbBr3)2 perovskite film is up to 80%. Figure S1 (Supporting Information) shows the PL device based on these Ava(MAPbBr3)4 perovskite films, which exhibits pure green color. The time-resolved photo­luminescence decay curves of these Ava(MAPbBr3)n films are given in Figure 3C, showing that these highly luminescent Ava(MAPbBr3)n perovskite films have longer PL lifetimes

Figure 3.  A) UV–vis spectra, B) fluorescence spectra, and C) Time-resolved photoluminescence decay curves of Ava(MAPbBr3)n perovskite films compared to regular MAPbBr3 perovskite films. These transients follow biexponential decay. The weighted average of lifetime increases approximately from about 27 ns for MAPbBr3 to 32, 38, 49 ns for Ava(MAPbBr3)n with n = 8, 4, 2.

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than the pure MAPbBr3. These optical property measurements confirm that the Ava(MAPbBr3)n films contain much less trap states or intraband states as the Ava amount increases. Both UV–vis and PL results of Ava(MAPbBr3)n films have revealed a typical bandgap tuning and surface passivation similar to quantum effect observed in semiconductor quantum dots. The mechanism behind these observed optical property tuning could be attributed to the chemical management of perovskite’s composition or to the well-known quantum confinement as proposed in Figure 1. The size of the MAPbBr3 unit in the Ava(MAPbBr3)n framework is less than 6 nm based on the XRD result, which is in agreement with previous theoretical calculations, which predicted Bohr radius of around 5 nm for MAPbBr3.[46] To further clarify and confirm our proposed configuration in Figure 1, Raman spectroscopy was adapted here to clarify whether Ava is incorporated into perovskite lattice or just absorbed on the surface of perovskite films. Figure 4A shows the normal Raman spectra of the MAPbBr3 single crystals, MAPbBr3 film, MABr and Ava powders and Ava(MAPbBr3)2 film. For the Ava(MAPbBr3)2 sample, several peaks of Ava’s characteristic at e.g., 656, 1060, 1310 cm−1 are observed which

appear in the spectrum of Ava but not in MAPbBr3 films and single crystals, meaning that the Ava has been incorporated into the MAPbBr3 perovskites. Meanwhile, the MA+ related Raman peaks at 970, 1480, 1586, and 2970 cm−1 are also observed in the Ava(MAPbBr3)2 sample. The information suggest that the Ava is incorporated into the bulk perovskite films, not surface absorbed, because the Raman signals from the surface Ava would be too weak to be detected by normal Raman. Furthermore, when the MAPbBr3/Ava ratio of n in the Ava(MAPbBr3)n films is varied, the relative Raman peak intensities of MA+ and Ava change with an obvious Ava dependence as shown in Figure 4B,C. With the increase of Ava amount, the MA-related Raman peak decreases while the Ava-related Raman peak increases. This trend further confirms that Ava is incorporated into the bulk of MAPbBr3-based perovskite via replacement of some MA+. Furthermore νsym(NH3+) of the Ava appears at 2941 cm−1 while shifts to 2921 cm−1 when incorporated in MAPbBr3 lattice, as shown in Figure S2 (Supporting Information). The red-shift here is attributed to the formation of hydrogen bonding between NH3+ and Br− in the Ava(MAPbBr3)2 sample, which plays the key role to stable the

Figure 4.  Raman spectra of A) Ava, MABr, MAPbBr3 film, MAPbBr3 single crystal, and Ava(MAPbBr3)2 film, B) Ava(MAPbBr3)n perovskite films with different n values. The perovskite samples were placed in a sealed Raman cell filled with Argon atmosphere for measurements. C) Plot of integral intensity of Raman peaks at 2970 and 661 cm−1 as a function of Ava/MAPbBr3 molar ratio (1/n).

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into the crystal lattice of the MAPbBr3; (2) While AvaxMA1−xPbBr3 perovskites show XRD peak below 10°, no such peak is observed in our Ava(MAPbBr3)n film. These results elucidate that our proposed Ava(MAPbBr3)n is not AvaxMA1−xPbBr3, and we have produced a novel crosslinked configuration Ava(MAPbBr3)n as proposed in Figure 1. The above-mentioned results demonstrate that the bifunctional amino acids are effective agents for fabrication of Ava(MAPbBr3)n with controllable quantum confines and high luminescence. Based on such a model, we predict that addition of other amino acids including glycine, β-alanine, γ-aminobutyric acid, and 6-aminohexanioic acid can also effectively form different crosslinkage to tune the absorbance of the MAPbBr3 film. Unfortunately these amino acids cannot be dissolved well into MAPbBr3 DMF precursor solution like the Ava amino acid. The maximum dissolution ability of glycine, β-alanine, γ-aminobutyric acid, and 6-aminohexanioic acid with molar ratio to MAPbBr3 is about 0.01, 0.04, 0.36, and 0.24, respectively. Nevertheless, all of them also show significant absorbance and fluorescence change similar to the Ava(MAPbBr3)n samples as shown in Figure S5 (Supporting Information). Our proposed 3:1 fixed Pb/Br ratio Ava(MAPbBr3)n perovskite with monolayer Ava intercalation is somehow similar to AvaBr-(MAPbBr3)n, in which the Ava+ only occupied the A site without formation of PbCOO to replace Br with COO group. This AvaBr-(MAPbBr3)n with only monolayer Ava+ is some different from previously reported homologous 2D (RNH3)2An−1MnX3n+1 perovskite with bilayer long chain molecular.[31–33] The structure of our AvaBr-(MAPbBr3)n perovskite with (RNH3)AnMnX3n+1 composition is shown in Figure 5A. This 2D/3D AvaBr-(MAPbBr3)n perovskite can also be obtained by using AvaBr+n(MAPbBr3) precursor instead of the Ava+(nMAPbBr3) composition. Although the COO− group of Ava in Ava(MAPbBr3)n replace the Br atom as crosslinker while the Ava+ in AvaBr-(MAPbBr3)n only work as monolayer long chain molecular, the XRD pattern and optical properties of the AvaBr-(MAPbBr3)n (n = 2, 4, and 8) listed in Figure S6 (Supporting Information) show similar trends to those of the Ava(MAPbBr3)n (n = 2, 4, 8). But the absorption peak of AvaBr-(MAPbBr3)n has less exciton characteristic peaks as the Ava(MAPbBr3)n, which is consistent with the lower photoluminescence of AvaBr-(MAPbBr3)n compared to Ava(MAPbBr3)n samples. Such a similarity could be due to the fact that the AvaBr-(MAPbBr3)n and Ava(MAPbBr3)n have the similar atomic layer structured MAPbBr3 in the core while the surface PbCOO bond on the surface has less

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MAPbBr3 perovskite unit. Unfortunately, the formation of PbCOO bond cannot be directly confirmed by Raman spectrum as NH3+ group or by FTIR due to the strong IR absorption of lead halide perovskite. However, we found that our Ava(MAPbBr3)2 precursor solution would slowly form some unknown white precipitate when it is stored for long duration, which could be due to the formation of unknown PbCOO complex. The material characterization in Figure S3 (Supporting Information) reveals that this white precipitate is different from our 2D/3D perovskite of Ava(MAPbBr3)2 or MAPbBr3 with unknown composition, but the FTIR results especially the shift of νCOO in the precipitate compare to νCOOH in Ava revealed that PbCOO bond might form in this precipitate. Furthermore, the MAPbBr3 solution would totally become gel once we add citrate salt with crosslinkable COO group. These observations suggested that the COO− group can form bonds with MAPbBr3, most likely the PbCOO at the uncoordinated Br− position on the surface of [PbX6] units. In the MAPbBr3 or CsPbBr3 quantum dots, the surface Pb atom is uncoordinated if there is no extra Br ions and other surfactant. To fully coordinate these uncoordinated surface atoms, there is extra Br precursor such as HBr used in these lead halide perovskite quantum dot synthesis.[38] To the best of our knowledge, there is neither report of amino acid crosslinked configuration lead halide perovskite nor fabrication of lead halide perovskite using zwitterionic bifunctional Ava in perovskite. Nevertheless, Han group has developed well-known highly stable perovskite solar cells using AvaxMA1−xPbI3 proveskite prepared by using stoichiometric precursor of xAvaI+(1−x)MAI+PbI2 solutions. Their work has demonstrated that the Ava+ cations can replace some MA+ sites in the MAPbI3 perovskite. However this AvaxMA1−xPbI3 perovskite exhibits a strong additional new peak between 5° and 10° on the XRD pattern.[8] Here, it is important to note that the neutral zwitterionic Ava molecules in the xAva+MAPbBr3 precursor solution employed in our work is substantially different from the totally protonized Ava+ cations in the xAvaBr+(1−x)ABr+PbBr2 precursor solution reported in the work by Han group. For comparison, we have also tried using the 0.5AvaBr+0.5MABr+PbBr2 precursor. In this case, the prepared perovskites also show new peaks at between 5° and 10° on the corresponding XRD patterns but with different optical properties as shown in Figure S4 (Supporting Information). The above-mentioned results can be summarized as follows: (1) The Raman spectra suggest that the Ava has been incorporated

Figure 5. A) Schematic illustration of crystal structure of 2D/3D AvaBr-(MAPbBr3)n (here n is 2). B) The photoluminescence intensity of the Ava(MAPbBr3)n compared with the AvaBr-(MAPbBr3)n with the same absorbance. C) SEM images of the AvaBr-(MAPbBr3)2 and Ava(MAPbBr3)2, scale bar = 500 nm.

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impact on their electronic structure in the Ava(MAPbBr3)n. The configuration difference between Ava(MAPbBr3)n and AvaBr-(MAPbBr3)n could lead to the different layer thickness, the AvaBr-(MAPbBr3)n should have a larger layer thickness than those crosslinked Ava(MAPbBr3)n. The small-angle XRD peak of the AvaBr-(MAPbBr3)2 in Figure S7 (Supporting Information) shifts to 3.6° while the AvaBr-(MAPbBr3)n locates at 4.0°, which suggests 2.4 nm thickness and 2.2 nm thickness, respectively. According to our crosslinked configuration of Ava(MAPbBr3)n, the replacement of the MA+ by NH3+ group and the coordination between COOPb are beneficial to passivation of the surface and the smoothening of the Ava(MAPbBr3)n films. In contrast, the Ava+ in AvaBr-(MAPbBr3)n only works as a spacer with less weaker hydrogen bond between MA+ and COOH group to form the 2D structure and these atomic layered (MAPbBr3)n perovskite films without Ava crosslinkage are highly irreproducible in the PL intensity and film morphology. The PL intensity of the different batches of AvaBr-(MAPbBr3)n like AvaBr-(MAPbBr3)2 can varied over one magnitude as shown in Figure S8 (Supporting Information), while the Ava(MAPbBr3)n samples are highly reproducible. Even the best AvaBr-(MAPbBr3)n samples’ PL intensity are always lower than the corresponding AvaBr-(MAPbBr3)n samples as shown in Figure 5B. Besides, the AvaBr-(MAPbBr3)n also exhibited varied morphology, and it is found that the smooth freshly grown AvaBr-(MAPbBr3)n, samples quickly turn coarse. Figure 5C shows SEM image of the AvaBr-(MAPbBr3)2 perovskite films with varying crystal sizes, while the Ava(MAPbBr3)2 perovskite films look ultrasmooth.

Although the Ava+ in the AvaBr-(MAPbBr3)n cannot effectively crosslink the (MAPbBr3)n layers as the PbCOO bond in Ava(MAPbBr3)n, the hydrogen bond between MA+ and COOH might still help effective forming the luminescent 2D/3D AvaBr-(MAPbBr3)n with the typical MAPbBr3 XRD patterns. Once we tried switching to using other alky amine without COOH group such as pentylamine (PA) and pentyleammonium bromide (PABr), it is unsuccessful to in situ fabricate the planar perovskite films with the composition of PABr-(MAPbBr3)2 and PA(MAPbBr3)2 although the pentylamine has the similar chain length as the Ava and amine are widely adapted in solution synthesis of MAPbBr3 QDs. Figure 6 lists the XRD, optical properties and SEM images of the perovskite films with composition of PABr-(MAPbBr3)2 and PA(MAPbBr3)2. XRD patterns reveal that both PABr-(MAPbBr3)2 and PA(MAPbBr3)2 exhibit the standard MAPbBr3 XRD peaks. However, PABr-(MAPbBr3)2 and PA(MAPbBr3)2 have some unknown impurity peaks below 14° especially in PA(MAPbBr3)2, the obvious below 10° peak in PABr-(MAPbBr3)2 indicated the formation of mixture of 3D and 2D perovskite structure rather than the homologous 2D/3D perovskites when we used PABr. The mixed absorption peaks of PABr-(MAPbBr3)2 and PA(MAPbBr3)2 at 432 nm and 450 nm are some similar to previous reported solution synthesis 2D perovskite nanoplates with impurities.[47] Meanwhile, SEM image of PABr-(MAPbBr3)2 reveal that the films are consisted with small particles, which is consistent with the XRD and UV–vis results. All those results demonstrated that the PA or PABr with composition of (RNH3)AnMnX3n+1 or (RNH3)AnMnX3n can only lead to formation of mixed 2D and 3D perovskite nanoparticles rather than

Figure 6.  A) XRD patterns and B) UV–vis spectrum of PABr-(MAPbBr3)2, PA-(MAPbBr3)2, and MAPbBr3. C) SEM images of the PA-(MAPbBr3)2 and PABr-(MAPbBr3)2; (D) PL spectrum of PABr-(MAPbBr3)2, PA-(MAPbBr3)2, and Ava-(MAPbBr3)2, scale bar = 1 μm.

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3. Conclusion In summary, we have proposed and fabricated a crosslinked Ava(MAPbBr3)n perovskite with framework of atomic layer of MAPbBr3 units crosslinked by the NH3+ end groups of the Ava molecules occupying the MA+ positions on the surface of one unit and by the COO− end groups of the same Ava molecules occupying the Br− sites on the surface of a nearby unit. Such novel crosslinked 2D/3D lead halide perovskite can be prepared facilely by in-situ spin coating the MAPbBr3 precursor solution into which varying amount of zwitterionic bifunctional amino acid is introduced. Such perovskite films are ultrasmooth without obvious crystal grains and show quantum confine optical properties. Raman spectroscopic results also indicated that Ava has been incorporated into MAPbBr3 lattice forming the crosslinked 2D/3D Ava(MAPbBr3)n perovskite rather than the known hybrid AvaxMA1−xPbBr3 structure. The quantum confine induced optical properties of Ava(MAPbBr3)n can be easily tuned by varying the Ava/MA ratio, and PLQY of up to ≈80% has been obtained for Ava(MAPbBr3)2. This crosslinked 2D/3D perovskite films can also be successfully obtained when extending to other amino acids. The strategy of forming crosslinked 2D/3D lead halide perovskite would serve as a promising approach for in situ deposition of lead halide perovskites quantum dots films for potential photovoltaic and optoelectronic application in future.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements Y.Z. acknowledges the support of the NSFC (grants 51372151 and 21303103) and Huoyingdong grant (151046). B.W. acknowledges the support of MOST (grant 2012CB932902). Z.Q.T. acknowledges NSFC (grant 21321062). The authors are grateful to Professors Y. B. Jiang and Y. Q. Li at Xiamen University for valuable discussions on florescence spectroscopy. Received: July 15, 2016 Revised: September 15, 2016 Published online:

Adv. Funct. Mater. 2016, DOI: 10.1002/adfm.201603568

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the homologous 2D perovskite in the one-step deposition method using Ava or AvaBr. Such difference between PABr and AvaBr might could be attributed to the effective hydrogen bond between COOH group and MA+. Furthermore, the PL performance of PABr or PA-based films is much weaker than Ava-based film. The PL intensity of PABr-(MAPbBr3)2 and PA(MAPbBr3)2 is almost three order of magnitudes lower than Ava-(MAPbBr3)2. Both the AvaBr-(MAPbBr3)n and the PA/PABr-(MAPbBr3)n results clearly demonstrate the importance of amino acidinduced crosslinked configuration to in situ formation of Ava(MAPbBr3)n perovskite quantum confined planar films by facile one-step method although the regular amine is effective to help form the perovskite QDs in the regular solution synthesis.

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Adv. Funct. Mater. 2016, DOI: 10.1002/adfm.201603568