Synthesis and Stabilization of Colloidal Perovskite Nanocrystals by

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Synthesis and stabilization of colloidal perovskite nanocrystals by multidentate polymer micelles Shaocong Hou, Yuzheng Guo, Yuguo Tang, and Qimin Quan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 19 May 2017 Downloaded from http://pubs.acs.org on May 21, 2017

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ACS Applied Materials & Interfaces

Synthesis and Stabilization of Colloidal Perovskite Nanocrystals by Multidentate Polymer Micelles Shaocong Hou,1 Yuzheng Guo,1,2,3 Yuguo Tang,2 and Qimin Quan1* 1

Rowland Institute at Harvard University, Cambridge, MA 02142, USA

2

Suzhou Institute of Biomedical Engineering and Technology, Chinese Academy of Science,

Suzhou 215163, P. R. China 3

College of Engineering, Swansea University, Fabien Way, Swansea, SA1 8EN, UK

KEYWORDS: Colloidal perovskite nanocrystals, multidentate ligands, polymer micelles, hybrid nanostructure, stability

ABSTRACT: Lead halide perovskites have emerged as low-cost, high-performance optical and optoelectronic materials, however, their material stability has been a limiting factor for broad applications. Here, we demonstrate stable core-shell colloidal perovskite nanocrystals using a novel, facile and low-cost co-polymer templated synthesis approach. The block co-polymer serves as a confined nano-reactor during perovskite crystallization and passivates the perovskite surface by forming a multidentate capping shell, thus significantly improving its photo-stability in polar solvents. Meanwhile, the polymer nanoshell provides an additional layer for further surface modifications, paving the way to functional nanodevices that can be self-assembled or lithographically defined.

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Lead halide perovskites are emerging as high-performance materials for photovoltaics, lightemitting devices and displays, due to their long carrier lifetime, low defect density, high charge carrier mobility and high fluorescent quantum yield, achievable in a low-cost, solution-based process.1-4 However, their inferior stability in polar solvents and humid environments, as well as their rapid photodegradation,5-6 has placed a grand challenge to practical applications. Device capsulation7 has been a viable way to improve stability, however, it only works at the macroscopic level. Embedding or crosslinking the perovskite crystals in a hydrophobic matrix8-12 has been proposed to work at the microscale level, which often comes at a price of sacrificing their optical properties due to incompatibility between perovskite nanocrystals and the encapsulation matrix unless proper interfacial layers were controlled. In this work, we present a novel, solution-based, co-polymer templated synthesis strategy to create perovskite nanocrystals with a multidentate polymer nanoshell. Our perovskite nanocrystals exhibit moderate quantum yield and long-term photo-stability in proton-donating polar solvents over one month. This demonstration shows new avenues for future optimization of the stability issue and opens up new opportunities for biomedical applications. Hot injection13 and low temperature supersaturation recrystallization14-15 are the prevalent methods for colloidal perovskite nanocrystal synthesis, where small-molecule ligands, such as oleic acid and oleylamine, form weakly monodentate bonds to ionic perovskite nanocrystal. Their intrinsic dissolubility equilibrium and dynamic ligand binding processes lead to serious degradation under high humidity and polar solvents.4 In particular, proton-donating solvents lead to easy cleavage of the weakly capped ligands and lead species from nanocrystal surface.16-17


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Figure 1. Synthesis of lead halide perovskite nanocrystals. (a) Synthetic strategies for perovskite nanocrystals using amphiphilic block co-polymer micelles as nanoreactors. (b) UV-vis absorption and fluorescence spectrum of as-prepared CsPbBr3 nanocrystal colloids. Inset: block polymer micelle solution (left), block polymer micelle encapsulated PbBr2 (middle) and CsPbBr3 nanocrystals (right) under white lamp (upper) or under UV lamp (bottom). (c) Time evaluation of fluorescence peak positions and FWHMs during reaction. To overcome these problems, we create a nanosized shell of polymer ligands that bind strongly to the nanocrystal core via multidentate coordinating groups. The di-block copolymer, polystyrene-block-poly-2-vinylpyridine (PS-b-P2VP), was chosen as the micelle template. The


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efficient binding between the P2VP block and the lead precursor (PbBr2) via intramolecular metal-coordinating interaction increases the solubility of the PbBr2 precursor. The P2VP block can also control the crystal nucleation and growth, and finally bind to the as-reacted perovskite nanocrystals via multidentate pyridine groups.18-19 Moreover, the long-chain hydrophobic PS block can sterically stabilize the lead precursors (during reaction), increase the colloidal stability (after reaction) and protects the enclosed perovskite nanocrystals from chemical attacks from unfavorable environment (e.g. moisture, polar solvents).10-11 The synthesis routine is illustrated in Figure 1a. First, PS-b-P2VP was dissolved in a selective nonpolar solvent to the PS block (i.e. toluene) to promote self-assembly into inverse micelles (core: P2VP; shell: PS). Next, lead halide precursors (PbBr2, ca. 0.1 mg/mL), otherwise insoluble in toluene, were preferentially uptaken by forming coordinating complex with the P2VP block. The other precursor, CsBr, was dissolved in methanol (15 mg/mL), which provided a higher mobility to diffuse into the PbBr2-uptaken micelle.19 After CsBr was added, a solution phase reaction of two precursors took place immediately, followed by fast crystallization within 5 min, yielding one or more inorganic perovskite CsPbBr3 nanocrystals within the micelle, exhibiting strong fluorescence under UV excitation. (Figure 1b) The kinetics of the reaction process was studied by monitoring the photoluminescence (PL) of the colloidal solution in real time. (Figure S1) The fluorescence spectrum during the reaction indicated two distinct growth stages, as shown in Figure 1c. The PL energy decreased rapidly as the reaction started, indicating a fast growth of the nanocrystals in size. The decreased full-width half-maximum (FWHM) implied a size-distribution focusing process.20 After ~75 s, an increase in FWHM was observed, which corresponded to the defocusing of the size distribution. This was due to the decreased precursor concentration in the micelle, leading to the onset of Ostwald ripening process. We note that the


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focusing stage was not observed in CsPbBr3 synthesis by hot-injection method using small molecule ligand (e.g. oleic acid and oleylamine).21 We speculate that current polymer ligand and room temperature reaction have led to the focusing effect, by which the size dispersity could be potentially optimized. The optimal capping capacity of P2VP was crucial for the micelle templated synthesis, since weaker capping ligands did not help to uptake lead precursors and to confine nanocrystal growth, while stronger capping ligands, such as amines, might inhibit nucleation completely at low temperature.22

Figure 2. Structure of perovskite nanocrtals. (a-c) TEM of polymer micelles containing PbBr2 precursors (a), multiple CsPbBr3 nanocrystals (b) and single CsPbBr3 nanocrystal (c). Scale bar: 10 nm. (d) High-resolution TEM of the CsPbBr3 nanocrystal. (e) XRD of the complex PbBr2:PSb-P2VP and CsPbBr3:PS-b-P2VP film. (Square boxes: CsPbBr3 orthorhombic phase. Circle: unreacted CsBr residue.


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For perovskite nanocrystals, fast reactions of PbBr2 and CsBr usually led to a high degree of supersaturation of primary particles,18-19 facilitating the formation of multiple small nanocrystals (ca. 10 nm-diameter) in one micelle (Figure 2b). Increasing the diffusional flow

18-19

via rapid

mixing and reducing the total volume of the precursor solution to the µL level, would instead lead to a single large perovskite crystal (ca. 25 nm-diameter, Figure 2c and S2). These multiple small nanocrystals (ca. 10 nm) in a micelle had much brighter fluorescence than a single large nanocrystal (ca. 25 nm). High-resolution TEM image (Figure 2d) and XRD (Figure 2e) of assynthesized nanocrystals displayed an orthorhombic crystalline phase of CsPbBr3.23 However, it is possible that other phases (such as cubic and monoclinic) also exist, since the measured XRD peaks were broad.13, 15 The reaction product and its optical property highly depended on the reaction temperature and the precursor ratio. (Figure S3-S5) Low temperature (-15 oC) led to serious nanocrystal aggregation and thus very low photoluminescence quantum yield (PLQY~5%), because stiff polymer chains lacked enough mobility at low temperature to adapt a proper configuration during the nanocrystal growth. On the other hand, higher temperature above 40 oC also led to low PLQY due to the overgrowth of the nanocrystal to a size much larger than the Bohr exciton diameter. The reaction conducting at between 4 oC and 20 oC yields smaller confined CsPbBr3 nanocrystals (10 nm-diameter) with excellent dispersibility. PLQY up to 51% was obtained, comparable to previous reports (Table S1).

8, 10-12, 24-27

In our synthesis, overstoichiometrical

CsBr precursors was used (by 8:1 molar ratio or more) in order to fully convert all PbBr2contained micelles due to much smaller solubility of CsBr in toluene. However, excess amount (12:1) of CsBr yielded larger crystals with weaker exciton confinement, resulting in significant drop in PLQY. Average fluorescence lifetime (τave), which accounts for both radiative


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recombination and non-radiative trap-assisted Shockley-Read-Hall recombination processes,

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increased with PLQY (Figure S5). This indicates that different synthesis conditions resulted in a similar radiative lifetime (τr = τave / PLQY), but very different non-radiative lifetimes (τnr). Although lead halide perovskite is highly defect-tolerant,29-30 the surface defects were nonnegligible in perovskite nanocrystals especially under non-optimal synthesis conditions.31 Hence, the non-radiative process, probably assisted by defects or traps, in the polymer/perovskite hybrid structure exhibited much stronger dependence on the surface passivation than the radiative recombination process.28 Therefore, the non-radiative processes could be suppressed by effective passivation using multidentate polymer ligands under optimized synthesis conditions. To investigate the colloidal stability of our perovskite/polymer core/shell nanocrystals, five common solvents with increasing polarity, including hexane, toluene, ethanol, IPA, and methanol, were chosen. Figure 3a shows the fluorescent images taken immediately after mixing different volume ratios of the solvents. Our polymer/perovskite colloids showed excellent resistance towards all solvents at 1/1 ratio in a short term (~ 2 min), as evidenced by the retained strong green fluorescence. With increasing volume ratio of colloid/solvent up to 1/5, the colloidal stability of the polymer micelle decreased due to the change of polarity of the solvent mixture. Complete sediment of the polymer/perovskite nanocrystals in the hexane/toluene mixture and partial sediment in alcohols were observed. The same level of fluorescence was maintained in all solvents except methanol, which has the highest solubility to CsBr components. Our polymer/perovskite nanocrystal maintained stable fluorescence even after adding 10 parts of ethanol or IPA.


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Figure 3. Stability of perovskite colloid and film. (a) Fluorescent images of perovskite colloids mixed with different solvents (toluene, hexane, ethanol, IPA, methanol) at 1/1, 1/5 and 1/10 v/v ratio. (b) Time evolution of fluorescence intensity after adding 1/1 v/v solvents into perovskite colloids. (c) Time evolution of fluorescence intensity after immersing perovskite/polymer hybrid films in different solvents. To quantify the stability of our polymer/perovskite colloids, the fluorescence intensity was monitored after mixing with two polar solvents (ethanol and IPA) with a fixed volume ratio (1:1) (Setup shown in Figure S6). Figure 3b shows the comparison of CsPbBr3 nanocrystals with our


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multidentate co-polymer ligand and small-molecule ligands (oleic acid and oleylamine) prepared by classic hot-injection method. The fluorescence of the hot-injection nanocrystals quenched immediately after mixing with both solvents, and it totally disappeared within three hours, indicating their rapid degradation in polar solvents. In comparison, our multidentate polymer/perovskite nanocrystal colloids exhibited stable fluorescence after more than 25 hours in ethanol. Remarkably, the fluorescence was maintained up to 50 days after adding IPA, the longest period of our experiment. Our polymer-capped perovskite nanocrystals also had a reasonable stability towards water compared to small-molecule ligand-capped ones (Figure S7). Moreover, our polymer/perovskite film exhibited stable fluorescence up to 50 days in both ethanol and IPA, with no sign of degradation even afterwards (Figure 3c). This clearly demonstrates significant improvement in stability over the best reported results using smallmolecule ligands (Table S1).27, 32 The degradation mechanism of small-molecule ligand-capped perovskite nanocrystals in polar solvents is highly related to their dynamic dissolubility equilibrium, ligand equilibrium processes and fast proton exchange between ligands and protondonating solvents.16-17, 33 Compared to the monodentate ligands, each P2VP chain in our current approach has multiple valence-bound 2VP units to coordinate to nanocrystal surface. In addition, polymeric micelles have much slower structural relaxation time than small-molecular surfactant assembles.18 Thus, the multidentate polymer ligands cannot be easily detached from the surface. Moreover, the robust hydrophobic shells formed by co-polymer ligands slowed down the diffusion of the polar solvents into the core.


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Figure 4. DFT simulation of P2VP on the perovskite surface. Simulated structure of (a) a single 2VP monomer, (b) oligomer with four 2VP units and (c) block oligomer (P2VP4-b-PSt4) with four 2VP units and four styrene (St) units bind on perovskite surface (100). (d) Partial density of state (PDOS) of an oligomer with four 2VP units on perovskite surface. (e) Binding energy of different number of 2VP units on the perovskite surface. Finally, we used density functional theory (DFT) to understand the interaction between the ligands and perovskite surface. For a single 2VP monomer, the pyridine ring was face-on packed on the (100) and (110) surface of CsPbBr3 perovskite crystal (Figure 4a and Figure S8). Electron localization analysis showed there was 0.4 electron overlapping between the pyridinc N atom and nearest Pb atom on the surface. A strong coordinating N-Pb bond with a bond length of 2.1-


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2.3 Å was formed. In comparison with the non-coordinating styrene monomer (St, Figure S9) dominated by Van der Waals interaction, N-Pb bond in 2VP case contributed around 40% extra binding energy (0.17 eV). When relaxing an oligomer chain containing four 2VP units on the (100) surface of the perovskite crystal, a distorted chain conformation was observed (Figure 4b). The 2VP units adjusted to an edge-on configuration on the surface due to large steric hindrance. Interestingly, all four nitrogen atoms of the 2VP units were prone to face to the surface due to a stronger coordinating interaction in the optimal configuration, and two of them formed coordinate bonds to the crystal surface with roughly matched lattice distance. The binding energy of the oligomer (four 2VP units) was as high as 1.0 eV without introducing any shallow or deep trap states within the bandgap of the perovskite (Figure 4d). To mimic the block polymer, an oligomer (2VP4-bSt4) containing equal number of 2VP units and St units was simulated in the same way, and two Pb-N coordinate bonds were well maintained with slight change of bond angle (Figure 4c). The binding energy of different numbers of 2VP groups in the oligomer are summarized in Figure 4e, where the binding energy increases monotonically as the 2VP number increases. Simulated structures shown in Figure S9-S12. Given that more than five hundreds of 2VP units exist in our PS-b-P2VP polymers (St547-b-2VP542), a considerable number of 2VP units can bind to the surface of perovskite nanocrystals in spite of their large steric hindrance. Thus, multidentate polymer ligands are much less likely to detach from the surface of perovskite nanocrystals than small-molecule monodentate ligands. Among several proposed passivation mechanisms,15, 34-35 our simulation is consistent with the Lewis acid-base passivation mechanism, that is, weak Lewis bases (such as pyridine and thiophene) were found to bind and passivate the acidic halide vacancies (Pb site on 100 surface).35


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In summary, we have presented a new co-polymer templated synthesis approach to prepare photo-stable perovskite nanocrystals. The confined growth of the perovskite nanocrystal by the co-polymer micelle leads to the perovskite/polymer core/shell nanostructure. Furthermore, the formation of strong multidentate bonds effectively passivate the perovskite nanocrystal surface. The hydrophobic polymer shell also serves as a barrier to polar solvents. Taken together, the multidentate core/shell perovskite nanocrystal exhibits orders of magnitudes improved stability in both the colloidal and thin film forms. This micelle-templated synthesis approach works well for a variety of PS-b-P2VP with different block lengths. (Figure S12) Considering a wide variety of functional polymers, such as conjugated polymers or biomacromolecules, our polymertemplated synthesis strategy offers a promising way to realize complex nanostructures for optoelectronics, as well as new applications in biomedicine. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge online. Materials, synthesis and density functional simulation details are described. Material characterizations such as scanning electron microscopy, X-ray diffraction, absorption/fluorescence spectrum and fluorescence life time measurements were provided. AUTHOR INFORMATION Corresponding Author *Email: [email protected] (Q.Q.) The authors declare no competing financial interest. Author Contributions


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S.H and Q.Q conceived the experiment. S.H performed the experiment. Y.G performed DFT simulation. Q.Q and Y.T supervised the project. All authors participated revising the manuscript. ACKNOWLEDGMENT The authors thank Dr. James Foley at Rowland Institute at Harvard University for his help and discussion on fluorescence quantum yield and lifetime measurement. The authors also thank Dr. Dan Congreve and Dr. Ye Tao for helpful discussion on the manuscript. The research was supported in part by the Rowland Junior Fellowship Award at Rowland Institute at Harvard University. REFERENCES 1. Lee, M. M.; Teuscher, J.; Miyasaka, T.; Murakami, T. N.; Snaith, H. J., Efficient hybrid solar cells based on meso-superstructured organometal halide perovskites. Science 2012, 338, 643647. 2. Tan, Z. K.; Moghaddam, R. S.; Lai, M. L.; Docampo, P.; Higler, R.; Deschler, F.; Price, M.; Sadhanala, A.; Pazos, L. M.; Credgington, D.; Hanusch, F.; Bein, T.; Snaith, H. J.; Friend, R. H., Bright light-emitting diodes based on organometal halide perovskite. Nat Nanotechnol 2014, 9, 687-692. 3. Deschler, F.; Price, M.; Pathak, S.; Klintberg, L. E.; Jarausch, D. D.; Higler, R.; Huttner, S.; Leijtens, T.; Stranks, S. D.; Snaith, H. J.; Atature, M.; Phillips, R. T.; Friend, R. H., High Photoluminescence Efficiency and Optically Pumped Lasing in Solution-Processed Mixed Halide Perovskite Semiconductors. J Phys Chem Lett 2014, 5, 1421-1426. 4. Zhang, F.; Zhong, H.; Chen, C.; Wu, X. G.; Hu, X.; Huang, H.; Han, J.; Zou, B.; Dong, Y., Brightly Luminescent and Color-Tunable Colloidal CH3NH3PbX3 (X = Br, I, Cl) Quantum Dots: Potential Alternatives for Display Technology. Acs Nano 2015, 9, 4533-4542. 5. Wang, Y.; Li, X.; Sreejith, S.; Cao, F.; Wang, Z.; Stuparu, M. C.; Zeng, H.; Sun, H., Photon Driven Transformation of Cesium Lead Halide Perovskites from Few-Monolayer Nanoplatelets to Bulk Phase. Adv Mater 2016, 28, 10637-10643. 6. Bella, F.; Griffini, G.; Correa-Baena, J. P.; Saracco, G.; Gratzel, M.; Hagfeldt, A.; Turri, S.; Gerbaldi, C., Improving efficiency and stability of perovskite solar cells with photocurable fluoropolymers. Science 2016, 354, 203-206. 7. You, J.; Meng, L.; Song, T. B.; Guo, T. F.; Yang, Y. M.; Chang, W. H.; Hong, Z.; Chen, H.; Zhou, H.; Chen, Q.; Liu, Y.; De Marco, N.; Yang, Y., Improved air stability of perovskite solar cells via solution-processed metal oxide transport layers. Nat Nanotechnol 2016, 11, 75-81. 8. Palazon, F.; Akkerman, Q. A.; Prato, M.; Manna, L., X-ray Lithography on Perovskite Nanocrystals Films: From Patterning with Anion-Exchange Reactions to Enhanced Stability in Air and Water. Acs Nano 2016, 10, 1224-1230.


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Synthesis and Stabilization of Colloidal Perovskite Nanocrystals by

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