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Colloidal CsPbX3 (X = Cl, Br, I) Nanocrystals 2.0: Zwitterionic Capping Ligands for Improved Durability and Stability

Colloidal CsPbX3 (X = Cl, Br, I) Nanocrystals 2.0: Zwitterionic Capping Ligands for Improved... Letter This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes. Cite This: ACS Energy Lett. 2018, 3, 641−646 Colloidal CsPbX (X = Cl, Br, I) Nanocrystals 2.0: Zwitterionic Capping Ligands for Improved Durability and Stability †,‡ †,‡ †,‡ # †,‡ Franziska Krieg, Stefan T. Ochsenbein, Sergii Yakunin, Stephanie ten Brinck, Philipp Aellen, †,‡ †,‡ †,‡ †,‡ †,‡ Adrian Süess, Baptiste Clerc, Dominic Guggisberg, Olga Nazarenko, Yevhen Shynkarenko, § § # ,†,‡ Sudhir Kumar, Chih-Jen Shih, Ivan Infante, and Maksym V. Kovalenko* Institute of Inorganic Chemistry, Department of Chemistry and Applied Bioscience, ETH Zürich, Vladimir Prelog Weg 1, CH-8093 Zürich, Switzerland Laboratory for Thin Films and Photovoltaics, Empa − Swiss Federal Laboratories for Materials Science and Technology, Uberlandstrasse 129, CH-8600 Dübendorf, Switzerland Institute of Chemical and Bioengineering, Department of Chemistry and Applied Bioscience, ETH Zürich, Vladimir Prelog Weg 1, CH-8093 Zürich, Switzerland Department of Theoretical Chemistry, Faculty of Science, Vrije Universiteit Amsterdam, de Boelelaan 1083, 1081 HV Amsterdam, The Netherlands * Supporting Information ABSTRACT: Colloidal lead halide perovskite nanocrystals (NCs) have recently emerged as versatile photonic sources. Their processing and optoelectronic applications are hampered by the loss of colloidal stability and structural integrity due to the facile desorption of surface capping molecules during isolation and purification. To address this issue, herein, we propose a new ligand capping strategy utilizing common and inexpensive long-chain zwitterionic molecules such as 3-(N,N-dimethyloctadecylammonio)- propanesulfonate, resulting in much improved chemical durability. In particular, this class of ligands allows for the isolation of clean NCs with high photoluminescence quantum yields (PL QYs) of above 90% after four rounds of precipitation/redispersion along with much higher overall reaction yields of uniform and colloidal dispersible NCs. Densely packed films of these NCs exhibit high PL QY values and effective charge transport. −2 Consequently, they exhibit photoconductivity and low thresholds for amplified spontaneous emission of 2 μJcm under femtosecond optical excitation and are suited for efficient light-emitting diodes. emiconducting lead halides with perovskite crystal ligands, typically a pair consisting of an anion (Br or oleate, − + structure, recently known as photovoltaic materials OA ) and a cation (oleylammonium, OLAH ), and the 3,16 showing power conversion efficiencies exceeding S oppositely charged NC surface ions (Scheme 1a). Together 1,2 22%, also hold great promise as versatile photonic sources with a mutual equilibrium between the ionized and molecular − + in the form of colloidal nanocrystals (NCs). Fully inorganic forms of these ligands (OA + OLAH ⇋ OLA + OAH or + − CsPbX (X = Cl, Br, or I, or a mixture thereof) have become OLAH +Br ⇋ OLA + HBr, Scheme 1a), these dynamics popular choices owing to their chemical stability and broadly cause rapid desorption of the protective ligand shell upon tunable photoluminescence (PL, 400−700 nm), small PL full isolation and purification of colloids, which is practically width at half-maxima (fwhm, 12−40 nm for blue-to-red), and observed as a loss of colloidal stability and a rapid decrease in 3,4 high PL quantum yields (QYs = 50−90%). Their intrinsic PL QY. This eventually also leads to the loss of structural 5,6 defect tolerance, i.e., the rather benign nature of surfaces with integrity, i.e., sintering of NCs into bulk polycrystalline respect to PL efficiency, is a particularly important asset for 7 8−13 materials. Thus far, the strategies to address such problems employing these NCs in displays, light-emitting diodes, 14,15 and potentially lasers. A highly pressing challenge related to organic−inorganic Received: January 9, 2018 interfaces was identified in the early days of CsPbX NCs. Accepted: February 9, 2018 Highly dynamic binding exists between the surface capping Published: February 9, 2018 © 2018 American Chemical Society 641 DOI: 10.1021/acsenergylett.8b00035 ACS Energy Lett. 2018, 3, 641−646 ACS Energy Letters Letter Scheme 1. (a) Depiction of Conventional Ligand Capping of and ammonium capping ligands (e.g., OLA+OAH), both Perovskite NCs Using Long-Chain Molecules with Single favoring stronger adhesion to the NC surface. First, the − − + a Head Groups, In the Ionized Form (OA or Br , OLAH ) cationic and anionic groups have no possibility of mutual or and (b) a Novel Strategy wherein Cationic and Anionic external neutralization by Brønsted acid−base equilibria. Groups Are Combined in a Single Zwitterionic Molecule Second, the binding to the NC surface is kinetically stabilized by the chelate effect. In agreement with this argument, one can explain also the effective bicarboxylate binding reported recently by Bakr et al. for CsPbI NCs. In the proof-of-principle experiment, we fully replaced the OAH and OLA by a zwitterionic ligand (Figure 1a; b; see detailed methods in the Supporting Information and related Figures S1−S7). In a typical synthesis of 10 nm CsPbBr NCs, cesium 2-ethylhexanoate (0.2 mmol), lead(II) 2-ethylhexanoate (0.24 mmol), and 3-(N,N-dimethyloctadecylammonio)- propanesulfonate (0.1 mmol, ligand) were combined in dried mesitylene (∼6 mL) and heated to 130 °C under inert gas. At this point, a trioctylphosphine-Br adduct (TOP-Br , 0.3 mmol) 2 2 dissolved in toluene (0.5 mL) was injected into the reaction mixture, which was then immediately cooled. The crude solution was centrifuged to remove insolubles (if any) and mixed with ethyl acetate (12 mL) to precipitate NCs. The NCs were isolated by centrifuging, redispersed in toluene (3 mL), and centrifuged again to remove a fraction of larger NCs The net effect of two possible sets of equilibria is facile ligand (below 10% by weight; if any). Afterward, higher purity could desorption during purification. Examples of long-chain sulfobetaines, phosphocholines, and γ-amino acids tested in this work are depicted be attained without the loss of structural integrity and a high PL left to right (n =1): 3-(N,N-dimethyloctadecylammonio)- QY could be retained by repeatedly (i.e., up to 3 more times) propanesulfonate, N-hexadecylphosphocholine, and N,N- adding a nonsolvent (e.g., 6 mL of ethyl acetate, 3 mL of dimethyldodecylammoniumbutyrate. acetone, or 1 mL of acetonitrile), centrifuging the mixture, and redispersing the precipitate in toluene (3 mL). In contrast, 7,9,17−24 conventional OA/OLA-capped CsPbBr NCs fully transform included embedding of NCs into a solid matrix or using 25−29 into poorly luminescent bulk material upon analogous washing. molecular additives in colloidal solutions of NCs. Zwitterionic-ligand-capped CsPbBr NCs can form much more In this work, we present a general approach for the efficient concentrated colloids (up to 50−100 mg/mL) than their OA/ surface ligand capping of CsPbX NCs using zwitterionic long- OLA-capped counterparts. Furthermore, the typical synthesis chain molecules, which are readily available commercially (i.e., sulfobetaines, phosphocholines, γ-amino acids, etc., Scheme yield of clean dispersible CsPbBr NCs is ca. 80% compared to only 10−20% for rather impure NCs prepared with the 1b). For instance, 3-(N,N-dimethyloctadecylammonio)- conventional OA/OLA capping. The obtained CsPbBr NCs propanesulfonate is a long-chain sulfobetaine, broadly used as showed a phase-pure orthorhombic crystal structure, identical a low-cost detergent in, for example, shower gels, protein 3,31,32 to NCs received by OA/OLA synthesis (Pnma space isolation, and antibacterial coatings. There are two major group; see powder X-ray diffraction patterns in Figure S4). The structural differences with respect to conventional carboxylate Figure 1. Synthesis of zwitterionic-capped CsPbX NCs, exemplified for CsPbBr : (a) reaction equation, (b) typical TEM images of CsPbBr 3 3 3 NCs, (c) absorbance and emission spectra, (d) QY of NCs covered with the 3-(N,N-dimethyloctadecylammonio)propanesulfonate and OA/ OLA after two steps of purification on day 1 and after storage for 28 days. 642 DOI: 10.1021/acsenergylett.8b00035 ACS Energy Lett. 2018, 3, 641−646 ACS Energy Letters Letter mass fraction of organic ligands was estimated to be ca. 11% using thermogravimetric analysis (Figure S5), corresponding to −2 a ligand density of ca. 1.7 nm (for 11 nm NCs). Solution nuclear magnetic resonance (NMR) spectra were acquired at various stages of the purification, confirming the formation of R P(OOCR) [trioctyl-λ -phosphanediyl bis(2- 3 2 ethylhexanoate), Figure S6] and the complete removal of the free zwitterionic ligand, reagents, and reagent byproducts (Figure S7). Solution NMR fails to accurately resolve the resonances attributed to surface-immobilized ligand molecules because the slow tumbling of NCs in solution results in significant signal broadening. Therefore, the purified NCs were decomposed to liberate the ligands by their complete ionic dissolution in deuterated dimethyl sulfoxide (DMSO-d ). The NMR spectra of the resulting solution point to the zwitterionic ligand as the sole surface-bound species (Figure S7). Analogous findings on the preferential and exclusive binding of sulfobetaine and on colloidal durability were obtained when 2-ethylhexanoate was replaced with the oleate in the synthesis (Figure S8). We also tested halide sources such as oleylammonium bromide (OLAHBr) as alternatives to TOP- Br . The OLA was found as a co-ligand at the surface (Figure Figure 2. Top and side views of a binding site in a model CsPbBr S9), presumably in the form of OLAHBr. The sulfobetaine-to- NC (∼3 nm) computed at the DFT/PBE level of theory, using OLA ratio was ca. 1.5. Diffusion-ordered NMR spectroscopy the CP2K software package. All structures have been fully (DOSY NMR, Table S2), which probes the diffusion speed of relaxed. Cs atoms are drawn in gray, Pb in orange, Br in magenta, the detected molecules, estimated that the diffusion coefficients N in blue, C in light blue, O in red, S in yellow, and H in white. The for the broad resonances obtained from the zwitterionic ligand binding site is circled in white for different ligands: (from left to + + − were nearly identical to the value independently calculated right) conventional ligands OLAH Br and OLAH OA and the using the Stokes−Einstein equation for the actual size of the zwitterionic C -sulfobetaine. For computational advantage, the + − −11 2 OLAH is replaced by methylammonium, the OA by acetate, and NCs (i.e., 5.17 vs 4.99 × 10 m /s for the 11 nm NCs). The the side chain in the zwitterion by a butyl group. At the bottom, the motion of a free ligand molecule, on the contrary, is 2 orders of electronic structure of each NC is shown by depicting the magnitude faster (Table S2).The photophysical qualities (i.e., molecular orbitals (MOs) close to the valence and conduction PL QYs, PL fwhm, and PL lifetimes) of the sulfobetaine-capped bands. The contribution of each atom type to a given MO is CsPbX NCs were commensurate with those of standard OLA/ represented with a different color (Cs in gray, Pb in orange, and Br OA-capped NCs (see Figure 1c for CsPbBr NCs, Figure S10 in magenta). In this plot, the contribution from the ligands is for time-resolved PL, and Figures S11 and 12 for chlorides and negligible compared to the full NC due the large number of MOs of iodides). The utility of other zwitterionic ligandsphospho- the latter. In Figure S17 we, however, illustrate the relative energy cholines and γ-amino acidsis illustrated in Figure S13. The alignment of the NC versus the frontier orbitals of the ligands. decisive role of zwitterionic surface capping for improving the chemical durability of perovskite NCs can be illustrated by a methodology employed are provided in the Supporting comprehensive study relating the optical characteristics, Information. All of the relaxed species comfortably fit the foremost the PL QYs, to the variation in the number of perovskite crystal structure, with the ammonium group in the + − + − washing steps, solvents, and aging period; see Figures 1d, S14, OLAH Br and OLAH OA engaging in hydrogen bond and 15 and additional discussions and details in the Supporting interactions with the corresponding anion. Remarkably, the Information. The retention of PL QYs above 60% for the dimethylammonium group of the zwitterion, which can be CsPbBr NCs was considered a benchmark for stability. Briefly, expected as rather bulky, also can be easily accommodated in a standard OLA/OA-cappedNCs exhibitedsuchPLQYs cation site at the surface. For all species, the binding energy was (∼80%) only when the number of washing steps did not computed to be ca. 40−45 kcal/mol, suggesting good affinity of exceed two and only for one antisolvent: ethyl acetate. Even in all of the ion pairs to the surface. However, there is no this best case, the PL QY dropped to ca. 20% after 28 days of substantial energetic difference between the conventional and storage under ambient conditions. On the contrary, the zwitterionic passivation. This supports the theory proposed sulfobetaine-capped CsPbBr NCs, washed twice with ethyl earlier in the introduction that the experimentally observed acetate, acetone, or acetonitrile as antisolvents, retained PL QYs improvements are due to the chelate effect. We also analyzed in the range of 70−90% for 28−50 days. These NCs could even the electronic structure to verify whether the different kinds of moderately tolerate washing with alcohol (i.e., ethanol), passivation could lead to the formation of localized surface showing a PL QY of 65% after two washings and ca. 40% states. For all cases, the bandgap of the perovskite remained after 28 days. The absorption and PL spectra as well as PL intact and free of midgap states. The HOMO−LUMO levels of lifetimes of the zwitterionic-capped NCs remained largely the ligands used were calculated and found to reside within the unchanged during intense washing for up to four times (Figure valence band and conduction band, respectively (Figure S17). S16). Interestingly, the spacing between the cationic and anionic By means of density functional theory (DFT), we analyzed head groups of the sulfobetaines, namely, three or four carbon + − the passivation of CsPbBr NCs capped with OLAH Br , atoms, has observable experimental effects. The C -sulfobe- 3 3 + − OLAH OA , and C -sulfobetaine (Figure 2). The details of the taines were better suited for the synthesis of the Cl- and Br- 643 DOI: 10.1021/acsenergylett.8b00035 ACS Energy Lett. 2018, 3, 641−646 ACS Energy Letters Letter Figure 3. (a) Amplified spontaneous emission (ASE) spectra showing evolution of the ASE band and (b) the threshold behavior for the intensity of the ASE band. (c) Photoconductivity spectrum inset: photo of a colloidal solution and drop-casted film of standard OA/OLA NCs (left) and C -sulfobetaine-covered NCs (right). (d) Bias dependence of photoresponse, with the inset showing the scheme of a photodetector made from the substrate with an interdigitated electrode and a drop-casted film of NCs. (e) Corresponding work functions and HOMO− LUMO gaps and (f) current density and luminance vs applied voltage of a LED. Inset: electroluminescence spectrum measured at 3.5 V. containing perovskites, while their C counterparts performed photoconductivity is also revealed in the linearity of the better for synthesizing the iodides, presumably owing to the photocurrent vs incident light intensity plot (at least over 3 larger cation−anion distances at the NC surface. This orders of magnitude in intensity) and in the relatively large comparison held true for both the oleyl and octadecyl side bandwidth of about 90 Hz (Figure S19). chains; see the discussion in the Supporting Information and Efficient charge transport and high PL QYs are required Figures S12 and 13. characteristics for the eventual use of perovskite NC films in A major and very typical issue for CsPbBr NCs is facile light-emitting diodes (LEDs). To assess the potential of the room-temperature sintering, which quickly renders the material sulfobetaine-capped NCs, we used a device structure similar to 37 12 polycrystalline and nonluminescent. In contrast, zwitterionic- that of Li et al. (Figure 3e). The current density passing capped CsPbBr in the form of thin, densely packed films through the devices was rather high, limiting the peak external remains in the range of 70−80% of the solution PL QY values, quantum efficiency (EQE) to 2.5% at 3.5 V (J = 21.7 mA/cm , and almost 60% of the initial QY in films stored under ambient L = 1641 cd/m , Figure 3f; see the statistics in Figure S20 and conditions is retained even after 10 months. The retention of the plot of EQE/current efficiency vs voltage in Figure S21), the quantum confinement in the thin films and the absence of trailing behind the most efficient CsPbBr NC LEDs in terms 10,12 sintering are apparent from the optical absorption spectra of EQEs (8.73 and 6.27%). At the same time, the peak (Figure S18). Strong coupling between neighboring NCs luminance of such devices exceeded 24 000 cd/m (Figure 3f), facilitates exciton−exciton interactions, enabling multiexciton significantly brighter than the aforementioned efficient LEDs 10,12 processes, which favor optical gain in the compact NC medium. (1660 and 15 185 cd/m , respectively) but lagging behind When the optical pumping levels substantially exceed one the Cs/formamidinium mixed-cation bromide perovskite NC exciton per NC, the population inversion of biexcitonic states is LED (55 005 cd/m ). The electroluminescence wavelength observed as emergence of an amplified spontaneous emission and fwhm were 516 and 16 nm, respectively (at 3.5 V; inset in band (ASE, Figure 3a). The ASE threshold of 2 μJcm (with Figure 3f). Despite sulfobetaine being a long-chain ligand, the 100 fs pulses, Figure 3b) is one of the lowest values reported for charge transport is not severely impeded, seen as high 14,38−41 solution-processed NC films. photoconductivity and high current densities in the LEDs. Dense NC packing and hence improved electronic coupling The current densities in our LEDs (current−voltage character- enable the observation of photoconductivity. In CsPbBr NC istics, Figures 3f and S22) are higher than those reported in films, the photoresponsivity spectrum closely resembles the LEDs from Li et al. (EQE = 6.27%) but without concomitant optical absorption spectrum, with typical responsivities (R)of increase in luminance, thus leading to a lower EQE (2.5%). about 0.5 A/W (Figure 3c). Hence, a photoconductive gain This reduced efficiency may be due to imbalance between −1 close to unity can be estimated (from G = R·hν·e , where hν electrons and holes at higher current densities. and e are the photon energy and electron charge, respectively). In conclusion, a novel class of capping ligands for perovskite This finding is corroborated by the observation of high PL QYs NCs is proposed, wherein each ligand molecule is capable of in these films. G > 1 can be expected only in the presence of coordinating simultaneously to the surface cations and anions. secondary, i.e., trap-assisted, photocurrent. The photocurrent vs Colloidal perovskite NCs prepared with tightly bound ligands bias dependence shows saturation above 30−40 V (Figure 3d), and without large quantities of excessive capping ligands will indicating efficient charge collection. The apparently trap-free serve as an ideal platform for further engineering of these NCs. 644 DOI: 10.1021/acsenergylett.8b00035 ACS Energy Lett. 2018, 3, 641−646 ACS Energy Letters Letter (5) Correa-Baena, J.-P.; Saliba, M.; Buonassisi, T.; Gratzel, ̈ M.; Abate, This may include the development of core−shell NC A.; Tress, W.; Hagfeldt, A. Promises and challenges of perovskite solar morphologies with enhanced thermal and environmental cells. Science 2017, 358, 739−744. stability, as critically needed for applications in displays and (6) ten Brinck, S.; Infante, I. Surface Termination, Morphology, and lighting, or even for rendering perovskite NCs water- Bright Photoluminescence of Cesium Lead Halide Perovskite compatible for biomedical applications. Nanocrystals. ACS Energy Lett. 2016, 1, 1266−1272. (7) Zhou, Q.; Bai, Z.; Lu, W. G.; Wang, Y.; Zou, B.; Zhong, H. In Situ ASSOCIATED CONTENT Fabrication of Halide Perovskite Nanocrystal-Embedded Polymer Composite Films with Enhanced Photoluminescence for Display * Supporting Information Backlights. Adv. Mater. 2016, 28, 9163−9168. The Supporting Information is available free of charge on the (8) Pan, J.; Quan, L. N.; Zhao, Y.; Peng, W.; Murali, B.; Sarmah, S. P.; ACS Publications website at DOI: 10.1021/acsenergy- Yuan, M.; Sinatra, L.; Alyami, N. M.; Liu, J.; Yassitepe, E.; Yang, Z.; lett.8b00035. Voznyy, O.; Comin, R.; Hedhili, M. N.; Mohammed, O. F.; Lu, Z. H.; Kim, D. H.; Sargent, E. H.; Bakr, O. M. Highly Efficient Perovskite- Experimental methods and supplementary figures (PDF) Quantum-Dot Light-Emitting Diodes by Surface Engineering. Adv. Mater. 2016, 28, 8718−8725. AUTHOR INFORMATION (9) Zhang, X.; Sun, C.; Zhang, Y.; Wu, H.; Ji, C.; Chuai, Y.; Wang, P.; Wen, S.; Zhang, C.; Yu, W. W. Bright Perovskite Nanocrystal Films for Corresponding Author Efficient Light-Emitting Devices. J. Phys. Chem. Lett. 2016, 7, 4602− *E-mail: [email protected]. ORCID (10) Chiba, T.; Hoshi, K.; Pu, Y. J.; Takeda, Y.; Hayashi, Y.; Ohisa, S.; Sergii Yakunin: 0000-0002-6409-0565 Kawata, S.; Kido, J. High-Efficiency Perovskite Quantum-Dot Light- Emitting Devices by Effective Washing Process and Interfacial Energy Sudhir Kumar: 0000-0002-2994-7084 Level Alignment. ACS Appl. Mater. Interfaces 2017, 9, 18054−18060. Chih-Jen Shih: 0000-0002-5258-3485 (11) Deng, W.; Xu, X.; Zhang, X.; Zhang, Y.; Jin, X.; Wang, L.; Lee, Ivan Infante: 0000-0003-3467-9376 S.-T.; Jie, J. Organometal Halide Perovskite Quantum Dot Light- Maksym V. Kovalenko: 0000-0002-6396-8938 Emitting Diodes. Adv. Funct. Mater. 2016, 26, 4797−4802. (12) Li, J.; Xu, L.; Wang, T.; Song, J.; Chen, J.; Xue, J.; Dong, Y.; Cai, Notes B.; Shan, Q.; Han, B.; Zeng, H. 50-Fold EQE Improvement up to The authors declare no competing financial interest. 6.27% of Solution-Processed All-Inorganic Perovskite CsPbBr QLEDs via Surface Ligand Density Control. Adv. Mater. 2017, 29, 1603885. ACKNOWLEDGMENTS (13) Li, G.; Rivarola, F. W.; Davis, N. J.; Bai, S.; Jellicoe, T. C.; de la Pena, F.; Hou, S.; Ducati, C.; Gao, F.; Friend, R. H.; Greenham, N. C.; This work was financially supported by the European Union Tan, Z. K. Highly Efficient Perovskite Nanocrystal Light-Emitting through the FP7 (ERC Starting Grant NANOSOLID, GA No. Diodes Enabled by a Universal Crosslinking Method. Adv. Mater. 306733) and by the Swiss Federal Commission for Technology 2016, 28, 3528−34. and Innovation (CTI-No. 18614.1 PFNM-NM). The authors (14) Yakunin, S.; Protesescu, L.; Krieg, F.; Bodnarchuk, M. 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Colloidal CsPbX3 (X = Cl, Br, I) Nanocrystals 2.0: Zwitterionic Capping Ligands for Improved Durability and Stability

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Letter This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes. Cite This: ACS Energy Lett. 2018, 3, 641−646 Colloidal CsPbX (X = Cl, Br, I) Nanocrystals 2.0: Zwitterionic Capping Ligands for Improved Durability and Stability †,‡ †,‡ †,‡ # †,‡ Franziska Krieg, Stefan T. Ochsenbein, Sergii Yakunin, Stephanie ten Brinck, Philipp Aellen, †,‡ †,‡ †,‡ †,‡ †,‡ Adrian Süess, Baptiste Clerc, Dominic Guggisberg, Olga Nazarenko, Yevhen Shynkarenko, § § # ,†,‡ Sudhir Kumar, Chih-Jen Shih, Ivan Infante, and Maksym V. Kovalenko* Institute of Inorganic Chemistry, Department of Chemistry and Applied Bioscience, ETH Zürich, Vladimir Prelog Weg 1, CH-8093 Zürich, Switzerland Laboratory for Thin Films and Photovoltaics, Empa − Swiss Federal Laboratories for Materials Science and Technology, Uberlandstrasse 129, CH-8600 Dübendorf, Switzerland Institute of Chemical and Bioengineering, Department of Chemistry and Applied Bioscience, ETH Zürich, Vladimir Prelog Weg 1, CH-8093 Zürich, Switzerland Department of Theoretical Chemistry, Faculty of Science, Vrije Universiteit Amsterdam, de Boelelaan 1083, 1081 HV Amsterdam, The Netherlands * Supporting Information ABSTRACT: Colloidal lead halide perovskite nanocrystals (NCs) have recently emerged as versatile photonic sources. Their processing and optoelectronic applications are hampered by the loss of colloidal stability and structural integrity due to the facile desorption of surface capping molecules during isolation and purification. To address this issue, herein, we propose a new ligand capping strategy utilizing common and inexpensive long-chain zwitterionic molecules such as 3-(N,N-dimethyloctadecylammonio)- propanesulfonate, resulting in much improved chemical durability. In particular, this class of ligands allows for the isolation of clean NCs with high photoluminescence quantum yields (PL QYs) of above 90% after four rounds of precipitation/redispersion along with much higher overall reaction yields of uniform and colloidal dispersible NCs. Densely packed films of these NCs exhibit high PL QY values and effective charge transport. −2 Consequently, they exhibit photoconductivity and low thresholds for amplified spontaneous emission of 2 μJcm under femtosecond optical excitation and are suited for efficient light-emitting diodes. emiconducting lead halides with perovskite crystal ligands, typically a pair consisting of an anion (Br or oleate, − + structure, recently known as photovoltaic materials OA ) and a cation (oleylammonium, OLAH ), and the 3,16 showing power conversion efficiencies exceeding S oppositely charged NC surface ions (Scheme 1a). Together 1,2 22%, also hold great promise as versatile photonic sources with a mutual equilibrium between the ionized and molecular − + in the form of colloidal nanocrystals (NCs). Fully inorganic forms of these ligands (OA + OLAH ⇋ OLA + OAH or + − CsPbX (X = Cl, Br, or I, or a mixture thereof) have become OLAH +Br ⇋ OLA + HBr, Scheme 1a), these dynamics popular choices owing to their chemical stability and broadly cause rapid desorption of the protective ligand shell upon tunable photoluminescence (PL, 400−700 nm), small PL full isolation and purification of colloids, which is practically width at half-maxima (fwhm, 12−40 nm for blue-to-red), and observed as a loss of colloidal stability and a rapid decrease in 3,4 high PL quantum yields (QYs = 50−90%). Their intrinsic PL QY. This eventually also leads to the loss of structural 5,6 defect tolerance, i.e., the rather benign nature of surfaces with integrity, i.e., sintering of NCs into bulk polycrystalline respect to PL efficiency, is a particularly important asset for 7 8−13 materials. Thus far, the strategies to address such problems employing these NCs in displays, light-emitting diodes, 14,15 and potentially lasers. A highly pressing challenge related to organic−inorganic Received: January 9, 2018 interfaces was identified in the early days of CsPbX NCs. Accepted: February 9, 2018 Highly dynamic binding exists between the surface capping Published: February 9, 2018 © 2018 American Chemical Society 641 DOI: 10.1021/acsenergylett.8b00035 ACS Energy Lett. 2018, 3, 641−646 ACS Energy Letters Letter Scheme 1. (a) Depiction of Conventional Ligand Capping of and ammonium capping ligands (e.g., OLA+OAH), both Perovskite NCs Using Long-Chain Molecules with Single favoring stronger adhesion to the NC surface. First, the − − + a Head Groups, In the Ionized Form (OA or Br , OLAH ) cationic and anionic groups have no possibility of mutual or and (b) a Novel Strategy wherein Cationic and Anionic external neutralization by Brønsted acid−base equilibria. Groups Are Combined in a Single Zwitterionic Molecule Second, the binding to the NC surface is kinetically stabilized by the chelate effect. In agreement with this argument, one can explain also the effective bicarboxylate binding reported recently by Bakr et al. for CsPbI NCs. In the proof-of-principle experiment, we fully replaced the OAH and OLA by a zwitterionic ligand (Figure 1a; b; see detailed methods in the Supporting Information and related Figures S1−S7). In a typical synthesis of 10 nm CsPbBr NCs, cesium 2-ethylhexanoate (0.2 mmol), lead(II) 2-ethylhexanoate (0.24 mmol), and 3-(N,N-dimethyloctadecylammonio)- propanesulfonate (0.1 mmol, ligand) were combined in dried mesitylene (∼6 mL) and heated to 130 °C under inert gas. At this point, a trioctylphosphine-Br adduct (TOP-Br , 0.3 mmol) 2 2 dissolved in toluene (0.5 mL) was injected into the reaction mixture, which was then immediately cooled. The crude solution was centrifuged to remove insolubles (if any) and mixed with ethyl acetate (12 mL) to precipitate NCs. The NCs were isolated by centrifuging, redispersed in toluene (3 mL), and centrifuged again to remove a fraction of larger NCs The net effect of two possible sets of equilibria is facile ligand (below 10% by weight; if any). Afterward, higher purity could desorption during purification. Examples of long-chain sulfobetaines, phosphocholines, and γ-amino acids tested in this work are depicted be attained without the loss of structural integrity and a high PL left to right (n =1): 3-(N,N-dimethyloctadecylammonio)- QY could be retained by repeatedly (i.e., up to 3 more times) propanesulfonate, N-hexadecylphosphocholine, and N,N- adding a nonsolvent (e.g., 6 mL of ethyl acetate, 3 mL of dimethyldodecylammoniumbutyrate. acetone, or 1 mL of acetonitrile), centrifuging the mixture, and redispersing the precipitate in toluene (3 mL). In contrast, 7,9,17−24 conventional OA/OLA-capped CsPbBr NCs fully transform included embedding of NCs into a solid matrix or using 25−29 into poorly luminescent bulk material upon analogous washing. molecular additives in colloidal solutions of NCs. Zwitterionic-ligand-capped CsPbBr NCs can form much more In this work, we present a general approach for the efficient concentrated colloids (up to 50−100 mg/mL) than their OA/ surface ligand capping of CsPbX NCs using zwitterionic long- OLA-capped counterparts. Furthermore, the typical synthesis chain molecules, which are readily available commercially (i.e., sulfobetaines, phosphocholines, γ-amino acids, etc., Scheme yield of clean dispersible CsPbBr NCs is ca. 80% compared to only 10−20% for rather impure NCs prepared with the 1b). For instance, 3-(N,N-dimethyloctadecylammonio)- conventional OA/OLA capping. The obtained CsPbBr NCs propanesulfonate is a long-chain sulfobetaine, broadly used as showed a phase-pure orthorhombic crystal structure, identical a low-cost detergent in, for example, shower gels, protein 3,31,32 to NCs received by OA/OLA synthesis (Pnma space isolation, and antibacterial coatings. There are two major group; see powder X-ray diffraction patterns in Figure S4). The structural differences with respect to conventional carboxylate Figure 1. Synthesis of zwitterionic-capped CsPbX NCs, exemplified for CsPbBr : (a) reaction equation, (b) typical TEM images of CsPbBr 3 3 3 NCs, (c) absorbance and emission spectra, (d) QY of NCs covered with the 3-(N,N-dimethyloctadecylammonio)propanesulfonate and OA/ OLA after two steps of purification on day 1 and after storage for 28 days. 642 DOI: 10.1021/acsenergylett.8b00035 ACS Energy Lett. 2018, 3, 641−646 ACS Energy Letters Letter mass fraction of organic ligands was estimated to be ca. 11% using thermogravimetric analysis (Figure S5), corresponding to −2 a ligand density of ca. 1.7 nm (for 11 nm NCs). Solution nuclear magnetic resonance (NMR) spectra were acquired at various stages of the purification, confirming the formation of R P(OOCR) [trioctyl-λ -phosphanediyl bis(2- 3 2 ethylhexanoate), Figure S6] and the complete removal of the free zwitterionic ligand, reagents, and reagent byproducts (Figure S7). Solution NMR fails to accurately resolve the resonances attributed to surface-immobilized ligand molecules because the slow tumbling of NCs in solution results in significant signal broadening. Therefore, the purified NCs were decomposed to liberate the ligands by their complete ionic dissolution in deuterated dimethyl sulfoxide (DMSO-d ). The NMR spectra of the resulting solution point to the zwitterionic ligand as the sole surface-bound species (Figure S7). Analogous findings on the preferential and exclusive binding of sulfobetaine and on colloidal durability were obtained when 2-ethylhexanoate was replaced with the oleate in the synthesis (Figure S8). We also tested halide sources such as oleylammonium bromide (OLAHBr) as alternatives to TOP- Br . The OLA was found as a co-ligand at the surface (Figure Figure 2. Top and side views of a binding site in a model CsPbBr S9), presumably in the form of OLAHBr. The sulfobetaine-to- NC (∼3 nm) computed at the DFT/PBE level of theory, using OLA ratio was ca. 1.5. Diffusion-ordered NMR spectroscopy the CP2K software package. All structures have been fully (DOSY NMR, Table S2), which probes the diffusion speed of relaxed. Cs atoms are drawn in gray, Pb in orange, Br in magenta, the detected molecules, estimated that the diffusion coefficients N in blue, C in light blue, O in red, S in yellow, and H in white. The for the broad resonances obtained from the zwitterionic ligand binding site is circled in white for different ligands: (from left to + + − were nearly identical to the value independently calculated right) conventional ligands OLAH Br and OLAH OA and the using the Stokes−Einstein equation for the actual size of the zwitterionic C -sulfobetaine. For computational advantage, the + − −11 2 OLAH is replaced by methylammonium, the OA by acetate, and NCs (i.e., 5.17 vs 4.99 × 10 m /s for the 11 nm NCs). The the side chain in the zwitterion by a butyl group. At the bottom, the motion of a free ligand molecule, on the contrary, is 2 orders of electronic structure of each NC is shown by depicting the magnitude faster (Table S2).The photophysical qualities (i.e., molecular orbitals (MOs) close to the valence and conduction PL QYs, PL fwhm, and PL lifetimes) of the sulfobetaine-capped bands. The contribution of each atom type to a given MO is CsPbX NCs were commensurate with those of standard OLA/ represented with a different color (Cs in gray, Pb in orange, and Br OA-capped NCs (see Figure 1c for CsPbBr NCs, Figure S10 in magenta). In this plot, the contribution from the ligands is for time-resolved PL, and Figures S11 and 12 for chlorides and negligible compared to the full NC due the large number of MOs of iodides). The utility of other zwitterionic ligandsphospho- the latter. In Figure S17 we, however, illustrate the relative energy cholines and γ-amino acidsis illustrated in Figure S13. The alignment of the NC versus the frontier orbitals of the ligands. decisive role of zwitterionic surface capping for improving the chemical durability of perovskite NCs can be illustrated by a methodology employed are provided in the Supporting comprehensive study relating the optical characteristics, Information. All of the relaxed species comfortably fit the foremost the PL QYs, to the variation in the number of perovskite crystal structure, with the ammonium group in the + − + − washing steps, solvents, and aging period; see Figures 1d, S14, OLAH Br and OLAH OA engaging in hydrogen bond and 15 and additional discussions and details in the Supporting interactions with the corresponding anion. Remarkably, the Information. The retention of PL QYs above 60% for the dimethylammonium group of the zwitterion, which can be CsPbBr NCs was considered a benchmark for stability. Briefly, expected as rather bulky, also can be easily accommodated in a standard OLA/OA-cappedNCs exhibitedsuchPLQYs cation site at the surface. For all species, the binding energy was (∼80%) only when the number of washing steps did not computed to be ca. 40−45 kcal/mol, suggesting good affinity of exceed two and only for one antisolvent: ethyl acetate. Even in all of the ion pairs to the surface. However, there is no this best case, the PL QY dropped to ca. 20% after 28 days of substantial energetic difference between the conventional and storage under ambient conditions. On the contrary, the zwitterionic passivation. This supports the theory proposed sulfobetaine-capped CsPbBr NCs, washed twice with ethyl earlier in the introduction that the experimentally observed acetate, acetone, or acetonitrile as antisolvents, retained PL QYs improvements are due to the chelate effect. We also analyzed in the range of 70−90% for 28−50 days. These NCs could even the electronic structure to verify whether the different kinds of moderately tolerate washing with alcohol (i.e., ethanol), passivation could lead to the formation of localized surface showing a PL QY of 65% after two washings and ca. 40% states. For all cases, the bandgap of the perovskite remained after 28 days. The absorption and PL spectra as well as PL intact and free of midgap states. The HOMO−LUMO levels of lifetimes of the zwitterionic-capped NCs remained largely the ligands used were calculated and found to reside within the unchanged during intense washing for up to four times (Figure valence band and conduction band, respectively (Figure S17). S16). Interestingly, the spacing between the cationic and anionic By means of density functional theory (DFT), we analyzed head groups of the sulfobetaines, namely, three or four carbon + − the passivation of CsPbBr NCs capped with OLAH Br , atoms, has observable experimental effects. The C -sulfobe- 3 3 + − OLAH OA , and C -sulfobetaine (Figure 2). The details of the taines were better suited for the synthesis of the Cl- and Br- 643 DOI: 10.1021/acsenergylett.8b00035 ACS Energy Lett. 2018, 3, 641−646 ACS Energy Letters Letter Figure 3. (a) Amplified spontaneous emission (ASE) spectra showing evolution of the ASE band and (b) the threshold behavior for the intensity of the ASE band. (c) Photoconductivity spectrum inset: photo of a colloidal solution and drop-casted film of standard OA/OLA NCs (left) and C -sulfobetaine-covered NCs (right). (d) Bias dependence of photoresponse, with the inset showing the scheme of a photodetector made from the substrate with an interdigitated electrode and a drop-casted film of NCs. (e) Corresponding work functions and HOMO− LUMO gaps and (f) current density and luminance vs applied voltage of a LED. Inset: electroluminescence spectrum measured at 3.5 V. containing perovskites, while their C counterparts performed photoconductivity is also revealed in the linearity of the better for synthesizing the iodides, presumably owing to the photocurrent vs incident light intensity plot (at least over 3 larger cation−anion distances at the NC surface. This orders of magnitude in intensity) and in the relatively large comparison held true for both the oleyl and octadecyl side bandwidth of about 90 Hz (Figure S19). chains; see the discussion in the Supporting Information and Efficient charge transport and high PL QYs are required Figures S12 and 13. characteristics for the eventual use of perovskite NC films in A major and very typical issue for CsPbBr NCs is facile light-emitting diodes (LEDs). To assess the potential of the room-temperature sintering, which quickly renders the material sulfobetaine-capped NCs, we used a device structure similar to 37 12 polycrystalline and nonluminescent. In contrast, zwitterionic- that of Li et al. (Figure 3e). The current density passing capped CsPbBr in the form of thin, densely packed films through the devices was rather high, limiting the peak external remains in the range of 70−80% of the solution PL QY values, quantum efficiency (EQE) to 2.5% at 3.5 V (J = 21.7 mA/cm , and almost 60% of the initial QY in films stored under ambient L = 1641 cd/m , Figure 3f; see the statistics in Figure S20 and conditions is retained even after 10 months. The retention of the plot of EQE/current efficiency vs voltage in Figure S21), the quantum confinement in the thin films and the absence of trailing behind the most efficient CsPbBr NC LEDs in terms 10,12 sintering are apparent from the optical absorption spectra of EQEs (8.73 and 6.27%). At the same time, the peak (Figure S18). Strong coupling between neighboring NCs luminance of such devices exceeded 24 000 cd/m (Figure 3f), facilitates exciton−exciton interactions, enabling multiexciton significantly brighter than the aforementioned efficient LEDs 10,12 processes, which favor optical gain in the compact NC medium. (1660 and 15 185 cd/m , respectively) but lagging behind When the optical pumping levels substantially exceed one the Cs/formamidinium mixed-cation bromide perovskite NC exciton per NC, the population inversion of biexcitonic states is LED (55 005 cd/m ). The electroluminescence wavelength observed as emergence of an amplified spontaneous emission and fwhm were 516 and 16 nm, respectively (at 3.5 V; inset in band (ASE, Figure 3a). The ASE threshold of 2 μJcm (with Figure 3f). Despite sulfobetaine being a long-chain ligand, the 100 fs pulses, Figure 3b) is one of the lowest values reported for charge transport is not severely impeded, seen as high 14,38−41 solution-processed NC films. photoconductivity and high current densities in the LEDs. Dense NC packing and hence improved electronic coupling The current densities in our LEDs (current−voltage character- enable the observation of photoconductivity. In CsPbBr NC istics, Figures 3f and S22) are higher than those reported in films, the photoresponsivity spectrum closely resembles the LEDs from Li et al. (EQE = 6.27%) but without concomitant optical absorption spectrum, with typical responsivities (R)of increase in luminance, thus leading to a lower EQE (2.5%). about 0.5 A/W (Figure 3c). Hence, a photoconductive gain This reduced efficiency may be due to imbalance between −1 close to unity can be estimated (from G = R·hν·e , where hν electrons and holes at higher current densities. and e are the photon energy and electron charge, respectively). In conclusion, a novel class of capping ligands for perovskite This finding is corroborated by the observation of high PL QYs NCs is proposed, wherein each ligand molecule is capable of in these films. G > 1 can be expected only in the presence of coordinating simultaneously to the surface cations and anions. secondary, i.e., trap-assisted, photocurrent. The photocurrent vs Colloidal perovskite NCs prepared with tightly bound ligands bias dependence shows saturation above 30−40 V (Figure 3d), and without large quantities of excessive capping ligands will indicating efficient charge collection. The apparently trap-free serve as an ideal platform for further engineering of these NCs. 644 DOI: 10.1021/acsenergylett.8b00035 ACS Energy Lett. 2018, 3, 641−646 ACS Energy Letters Letter (5) Correa-Baena, J.-P.; Saliba, M.; Buonassisi, T.; Gratzel, ̈ M.; Abate, This may include the development of core−shell NC A.; Tress, W.; Hagfeldt, A. Promises and challenges of perovskite solar morphologies with enhanced thermal and environmental cells. Science 2017, 358, 739−744. stability, as critically needed for applications in displays and (6) ten Brinck, S.; Infante, I. Surface Termination, Morphology, and lighting, or even for rendering perovskite NCs water- Bright Photoluminescence of Cesium Lead Halide Perovskite compatible for biomedical applications. Nanocrystals. ACS Energy Lett. 2016, 1, 1266−1272. (7) Zhou, Q.; Bai, Z.; Lu, W. G.; Wang, Y.; Zou, B.; Zhong, H. In Situ ASSOCIATED CONTENT Fabrication of Halide Perovskite Nanocrystal-Embedded Polymer Composite Films with Enhanced Photoluminescence for Display * Supporting Information Backlights. Adv. Mater. 2016, 28, 9163−9168. The Supporting Information is available free of charge on the (8) Pan, J.; Quan, L. N.; Zhao, Y.; Peng, W.; Murali, B.; Sarmah, S. P.; ACS Publications website at DOI: 10.1021/acsenergy- Yuan, M.; Sinatra, L.; Alyami, N. M.; Liu, J.; Yassitepe, E.; Yang, Z.; lett.8b00035. Voznyy, O.; Comin, R.; Hedhili, M. N.; Mohammed, O. F.; Lu, Z. H.; Kim, D. H.; Sargent, E. H.; Bakr, O. M. Highly Efficient Perovskite- Experimental methods and supplementary figures (PDF) Quantum-Dot Light-Emitting Diodes by Surface Engineering. Adv. Mater. 2016, 28, 8718−8725. AUTHOR INFORMATION (9) Zhang, X.; Sun, C.; Zhang, Y.; Wu, H.; Ji, C.; Chuai, Y.; Wang, P.; Wen, S.; Zhang, C.; Yu, W. W. Bright Perovskite Nanocrystal Films for Corresponding Author Efficient Light-Emitting Devices. J. Phys. Chem. Lett. 2016, 7, 4602− *E-mail: [email protected]. ORCID (10) Chiba, T.; Hoshi, K.; Pu, Y. J.; Takeda, Y.; Hayashi, Y.; Ohisa, S.; Sergii Yakunin: 0000-0002-6409-0565 Kawata, S.; Kido, J. High-Efficiency Perovskite Quantum-Dot Light- Emitting Devices by Effective Washing Process and Interfacial Energy Sudhir Kumar: 0000-0002-2994-7084 Level Alignment. ACS Appl. Mater. Interfaces 2017, 9, 18054−18060. Chih-Jen Shih: 0000-0002-5258-3485 (11) Deng, W.; Xu, X.; Zhang, X.; Zhang, Y.; Jin, X.; Wang, L.; Lee, Ivan Infante: 0000-0003-3467-9376 S.-T.; Jie, J. Organometal Halide Perovskite Quantum Dot Light- Maksym V. Kovalenko: 0000-0002-6396-8938 Emitting Diodes. Adv. Funct. Mater. 2016, 26, 4797−4802. (12) Li, J.; Xu, L.; Wang, T.; Song, J.; Chen, J.; Xue, J.; Dong, Y.; Cai, Notes B.; Shan, Q.; Han, B.; Zeng, H. 50-Fold EQE Improvement up to The authors declare no competing financial interest. 6.27% of Solution-Processed All-Inorganic Perovskite CsPbBr QLEDs via Surface Ligand Density Control. Adv. Mater. 2017, 29, 1603885. ACKNOWLEDGMENTS (13) Li, G.; Rivarola, F. W.; Davis, N. J.; Bai, S.; Jellicoe, T. C.; de la Pena, F.; Hou, S.; Ducati, C.; Gao, F.; Friend, R. H.; Greenham, N. C.; This work was financially supported by the European Union Tan, Z. K. Highly Efficient Perovskite Nanocrystal Light-Emitting through the FP7 (ERC Starting Grant NANOSOLID, GA No. Diodes Enabled by a Universal Crosslinking Method. Adv. Mater. 306733) and by the Swiss Federal Commission for Technology 2016, 28, 3528−34. and Innovation (CTI-No. 18614.1 PFNM-NM). The authors (14) Yakunin, S.; Protesescu, L.; Krieg, F.; Bodnarchuk, M. I.; thank ScopeM for use of the electron microscopes, the MoBiAS Nedelcu, G.; Humer, M.; De Luca, G.; Fiebig, M.; Heiss, W.; molecular and biomolecular analytical service of ETH Zurich Kovalenko, M. V. Low-threshold amplified spontaneous emission and for mass spectrometry measurements and elemental analysis lasing from colloidal nanocrystals of caesium lead halide perovskites. and Prof. Manfred Fiebig and his research group for access to Nat. Commun. 2015, 6, 8056. their femtosecond laser and for experimental assistance. Ms. (15) Pan, J.; Sarmah, S. P.; Murali, B.; Dursun, I.; Peng, W.; Parida, Laura Piveteau is gratefully acknowledged for stimulating M. R.; Liu, J.; Sinatra, L.; Alyami, N.; Zhao, C.; Alarousu, E.; Ng, T. K.; discussions. I.I. acknowledges The Netherlands Organization Ooi, B. S.; Bakr, O. M.; Mohammed, O. F. 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Published: Feb 9, 2018

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