A General Mild Reaction Creates Highly Luminescent Organic-Ligand

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A General Mild Reaction Creates Highly Luminescent Organic-LigandLacking Halide Perovskite Nanocrystals for Efficient Light-Emitting Diodes Bin-Bin Zhang, Shuai Yuan, Ju-Ping Ma, Yang Zhou, Jingshan Hou, Xueyuan Chen, Wei Zheng, Huaibin Shen, Xue-Chun Wang, Baoquan Sun, Osman M. Bakr, Liang-Sheng Liao, and Hong-Tao Sun J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b08140 • Publication Date (Web): 30 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

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Journal of the American Chemical Society

A General Mild Reaction Creates Highly Luminescent OrganicLigand-Lacking Halide Perovskite Nanocrystals for Efficient LightEmitting Diodes Bin-Bin Zhang,†,¶ Shuai Yuan,‡,¶ Ju-Ping Ma,†,¶ Yang Zhou,†,¶ Jingshan Hou,§ Xueyuan Chen,∥ Wei Zheng,∥ Huaibin Shen,⊥ Xue-Chun Wang,‡ Baoquan Sun,‡ Osman M. Bakr,# Liang-Sheng Liao,*,‡ and Hong-Tao Sun*,†,∥ †College

of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China

‡Jiangsu

Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of Functional Nano & Soft Materials (FUNSOM) & Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215123, China §School

of Materials Science and Engineering, Shanghai Institute of Technology, Shanghai 201418, China

∥CAS

Key Laboratory of Design and Assembly of Functional Nanostructures, and Fujian Key Laboratory of Nanomaterials, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China ⊥Key

Laboratory for Special Functional Materials of Ministry of Education, Henan University, Kaifeng 475004, China #Division

of Physical Sciences and Engineering, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia ABSTRACT: The presence of labile bulky insulating hydrocarbon ligands in halide perovskite nanocrystals (NCs) passivates surface traps, but concurrently makes charge transport difficult in optoelectronic devices. Early efforts routinely rely on the replacement of long-chain ligands with short-chain cousins, leading to notable changes in NC’s sizes and photophysical properties and thus making it hard to obtain devices with nearly designed emissions. Here we report a general solution-phase ligand-exchange strategy to produce organic-ligand-lacking halide perovskite NCs with high photoluminescence quantum yields and good stability in ambient air. We demonstrate that the ligand exchange can be achieved by a well-controlled mild reaction of thionyl halide with the carboxylic and amine groups on the NC’s surface, resulting in nearly dry NCs with well passivated surfaces and almost unaltered emission characteristics. Consequently, we achieve exceptionally high-performance blue perovskite NC LEDs with an external quantum efficiency of up to 1.35% and an extremely narrow full width at half maximum of 14.6 nm. Our work provides a systematic framework for preparing high-quality organic-ligand-lacking perovskite NC inks that can be directly cast as films featuring effective charge transport, thereby providing the foundation for further development of a wide range of efficient perovskite optoelectronic devices.

detectors.24,25 In particular, the development of highefficiency and low-cost perovskite LEDs will further progress in applications such as lighting, display, and visible-light optical communications.26-28 The common feature of almost all as-synthesized colloidal lead halide perovskite NCs with near-unity PLQYs is the presence of labile organic capping ligands such as oleic acid and/or oleylamine that provides passivation of surface traps and concurrently makes charge injection extremely difficult because of the ligand’s insulating nature.15,26-29 Although

INTRODUCTION More than five years of research into colloidal lead halide perovskite nanocrystals (NCs) has led to the realization of impressive near-unity photoluminescence quantum yields (PLQYs) in the visible and near-infrared spectral regions.1-14 This has garnered intense interest for their applications in solution-processed optoelectronic devices including light-emitting diodes (LEDs),15-19 lasers,20 solar cells,21-23 and photon and radiation

1 ACS Paragon Plus Environment

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reaction of thionyl halide with the carboxylic and amine groups on the NC’s surface, which can effectively graft the surfaces with passivating halide anions. We chose thionyl halides (SOCl2, SOBr2 or mixture thereof) as passivating agents with the idea that they could maximize the amount of halide ions required for perfect coordination of Pb2+ ions during the replacement reaction occurring at the NC’s surface.8,53,54 By fine-tuning the amount of thionyl halides used, we can readily control the severity of the reaction, thus circumventing excessive etching of NCs and yielding organic-ligand-lacking NCs with nearly identical size, PL emission lineshape, and well retained PLQYs with respect to the unexchanged cousins. Specifically, the PLQYs are maintained over 80% for the exchanged CsPbCl3 and CsPbBr3 NCs and ~58% for the mixed-halide cousins. Most attractively, our concept used here can produce stable organic-ligand-lacking NC inks that can be directly cast as films. As a result, we are able to achieve a blue LEDs with an emission peak at 460 nm, a narrow full width at half maximum (FWHM) of 14.6 nm, and a high peak external quantum efficiency (EQE) of 1.35%.

recently identified ligands that bind more strongly to the surface of NCs, such as quaternary ammonium and phosphonate,30-32 improve luminescence and stability, they still possess insulating hydrocarbon chains.33,34 It is a long-held view that halide perovskite NCs without sufficient protection by organic agents cannot luminesce efficiently,35,36 are susceptible to degradation and instability, and thus are not suitable for LEDs.26-28 To overcome this challenge, early efforts to exploit perovskite NCs in LEDs were routinely based on the replacement of long-chain ligands with short-chain counterparts, by which the external quantum efficiency (EQE) of perovskite NC LEDs steadily increases.34,37-44 However, such a replacement process readily results in regrowth of NCs because of the ionic nature of the lattices, leading to notable changes in NC’s size and emission characteristics that make it difficult to obtain LEDs with nearly designed emission features.29,34,40,44-47 An alternative strategy for preparing highly luminescent and stable colloidal perovskite NCs without a coating of a high density of bulky insulating hydrocarbon chains is thus urgently required for their implementation in efficient optoelectronic devices.48

RESULTS AND DISCUSSION

Replacing the long-chain, insulating organic ligands with conductive inorganic ligands has been proposed to increase charge carrier mobility, which has worked well in NC-based photovoltaics, LEDs and field-effect transistor devices.49,50 In particular, this strategy has been proven to be powerful in engineering the surface structure of coreshell NCs, demonstrating the capability of suppressing exciton quenching at the interface with charge transfer layers; the intrinsic reaction mechanism is based mainly on the difference in ligand binding energies before and after the exchange, which is thus largely governed by the composition of NC’s surfaces.51 However, owing to the difficulty in attaining core-shell halide perovskite NCs and weak bonding energies between constituent atoms caused by the ionic nature of the lattice,52 it is challenging to apply this strategy to perovskite NCs because of the ready deterioration of surface passivation.

A schematic of the solution-phase ligand exchange process is shown in Figure 1a. We chose all-inorganic halide perovskite (CsPbX3, X=halide ions) NCs, originally capped with oleic acid and oleylamine ligands, as an example system to exemplify our concept. We used SOCl2, SOBr2 or their mixture as an efficient halogenating reagent, among which SOCl2 has been commonly used in organic synthetic chemistry.55,56 Very recently, SOCl2 was used as a chlorination reagent to remove oleic acid ligands of CdSe-ZnS core-shell colloidal NCs because of its capability of reacting with the carboxylic group of oleic acid.49 We thus hypothesized that thionyl halides could be used for resurfacing halide perovskite NCs, probably resulting in nearly dry, perfect surfaces. We began by challenging our hypothesis using phasepure Ni2+-doped CsPbCl3 (Ni-CsPbCl3) NCs as a model system (Figure S1). The reason for choosing Ni-CsPbCl3 in our study is twofold. First, these NCs show a near-unity PLQY, suggesting the negligible nonradiative recombination channels in them that is important for some optoelectronic applications.6 Second, the photophysical properties of Ni-CsPbCl3 CsPbCl3 NCs are extremely sensitive to structural defects; for example, Cl vacancies in Ni-CsPbCl3 NCs, different from other types of defects, can seriously deteriorate the band-edge emission because of the formation of deep trap states in the band gap,6 which leads us to use PL properties as a probe to examine whether Cl vacancies occur when removing longchain capping ligands. In a typical experiment, 1 μL SOCl2 was added into 1 mL of NC solution (~ 1.83 × 10-6 mol/L) (process (1) in Figure 1a). After mixing evenly, 1 mL of methyl acetate (MA) was added to precipitate the NCs (process (2) in Figure 1a). By repeating the above processes, we can obtain NCs with decreased

We reasoned that the ligand exchange reaction involving inorganic ligands could readily trigger the formation of atomic point defects and/or more complicated defects such as vacancy complexes due to the removal of coordinated halide ions on NC’s surfaces, thus resulting in significant trap-mediated nonradiative recombination owing to the introduction of midgap states. We thus hypothesized that, if judiciously choosing halidebearing inorganic passivating ligands that can effectively complete the perfect coordination of metal cations when replacing long-chain ligands, we could have chances to produce highly luminescent, stable perovskite NCs with organic-ligand-lacking, near-perfect surfaces that could favor effective charge transport. Here we report a general solution-phase ligandexchange process to produce organic-ligand-lacking lead halide perovskite NCs with high PLQYs and good stability in ambient air. This is based on a well-controlled mild


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Figure 1. (a) Schematic of the mild reaction and ligand exchange process. Process (1) (a mild reaction on the NC’s surface): the bulky oleic acid and oleylamine ligands react with SOCl2, SOBr2 or their mixture and then are replaced by halide anions. Process (2): after ligand exchange, Ni-CsPbX3 NCs are precipitated via the addition of MA and are separated by centrifugation. Free organic and other residues are taken away by the solvent. (b) Normalized absorption and PL spectra of Ni-CsPbCl3 NCs solution before and after different times of ligand exchange. (c) PLQYs of Ni-CsPbCl3 NCs solution in hexane after different times of washing by MA or SOCl2. For the MA washing, the NCs were merely treated by MA to remove the organic ligands, whereas for the SOCl2 washing, the process is shown as in (a). The average PLQY and error bar were obtained by three parallel experiments. (d, e) TEM images of Ni-CsPbCl3 NCs (d) before ligand exchange and (e) after four times of exchange with SOCl2.

To understand the change in organic ligands, we took the X-ray photoelectron spectroscopy (XPS) measurement of Ni-CsPbCl3 NCs before and after ligand exchange. Notably, after the first exchange the ratio of C/Cs decreases from 73.9 to 13.1, while that of N/Cs from 3.4 to 0.7. However, these ratios do not show a significant change in the following exchange cycles (Figure 2a). We surmise that the removal and re-capturing of organic ligands could occur simultaneously during the reaction, and the balance between them can be readily established after the first exchange. Additionally, we note that the Ni 2p signal is undetectable in the as-synthesized NCs because of the shielding effect of organic ligands, but becomes notable in the exchanged cousins (Figure S7), further indicating a significant drop in the density of organic ligands after the exchange reaction. The absence of a sulfur 2p peak in XPS indicates that S does not exist at the NC’s surface (Figure S8). Additionally, the chlorine 2p peak in XPS indicates that the content of Cl- is almost constant before and after one time of ligand exchange by SOCl2 (Figure S9). All these observations are in a good consistence with the Fourier-transform infrared spectroscopy (FTIR) spectra (Figure 2b). The intensity of C-Hx stretching vibrations at 2921 and 2850 cm-1,18,29,49 stemming from oleic acid and oleylamine ligands, dramatically decreases after the first exchange, and then slowly decreases from SOCl2-2 to SOCl2-4. The C=O stretching vibration at 1743 cm-1 and O=CO- vibration

densities of organic ligands, as reflected by the conversion of oily product to nearly dry counterpart. The final product was denoted SOCl2-x, where x is times of ligand exchange between NCs and SOCl2. Interestingly, we find that the removal of organic ligand cannot significantly affect the absorption and emission lineshape and lifetimes of NCs (Figure 1b, Figure S2, S3 and Table S1). We note that only using MA to remove organic ligands leads to decreased PLQYs and shortened lifetimes after repeated washing, signifying the occurrence of new defects that result in nonradiative recombination (Figure 1c, Figure S4 and Table S2). By contrast, the PLQYs of SOCl2-x are not dramatically different from the unexchanged cousins; even after four times of exchange, the PLQY is as high as 81.4% (Figure 1c). All these observations suggest that such a mild reaction cannot significantly introduce new nonrecombination channels. We also find that the average size of the exchanged NCs preserves well with respect to the unexchanged ones (Figure 1d, e). We underscore that the amount of SOCl2 used is critical for maintaining the size of the NCs; when using 2 μL SOCl2 for this exchange, the NCs are etched, similar to the case of CdSe-ZnS core-shell NCs (Figure S5).49 Importantly, beyond the expectation that halide perovskite NCs without enough protection by organic ligands are instable, our exchanged NCs features excellent stability in ambient air (temperature: 18~24 °C; relative humidity: 49~71%) (Figure S6).



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(Ni-CsPbCl2Br) NCs. Interestingly, after the first exchange the exciton absorption shifts from 429 to 413 nm, and the band-edge emission shifts from 440 to 419 nm; repeated exchange renders the absorption and emission shift to even shorter wavelengths, accompanied by peak splitting (Figure 3a). We note that, with the increased times of exchange, the NC’s size does not show a significant change (Figure S11), while the Cl/Br ratio gradually increases (Figure S12). All these facts suggest that Br- ions in NCs are replaced by Cl-, narrowing the band gap and thus resulting in blueshift of emissions. Collectively, we conclude that SOCl2 indeed plays a role in supplying Clions in the exchange reaction; for CsPbCl3 NCs this can suppress the formation of Cl-vacancy-related surface traps, while for CsPbCl2Br NCs the enriched Cl- ions at the surface make them diffuse into the core or even cause phase separation, as suggested by the spectral shift and splitting in the PL spectra (Figure 3a). We point out that the near perfect surface of exchanged Ni-CsPbCl3 NCs may be the reason for retarding their degradation when exposed in ambient air (Figure S6).

Figure 2. (a) Molar ratios of carbon and nitrogen to Cs in NiCsPbCl3 NCs before and after different times of ligand exchange. (b) FTIR spectra of Ni-CsPbCl3 NCs before and after different times of ligand exchange. (c) 1H NMR spectra of Ni-CsPbCl3 NCs before ligand exchange and after four times of exchange with SOCl2. This spectrum contains resonances from native oleyl vinyl protons (5), ODE, solvent (*) and unknown impurities (×).

We note that our method can be extended to other halide perovskite NCs, including CsPbBr3 and Cl/Brmixed perovskite NCs. For CsPbBr3 NCs, we instead used SOBr2 as a halogenating reagent; even after three times of

at 1537 cm-1 from the oleic acid group and the –N-H vibration at 1633 cm-1 from the oleyl amine group follow the similar trend to that of C-Hx stretching vibrations.18 Based on these observations, we conclude that SOCl2 can react not only with the carboxylic group, but also with the oleylamine group. To gain deep insight into the exact change in the organic ligands, we next characterized the typical samples by quantitative 1H nuclear magnetic resonance (NMR) spectroscopy. Unsaturated ligand pairs and octadecene (ODE) both have alkene resonances in the 4-6 ppm range, which are located downfield from the many overlapping alkyl resonances in the 0-3 ppm region and thus are useful for quantitative studies.13,18,29 Mesitylene was used as an internal standard, and the resonance at a chemical shift of 5.3 ppm (resonance 5 as shown in Figure 2c), ascribed to native oleyl vinyl protons, was used for the determination of ligand density (see details in the Experimental Section). By this method, the ligand density in the unexchanged NCs is 25.31/nm2, whereas that in SOCl2-4 is decreased to 3.49/nm2. This suggests that only a limited number of organic ligands cap the exchanged NCs, which is thus believed to result in opening of the charge injection channel. We underscore that the exchanged NCs demonstrate better colloidal stability in hexane than the unexchanged cousins (Figure S10).

Figure 3. (a) Normalized absorption and PL spectra of NiCsPbCl2Br NCs solution before and after different times of ligand exchange. (b) PLQYs of CsPbBr3 NCs solution in hexane after different times of washing by MA or SOBr2 and Ni-CsPbCl1.7Br1.3 NCs solution in hexane after different times of washing by MA or SOCl2/SOBr2. (c, d) FTIR spectra of CsPbBr3 NCs (c) and Ni-CsPbCl1.7Br1.3 NCs (d) before and after different times of ligand exchange.

To understand the reaction mechanism, especially the role of SOCl2 in the ligand-exchange process, we then designed a control experiment in which SOCl2 was used to exchange the organic ligands of Ni2+-doped CsPbCl2Br


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Journal of the American Chemical Society To know the impact of the removal of organic ligands on the charge injection, we then measured the currentvoltage (I-V) trace of the Ni-CsPbCl1.7Br1.3 NCs before and after one-cycle ligand exchange. The measured I-V character in the Ohmic region for an electron-only device with an architecture of ITO/ZnMgO/Ni-CsPbCl1.7Br1.3 perovskite NCs/(1,3,5-triazine-2,4,6-triyl)-tris(benzene-3,1diyl)tris (diphenyl-phosphineoxide) (PO-T2T)/Liq/Al indicates that the organic-ligand-lacking NCs exhibit enhanced conductivity with respect to the as-synthesized ones (Figure 5). To ascertain whether such organicligand-lacking NCs can be used for optoelectronic devices, we next fabricated LEDs with the device architecture of ITO/PEDOT:PSS/Poly-TPD/perovskite NCs/1,3,5-tris(2-Nphenylbenzimi dazolyl) benzene (TPBi)/LiF/Al, as shown in Figure 6a, 6b. We point out that the LED using asprepared NCs cannot show any electroluminescence due to the insulating nature of capping agents. The devices prepared by the SOCl2/SOBr2-1 or MA-1 NCs show similar current densities versus applied voltages, and their turnon voltages are comparable, 3.76 V for the exchanged NiCsPbCl1.7Br1.3 NCs and 3.74 V for the MA-washed cousins (Figure 6c). However, the luminance of the device with the exchanged NCs is nearly one order of magnitude larger than that with the MA-washed one when the applied voltage is over 4.4 V. Additionally, the device with the exchanged NCs gives a peak EQE up to 1.35%, which is among the highest values in this spectral range for allinorganic perovskite LEDs (Figure 6d, Table S3). We note that the blue LEDs shown here demonstrates electroluminescence peaking at 460 nm and with an FWHM of 14.6 nm (Figure 6d), which only redshifts ~ 3.5 nm compared with the photoluminescence of assynthesized NCs (Figure S15). The slight spectral shift can be associated with the minor modification of NC’s composition, considering that the NC’s size is almost unchanged (Figure S16). We thus are optimistic that such

Figure 4. AFM images of Ni-CsPbCl1.7Br1.3 NC films on ITO/PEDOT:PSS/Poly-TPD prepared by the SOCl2/SOBr2-1 (a) and MA-1 (b) NCs.

ligand exchange, the PLQY of exchanged NCs, with organic ligand-lacking surfaces and well retained morphology and emission lineshape, can be maintained over 80% (Figure 3b, 3c, and Figure S13, S14). In marked contrast, only using MA, after three times of washing the PLQY of CsPbBr3 NCs notably decreases and the morphology notably changes (Figure 3b and Figure S14). We next used the mixture of SOCl2 and SOBr2 as halogenating reagents for replacing the surface ligands of Ni2+-doped CsPbCl1.7Br1.3 (Ni-CsPbCl1.7Br1.3) NCs. By finetuning the relative amount of SOCl2 and SOBr2 (see details in the Experimental Section), after the first exchange we can obtain highly luminescent, ligandlacking NCs with slightly changed emission lineshape (Figure 3b, 3d, and Figure S15, S16); for such mixedhalide perovskite NCs, repeated exchange causes the changes in the emission characteristics, mainly due to the difficulty in the control of halide stoichiometry. We note that for all these NCs studied one cycle of exchange is sufficient for significant removal of organic ligands while maintaining photophysical properties nearly unchanged. Considering that the film quality plays an important role in governing the performance of perovskite LEDs, we next characterized the morphology of perovskite NC films by the atomic force microscopy (AFM). We spin-coated the NCs on indium tin oxide (ITO)/poly(3,4ethylenedioxythiophene) polystyrene sulfonate (PEDOT: PSS)/poly(4-butylphenyl-diphenyl-amine) (Poly-TPD), a partial structure used for the following LED construction. Interestingly, we find that the uniformity of NiCsPbCl1.7Br1.3 film prepared by NCs after one-cycle ligand exchange with SOCl2 and SOBr2 (denoted SOCl2/SOBr2-1) is much better than that prepared by NCs after one-cycle MA washing (denoted MA-1). The root-mean-square roughness (σRMS) of the SOCl2/SOBr2-1 NC film is only 3.15 nm (Figure 4a). By contrast, the MA-1 NC film is discontinuous and shows a much higher σRMS (4.28 nm) (Figure 4b). We surmise that the rich surface defects in the MA-1 NCs could trigger the agglomeration of NCs and thus lead to poor quality of the resultant film, which, however, could be greatly suppressed in the SOCl2/SOBr21 NCs because of their well passivated surfaces.

Figure 5. Current-voltage (I-V) character of Ni-CsPbCl1.7Br1.3 NCs before and after one-cycle ligand exchange with SOCl2/SOBr2. Inset shows a schematic illustration of the energy levels for the electron-only device.



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Figure 6. (a) Schematic of the device structure. (b) Cross-sectional TEM image of the LED. (c) J–L–V characteristics of the MA-1 and SOCl2/SOBr2-1 LED. (d) EQEs of the MA-1 and SOCl2/SOBr2-1 LEDs. Inset shows a typical electroluminescence spectrum of the LED with an applied voltage of 4 V. (e) EL spectra of the SOCl2/SOBr2-1 LED as a function of the operation time under a constant voltage of 3.7 V. (f) Integrated EL intensity of the SOCl2/SOBr2-1 LED at different times under a constant voltage of 3.7 V. 1.35%, which is among the highest values in this spectral range for all-inorganic perovskite LEDs and could be further increased by optimization of the device architecture. Our work offers a unique opportunity for highly luminescent, organic-ligand-lacking, stable perovskite NCs inks that can be directly cast as films with effective charge transport. We envisage that the results and approach presented here can be extended to the development of a broad range of high-performance perovskite optoelectronic devices.

a slight shift can be further suppressed by minor alteration of the ratio of SOCl2/SOBr2 used. Our results highlight that the ligand-exchange method used here cannot significantly impact the emission characteristics of NCs, thus making it possible to obtain optoelectronic devices with nearly designed emission features. We point out that the emission FWHM of our LEDs is the narrowest among all reported all-inorganic perovskite NC LEDs (Table S3). We next studied the spectral stability of the LEDs under a constant applied voltage of 3.7 V. We find that the EL peak wavelength virtually does not change under continuous operation for 130 s (Figure 6e), which is superior to other reported Cl/Br-mixed perovskite LEDs in which the spectral shift usually occurs.57,58 The half-lifetime (T50) of our LED is 51.5 s when the applied voltage is 3.7 V (Figure 6f).

EXPERIMENTAL SECTION Chemicals. Cesium carbonate (Cs2CO3, Aladdin, 99.99%), lead(II) chloride (PbCl2, Alfa, 99.999%), lead(II) bromide (PbBr2, Aladdin 99.0%), nickel(II) chloride hydrate (NiCl2·xH2O, Alfa, 99.995%; note that NiCl2·xH2O was dried under vacuum for 12 h at 100 °C to remove water.), oleylamine (OAm, Acros, 80−90%), oleic acid (OA, Alfa, 90%), tri-n-octylphosphine (TOP, Alfa, 90%), 1octadecene (ODE, Alfa, 90%), benzoyl bromide (Aladdin, ≥98.0%), methyl acetate (MA, Alfa, ≥99%), thionyl chloride (SOCl2, Aladdin, ≥99%), thionyl bromide (SOBr2, Alfa, 97%), and n-hexane (Hex, Aladdin, ≥98.0%), were used without purification unless otherwise noted.

CONCLUSIONS To summarize, we have developed a general solutionphase ligand exchange method that leads to removal of most organic ligands and simultaneously grafts the NC’s surface with passivating halide anions. We showed that the ligand exchange can be readily realized by a wellcontrolled mild reaction of thionyl halide with the carboxylic and amine groups on the NC’s surface, resulting in organic-ligand-lacking, highly luminescent, stable perovskite NCs with nearly unaltered emission characteristics. As a consequence, we achieved a blue LED with electroluminescence peaking at 460 nm and an FWHM of 14.6 nm. The peak EQE of the LEDs reaches

Preparation of Cs-oleate Precursor. Cs2CO3 (0.4073 g), OA (1.35 mL), and ODE (15 mL) were mixed in a 50 mL three-necked bottle and dried for 1h at 120 °C to remove the water and oxygen. Then the mixture was heated to 150 °C under N2 atmosphere until all Cs2CO3 reacted with


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Journal of the American Chemical Society centrifuging for 5 min at 12000 rpm; the obtained NCs were dispersed in 1 mL hexane. This process was repeated 1~3 times to replace the organic capping ligands by halide ions through a mild reaction, and the final product was dispersed in hexane. For Ni-CsPbCl3 and CsPbBr3 NCs, the exchanged product was denoted SOX2-x, where X and x are halide ions and times of ligand exchange between NCs and SOX2, respectively. For CsPbBr3 NCs, 0.25 μL SOBr2 was added into 1 ml of purified NC solution (~ 5.91 × 10-6 mol/L). The ligand exchange progress was as same as that used for Ni-CsPbX3 NCs.

OA. The Cs-oleate precursor solution was kept in an oil bath at 150 °C. Synthesis of Undoped and Ni2+-Doped CsPbCl3 NCs. For the synthesis of undoped CsPbCl3 NCs, PbCl2 (0.1232 g), ODE (10 mL), OAm (1.6 mL), OA (1.6 mL), and TOP (2 mL) were loaded into a 50 mL three-neck flask, dried under vacuum for 1h at 120 °C, and then heated at 120 °C under N2 atmosphere. Meanwhile, 1.6 mL of dried OAm and 1.6 mL of dried OA were slowly injected into the mixed solution. After 10 min, the temperature was raised to 210 °C and kept at this temperature for 5 min. Finally, the Cs-oleate precursor (0.9 mL) was quickly injected, and 1 min later the reaction mixture was cooled by an icewater bath. The synthesis of Ni2+-doped CsPbCl3 NCs was similar to that of undoped CsPbCl3 NCs except for the use of NiCl2 (0.1146 g).

Note that the concentration of perovskite NCs are typically determined by means of spectrophotometry, where absorbance can be recalculated into molar concentrations by means of the Lambert–Beer’s law if the absorption cross section or the molar extinction coefficient is known. The absorption cross-section of CsPbCl3 (∼10 nm edge-length) is 4.2×10-14 cm2 at 380 nm.9 For the Cl/Br mixed perovskite NCs, we assume that they have the similar absorption cross section of the Cl cousin. For the CsPbBr3 NCs (∼11 nm edge-length), the absorption cross-section is 1.3×10-14 cm2 at 499 nm.59 Meanwhile, the molar extinction coefficient is related to the absorption cross section, following the Eq. (1),60

Synthesis of Ni2+-Doped CsPbCl2Br and CsPbCl1.7Br1.3 NCs. For a typical synthesis of Ni2+-doped CsPbCl2Br NCs, PbBr2 (0.1623 g), NiCl2 (0.1146 g), ODE (10 mL), OAm (1.6 mL), OA (1.6 mL), and TOP (2 mL) were loaded into a 50 mL three-neck flask, and dried under vacuum for 1h at 120 °C, and then heated at 120 °C under N2 atmosphere. Meanwhile, 1.6 mL of dried OAm and 1.6 mL of dried OA were slowly injected into the mixed solution. After 10 min, the temperature was raised to 210 °C and kept at this temperature for 5 min. Finally, the Cs-oleate precursor (0.9 mL) was quickly injected and 1 min later the reaction mixture was cooled by an ice-water bath. The synthesis of Ni2+-doped CsPbCl1.7Br1.3 NCs was similar to that of Ni2+-doped CsPbCl2Br NCs except for additional usage of 60 µL benzoyl bromide.

𝑁𝐴 ∙ σ

ε = 1000 ∙ ln 10

(1)

where ε is the molar extinction coefficient in M-1 cm-1, σ the absorption cross section in cm2, NA the Avogadro’s number. The concentration of perovskite NCs was calculated as follows: 𝐴=ε∙𝑙∙𝑐

Synthesis of CsPbBr3 NCs. PbBr2 (0.1623 g), ODE (10 mL), OAm (1.0 mL), and OA (1.0 mL) were loaded into a 50 mL three-neck flask, dried under vacuum for 60 min at 120 °C, and then heated at 120 °C under N2 atmosphere. After 10 min, the temperature was raised to 170 °C and kept at this temperature for 5 min. Finally, the Cs-oleate precursor (0.9 mL) was quickly injected and 5 s later the reaction mixture was cooled by an ice-water bath.

(2)

where A is the absorbance, ε the molar extinction coefficient, l length of the beam in the absorbing medium in cm, c the concentration of perovskite NCs in mol/L (M). Structure, Morphology, Absorption, Luminescence, FTIR, XPS and EDS Characterization. X-ray diffraction (XRD) data were recorded using a Bruker D2 PHASER diffractometer with Cu Kα radiation (λ = 1.54056 Å). Transmission electron microscopy (TEM) measurements were performed using an FEI Tecnai G20 S-TWIN TMP microscope operating at an accelerating voltage of 200 kV. The absorption spectra were taken by a double-beam UVvis-NIR spectrophotometer (Cary 5000, Agilent). PL spectra were recorded on a spectrometer equipped with continuous (450 W) xenon lamp (FLS980, Edinburgh Instrument). Room-temperature PLQYs were measured using an integration sphere incorporated into a spectrofluorometer (FluoroLog, Horiba) equipped with a 450 W xenon lamp. Time-resolved PL measurements were acquired on a Lifespec II setup (Edinburgh Instrument, UK) with the excitation of a picosecond pulsed 373 nm laser (pulse width: 43 ps), and the excitation energy density is 4.0 μJ cm-2. FTIR spectra were measured using a Bruker VERTEX 70v FTIR spectrometer. XPS data were recorded on a Thermo Scientific ESCALAB 250Xi spectrometer. X-ray energy dispersion spectroscopy (EDS)

Isolation and Purification of Perovskite NCs. The crude NCs solutions were centrifuged for 5 min at 12000 rpm. The supernatant was discarded and after repeating the previous step two more times, the precipitate was dispersed in 4 mL of hexane and centrifuged again for 5 min at 12000 rpm. For CsPbBr3 NCs, this supernatant was collected as the final stable colloidal NC solution. But for others, the supernatant was discarded; then, 4 mL hexane was added to the precipitate, and the resulting mixture was centrifuged for 5 min at 12000 rpm; finally, the precipitate was discarded, and the supernatant was collected as the final stable colloidal NCs solution. Solution-Phase Ligand Exchange Method. For NiCsPbX3 (Cl or mixed Cl/Br) NCs, 1 μL SOX2 (SOCl2 or mixture SOCl2/SOBr2 with a volume ratio of SOCl2/SOBr2 of 1:0.3) was added into 1 ml of purified CsPbX3 NCs solution (~ 1.83 × 10-6 mol/L), After mixing evenly, 1 mL of MA was added and the precipitate was collected by



were obtained using a Hitachi SU8010 scanning electron microscope.

AUTHOR INFORMATION Corresponding Author

Evaluation of the Ligand Concentration. The concentrations of oleyl ligands in deuterated chloroform stock solutions was determined using a combination of 1H NMR, UV−visible absorption and TEM. NMR spectra were recorded on a Varian Unity INOVA 400 spectrometer (400 MHz). The prepared perovskite NCs were dried into powders, which were redissolved in deuterated chloroform (30mg mL-1, 6 mL); mesitylene dissolved in deuterated chloroform (0.2 M, 10 μL) was added to a known volume of the NC stock solution and used as an internal standard. Finally, the clear mixture solutions were transferred into NMR tubes. In 1H NMR spectroscopy, the area of the signals is directly proportional to the number of hydrogens to which the peak corresponds. Hence, the concentration of ligands can be determined by Eq. (3),61 𝐶(𝑥) =

𝐼(𝑥) ∙ 𝑁(𝑠𝑡𝑑) 𝐼(𝑠𝑡𝑑) ∙ 𝑁(𝑥)

𝐶(𝑠𝑡𝑑)

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*E-mail: [email protected] *E-mail: [email protected]

Author Contributions ¶B.-B.

Zhang, S. Yuan, J.-P. Ma, and Y. Zhou contributed equally to this work.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (grant nos. 11874275, 11574225 and U1805252), a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and a project supported by CAS Key Laboratory of Design and Assembly of Functional Nanostructures, and Fujian Key Laboratory of Nanomaterials, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences.

(3)

where C(x) and C(std) are the concentration in mol/L, N(x) and N(std) are the number of protons generating the selected signals for integration, I(x) and I(std) are the integral intensities for the peaks of the analyte and internal standard, all respectively. The molar concentration of CsPbCl3 NCs in stock solution was determined by diluting 100 μL of solution to a known volume with hexane and measuring its absorbance at 380 nm.

REFERENCES (1) Akkerman, Q. A.; Raino, G.; Kovalenko, M. V.; Manna, L. Genesis, Challenges and Opportunities for Colloidal Lead Halide Perovskite Nanocrystals. Nat. Mater. 2018, 17 (5), 394-405. (2) Shamsi, J.; Urban, A. S.; Imran, M.; De Trizio, L.; Manna, L. Metal Halide Perovskite Nanocrystals: Synthesis, Post-Synthesis Modifications, and Their Optical Properties. Chem. Rev. 2019, 119 (5), 3296-3348. (3) Protesescu, L.; Yakunin, S.; Bodnarchuk, M. I.; Krieg, F.; Caputo, R.; Hendon, C. H.; Yang, R. X.; Walsh, A.; Kovalenko, M. V. Nanocrystals of Cesium Lead Halide Perovskites (CsPbX3, X= Cl, Br, and I): Novel Optoelectronic Materials Showing Bright Emission with Wide Color Gamut. Nano Lett. 2015, 15 (6), 36923696. (4) Koscher, B. A.; Swabeck, J. K.; Bronstein, N. D.; Alivisatos, A. P. Essentially Trap-Free CsPbBr3 Colloidal Nanocrystals by Postsynthetic Thiocyanate Surface Treatment. J. Am. Chem. Soc. 2017, 139 (19), 6566-6569. (5) Ahmed, T.; Seth, S.; Samanta, A. Boosting the Photoluminescence of CsPbX3 (X= Cl, Br, I) Perovskite Nanocrystals Covering a Wide Wavelength Range by PostSynthetic Treatment with Tetrafluoroborate Salts. Chem. Mater. 2018, 30 (11), 3633-3637. (6) Yong, Z. J.; Guo, S. Q.; Ma, J. P.; Zhang, J. Y.; Li, Z. Y.; Chen, Y. M.; Zhang, B. B.; Zhou, Y.; Shu, J.; Gu, J. L.; Zheng, L. R.; Bakr, O. M.; Sun, H. T. Doping-Enhanced Short-Range Order of Perovskite Nanocrystals for Near-Unity Violet Luminescence Quantum Yield. J. Am. Chem. Soc. 2018, 140 (31), 9942-9951. (7) Milstein, T. J.; Kroupa, D. M.; Gamelin, D. R. Picosecond Quantum Cutting Generates Photoluminescence Quantum Yields Over 100% in Ytterbium-Doped CsPbCl3 Nanocrystals. Nano Lett. 2018, 18 (6), 3792-3799. (8) Dutta, A.; Behera, R. K.; Pal, P.; Baitalik, S.; Pradhan, N. Near-Unity Photoluminescence Quantum Efficiency for All CsPbX3 (X= Cl, Br and I) Perovskite Nanocrystals: A Generic Synthesis Approach. Angew. Chem. 2019, 131 (17), 5608-5612. (9) Mondal, N.; De, A.; Samanta, A. Achieving Near-Unity Photoluminescence Efficiency for Blue-Violet-Emitting Perovskite Nanocrystals. ACS Energy Lett. 2018, 4 (1), 32-39.

Device Fabrication and Measurements. PEDOT:PSS solutions (CLEVIOS, Al 4083) were spin-coated onto the pre-patterned ITO glass substrates at 4000 rpm for the 40 s and baked at 150 °C in air for 15 min. The HTLs layer was prepared by spin-coating Poly-TPD (America Dye Source) chlorobenzene solution with a concentration of 8 mg mL−1 at 4000 rpm for 35 s in the nitrogen-filled glovebox and baked at 130 °C for 30 min to remove the residual solvent. SOCl2/SOBr2-1 NCs in hexane with a concentration of 12 mg mL−1 were deposited by spin coating at 3000 rpm for 45 s. Finally, TPBi (25 nm), LiF (2 nm) and Al (60 nm) were deposited using a thermal evaporation system under a pressure below ∼1× 10–6 mBar. AFM measurements were conducted on a Cypher-S atomic force microscope. TEM analysis on the cross section of the LED was carried out using an FEI Talos F200X system at 200 kV (FEI, Hillsboro, OR, USA). The EL spectra and J-V-L characteristics were collected by using a Keithley 2400 source meter and a spectrometer (Photo Research, PR670) for light output measurements in air at room temperature.

ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website. Detailed method for the calculation of the average lifetime, XRD, PL spectra, time-resolved PL, TEM images, particle size distribution, XPS spectra, colloidal stability, EDS, and supplementary tables.


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Figure 1. (a) Schematic of the mild reaction and ligand exchange process. Process (1) (a mild reaction on the NC’s surface): the bulky oleic acid and oleylamine ligands react with SOCl2, SOBr2 or their mixture and then are replaced by halide anions. Process (2): after ligand exchange, Ni-CsPbX3 NCs are precipitated via the addition of MA and are separated by centrifugation. Free organic and other residues are taken away by the solvent. (b) Normalized absorption and PL spectra of Ni-CsPbCl3 NCs solution before and after different times of ligand exchange. (c) PLQYs of Ni-CsPbCl3 NCs solution in hexane after different times of washing by MA or SOCl2. For the MA washing, the NCs were merely treated by MA to remove the organic ligands, whereas for the SOCl2 washing, the process is shown as in (a). The average PLQY and error bar were obtained by three parallel experiments. (d, e) TEM images of Ni-CsPbCl3 NCs (d) before ligand exchange and (e) after four times of exchange with SOCl2. 135x82mm (300 x 300 DPI)


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Figure 2. (a) Molar ratios of carbon and nitrogen to Cs in Ni-CsPbCl3 NCs before and after different times of ligand exchange. (b) FTIR spectra of Ni-CsPbCl3 NCs before and after different times of ligand exchange. (c) 1H NMR spectra of Ni-CsPbCl3 NCs before ligand exchange and after four times of exchange with SOCl2. This spectrum contains resonances from native oleyl vinyl protons (5), ODE, solvent (*) and unknown impurities (×). 84x69mm (300 x 300 DPI)



Figure 3. (a) Normalized absorption and PL spectra of Ni-CsPbCl2Br NCs solution before and after different times of ligand exchange. (b) PLQYs of CsPbBr3 NCs solution in hex-ane after different times of washing by MA or SOBr2 and Ni-CsPbCl1.7Br1.3 NCs solution in hexane after different times of washing by MA or SOCl2/SOBr2. (c, d) FTIR spectra of CsPbBr3 NCs (c) and Ni-CsPbCl1.7Br1.3 NCs (d) before and after different times of ligand exchange. 84x90mm (300 x 300 DPI)


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Figure 4. AFM images of Ni-CsPbCl1.7Br1.3 NC films on ITO/PEDOT:PSS/Poly-TPD prepared by the SOCl2/SOBr2-1 (a) and MA-1 (b) NCs. 84x39mm (300 x 300 DPI)



Figure 5. Current-voltage (I-V) character of Ni-CsPbCl1.7Br1.3 NCs before and after one-cycle ligand exchange with SOCl2/SOBr2. Inset shows a schematic illustration of the energy levels for the electron-only device. 60x60mm (300 x 300 DPI)


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Figure 6. (a) Schematic of the device structure. (b) Cross-sectional TEM image of the LED. (c) J–L–V characteristics of the MA-1 and SOCl2/SOBr2-1 LED. (d) EQEs of the MA-1 and SOCl2/SOBr2-1 LEDs. Inset shows a typical electrolumines-cence spectrum of the LED with an applied voltage of 4 V. (e) EL spectra of the SOCl2/SOBr2-1 LED as a function of the operation time under a constant voltage of 3.7 V. (f) Integrated EL intensity of the SOCl2/SOBr2-1 LED at different times under a constant voltage of 3.7 V. 135x83mm (300 x 300 DPI)



TOC 82x42mm (300 x 300 DPI)


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A General Mild Reaction Creates Highly Luminescent Organic-Ligand

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