In Situ Preparation of Metal Halide Perovskite Nanocrystal Thin Films

Mar 23, 2017 - †Department of Electrical Engineering, ‡Princeton Institute for Science and Technology of .... CsPb(Br x I 1-x ) 3 quantum dots and...
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In Situ Preparation of Metal Halide Perovskite Nanocrystal Thin Films for Improved LightEmitting Devices Lianfeng Zhao,† Yao-Wen Yeh,‡ Nhu L. Tran,§ Fan Wu,‡ Zhengguo Xiao,† Ross A. Kerner,† YunHui L. Lin,† Gregory D. Scholes,§ Nan Yao,‡ and Barry P. Rand*,†,∥ †

Department of Electrical Engineering, ‡Princeton Institute for Science and Technology of Materials, §Department of Chemistry, and Andlinger Center for Energy and the Environment, Princeton University, Princeton, New Jersey 08544, United States

∥

S Supporting Information *

ABSTRACT: Hybrid organic−inorganic halide perovskite semiconductors are attractive candidates for optoelectronic applications, such as photovoltaics, light-emitting diodes, and lasers. Perovskite nanocrystals are of particular interest, where electrons and holes can be confined spatially, promoting radiative recombination. However, nanocrystalline films based on traditional colloidal nanocrystal synthesis strategies suffer from the use of long insulating ligands, low colloidal nanocrystal concentration, and significant aggregation during film formation. Here, we demonstrate a facile method for preparing perovskite nanocrystal films in situ and that the electroluminescence of lightemitting devices can be enhanced up to 40-fold through this nanocrystal film formation strategy. Briefly, the method involves the use of bulky organoammonium halides as additives to confine crystal growth of perovskites during film formation, achieving CH3NH3PbI3 and CH3NH3PbBr3 perovskite nanocrystals with an average crystal size of 5.4 ± 0.8 nm and 6.4 ± 1.3 nm, respectively, as confirmed through transmission electron microscopy measurements. Additive-confined perovskite nanocrystals show significantly improved photoluminescence quantum yield and decay lifetime. Finally, we demonstrate highly efficient CH3NH3PbI3 red/near-infrared LEDs and CH3NH3PbBr3 green LEDs based on this strategy, achieving an external quantum efficiency of 7.9% and 7.0%, respectively, which represent a 40-fold and 23-fold improvement over control devices fabricated without the additives. KEYWORDS: organic−inorganic hybrid perovskites, nanocrystal synthesis, in situ preparation, light-emitting diodes, perovskite stability

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synthesis of colloidal nanocrystals in solution, but they coat the nanocrystal surface and reduce device performance.30 Finally, it is difficult to achieve uniform, dense films of colloidal nanocrystals. This is because nanocrystal suspensions saturate at low concentration (approximately 0.5 mg/mL),24 and, during film formation, substantial aggregation or even transformation into bulk phases compromises film uniformity and prevents the formation of pinhole-free films.31,32 Some alternative “in situ” synthesis methods have been proposed, where perovskite nanocrystals can be formed within a polymer matrix during film preparation.33,34 However, LEDs based on perovskite−polymer matrices suffer from high turn-on voltage and low power efficiency sourcing from large resistivity and injection barriers. Consequently, there is a need for facile,

hotovoltaics based on organic−inorganic hybrid perovskite semiconductors have achieved substantial progress over recent years.1−8 Beyond photovoltaics, perovskites have also been intensively investigated as emerging materials for other optoelectronic applications, such as light-emitting diodes (LEDs) and lasers because of their tunable band gap, high color purity, and low material cost.9−23 In particular, the synthesis of nanocrystalline perovskites has drawn great attention and shown promising application for LEDs because the photoluminescence (PL) of perovskites is enhanced through control of crystal size.24,25 Both organic−inorganic hybrid methylammonium lead halide perovskite and fully inorganic cesium lead halide perovskite nanocrystals have been successfully synthesized.26−28 However, traditional solution synthesis methods for perovskite nanocrystals have three main drawbacks for device applications. First, the synthesis process is complicated, typically requiring several cycles of purification by washing and centrifugation.29 Additionally, long and insulating ligands (such as oleylamine and oleic acid) are required for the © 2017 American Chemical Society

Received: January 18, 2017 Accepted: March 15, 2017 Published: March 23, 2017 3957

DOI: 10.1021/acsnano.7b00404 ACS Nano 2017, 11, 3957−3964

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Figure 1. (a) SEM image and (b) AFM image of MAPbI3 films without FPMAI additives. (c) XRD patterns of MAPbI3 films with and without FPMAI additives. (d) SEM image and (e) AFM image of MAPbI3 films with FPMAI additives. (f) F 1s XPS spectra of MAPbI3 films with and without FPMAI additives.

Figure 2. (a) Cross-sectional TEM image and (c) high-resolution cross-sectional TEM image of MAPbI3 films without FPMAI additives. (b) Cross-sectional TEM image and (d) high-resolution cross-sectional TEM image of MAPbI3 films with FPMAI additives. (e) Histogram of MAPbI3 crystal size distribution determined using the short diameter of each particle, with a normal distribution fit.

alternative synthetic strategies for perovskite nanocrystals that can overcome these shortcomings. In this work, we demonstrate an in situ strategy for the synthesis of metal halide perovskite nanocrystal thin films. Rather than casting films from preprepared nanocrystal suspensions, we promote in situ growth of perovskite nanocrystals during the film formation process by incorporating bulky organoammonium halides as additives into the starting precursor solution, which act to confine the crystal growth of 3D perovskites and passivate the surface of perovskite nanocrystals, increasing time-resolved PL (TRPL) decay lifetime and quantum yield (QY) significantly. Using this

strategy, highly efficient methylammonium lead iodide (MAPbI3) and methylammonium lead bromide (MAPbBr3) LEDs are demonstrated with peak external quantum efficiency (EQE) as high as 7.9% and 7.0%, respectively. These values represent a 40-fold and 23-fold improvement over control devices fabricated without the additives.

RESULTS AND DISCUSSION A one-step solvent exchange method is used to prepare MAPbI3 perovskite films.35,36 Stoichiometric perovskite precursors were premixed in dimethylformamide (DMF), and a solvent exchange step is performed during spin coating by 3958

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Figure 3. (a) Cross-sectional STEM image and (d) corresponding elemental EDS map showing the distribution of Pb for MAPbI3 films without FPMAI additives. (b) Cross-sectional STEM image and (e) corresponding elemental EDS map showing the distribution of Pb for MAPbI3 films with FPMAI additives. Electron diffraction patterns of MAPbI3 (c) nanocrystal and (f) matrix.

observed for samples with FPMAI additives, while no F 1s peak is observed for samples without FPMAI additives, proving that the FPMAI additives are indeed incorporated within the film. Furthermore, both samples show the same Pb 4f and I 3d spectra (Figure S1, Supporting Information), indicating identical oxidation states of Pb2+ and I−. To further understand the morphology and composition of additive-induced ultrasmooth perovskite films, cross-sectional transmission electron microscopy (TEM) measurements of MAPbI3 perovskite films with and without FPMAI were performed, as shown in Figure 2. The view in Figure 2a shows that, without the processing additive, the stoichiometric solution produces a compact polycrystalline film, featuring perovskite grains exhibiting various orientations. Figure 2c shows a high-resolution TEM (HRTEM) image for a perovskite film without additives, with lattice fringes spaced by 0.32 nm. This fringe spacing corresponds to the (004) plane spacing for MAPbI3 crystals.40 In contrast, MAPbI3 perovskite films processed with FPMAI possess a “nanocrystal-in-matrix” morphology, as shown in Figure 2b. The lattice fringes of one representative nanocrystal are shown in Figure 2d, where the interplane spacing of 0.22 nm is indexed to the (224) plane of MAPbI3.41 Figure 2e shows the size distribution of MAPbI3 perovskite nanocrystals. An average crystal size of 5.4 nm is achieved, with a standard deviation of 0.8 when fitting with a normal distribution. The crystal size distribution is narrower than that reported using other in situ fabrication techniques,33,34 although still broader than that of nanocrystals synthesized in solution, where narrow size distributions are achieved through centrifugation.29 To further investigate the morphology and distribution of MAPbI3 perovskites throughout the film, scanning transmission electron microscopy (STEM) and energy-dispersive X-ray spectroscopy (EDS) measurements were carried out for elemental mapping, with results shown in Figure 3. The dense perovskite films formed without additives are further confirmed with the uniform distribution of Pb (Figure 3a and

dropping toluene onto the spinning sample, resulting in uniform perovskite films, as observed by a scanning electron microscope (SEM) (Figure 1a). The toluene extracts the DMF solvent and freezes the morphological evolution during crystallization.36 Although full coverage is achieved, the perovskite film is rough with grains ranging from 30 to 170 nm and notably with a large volume of unpassivated grain boundaries.37 The root-mean-square (RMS) roughness is extracted to be 5.7 nm using an atomic force microscope (AFM) (Figure 1b). Bulky organoammonium halides inhibit the growth of threedimensional (3D) perovskites, resulting in layered Ruddlesden−Popper phases if the precursors satisfy the stoichiometry of that phase.38 Here, we use a stoichiometric 3D MAPbI3 perovskite precursor solution with 20 mol % bulky 4fluorophenylmethylammonium iodide (FPMAI) added as additives. Using the same film preparation method, the resulting perovskite films are ultrasmooth (Figure 1d) with only 1.2 nm RMS roughness (Figure 1e). Furthermore, the grains become so small that they can no longer be properly resolved by AFM with a tip radius of ∼20 nm, in agreement with our previous work using butylammonium iodide as an additive.36,39 The motivation for the use of FPMAI in the current study is that the presence of fluorine provides us with a distinct chemical signature of the bulky additive. X-ray diffraction (XRD) measurements show the same tetragonal perovskite crystal structure for samples with and without FPMAI additives (Figure 1c), indicating that the FPMAI additives are not incorporated into the grains in Ruddlesden−Popper phases, but rather allow the nanocrystals to retain a 3D MAPbI3 perovskite structure. The full width at half-maximum (fwhm) of the (110) peak at 14° is larger for samples with FPMAI additives, in agreement with the smaller crystal size seen in the AFM results. X-ray photoelectron spectroscopy (XPS) analysis was used to verify the presence of FPMAI additives in the film. Figure 1f shows the F 1s spectra of samples with and without additives. A prominent F 1s peak is 3959

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“nanocrystal-in-matrix” morphology, as confirmed using TEM in recent reports.15,16 In addition, when excess MAI is used in MAPbI3 precursor solutions instead of a bulky organoammonium halide, the resulting perovskite films are still compact and polycrystalline with large grains, with MAI at crystallite surfaces or, owing to its high vapor pressure, ultimately exiting the film.37 The fact that our technique of using bulky organoammonium halide additives results in “nanocrystal-in-matrix” morphology distinguishes our strategy from other recent work. To investigate the effects of our strategy on the optical properties of the perovskite films, we characterized the steadystate PL spectra, PLQY, and TRPL of MAPbI3 films with and without the FPMAI additives. Figure 4a and b show the steady-

d). In contrast, the distribution of Pb for MAPbI3 perovskites formed using FPMAI additives is consistent with the nanocrystal distribution observed in the STEM image (Figure 3 b and e), proving that MAPbI3 perovskites are confined into nanocrystals embedded in the additive-formed matrix. The corresponding electron diffraction patterns of the MAPbI3 nanocrystal and the matrix are shown in Figure 3c and f, respectively, showing that the MAPbI3 nanocrystals are crystalline while the matrix is amorphous. Other elemental (F, N, I, C) mapping results are shown in Figure S2 in the Supporting Information. The distribution of N and I is also consistent with the “nanocrystal-in-matrix” morphology, while the distribution of C is not, and the signal of C is very strong in the matrix area. We thus believe the matrix is primarily composed of excess bulky ligands, rather than crystalline or amorphous MAPbI3. Due to the excess FPMAI used in the precursor and our assumption that the bulky ligand attaches over the entire 3D perovskite nanocrystal surface, the distribution map of F is not well resolved by EDS. On the basis of our observations regarding the different morphologies of perovskite films formed with and without additives, we propose the following mechanism for the additiveconfined perovskite crystallization process. Without additives, perovskite crystals grow from different nucleation sites and aggregate, forming compact polycrystalline perovskite films (confirmed by the TEM image in Figure 2a) that possess a large volume of unpassivated grain boundaries.37 However, when bulky organoammonium halide additives are introduced in the processing, the outward growth of perovskite crystals from the initial nucleation sites is strongly suppressed. This results in sub-10 nm perovskite nanocrystals with additives capping the nanocrystal surface, forming a “nanocrystal-inmatrix” morphology. On the basis of our understanding of the mechanism of the additive-confined perovskite crystallization process, maintaining a stoichiometric ratio of the 3D MAPbI3 precursors with a suitable amount (20 mol % in this work) of bulky organoammonium halide additives is key to the success of this in situ nanocrystal preparation strategy. We confirmed by XRD and TEM that, at this percentage of loading, the bulky additives do not incorporate into the 3D tetragonal MAPbI3 perovskite structure, but only serve to confine the perovskite crystal growth. These observations indicate that the 3D tetragonal perovskite nucleates and grows prior to the lower dimensional Ruddlesden−Popper phase. However, at higher loadings of FPMAI additives (e.g., 40 or 60 mol %), the structure of the resulting films shifts to that of the lower dimensional Ruddlesden−Popper phase (Figure S3). Additionally, adopting a solvent exchange technique during film processing is necessary because it freezes the subsequent morphological evolution during crystallization.36 Figure S4a shows the XRD pattern for the film processed with 20 mol % FPMAI additives but without using the solvent exchange technique. Without the solvent exchange, films still retain the 3D tetragonal MAPbI3 perovskite structure, but the narrower fwhm of the (110) peak at 14° indicates a larger crystal size. This is because the gel is given more time to age and crystallites are given more time to grow, resulting in a film without full coverage (Figure S4b). It should be noted that, when precursor solutions with bulky organoammonium halides are prepared according to the stoichiometry of the layered Ruddlesden−Popper phase, the resulting perovskite films are compact polycrystalline films with the Ruddlesden−Popper phase structure, as opposed to the

Figure 4. (a) Steady-state PL spectra and (b) TRPL traces of MAPbI3 films with and without FPMAI additives.

state PL spectra and TRPL data, respectively. With FPMAI additives, the steady-state PL emission peak is blue-shifted from 764 nm to 749 nm. It is not a quantum confinement effect, since this would only manifest for nanocrystal diameters approaching the Bohr radius, approximately 2.1 nm.42 Instead, this may be understood as an effect of increased disorder.43 As shown in Figure 2d, the sub-10-nm nanocrystals contain approximately 15 lattice plane spacings. While this size is not small enough to induce quantum confinement and a subsequent sensitive dependence of band gap on size, the long-range, infinite lattice periodicity of a bulk material is nevertheless disturbed in the nanoscale system. Also, the dielectric constant of the local environment for these nanocrystals (which is the organic matrix) is substantially lower than the perovskite, thus further confining the wave function to the crystallite. Additionally, the reduced absorption coefficient and thus the reabsorption may also induce the PL shift. At an excitation intensity of 2 mW/cm2, QY is increased from 0.1% to 4.4%, while the TRPL decay lifetime is increased from 3 to 9 ns for films processed with FPMAI additives (Supporting Tables S1 and S2), indicating that the FPMAI 3960

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Figure 5. (a) EL spectra, (b) J−R−V curves, and (c) EQE curves of MAPbI3 red LEDs with and without FPMAI additives. Inset in (a) shows a working LED based on MAPbI3 with additives. (d) EL spectra, (e) J−L−V curves, and (f) EQE curves of MAPbBr3 LEDs with and without PEABr additives. Inset in (d) shows a working LED based on MAPbBr3 with additives.

Figure 6. Operational stability of (a) MAPbI3 and (b) MAPbBr3 LEDs at a constant current density of 3 mA/cm2.

inset. Similar to the PL spectra, the EL emission peak is also blue-shifted from 764 nm to 749 nm for devices with FPMAI additives. Figure 5b shows the current density−radiance− voltage (J−R−V) curves for devices with and without FPMAI additives. A maximum radiance as high as 72 W/(sr·m2) is achieved, which is 12 times higher than that of a device without additives. The angular emission intensity of the device follows a Lambertian profile (Figure S7). The EQE vs current density curves are shown in Figure 5c. Without the FPMAI additives, a peak EQE of 0.2% was observed. With the additives, a peak EQE of 7.9% was achieved, a 40-fold improvement. This device performance enhancement is consistent with the improvements in PLQY and lifetime observed due to FPMAI surface passivation effects. However, it should be noted that the bimolecular recombination pathway responsible for radiative recombination that controls the value of both EQE and PLQY is dependent on carrier density. The peak EQE is achieved at a condition with a higher carrier density than the condition when we perform the PLQY measurement, which explains the higher value of peak EQE than the measured PLQY. To demonstrate the versatility of our technique, we employed similar in situ preparation strategies with another kind of bulky organoammonium halide (phenethylammonium

additives can passivate trap states and reduce nonradiative recombination. One likely channel for nonradiative recombination is quenching on undercoordinated Pb sites.44 It has been reported that excess MAI in MAPbI3 can form a layer at grain boundaries, which can help suppress nonradiative recombination.37 Given the similar nature of excess MAI and FPMAI, we suspect that FPMAI passivates undercoordinated Pb atoms in a similar manner. To examine the effect of the film composition on device functionality, MAPbI3 LEDs were fabricated with a device structure of ITO (150 nm)/poly-TPD (25 nm)/MAPbI3 perovskite (50 nm)/TPBi (40 nm)/LiF (1.2 nm)/Al (100 nm) (ITO: indium tin oxide; poly-TPD: poly[N,N′-bis(4butylphenyl)-N,N′-bis(phenyl)benzidine]; TPBi: 2,2′,2″-(1,3,5benzinetriyl)tris(1-phenyl-1H-benzimidazole)), where polyTPD serves as a hole-transporting and electron-blocking layer and TPBi as an electron-transporting and hole-blocking layer. Figure S5 shows cross-sectional TEM images of the LEDs, which confirms the film thicknesses. The corresponding schematic energy band diagram of the device is shown in Figure S6. Figure 5a shows the electroluminescence (EL) spectra of devices with and without FPMAI additives, with an image of a working LED (with FPMAI additives) shown as an 3961

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were filtered and dried under low heat. Recrystallization, filtration, and drying were performed inside a N2-filled glovebox. Perovskite Film Deposition and Device Fabrication. PbI2 (PbBr2) and MAI (MABr) were dissolved in DMF (Sigma-Aldrich, 99.8% anhydrous) to obtain a 0.4 M MAPbI3 (MAPbBr3) solution. Additives (FPMAI or PEABr) were mixed with the perovskite precursor (MAPbI3 or MAPbBr3) in a 0.2:1 molar ratio. Poly-TPD (6 mg/mL in chlorobenzene) or PVK (6 mg/mL in chlorobenzene) was spin-coated on glass substrates with prepatterned ITO at 1500 rpm for 70 s followed by thermal annealing at 150 or 120 °C for 20 min. Poly-TPD was then treated with O2 plasma for 1 s to improve wetting. Perovskite films (with or without additives) were deposited on poly-TPD or PVK by spin coating at 6000 rpm. A solvent exchange step is performed after 3.5 s by dropping toluene on the spinning samples. Then, samples were annealed at 70 °C for 5 min. TPBi, LiF, and Al layers were thermally evaporated with thicknesses of 40, 1.2, and 100 nm, respectively. Device area is 0.1 cm2. Material and Device Characterization. Film surface morphologies were surveyed by an FEI Verios 460 XHR SEM. Crystal structures of the films were studied by using a Bruker D8 Discover Xray diffractometer with an incident wavelength of 0.154 nm. Surface topography of the films was imaged with a Bruker Nanoman AFM. Chemical contents of the films were characterized by using a Thermo Scientific Kalpha XPS System. Samples were directly transferred to the XPS chamber without air exposure in order to avoid surface contamination and oxidation. Cross-section TEM lamella samples of the devices were prepared by an FEI Helios DualBeam microscope. HRTEM images, electron nanodiffraction, and EDS measurements were carried out in an FEI Talos (S)TEM at 200 kV. Steady-state and TRPL measurements were carried out using an FLS980 spectrometer (Edinburgh Instruments). Samples were excited at 470 nm from a Xe arc lamp for the steady-state PL measurements. For the TRPL measurements, MAPbI3 samples with (without) additives were excited at 634.8 nm by a pulsed laser diode with a 500 ns pulse period and detection wavelength of 749 nm (764 nm); MAPbBr3 samples with (without) additives were excited at 374.4 nm by a pulsed laser diode with a 500 ns pulse period and detection wavelength of 530 nm (521 nm). PLQY was measured using a Petite integrating sphere coupled to the PTI QuantaMaster 400 Steady State fluorometer system. Excitation wavelengths of 450 and 400 nm were used for MAPbI3 and MAPbBr3 films, respectively. The following settings were kept the same for all films: bandpass values of 5 nm for both the excitation and emission slits, step increments of 1 nm, and integration times of 0.5 s per data point. The excitation intensity was 2 mW/cm2. Characteristics of perovskite LEDs were measured in a N2 glovebox using a custom motorized goniometer consisting of a Keithley 2400 sourcemeter unit, a picoammeter (4140B, Agilent), a calibrated Si photodiode (FDS100-CAL, Thorlabs), and a calibrated fiber-optic spectrophotometer (UVN-SR, StellarNet Inc.).

bromide, PEABr) as an additive to form MAPbBr3 nanocrystalline films. An average particle size of 6.4 nm is achieved, with a standard deviation of 1.3 nm (Figure S8). Consistent with our findings on the MAPbI3 system, MAPbBr3 films processed with additives also show a blue-shifted PL peak wavelength, increased PLQY (from 0.4% to 10.9%), and increased TRPL decay lifetime (from 1.6 ns to 9.5 ns), compared to samples processed without additives (Figure S9, Tables S1 and S2). Green LEDs were fabricated with a similar device structure to MAPbI3 LEDs except for substituting poly(N-vinylcarbazole) (PVK) for poly-TPD as the hole-transporting and electronblocking layer. Electroluminescence spectra, current density− luminance−voltage (J−L−V), and EQE curves are shown in Figure 5d−f. The peak luminance is increased from 220 cd/m2 to 11400 cd/m2 and the EQE is increased from 0.3% to 7.0% by introducing the additives into the perovskite layer. Concerning stability, devices employing our in situ preparation strategies show significantly improved operational stability. As shown in Figure 6a, the EQEs of the additiveconfined MAPbI3 LEDs show minimal degradation over 10 h under a constant current density of 3 mA/cm2, while the EQEs of MAPbI3 LEDs without additives are not functional within 2 h of constant stress. Considering that ion migration is known to occur in perovskite devices,45 this process can alter the internal field profile, induce unintended chemical interactions,8 and create defects,46 all of which can degrade device performance. It is possible that the bulky additives improve stability either because they encapsulate the particle or because the matrix that separates perovskite particles impedes ionic motion. Furthermore, the additive-confined MAPbI3 LEDs show no obvious efficiency degradation after storage in N2 for four months (Supporting Figure S10), further suggesting improved material stability. The MAPbBr3 LEDs with additives also show improved device stability compared to devices without additives, although the MAPbBr3 LEDs are considerably less stable than the MAPbI3 LEDs (Figure 6b), which is consistent with a previously reported study showing that MAPbBr3 is intrinsically less stable than MAPbI3.47

CONCLUSION In summary, we have described and demonstrated a facile, in situ perovskite nanocrystal preparation method where bulky organoammonium halides are used as additives to confine the crystal growth of perovskites, achieving perovskite nanocrystals with sub-10-nm crystal size. Furthermore, these additives have passivation effects on perovskite surfaces, improving the PLQY and TRPL decay lifetime from 0.4% to 11% and 1.6 ns to 9.5 ns (Tables S1 and S2). With this strategy, high-performance MAPbI3 and MAPbBr3 LEDs are demonstrated, with a peak EQE of 7.9% and 7.0%, respectively, which demonstrate the generality and promising potential of this perovskite thin film preparation strategy for optoelectronic device applications.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b00404. Additional figures and tables (PDF)

AUTHOR INFORMATION Corresponding Author

METHODS

*E-mail: [email protected].

Synthesis of Organic Cation Salts. MAI, FPMAI, MABr, and PEABr crystals were synthesized by mixing methylamine, 4fluorophenylmethylamine, and phenethylamine (Sigma-Aldrich) with HI or HBr (Sigma-Aldrich) in a 1:1 molar ratio, respectively. The reaction was performed in an ice bath while stirring for 3 h. The solvent of the resulting solution was evaporated using a rotary evaporator. The MAI, FPMAI, MABr, and PEABr were recrystallized from an isopropyl alcohol/toluene mixture. Finally, the large crystals

ORCID

Lianfeng Zhao: 0000-0003-0967-6536 YunHui L. Lin: 0000-0002-0817-8757 Gregory D. Scholes: 0000-0003-3336-7960 Barry P. Rand: 0000-0003-4409-8751 Notes

The authors declare no competing financial interest. 3962

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