Zn-Alloyed CsPbI3 Nanocrystals for Highly Efficient Perovskite Light

Feb 11, 2019 - Department of Chemistry and Physics, Louisiana State University , Shreveport , Louisiana 71115 , United States. ± Beijing Institute of...
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Letter Cite This: Nano Lett. XXXX, XXX, XXX−XXX

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Zn-Alloyed CsPbI3 Nanocrystals for Highly Efficient Perovskite LightEmitting Devices Xinyu Shen,† Yu Zhang,*,† Stephen V. Kershaw,∥ Tianshu Li,‡ Congcong Wang,†,§ Xiaoyu Zhang,†,‡ Wenyan Wang,§ Daguang Li,† Yinghui Wang,§ Min Lu,† Lijun Zhang,‡ Chun Sun,† Dan Zhao,† Guanshi Qin,† Xue Bai,† William W. Yu,†,⊥ and Andrey L. Rogach*,∥,±

Nano Lett. Downloaded from pubs.acs.org by UNIV OF GLASGOW on 02/11/19. For personal use only.

†

State Key Laboratory of Integrated Optoelectronics and College of Electronic Science and Engineering, ‡Key Laboratory of Mobile Materials MOE, State Key Laboratory of Automotive Simulation and Control, and College of Materials Science, and §College of Physics, Jilin University, Changchun 130012, China ∥ Department of Materials Science and Engineering, and Centre for Functional Photonics (CFP), City University of Hong Kong, Kowloon, Hong Kong SAR ⊥ Department of Chemistry and Physics, Louisiana State University, Shreveport, Louisiana 71115, United States ± Beijing Institute of Technology, School of Materials Science and Engineering, Beijing 100081, China S Supporting Information *

ABSTRACT: We alloyed Zn2+ into CsPbI3 perovskite nanocrystals by partial substitution of Pb2+ with Zn2+, which does not change their crystalline phase. The resulting alloyed CsPb0.64Zn0.36I3 nanocrystals exhibited an improved, close-tounity photoluminescence quantum yield of 98.5% due to the increased radiative decay rate and the decreased non-radiative decay rate. They also showed an enhanced stability, which correlated with improved effective Goldschmidt tolerance factors, by the incorporation of Zn2+ ions with a smaller radius than the Pb2+ ions. Simultaneously, the nanocrystals switched from n-type (for CsPbI3) to nearly ambipolar for the alloyed nanoparticles. The hole injection barrier of electroluminescent LEDs was effectively eliminated by using alloyed CsPb0.64Zn0.36I3 nanocrystals, and a high peak external quantum efficiency of 15.1% has been achieved. KEYWORDS: Lead halide perovskite nanocrystals, alloying, improved photoluminescence quantum yield, enhanced stability, light-emitting devices

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cesses.20−24 However, while the surface modification and passivation by organic molecules may eliminate some of the defect states, the addition of such a dielectric coating reduces the conductivity of PNC films and introduces a barrier for charge injection. Ligand exchange and purification processes are the effective ways to enhance the conductivity of PNCs, but these frequently decrease the PL QY by incorporating more defect states in the particles.21,25 Alternatively, metal ion doping is another effective way to decrease defect states and improve the performance of PNCs and PNC-based LEDs.8,26,27 However, the limited improvement of device EQE via these strategies indicates that new methods are necessary. Although radiative decay rate is another important factor, which can affect the performance of PNCs, it is often difficult to modulate this intrinsic property of materials.28 Several methods, including incorporation with photonic crystals, increasing the material refractive index, and using

ight-emitting devices (LEDs) based on lead halide perovskite nanocrystals (PNCs) with high photoluminescence quantum yield (PL QY) exhibit extremely high color purity and easily tunable emission, which make them promising for display and solid-state lighting applications.1−4 During the past few years in particular, there has been impressive progress for the external quantum efficiency (EQE) of perovskite LEDs.5−13 In spite of this, there is still a lot of space for further improvements of PNC-based LED performance to satisfy the criteria for their commercial applications.14 Although lead halide perovskites have been considered defecttolerant,15,16 a great boon from the point of view of photoluminescent performance, the existence of defect states in perovskite active layers is nonetheless an important issue for LEDs and cannot be ignored because such defects may reduce the carrier injection efficiency and radiative recombination probability.2,14,17,18 Similarly motivated studies of trap elimination in perovskite films via (anion) composition control have reported recently.19 Traditional methods aimed at improving the LED performance include surface passivation, ligand exchange, and optimized PNC purification pro© XXXX American Chemical Society

Received: October 28, 2018 Revised: January 29, 2019

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DOI: 10.1021/acs.nanolett.8b04339 Nano Lett. XXXX, XXX, XXX−XXX

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Figure 1. (a−e) TEM images of CsPbI3 PNCs with the ZnI2-to-PbI2 precursor ratios of (a) 0, (b) 0.5, (c) 0.75, (d) 1, and (e) 1.5, with their corresponding HRTEM images shown in the insets. (f) XRD patterns of CsPbI3 PNCs with ZnI2-to-PbI2 ratios of 0, 0.5, 0.75, 1, and 1.5. (g) Elemental mappings of Cs, Pb, Zn, and I elements in the Zn-alloyed CsPbI3 PNCs; the last frame on the row shows overlapped TEM/elemental mapping images.

(the feed ratios of ZnI2 to PbI2 are 0, 0.5, 0.75, 1, and 1.5), perovskite PNCs still retain a cubic morphology. They exhibited average diameters of 13.2 ± 1.3, 12.4 ± 1.6, 12.7 ± 1.5, 9.5 ± 1.3, and 8.1 ± 1.3 nm, respectively, and lognormal distribution histograms (as shown in Figure S1a−e). The decrease in particle size at high Zn content may be due to the increased uptake of iodide ions, which inhibits the further growth of the perovskite.33,34 The high-resolution TEM (HRTEM) images (bottom right insets in Figure 1a−e) revealed that CsPbI3 PNCs both without and with ZnI2 precursor were highly crystalline and displayed lattice fringes for the (100) plane, which can be also seen in the enlarged powder X-ray diffraction (XRD) data at ranges of 13.0− 17.0°(the right panel of Figure 1f; the calculation results are listed in Table S1). As shown in Figure 1f, diffraction peaks appeared at 14.1, 20.0, 24.6, 28.6, 35.2, 41.0 and 43.2°, which could be assigned to (100), (110), (111), (200), (210), (211), (220), and (300) planes of the cubic perovskite structure.35 These diffraction peaks shifted to higher angles without the introduction of any extra peaks as the proportion of ZnI2 precursor increased, indicating that the incorporation of the Zn source did not change the crystalline forms of the PNCs but only contracted the lattice.36 Because the ionic radius of Zn2+ (74 pm) is smaller than that of Pb2+ (119 pm), the reduced lattice constant suggests Zn2+ cations replace Pb2+ cations. If Zn atoms resided in interstitial sites, we would observe a lattice expansion with increasing Zn content. Elemental mapping was performed on CsPbI3 PNCs with ZnI2 precursor, as shown in Figure 1g. All of the corresponding mapping images of Cs, Pb, Zn, and I were overlapping with the

localized surface plasmon effects, have been proposed to improve the radiative decay rate.28−31 However, some of these can also prejudice the overall performance; for example, materials with a higher refractive index may have improved radiative rates but suffer from poorer external coupling, and additionally, the more-complex hybrid device structures may be hard to realize (i.e., fabricate in high yield and at low cost) in LED devices. Here, we propose a much simpler approach by incorporating Zn2+ into CsPbI3 PNCs, forming a series of alloyed CsPb1−xZnxI3 (0 ≤ x < 1) PNCs, which resulted in a progressive lattice contraction but without changing the crystalline forms of the perovskites and improved effective tolerance factors and stabilities. Importantly, Zn2+ ions were confirmed to be effective in elimination of the defect states in alloyed PNCs, resulting in a 4.1-fold increase of their radiative decay rates. Consequently, the PL QY of the alloyed CsPb1−xZnxI3 PNCs was enhanced to near-unity. Simultaneously, the PNCs converted from n-type because of I vacancies to nearly ambipolar, and the hole injection barrier of LEDs was eliminated by alloying with Zn2+, similar to what has been reported recently.32 As a consequence of all these beneficial improvements, the LEDs based on the Zn-alloyed PNCs exhibited an excellent peak EQE value of 15.1%. CsPbI3 PNCs alloyed with Zn2+ were synthesized by using ZnI2 as a Zn source via a typical hot injection reaction.1 The morphologies of as-prepared CsPbI3 PNCs without and with ZnI2 precursors were investigated through transmission electron microscopy (TEM) (Figure 1a−e). As shown in Figure 1a−e, with the increasing amount of ZnI2 precursor B

DOI: 10.1021/acs.nanolett.8b04339 Nano Lett. XXXX, XXX, XXX−XXX

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Figure 2. (a) XPS spectra of CsPbI3 PNCs synthesized with and without ZnI2 precursors. (b−d) High-resolution XPS spectrum of Zn (2p1/2 and 2p1/2), Pb (4f5/2 and 4f7/2), and I (3d3/2 and 3d5/2) of CsPbI3 PNCs synthesized with and without ZnI2 precursors.

TEM image, illustrating the existence of Zn2+ in the CsPbI3 PNCs. To further explore the chemical states of Zn2+ in the PNCs, X-ray photoelectron spectroscopy (XPS) has been conducted on CsPbI3 PNCs produced with and without ZnI2 precursor. As shown in Figure 2a, both kinds of PNCs exhibited XPS peaks of Cs, Pb, I, O, C, and N. However, two additional peaks in CsPbI3 PNCs synthesized with ZnI2 precursor appeared at 1021.4 and 1044.6 eV, which could be assigned to the Zn 2p signals of Zn2+, proving that Zn2+ ions have been incorporated into CsPbI3 PNCs.37 This is further supported by the high-resolution XPS spectra of Zn 2p shown in Figure 2b. Moreover, the high-resolution XPS spectra involving Pb 4f and I 3d signals were analyzed in detail to reveal the existence of Zn in the host CsPbI3 PNCs. As shown in Figure 2c, the Pb 4f spectra of pure CsPbI3 PNCs have two signature peaks of Pb2+ 4f7/2 and Pb2+ 4f5/2 at 138.3 and 143.2 eV.38,39 In addition, the I 3d spectra of bare CsPbI3 PNCs showed two main peaks of I− 3d5/2 and I− 3d3/2 at 619.2 and 630.6 eV (Figure 2d).40,41 In contrast, as Zn2+ ions were introduced, the binding energy of Pb2+ 4f7/2 and Pb2+ 4f5/2 as well as I− 3d3/2 and I− 3d5/2 move to lower energy. The binding energy difference between Pb 4f7/2 and I 3d5/2 is decreased by 0.2 eV when the Zn is present. These results indicate that some Zn2+ ions have entered into the lattice of the CsPbI3 perovskite host and most probably substituted on the Pb2+ sites.42 Moreover, Zn2+ has a stronger chemical interaction with I− than Pb2+, which can lead to a partial reduction in the interaction between extranuclear and core electrons in both neighboring Pb2+ and I− ions.42 As shown in Figure S2b−e, the energy-dispersive X-ray spectroscopy (EDS) spectrum indicates a Zn-to-(Zn plus Pb) atomic ratio of 0.12, 0.27, 0.36, and 0.49 corresponding to

Figure 1b−e, values that were further supported by inductively coupled plasma mass spectrometry (ICP-MS) and XPS measurements (Tables S2 and S3). These measurements together substantiate the formation of alloyed CsPb1−xZnxI3 (0 ≤ x < 1) PNCs.43 The optical properties of Zn2+-alloyed CsPbI3 PNCs are summarized in Figure 3; the photos given in Figure 3a show that the obtained PNCs dispersed in toluene emit red light under UV-light irradiation (365 nm). Their emission intensity increased at first and then decreased with the increasing concentration of Zn2+ in PNCs. The normalized absorption and PL spectra of alloyed CsPb1−xZnxI3 (0 ≤ x < 1) and bare CsPbI3 PNCs are compared in Figure 3a. Both the absorption peak and the PL peak of the PNCs shifted to the blue with increasing Zn2+ content in CsPbI3 PNC hosts. The absorption peak, PL peak, and PL QY as functions of x in CsPb1−xZnxI3 (0 ≤ x < 1) PNCs are summarized in Figure 3b. The absorption peak changed from 678 to 675, 671, 668, and 662 nm as x increased from 0 to 0.12, 0.27, 0.36, and 0.49, respectively. Simultaneously, the PL peak varied from 690 to 688, 684, 682, and 676 nm. The blue shifts of absorption and PL peaks mainly result from lattice contraction and decreased PNC size.44,45 The PL QY increased from 61.3% to 68.2, 76.1, and 98.5% when the Zn-to-(Zn plus Pb) ratios varied from 0 to 0.12 and 0.27 and 0.36 and then decreased to 78.0% when the Zn-to-(Zn plus Pb) ratio further increased to 0.49. To further investigate the radiative mechanism of an excellent near unity PL QY for alloyed PNCs, PL lifetimes of pure CsPbI3 and alloyed CsPb1−xZnxI3 (0 ≤ x < 1) PNCs were measured. As shown in Figure 3c, it is observed that the PL decay of alloyed CsPb1−xZnxI3 (0 ≤ x < 1) PNCs is faster than that of pure CsPbI3 PNCs. Their average lifetimes () C

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where A and B are scaling factors, I0 is the intensity at 0 K, kB is the Boltzmann constant, and Etrap is the trap related energy.17,47,48 The calculated values are 24.4 meV for CsPbI3 PNCs and 47.6 meV for CsPb0.64Zn0.36I3 PNCs. To explore whether the increase of exciton binding energy is due to the decrease of particle size or the introduction of Zn2+, two kinds of CsPbI3 PNCs with different sizes as 7.5 ± 1.2 and 9.2 ± 1.4 nm as well as CsPb0.73Zn0.27I3 PNCs with the size of 9.2 ± 1.2 nm were prepared. Their TEM images, dimension distribution, and temperature-dependent PL spectra are shown in Figure S5. The normalized integrated PL intensity of CsPbI3 PNCs with different sizes, and of CsPbI 3 , CsPb 0.73 Zn 0.27 I 3 and CsPb0.64Zn0.36I3 PNCs with similar size versus temperature are shown in Figure S6. As shown in Table S5, the calculated exciton binding energies of CsPbI3 PNCs with different sizes (7.5 ± 1.2, 9.2 ± 1.4, and 13.2 ± 1.3 nm) are 32.0, 29.3, and 24.4 meV, respectively. At the same time, exciton binding energies are 29.3, 39.8, and 47.6 meV for CsPbI 3 , CsPb0.73Zn0.27I3, and CsPb0.64Zn0.36I3 PNCs with similar size, respectively. This indicates that the increase of exciton binding energy in the alloys originates from the incorporation of Zn and not simply from any accompanying lattice contraction. Furthermore, according to the Fermi Golden Rule and assuming a Boltzmann distribution of exciton energies, the following equation can be obtained for the radiative decay rate:

Figure 3. (a) Absorption and PL spectra of bare CsPbI3 and alloyed CsPb1−xZnxI3 (0 ≤ x < 1) PNCs with different Zn-to-(Zn plus Pb) ratios. Insets show photographs of the corresponding colloidal samples under UV-light (365 nm) irradiation. (b) The absorption peak and PL peak (excitation at 405 nm) maxima and PL QYs of bare CsPbI3 and alloyed CsPb1−xZnxI3 (0 ≤ x < 1) PNCs with different Zn-to-(Zn plus Pb) ratios. (c) The PL decay time of bare CsPbI3 and alloyed CsPb1−xZnxI3 (0 ≤ x < 1) PNCs with different Zn-to-(Zn plus Pb) ratios.

kr =

(3)

where n, ω, ε0, m0, μ, M, and ΔE are the refractive index, the optical transition frequency, the vacuum permittivity, the freeelectron mass, the equivalent mass of the exciton, the mass of the exciton, and the energy line width, respectively (for the detailed derivation, see the Supporting Information). As can be seen from the above equation, it is related to exciton binding energy and the PNC refractive index. However, the refractive index decreased slightly with the incorporation of Zn, being 1.93 at 690 nm and 1.88 at 682 nm for CsPbI3 and alloyed CsPb0.64Zn0.36I3 PNCs, respectively. If we only consider the factor of exciton binding energy, the radiative decay rate would be expected to increase by 2.72-fold after the incorporation of Zn according to the above equation. In the event that we actually observe a 4.1-fold improvement although it is not the only factor in play, the increase of the exciton binding energy with the incorporation of Zn2+ is probably the main single factor for the improvement of the radiative decay rates in the alloyed PNCs.49−51 As for the decrease of the apparent non-radiative decay rate, we keep in mind that it ignores the presence of dark (nonemitting) dots (and any potential changes in this dark fraction) but would over-estimate the true non-radiative rate by some factor between 0 and 1 depending on the dark fraction itself (see the Supporting Information for details).46,52 The apparent non-radiative decay rates decreased when the Zn-to-(Zn plus Pb) ratio increased from 0 to 0.36; however, at Zn contents higher than 0.49, the apparent non-radiative rates dramatically increased. This may indicate that the defect density in the PNCs drops through an optimum minimum value but then increases again at higher Zn contents, but other factors such as a change in the dark fraction of emitters could also be playing a role alongside this. That said, the relatively high PLQYs in these materials points to the probability that the dark fraction of PNCs is low.

[calculated via numerical integration (see eq S1) of the decay curves rather than taking an average from a multi-exponential fit], radiative decay rates (kr) and the apparent (see the Supporting Information) non-radiative decay rates (knr) are given in Table S4. The decreased from 215.2 ns to 211.8, 187.3, 85.0, and 61.3 ns as the Zn-to-(Zn plus Pb) ratio increased from 0 to 0.12, 0.27, 0.36, and 0.49. The radiative decay rate increased 4.1-fold, and the apparent non-radiative decay rate decreased 10-fold in CsPb0.64Zn0.36I3 PNCs, indicating that the incorporation of Zn indeed affects the intrinsic optical properties of PNCs. The temperature-dependent PL spectra of unalloyed CsPbI3 and alloyed CsPb0.64Zn0.36I3 PNCs (which is the sample with the close to unity PL QY of 98.5%) are shown in Figure S3. Figure S4a shows the emission energy as a function of temperature, and the solid line was obtained by fitting the temperature-dependent peak energies to Varshni’s equation: Eg = Eg (0) − αT 2/(β + T )

ne 2ω 2 ij μ yz3/2 1 − e−Δ E / kT f jj E Bzz ΔE 3ε0m0c 3 0 k M {

(1)

where α is the temperature coefficient, Eg(0) is the bandgap at 0 K, and β approximates the Debye temperature of the material.46,47 The calculated α values are −2 × 10−4 K−2 and −1.3 × 10−4 K−2 for CsPbI3 and CsPb0.64Zn0.36I3 PNCs, respectively. At the same time, the Debye temperatures were 280 and 260 K for CsPbI3 and CsPb0.64Zn0.36I3 PNCs, respectively. The differences between two PNC samples in temperature coefficient and the Debye temperature illustrate that the incorporation of Zn affects the intrinsic properties of PNCs. Simultaneously, the exciton binding energy, EB, is obtained from normalized integrated PL of PNCs versus temperature in Figure S4b according to the following equation: I0 I(T) = −E B / kBT (2) + Be−Etrap / kBT 1 + Ae D

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Figure 4. (a) Device energy-level diagrams for each functional layer in the LEDs. (b) PL and EL spectra for CsPbI3 and alloyed CsPb0.64Zn0.36I3 PNCs in the films and LEDs. The inset shows the corresponding CIE coordinates for the EL spectra. (c) J−V−L curves for LEDs based on CsPbI3 and alloyed CsPb0.64Zn0.36I3 PNCs; the photos in the insets show light from the respective working devices. (d) EQEs of these LEDs.

Figure S8. When the alloy ratio, x, increased from 0 to 0.12, 0.27, 0.36 and 0.49, the t′ increased from 0.807 to 0.820, 0.837, 0.847, and 0.863, which indicates that the stability of the alloyed CsPb1−xZnxI3 (0 ≤ x < 1) PNCs is enhanced, and alloyed CsPb1−xZnxI3 (0 ≤ x < 1) PNCs would undergo less octahedral rotation deformation. Formation of defects has been adequately suppressed, which can be further supported by the variations in the TEM images, XRD characteristic peaks, and PLQY values with time (Figures S9 and S10). Figure S11a shows the estimation of optical bandgaps from Tauc plots for the CsPbI3 and CsPb1−xZnxI3 (0 ≤ x < 1) PNC thin films. The optical bandgaps (Eg) increased with increasing x. The ultraviolet photoelectron spectra (UPS) of CsPbI3 and CsPb0.64Zn0.36I3 PNCs are given in Figure S13b. For unalloyed CsPbI3 PNCs, their Fermi level (−4.17 eV) and conduction band minimum (CBM, −3.96 eV) were close, clearly identifying their n-type behavior (Figure 4a). As for alloyed CsPb0.64Zn0.36I3 PNCs, the Fermi level shifted to −4.43 eV, while the CBM and valence band maximum (VBM) values do not have an obvious change, demonstrating the switch from ntype to progressively more nearly ambipolar nature for CsPbI3 PNCs after sufficient Zn2+ alloying. The “electron-only” and “hole-only” devices were fabricated and compared (Figure S11c,d; see the figure insets for the device layer structures). The carrier mobility of the PNC films was obtained by fitting the space-charge-limited-current region (SCLC) with Child’s law:

To quantitatively analyze the defect density in PNCs, capacitor-like devices formed by sandwiching the PNC films between indium tin oxide (ITO) and gold (Au) were fabricated, and the evolution of the space-charge-limited current for different biases was characterized (Figure S7). The sharp rise of the current dentisity−voltage (J−V) curve relates to a trap-filled limit, where all the defects are occupied by charge carriers. The defect density is calculated according to the following equation: Nt =

2εε0VTFL eL2

(4)

where ε and ε0 are the relative dielectric constant of the PNCs and the vacuum permittivity, respectively, VTFL is the trap-filled limit voltage, L is the thickness of the obtained PNC film, and e is the elementary electronic charge.53,54 By assuming that ε = 6.3,1 the defect density of CsPbI3 and CsPb0.64Zn0.36I3 PNC films were estimated to be 1.26 × 1017 and 1.75 × 1016 cm−3, respectively. This means that the defect density has been substantially decreased by almost an order of magnitude upon the incorporation of Zn. The defects might be attributed to the existence of excess Pb in CsPbI3 PNCs as a type of nonradiative center which was reported in previous studies.2,55 The increased element ratio of I to (Zn plus Pb) as seen from EDS, ICP-MS, and XPS analysis indicates that the surface of perovskite nanocrystals changed from a lead-rich state to an iodine-rich state, and in this process, non-radiative defects on the surface are suppressed. The effective Goldschmidt tolerance factor t′ (see the Supporting Information for a discussion of their modified form for alloys) for the bare and alloyed PNCs are summarized in

J= E

9εε0μV 2 8L3

(5) DOI: 10.1021/acs.nanolett.8b04339 Nano Lett. XXXX, XXX, XXX−XXX

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Nano Letters where μ, J, and V are the carrier mobility, current density, and applied voltage, respectively.53 The electron mobilities of CsPbI3 and CsPb0.64Zn0.36I3 PNC films were 1.0 × 10−3 and 6.5 × 10−4 cm2 V−1 s−1, respectively. The hole mobilities of CsPbI3 and CsPb0.64Zn0.36I3 PNC films were 2.5 × 10−4 and 3.2 × 10−4 cm2 V−1 s−1, respectively. With the incorporation of Zn, the electron mobility decreased while the hole mobility increased; that is to say that the PNCs changed from n-type to nearly ambipolar, which is complementary to the result from UPS. Furthermore, in the Zn alloy case, the two carrier mobilities were more balanced (within a factor of around 2), while there was more of a disparity (4-fold) for the Zn-free case. Consequently, the CsPbI3 and CsPb0.64Zn0.36I3 PNCs were adopted as light emitters for LED fabrication. As shown in the energy diagrams in Figure 4a, the LED structures were ITO/ ZnO/polyethylenimine (PEI)/PNCs/p-type 4,4′,4″-tris(carbazol-9-yl) triphenylamine (TCTA)/ MoO3/Au (for details, see the Supporting Information). The thicknesses of ITO, ZnO/PEI, NCs, TCTA, and MoO3/Au were 200, 60, 50, 60, and 50 nm, respectively (Figure S12). The normalized electroluminescence (EL) and PL spectra of bare CsPbI3 and CsPb0.64Zn0.36I3 PNCs in film-device forms are shown in Figure 4b; they illustrate that the EL is indeed from the PNCs without any noticeable contribution from other charge transport materials and that there is no significant change in the spectrum under the two different excitation regimes.56,57 Symmetric emissions of LEDs based on CsPbI3 and CsPb0.64Zn0.36I3 PNCs correspond to Commission Internationale de l’Eclairage (CIE) 1931 color coordinates of (0.72, 0.28) and (0.70, 0.28), respectively (inset of Figure 4b). The current density−voltage-light intensity (J−V−L) curves of LEDs with 9 mm2 emitting areas are shown in Figure 4c. The turn-on voltage of LEDs with CsPb0.64Zn0.36I3 PNCs is 2.0 V, slightly lower than that of pure CsPbI3 PNC-based LEDs (2.1 V), indicating that highly efficient, reduced-barrier-charge injection into the PNC emitters is achieved.58 Simultaneously, the LEDs with alloyed CsPb0.64Zn0.36I3 PNCs have much stronger luminance than the unalloyed PNC ones in the whole voltage range and have a maximum luminance of 2202 cd m−2 with a voltage and current density of 6.7 V and 445 mA cm−2, respectively. In addition, the peak EQE of CsPb0.64Zn0.36I3 PNC-based LEDs reached 15.1% (Figure 4d), significantly larger than that of LEDs based on CsPbI3 PNCs. The increase of EQE indicates that the radiative efficiency has been improved mostly by suppressing the effective non-radiative decay rate and at the same time increasing the radiative decay rate of CsPb0.64Zn0.36I3 PNCs, which has also been confirmed by the above results (Figures S7 and 3). In conclusion, alloying Zn2+ into CsPbI3 PNCs reduced the non-radiative decay rates by suppressing the defects in CsPbI3 PNCs and increased the radiative decay rates by enhancing the exciton binding energy of the PNCs. Furthermore, the alloying improved the stability by lattice contraction, which not only improved the PL QY but also maintained the α phase of the PNCs for 70 days in the open air at the same time. A further consequence of the alloying, the switch from n-type to nearly ambipolar behavior in the PNC layer and the elimination of the hole injection barrier in LEDs, led to the best-performing device achieving a high luminance of 2202 cd m−2, equating to a 4-fold increase versus the zinc-free material, a low turn-on voltage of 2.0 V, and a peak EQE of 15.1%. All of these factors

combined together point out on the excellent prospects for Znalloyed PNCs in LED applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.8b04339.

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Additional details on the experimental methods and materials; figures showing log-normal distributions, variations of EDX spectra, temperature-dependent PL spectra, emission peak center energy, normalized integrated PL, TEM images, size histograms, normalized integrated PL intensity, current density−voltage characteristics, effective Goldschmidt tolerance factors, XRD patterns, Tauc plots, UPS spectra, and cross-sectional SEM analysis; tables showing XRD scattering angles and lattice constants, ICP-MS analysis, PL lifetime average lifetimes, radiative decay rates, PL QYs, and apparent non-radiative decay rates, exciton binding energy (PDF)

AUTHOR INFORMATION

Corresponding Authors

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

Yu Zhang: 0000-0003-2100-621X Stephen V. Kershaw: 0000-0003-0408-4902 Yinghui Wang: 0000-0002-4392-7995 Lijun Zhang: 0000-0002-6438-5486 Chun Sun: 0000-0003-0046-8407 Xue Bai: 0000-0003-2309-521X William W. Yu: 0000-0001-5354-6718 Andrey L. Rogach: 0000-0002-8263-8141 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (grant nos. 61675086, 61475062, 61722504, 51772123, 11674127, and 51702115), National Key Research and Development Program of China (grant no. 2017YFB0403601), Jilin Province Science Fund for Excellent Young Scholars (grant no. 20170520129JH), China Postdoctoral Science Foundation (grant no. 2017M611319), National Postdoctoral Program for Innovative Talents (grant no. BX201600060), BORSF RCS, Institutional Development Award (grant no. P20GM103424), the Research Grant Council of Hong Kong S.A.R. (GRF project CityU11337616), the Special Project of the ProvinceUniversity Co-Constructing Program of Jilin University (grant no. SXGJXX2017-3), and the Talent Introduction Plan of Overseas Top Ranking Professors by the State Administration of Foreign Expert Affairs (grant no. MSBJLG040).

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REFERENCES

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DOI: 10.1021/acs.nanolett.8b04339 Nano Lett. XXXX, XXX, XXX−XXX