Bright Perovskite Nanocrystal Films for Efficient ... - ACS Publications

Oct 19, 2016 - Department of Chemistry and Physics, Louisiana State University, Shreveport, Louisiana 71115, United States. •S Supporting Informatio...
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Bright Perovskite Nanocrystal Films for Efficient Light-Emitting Devices Xiaoyu Zhang,† Chun Sun,† Yu Zhang,*,† Hua Wu,† Changyin Ji,† Yahui Chuai,† Peng Wang,† Shanpeng Wen,† Chunfeng Zhang,‡ and William W. Yu*,†,§ †

State Key Laboratory on Integrated Optoelectronics and College of Electronic Science and Engineering, Jilin University, Changchun 130012, China ‡ National Laboratory of Solid State Microstructures, School of Physics, Nanjing University, Nanjing 210093, China § Department of Chemistry and Physics, Louisiana State University, Shreveport, Louisiana 71115, United States S Supporting Information *

ABSTRACT: The high photoluminescence efficiency, high color purity, and easy tunable bandgap make inorganic perovskite nanocrystals very attractive in luminescent display applications. Here, we report a color-saturated, red lightemitting diode (LED) using an inverted organic/inorganic hybrid structure and perovskite nanocrystals. We demonstrated that through a simple post treatment to the perovskite nanocrystals with polyethylenimine, the surface defects of the perovskite nanocrystals could be well passivated, leading to great enhancements on their absolute photoluminescence quantum yield and photoluminescence lifetime. Through using a well-passivated perovskite nanocrystal film and optimizing the charge balance, we achieved an electroluminescence LED with a current efficiency of 3.4 cd A−1, corresponding to an external quantum efficiency (EQE) of 6.3%, which is the highest value reported among perovskite NC LEDs so far.

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comparison, other devices using similar perovskite NC emitters without additional treatments only showed low efficiencies around 1%.22−25 Red LEDs are key optoelectronic components that can enable wide-gamut full-color displays. However, no fabrication of efficient red LEDs using perovskite emitters has been reported so far. Thus, we adopted the red-emitting CsPb(Br/ I)3 NCs as emitters in order to fabricate efficient red LED devices. Inspired by the success in triple-cation (inorganic Cs+/ methylammonium, MA/formamidinium, FA) perovskite based solar cells,26 we investigated the effect of cation mixtures on LED performance. This was enabled by a simple post-treatment to the synthesized inorganic perovskite NCs using a polymer (polyethylenimine, PEI) containing MA groups.27 With the introduction of this polymer, we found that the orientation of CsPb(Br/I)3 NC film was changed, together with enhancements on air-stability, absolute PL QY, and PL lifetime of the NC emitters. These favorable changes make PEI modified CsPb(Br/I)3 NC films suitable for electroluminescence LED applications. PEI has been used to produce low work function electrodes28 in broad application fields including solar cells,29 LEDs,30 photodetectors,31 and field effect transistors.32 In our CsPb(Br/

ulk halide perovskites are crystalline semiconductors with exceptional optoelectronic properties, such as low trap densities, long carrier diffusion lengths, high and balanced charge-carrier mobilities.1−3 Successes have been achieved for employing this class of materials in photovoltaic cells, lasers, photodetectors, and light-emitting diodes (LEDs).4−7 Very recently, through preventing the formation of metallic lead atoms and confining the exciton in uniform MAPbBr3 nanograins (average diameter of 99.7 nm), Lee and co-workers boosted the current efficiency (CE) and external quantum efficiency (EQE) of green perovskite LEDs to 42.9 cd A−1 and 8.53%, respectively.8 This optoelectronic performance benefited a lot from the reduced nanograin size; hence, perovskite nanocrystals (NCs) with size around 10 nm and photoluminescence quantum yield (PL QY) up to 90% may have the potential for efficient light-emitting applications,9−11 which will improve radiative recombination by confining the excitons in the NCs.12−14 The luminescence efficiency of the quantum dot (QD) emissive layer is another key parameter for achieving efficient LEDs. However, the PL QY values of QD films are generally low, which is the result of self-quenching.15,16 Strategies to overcome self-quenching in QD films typically include the growth of a protective shell,17 capping with insulating organic ligands18 and incorporation into a polymer matrix.19,20 Tan and co-workers applied a cross-linking method to a CsPbI3 NC film and raised its PL QY to 85%, which enabled a highly efficient perovskite NC LED with a remarkable EQE of 5.7%;21 in © 2016 American Chemical Society

Received: September 11, 2016 Accepted: October 19, 2016 Published: October 19, 2016 4602

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Figure 1. Characterization of CsPb(Br/I)3 NC films with or without PEI. (a) Comparison of absorption and photoluminescence (365 nm excitation wavelength) spectra for a pure CsPb(Br/I)3 NC film and the same one with 0.4% PEI, with inset showing the photograph of NC films with (right) and without (left) PEI. (b) Evolution of PL decay curves for CsPb(Br/I)3 NC films as the PEI mass fraction increases. (c) SEM image showing the top view of the CsPb(Br/I)3 NC film on glass substrate, and the same film but modified with 0.4% PEI is given in (d). All white scale bars represent 100 nm. (e) Evolution of XRD patterns for CsPb(Br/I)3 NC films as the PEI content increases. (f) XRD patterns of a CsPb(Br/I)3 NC film deposited on top of 0.4% PEI.

from the same issues mentioned above; the PL QY value of a pure CsPb(Br/I)3 NC film on glass was only around 15% in air. By embedding small amounts of Cs in a MAPbI3 structure or FA perovskite, either the stability or optoelectronic property can be enhanced.34−36 Thus, we suppose that it may also work if a trace amount of MA were introduced to Cs perovskite; fortunately, this hypothesis was borne out: both the air-stability and PL QY of CsPb(Br/I)3 NC emissive layer were improved through simply depositing the NC film upon a polyethylenimine (PEI, containing MA groups, Figure S1) film. Normalized UV−vis absorption and PL emission spectra of CsPb(Br/I)3 NC films with and without PEI are given in Figure 1a, together with a photograph of the two NC films emitting bright red light (365 nm excitation wavelength) with high color purity (the full-width at half-maximum, fwhm, was 33 nm) as an inset. Figure S2 presents typical transmission electron microscopy (TEM) images of CsPb(Br/I)3 NCs, showing the presence of monodisperse cubicshape NCs with an edge length of 12−13 nm. No obvious changes were observed from PL spectra of the two samples; however, a big difference between

I)3 NC LEDs, when incorporating PEI between the electrontransporting layer (ETL) and the emissive layer, it facilitated the electron injection and prevented the charging of NCs. Through employing proper charge-injection layers (CILs) to alter the band alignments, we realized a balanced charge carrier injection into the emissive layer, which was proved by the low efficiency roll-off; through optimizing the thickness of chargetransport layers (CTLs), we achieved charge-transport balance, which was proved by the high EQE and pure electroluminescence (EL) spectra of the LED. As a result, our CsPb(Br/I)3 NC LED gave a low turn-on voltage at 1.9 V, a maximum luminance of 2015 cd m−2, a current efficiency (CE) of 3.4 cd A−1, and an EQE up to 6.3%, making this device the best-performing perovskite NCs based LED so far. Previous studies indicate that charge recombination centers in perovskites are almost exclusively localized on the surfaces of the crystals rather than in the bulk,33 and the easily formed lead (Pb) atoms in perovskites can cause strong exciton quenching by increasing the nonradiative decay rate and decreasing the radiative decay rate.8 The inorganic perovskite NCs also suffer 4603

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The Journal of Physical Chemistry Letters Table 1. Fitted PL Lifetimes of Different CsPb(Br/I)3 NC Films mass fraction of PEI (%)

τ1 (ns)

f1 (%)

τ2 (ns)

f 2 (%)

τ3 (ns)

f 3 (%)

χ2

τavg (ns)

QY (%)

0 0.04 0.1 0.2 0.4

2.6 4.1 2.9 5.2 7.4

23.28 21.38 14.59 6.87 2.75

21.2 15.4 19.5 29.0 34.3

50.26 48.83 40.29 32.02 19.00

66.9 73.0 72.4 133.3 178.8

26.47 29.79 45.12 61.11 78.25

1.31 1.36 1.17 1.04 1.07

26.2 29.0 50.8 91.1 147.5

15 20 30 50 70

Figure 2. Structure and energy levels of the CsPb(Br/I)3 NC LED. (a) Device structure (right) and cross-sectional SEM image (left) of a CsPb(Br/ I)3 NC LED. (b) UPS spectra of ZnO NCs and ZnO/PEI film deposited on ITO glass substrate. (c) The corresponding energy level diagram. (d) Overall energy band diagram of the LED structure. The ZnO and perovskite NC energy bands were determined from UPS and optical absorption measurements; others were taken from ref 46. The red dash line represents for energy levels of the ZnO NCs.

PL lifetimes and PL QYs indicate an overall reduction in the rate of nonradiative recombination within the NC films and are evidence that the pathways to nonradiative recombination at the trap sites have been substantially reduced.37 A remarkable high PL QY of ∼95% has been achieved for the CsPb(Br/I)3 NC film using 2.0% PEI, which is the highest value among noncore−shell structured NC films (Figure S4). Previous study on CsPbI3 NC films has shown that a 0.3 nm increase in the NC particle distance may lead to a smaller degree of PL quenching by Förster resonant energy transfer (FRET), and caused a 3-fold increasing in PL QY values.21 In an attempt to get a better understanding of the PEI-induced PL enhancement of CsPb(Br/I)3 NC films here, the morphology evolution of the CsPb(Br/I)3 NC films was manifested by scanning electron microscope (SEM) images. As presented in Figure 1c, NCs connected to other forming large sized islands (aggregated) for the CsPb(Br/I)3 NC only film, exhibiting a poor coverage above the substrate. However, for the 0.4% PEIsupported film (Figure 1d), a smooth and pinhole-free surface was observed, indicating a high-quality thin film. In brief, SEM images reveal that the crystal distance is larger for pure CsPb(Br/I)3 NC films, which lead to less effective FRET to

their PL QY values was obvious. The absolute PL QYs of the films were determined using a fluorescence spectrometer with an integrated sphere excited by a 405 nm wavelength light in air. With the support of PEI (dissolved in 2-methoxyethanol with a mass fraction of 0.4% in this case), the NC film’s PL QY increased from 15% to 70%. The PL QY of pure NC film (without PEI) dropped to 7% in less than 1 h, the one with PEI dropped to 35% over 24 h in air (Figure S3). In order to gain a better understanding of the increase in PL QY, we studied the recombination processes within the NC layer of the samples using time-resolved PL spectroscopy. Figure 1b presents PL decay curves of CsPb(Br/I)3 NC films deposited on PEI films that were prepared from solutions of a series of polymer mass fractions. The decay fitting results are listed in Table 1, together with PL QY values of the corresponding NC films. The data reveal an obvious correlation between the average PL lifetimes of the CsPb(Br/I)3 NC films and the PEI mass fractions. The average PL lifetime increased from 26.2 to 147.5 ns when the mass fraction of PEI in solution changed from 0 to 0.4% (weight percentage of PEI in 2methoxyethanol), whereas the PL QY values of the NC films increased from 15 to 70% at the same time. Both the increased 4604

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Figure 3. Device performance. (a) Current density and brightness versus driving voltage. (b) External quantum efficiency and current efficiency versus luminance. (c) Device electroluminescence (at a voltage of 3 V) and photoluminescence spectra. (d) Corresponding CIE coordinates for the electroluminescence spectrum.

NC’s VBM and CBM values of −5.97 and −4.08 eV, respectively. A schematic diagram and a cross-sectional SEM image of the inverted device are shown in Figure 2a, with a multilayer structure consisting of patterned ITO (cathode), ZnO NC film (electron transport layer, ETL, 50 nm), CsPb(Br/I)3 NCs (emitting layer, 60 nm), 4,4′-bis(carbazole-9-yl)biphenyl and 4,4′,4″-tris(carbazol-9-yl)triphenylamine organic molecules (hole transport layer, HTL, 50 nm), and MoOx/Au (anode). Because of the excellent optical and electrical properties, ZnO NCs have shown great potentials in LED applications as ETL and hole blocking layer (HBL) materials39−42 and supported the best performed QLEDs up to now.43 However, this stable metal oxide is limited by a fixed electron injection level.44 Here, this limitation was overcome with two modifications: we employed smaller ZnO NCs (around 2 nm, Figure S6) with higher CBM values, and further reduced the CBM after PEI was deposited. Normally, physisorption of PEI will turn the modified conductors into efficient electron-selective electrodes.45 Figure 2b displays the results of UPS spectra taken from ZnO film, and 0.4% PEI-coated ZnO film on top of ITO. When the 3.66 eV optical energy bandgap of ZnO film is considered (Figure S6), we were able to calculate the CBM and VBM values. As summarized in Figure 2c, a reduction of 0.44 eV in the CBM was achieved after the polymer modification, which will provide an efficient electron injection within the device. Either for PL or electroluminescence (EL), charging was known as a common source of luminescence quenching.43 Because the CBM value of ZnO (−3.96 eV) is higher than that of CsPb(Br/I)3 NCs (−4.08 eV), for perovskite NC emitters in a direct contact with the ZnO film, a spontaneous charge transfer occurs and leaves the positively charged CsPb(Br/I)3 NCs with lower emission efficiency. As shown in Figure S7, CsPb(Br/I)3 NCs from the samples with a 0.4% PEI layer have

nonradiative sites thus higher PL QYs. However, the more compact NC film using PEI has shown brighter PL, implying that PEI plays a special role in PL enhancement. X-ray diffraction (XRD) was used to determine whether the PEI-induced elimination of charge recombination centers will change the crystal structures of the CsPb(Br/I)3 NCs. Figure 1e presents XRD patterns of the CsPb(Br/I)3 NC film and CsPb(Br/I)3 NC/PEI film on quartz substrates, whereas Figure 1f shows the detailed XRD pattern of a CsPb(Br/I)3 NC film deposited on 0.4% PEI substrate, exhibiting peaks at 14.57°, 20.73°, 25.29°, 29.40°, 32.90°, 36.18°, 41.80°, and 44.47° that can be assigned to (100), (110), (111), (200), (210), (211), (220), and (300) planes, respectively. All these peaks can be found in XRD patterns of every NC film, and no peak shift has been observed between different samples, indicating that the crystal structure of CsPb(Br/I)3 NCs remains unchanged with PEI passivation. Note that CsPb(Br/I)3 NCs are assembled with preferential orientation of (100) and (200) planes, but this preferential orientation disappears gradually as the PEI mass fraction increases. We suspect this phenomenon to be originated from the basic N atom with lone pair electrons in alkylamine groups interacting with the PbI6-octahedra, which is similar to a previously reported study.38 Highly luminescent CsPb(Br/I)3 NC films have been successfully obtained; we then used these bright films in light-emitting device (LED) as the emissive layer. We adopted an inverted LED design in which the emitting layer was sandwiched between the hybrid inorganic/organic chargetransport layers (CTLs). Ultraviolet photoelectron spectroscopy (UPS) measurements (Figure S5a) were developed to CsPb(Br/I)3 NC films in order to map the NC’s valence band maximum (VBM) and conduction band minimum (CBM). The Tauc plot of a CsPb(Br/I)3 NC film on quartz substrate (Figure S5b) showed a bandgap of 1.89 eV. Thus, we got the 4605

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Figure 4. Characterization of RGB perovskite NC LEDs. (a) UPS spectra of CsPbCl3, CsPbBr3, and CsPbI3 NC films deposited on ITO substrate. (b) Energy band diagram of CsPbCl3, CsPbBr3, and CsPbI3 NC films, with their Tauc plots as inset. (c), (d), and (e) represent for the current− voltage−luminance curves of blue-, green-, and red-emitting devices, respectively. (f) External quantum efficiency and current efficiency versus current density of the RGB LEDs. (g) Electroluminescence spectra of three color emitters.

much slower PL decay (46.8 ns) compared with those without PEI (20.4 ns), indicating that PEI helps to maintain charge neutrality of CsPb(Br/I)3 NC emitters and preserve their superior emissive properties. In order to obtain well performed LEDs, both charge transport and injection have to be balanced. Injection asymmetry can easily cause charging of the NCs even without a bias, which will harm the LED efficiency and lead to the efficiency roll-off.40 The inverted device architecture takes advantage that it enables the systematic engineering of HTLs with conventional organic materials achieving balanced charge transport and injection, which is the reason that the majority of well performed QLEDs rely on this type of structure.46 Because the electron injection layer (EIL) has suppressed charging of the NC film successfully, we must find a hole injection layer (HIL) that can also help to avoid NCs getting charged. Figure 2d shows the flat-band energy level diagram of each layer used

in our CsPb(Br/I)3 NC LEDs. The CsPb(Br/I)3 NCs have a VBM of −5.97 eV, and that of the chosen HIL material nearby is −6.0 eV. The organic material CBP, in short for 4,4′bis(carbazole-9-yl)biphenyl, was used as the HIL material here. CBP has a high CBM value, which can also help in blocking electrons.46 Benefitted from the little difference in VBM between the emitter and the HIL, the undesired spontaneous hole injection is inhibited, which can be proven by the PL decay curves. As exhibited in Figure S7b, the NC PL lifetime was slightly increased from 24.9 to 26.5 ns when CBP was introduced, indicating suppressed NC charging. At last, 4,4′,4″tris(carbazol-9-yl)triphenylamine (TCTA) was adopted for fabricating the HTL, in consideration of its proper VBM value, which will facilitate the hole transport from the anode to the CBP layer (a sizable energetic barrier existed between them). Atomic force microscope (AFM) images of each layer within the device are given in Figure S8, showing that the root 4606

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The Journal of Physical Chemistry Letters Table 2. Characteristics of Perovskite NC LEDsa λmax (nm)

mass fraction of PEI

fwhm (nm)

Von (V)

Lmax (cd m−2)

648 648 648 404 516 688

0.04% 0.2% 0.4% 0.2% 0.2% 0.2%

33 33 33 14 20 36

1.9 1.9 1.9 3.4 2.4 1.9

2450 2216 2100 11 3019 435

ηEQE (%) Peak, at 100 mA cm−2 3.08, 6.30, 4.22, 0.61, 0.40, 7.25,

2.34 2.83 2.62 0.54 0.40 4.23

ηP (lm W−1) Peak, at 100 mA cm−2

ηA (cd A−1) Peak, at 100 mA cm−2

1.22, 0.71 4.05, 0.83 2.66, 0.85 0.0041, 0.0031 1.01, 0.85 0.78, 0.29

1.67, 1.27 3.42, 1.54 2.29, 1.41 0.0049, 0.0043 1.32, 1.31 0.49, 0.29

a λmax = electroluminescence emission peak wavelength, fwhm = full width at half-maximum, Von = turn-on voltage, Lmax = maximum luminescence, ηEQE = external quantum efficiency, ηP = power efficiency, and ηA = current efficiency.

color coordinates of (0.71, 0.29) (Figure 3d), which are close to the spectral locus and represent color-saturated red emission. Figure S10 shows normalized EL spectra of a device at different applied voltages within a current−voltage characterization process. All EL spectra are located at the same wavelength of 648 nm, and the bandwidths of the EL spectrum broadens from 33 to 40 nm along with the increase of the applied voltage from 3 to 7 V. But when the characterization process duration becomes longer, such as 1 min, the device emission red-shifts gradually, and returns toward the original state after resting (Figure S11). This phenomenon is similar to previous observations in both LEDs21 and light-emitting electrochemical cells (LECs),47 which was suggested to be originated from phase segregation into purer halide phases.48 In the study of perovskite LECs, the EL emission peak shift reduced from 33 to 3 nm through lowering the interface charge and suppressing the halide ions mobility. Thus, we believe the strategy that helps to suppress the ions mobility such as assembling perovskite NCs into wide-bandgap conductive polymers may be a solution to solve this problem, and studies on reducing hysteresis of perovskite solar cells may also be helpful.49 Using this optimized device structure, we were able to fabricate red, green, and blue (RGB) LEDs by adopting different perovskite NC emitters and 0.2% PEI. The absorbance and PL spectra of CsPbCl3, CsPbBr3, and CsPbI3 NC films employed here are given in Figure S12. As shown in Figure 4a, UPS measurement was used to determine the CBM and VBM values of the three NC samples, and the characterization results are summarized in Figure 4b. Figure 4 also illustrates the performances of NC LEDs using different perovskite emitters in terms of current−voltage−luminance, EQE, current efficiency, and EL spectra (please see Figure 4c,d,e,f,g), and the detailed performance parameters are summarized in Table 2. The luminance reached 435, 3019, and 11 cd m−2 at the maximum in red-, green-, and blue-emitting devices, respectively. In addition, the EQEs of the RGB devices were 7.25, 0.40, and 0.61%, corresponding to 0.49, 1.32, and 0.0049 cd A−1 in current efficiencies, respectively. It is noteworthy that LEDs exhibited very low turn-on voltages of 1.9 and 2.4 V for red and green devices, corresponding to the bandgap of each NC. However, a much higher Von than the band gap energy was observed for the blue-emitting device, which also exhibited a low luminance and poor efficiency. We suspect several reasons for this inefficient blue emission, including poor solubility of the NCs (making it difficult to form a film with enough thickness and full coverage), low PL QY, and the increased hole injection energy barrier. This work demonstrates that Cs−Pb-halide perovskite NCs can exhibit efficient electroluminescence with well-designed

mean squared roughness (Rrms) values for ZnO NC, 0.4% PEI, CsPb(Br/I)3 NC, and CBP films are 1.13, 1.56, 1.98, and 1.48 nm, respectively. Figure 3a presents the voltage-dependent variations of luminance and current density for CsPb(Br/I)3 NC LEDs with different PEI contents. All devices show turn-on voltages (Von, 1.9 V) near the band gap energy of the emitter, which suggests that this device architecture results in highly efficient charge carrier injection from CTLs into NCs. The device using 0.04% PEI demonstrates the highest brightness, and reaches its luminance maximum of 2450 cd m−2 at the voltage and current density of 7.3 V and 515 mA cm−2, respectively. The external quantum efficiency (EQE) varies across the devices using PEI solution of different mass fractions. As shown in Figure 3b, EQEs of devices using 0.2% and 0.4% PEI decrease as the devices are driven to higher luminance values, while the 0.04% PEI device shows a rise of EQE as the luminescence increases, with rollover appearing only for operation at high brightness. In terms of achieving the maximum EL efficiency, the 0.2% PEI device demonstrates the best performance, reaching an EQE of 6.3% at a current density of 8 mA cm−2 with a brightness of 277 cd m−2. To our knowledge, the peak EQE of this device is the highest value for perovskite NC LEDs up to now. The high EQE value can be maintained in a wide range of current densities; 2.83% was sustained when the current density reached 100 mA cm−2. The low efficiency roll-off suggests that NCs within the emissive layer still keep low charging degree even at high current densities, and carrier loss due to the nonradiative recombination is suppressed, meaning that balance and efficient charge injection happen within the device as expected. The reproducibility of the optimized devices was very high; over 80% of the LEDs provided the brightness over 2000 cd m−2. Of interest is that changing the emissive layer thickness has a big impact on device performance.39 As shown in Figure S9, although LEDs with different emissive layer thicknesses exhibit similar current−voltage characteristics, the device performances are obviously different from each other. Decreasing film thickness of CsPb(Br/I)3 NCs from 60 to 45 nm causes higher current density and luminescence at higher bias, but leads to a remarkable drop in EQE. Figure 3c presents EL and PL spectra from the same LED using 0.4% PEI. The device displayed EL solely from CsPb(Br/ I)3 NCs without any noticeable contribution from charge transport materials, and no peak shift was observed from the PL spectrum. This indicates that CsPb(Br/I)3 NCs serve as the primary exciton recombination centers during the device operation and further suggests that the balanced charge carrier transport is achieved. The symmetric emission peak at 648 nm with a narrow full-width at half-maximum (fwhm) of 33 nm corresponds to Commission Internationale de l’Eclairage (CIE) 4607

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in 2-methoxyethanol in different mass fractions) was spincoated onto the ZnO film at a speed of 3000 rpm and annealed at 110 °C for 10 min. Perovskite NC active layers were spincast from its dispersion at 1000 to 3000 rpm. 4,4′Bis(carbazole-9-yl)biphenyl (CBP), 4,4′,4″-tris(carbazol-9-yl)triphenylamine (TCTA), MoO3, and Au were then sequentially deposited by thermal evaporation in a vacuum deposition clamber (1 × 10−7 Torr). Characterizations. Transmission electron microscopy (TEM) images were obtained on a FEI Tecnai F20 microscope. Scanning electron microscopy (SEM) images were taken with a JEOL JSM-7500F system. Atomic force microscopy (AFM) images were recorded using a VEECO DICP-II microscope. Ultraviolet photoelectron spectroscopy (UPS) spectra were collected using a PREVAC system. XRD data werre collected on a Bruker SMART-CCD diffractometer. Absorption spectra were measured on a PerkinElmer Lambda 950 spectrometer and photoluminescence (PL) spectra on a Cary Eclipse spectrofluorimeter. The PL efficiencies of films were measured using an integrating sphere with a 405 nm excitation source. Time-resolved PL measurements were performed on a timecorrelated single-photon counting (TCSPC) system under right-angle sample geometry using mini-τ miniature fluorescence lifetime spectrometer (Edinburgh Instruments). A 379 nm picosecond diode laser (EPL-375, repetition rate 5 MHz, 64.8 ps) was used to excite the samples. A Keithley 2612B sourcemeter was used to measure the current−voltage characteristics of the device. The light output was measured by a calibrated Newport 1936-R power meter with a 918D-SL0D3R silicon photodetector positioned at a fixed distance and directed toward the ITO glass side of the device. The LED brightness was determined from the fraction of light that reached the photodetector. The EL spectra were recorded with a Maya spectrometer (Ocean Optics) coupled to an optical fiber.

LED architectures. The easy processed PEI layer plays multiple and important roles in the LEDs. It helps to passivate surface defects of NC emitters, facilitates the electron injection, and prevents the NC emitters from charging. By optimizing charge injection and transport, we demonstrated EQE values of 6.30% and 7.25% for devices employing CsPb(Br/I)3 and CsPbI3 NCs, respectively, which are the best-performing perovskite NC LEDs so far.

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EXPERIMENTAL SECTION Materials. Oleic acid (OA, 90%) and 1-octadecene (ODE, 90%) were purchased from Alfa Aesar. Oleylamine (OLA, 80− 90%) was purchased from Aladdin. All other chemicals were obtained from Sigma-Aldrich. Synthesis of ZnO NCs. The ZnO NCs were synthesized using a reported procedure.50 A mixture of 0.4403 g of zinc acetate and 30.0 mL of ethyl alcohol were loaded into a 250 mL three-neck flask. After 10 min nitrogen flow, the flask was heated with stirring to make the mixture boiling. After 30 min, zinc acetate powder completely dissolved and the solution became colorless and transparent. Then, the flask was cooled to room temperature naturally and the solution turned into a white turbid solution. 0.2 g of sodium hydroxide in 10 mL of ethyl alcohol was injected into the flask swiftly and the stirring was continued for another 4 h. The product was then purified. Typically, 5 mL of ZnO NC solution was loaded into a 50 mL centrifuge tube and hexane was added to fill up the tube. After centrifugation, the precipitate was dissolved in 3 mL of ethyl alcohol. The purification was executed one more time and the finally obtained ZnO NCs were dissolved in 3 mL of ethyl alcohol and stored in a nitrogen filled glovebox. Synthesis of CsPbX3 NCs. The synthesis procedures were carried out by following a published method.9 First, cesium oleate was made. Cs2CO3 (0.80 g), OA (2.50 mL) and ODE (30.0 mL) were added into a 100 mL three-neck flask, degassed and dried under vacuum for 1 h at 120 °C. Then, the mixture was heated to 150 °C under N2 until a clear solution was obtained. For the synthesis of CsPbX3 NCs, 10.0 mL of ODE and 0.188 mmol of PbX2 (in detail, 0.115 g of PbI2 and 0.046 g of PbBr2 for CsPb(Br/I)3 NCs, 0.172 g of PbI2 for CsPbI3 NCs, 0.138 g of PbBr2 for CsPbBr3 NCs, 0.104 g of PbCl2 for CsPbCl3 NCs, respectively) were loaded into a 50 mL threeneck flask, degassed and dried by applying vacuum for 1 h at 120 °C. Dried OLA (1.0 mL) and dried OA (1.0 mL) were injected to the flask at this temperature. After the solution became clear, the temperature was raised to 180 °C and 0.80 mL of cesium oleate solution (0.10 M in ODE, preheated to 100 °C) was quickly injected. Five seconds later, the reaction mixture was cooled down to room temperature by an ice-water bath. For CsPbCl3 NCs, 150 °C was adopted and 2.0 mL of trioctylphosphine (TOP) was used. For CsPbI3 NCs, 160 °C was applied to obtain the product. The reaction mixture was separated by centrifuging for 10 min at 5000 rpm. Then the precipitate was redispersed in 2.0 mL of toluene and centrifuged again for 10 min at 12 000 rpm, and redispersed in 2.0 mL of toluene for applications. Device Fabrication. Patterned ITO-coated glass was cleaned with soap, deionized water, ethanol, chloroform, acetone, and isopropanol successively, and then was treated in UV-ozone for 15 min. A solution of ZnO NCs was spincoated onto the ITO glass at 1000 rpm for 1 min and annealed in air at 150 °C for 10 min. Then, the substrate was transferred into a glovebox. A solution of polyethylenimine (PEI) (dissolve

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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.jpclett.6b02073. Additional TEM images, AFM images, and optical characterizations of various products. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions

(X.Z. and C.S.) These authors contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (51272084, 61225018, 61475062, 61675086), the Jilin Province Key Fund (20140204079GX), NSF, BORSF RCS, and the National Postdoctoral Program for Innovative Talents (BX201600060).

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