Efficient and High-Color-Purity Light-Emitting Diodes Based on In Situ

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Efficient and High-Color-Purity Light-Emitting Diodes Based on In-Situ Grown Films of CsPbX3 (X = Br, I) Nanoplates with Controlled Thicknesses Junjie Si, Yang Liu, Zhuofei He, Hui Du, Kai Du, Dong Chen, Jing Li, Mengmeng Xu, He Tian, Haiping He, Dawei Di, Changqing Lin, Yingchun Cheng, Jianpu Wang, and Yizheng Jin ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b05191 • Publication Date (Web): 18 Oct 2017 Downloaded from http://pubs.acs.org on October 18, 2017

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Efficient and High-Color-Purity Light-Emitting Diodes Based on In-Situ Grown Films of CsPbX3 (X = Br, I) Nanoplates with Controlled Thicknesses Junjie Si,†‡ Yang Liu,†‡ Zhuofei He,§ Hui Du,† Kai Du,ǂ Dong Chen,§ Jing Li,ǁ Mengmeng Xu,⁜ He Tian,ǂ Haiping He,ǁ Dawei Di,+ Changqing Lin,⁜ Yingchun Cheng,⁜ Jianpu Wang,⁜ and Yizheng Jin*§ †

State Key Laboratory of Silicon Materials, Centre for Chemistry of

High-Performance & Novel Materials, School of Materials Science and Engineering, § Centre for Chemistry of High-Performance & Novel Materials, State Key Laboratory of Silicon Materials, Department of Chemistry, ǂ Center of Electron Microscope, State Key Laboratory of Silicon Material, School of Material Science and Engineering,

ǁ

State Key Laboratory of Silicon Materials, School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China. +Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, UK.

⁜

Key

Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Centre for Advanced Materials (SICAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing 211816, China.

KEYWORDS: perovskite, light-emitting diode, in-situ grown, nanoplate, color purity, quantum well, efficiency 1

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ABSTRACT

We report a facile solution-based approach to the in-situ growth of perovskite films consisting

of

monolayers

phenylbutylammonium

of

(PBA)

CsPbBr3 nanoplates cations,

i.e.

passivated

two-dimensional

by

bulky layered

PBA2(CsPbBr3)n-1PbBr4 perovskites. Optimizing film-formation processes leads to layered perovskites with controlled n values in the range of 12-16. The layered perovskite emitters show quantum-confined bandgap energies with a narrow distribution, suggesting the formation of thickness-controlled quantum-well (TCQW) structures. The TCQW CsPbBr3 films exhibit smooth surface features, narrow emission linewidths, low trap densities and high room-temperature photoluminance quantum yields, resulting in high-color-purity green LEDs with remarkably high external quantum efficiencies (EQEs) of up to 10.4%. The solution-based approach is extended to the preparation of TCQW CsPbI3 films for high-color-purity red perovskite LEDs with high EQEs of up to 7.3%.

2


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Light-emitting diodes (LEDs) based on solution-processed metal halide perovskite semiconductors have attracted increasing interest.1-5 In general, bulk crystals or films of three-dimensional metal halide perovskites show narrow emission linewidths and high color purity. However, they show high PL QYs only at high excitations, which is a major factor hindering their applications in high-efficiency perovskite LEDs.6-8 An effective approach to addressing this issue is by using nano-structured emitters,7-23 which spatially confine injected charge carriers to encourage radiative recombination processes. Recent advances in synthetic chemistry result in colloidal solutions of perovskite quantum dots with narrow size distributions or perovskite nanoplates with controlled thicknesses.10,13,22,24-27 These colloidal perovskite nanostructures exhibit remarkable optical properties, such as high photoluminescence quantum yields (PL QYs) and narrow emission line-widths. However, the efficiencies of PeLEDs based on colloidal quantum dots or nanoplates remain modest. A major challenge lies in developing surface ligands which fulfill the requirements of both solution-processing properties for film formation and solid-state optoelectronic properties for LED operation.12,16 An alternative approach is to in-situ grow perovskite nano-emitters from precursor solutions. A few good examples are independent studies from Yuan et al.,7 W ang et al.14 and Xiao et al.,19 which demonstrated high-efficiency LEDs based on Ruddlesden-Popper perovskites with a general formula L2(SPbX3)n-1PbX4, where L, S, X and n represent bulky cations with long-chain ammonium groups (phenylethylammonium, 1-naphthylmethylammonium or n-butylammonium cations), small methylammonium (or formamidindium) cations, halide ions (I- or Br-), and the 3


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number of PbX4 octahedral layers within a crystallite, respectively. These Ruddlesden-Popper perovskite films contain a mixed phase of nanocrystallites with different n values, which exhibit size-dependent emission due to quantum confinement effects28. In other words, the emissive films were ensembles of multiple quantum wells (MQWs) with different bandgap energies. The full widths at half maximum (FWHMs) of the emission peaks from the MQW perovskite films were wider than those from the bulk perovskite films or crystals9,29-30 or those from the colloidal perovskite nanostructures with proper size control.12,16 As a result, the high-efficiency MQW PeLEDs exhibited deteriorated color purity.

Here

we

report

a

facile

solution-based

approach

to

in-situ

grow

thickness-controlled quantum-well (TCQW) CsPbBr3 films, i.e. monolayers of CsPbBr3 nanoplates with well-controlled thicknesses in the quantum-confined regime. The TCQW CsPbBr3 films exhibit relative smooth surface features, high room-temperature PL QYs achievable at low excitation levels and narrow emission linewidths. These merits enable us to fabricate green PeLEDs with remarkably high peak external quantum efficiencies (EQEs) of up to 10.4%, and high-color-purity emission comparable to those from the state-of-the-art colloidal perovskite nanostructures. Furthermore, we demonstrate that the same strategy can be exploited to produce high-quality TCQW CsPbI3 films, which result in red PeLEDs with high EQEs of up to 7.3%.

RESULTS AND DISCUSSION 4


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Figure1a shows a cross-sectional transmission electron microscope (TEM) image of our device. The PeLEDs consist of multiple layers of, in the following order, glass substrate with an indium tin oxide (ITO) coating, nickel oxide (NiO, ~13 nm), poly (9,9-dioctyl-fluorene-co-N-(4-butylphenyl) diphenylamine) /poly (9-vinlycarbazole) (TFB/PVK,

~18

nm),

2,2’,2’’-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole)

perovskite, (TPBi,

~45

nm),

calcium (Ca, ~7 nm) and aluminum (Al, ~70 nm). The perovskite layer was deposited from a precursor solution of phenylbutylammonium bromide (PBABr), CsBr and PbBr2 with a molar ratio of 1:1.75:1.4 dissolved in dimethyl sulfoxide (DMSO), followed by annealing at 100 °C for 50 min. The procedures led to continuous and pinhole-free perovskite films with uniform thicknesses of 9.1±1.0 nm (Figure 1a and S1, Supporting Information). High-resolution TEM (HRTEM) observations on the cross-sectional sample (Figure 1b and S2, Supporting Information) illustrate that the perovskite film consists of a monolayer of nanocrystals. The nanocrystal shown in Figure 1b displays well-resolved lattice fringes with interplanar spacing of 0.59 and 0.41 nm, which correspond to the {020} and {200} planes, respectively, of orthorhombic γ-CsPbBr3 (Figure 1c).31-32 The crystal is oriented with {020} planes parallel to the substrate surface. The number of PbBr4 sheets is determined to be 12. We analyzed several other nanocrystals with well-resolved lattice fringes (Figure S2). The results show that the structure of all nanocrystals matches that of orthorhombic γ-CsPbBr3. Majority of the nanocrystals share the same orientation of the nanocrystal shown in Figure 1b, i.e. with the b-axis perpendicular to the substrate surface. The 5


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number of PbBr4 sheets of all nanocrystals is in the range of 12-16.

Figure 1. TEM analyses of the perovskite nanoplates. a) A cross-sectional TEM 6


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image of a multi-layered device. Scale bar: 20 nm. b) A HRTEM image of the cross-sectional sample. Scale bar: 5 nm. Inset is the corresponding fast Fourier transform (FFT) pattern. c) Corresponding orthorhombic crystal structure. d) A typical TEM image of the perovskite nanoplates transferred onto a copper grid. Scale bar: 20 nm. e) A typical HRTEM image of the perovskite nanoplates transferred onto a copper grid. Scale bar: 5 nm. Inset is the corresponding FFT pattern. f) A schematic showing the structure of the perovskite nanoplates.

The perovskite nanocrystals were transferred onto copper grids (See experimental section for details) to gain more morphological information. Figure 1d shows that the shapes of the perovskite nanostructures are square or rectangular with varied domain sizes of 10-50 nm. HRTEM analyses (Figure 1e and S3, Supporting Information) suggest that the structure of these nanocrystals matches that of γ-CsPbBr3. Grazing-angle Fourier transform infrared spectroscopy (FTIR) experiments suggest presence of PBA cations in the films (refer to Figure S4, Supporting Information for details). It is likely that the bulky PBA cations impeded further crystal growth of the CsPbBr3 nanoplates through surface passivation or modification. In this regard, the perovskite

nanostructures

are

considered

as

two-dimensional

layered

PBA2(CsPbBr3)n-1PbBr4 with n values in the range of 12-16, i.e. n ∈ [12, 16]. The layered Ruddlesden–Popper perovskites naturally form quantum-well structures when their thicknesses are in the quantum confinement regime.33-36 Given that Bohr radius of CsPbBr3 is ~7 nm24 and the thicknesses of the inorganic CsPbBr3 7


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nanoplates in the PBA2(CsPbBr3)n-1PbBr4 (n ∈ [12, 16]) perovskites are in the range of 7.1-9.4 nm, we expect the electronic properties of the perovskite films to be influenced by quantum confinement effects. Therefore we studied optical properties of the films of PBA2(CsPbBr3)n-1PbBr4 (n ∈ [12, 16]) perovskites. Figure 2a shows a UV-Vis absorption spectrum. The rising edge and plateau of the absorption spectrum are fitted to the sigmoidal formula to account for the effects of broadening.37 This fitting gives rise to a bandgap of ~2.43 eV. The PL spectrum shown in Figure 2b displays a peak centered at 512 nm, corresponding to an optical bandgap of 2.42 eV. The bandgaps determined by optical measurements are considerably larger than the bandgap of bulk CsPbBr3, 2.25 eV,32 suggesting the presence of quantum confinement effects.24,38 Furthermore we carried out density functional theory (DFT) calculations (see Figure S5, Supporting Information for details) for the PBA2(CsPbBr3)n-1PbBr4 perovskites. The results confirm that the bandgaps of the in-situ grown CsPbBr3 nanoplates, i.e. the PBA2(CsPbBr3)n-1PbBr4 (n ∈ [12, 16]) perovskites, are influenced by quantum confinement effects. The PL FWHM is determined to be ~82 meV, which is narrower than those of the green MQW perovskite films8,14,19-21. The narrow emission linewidths suggest that the thicknesses of the perovskite nanoplates are well-controlled within a regime where the bandgap is not very sensitive to the exact n values. To conclude, the in-situ grown films of monolayers of two-dimensional PBA2(CsPbBr3)n-1PbBr4 nanoplates (n ∈ [12, 16]) with well-controlled thicknesses show a narrow distribution of quantum-confined bandgap energies, and thereby can be described as TCQW structures. 8


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Figure 2. Optical properties of the TCQW CsPbBr3 films. a) UV-Vis absorption spectrum. b) PL spectrum. c) Excitation-intensity-dependent PL QY. Excitation wavelength: 445 nm. d) Temperature-dependent radiative lifetimes. The data were extract from Figure S6 (Supporting Information).

Figure 2c shows light-intensity-dependent PL QY of a TCQW CsPbBr3 film under a 445-nm laser excitation. The PL QYs are >50% when the excitation level is below 40 mW cm-2. The highest value is determined to be ~56%. Remarkably, the PL QY remains to be ~53% at an excitation power density as low as 0.09 mW cm-2. This feature is different from that of the bulk perovskite films or the MQW perovskite films, which show much lower PL QYs at such a low excitation power density.6-8,14 9


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These facts indicate low defect density and excellent emissive properties of the TCQW CsPbBr3 films. Furthermore, we characterized PL and PL decay of the TCQW CsPbBr3 films at low temperatures (Figure S6, Supporting Information). The temperature-dependent PL intensities indicate that the PL QYs of the perovskite film increase at low temperatures (Figure S6c). At 70 K, the PL QY is estimated to be ~80%. The extracted PL QY and PL lifetime at different temperatures were used to calculate the corresponding radiative lifetime (see Figure S6 for details). The results (Figure 2d) reveal that the radiative lifetime increases linearly with temperature. This is an indication of exciton recombination being localized in two-dimensional quantum wells,39 which is line with with the formation of TCQW structures. The morphological and structural properties of the films of two-dimensional layered PBA2(CsPbBr3)n-1PbBr4 perovskites were investigated. For the sake of consistency, all the perovskite films were deposited onto the multi-layers of NiO/TFB/PVK. Atomic force microscopy (AFM) measurements (Figure 3a and S7, Supporting Information) illustrate that the root-mean-square (RMS) surface roughness of the perovskite film and the underlying PVK layer is ~1.3 nm and ~1.0 nm, respectively. The relatively smooth surface features of the perovskite film are in agreement with the uniform film thicknesses observed in the cross-sectional samples. Figure 3b displays X-ray diffraction (XRD) pattern of a perovskite film. Two prominent broadened peaks at 15.2° and 30.4° with FWHMs of 1.8° and 1.1°, respectively, can be assigned to {020} reflection series from the CsPbBr3 nanocrystallites.31-32,40 Diffraction peaks at lower Bragg angles (2θ < 12°) are absent. 10


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This observation rules out the presence of Ruddlesden-Popper perovskites with very small n values, e.g. perovskites with n = 1, 2 or 3.41-42 This result is consistent with HRTEM observations that only layered Ruddlesden-Popper perovskites with n values in the range of 12-16 are found.

Figure 3. Morphological and structural properties of the TCQW CsPbBr3 films. a) A typical AFM image. b) XRD pattern.

The key to obtain TCQW CsPbBr3 films is to achieve exquisite control over the n values of the layered perovskites. We investigated a number of parameters associated with film processing. The ratio of PBA cations to Cs+ in the precursor solutions was identified to be critical. For example, doubling the concentration of PBABr in the precursor solution (Figure S8, Supporting Information) led to the occurrence of three blue-shifted peaks at 433, 462 and 493 nm, respectively, in the PL spectrum and a series of Bragg peaks at low angles (2θ < 12°) in the XRD pattern. These features are attributed to the formation of perovskite films with MQW structures.41-42 Halving the 11


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concentration of PBABr in the precursor solution resulted in a slightly red-shifted emission peak at 515 nm and decreased FWHMs of the XRD reflection peaks (Figure S8), suggesting formation of layered perovskites with larger n values. However, this film showed a low PL QY of ~9% at an excitation intensity of 0.35 mW/cm2. Furthermore, we found that the in-situ growth of perovskite films was highly dependent on the surface properties of the underlying layers. Figure S9 (Supporting Information) shows PL spectra, PL decay curves and PL QYs of perovskite films deposited onto various substrates (from the same precursor solution), including quartz, NiO, NiO/PVK and NiO/TFB/PVK. The film formed onto the trilayers of NiO/TFB/PVK demonstrated the highest PL QY (~55%) and the longest PL lifetime (~22 ns). The differences in the optical properties suggest that the surface properties of the underlying layers play important roles in the formation of perovskite films. In addition, directly contacting a perovskite film with NiO can cause strong luminescence quenching. The uniform thicknesses, room-temperature PL QYs of ~55% achievable at low excitation levels and narrow emission linewidth of the optimized TCQW CsPbBr3 films make them very attractive for LED applications. Figure 4a shows a schematic of the flat-band energy-level diagram of the multiple layers of our PeLEDs. The energy levels of the TCQW CsPbBr3 layer were obtained by combining the valence-band position extracted from ultraviolet photoelectron spectroscopy characterizations (Figure S10, Supporting Information) and the optical bandgap from PL measurements. Other energy-level values were taken from literature.9,43-45 The 12


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trilayer-structured hole-transporting layers of NiO/TFB/PVK were adopted due to both the optimal surface properties which facilitate the formation of high-quality TCQW CsPbBr3 films (Figure S9) and the “ladder-like” energy structures which allow efficient hole injection. The TPBi layer with an electron affinity of ~2.9 eV and an ionization potential of ~6.4 eV was used as an electron-transporting and hole-blocking layer. Figure 4b shows a typical electroluminescence (EL) spectrum of our PeLEDs. A symmetric EL peak at 514 nm with a narrow FWHM of 17 nm (82 meV), which corresponds to Commission Internationale de l’Eclairage (CIE) color coordinates of (0.07, 0.74) (Figure S11, Supporting Information), inherits the high PL color purity of the TCQW CsPbBr3 films. EL spectrum of a control device without the perovskite layer (Figure S12, Supporting Information) shows a peak at 442 nm. The absence of this peak at 442 nm in the EL spectra of our devices implies that the hole-transporting layers are not in direct contact with the electron-transporting layer. The injected carriers are effectively confined within the TCQW CsPbBr3 layer for efficient radiative

recombination.

Current

density−luminance−voltage

curves

and

EQE−luminance curve of a representative PeLED are shown in Figure 3c and 3d, respectively. The current density and luminance rose rapidly after turn-on at ~2.8 V, yielding a maximum brightness of ~14000 cd m-2 at 8 V. The peak EQE of this device was 9.7%. The peak EQE of our champion devices is 10.4% (Figure S13), which is among some of the highest values for green PeLEDs reported to date.16,19,46 The processing of the TCQW CsPbBr3 films, which involves one-step 13


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spin-coating from a precursor solution, is facile and highly controllable. This advantage contributes to excellent reproducibility of device performance. Figure 4e shows a histogram of peak EQEs for 32 devices from 5 batches, demonstrating an average value of ~9.7% with a low relative standard deviation of 3.5%. Furthermore, the color purity of our devices, as reflected by the narrow FWHM of ~82 meV, is comparable to those of the PeLEDs based on colloidal quantum dots with controlled sizes and is better than those of the PeLEDs based on MQW perovskites (Figure 4f and Table S1, Supporting Information).1-2,8-14,16-21,29-31 Our device processing can be applied to the fabrication of LEDs with an area of ~56 mm2 (inset of Figure 4b). This device demonstrated a peak EQE of ~9.0%, which was close to those of the small-area devices (3.24 mm2). This device was stored in a glovebox for 30 days. No significant decrease of device efficiency was observed (Figure S14, Supporting Information), indicating good shelf stability of our PeLEDs.

14


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Figure 4. Device characteristics of the TCQW CsPbBr3 PeLEDs. a) Flat-band energy level diagram. b) EL spectrum at an applied bias of 4 V. Inset shows a photograph of a 56-mm2 device (The logo credit to Zhejiang University). c) Current density−luminance−voltage characteristics and d) the corresponding EQE-luminance curve of a device. e) A histogram of peak EQEs for 32 devices from 5 batches. f) Summary of peak EQEs and FWHMs for our device and other representative green PeLEDs (see Table S1 for details). 15


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In order to test the versatility of TCQW structures for different perovskite materials, we deposited films based on CsPbI3 nano-emitters onto trilayered hole-transporting

layers

of

NiO/poly

(N,

N’-bis(4-butylphenyl)-N,N’-bis(phenyl)-benzidine) (poly-TPD)/PVK by the same approach. The perovskite films show relatively smooth surfaces with a RMS roughness of ~2 nm, absence of diffraction peaks at lower Bragg angles (2θ < 12°), quantum-confined PL emission at 1.83 eV with a small FWHM of 90 meV and a high PL QY of ~49% at room temperature (Figure 5 a-c). These features are consistent with TCQW structures. EL spectrum of a TCQW CsPbI3 PeLED (Figure S15, Supporting Information) shows a symmetric emission peak at 683 nm with a FWHM of 90 meV, corresponding to CIE color coordinates of (0.72, 0.28) (Figure S10). Figure 4d shows current density−luminance−voltage curves of a TCQW CsPbI3 PeLED. The peak EQE of this device reached 7.3%, which is a record for red PeLEDs.1, 11, 15, 47 A histogram for 16 devices (Figure S15, Supporting Information) shows an average peak EQE of 6.6% with a low relative standard deviation of 5.3%.

16


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Figure 5. TCQW CsPbI3 films and the corresponding PeLEDs. a) A typical AFM height image of a TCQW CsPbI3 film. b) XRD pattern of a TCQW CsPbBr3 film. c) PL spectrum of a TCQW CsPbI3 film. The PL QY was measured with a 445-nm excitation (power density: 10 mW/cm2). d) Current density−luminance−voltage curves of a TCQW CsPbI3 PeLED.

CONCLUSION

This work demonstrated exquisite control over the structure and the optoelectronic properties of perovskite quantum emitters by a facile solution-based method. The optimal TCQW CsPbX3 (X= Br, I) films with controlled thicknesses, 17


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narrow emission linewidths, low trap densities and high PL QYs led to high-color-purity and high-efficiency green and red PeLEDs, representing an important step towards display applications. Considering the versatility of solution-processed metal halide perovskites, we expect that the application of TCQW films can be extended to other efficient and high-color-purity PeLEDs with tunable emission wavelengths.

METHODS

PVK (molecular weight: 25,000–50,000 g mol−1) and PbBr2 (99.999%) were purchased from Sigma Aldrich. Phenylbutylamine (PBA, >98.0%), chlorobenzene (99.8%) and m-xylene (99+%) were purchased from Acros. CsBr (99.999%), DMSO (99.9%) and hydrobromic acid (48 wt.% in water) were purchased from Alfa-Aesar. TPBi (98%) was purchased from J&K Chemical Ltd. Poly-TPD and TFB were purchased from American Dye Source. PBABr was synthesized by adding hydrobromic acid (22.66 mmol) into a solution of phenylbutylamine (18.89 mmol) in methanol (20 mL) at 0 °C and then stirred for 2 h. Next, the solution was evaporated at 50 °C to obtain precipitates, which were washed thrice with diethyl ether and then vacuum-dried at 30 °C for 24 h. PBAI was synthesized by using a similar method except that hydrobromic acid was substituted by hydroiodic acid. The precursor solutions for the TCQW CsPbBr3 films and the TCQW CsPbI3 films were prepared by dissolving PBABr (14.2 mg), CsBr (23.1 mg) and PbBr2 (31.8 18


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mg) in DMSO (1 mL) and PBAI (19.5 mg), CsI (36.6 mg) and PbI2 (65.0 mg) in DMSO (1 mL), respectively. Regarding the fabrication of green PeLEDs, the NiO layers were deposited onto ITO coated glass substrates by following a literature method.44 The TFB layers were deposited by spin-coating polymer solutions (in m-xylene, 8 mg/mL) at 2000 rpm, annealed at 150 °C for 30 min and then spin-rinsed by chlorobenzene to produce ultrathin layers, which improved hole injection.48 After that, the PVK layers were deposited by spin-coating polymer solutions (in chlorobenzene, 8 mg/mL) at 2000 rpm, followed by annealing at 150 °C for 30 min. The TCQW CsPbBr3 films were prepared by spin coating the precursor solutions at 5000 rpm, followed by annealing at 100 °C for 50 min. Finally, the TPBi, Ca and Al layers were deposited using a thermal evaporation system (Travato C300) through a shadow mask under a base pressure of ∼2× 10−7 torr. The device areas were defined by overlapping areas of the ITO films and the top electrodes. The fabrication of red PeLEDs was similar to that of green PeLEDs, except that layers of poly-TPD (~25 nm) and PVK (~6 nm) were deposited onto the NiO layers and the perovskite films were annealed at 130 °C for 5 min. The cross-sectional samples were prepared by using a focused-ion-beam equipment (Quata 3D FEG) and then characterized by using a Tecnai G2 F20 microscope operated at 200 KeV. TEM observations on the CsPbBr3 nanoplates were conducted using a Hitachi HT7700 microscope operated at 80 KeV or a Tecnai G2 F20 microscope operated at 200 KeV. The samples were prepared by dipping a 19


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TCQW CsPbBr3 film (deposited onto the multilayers of NiO/TFB/PVK) in chlorobenzene, which dissolved the underlying organic layers. Then the perovskite nanocrystals were transferred by drop-casting the chlorobenzene suspension onto carbon coated copper grids (300 mesh, Ted Pella, Inc., 01824G). We note that all TEM experiments should be conducted at low-dose conditions to minimize beam damage to the perovskite samples. Grazing-angle FTIR spectra were obtained by using a Thermo Fisher IS50 equipped with a Smart SAGA (specular apertured grazing angle) reflectance accessory spectrophotometer and a liquid nitrogen cooled HgCdTe (MCT) Detector. The perovskite film was spin-coated onto a gold substrate because of its total reflection in mid-infrared region. AFM measurements were conducted on a Cypher-S atomic force microscope located in a nitrogen-filled glove box. XRD measurements were performed on an X'Pert PRO system operated at 40 keV and 40 mA with Cu Kɑ radiation (λ = 1.5406 Å). The calculations of bandgaps were performed by using DFT implemented in Quantum ESPRESSO package.49 Generalized gradient approximation (GGA) of the exchange-correlation

functional

with

ultrasoft

pseudopotentials

energy

in

Perdew-Burke-Ernzerhof (PBE) parameterization was employed.50-51 To preventing the slab-slab interaction, a 20 Å thick vacuum slab was used. The kinetic energy cutoff was set to be 340 eV and 2×2×1 Monkhost-Pack mesh was used for Brillouin zone sampling. During optimization, the Pb and Br atoms were fixed, and other atoms 20


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were relaxed until the force on each of them was less than 0.5 eV/Å. The electronic self-consistent field (SCF) step was converged when the total energy is less than 1.4×10-5 eV. Steady-state and time-resolved PL spectra were obtained by using an Edinburgh Instruments (FLS920) spectrometer. A pulsed laser diode with a wavelength of 404.2 nm and a pulse width of 58.6 ps was used as the excitation light source. The excitation fluence was ~4 nJ cm-2. PL transients were measured by the standard time-correlated single-photon counting technique. The temperature-dependent optical measurements were performed in a closed-cycle helium cryostat. PL QYs of the perovskite films were obtained by a three-step method. 52 The setup combined a Xenon lamp, optical fiber, a QE65000 spectrometer (Ocean Optics) and a home-designed integrating sphere. UPS spectra were collected on a Thermo ESCALAB-250Xi spectrometer with an applied bias of −10 V. A He I ultraviolet radiation source (21.2 eV) was used. The overall resolution of the instrument is 0.1 eV. All PeLED devices were characterized at room temperature in a nitrogen-filled glovebox. A Keithley 2400 source meter and an integration sphere (FOIS-1) coupled with a QE-Pro spectrometer (Ocean Optics) were used for the measurements.53 The devices were swept from zero bias to forward bias.

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ASSOCIATED CONTENT

The authors declare no competing financial interest.

Supporting Information The supporting Information is available free of charge on the ACS publications website at DOI: XX.XXXX/XXXXX Additional XRD patterns, HRTEM images, AFM images, UPS spectrum, further optical characterization and EQE −Luminance measurements (PDF)

AUTHOR INFORMATION

Corresponding Author

*

E-mail: [email protected]

Author Contributions ‡

These authors contributed equally to this work.

ACKNOWLEDGMENT This work was financially supported by the National Key Research and Development Program of China (2016YFB0401602), the National Natural Science Foundation of China (51522209, 91433204, 11474164 and 61634001), the National Basic Research Program of China-Fundamental Studies of Perovskite Solar Cells (2015CB932200), 22


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the Natural Science Foundation of Jiangsu Province (BK20150043), the Joint Research Program between China and European Union (2016YFE0112000) and the Fundamental Research Funds for the Central Universities (2017XZZX001-03A).

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Our work demonstrates in-situ growth of thickness-controlled quantum-well (TCQW) CsPbX3 (X= Br, I) films, i.e. monolayers of perovskite nanoplates with well-controlled thicknesses in the quantum confinement regime. The TCQW films show smooth surfaces, narrow emission linewidths, low trap densities and high room-temperature PL QYs, leading to high-color-purity green and red perovskite LEDs with peak EQEs of 10.4% and 7.3%, respectively.

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