Ultra-bright and highly efficient inorganic based perovskite light-emitting diodes

Abstract

Inorganic perovskites such as CsPbX₃ (X=Cl, Br, I) have attracted attention due to their excellent thermal stability and high photoluminescence quantum efficiency. However, the electroluminescence quantum efficiency of their light-emitting diodes was <1%. We posited that this low efficiency was a result of high leakage current caused by poor perovskite morphology, high non-radiative recombination at interfaces and perovskite grain boundaries, and also charge injection imbalance. Here, we incorporated a small amount of methylammonium organic cation into the CsPbBr₃ lattice and by depositing a hydrophilic and insulating polyvinyl pyrrolidine polymer atop the ZnO electron-injection layer to overcome these issues. As a result, we obtained light-emitting diodes exhibiting a high brightness of 91,000 cd m^-2 and a high external quantum efficiency of 10.4% using a mixed-cation perovskite Cs_0.87 MA_0.13 PbBr₃ as the emitting layer. To the best of our knowledge, this is the brightest and most-efficient green perovskite light-emitting diodes reported to date.

ARTICLE

Received 13 Sep 2016 | Accepted 15 Apr 2017 | Published 7 Jun 2017

Liuqi Zhang1, Xiaolei Yang1, Qi Jiang1, Pengyang Wang1, Zhigang Yin1,2, Xingwang Zhang1,2, Hairen Tan3, Yang (Michael) Yang4, Mingyang Wei3, Brandon R. Sutherland3, Edward H. Sargent3 & Jingbi You1,2

Inorganic perovskites such as CsPbX3 (X Cl, Br, I) have attracted attention due to their

excellent thermal stability and high photoluminescence quantum efciency. However, the electroluminescence quantum efciency of their light-emitting diodes was o1%. We posited that this low efciency was a result of high leakage current caused by poor perovskite morphology, high non-radiative recombination at interfaces and perovskite grain boundaries, and also charge injection imbalance. Here, we incorporated a small amount of methylammonium organic cation into the CsPbBr3 lattice and by depositing a hydrophilic and insulating polyvinyl pyrrolidine polymer atop the ZnO electron-injection layer to overcome these issues. As a result, we obtained light-emitting diodes exhibiting a high brightness of 91,000 cd m 2 and a high external quantum efciency of 10.4% using a mixed-cation perovskite Cs0.87MA0.13PbBr3 as the emitting layer. To the best of our knowledge, this is the brightest and most-efcient green perovskite light-emitting diodes reported to date.

DOI: 10.1038/ncomms15640 OPEN

Ultra-bright and highly efcient inorganic based perovskite light-emitting diodes

1 Key Laboratory of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China. 2 College of Materials Science and Opto-electronic Technology, University of Chinese Academy of Sciences, Beijing 100049, China. 3 Department of Electrical and Computer Engineering, University of Toronto, 35 St George Street, Toronto, Ontario, Canada M5S 1A4. 4 College of Optical Science and Engineering, Zhejiang University, Hangzhou 310027, China. Correspondence and requests for materials should be addressed to X.Z.(email: mailto:[email protected]

Web End [email protected] ) or toJ.Y. (email: mailto:[email protected]

Web End [email protected] ).

NATURE COMMUNICATIONS | 8:15640 | DOI: 10.1038/ncomms15640 | http://www.nature.com/naturecommunications

Web End =www.nature.com/naturecommunications 1

ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms15640

Organicinorganic perovskites have received extensive attention in recent years in view of their attractive electrical and optical properties. Solution-processed

perovskite solar cells have demonstrated a high certied power conversion efciency of 22.1%, which is comparable to photovoltaics made from traditional inorganic semiconductor materials such as Si, CIGS and CdTe (refs 18). In addition, they have been utilized as efcient low-threshold gain media in optically pumped lasers9,10. Perovskite materials exhibit high photoluminescence quantum yield (PLQY, 490% in solution for nanocrystals) and high colour purity with narrow emission linewidths o20 nm (refs 1115). These features make them promising candidates as new materials for light-emitting diodes (LEDs).

Electroluminescence (EL) from trihalide organicinorganic perovskite-based LEDs (PeLEDs) was rst reported in 2014 (ref. 16). The peak brightness of these LEDs was of order 300 cd m 2 at green wavelengths, and the external quantum efciency (EQE) from CH3NH3PbI3 xClx (754 nm emission, red)

and CH3NH3PbBr3- (517 nm emission, green) based LEDs were0.76 and 0.1%, respectively16. By interface engineering and perovskite layer optimization, the EQE was increased to over 3% (refs 1727). A breakthrough in organicinorganic PeLEDs was achieved by controlling the crystallization process of CH3NH3PbBr3 by adopting a nanocrystal pinning method. As a result, a dense lm with small crystal domains (o100 nm) was obtained, effectively conning charge carriers. These devices demonstrated an impressive EQE of 8.53% (ref. 28). More recently, by conning electrons and holes to two-dimensional (2D) perovskites29, Yuan et al. and Wang et al. obtained near-infrared EQEs of 8.8 and 11.7%, respectively30,31. Combined with nanocrystal pinning and 2D perovskites, Rand et al. also achieved close to 10% EQE of organicinorganic LEDs32.

Compared with monovalent organic cation-based lead-halide perovskites, all inorganic perovskites exhibit improved thermal stability and more efcient PL, making them attractive in a number of optoelectronic device applications14,15,33,34. Inorganic perovskites such as CsPbX3 (X Cl, Br, I) have attracted great

attention due to their improved thermal stability and higher PLQY in comparison with organic-cation perovskites. Recently, CsPbX3 nanocrystals have been successfully synthesized and used as emitting materials for LEDs14,15. CsPbX3 thin lm-based LEDs have also been demonstrated34. However, the EQE of the LEDs based on these materials remain o1% (refs 15,34). During revising of this manuscript, Li et al. and Ling et al. both reported inorganic CsPbBr3 LEDs with about 6% EQE by surface passivation of perovskite nanocrystals and controlling perovskite thin lm morphology, respectively35,36. There is still much more room for improvement in brightness and efciency. The low efciency may arise from high leakage current due to poor morphology (high density of pinholes), signicant nonradiative recombination at the interface of perovskite/injection layers and within the perovskite layer itself and charge injection imbalance15,34.

In this report, we achieved high-quality dense CsPbBr3 perovskite thin lms by incorporating a small amount of organic methylammonium cation into the lattice and by using a hydrophilic insulating polymer interface layer on top of the ZnO electron-injecting electrode. We fabricated high performance mixed-cation perovskite LEDs with an active layer composition of Cs0.87MA0.13PbBr3. These LEDs exhibited a peak

brightness of 91,000 cd m 2 and a peak EQE of 10.43%. This represents the brightest and most-efcient green perovskite LEDs reported to date28,30,31.

ResultsMorphology of CsPbBr3 lms. CsPbBr3 perovskite thin lms were fabricated by spin-coating a CsBr:PbBr2 precursor from dimethyl sulfoxide (DMSO) onto substrates, followed by annealing at 100 C for 20 min to remove residual solvent and to induce perovskite crystallization. A high ratio of CsBr:PbBr2(2.2:1) precursor solution was used to guarantee the formation of pure phase CsPbBr3, while the excess CsBr would be readily precipitated during solution stirring33. As shown in Fig. 1a, the perovskite lm directly deposited onto the electron-injection layer of ZnO exhibited a high density of pinholes. To improve the surface morphology, we rst introduced a thin hydrophilic insulating polymer, polyvinyl pyrrolidine (PVP), between the ZnO and perovskite layers. The density of pinholes was largely reduced by inserting the PVP intermediate layer, as shown in Fig. 1b. Real-time contact angle results showed that PVP-modied ZnO lms have increased hydrophilicity (Supplementary Fig. 1). As a result, the PVP-modied substrate has better wetting of the hydrophilic perovskite precursor solution, leading to uniform growth of perovskite lms with reduced pinholes. Although the perovskite lm surface coverage was signicantly improved by introducing the PVP intermediate layer, there was still an appreciable density of pinholes. To improve further the morphology of the perovskite lm, we added a small amount of CH3NH3Br (MABr) into the precursor solution. We hypothesized that molecular pinning would help reduce pinholes by better controlling the crystallization kinetics of the CsPbBr3 lms37,38. Figure 1c shows that the morphology of the perovskite lm was improved considerably once we did add MABr, leading to now a negligible density of pinholes, which could be good for reducing current leakage in LEDs.

Crystal and band structure of CsPbBr3 lms. We characterized the crystal structure of CsPbBr3 lms deposited on bare ZnO and ZnO/PVP substrates, and on ZnO/PVP with the MABr additive. X-ray diffraction pattern of these three lms were almost identical, with all crystallographic signatures matching that of the pure CsPbBr3 phase (Supplementary Fig. 2). The band structure of CsPbBr3 lms with and without the MABr additive were determined using ultraviolet photoelectron spectroscopy (UPS; Supplementary Fig. 3, Supplementary Table 1) in combination with linear absorption measurements (Supplementary Fig. 4). The conduction and valence band relative to vacuum of CsPbBr3 with MABr are located at 3.37 and 5.71 eV, respectively. This could form a good alignment with electron-injecting layer such as ZnO ( 3.84 eV; Supplementary

Fig. 5) and hole-injecting such as CBP ( 6.0 eV)39 (Fig. 3b).

Chemical states of CsPbBr3 lms. We also carried out X-ray photoelectron spectroscopy (XPS) measurements on CsPbBr3 lms with and without the MABr additive (Supplementary Figs 6 and 7). The Pb 4f core level from pure CsPbBr3 can be t to four peaks (Supplementary Fig. 7). Two main peaks are located at 138.9 eV (Pb 4f7/2) and 142.8 eV (Pb 4f5/2), which correspond to

PbBr bonding28,40. Two additional weaker peaks at 137.1 and 141.9 eV can be attributed to Pb metallic states28,40. After incorporating MABr into the CsPbBr3 lattice, only PbBr peaks were found, indicating that Pb metallic states, which are known to function as non-radiative recombination centres28,40, have been suppressed.

Photoluminescence of CsPbBr3 lms. We carried out steady-state PL on CsPbBr3 thin lms deposited from different conditions (Fig. 2a). The CsPbBr3 lms directly deposited on ZnO showed weak green emission at 524 nm with a full width at

2 NATURE COMMUNICATIONS | 8:15640 | DOI: 10.1038/ncomms15640 | http://www.nature.com/naturecommunications

Web End =www.nature.com/naturecommunications

NATURE COMMUNICATIONS | DOI: 10.1038/ncomms15640 ARTICLE

a b c

Figure 1 | Morphology of CsPbBr3 lms deposited under different conditions. (ac) Planar SEM images of CsPbBr3 deposited on ZnO, ZnO/PVP and Cs0.87MA0.13PbBr3 on ZnO/PVP, respectively, here PVP is polyvinyl pyrrolidine. The scale bar is 2 mm in all images.

ZnO/CsPbBr3 ZnO/PVP/CsPbBr3

ZnO/PVP/Cs0.87MA0.13PbBr3

40

a b

1

Normalized intensity (a. u.)

0.1

PL intensity (a.u.)

0.01

1E3

1E4

350 400 450 500 550 600 650 700

1E5

Wavelength (nm)

c

d

60

50

40

PLQY (%)

30

20

10

0 0 1 2 3 4 5

Excitation power density (mW cm2)

Figure 2 | PL behaviour of CsPbBr3 lms deposited under different conditions. (a) Steady-state PL of CsPbBr3 lms on ZnO, ZnO/PVP and Cs0.87MA0.13PbBr3 lm on ZnO/PVP, respectively, here PVP is polyvinyl pyrrolidine, MA is CH3NH3. (b) Time-resolved PL of CsPbBr3 lms on ZnO and

ZnO/PVP and Cs0.87MA0.13PbBr3 on ZnO/PVP. (c) PL image of Cs0.87MA0.13PbBr3 lms on ZnO/PVP under ultraviolet lamp excitation. (d) PLQY of Cs0.87MA0.13PbBr3 as a function of excitation power density.

half maximum (FWHM) of 24 nm. Perovskites deposited onto the PVP-modied ZnO showed a dramatic increase in PL intensity, indicating that non-radiative recombination in the perovskite layer or at the interfaces has been signicantly suppressed. The enhancement of PL is posited to arise from several phenomena. First, the improved morphology may reduce non-radiative recombination at grain boundaries (Fig. 1a,b), leading to enhanced PL. Second, PVP could passivate ZnO surface defects41, which could act as non-radiative recombination traps at the interface of ZnO/perovskite. After PVP modication, the PL emission intensity and carrier lifetime of ZnO increased considerably (Supplementary Fig. 8), indicating a reduced surface defects of ZnO with PVP layer. Similar enhancement were also observed while coating PVP on perovskite surface, further conrming our argument (Supplementary Fig. 8).

The PL was further improved by introducing MABr into the CsPbBr3 lattice. This increase in PL intensity is consistent with the reduction of perovskite grain boundaries, as well as the suppression of Pb metallic recombination centres, which were conrmed by scanning electron microscopy (SEM) and XPS results, respectively (Fig. 1c and Supplementary Fig. 7). The emission peak from CsPbBr3 has been slightly shifted from 524 to 526 nm after adding MABr, indicating that a small fraction of MA cations have been introduced into the CsPbBr3 crystal lattice.

A single Gaussian emission peak at 526 nm shows that the CsPbBr3 layer with MABr additive is a pure perovskite phase, which could be ascribed to the formation of an alloy phase of Cs1 xMAxPbBr3. We estimate that the MA content in this

alloyed perovskite is 0.13, that is, Cs0.87MA0.13PbBr3-based on the

band-edge emission as shown in Fig. 2a, and the band-edge

NATURE COMMUNICATIONS | 8:15640 | DOI: 10.1038/ncomms15640 | http://www.nature.com/naturecommunications

Web End =www.nature.com/naturecommunications 3

ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms15640

a b

1

2

Al

3

Energy (eV)

MoO3

CBP

4

ITO

CBP

Cs0.87MA0.13PbBr3

5

ZnO

PVP

Perovskite

PVP

6

ZnO

MoO3/Al

Glass/ITO

7

8

c

Al

CBP/MoO3

Cs0.87MA0.13PbBr3

ZnO/PVP

ITO

Figure 3 | Device structure of CsPbBr3 inorganic-based perovskite LEDs. (a) Device structure, glass/ITO/ZnO/PVP/CsPbBr3/CBP/MoO3/Al, here PVP is polyvinyl pyrrolidine, CBP is 4,40-Bis(N-carbazolyl)-1,10-biphenyl. ZnO are CBP/MoO3 are used as the electron and hole injection layers, respectively. PVP was used to improve peorvskite morphology and also passivate the interface defects and improve charge injection balance. (b) Band alignment of each functional layer. (c) Cross-sectional SEM image of the LEDs, scale bar is 500 nm.

emission form pure MAPbBr3 (540 nm)28 according to the linear relationship Eg,Cs1 xMAxPbBr3 (1 x)E

g,CsPbBr3

xEg,MAPbBr3

(ref. 42). The nal MABr content in CsPbBr3 as estimated from the change in bandgap is approximately consistent with the initial precursor composition where CsBr:PbBr2:MABr 2.2:1:0.1 and

only 1 mol CsBr contributes to the formation of CsPbBr3 and the initial MABr ratio is 0.1 mol. The linear absorption of CsPbBr3 with and without MABr was consistent with the PL results (Supplementary Fig. 4). We further observed that the PL FWHM narrowed from 24 nm to 18 nm after introducing MABr. This indicates that the MABr additive has improved the sharpness of the perovskite band edge.

We next acquired time-resolved PL decay spectra of the different perovskite layers (Fig. 2b). The time-resolved PL curves were t to bi-exponential decays, where the fast decay component is associated with trap-assisted recombination at grain boundaries or surfaces, and the slow decay is ascribed to radiative recombination inside the bulk perovskite phase28,43. For the ZnO/CsPbBr3, PVP/CsPbBr3 and PVP/Cs0.87MA0.13PbBr3 lms, the decay times are (t1 1.2 ns, t2 4.6 ns), (t1 2.1 ns,

t26.4 ns) and (t1 1.8 ns, t2 7.5 ns), respectively.

Generally, it was found that the PL lifetime of the perovskite lm is increased after PVP modication, and further increased after the addition of MABr. We observed that the CsMA mixed perovskite has a marginally faster decay component in comparison with pure Cs perovskite. We hypothesize that this may be a result of increased surface defects in the CsMA mixed perovskite. However, the slow decay component of CsMA exhibited a longer lifetime, indicative of less bulk defects, consistent with the observed reduction of Pb metallic states (Supplementary Fig. 7). Although CsMA sample showed shorter lifetime in fast decay component compared with pure Cs, stronger PL from CsMA samples (Fig. 3a) indicated that the overall defects including surface and bulk defects in CsMA are

less than that of in pure Cs. The Cs0.87MA0.13PbBr3 lms show

bright and uniform green PL under ultraviolet lamp excitation (Fig. 2c). Both PVP interface engineering and MABr lattice incorporation enhanced the PL emission of the perovskite lm, which is benecial to realize high performance LEDs.

The PLQY of Cs0.87MA0.13PbBr3 was measured as a function of

excitation power density (Fig. 2d). As seen in other perovskites, the PLQY increases with excitation power30,31. This is attributed to state-lling of recombination centres in the perovskite layer30,31. Our inorganic-based perovskite materials, Cs0.87MA0.13PbBr3, exhibited high quantum yield (35.8%) even at low light intensity (0.07 mW cm 2). This is signicantly higher than previous reports at a similar order of power excitation, indicative of reduced non-radiative recombination centres in the perovskite layer30,31. Upon increasing the excitation intensity to 4.70 mW cm 2, the quantum efciency increased to as high as 55%. The high PL quantum yield suggests promise for high EQE LEDs.

Light-emitting diodes based on CsPbBr3 lms. We fabricated LEDs consisting of glass/indium tin oxide (ITO)/ZnO/PVP/ CsPbBr3/CBP/MoO3/Al (Fig. 3a), where CsPbBr3 is the emitting layer, Apart from ITO and MoO3/Al, which were deposited in vacuum, all layers were solution processed via spin coating. The band alignment of the CsPbBr3 LEDs could be drawn as shown in

Fig. 3b based on the band structure of CsPbBr3 and ZnO (Supplementary Figs 3 and 5), and also the valence band of CBP ( 6.0 eV)39. ZnO and CBP/MoO3 are used as the electron and

hole injection layers, respectively. In addition to improve perovskite morphology and also passivate the interface defects, which has been illustrate above (Figs 1 and 2). PVP layer could also induce an electron-injection barrier (Fig. 3b), which could improve charge injection balance, this will be discussed later. The

4 NATURE COMMUNICATIONS | 8:15640 | DOI: 10.1038/ncomms15640 | http://www.nature.com/naturecommunications

Web End =www.nature.com/naturecommunications

NATURE COMMUNICATIONS | DOI: 10.1038/ncomms15640 ARTICLE

a

b

540

0.8

0.6

0.4

0.2

0.0

0.0 0.2 0.4

x

560

EL intensity (a.u.)

500

520

580

y

600

620

700

(0.11, 0.78)

480

460

350 400 450 500

Wavelength (nm)

550 600 650 700

420

0.6 0.8

c

d

107

40

103

ZnO/CsPbBr3

10

EQE (%)

101

105

Current density (mA cm2 )

Luminance (cd m2 )

Current efficiency (cd A1 )

30

8

101

103

6

20

103

101

4

105

10

10-1

2

107

10-3

0

0

0 2 4 6 8

1E3 0.01 0.1 1 10 100 1,000

Voltage (V)

Current density (mA cm2)

Figure 4 | EL performance of the devices. (a) EL spectra of Cs0.87MA0.13PbBr3-based devices under varying voltage bias (the emission image is shown in inset). (b) The corresponding CIE coordinate. (c) IV and voltagelight intensity (LV) curves for the devices with and without PVP buffer layer or with and without CH3NH3Br (MABr) additive, that is, ZnO/CsPbBr3, ZnO/PVP/CsPbBr3 and ZnO/PVP/Cs0.87MA0.13PbBr3, respectively, here PVP is polyvinyl pyrrolidine. (d) Current efciency and EQE of devices with and without PVP buffer layer, with and without CH3NH3Br (MABr) additive, that is,

ZnO/CsPbBr3, ZnO/PVP/CsPbBr3 and ZnO/PVP/Cs0.87MA0.13PbBr3, respectively.

electrons and holes injected from each side recombine radiatively in the perovskite layer, resulting in photon emission. A cross-sectional SEM image of a typical device showed a clear sandwich structure (Fig. 3c). The thicknesses of the ZnO/PVP, CsPbBr3 and 4,40-Bis(N-carbazolyl)-1,10-biphenyl (CBP)/MoO3 layers are B45 nm, 100 nm and 80 nm, respectively.

Electroluminescence of light-emitting diodes. The EL spectra of CsPbBr3 and Cs0.87MA0.13PbBr3-based devices are centred at 516

and 520 nm, respectively (Supplementary Fig. 9). Compared to the PL emission at 526 nm for Cs0.87MA0.13PbBr3, the EL

emission showed a slight blue shift to 520 nm, which has also been observed in other perovskite-based LEDs16,44. The blue shift in the EL spectrum could be ascribed to free carrier emission, as already demonstrated in several perovskite systems16,44,45. For the devices using Cs0.87MA0.13PbBr3 as an emitting layer, the EL

spectrum as a function of voltage bias was measured (Fig. 4a), and an EL image of the device under operation was taken (inset of Fig. 4a). The EL showed very narrow emission (FWHM 18 nm)

and high colour purity. This spectral line width is narrower than that of previously reported perovskite nanocrystal-based LEDs14,15. The devices exhibited saturated and pure colour(90.2%) at green wavelengths, with Commission Internationale de lEclairage (CIE) chromaticity coordinates at (0.11, 0.78) (Fig. 4b).

We measured the voltagecurrent (IV) curve of the devices (Fig. 4c) and found that the control devices (without both PVP and MABr) showed higher injection current. This could be two reasons: one is high density of pinholes in the perovskite layer

which results in a signicant electron and hole injection leakage; and another could be the imbalanced charge injection. After introducing the PVP buffer layer and the MABr additive, the injection current of CsPbBr3 device was signicantly reduced, indicating that the current leakage and charge injection imbalance has been suppressed. The turn-on voltage was slightly increased after inserting an immediate layer of PVP, which could be mainly due to the injection barrier caused by the insulating nature of PVP layer46. The further minor increase of turn-on voltage after MABr incorporation might be ascribed to deeper valence band of Cs0.87MA0.13PbBr3 (5.71 eV) compared to CsPbBr3

(5.50 eV) (Supplementary Fig. 3, Supplementary Table 1)47. Similar phenomenon has also been found by Sun et al. in FACs mixture nanocrystal-based LEDs48. Although the turn-on voltage increased after incorporating PVP buffer layer and adding MABr salt, it can be calculated that the current efciency are signicantly increased from 0.02 cd A 1 (CsPbBr3) to 1 cd A 1 (PVP/Cs0.87MA0.13PbBr3) at small brightness (1 cd m 2). The increase of current efciency indicated the non-radiative recombination has been suppressed, which will be discussed later. The control devices showed a maximum brightness of 300 cd m 2 (Fig. 4c). The maximum brightness was dramatically increased to 11600 cd m 2 by introducing the PVP intermediate layer. Consistent with this was an observed increase in the current efciency from0.26 cd A 1 to 7.19 cd A 1 (Fig. 4d). The EQE of the LEDs from control devices of CsPbBr3 on ZnO is o0.1%. The addition of the PVP buffer layer improved the quantum efciency to 2.4% (Fig. 4d).

NATURE COMMUNICATIONS | 8:15640 | DOI: 10.1038/ncomms15640 | http://www.nature.com/naturecommunications

Web End =www.nature.com/naturecommunications 5

ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms15640

Table 1 | Device performance with and without PVP intermediate layers or CH3NH3Br (MABr) additive.

Devices Vth(V) Lmax(cd m 2) Current efciency(Cd A 1)

EQE (%)

ZnO/CsPbBr3 2.3 350 0.26 0.09 ZnO/PVP/CsPbBr3 2.6 11,600 7.19 2.41

ZnO/PVP/Cs0.87MA0.13PbBr3 2.9 91,000 33.9 10.43

The MABr salt additive further improves both the maximum brightness and EQE. The maximum brightness of Cs0.87MA0.13PbBr3-based LEDs increased to as high as

91,000 cd m 2. The current efciency and quantum efciency were increased to 33.9 cd A 1 and 10.43%, respectively (Fig. 4d).

The best devices exhibited an internal quantum efciency (IQE) of 47% calculated by IQE 2n2EQE, where n is the refractive of

glass (1.5)31. The device performance parameters are summarized in Table 1. To the best of our best knowledge, these devices are the brightest and most-efcient perovskite-based LEDs emitting at green wavelengths reported to date.

These devices also demonstrated high reproducibility (Supplementary Fig. 10). The high performance in the ZnO/PVP/Cs0.87MA0.13PbBr3 LED is a result of careful

optimization of the interfaces and perovskite layers (Supplementary Figs 1113, Supplementary Tables 2 and 3). It was found that a high amount of MABr compositionally mixed with CsPbBr3 led to a signicant decrease in LED performance.

We attribute this to poor morphology (Supplementary Fig. 14).

We tested the stability of CsPbBr3 LEDs (Supplementary Fig. 15). Consistent with what was observed in previous perovskite LED reports33,49, the devices decayed after several minutes. The decay mechanism is hypothesized to be a result of ion migration under steady-state voltage bias. To improve device stability, we attempted to incorporate ion-migration-inhibiting polymers50, such as PVP, into the perovskite layer. The results indeed showed that the stability had been signicantly improvedthe output was stable for several hours. While encouraging, the efciency of this polymer-blended device is reduced in comparison with the ZnO/PVP/Cs0.87MA0.13PbBr3

devices (Supplementary Fig. 16). We will study in more depth the underlying phenomena leading to this stability/efciency compromise in our future work. We have also tested the transient light emission response of our perovskite LEDs (Supplementary Fig. 17). Nearly instantaneous turn-on was achieved with a response time about 18 ms to reach their maximum output light intensities. Such a fast turn-on is comparable to that of conventional LEDs51.

DiscussionWe found that the improvement via PVP could be due to three reasons. First, the improvement of the perovskite lm morphology, as shown in Fig. 1a,b, leads to reduced pinholes which minimize current leakage (Fig. 4a). Second, the suppression of non-radiative recombination at the ZnO/perovskite interface, which has been conrmed by PL results (Fig. 2a, Supplementary Fig. 8), improves the radiative efciency.

In addition to improvement of perovskite lm morphology and passivation of defects at interface, PVP could improve the charge injection balance in our perovskite LEDs, and thus enhancing devices EL efciency. Similar mechanism has been proposed in previous reports, while using PMMA insulting layer in quantum dot LEDs46. It was found that the injection current from electron only devices is much higher than that of the current from hole only devices, while using ZnO and CBP as the main injection

layers, respectively (Supplementary Fig. 18). These results indicated that the electron injected by ZnO is faster than holes injected by CBP, which could be due to the different carrier mobility of ZnO and CBP (ref. 46). This will lead to charge injection imbalance and also the excess electron current while using these layer as electron- and hole-injection layers in LEDs, and thus degrading EL efciency46. Insulting PVP layer can slow down electron-injection via an energy barrier (Fig. 3b, Supplementary Fig. 18), an improvement of charge balance could be anticipated by inserting a PVP layer on ZnO surface as the electron-injection layer. In fact, the reduction of device injection current via PVP insertion conrmed the improvement of charge injection balance (Fig. 4c)46, and thus improving EL emission efciency.

There could be two key improvements of leading to superior performance for MACs mixed perovskite devices. The rst one is ascribed to the suppression of non-radiative recombination centres by eliminating the Pb metallic phase by compositionally blending CsPbBr3 with MABr to form the compound

Cs0.87MA0.13PbBr3. PL (Fig. 2a) and time-resolved PL (Fig. 2b)

both indicated that the less defects in CsMA lms. Second, the key advance that led to the dramatic improvement in EL brightness and efciency is the reduction of leakage current via improved morphology as a result of both PVP-modied ZnO and the MABr additive (Fig. 1c).

In summary, we have obtained high-quality Cs0.87MA0.13PbBr3

perovskite light-emitting thin lms with minimized pinholes through electron-injecting interface passivation and perovskite composition modulation. These strategies jointly reduced the device leakage current. Furthermore, the non-radiative recombination centres at the interfaces and in the perovskite lm were suppressed and also the charge injection balance were improved. As a result of these advances, we obtained ultra-bright and highly efcient inorganic perovskite-based LEDs. With additional optimizations to the perovskite and interfacial layers, the inorganic perovskite-based LEDs have promise to reach 20% EQE, making them competitive with materials such as semi-conducting organics and colloidal quantum dots.

Methods

Preparation of perovskite solution and ZnO nanoparticles. CsBr (Sigma Aldrich, 99.9%) and PbBr2 (Aldrich, 99.99%) (CsBr:PbBr2 molar ratio of 2.2:1) solutions were prepared using DMSO as a solvent. The solution concentration is0.5 M (CsBr 1.1 M, PbBr2 0.5 M). A high ratio of CsBr:PbBr2 was used to suppress the formation of non-CsPbBr3 phases. For lms which incorporated the CH3NH3Br additive, 0.05 M CH3NH3Br was added to the solution (CsBr:PbBr2:MABr 2.2:1:0.1). Comparative studies used additional ratios of

CsPbBr3 solutions, such as CsBr:PbBr2:MABr 2:1:0.1, 2.1:1:0.1, 2.2:1:0.1, 2.4:1:0.1

and 2.2:1:0.2. The precursor solutions were stirred at 45 C overnight. And then the solution was stand for 4 h at room temperature, precipitates were formed in the CsBr-rich solution, top transparent solution was decanted and ltered for using. The details of precursor preparation procedures were shown in Supplementary Fig. 19. The ZnO nanoparticles were synthesized using a previously developed method8. The synthesized ZnO nanoparticles were dispersed in methanol and n-butanol to form a 2% ZnO nanoparticle solution.

Light-emitting diode fabrication. The glass/ITO substrate was sequentially washed with isopropanol, acetone, distilled water and isopropanol. The sheet resistance of ITO is 15 O per square. ZnO nanoparticles of concentration 2% by weight were spin-coated onto ITO substrates at 2,000 r.p.m. for 30 s and then annealed at 150 C for 15 min. For control devices, perovskite precursor (CsBr:PbBr2 2.2:1) was spin-coated onto ZnO at 2,000 r.p.m. for 2 min, and then

annealed at 100 for 20 min. After, a 2wt% CBP solution was spin-coated onto the perovskite. The devices were transferred into a vacuum chamber for MoO3/Al deposition. For PVP interface-modied devices, 0.5 wt% PVP solution in DMSO was spin-coated onto ZnO at 2,000 r.p.m. for 1 min, and then annealed at 150 C for 15 min to induce crosslinking. However, we found that the PVP is slightly washed away during spin-coating of the DMSO solution. The PVP thickness before and after DMSO solution washing are 8.4 and 5.0 nm, respectively, which were measured by ellipsometer (Supplementary Fig. 20). For MABr additive devices, the ratio of CsBr:PbBr2:MABr is 2.2:1:0.1, the nal composition is Cs0.87MA0.13PbBr3

6 NATURE COMMUNICATIONS | 8:15640 | DOI: 10.1038/ncomms15640 | http://www.nature.com/naturecommunications

Web End =www.nature.com/naturecommunications

NATURE COMMUNICATIONS | DOI: 10.1038/ncomms15640 ARTICLE

as determined by band-edge emission measurements. The solution concentration was 0.5 M. The device active area was 0.108 cm2.

Materials and device characterization. A eld emission SEM (FEI NanoSEM650) was used to acquire SEM images. The instrument uses an electron beam accelerated at 500 V to 30 kV, enabling operation at a variety of currents. Absorption measurement were carried out by Hitachi ultravioletvisible U-4100 spectrophotometer. Absorbance was determined from a transmittance measurement using integrated sphere. PL measurements were carried out by FLS980 Spectrometer. The X-ray diffraction patterns (y2y scans) were taken on a

Rigaku D/MAX-2500 system using Cu ka (l 1.5405 ) as the X-ray source. Scans

were taken with 0.5 mm wide source and detector slits, and X-ray generator settings at 40 kV and 30 mA. XPS was performed on the Thermo Scientic ESCALab 250Xi using 200 W monochromated Al Ka (1,486.6 eV) radiation. The 500 mm X-ray spot was used for XPS analysis. The base pressure in the analysis chamber was

B3 10 10 mbar. Typically the hydrocarbon C1s line at 284.8 eV from adventi

tious carbon is used for energy referencing. UPS samples were analyzed on a Thermo Scientic ESCALab 250Xi. The gas discharge lamp was used for UPS, with helium gas admitted and the HeI (21.22 eV) emission line employed. The helium pressure in the chamber during analysis was B2E 8 mbar. The data was acquired

with a 10 V bias. The work function of the measured sample can be calculated

from following equation: hn f E

Fermi

Ecutoff, here, hn 21.22 eV,

EFermi 21.08 eV (using Ni as the standard sample for calibration), E

cut-off is the

cut-off shown in the corresponding Figures. The PLQY was measured using a Horiba Fluorolog system equipped with a single grating and a Quanta-Phil integration sphere coupled to the Fluorolog system with optic bre bundles30. The following settings were applied for PLQY measurements: an excitation wavelength of 400 nm; bandpass values of 10 and 5 nm for the excitation and emission slits, respectively; step increments of 1 nm and integration time of 0.5 s per data point. The excitation power density in the power-dependent PLQY characterization was tuned by varying the slit width on the Fluorolog monochromator. JV characteristics of LEDs were taken using a Keithley 2,400 source metre. Two Keithley 2,400 source metre units linked to a calibrated silicon photodiode were used to measure the currentvoltagebrightness characteristics. The measurement system has been carefully calibrated by efcient InGaN/GaN LEDs with a similar photon response by PR-650 SpectraScan. A Lambertian prole was assumed in the calculation of EQE28,30,31.

Data availability. The data that support the ndings of this study are available from the corresponding author upon reasonable request.

References

1. Kojima, A., Teshima, K., Shirai, Y. & Miyasaka, T. Organometal halide perovskites as visible-light sensitizers for photovoltaic cells. J. Am. Chem. Soc. 131, 60506051 (2009).

2. Kim, H. S. et al. Lead iodide perovskite sensitized all-solid-state submicron thin lm mesoscopic solar cell with efciency exceeding 9%. Sci. Rep. 2, 591 (2012).

3. Lee, M. M., Teuscher, J., Miyasaka, T., Murakami, T. N. & Snaith, H. J. Efcient hybrid solar cells based on meso-superstructured organometal halide perovskites. Science 338, 643647 (2012).

4. Burschka, J. et al. Sequential deposition as a route to high-performance perovskite-sensitized solar cells. Nature 499, 316319 (2013).

5. Jeon, N. J. et al. Solvent engineering for high-performance inorganicorganic hybrid perovskite solar cells. Nat. Mater. 13, 897903 (2014).

6. Yang, W. S. et al. High-performance photovoltaic perovskite layers fabricated through intramolecular exchange. Science 348, 12341237 (2015).

7. Bi, D. et al. Efcient luminescent solar cells based on tailored mixed-cation perovskites. Sci. Adv. 2, e1501170 (2016).

8. You, J. et al. Improved air stability of perovskite solar cells via solution-processed metal oxide transport layers. Nat. Nanotechnol. 11, 7581 (2016).

9. Xing, G. et al. Low-temperature solution-processed wavelength-tunable perovskites for lasing. Nat. Mater. 13, 476480 (2014).

10. Zhu, H. et al. Lead halide perovskite nanowire lasers with low lasing thresholds and high quality factors. Nat. Mater. 14, 636643 (2015).

11. Stranks, S. D. & Snaith, H. J. Metal-halide perovskites for photovoltaic and light-emitting devices. Nat. Nanotechnol. 10, 391402 (2015).

12. Deschler, F. et al. High photoluminescence efciency and optically pumped lasing in solution-processed mixed halide perovskite semiconductors. J. Phys. Chem. Lett. 5, 14211426 (2014).

13. Zhang, F. et al. Brightly luminescent and color-tunable colloidal CH3NH3PbX3 (X Br, I, Cl) quantum dots: potential alternatives for display technology. ACS

Nano 9, 45334542 (2015).14. Protesescu, L. et al. Nanocrystals of cesium lead halide perovskites (CsPbX3,

X Cl, Br, and I): novel optoelectronic materials showing bright emission with

wide color gamut. Nano Lett. 15, 36923696 (2015).15. Song, J. et al. Quantum dot light-emitting diodes based on inorganic perovskite cesium lead halides (CsPbX3). Adv. Mater. 27, 71627167 (2015).

16. Tan, Z. K. et al. Bright light- emitting diodes based on organometal halide perovskite. Nat. Nanotechnol. 9, 687692 (2014).

17. Li, G. et al. Efcient light-emitting diodes based on nanocrystalline perovskite in a dielectric polymer matrix. Nano Lett. 15, 26402644 (2015).

18. Jaramillo-Quintero, O. A., Sanchez, R. S., Rincon, M. & Mora-Sero, I. Bright visible-infrared light emitting diodes based on hybrid halide perovskite with Spiro-OMeTAD as a hole-injecting layer. J. Phys. Chem. Lett. 6, 18831890 (2015).

19. Sadhanala, A. et al. Blue-green color tunable solution processable organolead chloride bromide mixed halide perovskites for optoelectronic applications.

Nano Lett. 15, 60956101 (2015).20. Gil-Escrig, L. et al. Efcient photovoltaic and electroluminescent perovskite devices. Chem. Commun. 51, 569571 (2015).

21. Li, J., Bade, S. G. R., Shan, X. & Yu, Z. Single-layer light-emitting diodes using organometal halide perovskite/poly(ethylene oxide) composite thin lms. Adv. Mater. 27, 51965202 (2015).

22. Kim, Y. H. et al. Multicolored organic/inorganic hybrid perovskite light-emitting diodes. Adv. Mater. 27, 12481254 (2015).

23. Hoye, R. L. Z. et al. Enhanced performance in uorene-free organometal halide perovskite light-emitting diodes using tunable, low electron afnity oxide electron injectors. Adv. Mater. 27, 14141419 (2015).

24. Aygler, M. F. et al. Light-emitting electrochemical cells based on hybrid lead halide perovskite nanoparticles. J. Phys. Chem. C 119, 1204712054 (2015).

25. Yu, J. C. et al. High-performance planar perovskite optoelectronic devices:a morphological and interfacial control by polar solvent treatment. Adv. Mater. 27, 34923500 (2015).

26. Xing, J. et al. High-efciency light-emitting diodes of organometal halide perovskite amorphous nanoparticles. ACS Nano 10, 66236630 (2016).

27. Wang, J. et al. Interfacial control toward efcient and low-voltage perovskite light-emitting diodes. Adv. Mater. 27, 23112316 (2015).

28. Cho, H. et al. Overcoming the electroluminescence efciency limitations of perovskite light-emitting diodes. Science 350, 12221225 (2015).

29. Dou, L. et al. Atomically thin two-dimensional organic-inorganic hybrid perovskites. Science 349, 15181521 (2015).

30. Yuan, M. et al. Perovskite energy funnels for efcient light-emitting diodes. Nat. Nanotechnol. 11, 872877 (2016).

31. Wang, N. et al. Perovskite light-emitting diodes based on solution-processed self-organized multiple quantum wells. Nat. Photon. 10, 699704 (2016).32. Xiao, Z. Efcient perovskite light-emitting diodes featuring nanometre-sized crystallites. Nat. Photonics. 11, 108115 (2017).

33. Kulbak, M., Cahen, D. & Hodes, G. How important is the organic part of lead halide perovskite photovoltaic cells? Efcient CsPbBr3 cells. J. Phys. Chem. Lett.

6, 24522456 (2015).34. Yantara, N. et al. Inorganic halide perovskites for efcient light-emitting diodes.J. Phys. Chem. Lett. 6, 43604364 (2015).35. Li, J. et al. 50-fold EQE improvement up to 6.27% of solution-processed all-inorganic perovskite CsPbBr3 QLEDs via surface ligand density control.

Adv. Mater. 29, 1603885 (2016).36. Ling, Y. et al. Enhanced optical and electrical properties of polymer-assisted all-inorganic perovskites for light-emitting diodes. Adv. Mater. 28, 89838989 (2016).

37. Meng, L., You, J., Guo, T. & Yang, Y. Recent advances in the inverted planar structure of perovskite solar cells. Acc. Chem. Res. 49, 155165 (2016).

38. Jeon, N. J. et al. Compositional engineering of perovskite materials for high-performance solar cells. Nature 517, 476480 (2015).

39. Veldhuis, S. A. et al. Perovskite materials for light-emitting diodes and lasers. Adv. Mater. 28, 68046834 (2016).

40. Zhang, W. et al. Enhanced optoelectronic quality of perovskite thin lms with hypophosphorous acid for planar heterojunction solar cells. Nat. Commun. 6, 10030 (2015).

41. Small, C. E. et al. High-efciency inverted dithienogermole thienopyrrolodione-based polymer solar cells. Nat. Photon. 6, 115120 (2012).

42. Noh, J. H., Im, S. H., Heo, J. H., Mandal, T. N. & Seok, S. I. Chemical management for colorful, efcient, and stable inorganicorganic hybrid nanostructured solar cells. Nano Lett. 13, 1764 (2013).

43. Shi, D. et al. Low trap-state density and long carrier diffusion in organolead trihalide perovskite single crystals. Science 347, 519522 (2015).

44. Palma, A. L. et al. Mesoscopic perovskite light-emitting diodes. ACS Appl. Mater. Interfaces 8, 2698926997 (2016).

45. DInnocenzo, V. et al. Excitons versus free charges in organo-lead tri-halide perovskites. Nat. Commun. 5, 3586 (2014).

46. Dai, X. et al. Solution-processed, high-performance light-emitting diodes based on quantum dots. Nature 515, 9699 (2014).

47. Endres, J. et al. Valence and conduction band densities of states of metal halide perovskites: a combined experimental theoretical study. J. Phys. Chem. Lett.

7, 27222729 (2016).48. Zhang, X. et al. Hybrid perovskite light-emitting diodes based on perovskite nanocrystals with organicinorganic mixed cations. Adv. Mater 29, 1606405 (2017).

NATURE COMMUNICATIONS | 8:15640 | DOI: 10.1038/ncomms15640 | http://www.nature.com/naturecommunications

Web End =www.nature.com/naturecommunications 7

ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms15640

49. Zhang, X. et al. Enhancing the brightness of cesium lead halide perovskite nanocrystal based green light-emitting devices through the interface engineering with peruorinated monomers. Nano Lett. 16, 14151420 (2016).

50. Zhou, Q. et al. In situ fabrication of halide perovskite nanocrystal embedded polymer composite lms with enhancednphotoluminescence for display backlights. Adv. Mater. 28, 91639168 (2016).

51. Li, J. et al. Single-layer halide perovskite light-emitting diodes with sub-band gap turn-on voltage and high brightness. J. Phys. Chem. Lett. 7, 40594066 (2016).

Acknowledgements

We thank Prof Junxi Wang and Prof Hua Yang from Institute of Semiconductors, Chinese Academy of Sciences for kindly providing GaN/InGaN LEDs to us for calibration of our measurement system, and for helping us with the CIE measurement. We also thank Prof Lu Li and Prof Xing Yang from Chongqing University of Arts and Science for brightness calibration measurement. We thank Prof Bo Wang from Beijing University of Technology for helping with contact angle measurements. We also would like to Prof Haibo Zeng from Nanjing University of Science and Technology for fruitful discussions. This work is supported by National 1,000 Young Talents awards, National Key Research and Development Program of China (Grant No. 2016YFB0700700), National Natural Science Foundation of China (Grant Numbers: 61634001, 61574133), Beijing Municipal Science & Technology Commission (Grant No. Z151100003515004) and Young top-notch talent project of Beijing. H.T. acknowledges the Netherlands Organisation for Scientic Research (NWO) for a Rubicon grant (680-50-1511) to support his postdoctoral research at University of Toronto.

Author contributions

J.Y. conceived the idea, designed the experiment and analyzed the data. L.Z. fabricated devices and collected all data. X.Y., Q.J., P.W., Z.Y. were involved in data analysis. H.T. and M.W. carried out PLQY measurements, Y.Y. is responsible for LED response

measurements. J.Y., L.Z. and X.Z. co-wrote the manuscript. H.T., B.R.S. and E.H.S. improved the manuscript. J.Y. and X.Z. directed and supervised the project. All authors contributed to discussions and nalizing the manuscript.

Additional information

Supplementary Information accompanies this paper at http://www.nature.com/naturecommunications

Web End =http://www.nature.com/ http://www.nature.com/naturecommunications

Web End =naturecommunications

Competing interests: The authors declare no competing nancial interests.

Reprints and permission information is available online at http://npg.nature.com/reprintsandpermissions/

Web End =http://npg.nature.com/ http://npg.nature.com/reprintsandpermissions/

Web End =reprintsandpermissions/

How to cite this article: Zhang, L. et al. Ultra-bright and highly efcient inorganic based perovskite light-emitting diodes. Nat. Commun. 8, 15640 doi: 10.1038/ncomms15640 (2017).

Publishers note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional afliations.

This work is licensed under a Creative Commons Attribution 4.0 International License. The images or other third party material in this article are included in the articles Creative Commons license, unless indicated otherwise in the credit line; if the material is not included under the Creative Commons license, users will need to obtain permission from the license holder to reproduce the material. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/

Web End =http://creativecommons.org/licenses/by/4.0/

r The Author(s) 2017

8 NATURE COMMUNICATIONS | 8:15640 | DOI: 10.1038/ncomms15640 | http://www.nature.com/naturecommunications

Web End =www.nature.com/naturecommunications