Polymer-assisted In-situ Growth of All-Inorganic Perovskite

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Functional Inorganic Materials and Devices

Polymer-assisted In-situ Growth of All-Inorganic Perovskite Nanocrystal Film for Efficient and Stable Pure Red Light-emitting Devices Wanqing Cai, Ziming Chen, Zhenchao Li, Lei Yan, Donglian Zhang, Linlin Liu, Qing-Hua Xu, Yuguang Ma, Fei Huang, Hin-Lap Yip, and Yong Cao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b13418 • Publication Date (Web): 07 Nov 2018 Downloaded from http://pubs.acs.org on November 7, 2018

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ACS Applied Materials & Interfaces

Polymer-assisted In-situ Growth of All-Inorganic Perovskite Nanocrystal Film for Efficient and Stable Pure Red Light-emitting Devices Wanqing Cai,†1 Ziming Chen,†1 Zhenchao Li,1 Lei Yan,1 Donglian Zhang,1 Linlin Liu,1 Qing-hua Xu,2 Yuguang Ma,1 Fei Huang,1 Hin-Lap Yip,*1 Yong Cao1 1

State Key Laboratory of Luminescent Materials and Devices, Institute of Polymer Optoelectronic Materials and Devices, School of Materials Science and Engineering, South China University of Technology, 381 Wushan Road, Guangzhou 510640, P. R. China 2

Department of Chemistry, National University of Singapore, 117543 Singapore

* Correspondence: [email protected] (H.-L. Y.) † These authors contributed equally to this work.

ABSTRACT: In the past few years, a substantial progress has been made for perovskite light-emitting devices. Both pure green and infrared thin film perovskite light-emitting devices with external quantum efficiency over 20% have been successfully achieved. However, pure red and blue thin film perovskite light-emitting diodes still suffer from inferior efficiency. Therefore, the development of efficient and stable thin film perovskite light-emitting diodes with pure red and blue emissions is urgently needed for possible applications as a new display technology and solid-state lighting. Here, we demonstrate an efficient light-emitting diode with pure red emission based on polymer-assisted in-situ growth of high quality all-inorganic CsPbBr0.6I2.4 perovskite nanocrystal film with homogenous distribution of nanocrystals in size of 20 ~ 30 nm. With this method, we can dramatically reduce the formation temperature of CsPbBr0.6I2.4 and stabilize its perovskite phase. Eventually, we successfully demonstrate a pure-red-emission perovskite light-emitting diode with a high external quantum efficiency of 6.55% and luminance of 338 cd/m2. Furthermore, the device obtains an ultra-low turn-on voltage of 1.5 V and a half-lifetime of over 0.5 hours at a high initial luminance of 300 cd/m2. Keywords: Low-temperature process, cesium-based perovskite, perovskite-polymer hybrid, perovskite LED, pure red-emitting

INTRODUCTION

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In recent years, organic-inorganic hybrid perovskites have been considered as promising semiconductors for optoelectronic applications. The best perovskite solar cells have power conversion efficiencies more than 23%,1 which make them excellent candidates for solar energy harvesting. In addition to their potential photovoltaic applications, organic-inorganic hybrid perovskites also have brilliant prospects for light-emitting diodes (LEDs) application because of their high color purity, tunable bandgap, low non-radiative recombination rates, and also high and balanced electron/hole mobility.2-6 In addition, the low material cost and solution processability of perovskite LEDs (PeLEDs) make them suitable for large-scale fabrication. Over the past several years, substantial progress has been made in the development of thin film near-infrared perovskite LEDs, which showed a maximum external quantum efficiency (EQE) of more than 20%.7 To fulfill the demands of white-light illumination and vivid display, LEDs with excellent color purity of red, green and blue are needed. Of the PeLEDs with visible emissions, efficient quasi-2D perovskite LEDs with green emission have been demonstrated to have an EQE over 20%.8 Typically, perovskites with mixed halide have been used to fabricate pure red and blue thin film PeLEDs (CH3NH3PbI3-xBrx for red and CH3NH3PbBr3-xClx for blue); however, in most cases, these devices have exhibited inferior performance.10-14 Therefore, the development of efficient and stable perovskite LEDs with pure red and blue emissions is an urgent target in this field. For pure red-emission PeLEDs, the inferior efficiency of methylammonium (MA)-based system is mainly due to the low photoluminescence (PL) quantum efficiency (PLQE) of the perovskite layer.15-17 Furthermore, the poor thermal stability of CH3NH3PbI3-xBrx perovskite film is susceptible to the sublimation of organic compound and the perovskite phase segregation, which affects the quality of the perovskite devices.18-21 Alternately, an all-inorganic CsPbI3 perovskite with better thermal stability could be a good candidate for stable and highly efficient red-emission perovskite LEDs.22-31 However, CsPbI3 thin films also suffer from phase stability problem. The cubic perovskite phase (black phase), which forms at temperatures over 300 °C, tends to transform to the orthorhombic non-perovskite phase (yellow phase) at room temperature.32,33 Interestingly, CsPbI3 nanocrystals with reduced crystal size tends to be more stable in terms of phase stability at room temperature.34-37 Therefore, it is possible to achieve efficient red-emission perovskite LEDs by modulating the nanostructure of the CsPbI3 perovskite. Li et al. reported that the use of cross-linkable ligands to produce CsPbI3 nanocrystal-based red PeLEDs could achieve a EQE of 5.7% and a maximum luminance of 206 cd/m2.9 In addition, efficient red PeLED with an EQE of 3.7% and maximum luminance of 440 cd/m2 has been achieved 2


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using cesium (Cs)-based self-organized perovskite multiple quantum wells.38 In addition, Tian et al. reported a spectrally stable red PeLED with a decent EQE of 6.23% through combining Cs-based quasitwo-dimensional perovskite and poly (ethylene oxide) together.39 Moreover, Lu et al. demonstrated flexible perovskite LEDs with EQE of 8.2% based on CsPbI3 nanocrystals, using a photopolymer as the substrate and an ultra-smooth Ag film as the bottom electrode.41 Although good progress has been made in CsPbI3 LEDs, their emission in deep-red region (around 700 nm) is not ideal as our eyes are not very sensitive to the deep-red emission and may limit their applications.36,40 Therefore, a precise color control for pure red emission (generally, in the range of 640 nm to 670 nm) in PeLEDs is necessary. In this study, we report a new methodology to fabricate high quality Cs-based perovskite nanocrystal films through in-situ growth of the nanocrystals within a polymer film matrix. Light emitting devices based on the resultant perovskite hybrid films show a pure red emission color with high efficiency and stability. We found that the polymer, poly(2-ethyl-2-oxazoline) (PEOXA), which contains functional groups that can form coordination bonds with the metal cations in the perovskite, could reduce the formation temperature and improve the phase stability of the perovskite nanocrystals. With this process, we produced a pure-red perovskite LED with a maximum EQE of 6.55% and luminance of 338 cd/m2. Furthermore, the device had an ultra-low turn-on voltage of 1.5 V and a half-lifetime of over 0.5 hours at a high initial luminance of 300 cd/m2.

RESULTS AND DISCUSSION Inspired by our previous work of using polymer, PEOXA, to modulate the crystal formation of MAPbI3 for both solar cell and LED applications as well as other demonstrations of using functional polymers to stabilize the perovskite nanocrystals, we fabricated Cs-based perovskite/polymer hybrid film by introducing PEOXA to the perovskite precursor solution.42-45 Our first attempt is focused on the composition of CsPbI3. Figure S1 shows the absorption and PL of the annealed hybrid films with different concentrations of PEOXA. Without the addition of PEOXA, there is no distinct absorption and emission from the CsPbI3 perovskite, suggesting the absence of CsPbI3 crystals. Encouragingly, when we increased the PEOXA concentration to more than 5%, both characteristic absorption edges and emissions at around 700 nm from the CsPbI3 perovskite were observed, illustrating that the introduction of PEOXA could help to promote the formation of the cubic phase of CsPbI3. However, in this case, the emission peak of CsPbI3 was still located in the deep-red range. To obtain a pure red perovskite emitter 3



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with a Commission Internationale de l’Eclairage (CIE) of (0.73, 0.27), the emission spectrum of the perovskite film should be further finely tuned and it could be achieved by the introduction of Br into the CsPbI3 perovskite to hypsochromically shift the emission from the deep-red to the pure red region.46,47 By introducing different amounts of Br into the CsPbI3 lattice to form PEOXA/CsPbBrxI3-x (0 ≤ x ≤ 3) hybrid films, we found that the emission can be easily tuned from deep-red to orange, as shown in Figure S2a. Among them, the CsPbBr0.6I2.4 perovskite shows excellent color purity with an emission peak at 668 nm and a CIE of (0.73,0.27), so in the rest of our study we focused on optimizing the hybrid film and LED devices based on this selected composition. The absorption spectrum of the CsPbBr0.6I2.4 perovskite films with various PEOXA concentrations are shown in Figure S2b. Similar to the case of CsPbI3, the absorption edge appeared after adding 5% PEOXA in the film. A series of photos of the CsPbBr0.6I2.4 perovskite hybrid films blended with different amount of PEOXA taken under and without UV light excitation are shown in Figure 1a. It obviously shows that the PL intensity of the perovskite films is gradually increased with the PEOXA concentration. In order to correlate the film structure to the PL property of the hybrid CsPbBr0.6I2.4 perovskite films, we studied the morphology of the films at various PEOXA concentrations using atomic force microscopy (AFM), scanning electron microscopy (SEM) and also used X-ray diffraction spectroscopy (XRD) to study the crystal structure. The AFM images in Figures S3 and SEM images in Figure S4 show that the pristine CsPbBr0.6I2.4 film without PEOXA had a poor morphology and a large roughness (RMS = 11.6 nm). After the introduction of PEOXA, high-coverage films with small perovskite nanocrystals were formed, resulting in a small roughness of around 3 nm. As the PEOXA concentration increased, the decrease in the crystal size was in the range of around 300 nm (5%-PEOXA sample) to 30 nm (45%-PEOXA sample). This result was consistent with previous reports that the introduction of PEOXA improves the film coverage and decreases the crystal size of CH3NH3PbI3 perovskite thin films.46 Interestingly, in the previous study of CH3NH3PbI3 systems, all the CH3NH3PbI3 perovskite disappeared in the samples with high PEOXA concentrations of over 10%, as PEOXA suppressed the growth of CH3NH3PbI3 crystals.47 However, there was no obvious crystal-growth suppression in the CsPbBr0.6I2.4 samples in our case with very high PEOXA concentrations. When the concentration of the PEOXA was further increased to 55%, the perovskite crystals tend to growth into larger size again probably due to severe phase separation between the polymer and perovskite phases at higher polymer blending ratios. The corresponding PLQEs of the CsPbBr0.6I2.4 films with different concentrations of PEOXA were measured and are shown in Figure 1b. The PLQE of CsPbBr0.6I2.4 film increased with 4


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increasing PEOXA concentration up to 45%-PEOXA, showing a maximum PLQE of 44%, further increasing the PEOXA concentration resulted in a drop of PLQE. We also extracted the trap densities of perovskite films with different concentrations of PEOXA. According to Figure S8, the trap densities of perovskite films with 0%, 5%, 15%, 25%, 35%, 45% and 55% are found to be 4.51017 cm-3, 2.71017 cm-3, 2.51017 cm-3, 2.31017 cm-3, 1.21017 cm-3, 8.41016 cm-3 and 5.91016 cm-3, respectively. We attributed the decrease of trap density after the introduction of PEOXA to the surface trap passivation effect provided by the electron lone pair in the oxygen (and/or nitrogen) atoms in PEOXA. Such passivation effect was also observed in our previous study, which also resulted in an increase in PLQE of the CH3NH3PbI3 perovskite films.48 However, although a better passivation effect was observed in the 55%-PEOXA case, the drop of PLQE is probably due to the increase in crystal size as revealed by the AFM and SEM studies, which leads to a slower radiative-recombination decay rate of the perovskite.48 This result suggested that crystal size control is a critical factor that affecting the PL property of the perovskite film and an optimal CsPbBr0.6I2.4 film with 45% PEOXA was achieved, which is readily to be used as the emissive layer for pure-red-emission PeLED.

Figure 1. (a) The photos of CsPbBr0.6I2.4 thin films with various concentration of PEOXA under day light (upper one) and UV light (below one). (b) PLQEs of CsPbBr0.6I2.4 films with various concentrations of PEOXA (excited at 1 mW/cm2).

Proper thermal annealing is important for optimizing the crystal quality of perovskite films. For CsPbI3 system, as mentioned previously, the phase-transition temperature from the yellow to black phases is over 300 °C.50,51 Encouragingly, we found that the introduction of PEOXA could dramatically reduce the formation temperature of cubic CsPbBr0.6I2.4 perovskites. To evaluate the effect of annealing temperature on the film formation property of the hybrid films, the thin films of CsPbBr0.6I2.4 perovskite with 45%-PEOXA were annealed at different temperatures ranging from 120 °C to 160 °C at a fixed 5



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time of 30 min. The XRD measurement, as shown in Figure 2a, illustrates that after the introduction of PEOXA, CsPbBr0.6I2.4 perovskite started to form when the annealing temperature reached 120 °C. We observed diffraction peaks (2θ) of 14.4°, 20.4°, and 29.1°, which are corresponding to the (100), (110), and (200) planes of CsPbBr0.6I2.4 crystal, respectively. The intensity of the (100) peak in the sample annealed at 150 °C was similar to that of the sample annealed at 300 °C, which meant the low-annealing temperature of 150 °C was enough for complete perovskite formation. Similar phenomenon was observed in previous reports that the coordination between oxygen-contained polymer and CsPbI3 as well as the confinement of CsI by organic materials could reduce the formation temperature of Cs-based perovskite.39,40 The SEM images presented in Figure 2b show that when we annealed the CsPbBr0.6I2.4 thin film with 45% PEOXA at 120 °C, no clear crystal shape could be observed. However, when we increased the annealing temperature to over 130 °C, smooth CsPbBr0.6I2.4 films with uniformly distributed small crystals were obtained, which agreed well with the XRD results. Furthermore, we used transmission electron microscope (TEM) to characterize the crystal distribution and crystal size of the CsPbBr0.6I2.4 film with 45% PEOXA. Figure 2c shows that the CsPbBr0.6I2.4 nanocrystals are dispersed homogenously across the whole perovskite film. As the nanocrystals are embedded in a PEOXA matrix, the crystal size is restricted in the range of round 20 nm ~ 30 nm, as shown in Figure 2d. It can be expected that the separation of nanocrystals by PEOXA can suppress their self-aggregation and hence be beneficial for their long-term stability in perovskite film. In such case, a high-quality CsPbBr0.6I2.4 film with homogenous nanocrystal dispersion was successfully formed.

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Figure 2. (a) XRD patterns of CsPbBr0.6I2.4 films with 45% PEOXA annealed at different temperatures. (b) SEM images of CsPbBr0.6I2.4 thin films with 45% PEOXA at annealing temperatures ranging from room temperature to 160 °C. (c) TEM of CsPbBr0.6I2.4 film with 45%-PEOXA. (d) High-resolution TEM image of crystals in (c), the repeated distance of 4.29 Å indicates the (110) plane of CsPbBr0.6I2.4 lattice.

The morphology study in Figure 2 suggests that on one hand thermal annealing is important to transform the unannealed amorphous film into crystalline film; on the other hand the crystallization temperature can be significantly reduced in the presence of the functional polymer. That triggered us to further study the interaction between PEOXA and perovskite precursor materials and to investigate its templating effect on the growth of the perovskite nanocrystals. Therefore, we employed Fouriertransform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) measurements to exploit the possible interactions within the hybrid films. As shown in Figure 3a, the C=O double bond on the pendant group of pure PEOXA film showed a specific absorption peak at 1655 cm-1, whereas in the unannealed perovskite thin film with 45% PEOXA, the signal of the C=O double bond shifted to 7



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1641 cm-1, suggesting a possible interaction between the C=O double bond and the cations like Cs+ and Pb2+ in the film due to the electron donating nature of the two electron lone pairs on the oxygen atom. XPS measurement was further performed to study the bonding properties between the PEOXA and perovskite precursor materials. Figures 3c and 3d showed that when we dramatically increased the concentration of PEOXA in the perovskite films from 15% to 45% in the unannealed samples, only one single peak of Cs3d was showed with a slight difference in the peak position, reflecting the insignificant interaction between Cs+ and PEOXA. However, in term of Pb4f, we observed two peaks at the binding energy of around 137.5 eV and 135.8 eV, reflecting the various chemical environment of Pb2+. We assigned the peak at 137.5 eV for Pb atom in isolated PbI2 or PbBr2 and assigned the peak at 135.8 eV to the Pb2+ that interacted with PEOXA, according to the peak intensity enhancement at 135.8 eV with the increase of PEOXA concentration. Therefore, the two electron lone pairs of oxygen in the C=O can possibility act as a Lewis base that coordinates with the Pb2+, which then acts as a Lewis acid, in the unannealed film.52-56 In addition, according to the FTIR result in Figure 3a, the specific absorption peak of C=O slightly shifts back to the 1649 cm-1 in an annealed film, illustrating that part of the C=O∙Pb2+ coordination was lost when the CsPbBr0.6I2.4 crystals started to grow. It is also worth to note that the nitrogen atom on the backbone of the polymer may also interact with the metal cations, or more likely providing a collective effect with the C=O for forming coordination bonds to the metal atom. Unfortunately, we cannot observe obvious shift of C-N stretch signal in FTIR as shown in Figure 3b, in both the annealed and unannealed films, suggesting that the nitrogen atom may play a minor role on interacting with the metal cations.

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Figure 3. FTIR measurement of a PEOXA and CsPbBr0.6I2.4 film with 45% PEOXA with or without annealing toward (a) C=O stretch signal and (b) C-N stretch signal, the curves were normalized by the intensity of C=O stretch signal. X-ray photoelectron spectroscopy measurements of (c) Cs3d and (d) Pb4f in a CsPbBr0.6I2.4 films with 15% and 45% PEOXA without annealing.

Based on the characterizations and analyses discussed above, we propose the following mechanism for the perovskite film formation. The schematic diagram presented in Figure 4 shows that in the asprepared film without annealing, the oxygen atoms in the PEOXA have already strongly coordinated with the Pb2+ in the PbI2 or PbBr2, which probably act as heterogeneous nucleation sites to facilitate the nucleation of the perovskite nanocrystal with a lower activation energy. During the annealing process, the nuclei gradually grow into CsPbBr0.6I2.4 nanocrystals, in the meanwhile, the degree of C=O∙Pb2+ coordinated interactions in the hybrid film reduces accordingly. The large amount of heterogeneous nucleation sites provided by the polymer also increases the number density of the nucleus in the film, while eventually leads to the formation of relatively small nanocrystals in the size of 20 nm ~ 30 nm in the optimized hybrid films. Besides the high PLQEs of the nanocrystal films, another advantage of the formation of small nanocrystals is their enhanced phase stability, which had been widely explored to improve the stability of all-inorganic perovskite nanocrystal-based solar cells.35 9



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Figure 4. Schematic diagram of polymer-induced in situ perovskite nanocrystal formation process.

To take one step further, we fabricated perovskite LEDs based on the device architecture of ITO/ZnMgO/perovskite/poly-TPD/MoO3/Ag, as shown in Figure 5a, to study the electroluminescent (EL) properties of the CsPbBr0.6I2.4 films with various concentrations of PEOXA. This device architecture offers a proper energy alignment for both electron and hole injection to the light emitter. As shown in Figure S5, we measured the valance band maximum (VBM) of the ZnMgO NP film and perovskite films using ultraviolet photoelectron spectroscopy (UPS), and then calculated their conduction band based on the corresponding optical bandgaps. The VBM of ZnMgO was -6.1 eV, as calculated by a linear extrapolation of the edge of the valence band spectra. The value of the optical bandgap of CsPbBr0.6I2.4 was calculated by the equation of Eg =

1240 λ ,

where λ is the absorption edge

which is confirmed by the intersection between the absorption edge and the background, as shown in Figure S6. The intersection of 655 nm illustrates the bandgap of CsPbBr0.6I2.4 is around 1.9 eV. Considering the VBM of CsPbBr0.6I2.4 was -5.4 eV, the conduction band minimum (CBM) of the perovskite was calculated as -3.5 eV. The energy level diagram of the whole LED device is shown in Figure 5a. The deep VBM (-6.1 eV) of ZnMgO made it an efficient hole-blocking layer that blocks the injected holes from the anode. Alternatively, the high value of the lowest unoccupied molecular orbital level of poly-TPD (-2.7 eV) made it as an effective electron blocking layer to block electrons injected from the cathode. Therefore, the injected charge carriers are better confined within the emission zone of the emissive perovskite layer. With this optimized device architecture, the LED devices began to light 10


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up at applied voltage as low as 1.5 V. Considering the bandgap of the perovskite emitter was around 1.9 eV, this sub-bandgap EL indicated that such device structure facilitated efficient injection and recombination of charge-carriers, probably through the Auger-assisted energy upconversion which was observed in quantum-dot and perovskite LEDs using ZnO as electron injection layer. 57-59

Figure 5. (a) Device architecture and energy diagram of each layer of the LED device. (b) Current density and luminance versus voltage characteristics of CsPbBr0.6I2.4 LEDs with different concentrations of PEOXA. The solid lines and dotted lines correspond to luminance and current density, respectively. (c) EQE versus voltage characteristic of CsPbBr0.6I2.4 LEDs with different concentrations of PEOXA. (d) EL-spectrum stability of the CsPbBr0.6I2.4 LED with 45% PEOXA. The inset is the photos of a lightup LED at voltage bias of 1.5 V (top) and 3.0 V (bottom). (e) CIE color coordinates of CsPbBr0.6I2.4 LED with 45% PEOXA.

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The performances of the hybrid CsPbBr0.6I2.4 LEDs are summarized in Table 1, Figures 5b and 5c. Due to the poor crystal quality, no EL from the 0%- and 5%-PEOXA samples were observed. When we increased the concentration of PEOXA from 15% to 45%, the device performance was enhanced. The highest luminance of 1669 cd/m2 was achieved in the device with 25% PEOXA at the relatively high current density of 965 mA/cm2. The EQE and current efficiency (CE) became the highest in the 45%PEOXA device, which reached an impressive EQE of 6.55% and a CE of 1.4 cd/A. However, increasing the concentration of PEOXA to 55% resulted in a drop in device performance. There are two possible reasons for these results. First, the EQE data followed the same trend as the PLQE data shown in Figure 1b, suggesting that device performance was governed by the emission properties of the perovskite film. Second, the increase in PEOXA concentration resulted in an improvement in film quality and the suppression of leakage current. As discussed above, the film quality and coverage were improved when we increased the PEOXA concentration from 0% to 45%, but the film morphology became poorer if we further increased the PEOXA concentration to 55%. The changes in the morphologies of the perovskite films also resulted in changes in leakage currents in the perovskite LEDs, as the existence of pinholes could possibly lead to direct contact between the hole and electron injection layers.26 Therefore, according to the J-V curves shown in Figure 5a, the leakage current of the perovskite LEDs reduced with increasing PEOXA concentrations, except when the concentration increased to 55%, in which the leakage current increased again due to poor film morphology. Table 1. Perovskite LED properties with various concentration of PEOXA. PEOXA Ratio

Von (V)

Lmax (cd/m2)

CEmax (cd/A)

EQEmax (%)

CIE (x,y)

0% and 5%

-

-

-

-

-

15%

1.7

664

0.28

1.04

(0.725,0.275)

25%

1.5

1669

0.65

2.42

(0.725,0.275)

35%

1.5

1135

0.83

3.78

(0.726,0.274)

45%

1.5

338

1.36

6.55

(0.726,0.274)

55%

1.6

70

0.26

1.23

(0.726,0.274)

The EL emission peak of the 45%-PEOXA device was located at 668 nm with a narrow full width at a half-maximum (FWHM) of 32 nm (Figure 5d). The LED device exhibited a very pure red emission with CIE color coordinates of (0.73, 0.27) (Figure 5e). In addition, no change in EL spectra was observed at different applied voltages, which suggested that the device obtained an excellent spectral 12


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stability. In addition, in the case of the 45%-PEOXA sample, when the device was operated at a high initial luminance of 300 cd/m2, a 0.5-hours half-lifetime could be obtained, as shown in Figure S6. We also summarize the published performance of perovskite LEDs with red emission in Table S1. Although a longer device lifetime has been reported before, our devices were operated at the higher initial luminance of 300 cd/m2, which is more associated to the actual application requirement for display application, particularly for vivid display. In summary, we demonstrated a new method based on polymer-assisted in situ growth of perovskite nanocrystals to prepare stable CsPbBr0.6I2.4 perovskite films at a relatively low annealing temperature (150 °C). We considered that strong interactions are formed between the functional C=O group in PEOXA and Pb2+ in PbI2 or PbBr2 by Lewis base and Lewis acid coordination, facilitated the growth of perovskite nanocrystal at a lower annealing temperature. The optimized hybrid films contained CsPbBr0.6I2.4 nanocrystals of sizes around 20-30 nm with homogenous distribution, which also showed a high PLQE of 44%. In addition, the small CsPbBr0.6I2.4 crystal size also showed improved phase stability. As a result, highly efficient and relatively stable perovskite LEDs with maximum EQE of 6.55% and luminance of 338 cd/m2 was achieved. Also, the LED showed a low turn-on voltage of 1.5 V and a CIE of (0.73, 0.27), illustrating that the pure-red emission was successfully achieved from this device. In addition, the CsPbBr0.6I2.4 LED with 45% PEOXA presented a half-lifetime of 0.5 hours with a high initial luminance of 300 cd/m2. This result also suggested that polymer/perovskite hybrid emitter could be one of the promising systems for improving the performance and stability of perovskite LEDs.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DIO: . Experimental details, UV-vis absorption, PL spectrum, Atomic force microscope images, scanning electron microscope images, valance band and conduction band calculation, stability test (PDF)

AUTHOR INFORMATION Corresponding Author Email: * [email protected] 13



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Author Contributions † W.C. and Z.C. contributed equally. Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT The work was financially supported by the Natural Science Foundation of China (No. 91733302 and No. 51573057,), and the Science and Technology Program of Guangzhou, China (Grant No. 2017A050503002).

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Table of Contents Pure red emissive perovskite light-emitting diode is a critical component for the applications of vivid display and white-light illumination etc. Here, we demonstrate an efficient method of polymer-assisted in-situ growth for all-inorganic perovskite, to obtain a high-quality perovskite film with homogenous distribution of CsPbBr0.6I2.4 nanocrystals. With this method, we successfully demonstrate a pure-redemission perovskite light-emitting diode with a decent efficiency of 6.55%, luminance of 338 cd/m2 and a half-lifetime of over 0.5 hours at a high initial luminance of 300 cd/m2.

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Polymer-assisted In-situ Growth of All-Inorganic Perovskite Nanocrystal Film for Efficient and Stable Pure Red Light-emitting Devices Wanqing Cai,†1 Ziming Chen,†1 Zhenchao Li,1 Lei Yan,1 Donglian Zhang,1 Linlin Liu,1 Qing-hua Xu,2 Yuguang Ma,1 Fei Huang,1 Hin-Lap Yip,*1 Yong Cao1 1

State Key Laboratory of Luminescent Materials and Devices, Institute of Polymer Optoelectronic Materials and Devices, School of Materials Science and Engineering, South China University of Technology, 381 Wushan Road, Guangzhou 510640, P. R. China 2

Department of Chemistry, National University of Singapore, 117543 Singapore

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Polymer-assisted In-situ Growth of All-Inorganic Perovskite

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