Self-Assembled High Quality CsPbBr3 Quantum Dot Films Toward

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Self-Assembled High Quality CsPbBr3 Quantum Dot Films Toward Highly Efficient Light-Emitting Diodes Shuai Yuan, Zhao-Kui Wang, Ming-Peng Zhuo, Qisheng Tian, Yan Jin, and Liang-Sheng Liao ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b05185 • Publication Date (Web): 10 Sep 2018 Downloaded from http://pubs.acs.org on September 10, 2018

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Self-Assembled High Quality CsPbBr3 Quantum Dot Films Toward Highly Efficient Light-Emitting Diodes Shuai Yuan1, Zhao-Kui Wang*1, Ming-Peng Zhuo1, Qi-Sheng Tian1, Yan Jin1, and Liang-Sheng Liao*1,2 1

Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute

of Functional Nano & Soft Materials (FUNSOM), Soochow University, Suzhou, Jiangsu 215123, China 2

Institute of Organic Optoelectronics, Jiangsu Industrial Technology Research

Institute (JITRI), Wujiang, Suzhou, Jiangsu 215211, P. R. China *Address correspondence to [email protected] (Z. K. Wang), [email protected] (L. S. Liao)

KEYWORDS: perovskite, in-situ grown, quantum dots, LED, trap passivation, excitonic binding energy

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ABSTRACT: Full inorganic cesium lead halide perovskites (IOPs) are regarded as attractive candidates for light-emitting diodes (LEDs) by their excellent luminescent conversion. However, unsatisfactory efficiency and stability are still the main drawbacks that hinder the commercialization progress of perovskite LEDs (PeLEDs). Here, we report an extremely uniform and flat CsPbBr3 film composing of self-assembly core-shell structured quantum dots (SCQDs) based on one-step precursor coating. The QDs size in the CsPbBr3 film is around 4.5 nm (smaller than Bohr radius), which significantly confine injected carriers and lead to a ultra-high exciton binding energy (Eb) of 198 meV. In addition, unfavorable surfacial defects are dramatically passivated by a thin surfacial-capping layer composed of long-chain ammonium groups (phenylalanine bromide, PPABr), resulting in an ultralow non-radiative recombination rate. Consequently, CsPbBr3 SCQDs film presents a high photoluminescence quantum yield (PLQY) of 85%. It enables the resulting green PeLEDs deliver a recorded external quantum efficiency (EQE) over 15% with ideal operational stability. Furthermore, the developed CsPbBr3 SCQDs film also demonstrates promising potential in multifunctional lighting sources such as flexible


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and smart devices.

Metal halide based perovskite materials have been successfully utilized in the fields of photovoltaics, detectors, lasers and light emitting diodes (LEDs).1-5 Particularly, lead halide perovskites present promising potential in LEDs owing to their charming merits of low-cost solution processing, adjustable bandgap and narrow emitting line width.6-12 Since the first demonstration of CsPbX3 (X=Cl, Br, I) quantum dots (QDs) based perovskite LEDs (PeLEDs) by Zeng’s group in 2015,8 the external quantum efficiency (EQE) of perovskite LEDs has approached to more than 10%.13 However, the unsatisfactory efficiency and stability are still the major obstacles that hinder the commercialization progress of PeLEDs. Compared to organic-inorganic hybrid perovskites, full inorganic cesium lead halide perovskites (IOPs) are regarded as more suitable emitters of PeLEDs because of their good thermal and chemical stabilities.12-16 These merits will enable PeLEDs present larger operation bias, higher luminance and better long-term stability. To achieve highly efficient IOPs-based PeLEDs, two main requirements are expected to be fulfilled for the emitting layer. One is superb luminescence properties with extremely high photoluminescence ACS Paragon Plus Environment

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quantum yield (PLQY). In this regard, IOP films should possess uniform grain size comparable with Bohr radius (BR), which is beneficial for increasing exciton binding energy (Eb), resulting in higher radiative rate and faster emitting lifetime.6,17 It should be noted that effective defect-passivation is highly demanded especially for IOPs QDs, since the numerous under-coordinated bonds would cause high possibility of exciton-capture and energy loss.18,19 The other critical factor is excellent film-formatting ability for the emitting layer. An ultra-flat emitting film without any pinholes can ensure ultra-low shunt current, resulting in higher device efficiency especially in the low injection region.11,13 Unfortunately, most IOPs films prepared by either precursor solution or colloid QDs suspension still hardly meet the aforementioned requirements. Since the crystalline nature of IOP films, in-situ deposited perovskite layers generally suffer from large grain size and many non-radiative recombination centers.20-23 Recently, some modifying strategies such as introducing polymer matrix24-26 and adding excess CsBr,20 have been reported to improve the film quality. Nevertheless, the limited increase of PLQY indicates that there is still a large room for its further improvement. Colloid IOP QDs are regarded as ideal alternatives toward LED application owing to their superb optoelectronic properties with near 100% luminescence conversion.13,

27-29

Unfortunately, their

corresponding solid-state films still hardly escape the weaknesses of film formation and electric properties due to immature ligand engineering.30-32 Despite each approach has respective drawbacks, probably, a tremendous advance could be taken forward based on a synthetic effect via combining the advantages of these methods. During


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preparation of this work, Zhao and coworkers have demonstrated the feasibility of such a strategy and achieved unneglectable devices’ performance based on hybrid perovskite.33

Herein, we report an extremely uniform and flat cesium lead bromide (CsPbBr3) film composing of self-assembly core-shell structured QDs (SCQDs) with effective defect-passivation. The formed QDs in a one-step precursor spin-coated CsPbBr3 film are 4.5 nm in average size (smaller than the BR (7 nm) of CsPbBr3), which effectively confine excited electron-hole pairs and lead to an ultra-high Eb of 198 meV. More importantly, the under-coordinated bonds on the surface of CsPbBr3 QDs are significantly passivated by a thin surfacial-capping layer composed of long-chain ammonium

group

(phenylalanine

bromide,

PPABr).

Consequently,

a

low

non-radiative recombination rate of 1.2 × 106 s-1 and a high PLQY of 85% for the CsPbBr3 SCQDs film are obtained. These merits enable the resulting green PeLEDs deliver a maximum EQE more than 15% with ideal operational lifetime. And a trial fabrication of relatively large-area (2 × 2 cm2) flexible PeLEDs demonstrates the promising potential of CsPbBr3 SCQDs film in multifunctional lighting

RESULTS AND DISCUSSION A hot casting technique was developed to deposit CsPbBr3 SCQDs film, where a subtly tuned precursor solution (mixture of PPABr, CsBr, and PbBr2 in DMSO) was dipped onto a spinning hot quartz substrate. We found that the spinning film turned to light green after 20 s at 5000 rpm despite the coordination interaction between DMSO


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and PbBr2,17 indicating a prompt formation of PPABr-CsBr-PbBr2 components due to easily produced Lewis acid-base adduct from amidogen and lead ions.17,33 The resulting CsPbBr3 SCQDs film with molar ratio of PPABr, CsBr and PbBr2 at 0.6, 1.1, 1 possessed full-coverage and pinhole-free surface, as characterized by scanning electron microscope (SEM) shown in Figure 1a. A further inspection based on improved resolution SEM image (Figure S1, Supporting Information) illustrated that the film was consisted of compactly stacked nanoparticles with average diameter around 15 nm, which is related to a growth-impede effect induced by the bulky molecule of PPABr. Generally, the long chain molecules inserted into the space between lead-halide octahedron can suppress the growth of crystal nucleus, which is highly beneficial for emitting properties because of decreased diffusion length and effective spatial confinement of charge carries.6,

11

Figure 1b shows the film

morphology of CsPbBr3 SCQDs film probed by atomic force microscope (AFM). The accurate root-mean-square (RMS) roughness of SCQDs film is just 0.6 nm, which is highly desirable for emitting layer by the foreseeable virtue of leakage screen. Figure 1c records the structural composition of CsPbBr3 SCQDs film revealed by X-ray diffraction (XRD). The smooth CsPbBr3 film presented a cubic phase rather than low-dimensional Ruddlesden-Popper phase. The main diffraction peaks located at 15.16°, 21.52°, 30.63°, 37.58° and 43.52° can be assigned to (100), (110), (200), (210), (211), and (202) plane, respectively,13 confirming the cubic phase of CsPbBr3 SCQDs film. The morphological and structural characteristics provide topological and compositional features of the CsPbBr3 SCQDs film. Interestingly, further calculation


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from (200) diffraction peak based on Scherrer Formula (Supporting information) indicates that the average grain size of CsPbBr3 film is just about 6 nm, which is very different from the direct observation.

To gain deep understanding of the CsPbBr3 SCQDs film, transmission electron microscopy (TEM) measurements were carried out by peeling off the as prepared film from quartz and dispersing in anisole before attaching on copper grid. Figure 2a shows a clear image of nearly monodispersed QDs with a heterogeneous capping layer. This structure is further confirmed by the magnified TEM image (Supporting Information, Figure S2). Figure 2b and c present the high revolution TEM image of a typical QD and corresponding fast Fourier transform (FFT) filtered image, respectively. The lattice constant is determined to be 0.31 nm, which is assigned to the (111) plane of cubic phased CsPbBr3 perovskite. The FFT pattern with discriminable diffract-spots recognized as (111) and (200) planes further identified the high crystallization of CsPbBr3 QDs film. Noticeably, the encapsuled capping layer of PPABr is amorphous in nature. A schematic of compositional feature of CsPbBr3 SCQDs film is shown in Figure 2d. Figure 2e gives a statistical histogram of QD size fitted from Figure 2a. CsPbBr3 QDs had narrow distributed size with an average value of 4.5 nm, which is closed to the calculated result from XRD pattern based on Scherrer analysis. As the average size is even smaller than the BR (7 nm) of CsPbBr3 perovskite,16 CsPbBr3 SCQDs film is expected to exhibit outstanding optical properties including absorption and photoluminescence (PL) spectra. Under excitation wavelength of 400 nm, the SCQDs film exhibited a green emission with a


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Lorentz-shaped PL peak located at 510.5 nm (Supporting Information, Figure S3). From the absorption edge at 510 nm, the energy band gap of CsPbBr3 SCQDs is extrapolated to be 2.43 eV. A separation between absorbance and emission spectra is desired in practical application either electroluminescence (EL) or PL by avoiding the self-absorption loss.

The amount of long-chain organic molecules dominates the formation mechanism of CsPbBr3 SCQDs film. We tuned and investigated the CsPbBr3 SCQDs film by adjusting the molar ratio of PPABr and PbBr2 from 0 to 1.2, as shown in Figure S4a-4d. From TEM images and statistical diameter distribution, the average particle size of CsPbBr3 particles continuously decreased from 15 nm, 4.5 nm to 3 nm for the molar ratio of 0.3, 0.6, and 1.2 samples (the pure CsPbBr3 can refer to Figure S6), respectively. Meanwhile, the thickness of long-chain organic molecules formed shell was increased. The morphology of CsPbBr3 particles showed obvious change from incomplete capped block (0.3) to SCQDs (0.6), and finally forming QDs-in-matrix distribution (1.2). At the beginning of nucleate stage, the long chain molecules inserted into the terminal space of lead-halide octahedron, which dramatically inhibited the further growing of CsPbBr3 crystal nucleus (CCN). With increasing the amount of PPABr, vast majority surface of CCN were pinned leading to significant growth suppression. With further increasing the amount of PPABr, the aggregation of molecular become more obvious in the space, which would severely affect the conductivity and result in bad carriers injection and Joule heat in device. The corresponding PL peak continuously shifted from 525 nm (pure CsPbBr3) to 505


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nm (Figure S4e and 4f). Similar results also can be extracted from the absorption spectra (Figure S4g). Such blueshift phenomenon confirms the quantum confinement effect of SCQDs film.

To better investigate the emitting properties of the CsPbBr3 SCQDs (0.6), we further conducted a set of optical measurements and characteristics. Figure 3a shows the excitation-power-density dependent PLQYs which were measured by using a fluorescence spectrometer with an integrated sphere. The CsPbBr3 SCQDs film presented extremely high PLQY over 80% in a wide range of excitation power density (2.45~28 mW/cm2). A maximum PLQY of 85%, which is indeed a superb performance in solid-state perovskite films, was approached at 3.5 mW/cm2. Even under extremely low excitation power density of 0.035 mW/cm2, the SCQDs film still maintained a high PLQY of 77%, implying easy trap-filling process and ultra-low trap density in the CsPbBr3 SCQDs film. Furthermore, transient PL measurements were performed as shown in Figure 3b. The CsPbBr3 SCQDs film exhibited a rapid radiative recombination path with average decay time of 12 ns (Supporting Information, Table S1). The fast emitting process means an excellent luminescent performance due to the feature of exciton recombination in the CsPbBr3 SCQDs film. To confirm the emitting property, temperature dependent PL spectra were implemented as shown in Figure 3c. Peak position and integrated intensity of PL spectra changed slightly as varying the temperature from 80 K to 310 K. Furthermore, the Eb of CsPbBr3 QDs was calculated by following equation:34


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It =

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I0 1 + A exp(

Eb ) kbT

(1)

where I0 is original PL intensity at 0 K, kb is Boltzmann constant and A is pre-exponential constant. The fitting results via collecting the integrated PL intensity at different temperature are presented in Figure 3d. The calculated Eb was as high as 198±0.8 meV, which is the highest value to the best of our knowledge. Since the exciton formation and dissociation is very fast, the conversion between excitons and free carriers are persisting in entire emitting process. To understand the kinetics of free carriers and excitons, the exciton dissociation rate (Pdiss) was calculated from the modified saha equation:35,36

Pdiss = v exp( −

Eb ) kbT

(2)

where v = Eb/h (~ 2 × 10-12/s) is the attempt frequency, and h the Planck constant. As a consequence, Pdiss was on the order of 109/s, and exciton yield was calculated to be 102/s by the ratio of Pf/Pdiss , where exciton formation rate (Pf) is on the order of approximately 1011/s.37,38 In this regard, the emitting process of CsPbBr3 SCQDs film is totally dominated by excitons rather than slow bi-molecule path. Furthermore, the exciton lifetime (τr) and other kinetics parameters were calculated by combining PLQY and transient PL results (supporting information). Importantly, the non-radiative recombination (knr) was as low as 1.2 × 106/s, which is two orders of magnitude lower than the reported ones.11,26 In general, once the particle size is decreased to nanoscale level, most ions will inevitably expose to the surface,


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generating numerous under-coordinated bonds as charged defects.39 Aside from the large Eb and short emitting lifetime, such a low non-radiative recombination rate is intimately associated with the ultralow defect density in the CsPbBr3 SCQDs film owing to the passivation of PPABr capping layer.

Aforementioned considering is further clarified by intentionally separating the PPABr capping layer from the CsPbBr3 SCQDs. The CsPbBr3 SCQDs were exfoliated from the substrate and dispersed into two batch of anisole for preparing two suspensions A and B. Solution B was processed by ultrasonic treatment (40 min) with the goal of removing PPABr. The corresponding TEM measurement verified the successfully skinning treatment (Supporting Information, Figure S5). As shown in

Figure 4a, Fourier transform infrared spectroscopy (FTIR) were carried out to compare the structural alteration of samples A and B. For comparison, the feature information of pristine PPABr and pure CsPbBr3 are also presented. The peaks located at 697.2 cm-1 and 747.6 cm-1 in PPABr were associated with the plane deformation of benzene ring.40,41 Meanwhile, pure CsPbBr3 also showed three diacritical peaks in the low wavenumber region. In sample A, signals presented at 701 cm-1 and 740 cm-1 were slightly differ with those in pristine PPABr. We ascribe it to the surfacial interaction between PPABr bulky molecule and CsPbBr3 QDs.11,42 In sample B, the peak positions and intensities were almost the superposition from pristine PPABr and pure CsPbBr3, indicating an absent bonding between two components. The in-situ TEM observation of aggregation process of CsPbBr3 QDs (Supporting Information, Figure S6) further revealed the blanked surface of skinned


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QDs. Since the active dangling bonds exposed outward without any passivation, CsPbBr3 QDs preferred to merge each other in order to reduce the surface energy. The aggregation phenomenon was also observed in degradation process. After two days’ aging, suspension A still maintained decent dispersity (Supporting Information,

Figure S7a). In contrast, aggregated yellow clusters were observed in suspension B, which was confirmed as CsPbBr3 by XRD (Supporting Information, Figure S7b). To further investigate the active surface of skinned QDs, X-ray photoelectron spectroscopy (XPS) were performed as shown in Figure 4b and 4c. Pb4f7/2 and Pb4f5/2 peaks were presented in both samples A and B. However, after removing PPABr capping layer, additional components located at 137.2 eV and 142.1 eV with respect to metallic Pb6,43 were observed in sample B. The generation of metallic Pb is attributed to the disrupted chemical bonding between PPABr and CsPbBr3. It is agreed well with the FTIR analysis. Due to the under-coordination states, the dangling bonds of metallic Pb located at QDs surface not only induce aggregation under external stimulus but also cause negative effect on emitting by capturing excitons. Optical properties including PLQY and transient PL measurements of solutions A and B were further conducted as shown in Figures S8 (Supporting Information). Suspension A still maintained a relative high luminescent efficiency of 70%. Compared to the solid film, the slight decreased PLQY is most likely ascribed to the changed chemical surrounding caused by anisole. The corresponding radiative lifetime (12.6 ns) was similar to that of solid-state film. In contrast, the PLQY of skinned QDs (suspension B) dramatically droped to 6% along with a ultrashort


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emitting time of 3 ns and a large knr of 1.19 × 108/s due to the surfacial quenching of metallic Pb.6 Moreover, the emitting center of suspension B showed a slight blue shift to 508 nm (Supporting Information, Figure S9). We ascribe it to the absent of image charge effect due to the large difference of dielectric constants between PPABr and CsPbBr3.44-46 To summarize, the outstanding optical properties of CsPbBr3 SCQDs film is beneficial from two main merits: (1) extremely high Eb induced by uniform and small size of QDs; (2) effective surfacial passivation originating from capped organic layer. These virtues would enable the CsPbBr3 SCQDs have promising potential in LEDs.

Prior to fabricate devices, we deposited SCQDs film on stacked layers of poly (3, 4-ethylenedioxythiophene):

poly

(styrenesulfonate)

(PEDOT:PSS)

and

poly(Nvinylcarbazole) (PVK) to verify its selectivity upon substrate. Relevant morphology and emitting properties pledge its further application in practical LEDs (Supporting Information, Figure S10). Figure 5a shows a schematic of CsPbBr3 SCQDs based PeLEDs with a structure of ITO/ PEDOT:PSS/ PVK/ CsPbBr3 (25 nm) /1,3,5-tris(N-phenylbenzimidazol-2-yl) benzene (TPBi)/ Liq/ Al. Here, PEDOT:PSS is utilized as a buffer layer to form ladder-shaped hole injection path between ITO and PVK (Supporting Information, Figure S11), the cross-sectional SEM image is shown in Figure S12 (Supporting Information). Current density-luminance-voltage (J-L-V) characteristics are shown in Figure 5b. The device presented an extremely low shunted current below 10-5 mA/cm2 (Supporting Information, Figure S13). As the bias was over turn-on voltage, both of current density and luminance increased


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sharply. The beginning luminance rapidly rised to 4 cd/m2 under an extremely low current density of 0.01 mA/cm2, implying an very low trap density in the CsPbBr3 SCQDs film. Owing to the feature of easy trap-filling, the device exhibited very high efficiency at starting regime as shown in Figure 5c. The highest EQE was up to 15.17% under the voltage of 3.9 V. In addition, the EQE still kept 5% at maximal luminance of 18600 cd/cm2 (@7.3 V). The good performance is attributed to the tough coulomb interaction between injected electron and hole induced by the large Eb. The EL peak was located at 512 nm without any alteration under various voltages as shown in

Figure 5d. And the line width of EL is as narrow as 21 nm corresponding to Commission Internationale del’Eclairage (CIE) colour coordinates of (0.063, 0.725) (Supporting Information, Figure S14). The angular emission intensity of the device followed a Lambertian profile (Supporting Information, Figure S15). Figure 5e plots a histogram of EQEs with an average value of 14% for 32 devices, indicating the superb repeatability of CsPbBr3 SCQDs based PeLEDs. The device’s performance based on pure CsPbBr3 film and SCQDs film with different amount of long-chain ammonium groups(PPABr) are exhibited in Figure S16 (Supporting Information). It is easily to be gained that with increasing the amount of PPABr, PeLEDs exhibited better EQE and max luminance, but the carriers injection and EQE drop dramatically in the case with PPABr at 1.2, which is attributed to its poor conductivity. To further verify the operational stability of SCQDs device, we drove a device under fixed current density of 2.5 mA/cm2 with original luminance at 1000 cd/m2, meanwhile, a device with pure CsPbBr3 film was tested under 12 mA/cm2 as comparison. The


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corresponding results are shown in Figure 5f. The T50 of those two devices are 1.2 h and 8 min, which confirms the ideal operational lifetime of SCQDs PeLED. A high luminance test with luminance at 14000 cd/m2 is shown in Figure S17 (Supporting Information), the T50 at such level is 5min, this result is truly inspiring. To verify the long-term stability of encapsulated device, especially towards materials’ chemical stability. Stored SCQDs and CsPbBr3 based devices were intermittently tested under safe range (50 cd/m2 at the beginning, current density 0.1 mA/cm2 for SCQDs based device and 1.1 mA/cm2 for CsPbBr3 based device) in order to avoid the destruction from Joule heat and ion migration under large injection current density (Figure 5g), The plot of normalized EQE as a function of time demonstrated that the device toughly held 80% of the initial efficiency after 12 days.

The CsPbBr3 SCQDs film with decent large area film formation is also regarded as a promising potential in functional lighting. We further fabricated flexible PeLED by attaching multilayer structure on PET/ITO substrates. Figure 6a shows the photographs of flexible PeLED with an emitting area of 2 × 2 cm2. And the emitting uniformity of the large area device from 5 different spots were carried out as shown in

Table S3 and Figure S18 (Supporting Information). Encouragingly, the average luminance error was less than 5%. As representative, the device performance recorded from spot 5 are presented in Figure 6b and Figure 6c. Compared to the small-area (0.1 cm2) device, the large-area device delivered slightly high driving voltage at same luminance. Moreover, the flexible device demonstrated good mechanical stability by keeping 90% of original efficiency after 50 times bending test with radius of 1 cm


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(Figure 6d). Considering the impressive PLQY of CsPbBr3 SCQDs based solid-state film, fluorescent application in terms of optical filter or luminescence solar concentrator (LSC)47 are expected. Figure 6e and Figure 6f show the photographs of a brief LSC composed of stacked layers of CsPbBr3 SCQDs/ PMMA under excitation of UV lamp (356 nm) and AM 1.5 solar simulator (100 mW/cm2), respectively. Combining the bright edge emitting and less overlap between absorption and PL, the CsPbBr3 SCQDs based LSC is considered as a promising candidate for energy glass, especially in field of exterior wall decoration of buildings.

CONCLUSION

In summary, we have demonstrated an extremely uniform and flat CsPbBr3 film composed of self-assembly core-shell structured QDs with efficient defect-passivation by a thin surfacial-capping layer of PPABr. The self-assembly CsPbBr3 SCQDs presented ultra-high Eb and low non-radiative recombination, resulting in a superb PLQY of 85%. The corresponding green PeLEDs exhibited a recorded peak EQE of 15.17%. It should be noted that, an ultra-high EQE of 12.4% was achieved under low injection current density of 0.01 mA/cm2 owing to the feature of easy trap-filling. In addition, the advantage of large-area film formation ability enable the CsPbBr3 SCQDs film be utilized in multifunctional lighting. The fabricated flexible LED with large emission area of 4 cm2 also demonstrated superior EQE more than 10%. The developed CsPbBr3 SCQDs film provides the diversification for fabricating high-performace and stable PeLEDs.


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METHODS Materials: CsBr, PbBr2, DMSO (anhydrous, 99.9%), hydrobromic acid (HBr), phenylalanine (PPA) and were purchased from Sigma-Aldrich. TPBi (99.9%) and Liq were purchased from You Xuan Chemical Co. PEDOT:PSS (Clevio P VP Al 4083) was

purchased

from

Heraeus.

PVK

was

purchased

from

Acros.

3-(decyldimethylammonio) propane-1-sulfonate was provided by San Jia Chemical Co. All the materials were directly used without any further purifcations. PPABr was synthesized by adding HBr into a solution of PPA in tetrahydrofuran at 0 ℃ and then stirred for 2 h, the excess HBr was used in order to ensure full reaction of PPA. Next, the solution was rotary evaporated at 50 ℃ to obtain white precipitates, which were redissolved in ethanol and washed with diethyl ether for 3 times. Finally, the product was vacuum-dried at room temperature for 24 h. The precursor solutions for the SCQDs film were prepared by dissolving PPABr, CsBr and PbBr2 in DMSO with different molar ratio of x:1.1:1 (x= 0, 0.3, 0.6, 1.2), here PbBr2 was fixed at 0.3 M. The solution was then stirred at 60 ℃ overnight. Then 3-(decyldimethylammonio) propane-1-sulfonate was added in precursor with a ratio of 1% wt. The completed precursor was then stirred for 1h.

SCQDs film deposition and characterization: The precursor was filtrated and heated at 70 ℃, then immediately dipped onto a spinning quartz wafer, the quartz was preheated at 90 ℃ and the rotating speed was set as 6000 rpm, followed by annealing at 70 ℃ for 5 min. SEM images of layers were measured with a Carl Zeiss Supra 55. The thicknesses of the SCQDs film were measured by an ellipsometry (Alpha-SE). AFM ACS Paragon Plus Environment

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measurements were conducted on a Cypher-S atomic force microscope. XRD spectra were obtained by X-ray diffractometer with a Cu Kα source (PANalytical B.V. Empyrean). Transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HR-TEM) and selected area electron diffraction (SAED) were performed using a JEM-2100F TEM (JEOM, Japan). Light absorption spectra of SCQDs layer was acquired by a UV-vis spectrometer (SPECORD S 600). PL spectra, PLQY, excitation power intensity and temperature dependent PL spectra were collected

by

fluorescence

spectrophotometer

(Fluromax

4).

The

temperature-dependent measurements were performed by using a closed-cycle nitrogen cryostat, excitation power intensity dependent PL was carried out by employing neutral filter, and the excitation power density was monitored by Thorlabs PM100D optical meter. PL decay lifetimes were acquired by fluorescence spectrophotometer (HORIB-FM-2015). FTIR measurement were conducted by using absorption infrared spectrometer (VERTX 70), here all samples were prepared by mixing anisole solution and KBr powder. XPS was performed on the Thermo Scientific ESCALab 250Xi using 200 W monochromated Al Ka (1,486.6 eV) radiation. The 500 mm X-ray spot was used for XPS analysis. Typically, the hydrocarbon C1s line at 284.8 eV from adventitious carbon was used for energy referencing.

Devices Fabrication and characterization: ITO-patterned glass substrates were cleaned sequentially with detergent water, acetone, and ethanol for 30 min by sonication, respectively, then dried with nitrogen and followed by oxygen plasma treatment for


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20 min before using. All the devices were fabricated with multilayers structure consisting of ITO/PEDOT: PSS/PVK/CsPbBr3 SCQDs film/TPBi/Liq/Al. The PEDOT:PSS solution was fltered by 0.2 µm hydrophilic poly(tetrafluoroethylene) syringe flters before using. The PEDOT:PSS solution was spin-coated onto ITO substrates at 5000 rpm for 35 s, following by annealing at 150 °C for 10 min in air to remove the residual water. Then substrates were moved to glove box for further deposition. PVK films were spin-coated onto PEDOT:PSS from a 6 mg/ml chlorobenzene solution at 5000 rpm, then annealing treatment were implemented at 120 ℃ for 15 min. Next, CsPbBr3 SCQDs film were spin-coated on the preheated substrates. Finally, the device fabrication process was finished by depositing TPBi (40 nm), Liq (2 nm), and Al (100 nm) layers in thermal evaporation chamber with vacuum pressure below 4 × 10-4 Pa. The active area of the small device is 0.1 cm2. For flexible and large-area (4 cm2 in emission area ) device, the PET/ITO substrates with pre-patterns were used. The EL spectra and electrical output characteristics of CsPbBr3 SCQDs based LEDs were collected under ambient condition using a Keithley 2400 sourcemeter and a Photo Research spectrometer PR650. The devices were swept from zero bias to forward bias.

Conflict of Interest: The authors declare no competing financial interest.

Acknowledgement: We acknowledge financial support from the Natural Science


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Foundation of China (Nos. 91733301, 61674109, 11675252, 11605278), the National

Key

R&D

Program

of

China

(No. 2016YFA0202400), and from

the Natural Science Foundation of Jiangsu Province (No. BK20130288). This project is also funded by the Collaborative Innovation Center of Suzhou Nano Science and Technology, and by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

Supporting charge

Information

Available:

This

material

is

available

free

of

via the Internet at http://pubs.acs.org.

Brief summary of SEM, AFM, TEM, PL, absorption information of CsPbBr3 SCQDs film and skinned CsPbBr3 SCQDs particle, device performance including J-V, EQE and operation lifetime, Scherrer analysis and recombination parameters.

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Figures and Tables


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Figure 1. Morphologic and compositional characters of as prepared CsPbBr3 SCQDs film. (a) SEM image, (b) AFM image, and (c) the corresponding XRD pattern, (PDF#54-0752, the vertical lines). Scale bar: 100 nm.


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Figure 2. TEM analysis about the structural information of CsPbBr3 SCQDs film. (a) Low resolution TEM image, (b) HRTEM images of a typical CsPbBr3 QD, (c) FFT-filtered HRTEM image (inset is corresponding FFT pattern), (d) schematic of compositional feather of SCQDs film, and (e) histogram of CsPbBr3 QDs size distribution with a distribution fit.


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Figure 3. Optical properties of CsPbBr3 SCQDs film. (a) Excitation power density dependent PLQY under exciting wavelength of 400 nm. Inset: photograph of CsPbBr3 SCQDs film excited under UV lamp (wavelength: 356 nm). (b) Transient PL of corresponding film with fitting. (c) Temperature dependent PL spectrum from 80 to 310 K, (d) the relevant integration of PL intensity.


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Figure 4. (a) FTIR spectra of PPABr, CsPbBr3 SCQDs (suspension A), skinned CsPbBr3 SCQDs (suspension B) and pure CsPbBr3. XPS spectra of (b) CsPbBr3 SCQDs and (c) skinned CsPbBr3 SCQDs.


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Figure 5. (a) Schematic of CsPbBr3 SCQDs based PeLEDs. (b) Current density and luminance as a function of driving voltage. (c) Current efficiency and EQE vs luminance characteristics. (d) EL spectra based on various voltage (inset: the photograph of the corresponding device operated under 4V). (e) Histogram of peak EQEs for 32 devices. (f) Operation stability of device with initial luminance at 1000 cd/m2, SCQDs LED at fixed current density of 2.5 mA/cm2 and pure CsPbBr3 LED at fixed current density of 12 mA/cm2. (g) Long-term stability with intermittent testing of stored devices. Original luminance at 50 cd/m2, the driven current density are 0.1 mA/cm2 for SCQDs device and 1.1 mA/cm2 for pure CsPbBr3 device, respectively.


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Figure 6. (a) Photographs of flexible PeLED with emission area of 2 cm × 2 cm. (b) I-V-L curves of the corresponding device. (c) CE and EQE as a function of luminance. (d) Normalized EQE as a function of bending times. Photographs of LSCs based on CsPbBr3 SCQDs film with PMMA coating layer deposited on glass substrate (7 cm × 7 cm), (e) under excitation of UV lamp (wavelength: 365 nm) and (f) a AM 1.5 solar simulator (100 mW/cm2).


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ToC Figure An extremely uniform and flat cesium lead bromide (CsPbBr3) film composing of self-assembly core-shell

structured

quantum

dots (SCQDs)

with effective

defect-passivation by phenylalanine bromide (PPABr) is reported. The formed QDs in CsPbBr3 film are 4.5 nm in average size, which effectively confine excited electron-hole pairs and lead to an ultra-high Eb of 198 meV. As a result, CsPbBr3 SCQDs films exhibit a low non-radiative recombination rate of 1.2 × 106 s-1 and a high PLQY of 85%, which enable the resulting green PeLEDs deliver a recorded EQE of more than 15% with ideal operational stability.


Self-Assembled High Quality CsPbBr3 Quantum Dot Films Toward

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