Highly Luminescent and Stable Perovskite Nanocrystals with

Jan 4, 2018 - All inorganic perovskite nanocrystals (NCs) of CsPbX3 (X = Cl, Br, I, or their mixture) are regarded as promising candidates for high-pe...
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Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 3784−3792

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Highly Luminescent and Stable Perovskite Nanocrystals with Octylphosphonic Acid as a Ligand for Efficient Light-Emitting Diodes Yeshu Tan,†,§ Yatao Zou,†,§ Linzhong Wu,† Qi Huang,† Di Yang,† Min Chen,† Muyang Ban,† Chen Wu,† Tian Wu,† Sai Bai,‡ Tao Song,*,† Qiao Zhang,*,† and Baoquan Sun*,† †

Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University, Suzhou 215123, People’s Republic of China ‡ Department of Physics, Chemistry and Biology (IFM), Linköping University, SE-581 83 Linköping, Sweden S Supporting Information *

ABSTRACT: All inorganic perovskite nanocrystals (NCs) of CsPbX3 (X = Cl, Br, I, or their mixture) are regarded as promising candidates for high-performance light-emitting diode (LED) owing to their high photoluminescence (PL) quantum yield (QY) and easy synthetic process. However, CsPbX3 NCs synthesized by the existing methods, where oleic acid (OA) and oleylamine (OLA) are generally used as surface-chelating ligands, suffer from poor stability due to the ligand loss, which drastically deteriorates their PL QY, as well as dispersibility in solvents. Herein, the OA/OLA ligands are replaced with octylphosphonic acid (OPA), which dramatically enhances the CsPbX3 stability. Owing to a strong interaction between OPA and lead atoms, the OPA-capped CsPbX3 (OPA-CsPbX3) NCs not only preserve their high PL QY (>90%) but also achieve a high-quality dispersion in solvents after multiple purification processes. Moreover, the organic residue in purified OPA-CsPbBr3 is only ∼4.6%, which is much lower than ∼29.7% in OA/OLA-CsPbBr3. Thereby, a uniform and compact OPACsPbBr3 film is obtained for LED application. A green LED with a current efficiency of 18.13 cd A−1, corresponding to an external quantum efficiency of 6.5%, is obtained. Our research provides a path to prepare high-quality perovskite NCs for highperformance optoelectronic devices. KEYWORDS: perovskite nanocrystals, octylphosphonic acid, photoluminescence, dispersibility, light-emitting diodes

1. INTRODUCTION Perovskite light-emitting diodes (LEDs) have attracted wide attentions over the past 3 years due to their solution processability, low cost, and tunable electroluminescence (EL) spectra with a narrow full width at half-maximum (FWHM).1−6 To boost the LEDs’ efficiency, low-dimensional perovskites, such as two-dimensional (2D) and colloidal nanocrystals (NCs) type perovskites, are regarded as preferable light emissive layer in the LEDs due to their large exciton banding energy and a better film morphology.7−9 External quantum efficiency (EQE) of the organic halide perovskite (OHP) quantum-well based LEDs has reached a record efficiency of 11.7% recently.10 Compared with the OHP ones, the all-inorganic perovskite (AIP) usually exhibits a better thermal and moisture stability.11−15 Yet, a smaller exciton binding energy (∼a few meV) and a discontinuous film morphology of the three-dimensional (3D) perovskite limit the LED device performance.16 Since Protesescu et al. reported the synthesis of AIP CsPbX3 (X = Cl, Br, I, or their mixture) NCs by the hot-injection method,12 efforts have been made to fabricate AIP NCs based LEDs.5,17−22 However, the present assynthesized CsPbX3 NCs, which are capped with surface ligands of oleic acid (OA) and oleylamine (OLA), suffer from © 2018 American Chemical Society

ligand loss during the purification process due to the weak interaction between these surface ligands and NCs. The photoluminescence (PL) quantum yield (QY) and dispersibility in solvents and the structural stability are deteriorated after the purification process of perovskite NCs.15 The capped organic ligands allow CsPbX3 NCs to be dispersed in organic solvents and passivate defect sites on the CsPbX3 NCs surface.23 Therefore, any ligand loss induces defect sites on the CsPbX 3 NCs surface, increasing the nonradiative recombination chances in LED devices.24 To suppress these defect sites, Tan and co-workers demonstrated an atomic layer deposition layer of alkyl aluminum to passivate CsPbBr3 NCs active layers surface defects and fabricated efficient perovskite LEDs.18 In addition, there are two innovative works to enhance the stability of AIP NCs by tuning their surface chemistry.25,26 Recently, Li et al.’s pioneering work reported CsPbBr3-based LEDs with an EQE of 6.27% by smartly controlling the surface ligands density.15 Mixed antisolvents of hexane/ethyl acetate with different ratios were selected to tune their polarity to Received: November 10, 2017 Accepted: January 4, 2018 Published: January 4, 2018 3784

DOI: 10.1021/acsami.7b17166 ACS Appl. Mater. Interfaces 2018, 10, 3784−3792

Research Article

ACS Applied Materials & Interfaces

Figure 1. CsPbX3 NCs synthesized using OPA as capping ligands. (a) Fluorescence photographs of the OPA-CsPbX3 NC solution excited under 365 nm UV lamp. (b) PL and UV−vis absorption of the OPA-CsPbX3 NCs with different halide compositions. (c) TEM image of OPA-CsPbBr3 NCs. Inset shows the corresponding HRTEM image.

show a current efficiency of 18.13 cd A−1, corresponding to an EQE of 6.5%, which is ∼8 times higher in comparison with the OA/OLA-CsPbBr3-based one. Our work provides a way to synthesize highly stable CsPbBr3 NCs by simply tuning the surface-anchoring ligands for all-solution-processed high efficiency optoelectronic devices.

precipitate CsPbX3 NCs out of the mother solution with minimal possibility to lose any surface ligand. Nevertheless, alkyl amine ligands are still involved in the synthesis process, which means the proton transfer among the acid−base ligands is not completely overcome. We noticed that the recent work demonstrated an amine-free method, where quaternary alkylammonium halides were used as precursors to eliminate the need of OLA to enhance the CsPbX3 NCs stability.27 However, the LED based on the bare OA-capped CsPbX3 NCs only yielded a peak EQE of 0.325% due to the moderate EL intensity. In this work, a novel strategy to synthesize high-quality CsPbBr3 NCs that can preserve both high PL QY and good dispersibility with less organic residue for high-performance LEDs is proposed. In our hot-injection method, octylphosphonic acid (OPA) is used as the capping ligand to synthesize CsPbX3 NCs. The OPA-capped CsPbBr3 NCs (OPA-CsPbBr3) exhibit a high PL QY up to ∼90%, a narrow PL FWHM (∼19 nm), and an obviously enhanced colloidal stability. Two chelating sites of OPA with lead ions provide a strong interaction with CsPbX3 NCs. More than ∼80% PL QY of the OPA-CsPbBr3 NCs remained even after four times of antisolvent-assisted washing, whereas the OA/OLA-capped CsPbBr3 NCs (OA/OLA-CsPbBr3) only display a low PL QY of ∼20% with the same purification steps, indicating the ultrastable characteristics of the OPA-capped perovskite NCs. Generally, the residual organic ligands degenerate the perovskite film morphology, increasing the leak current and blocking the charge injection in LEDs. There is only ∼4.6% of organic residue in the OPA-CsPbBr3 NCs after washing with an antisolvent for two times compared with ∼29.7% in the OA/ OLA-CsPbBr3 ones. Consequently, a uniform and compact OPA-CsPbBr3 NCs film with an impressive air stability can be obtained. Green LED devices based on the OPA-CsPbBr3 NCs

2. RESULTS AND DISCUSSION 2.1. Synthesis of Perovskite NCs. Here, OPA is used to replace OA/OLA to synthesize CsPbBr3 NCs with the hotinjection method.12 Normally, the noncoordinating solvent octadecene (ODE) is used to dissolve cesium acetate and lead salts in the presence of coordinating chelates of both fatty acids and amines. On the contrary, OPA can dissolve all of the precursor without amines due to its enhanced acid-dissociation characteristic. In the OPA system, all of the precursor turns into a colorless solution at temperature as low as ∼75 °C. However, a temperature of ∼110 °C is required to obtain a colorless solution in the OLA/OA system. The temperature difference reveals that OPA-lead couple has a better solubility in ODE. Cesium carbonate is replaced with cesium acetate to enhance the cesium solubility in ODE.13 Figure 1a shows the digital fluorescence photographs of all of the inorganic CsPbX3 NCs with different halides (Cl, Br, I, or mixed halide systems). It reveals that PL emission can be easily tuned from violet to red by controlling the halides ratios. The narrowest FWHM of ∼14 nm is observed in the PL emission spectra of CsPbX3 NCs (Figure 1b). Instead of the cubic shape of OA/OLA-CsPbX3 NCs, the OPA-CsPbX3 NCs exhibit a spherical shape, as shown in the transmission electronic microscopy (TEM) images in Figures S1a and 1c. We anticipate that the shape variation may be ascribed to the effects of different ligands on the nucleation and growth stages. Previous works have demonstrated that the CsPbX3 NCs structures are significantly changed by the surface 3785

DOI: 10.1021/acsami.7b17166 ACS Appl. Mater. Interfaces 2018, 10, 3784−3792

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ACS Applied Materials & Interfaces

Figure 2. Characteristics of CsPbBr3 NCs purified with varied cycle times. (a) TGA measurement of OPA- and OA/OLA-capped CsPbBr3 NC powder after washing with antisolvent twice. (b) PL QY of OPA-CsPbBr3 and OA/OLA-CsPbBr3 NC solution in hexane with one to eight purification cycles. Inset shows the PL spectra of OPA-CsPbBr3 NC solution with different purification cycles. (c) Photographs (top) of OPACsPbBr3 NC solution purified one to eight times and the corresponding fluorescence photographs (bottom) of excition under 365 nm UV lamp. (d−g) TEM images of OPA-CsPbBr3 NCs purified for two, four, six, and eight times, respectively.

ligands and reaction temperature.13,28 Herein, two main reasons can be accounted for the isotropic structure of OPA-CsPbX3 NCs. First, the strong binding strength between OPA and lead ions could accelerate the nucleation and growth stages; second, the shorter alkyl chain of OPA compared with OA/OLA could allow the nucleation seed to grow at a faster rate. Indeed, we observe that the reaction solution turns to greenish immediately upon injecting cesium precursor into the PbBr2 precursor solution in the OPA system. In comparison, the OA/ OLA-based solution changes color after 2−3 s. TEM image reveals that OPA-CsPbBr3 NCs exhibit an average diameter of ∼10.8 nm. A high-resolution transmission electronic microscopy (HRTEM) image (inset in Figure 1c) indicates the lattice spacing of the OPA-capped CsPbBr3 NCs is ∼5.8 Å, corresponding to the (100) crystal plane of the cubic phase. Besides the different shapes of NCs, it is found that the PL emission peak of OPA-CsPbBr3 shows ∼2 nm blue shift compared with the OA/OLA-CsPbBr3 ones (Figure S1b). Analogously, the NCs based on different halides ratio share similar structures with the CsPbBr3 ones, as shown in the TEM images in Figure S2. Figure S3a,b presents the 2D grazing incident angle X-ray diffraction (2D-GIXRD) patterns of the OA/OLA- and OPA-capped CsPbBr3 NCs films. Similar diffraction profiles with curved ring patterns are obtained, further confirming that changing the ligand does not affect the crystals structure. However, it is obviously observed that the 2D-GIXRD ring patterns of the OPA-CsPbBr3 NCs film for the crystal planes (100), (110) and (200) are enhanced (Figure S3c,d), indicating its improved crystallinity. 2.2. Perovskite Stability Characterizations. To apply perovskite NCs for a high-performance LED, any excess

organic ligand should be removed to offer high carrier transporting property in a film.15,29 Here, methyl acetate is used as the antisolvent to remove the extra organic residuals.30 As shown in Figure S4, the purified OA/OLA-CsPbBr3 NC powder shows agglomerated state, which may be correlated with the large amount of residual organic compounds. In contrast, the OPA-CsPbBr3 solid powder displays an isolated particle state. The thermogravimetric analysis (TGA) spectra are acquired to characterize the organic component in the purified perovskite powder. Both OPA-CsPbBr3 and OA/OLACsPbBr3 NCs are purified twice by methyl acetate before TGA characterization. OA/OLA-CsPbBr3 powder loses 30% weight, whereas the OPA-CsPbBr3 powder remains over 95% of the initial weight at ∼450 °C, as shown in Figure 2a. CsPbBr3 bulk displays a negligible weight loss below ∼450 °C; only organic compounds are decomposed below 450°C. Herein, there is only ∼4.6% residual organics in the OPA-CsPbBr3 NCs powder, whereas there is ∼29.7% organics in the OA/OLAbased one. To completely remove the residual organic compounds in the OA/OLA-chelated CsPbBr3 NCs, the purification processes are repeated for more cycles. Unfortunately, an agglomeration is obviously observed in the NCs solution when the purification cycle increases (Figure S5a) with the complete PL loss (Figure S5b). The OA/OLA-CsPbX3 NCs display a PL QY of only ∼20% after being washed four times, which is consistent with the previous report.15 The PL QY dependence on purification times is presented in Figure 2b. The drastic PL QY degradation for OA/OLA-CsPbBr3 NCs with postpurification should be correlated with the dissociation of OA/OLA from CsPbBr3 in the washing process. In comparison, the OPA-CsPbX3 NCs almost preserve their 3786

DOI: 10.1021/acsami.7b17166 ACS Appl. Mater. Interfaces 2018, 10, 3784−3792

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ACS Applied Materials & Interfaces

Figure 3. Stability measurement for the CsPbBr3 NC films. (a) Time dependence of PL intensity of OPA-CsPbBr3 and OA/OLA-CsPbBr3 NC films stored in ambient atmosphere (humidity ∼50%); (b) and (c) are the corresponding PL spectra, respectively. Insert illustrates the schematic of NCs structure change before and after air exposure. (d) Time dependence of PL intensity fluctuation of OA/OLA-CsPbBr3 and OPA-CsPbBr3 NC films with different ligands measured by confocal microscopy. The intensities are acquired from nine monitoring points as shown in the corresponding confocal images in (e) OA/OLA-CsPbBr3 film and (f) OPA-CsPbBr3 film. Both images are 50 × 50 μm2 in size.

S7c, indicating that the OPA-CsPbBr3 NCs display a strong tolerance against the polar solvent ethanol. Figure S7b,d illustrates the time-dependent UV−vis absorption and PL emission spectra of the two types of NCs solution with ethanol treatment. The PL intensity of the OA/OLA-CsPbX3 solution dramatically decreases within 1 min and then drops to quite a low level after only 20 min (Figure S7e). On the contrary, the OPA-CsPbX3 ones retain the initial PL properties even after 20 min. The stability of perovskite NCs films against a polar solvent treatment is analyzed. According to the fluorescence images in Figure S8, we can observe that the OPA-CsPbBr3 and OA/OLA-CsPbBr3 NC films display a different PL intensity decay behavior against a polar solvent. The OPA-CsPbBr3 NC film exhibits a strong resistance against ethanol, leading to the PL intensity remaining near 80% of its initial value after spincoating with ethanol. However, for the OA/OLA-CsPbBr3 NC film, the PL intensity dramatically decreases to 10% of its initial value after ethanol treatment. The excellent resistance of OPACsPbX3 against the polar solvent ethanol enables the fabrication of the all-solution-processed LEDs devices by spin-coating the ethanol dispersion of the ZnO nanoparticles onto the perovskite NCs films as an electron transport layer (ETL).32 Aside from the stability of CsPbBr3 NCs in a solution, the NC film’s stability toward ambient moisture and oxygen is another important issue to evaluate its quality.33,34 Figure S9 shows the time-dependent fluorescence photographs of the deposited OA/OLA-CsPbBr3 films and OPA-CsPbBr3 ones with two purification cycles in an ambient atmosphere (humidity ∼50%, temperature ∼25 °C). The corresponding

original properties (PL QY > 80%) when washed for same number of times. No aggregation is observed in the photographs of NCs washed from one to eight times, as shown in Figure 2c. Surprisingly, the OPA-CsPbBr3 NCs still exhibit 70% of PL QY after being washed for eight times, as shown in Figure 2b. Because of the strong ligand bonding in the OPA-CsPbBr3 NCs, they exhibit great resistivity to polar solvent. In addition, the TEM images also show similar OPACsPbX3 particles size after being washed for two to eight cycles, suggesting a high stability, as shown in Figure 2d−g. It is obvious that there is no PL emission peak shift with different cycles of purification, as shown in the inset spectra in Figure 2b. Moreover, due to easy proton transfer between OA and OLA,31 as schematic illustrated in Figure S6, postpurified OA/OLACsPbBr3 NCs solution shows poor stability. An obvious yellowish agglomeration phenomenon is observed (Figure S5c). In contrast, a transparent solution of OPA-CsPbBr3 NCs could be stored for more than 3 months without any obvious change in its initial dispersibility due to the absence of amine groups in the OPA-CsPbBr3 NCs solution (Figure S5d), indicating the excellent stability of the OPA-CsPbBr3 NCs. To verify our assumption, the stability of OPA-CsPbBr3 NCs against a polar solvent, such as ethanol, is tested. As shown in Figure S7a, an obvious color change is observed when 50 μL of ethanol is added into the OA/OLA-CsPbBr3 NCs solution (with the concentration of 0.05 mg mL−1 in hexane). This color change is ascribed to the degradation of NCs because ethanol can attack the CsPbBr3 NCs. However, there is no obvious color change upon ethanol addition into the OPA-CsPbBr3 NCs solution with the same concentration as shown in Figure 3787

DOI: 10.1021/acsami.7b17166 ACS Appl. Mater. Interfaces 2018, 10, 3784−3792

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Figure 4. CsPbBr3 NC films’ morphologies. Fluorescence photographs of (a) OA/OLA-CsPbBr3 and (b) OPA-CsPbBr3 NC films purified with two cycles. AFM height images of (c) OA/OLA-CsPbBr3 and (d) OPA-CsPbBr3 NC films with two purified cycles, respectively. Corresponding 3D images are presented in (e) and (f). Both AFM images are 500 × 500 nm2 in size.

Figure 5. Electrical output characteristics of CsPbBr3 NCs based LEDs. (a) Schematic device structure. (b) Energy-level diagram of the devices, energy values are taken from refs 32, 39. (c) EL spectra of OA/OLA- and OPA-capped NCs-based perovskite LEDs. Insert shows a photograph of a device with the Institution of Functional Nano & Soft Materials (FUNSOM) logo. (d) J−V−L curve of devices based on different ligands. (e) EQE− J−E curves of devices based on different ligands.

obvious PL red shift is observed in the time-dependent normalized PL spectra of the OA/OLA-CsPbBr3 films (Figure 3b) due to the NCs agglomeration with ligands loss in ambient environments (inset in Figure 3b). Benefiting from the strong interaction between NCs and ligands, the PL peak of the OPA-

time-dependent PL intensity is illustrated in Figure 3a. A rapid PL emission intensity degradation is observed in the OA/OLACsPbBr3 films upon time elapsing, whereas the OPA-CsPbX3 NCs film still preserves its initial 90% PL intensity in air for 3 days, indicating its strong water and oxygen resistance. An 3788

DOI: 10.1021/acsami.7b17166 ACS Appl. Mater. Interfaces 2018, 10, 3784−3792

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ACS Applied Materials & Interfaces CsPbBr3 NCs films shows no drift (Figure 3c). Figure 3d depicts the time-dependent PL intensity of the OA/OLACsPbBr3 and OPA-CsPbBr3 NCs films. The PL intensity is monitored from nine individual points as shown in the corresponding fluorescence images in Figure 3e,f. The less PL fluctuation observed in the OPA-CsPbBr3 film indicates its enhanced PL stability.35 High-quality NC films are indispensable for a highperformance optoelectronic device.36,37 The surface ligands and residual organics can dramatically influence the carriers transport in the perovskite film; herein, the conductivity of the OPA-CsPbBr3 and OA/OLA-CsPbBr3 films are measured, and the current−voltage curve is shown in Figure S10. A higher current in the OPA-CsPbBr3-based films under same bias is observed, indicating that the conductivity of the OPA-CsPbBr3 NC film is higher than that of the OA/OLA-CsPbBr3 one. The higher conductivity in the OPA-CsPbBr3 NC film should be attributed to the short chain of OPA and the less organic residual compounds in the perovskite film, as we discussed in the TGA part. Because of the less organic residue in the OPACsPbBr3 NCs solution, a compact and smooth perovskite film can be obtained, as shown in the fluorescence microscopy images in Figure 4b. In comparison, the OA/OLA-CsPbBr3 based films display strong striation defects due to the excess organic residue (Figure 4a). We anticipate that this obvious striation defects originate from the different surface tension and vapor pressure of the coexisting organic residues and NCs solvents, as demonstrated in previous work.38 The SEM images of the films based on perovskite NCs purified twice are shown in Figure S11. In the OPA-CsPbBr3 film, the NCs show a similar crystal size of ∼11 nm and a densely packed state. However, regarding the OA/OLA-CsPbBr3 film, the NCs show different sizes, which should be ascribed to the deterioration of the NCs during the purification process. In addition, the atomic force microscopy (AFM) topographic height image of the OPA-CsPbX3 films indicates isolated NCs uniformly distributed in the flat film with a lower root mean square (RMS) roughness of ∼2.5 nm (Figure 4d), whereas the OA/OLA-CsPbX3-based film displays large particle agglomeration, which leads to a rough morphology with the RMS roughness of ∼8.6 nm (Figure 4c). The three-dimensional (3D) AFM images of the CsPbBr3 films shown in Figure 4e,f provide a straightforward comparison of the difference in the morphology of the two films. 2.3. Light-Emitting Devices. LEDs with a multilayer structure that consists of indium tin oxide (ITO), poly(3,4ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS, ∼40 nm), poly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzi-dine] (poly-TPD) (∼35 nm), perovskite NCs (∼20 nm), 1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl) benzene (TPBi) (∼40 nm), LiF (∼1 nm), and Al (∼100 nm) are fabricated, as shown in Figure 5a. Poly-TPD and TPBi are selected as the hole transport layer (HTL) and ETL, respectively, due to a well-matched mobility and a favorable energy band alignment with the perovskite layer,19 as shown in Figure 5b. The LEDs display a pure green EL peak at 516 nm with a narrow FWHM of ∼19 nm (Figure 5c and inset photograph). In line with the PL shift of the OPA-CsPbBr3 NCs and OA/OLA ones, the EL peak shares the similar shift behavior. Figure 5d,e shows the electrical output characteristics of devices based on NCs chelated with OA/OLA and OPA. Detailed device performance parameters are summarized in Table 1. The champion OPA-CsPbBr3-based device yields a

Table 1. Detailed Performance Parameters of Devices Based on OPA-CsPbBr3 and OA/OLA-CsPbBr3 NC Films ligand OPA OA/OLA

VT (V) 2.8 3.2

Lmax (cd m−2) 7085 780

ηP (lm W−1) 14.24 1.65

ηA (cd A−1) 18.13 1.84

EQE (%) 6.5 0.86

current efficiency (ηA) of 18.13 cd A−1, corresponding to an EQE of 6.5%, which is ∼8 times higher than 0.86% for the OA/ OLA-CsPbX3-based one. The average EQE of over 20 OPACsPbBr3 LEDs is 4.6%. The EQE distribution is shown in Figure S12. Based on our best knowledge, the OPA-CsPbBr3-based device demonstrates a high level of current efficiency and EQE for all-inorganic CsPbBr3 NCs based LEDs so far, as summarized in Table S1. The significant efficiency improvement in the OPA-CsPbBr3based device can be attributed to the high PL QY of the OPAchelated NCs, as well as the high-quality perovskite film. For example, the rapid EL luminance increase with increasing bias indicates an effective charge carrier transport and radiative recombination in the OPA-CsPbBr3-based devices. As shown in the J−V−L curves, the leak current density of an OA/OLACsPbBr3 LED is about 1 magnitude higher than that of an OPA-CsPbBr3-based LED, leading to an inefficient radiative recombination and a poor device performance. Therefore, the highest luminescence for the OA/OLA-CsPbBr3-based device is only ∼780 cd m−2, which is ∼10 times smaller than ∼7085 cd m −2 for the OPA-CsPbBr 3 -based one. The poor film morphology of the OA/OLA-CsPbBr3 films causes microscale shorting problems resulting in a large leak in current density. Additionally, the OA/OLA-CsPbBr3-based devices display a 0.4 V higher (3.2−2.8 V) turn on voltage than the OPA-CsPbBr3based one, which may be ascribed to the excess insulating OA and OLA in the perovskite layer. Moreover, the ligand loss of OA/OLA-CsPbBr3 NCs results in high possibility of a nonradiative recombination at the surface trap sites, which deteriorate the device performance. Meanwhile, the device stability is tested to verify the advantages of the OPA-CsPbBr3based LEDs, as shown in Figure S13. The devices based on OPA-CsPbBr3 and OA/OLA-CsPbBr3 NC are measured under a constant current density of 2.5 mA cm−2 for 30 min. The EQE of the OA/OLA-CsPbBr3-based LED decreases dramatically to nearly 20% of its original value, whereas the EQE of the OPA-CsPbBr3-based one remains over 50% after 30 min, indicating a better stability.

3. CONCLUSIONS In summary, we have demonstrated a hot-injection method to achieve highly stable and luminescent CsPbBr3 NCs for efficient LED devices. The robust bonding strength between OPA and CsPbBr3 allows us not only to remove the extra nonchelating organic residue after purification but also to preserve its initial excellent properties of dispersibility and PL QY. As a result, the purified OPA-CsPbX3 NCs solution exhibits a stable high PL QY of 80%. In addition, it is found that there is only 4.6% of organic residue in purified OPA-CsPbBr3 NCs in comparison with 29.7% in OA/OLA-CsPbBr3 one. Herein, we fabricate an efficient green LED with the current efficiency of 18.13 cd A−1, corresponding to an EQE of 6.5%. The obvious performance improvement in OPA-based devices is ascribed to the better film morphology and low trap states density of the NCs films. We believe that the OPA-capped NCs 3789

DOI: 10.1021/acsami.7b17166 ACS Appl. Mater. Interfaces 2018, 10, 3784−3792

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concentration of 8 mg mL−1 was spin-coated on the PEDOT:PSS layers at 2000 rpm and baked at 130 °C for 30 min to remove the residual solvent. A 3 mg mL−1 of NCs in hexane were spin-coated on the HTLs at 4000 rpm for 60 s. Then, the NCs layer was baked at 50 °C for 2 min. The thickness of the perovskite NCs layer was ∼20 nm, which is confirmed by the cross-sectional SEM image in Figure S15. Finally, the substrates were transferred into a vacuum thermal evaporator with a pressure below ∼1.0 × 10−6 mBar chamber to deposit ∼40 nm TPBi, ∼1 nm LiF, and 100 nm Al cathodes. The active area of the device was 9 mm2. The J−V−L characteristics and the EL spectra of the LEDs were collected by a Keithley 2400 sourcemeter and a PhotoResearch spectrometer PR670 with an adhesive encapsulation in the dark room.

provide an effective strategy to synthesize ultrastable perovskite NCs. These excellent NCs could be further optimized to improve the performance and stability of the perovskite optoelectronic devices.

4. EXPERIMENTAL SECTION 4.1. Preparation of Cs-Oleate Solution. The solution of Csoleate was prepared as follows: CsOAc (0.315 g), 10 mL of octadecene (ODE), and 1 mL of OA were loaded into a 50 mL three-neck flask, degassed for 60 min at room temperature, heated under N2 to 100 °C, degassed for 60 min, and heated to 140 °C until all of the CsOAc was dissolved in the ODE. 4.2. Synthesis of CsPbX3 NCs. OA/OLA-CsPbX3 NCs were synthesized according to the previous reported method.12 PbBr2 (0.069 g), 0.5 mL of OA, and 0.5 mL of OLA were loaded into a 50 mL three-neck flask with 5 mL of ODE, degassed for 60 min, heated to 120 °C, and degassed for 60 min. The solution was heated to 160−180 °C when the Cs-oleate solution (0.4 mL) was quickly injected. The OPA-CsPbBr3 NCs were synthesized by the hot-injection method. First, octylphosphonic acid (0.1 g), trioctylphosphine oxide (TOPO, 1 g), and PbBr2 (0.069 g) were loaded into a 50 mL threeneck flask with 5 mL of ODE and degassed for 30 min. Second, the flask was heated to 100 °C, degassed for 30 min. Third, the solution was heated to 160−180 °C under N2 and Cs-oleate solution (0.4 mL) was quickly injected. The flask was quickly dipped in a ice−water bath to stop the reaction after 5 s. CsPbX3 NCs with various colors were synthesized with the same method by adjusting the lead−halide ratio in the lead source. TOPO here helps the OPA dissolve in ODE. It is worth noting that only pure TOPO without an OPA cannot be used to obtain high-quality perovskite NCs with good dispersion in a solvent, as shown in Figure S14, which may be ascribed to the weak interaction between TOPO and NCs. 4.3. Centrifugation of CsPbX3 NCs. The CsPbX3 NCs in a crude solution were isolated by adding 5 mL hexane and centrifuging under 3000 rpm for 3 min. Then, the supernatant was extracted, 15 mL of methyl acetate was added to the supernatant, and the mixture centrifuged under 8000 rpm for 5 min. The precipitant was then dispersed in 2 mL hexane. For more purified cycles, the isolating and centrifuging processes were repeated. Finally, the products were redispersed in 2 mL hexane for further use. 4.4. NCs Characteristics. The UV−vis absorption and PL of CsPbX3 were acquired by a UV−vis spectrometer (SPECORD S 600) and a PL spectrometer (FLUOROMAX-4) in hexane. The PL QY of the perovskite NCs were measured by PL spectrometer with an integrating sphere. The TEM and HRTEM images of NCs were characterized by a FEI Tecnai G2 F20 on Cu grids coated with a carbon film. The AFM characteristics of the NCs film were measured by a Veeco MultiMode V AFM microscope. The optical images of the NCs films were obtained by a Motic microscopy. The confocal PL images were taken from a TCS SP5 Confocal Systems. The GIXRD measurements were carried at the BL14B1 beamline of the Shanghai Synchrotron Radiation Facility with a wavelength of 1.38 Å. The 2DGIXRD patterns were imaged by a MarCCD detector mounted on a normal distance of ∼180 mm away from the sample at a grazing incident angle of 0.40°, with an exposure time ∼50 s. The 2D-GIXRD patterns were calculated via Fit 2D software with scattering vector q coordinates: q = 4π sin θ/λ, where θ was the half of the diffraction angle. 4.5. Device Fabrication and Characterization. The devices were fabricated on commercially available prepatterned ITO glasses by the spin-coating method. The ITO substrates were cleaned in acetone, ethanol, and deionized water for 25 min in that sequence, followed by an ozone plasma treatment for 15 min before the spin-coating process. PEDOT:PSS (CLEVIOS, Al 4083) was spin-coated on the cleaned substrates at 4000 rpm for 40 s and baked at 150 °C in air for 15 min. Then, the PEDOT:PSS-coated substrates were moved into a nitrogenfilled glovebox for further coating of HTLs and NCs layers. Poly-TPD (America Dye Source) dissolved in chlorobenzene with the

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b17166. TEM and PL images of OA/OLA-CsPbBr3 NCs, TEM images of OPA-CsPbX3 NCs with different halides, 2DGIWRD patterns of OA/OLA- and OPA-capped CsPbBr3 NCs films, photographs of dried NCs, photograph of OA/OLA-CsPbBr3 NCs after antisolvent washing, scheme of ligands evolution, characteristics of OPA-CsPbBr3 and OA/OLA-CsPbBr3 NCs tolerance against ethanol, characterization of stability test in air, EQE distribution of LED devices and a summary table for published electrical output characteristics of CsPbBr3based LEDs (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (T.S.). *E-mail: [email protected] (Q.Z.). *E-mail: [email protected] (B.S.). ORCID

Qiao Zhang: 0000-0001-9682-3295 Baoquan Sun: 0000-0002-4507-4578 Author Contributions §

Y.T. and Y.Z. contributed equally.

Author Contributions

The manuscript was written through contributions of all of the authors. All of the authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledged Prof. Yizheng Jin from Zhejiang Unversity for helpful discussion. This work was supported by the National Key Research and Development Program of China (2016YFA0202402), the National Natural Science Foundation of China (91123005, 61674108, 21274087, 61504089), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), the 111 Projects, and Collaborative Innovation Center of Suzhou Nano Science and Technology.

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REFERENCES

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