Highly Luminescent and Stable Perovskite Nanocrystals with

Subscriber access provided by READING UNIV

Article

Highly Luminescent and Stable Perovskite Nanocrystals with Octylphosphonic Acid as 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 ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17166 • Publication Date (Web): 04 Jan 2018 Downloaded from http://pubs.acs.org on January 4, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Highly Luminescent and Stable Perovskite Nanocrystals with Octylphosphonic Acid as Ligand for Efficient Light Emitting Diodes Yeshu Tan,1‡ Yatao Zou,1‡ Linzhong Wu,1 Qi Huang,1 Di Yang,1 Min Chen,1 Muyang Ban,1 Chen Wu,1 Tian Wu,1 Sai Bai,2 Tao Song,1* Qiao Zhang,1* Baoquan Sun1* 1

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 2

Department of Physics, Chemistry and Biology (IFM), Linköping University, SE-581 83 Linköping, Sweden

Keywords: perovskite nanocrystals, octylphosphonic acid, photoluminescence, dispersibility, light emitting diodes

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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 existed 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. Owe to the 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 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 that of ~29.7% in OA/OLA-CsPbBr3. Thereby, a uniform and compact OPA-CsPbBr3 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 achieved. Our research provides a path to prepare high quality perovskite NCs for high performance optoelectronic devices.


Page 2 of 32

Page 3 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. Introduction Perovskite light emitting diodes (LEDs) have attracted wide attentions over the past three years due

to their solution processability, low cost and tunable

electroluminescence (EL) spectra with narrow full width at half maximum (FWHM).1-6 To boost LEDs efficiency, low dimensional perovskites, such as two-dimensional (2D) and colloidal nanocrystals (NCs) type perovskites, are regarded as preferable light emissive layer in LEDs due to their large exciton banding energy and 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, all-inorganic perovskite (AIP) usually exhibits better thermal and moisture stability.11-15 Yet, the smaller exciton binding energy (~ a few meV) and discontinuous film morphology of 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 hot injection method,12 efforts have been made to fabricate AIP NCs based LEDs.5,

17-22

However, the present as-synthesized CsPbX3 NCs, which are capped

with surface ligands of oleic acid (OA) and oleylamine (OLA), suffer from ligand loss during purification process due to the weak interaction between these surface ligands and NCs. The photoluminescence (PL) quantum yield (QY) and dispersibility in solvents and structural stability are deteriorated after purification process of perovskite NCs.15 The capped organic ligands allow CsPbX3 NCs to be dispersed in organic solvents and passivate defect sites on CsPbX3 NCs surface.23 Therefore, any



ligand loss induces defect sites on CsPbX3 NCs surface, increasing non-radiative recombination chances in LED devices.24 To suppress these defect sites, Tan and coworkers 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 anti-solvents of hexane/ethyl acetate with different ratios were selected to tune their polarity to precipitate CsPbX3 NCs out of 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 acid-base ligands are not completely overcome. We noticed that 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 high PL QY up to ~90%, narrow PL full width at half maximum (FWHM) (~19 nm) and obvious enhanced colloidal stability. Two


Page 4 of 32

Page 5 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


chelating sites of OPA with lead ions provide strong interaction with CsPbX3 NCs. More than ~80% PL QY of OPA-CsPbBr3 NCs is remained even after four times of anti-solvent

assisted

washing,

while

OA/OLA

capped

CsPbBr3

NCs

(OA/OLA-CsPbBr3) only display low PL QY of ~20% with the same purification steps, indicating the ultra-stable characteristics of OPA capped perovskite NCs. Generally, the residual organic ligands degenerate perovskite film morphology, increasing the leak current and blocking the charge injection in LEDs. There are only ~4.6% of organic residue in OPA-CsPbBr3 NCs after washed with anti-solvent for twice in comparison with ~29.7% in OA/OLA-CsPbBr3 ones. Consequently, a uniform and compact OPA-CsPbBr3 NCs film with impressive air stability can be obtained. Green LED devices based on OPA-CsPbBr3 NCs 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 OA/OLA-CsPbBr3 based one. Our finding paves a way to achieve highly stable CsPbBr3 NCs by simply tuning surface anchoring ligands for all solution processed high efficiency optoelectronic devices. 2. Results and Discussion 2.1 Synthesis of Perovskite NCs Here, OPA is used to replace OA/OLA to synthesize CsPbBr3 NCs with hot-injection method.12 Normally, non-coordinating solvent 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 the precursor without amines due to its enhanced acid dissociation characteristic. In OPA system, all precursor



Page 6 of 32

turns into colorless solution at temperature as low as ~75 oC. However, a temperature of ~110 oC is required to obtain the colorless solution in OLA/OA system. The temperature difference reveals that OPA-lead has better solubility in ODE. Cesium carbonate is replaced with cesium acetate to enhance cesium solubility in ODE.13 Figure 1a shows the digital fluorescence photographs of all inorganic CsPbX3 NCs with different halides (Cl, Br and 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 PL emission spectra of CsPbX3 NCs (Figure1b). Instead of cubic shape of OA/OLA-CsPbX3 NCs, the OPA-CsPbX3 one exhibits sphere shape, as shown in the transmission electronic microscopy (TEM) image in Figure S1a and Figure 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 CsPbX3 NCs structures are significantly changed by the surface ligands and reaction temperature.13,

28

Herein, two main

reasons can be accounted for the isotropic structure of OPA-CsPbX3 NCs. Firstly, the strong binding strength between OPA and lead ions could accelerate the nucleation and growth stages; secondly, 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 OPA system. In comparison, the OA/OLA based one changes color after 2-3 seconds. TEM image reveals that OPA-CsPbBr3 NCs exhibit an average diameter of ~10.8 nm. A high-resolution transmission electronic


Page 7 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


microscopy (HRTEM) image (Inset in Figure 1c) indicates the lattice spacing of OPA capped CsPbBr3 NCs is ~5.8 Å, corresponding to the (100) crystal plane of cubic phase. Besides the different NCs shape, it is found that PL emission peak of OPA-CsPbBr3 shows ~2 nm blue shift compared with OA/OLA-CsPbBr3 ones (Figure S1b). Analogously, the different halides ratio based NCs shares similar structures with the CsPbBr3 ones, as shown TEM images in Figure S2. Figure S3a and b present 2D grazing incident angle X-ray diffraction (2D-GIXRD) patterns of OA/OLA and OPA capped CsPbBr3 NCs films. Similar diffraction profiles with curved ring patterns are obtained, further confirming that ligand changing does not affect the crystals structure. However, it is obviously observed that 2D-GIXRD ring patterns of OPA-CsPbBr3 NCs film for crystal plane (100), (110) and (200) are enhanced (Figure S3c and d), indicating its improved crystallinity. 2.2 Perovskite Stability Characterizations In order to apply perovskite NCs for 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 anti-solvent to remove the extra organic residuals.30 As shown in Figure S4, purified OA/OLA-CsPbBr3 NC powder shows agglomerated state, which may be correlated with large amount of residual organic compounds. In contrast, the OPA-CsPbBr3 solid powder displays isolated particle state. Thermogravimetric measurement (TGA) spectra was acquired to characterize the organic component in purified perovskite powder. Both OPA-CsPbBr3 and OA/OLA-CsPbBr3 NCs are purified twice by methyl acetate before TGA



characterization.

OA/OLA-CsPbBr3

powder

loses

30%

Page 8 of 32

weight,

while

the

OPA-CsPbBr3 powder remains over 95% of weight at ~450 °C, as shown in Figure 2a. CsPbBr3 bulk displays negligible weight loss below ~450 °C, only organic compounds are decomposed below 450°C. Herein, there is only ~4.6% residual organics in OPA-CsPbBr3 NCs powder, while there is ~29.7% organics in OA/OLA based one. In order to completely remove the residual organic compounds in OA/OLA chelated CsPbBr3 NCs, the purification processes are repeated for more cycles. Unfortunately, agglomeration is obviously observed in NCs solution when increasing purification cycle (Figure S5a) associated with complete PL loss (Figure S5b). The OA/OLA-CsPbX3 NCs display PL QY of only ~20% after washed four times, which is consistent with the previous report.15 The PL QY dependent on purification times is presented in Figure 2b. The drastic PL QY degradation for OA/OLA-CsPbBr3 NCs with post-purification should be correlated with the dissociation of OA/OLA from CsPbBr3 in washing process. In comparison, OPA-CsPbX3 NCs almost preserve their original properties (PL QY>80%) when they are washed for same 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 washed for eight times, as shown in Figure 2b. Because of the strong ligand bonding in OPA-CsPbBr3 NCs, they exhibit great resistivity to polar solvent. In addition, the TEM images also show similar OPA-CsPbX3 particles size after washed for two to eight cycles, suggesting high stability, as shown in Figure 2d-g. It is obvious that there is no PL emission peak


Page 9 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


shift with different cycles of purification, as inset spectra showed in Figure 2b. Moreover, due to the easy proton transfer between OA and OLA,31 as schematic illustrated in Figure S6, post-purified OA/OLA-CsPbBr3 NCs solution shows poor stability. Obvious yellowish agglomeration phenomenon is observed (Figure S5c). In contrast, transparent solution of OPA-CsPbBr3 NCs could be stored for more than three months without any obvious change of its initial dispersibility due to the absence of amine groups in OPA-CsPbBr3 NCs solution (Figure S5d), indicating the excellent stability of OPA-CsPbBr3 NCs. To verify our assumption, the stability of OPA-CsPbBr3 NCs against polar solvent, such as ethanol, is tested. As shown in Figure S7a, obvious color change is observed while 50 µL of ethanol is added into OA/OLA-CsPbBr3 NCs solution (with concentration of 0.05 mg mL-1 in hexane). This color change is ascribed to the NCs degradation because ethanol can attack CsPbBr3 NCs. However, there is no obvious color change upon ethanol addition into OPA-CsPbBr3 NCs solution with the same concentration as shown in Figure S7c, which indicates that the OPA-CsPbBr3 NCs displays strong tolerance against polar solvent of ethanol. Figure S7b and d illustrate 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 one minute, and drops to a quite low level after only 20 min (Figure S7e). On the contrary, the OPA-CsPbX3 ones remain the initial PL properties even after 20 min. The stability of perovskite NCs films against polar solvent treatment has been conducted. According to fluorescence images in Figure S8,



we can observe that OPA-CsPbBr3 and OA/OLA-CsPbBr3 NC films display different PL intensity decay behavior against polar solvent. OPA-CsPbBr3 NC film exhibits strong resistance against ethanol, leading to PL intensity remaining near 80% of its initial value after spin-coated with ethanol. While for OA/OLA-CsPbBr3 NC film, the PL intensity dramatically decreases to 10% of its initial value after ethanol treatment. The excellent resistance of OPA-CsPbX3 against polar solvent of ethanol enables the fabrication of all-solution processed LEDs devices by spin-coating the ethanol dispersion of ZnO nanoparticles onto the perovskite NCs films as electron transport layer (ETL).32 Aside from the stability of CsPbBr3 NCs in a solution, NCs film stability toward moisture and oxygen in ambient is another important issue to evaluate its quality.33-34 Figure S9 shows the time-dependent fluorescence photographs of deposited OA/OLA-CsPbBr3 films and OPA-CsPbBr3 ones with two purification cycles in ambient atmosphere (humidity ~50%, temperature ~25 oC). The corresponding time-dependent PL intensity is illustrated in Figure 3a. Rapid PL emission intensity degradation is observed in OA/OLA-CsPbBr3 films upon time elapsing, while OPA-CsPbX3 NCs film still preserves its initial 90% PL intensity in air for three days, which indicates its strong water and oxygen resistance. Obvious PL red shift is observed in the time-dependent normalized PL spectra of 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, PL peak of OPA-CsPbBr3 NCs films shows no drift (Figure 3c). Figure 3d depicts the


Page 10 of 32

Page 11 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


time-dependent PL intensity of OA/OLA- CsPbBr3 and OPA-CsPbBr3 NCs films. The PL intensity is monitored from nine individual points as shown in the corresponding fluorescence images in Figure 3e and f. Less PL fluctuation is observed in OPA-CsPbBr3 film, which indicates that OPA-CsPbBr3 film has enhanced PL stability.35 High-quality NCs films are indispensable for high-performance optoelectronic device.36-37 The surface ligands and residual organics can dramatically influence the carriers transport of the perovskite film, herein, the conductivity of OPA-CsPbBr3 and OA/OLA-CsPbBr3 films are measured, and current-voltage curve is shown in Figure S10. A higher current in OPA-CsPbBr3 based films under same bias is observed, which indicates that the conductivity of OPA-CsPbBr3 NC film is higher than that of OA/OLA-CsPbBr3 one. The higher conductivity in OPA-CsPbBr3 NC film should be attributed to the short chain of OPA and less organic residual compounds in perovskite film as we discussed in TGA part. Because of less organic residue in OPA-CsPbBr3 NCs solution, compact and smooth perovskite film can be obtained, as fluorescence

microscope

images

shows

in

Figure

4b.

In

comparison,

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 coexisted organic residues and NCs solvents, as previous work demonstrated.38 SEM images of films based on perovskite NCs purified twice are shown in Figure S11. In OPA-CsPbBr3 film, the NCs show the similar crystal size of ~11 nm and a densely packed state. However,



Page 12 of 32

regarding to OA/OLA-CsPbBr3 film, the NCs shows different sizes, which should be ascribed to NCs deterioration during the purification process. In addition, atomic force microscopy (AFM) topographic height image of 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), while the OA/OLA-CsPbX3 based one displays large particle agglomeration, which leads to rough morphology with a RMS roughness of ~8.6 nm (Figure 4c). The three-dimension (3D) AFM images of CsPbBr3 films shown in Figure 4e and f provide a straightforward comparison of film morphology difference between two films. 2.3 Light Emitting Devices LEDs with a multilayer structure that consists of indium tin oxide (ITO), poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS, ~40 nm), poly[N,N'-bis(4-butylphenyl)-N,N'-bis(phenyl)-benzi

(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) were fabricated, as shown in Figure 5a. Poly-TPD and TPBi were selected as hole transport layers (HTL) and ETL due to well-matched mobility and favorable energy band alignment with perovskite layer,19 as shown in Figure 5b. LEDs display pure green EL peak at 516 nm with narrow FWHM of ~19 nm (Figure 5c and inset photograph). In line with the PL shift of OPA-CsPbBr3 NCs and OA/OLA ones, EL peak shares the similar shift behavior. Figure 5d and 5e show the electrical output characteristics of devices based on NCs chelated with OA/OLA and OPA. Detailed device performance parameters are


Page 13 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


summarized in Table 1. The champion OPA- CsPbBr3 based device yields a current efficiency (ηA) of 18.13 cd A-1, corresponding to an EQE of 6.5%, which is ~8 times higher than 0.86% of the OA/OLA-CsPbX3 based one. The average EQE of over 20 OPA-CsPbBr3 LEDs devices 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 OPA-CsPbBr3 based device can be attributed to the high PL QY of OPA chelated NCs as well as the high-quality perovskite film. For example, the rapid EL luminance increase with increasing bias indicates the effective charge carrier transport and radiative recombination in OPA-CsPbBr3 based devices. As shown in J-V-L curves, the leak current density of OA/OLA-CsPbBr3 LED is about one magnitude higher compared to it of OPA-CsPbBr3 based LED, leading to inefficient radiative recombination and poor device performance. Therefore, the highest luminescence for OA/OLA-CsPbBr3 based device is only ~780 cd m-2, which is ~10 times smaller than ~7085 cd m-2 of OPA-CsPbBr3 based one. The poor film morphology of OA/OLA-CsPbBr3 films causes microscale shorting problems resulting in the large leak current density. Additionally, the OA/OLA-CsPbBr3 based devices display 0.4 V higher (3.2 V to 2.8 V) turn on voltage than OPA-CsPbBr3 based 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 high possibility of non-radiative recombination at surface trap sites, which deteriorate



device performance. Meanwhile, the device stability was tested to verify the advantages of OPA-CsPbBr3 based 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 OA/OLA-CsPbBr3 based LED decreases dramatically to nearly 20% of its original value, while the EQE of OPA-CsPbBr3 based one remains over 50% after 30 min, indicating better stability. 3. Conclusion In summary, we have demonstrated a hot-injection method to achieve a highly stable and luminescent CsPbBr3 NCs for efficient LED devices. The robust bonding strength between OPA and CsPbBr3 allows us not only to remove extra non-chelating organic residue after purification, but also 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 residual in purified OPA-CsPbBr3 NCs in comparison with 29.7% of OLA-OA-CsPbBr3 one. Herein, we fabricate an efficient green LED with current efficiency of 18.13 cd A-1, corresponding to an EQE of 6.5%. The obvious performance improvement of OPA based devices is ascribed to the better film morphology and low trap states density of NCs films. We believe that the OPA capped NCs provide an effective strategy to synthesize ultra-stable perovskite NCs. These excellent NCs could be further optimized to improve the devices performance and stability of perovskite optoelectronic devices. 4. Experimental section


Page 14 of 32

Page 15 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Preparation of Cs-oleate solution: The solution of Cs-oleate was prepared as follows: CsOAc (0.315g), 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 CsOAc was dissolved in ODE. 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 Cs-oleate solution (0.4 mL) was quickly injected. OPA-CsPbBr3 NCs were synthesized by hot injecting method. Firstly, octylphosphonic acid (0.1g), trioctylphosphine oxide (TOPO, 1g) and PbBr2 (0.069 g) were loaded into a 50 mL three-neck flask with 5 mL of ODE, degassed for 30 min. Secondly, the flask was heated to 100 °C, degassed for 30 min. Thirdly, 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 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 OPA dissolve in ODE. It is worth noting that only pure TOPO without OPA can’t 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.



Centrifugation of CsPbX3 NCs: The CsPbX3 NCs in 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 centrifuged under 8000 rpm for 5 min. Then the precipitant was dispersed in 2 mL hexane. For more purified cycles, the isolating and centrifuging processes were repeated. Finally, the products were re-dispersed in 2 mL hexane for further use. NCs characteristics: The UV-vis absorption and PL of CsPbX3 were acquired by UV-vis spectrometer (SPECORD S 600) and PL spectrometer (FLUOROMAX-4) in hexane. The PL QY of perovskite NCs were measured by PL spectrometer with integrating sphere. TEM and HRTEM images of NCs were characterized by a FEI Tecnai G2 F20 on Cu grids coated with carbon film. AFM characteristics of NCs film were measured by Veeco MultiMode V AFM microscope. The optical images of NCs films were obtained by a Motic microscopy. Confocal PL images were taken from TCS SP5 Confocal Systems. GIXRD measurements were carried at the BL14B1 beamline of the Shanghai Synchrotron Radiation Facility (SSRF) with a wavelength of 1.38 Å. 2D-GIXRD patterns were imaged by a MarCCD detector mounted at a normal distance of ~180 mm away from the sample at a grazing incident angle 0.40°, with an exposure time~50 s. The 2D-GIXRD patterns were calculated via Fit 2D software with scattering vector q coordinates: q=4πsinθ/λ, where θ was half of the diffraction angle. Device fabrication and characterization: Devices were fabricated on commercial available pre-patterned ITO glasses by spin-coating method. The ITO substrates were


Page 16 of 32

Page 17 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


cleaned in acetone, ethanol and deionized water for 25 min in 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 PEDOT:PSS coated substrates were moved into a nitrogen filled glove box for further coating of HTLs and NCs layers. Poly-TPD (America Dye Source) dissolved in chlorobenzene with concentration of 8 mg mL-1 was spin-coated on PEDOT:PSS layers at 2000 rpm and baked at 130 °C for 30 min to remove the residual solvent. 3 mg mL-1 of NCs in hexane were spin-coated on HTLs at 4000 rpm for 60s. Then the NCs layer was baked at 50 °C for 2 min. The thickness of perovskite NCs layer is ~20 nm, which is confirmed by the cross-section SEM image in Figure S15. Finally, the substrates were transferred into a vacuum thermal evaporator with a pressure below ~1.0E-6 mBar chamber to deposit ~40 nm TPBi, ~1 nm LiF and 100 nm Al cathodes. Active area of device was 9 mm2. The J-V-L characteristics and EL spectra of LEDs were collected by a Keithley 2400 sourcemeter and a PhotoResearch spectrometer PR670 with adhesive encapsulation in the dark room. Supporting Information: TEM and PL images of OA/OLA-CsPbBr3 NCs, TEM images of OPA-CsPbX3 NCs with different halides, 2D-GIWRD patterns of OA/OLA and OPA capped CsPbBr3 NCs films, photographs of dried NCs, photograph of OA/OLA-CsPbBr3 NCs after anti-solvent 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



Page 18 of 32

summary table for published electrical output characteristics of CsPbBr3 based LEDs. This material is available free of charge via the Internet at http://pubs.acs.org.

Author Information *Corresponding

Author:

[email protected];

[email protected];

[email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally.

Notes: The authors declare no competing financial interest.

Acknowledgment 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.


Page 19 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figures and Tables

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



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


Page 20 of 32

Page 21 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. Stability measurement for CsPbBr3 NC films. (a) Time-dependent of PL intensity of OPA-CsPbBr3 and OA/OLA-CsPbBr3 NC films stored in the 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-dependent 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 µm size.



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 nm size.


Page 22 of 32

Page 23 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. Electrical output characteristics of CsPbBr3 NCs based LEDs. (a) Schematic device structure. (b) Energy level diagram of devices, energy values are taken form reference.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 different ligands based devices. (e) EQE-J-E curves of devices based on different ligands.



Page 24 of 32

Table 1. Detailed performance parameters of devices based on OPA-CsPbBr3 and OA/OLA-CsPbBr3 NC films.

VT

Lmax

ηP

ηA

EQE

(V)

(cd m-2)

(lm W-1)

(cd A-1)

(%)

OPA

2.8

7085

14.24

18.13

6.5

OA/OLA

3.2

780

1.65

1.84

0.86

Ligand


Page 25 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


References 1.

Li, G.; Tan, Z.-K.; Di, D.; Lai, M. L.; Jiang, L.; Lim, J. H.-W.; Friend, R. H.;

Greenham, N. C., Efficient Light-Emitting Diodes Based on Nanocrystalline Perovskite in a Dielectric Polymer Matrix. Nano Lett. 2015, 15, 2640-2644. 2.

Tan, Z.-K.; Moghaddam, R. S.; Lai, M. L.; Docampo, P.; Higler, R.; Deschler, F.;

Price, M.; Sadhanala, A.; Pazos, L. M.; Credgington, D.; Hanusch, F.; Bein, T.; Snaith, H. J.; Friend, R. H., Bright Light-Emitting Diodes Based on Organometal Halide Perovskite. Nat. Nanotech. 2014, 9, 687-692. 3.

Ling, Y.; Yuan, Z.; Tian, Y.; Wang, X.; Wang, J. C.; Xin, Y.; Hanson, K.; Ma, B.;

Gao, H., Bright Light-Emitting Diodes Based on Organometal Halide Perovskite Nanoplatelets. Adv. Mater. 2016, 28, 305-311. 4.

Cho, H.; Jeong, S.-H.; Park, M.-H.; Kim, Y.-H.; Wolf, C.; Lee, C.-L.; Heo, J. H.;

Sadhanala, A.; Myoung, N.; Yoo, S.; Im, S. H.; Friend, R. H.; Lee, T.-W., Overcoming the Electroluminescence Efficiency Limitations of Perovskite Light-Emitting Diodes. Science 2015, 350, 1222-1225. 5.

Song, J.; Li, J.; Li, X.; Xu, L.; Dong, Y.; Zeng, H., Quantum Dot Light-Emitting

Diodes Based on Inorganic Perovskite Cesium Lead Halides (CsPbX3). Adv. Mater. 2015, 27, 7162-7167. 6.

Koscher, B. A.; Swabeck, J. K.; Bronstein, N. D.; Alivisatos, A. P., Essentially

Trap-Free CsPbBr3 Colloidal Nanocrystals by Postsynthetic Thiocyanate Surface Treatment. J. Am. Chem. Soc. 2017, 139, 6566-6569. 7.

Quan, L. N.; Zhao, Y.; Garcia de Arquer, F. P.; Sabatini, R.; Walters, G.; Voznyy,



O.; Comin, R.; Li, Y.; Fan, J. Z.; Tan, H.; Pan, J.; Yuan, M.; Bakr, O. M.; Lu, Z.; Kim, D. H.; Sargent, E. H., Tailoring the Energy Landscape in Quasi-2D Halide Perovskites Enables Efficient Green-Light Emission. Nano Lett. 2017, 17, 3701-3709. 8.

Yuan, M.; Quan, L. N.; Comin, R.; Walters, G.; Sabatini, R.; Voznyy, O.;

Hoogland, S.; Zhao, Y.; Beauregard, E. M.; Kanjanaboos, P.; Lu, Z.; Kim, D. H.; Sargent, E. H., Perovskite Energy Funnels for Efficient Light-Emitting Diodes. Nat. Nanotech. 2016, 11, 872-877. 9.

Grancini, G.; Roldan-Carmona, C.; Zimmermann, I.; Mosconi, E.; Lee, X.;

Martineau, D.; Narbey, S.; Oswald, F.; De Angelis, F.; Graetzel, M.; Nazeeruddin, M. K., One-Year Stable Perovskite Solar Cells by 2D/3D Interface Engineering. Nat. Commun. 2017, 8, 15684. 10. Wang, N.; Cheng, L.; Ge, R.; Zhang, S.; Miao, Y.; Zou, W.; Yi, C.; Sun, Y.; Cao, Y.; Yang, R.; Wei, Y.; Guo, Q.; Ke, Y.; Yu, M.; Jin, Y.; Liu, Y.; Ding, Q.; Di, D.; Yang, L.; Xing, G.; Tian, H.; Jin, C.; Gao, F.; Friend, R. H.; Wang, J.; Huang, W., Perovskite Light-Emitting Diodes Based on Solution-Processed Self-Organized Multiple Quantum Wells. Nat. Photon. 2016, 10, 699-704. 11. Huang, H.; Chen, B.; Wang, Z.; Hung, T. F.; Susha, A. S.; Zhong, H.; Rogach, A. L., Water Resistant CsPbX3 Nanocrystals Coated with Polyhedral Oligomeric Silsesquioxane and Their Use as Solid State Luminophores in All-Perovskite White Light-Emitting Devices. Chem. Sci. 2016, 7, 5699-5703. 12. Protesescu, L.; Yakunin, S.; Bodnarchuk, M. I.; Krieg, F.; Caputo, R.; Hendon, C. H.; Yang, R. X.; Walsh, A.; Kovalenko, M. V., Nanocrystals of Cesium Lead Halide


Page 26 of 32

Page 27 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Perovskites (CsPbX3, X = Cl, Br, and I): Novel Optoelectronic Materials Showing Bright Emission with Wide Color Gamut. Nano Lett. 2015, 15, 3692-3696. 13. Pan, A.; He, B.; Fan, X.; Liu, Z.; Urban, J. J.; Alivisatos, A. P.; He, L.; Liu, Y., Insight into the Ligand-Mediated Synthesis of Colloidal CsPbBr3 Perovskite Nanocrystals: The Role of Organic Acid, Base, and Cesium Precursors. ACS Nano 2016, 10, 7943-7954. 14. Zhang, F.; Zhong, H.; Chen, C.; Wu, X.-g.; Hu, X.; Huang, H.; Han, J.; Zou, B.; Dong, Y., Brightly Luminescent and Color-Tunable Colloidal CH3NH3PbX3 (X = Br, I, Cl) Quantum Dots: Potential Alternatives for Display Technology. ACS Nano 2015, 9, 4533-4542. 15. Li, J.; Xu, L.; Wang, T.; Song, J.; Chen, J.; Xue, J.; Dong, Y.; Cai, B.; Shan, Q.; Han, B.; Zeng, H., 50-Fold EQE Improvement up to 6.27% of Solution-Processed All-Inorganic Perovskite CsPbBr3 QLEDs via Surface Ligand Density Control. Adv. Mater. 2017, 29, 1603885. 16. Miyata, A.; Mitioglu, A.; Plochocka, P.; Portugall, O.; Wang, J. T.-W.; Stranks, S. D.; Snaith, H. J.; Nicholas, R. J., Direct Measurement of the Exciton Binding Energy and Effective Masses for Charge Carriers in Organic–Inorganic Tri-Halide Perovskites. Nat. Phys. 2015, 11, 582-587. 17. Shi, Z.; Li, Y.; Zhang, Y.; Chen, Y.; Li, X.; Wu, D.; Xu, T.; Shan, C.; Du, G., High-Efficiency and Air-Stable Perovskite Quantum Dots Light-Emitting Diodes with an All-Inorganic Heterostructure. Nano Lett. 2017, 17, 313-321. 18. Li, G.; Rivarola, F. W. R.; Davis, N. J. L. K.; Bai, S.; Jellicoe, T. C.; de la Peña, F.;



Hou, S.; Ducati, C.; Gao, F.; Friend, R. H.; Greenham, N. C.; Tan, Z.-K., Highly Efficient Perovskite Nanocrystal Light-Emitting Diodes Enabled by a Universal Crosslinking Method. Adv. Mater. 2016, 28, 3528-3534. 19. Pan, J.; Quan, L. N.; Zhao, Y.; Peng, W.; Murali, B.; Sarmah, S. P.; Yuan, M.; Sinatra, L.; Alyami, N. M.; Liu, J.; Yassitepe, E.; Yang, Z.; Voznyy, O.; Comin, R.; Hedhili, M. N.; Mohammed, O. F.; Lu, Z. H.; Kim, D. H.; Sargent, E. H.; Bakr, O. M., Highly Efficient Perovskite-Quantum-Dot Light-Emitting Diodes by Surface Engineering. Adv. Mater. 2016, 28, 8718-8725. 20. Zhang, X.; Xu, B.; Zhang, J.; Gao, Y.; Zheng, Y.; Wang, K.; Sun, X. W., All-Inorganic Perovskite Nanocrystals for High-Efficiency Light Emitting Diodes: Dual-Phase CsPbBr3-CsPb2Br5 Composites. Adv. Funct. Mater. 2016, 26, 4595-4600. 21. Shamsi, J.; Rastogi, P.; Caligiuri, V.; Abdelhady, A. L.; Spirito, D.; Manna, L.; Krahne, R., Bright-Emitting Perovskite Films by Large-Scale Synthesis and Photoinduced Solid-State Transformation of CsPbBr3 Nanoplatelets. ACS Nano 2017, 11, 10206-10213. 22. Di Stasio, F.; Imran, M.; Akkerman, Q. A.; Prato, M.; Manna, L.; Krahne, R., Reversible Concentration-Dependent Photoluminescence Quenching and Change of Emission Color in CsPbBr3 Nanowires and Nanoplatelets. J. Phys. Chem. Lett. 2017, 8, 2725-2729. 23. Anderson, N. C.; Hendricks, M. P.; Choi, J. J.; Owen, J. S., Ligand Exchange and the Stoichiometry of Metal Chalcogenide Nanocrystals: Spectroscopic Observation of Facile Metal-Carboxylate Displacement and Binding. J. Am. Chem. Soc. 2013, 135,


Page 28 of 32

Page 29 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


18536-18548. 24. Bae, W. K.; Park, Y.-S.; Lim, J.; Lee, D.; Padilha, L. A.; McDaniel, H.; Robel, I.; Lee, C.; Pietryga, J. M.; Klimov, V. I., Controlling the Influence of Auger Recombination on the Performance of Quantum-Dot Light-Emitting Diodes. Nat. Commun. 2013, 4, 2661. 25. Wang, C.; Chesman, A. S.; Jasieniak, J. J., Stabilizing the Cubic Perovskite Phase of CsPbI3 Nanocrystals by Using an Alkyl Phosphinic Acid. Chem. Commun. 2016, 53, 232-235. 26. Koh, W.-k.; Park, S.; Ham, Y., Phosphonic Acid Stabilized Colloidal CsPbX3 (X=Br, I) Perovskite Nanocrystals and Their Surface Chemistry. ChemistrySelect 2016, 1, 3479-3482. 27. Yassitepe, E.; Yang, Z.; Voznyy, O.; Kim, Y.; Walters, G.; Castañeda, J. A.; Kanjanaboos, P.; Yuan, M.; Gong, X.; Fan, F.; Pan, J.; Hoogland, S.; Comin, R.; Bakr, O. M.; Padilha, L. A.; Nogueira, A. F.; Sargent, E. H., Amine-Free Synthesis of Cesium Lead Halide Perovskite Quantum Dots for Efficient Light-Emitting Diodes. Adv. Funct. Mater. 2016, 26, 8757-8763. 28. Qu, L.; Peng, Z. A.; Peng, X., Alternative Routes toward High Quality CdSe Nanocrystals. Nano Lett. 2001, 1, 333-337. 29. Chiba, T.; Hoshi, K.; Pu, Y. J.; Takeda, Y.; Hayashi, Y.; Ohisa, S.; Kawata, S.; Kido, J., High-Efficiency Perovskite Quantum-Dot Light-Emitting Devices by Effective Washing Process and Interfacial Energy Level Alignment. ACS Appl. Mater. Interfaces 2017, 9, 18054-18060.



30. Swarnkar, A.; Marshall, A. R.; Sanehira, E. M.; Chernomordik, B. D.; Moore, D. T.; Christians, J. A.; Chakrabarti, T.; Luther, J. M., Quantum Dot–Induced Phase Stabilization of α-CsPbI3 Perovskite for High-Efficiency Photovoltaics. Science 2016, 354, 92-95. 31. De Roo, J.; Ibanez, M.; Geiregat, P.; Nedelcu, G.; Walravens, W.; Maes, J.; Martins, J. C.; Van Driessche, I.; Kovalenko, M. V.; Hens, Z., Highly Dynamic Ligand Binding and Light Absorption Coefficient of Cesium Lead Bromide Perovskite Nanocrystals. ACS Nano 2016, 10, 2071-81. 32. Dai, X.; Zhang, Z.; Jin, Y.; Niu, Y.; Cao, H.; Liang, X.; Chen, L.; Wang, J.; Peng, X., Solution-Processed, High-Performance Light-Emitting Diodes Based on Quantum Dots. Nature 2014, 515, 96-99. 33. Smith, I. C.; Hoke, E. T.; Solis-Ibarra, D.; McGehee, M. D.; Karunadasa, H. I., A Layered Hybrid Perovskite Solar-Cell Absorber with Enhanced Moisture Stability. Angew. Chem. 2014, 126, 11414-11417. 34. Protesescu, L.; Yakunin, S.; Bodnarchuk, M. I.; Bertolotti, F.; Masciocchi, N.; Guagliardi, A.; Kovalenko, M. V., Monodisperse Formamidinium Lead Bromide Nanocrystals with Bright and Stable Green Photoluminescence. J. Am. Chem. Soc. 2016, 138, 14202-14205. 35. Lee, S.; Park, J. H.; Lee, B. R.; Jung, E. D.; Yu, J. C.; Di Nuzzo, D.; Friend, R. H.; Song, M. H., Amine-Based Passivating Materials for Enhanced Optical Properties and Performance of Organic–Inorganic Perovskites in Light-Emitting Diodes. J. Phys. Chem. Lett. 2017, 8, 1784-1792.


Page 30 of 32

Page 31 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


36. Zou, Y.; Ban, M.; Cui, W.; Huang, Q.; Wu, C.; Liu, J.; Wu, H.; Song, T.; Sun, B., A General Solvent Selection Strategy for Solution Processed Quantum Dots Targeting High Performance Light-Emitting Diode. Adv. Funct. Mater. 2017, 27, 1603325. 37. Zhang, H.; Li, H.; Sun, X.; Chen, S., Inverted Quantum-Dot Light-Emitting Diodes Fabricated by All-Solution Processing. ACS Appl. Mater. Interfaces 2016, 8, 5493-5498. 38. Birnie, D. P., Rational Solvent Selection Strategies to Combat Striation Formation during Spin Coating of Thin Films. J. Mater. Res. 2001, 16, 1145-1154. 39. Wu, Z.; Bai, S.; Xiang, J.; Yuan, Z.; Yang, Y.; Cui, W.; Gao, X.; Liu, Z.; Jin, Y.; Sun, B., Efficient Planar Heterojunction Perovskite Solar Cells Employing Graphene Oxide as Hole Conductor. Nanoscale 2014, 6, 10505-10510.



Table of Contents Graphic


Page 32 of 32

Highly Luminescent and Stable Perovskite Nanocrystals with

Recommend Documents