Surface Ligand Engineering for Near-Unity Quantum Yield Inorganic

Article Cite This: Chem. Mater. XXXX, XXX, XXX−XXX

pubs.acs.org/cm

Surface Ligand Engineering for Near-Unity Quantum Yield Inorganic Halide Perovskite QDs and High-Performance QLEDs Guopeng Li,† Jingsheng Huang,‡ Hanwen Zhu,† Yanqing Li,*,‡ Jian-Xin Tang,*,‡ and Yang Jiang*,† †

School of Materials Science and Engineering, Hefei University of Technology (HFUT), Hefei, Anhui 230009, P. R. China Institute of Functional Nano & Soft Materials (FUNSOM), Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou, Jiangsu 215123, P. R. China

‡

Downloaded via UNIV OF NEW ENGLAND on August 21, 2018 at 23:52:10 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: Despite the great potential of all-inorganic CsPbX3 (X = Br or I) quantum dots (QDs) for light-emitting diodes (QLEDs), their emission properties have been impeded by the long insulating ligands on the QD surface. To address the problem, an efficient surface ligand engineering method has been executed by using a short conjugation molecular ligand phenethylamine (PEA) as ligands to synthesize CsPbX3 QDs and then treating the CsPbX3 QD films with phenethylammonium bromide (PEABr) or phenethylammonium iodide (PEAI). The results indicate that the short conjugation molecular ligand is successfully adsorbed on the surface of CsPbX3 QDs to instead long insulating ligands, resulting in the remarkable enhancement of the carrier injection and transport. The incorporation of phenethylamine (PEA) as synthetic ligand causes the fewer trap states in both CsPbBr3 and CsPbI3 QDs, exhibiting the near-unity photoluminescence quantum yields (PLQYs) of 93% and 95%, respectively. The luminance of CsPbBr3 and CsPbI3 QLEDs could be improved to 21470 and 1444 cd m−2, respectively, when the long insulating ligands were further replaced with conjugation molecular ligands. Particularly, the external quantum efficiency (EQE) of CsPbI3 QLEDs reaches 14.08%, which is among the highest efficiency of red perovskite LEDs.

■

performance.7,35 Ligand exchange (either a film-based posttreatment or a solution-phase pretreatment) has been well developed in traditional QD systems.36 Orders of magnitude improvements in electronic coupling have been successfully demonstrated. The ligand exchange process requires the use of polar solvents that can deliver short but more conductive ligands (be organic or inorganic, such as halides) to cross-link QD solids. Similar ideas in this vein, however, cannot be directly transferred to perovskite QDs. The II−VI and III−V QD cores show high stability against postligand treatments due to their covalent-bond-dominated characteristics.37 This is, however, not the case for perovskite QDs where the polar solvents might challenge the stability of the ionic perovskite QDs. Indeed, the structural stability of CsPbX3 QDs in the presence of polar solvents has limited the direct transfer of a typical ligand exchange process. To address the issue, researchers have developed different approaches that are compatible with perovskite QDs. Zeng’s group proposed a special purification process where a gentler hexane/acetate solvent pair was used as purification reagent to control the density of these insulating ligands.38 The external quantum efficiency (EQE) of CsPbBr3 QLEDs had a 50-fold

INTRODUCTION Solution-processed inorganic perovskite CsPbX3 (X = Cl, Br, or I) quantum dots (QDs) have emerged as one of most promising optoelectronic materials in a wide range of applications such as photovoltaics, photodetectors, lightemitting diodes (LEDs), and lasers, in view of their low-cost wet-chemical processing, extraordinary optical performance, and high stability. 1−9 Following pioneering work by Kovalenko’s group in 2015,1 advances in colloidal synthesis have enabled synthesis of CsPbX3 QDs with controllable sizes and compositions.10−33 The performance of CsPbX3 QDsbased optoelectronic devices has been simultaneously improved in parallel with the development of the abovementioned synthetic process.34 However, compared to their conventional counterparts of II−VI and III−V QDs, the full potential of CsPbX3 QDs has not been unlocked, especially in the field of LEDs.35,36 This is in part due to the striking differences in their intrinsic binding characteristics, e.g., either covalent or ionic. Following colloidal synthesis, CsPbX3 QDs can be stabilized with long organic ligands, such as oleylamine (OAm) and oleic acid (OA), in a similar fashion to II−VI and III−V QDs. For subsequent device applications, the long chain but insulating organic ligands between the QDs, however, hinder efficient charge carriers hopping in the QD solids. This has finally led to a transport concern that may finally compromise the device © XXXX American Chemical Society

Received: June 15, 2018 Revised: August 13, 2018

A

DOI: 10.1021/acs.chemmater.8b02544 Chem. Mater. XXXX, XXX, XXX−XXX

Article

Chemistry of Materials Scheme 1. Synthesis Process of PEA-Modified CsPbBr3 QDs

Figure 1. Characterization of CsPbBr3 QDs with different PEA contents. (a−d) TEM images for PEA contents of (a) 0.1, (b) 0.2, (c) 0.3, and (d) 0.4. Scale bar: 20 nm. (e) Photographs of various QDs octane solutions under daylight (top) and UV 365 nm lamp (bottom) (from left to right: 0.1, 0.2, 0.3, and 0.4 PEA). (f) Absorption and photoluminescence (PL) spectra, (g) XRD patterns, (h) time-resolved PL decay curves, and (I) PLQY and the averaged lifetime (τave).

ligands to synthesize CsPbBr3 QDs directly.41 After introduction of DDAB ligand, the carrier injection and transport are notably enhanced as well as the CsPbX3 QLEDs’ performance. Moreover, multidentate ligands, i.e., 3-(N,N-dimethyloctadecylammonio)propanesulfonate or 2,2′-iminodibenzoic acid, were used to prepare the CsPbX3 QDs with high photoluminescence quantum yields (PLQYs).42,43 Because of the chelate effect with the electronic coupling between ligands and QDs, the durability and stability of perovskite QDs can be improved effectively, which promotes the fabrication of highperformance CsPbX3 QLEDs. Compared with other strategies (e.g., the core−shell structures), the surface ligand engineering is more effective for the optimization of colloidal CsPbX3 (X = Cl, Br, or I) QDs. The main reasons are ascribed to the nature of ionic compound for CsPbX3 QDs and the highly dynamic binding between ligands and QDs.

improvement by executing the treating cycles twice. However, a certain amount of OA and OAm ligands is necessary to keep the stability and optical properties of colloidal CsPbBr3 QDs. This is evidenced by the fact that an increase in the treatment time would significantly degrade the CsPbBr3 QDs and the corresponding QLED performance because of the decrease in the density of OA and OAm ligands.38 In parallel with advances in improved purifications, surface ligand engineering with new-style ligands provides a facile and more diversified route to improving the carrier transport property as well as the QLED performance. The incorporation of new-style ligands can be simply divided into two categories: (1) the direct introduction as synthetic ligands and (2) the post-ligandexchange treatment. A halide ion pair ligand (didodecyldimethylammonium bromide (DDAB)) with a relatively short chain has been used to exchange the longchain ligands OA and OAm,39,40 which were also used as B


Article

Chemistry of Materials

Figure 2. Characterization of CsPbBr3 QDs. (a−d) HRTEM images of PEA-modified CsPbBr3 QDs with the PEA content of (a) 0.1, (b) 0.2, (c) 0.3, and (d) 0.4. Scale bar is 5 nm. The 0.58 and 0.41 nm correspond to the lattice plane spacings of (110) and (020) of orthorhombic perovskite structure. (e) Schematic diagram of CsPbBr3 orthorhombic structure.

Inspired by these encouraging results, a π-conjugation ligand, phenethylamine (PEA) with a benzene ring and short branch chain, has been explored in the surface ligand engineering of CsPbX3 (X = Br and I) QDs, which has a passivation effect on perovskite crystals and can improve their optical properties.44−51 To incorporate PEA ligand on the surface of CsPbX3 QDs sufficiently, PEA was first used as synthetic ligands to partially substitute OAm and prepare perovskite QDs, which had a near-unity PLQY for CsPbBr3 (93%) and CsPbI3 (95%), respectively. The effectiveness of the PEA-based modification was further verified by the performance improvement of CsPbX3 QLEDs. Besides, the πconjugation ligand was furtherly introduced to replace the insulating ligands (e.g., OA and OAm) via a gentle ligandexchange process with PEABr or PEAI. The device performance of CsPbBr3 QLEDs was improved for green emission at 516 nm, exhibiting a maximum luminance of 21470 cd m−2, a maximum EQE of 4.59%, and a maximum current efficiency (CE) of 14.54 cd A−1. Particularly, the red-emission CsPbI3 QLED reached a maximum luminance of 1444 cd m−2 and a maximum EQE of 14.08%, which is among the best performance of red perovskite LEDs based on pure I perovskite materials.34,52−54

■

size may be attributed to the less steric hindrance of PEA than OAm, which would promote the growth of QDs with a bigger size.55 However, it is evident in Figure 2a−d of high-resolution TEM (HRTEM) images that all the PEA-modified CsPbBr3 QDs remain the crystalline orthorhombic structure and cube morphology regardless of the variation in the PEA content. The X-ray diffraction (XRD) patterns in Figure 1g further confirm that the PEA-modified CsPbBr3 QDs keep the perovskite orthorhombic structure (see Figure 2e) in spite of the PEA-induced changes of the cube size.56 These facts manifest that the addition of PEA in the synthesis process would pay a more significant role on the nucleation and growth dynamics of CsPbBr3 QDs rather than their crystal structure and morphology due to the smaller steric hindrance of PEA. The X-ray photoelectron spectroscope (XPS) spectra shown in Figure S4a,b indicate that the protonation degree of amine ligand increases by adding the PEA, which would promote the amine ligands to bind with Br− ion at the surface of QDs and therefore decrease surface trap states. After adding the PEA, the ligand content decreases from 27% to 21% as shown in thermogravimetric analysis (TGA) curves (Figure S4c), which could be attributed to the size increase of QDs and the smaller molecular mass of PEA. The PEA-modified CsPbBr3 QDs exhibit a bright luminance under the UV lamp irradiation as shown in Figure 1e, while the increase in PEA content causes a slight red-shift of the optical properties. It is obvious in Figure 1f and Figure S2c that the photoluminescence (PL) peak of the PEA-modified CsPbBr3 QDs gradually shifts from 501 to 514 nm when changing the PEA content from 0.0 to 0.4. The exciton peak marked in Figure 1f is attributed to some CsPbBr3 QDs with smaller size. This is further supported by the TEM image in Figure S5. The time-resolved PL decay spectra and PLQYs of the PEAmodified CsPbBr3 QDs were measured and are shown in Figures 1h and 1i, revealing the variation of optical properties with respect to the increase in PEA content. The time-resolved PL decay curves in Figure 1h were fitted by a three-exponential decay (shown in Table S1) for reducing the goodness-of-fit χR2 below to 1.2.54,57 As summarized in Figure 1i and Table S1, the incorporation of PEA causes the similar variation tendency of PLQY and average lifetime (τave). The use of 0.2 PEA leads to the maximum PLQY of 93% and the peak τave of 11.03 ns. To

RESULTS AND DISCUSSION

The CsPbX3 (X = Br or I) QDs were synthesized according to previous reports shown in Scheme 1, in which the synthetic ligand OAm was partially substituted by PEA (see the chemical structure in Figure S1).12 As discussed below, the presence of PEA adsorbed on the surface of CsPbBr3 QDs has been proved by the Fourier transform infrared spectroscopy (FTIR) measurement. To verify the effect of PEA content on the physical properties of CsPbX3 QDs, a series of CsPbBr3 QDs were synthesized by altering the PEA content from 0.0 to 0.4 as determined with the PbBr2 solubility. Figure 1a−d displays the transmission electron microscope (TEM) images of CsPbBr3 QDs modified with various PEA contents (0.1− 0.4), and for comparison, the TEM image of CsPbBr3 QDs without PEA is presented in Figure S2a. Note that the cube size of PEA-modified CsPbBr3 QDs increases with the increase in the PEA content, which agrees with the trend of size variation in Figure S3. The mechanism of the increase in cube C


Article


Figure 3. PEA-modified CsPbBr3 QDs films before and after ligand-exchange with PEABr. (a) Schematic of the ligand exchange on the QD surface. (b) FTIR spectra, (c) XRD patterns, and (d) optical spectra (left) and photograph (right) of PEA-modified CsPbBr3 QDs film before and after ligand exchange with PEABr.

in PEA-modified CsPbBr3 QDs may originate from the improved electronic coupling between the ligands and QDs due to the insertion of π-conjugation benzene ring and the ligand shortening by partially replacing OAm with PEA. The improved electrical conduction of QLEDs with PEA-modified CsPbBr3 QDs is accompanied by the significant luminance increase from 402.5 to 2331 cd m−2 with respect to the increase in PEA content from 0.0 to 0.2 (see Table S2). Consequently, the efficiency of QLEDs can be dramatically improved with a maximum current efficiency (CE) of 12.41 cd A−1, power efficiency (PE) of 7.09 lm W1−, and EQE of 3.75%, which are consistent with the observation of higher PLQY and fewer traps in 0.2 PEA-modified CsPbBr3 QDs in Figure 1. The improved performance of QLEDs proves the effectiveness of incorporating PEA in the synthesis process of CsPbBr3 QDs. To further enhance the performance of CsPbBr3 QLEDs, increasing the PEA content in the CsPbBr3 QDs layer was attempted via a solid-state ligand-exchange method. As shown in Figure 3a, the ligand-exchange process was conducted by substituting PEABr (see Figure S1) for the long-chain ligands of OA and OAm. Here, ethyl acetate was used as the solvent to dissolve PEABr due to its low polarity, which would be advantageous to persevere the structure integrity and optical properties of CsPbBr3 QDs.4,38 For simplicity, the abbreviation EX is used to denote the ligand exchange. Figure 3b presents the FTIR spectra of 0.2 PEA-modified CsPbBr3 QDs without and with the EX to characterize the variation of surface ligands caused by the ligand-exchange process, and the marked peaks are identified in Note S1 (Supporting Information). It is noted that the benzene ring signals can be observed in PEA-modified CsPbBr3 QDs without the PEABr ligand-exchange process, implying the successful introduction of the PEA ligands on the surface of CsPbBr3 QDs. Upon the ligand-exchange process, the signals about PEA become stronger. At the same time, the peaks related to −CH2−, −CH3, and −CO signals are weakened, and the others corresponding to −NH2, −COO−, and −CC− (OA and OAm) disappear. The XPS spectra presented in Figure S7 also demonstrate that C and O element

further elucidate the effect of PEA modification on the recombination dynamics of CsPbBr3 QDs, the radiative recombination lifetime (τr), radiative (kr) and nonradiative (knr) decay rates were calculated based on the PLQY and τave. As listed in Table S1, the τr increases markedly from 9.09 ns to >11 ns with the PEA content of ≥0.2, while the kr inversely decreases from 0.11 to ∼0.08 ns−1. Such behaviors may be attributed to the weaker quantum confinement with the increase in the size of CsPbBr3 QDs as demonstrated in Figure 1.58 Meanwhile, the knr declines from 0.028 ns−1 (0.0 PEA) to 0.006 ns−1 (0.2 PEA), implying the suppression of the nonradiative decay with the incorporation of PEA during the synthesis process.41,42 According to the enhanced PLQY and time-resolved PL decay, it is reasonable to infer that the PEAmodified CsPbBr3 QDs possess fewer trap states. In addition, it should be pointed out that the shorter τave (5.44 ns) and larger knr (0.059 ns−1) observed in 0.1 PEA-modified CsPbBr3 QDs may be caused by the reabsorption between the multisized QDs with different bandgaps, rather than the increase of trap states by the PEA addition.3 To further evaluate the quality of PEA-modified CsPbBr3 QDs, the QLEDs were constructed by spin-coating the holeinjection layer (HIL) of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), the hole-transport layer of poly(N,N′-bis-4-butylphenyl-N,N′bisphenyl)benzidine (Poly-TPD), and the emission layer (EML) of CsPbBr3 QDs on the ITO glass and then evaporating the electron-transport layer (ETL) of 2,2′,2″(1,3,5-benzinetriyl)tris(1-phenyl-1-H-benzimidazole) (TPBi) and the bilayer cathode of LiF/Al. The device performances of CsPbBr3 QLEDs with different PEA contents are plotted in Figure S6, and the corresponding results are presented in Table S2. The current density−voltage (J−V) and luminance− voltage (V−L) curves in Figure S6a reveal that the QLEDs with PEA-modified CsPbBr3 QDs exhibit the lower driving voltage as compared to that without PEA, indicating a better carrier injection and transport in the EML of PEA-modified CsPbBr3 QDs.41−43 Such an enhanced electrical conductivity D


Article


Figure 4. Device performance of various CsPbBr3 QLEDs treated with PEABr. (a) Schematic of the device structure. (b) Current density−voltage (J−V) and luminance−voltage (L−V) characteristics. (c) EQE as a function of luminance. (d) CE as a function of luminance. (e) EL spectra. Inset: photograph of the working device with green emission. (f) CIE coordinates. Inset is the enlarged region.

Table 1. Performance of CsPbBr3 QLEDs Treated with PEABr PEA content

HTL

EL peak [nm]

Von [V]

max L [cd m−2]

max EQE [%]

max CE [cd A−1]

max PE [lm W−1]

0.0 0.1 0.2 0.3 0.4 0.2

Poly-TPD Poly-TPD Poly-TPD Poly-TPD Poly-TPD TFB/PVK

516 516 516 518 516 516

3.02 3.08 2.90 2.97 3.17 3.58

14950 16800 21470 19610 18470 13770

2.92 1.32 2.23 2.95 1.89 4.59

9.50 4.42 7.47 9.92 6.28 14.54

5.17 4.42 5.08 5.88 3.04 8.75

CIE coordinates (0.0906, (0.1037, (0.0969, (0.0992, (0.0991, (0.0896,

0.7608) 0.7593) 0.7624) 0.7658) 0.7618) 0.7581)

with the EX. Two distinct peaks are obvious in the XRD patterns, which match well with (110) and (220) crystallographic planes of the perovskite orthorhombic structure. The film XRD patterns are different from the powder XRD patterns presented in Figure 1g, which could be attributed to the higher oriented packing of CsPbBr3 QDs in the film state.59 Figure 3d shows the optical spectra of various CsPbBr3 QDs films together with the photographs under the UV irradiation, revealing the negligible change in the spectral shape but a slight red-shift of the absorption edges and PL peaks after the EX

contents are decreased and the N element content is almost invariable, while the Cs, Pb, and Br atom contents are increased after the ligand-exchange process. The variation in FTIR and XPS spectra indicates that the long-chain ligands (OA and OAm) have been partially replaced with PEA. Figure S8 shows the scanning electron microscope (SEM) images of 0.2 PEA-modified CsPbBr3 QDs layers before and after the EX, indicating that individual CsPbBr3 QDs in the films is maintained after the EX with PEABr. Figure 3c displays the XRD patterns of various PEA-modified CsPbBr3 QDs films E


Article


Figure 5. Device performance of CsPbI3 QLEDs. (a) J−V−L and (b) EQE curves of CsPbI3 QLED without PEA, with 0.2 PEA and with 0.2 PEA and PEAI treatment. (c) EL spectrum (inset is the photograph of a red device) of CsPbI3 QLED with 0.2 PEA and PEAI treatment and (d) CIE coordinates of the EL spectrum.

and electron transports for the more efficient radiative recombination.54,60 As shown in Figure S10 and Table 1, the QLED using a TFB/PVK HTL exhibits a maximum CE of 14.54 cd A−1 and an EQE of 4.59%. In addition, all the CsPbBr3 QDs with the EX have a bright emission at ∼516 nm with a full width at half-maximum (fwhm) of 19 nm (Figure 4e), which corresponds to a high color-purity green emission in Commission Internationale del’ Enclairage (CIE) diagram (Figure 4f). Besides the fabrication of highly bright green CsPbBr3 QLEDs with the PEA modification and the PEABr ligand exchange, the same strategy was explored to synthesize the CsPbI3 QDs for red-emission QLEDs. As shown in Figure S11, the addition of PEA would also increase the size of CsPbI3 QDs while preserving their cubic perovskite structure, high crystallinity, and narrow size distribution. Besides, the PLQY has a distinct improvement from 80% (no PEA) to 95% (0.2 PEA) by introducing PEA, which is in good agreement with the bright red emission of 0.2 PEA CsPbI3 QDs in Figure S12. And the 0.2 PEA modified CsPbI3 QDs have a τr of 31.579 ns, and a slow knr of 1.67 ps−1 (fitted from Figure S12), which indicates that the PEA ligands could also play a dominant role on the passivation of trap states in CsPbI3 QDs. The CsPbI3 QLEDs were fabricated by adopting the same device structure in Figure 4a, and the device performances are shown in Figure 5a,b. With the modification of PEA, the maximum luminance of CsPbI3 QLED is improved from 188 to 549 cd m−2, and the maximum EQE also increases from 2.45% to 7.36%, which could be attributed to the improved optical properties of CsPbI3 QDs with PEA ligand. Besides, the ligand-exchange process was also executed on the 0.2 PEA CsPbI3 QDs layer with PEAI. After the treatment, the Von decreases from 4.71 V (no PEA) and 3.81 V (0.2 PEA) to 3.45 V (0.2 PEA-EX) due to the improvement of the carrier transport and injection as

with PEABr. The PL shift may be attributed to the decreased distance between adjacent CsPbBr3 QDs due to the partial substitution of OA and OAm with shorter surface ligands. These results provide the direct evidence that the structure integrity and optical properties of CsPbBr3 QDs are preserved after this ligand-exchange process, which are accompanied by the uniform and bright luminance from the CsPbBr3 QDs films with the PEABr ligand-exchange (see the photographs in Figure 3d). The CsPbBr3 QLEDs treated with PEABr were fabricated with a device structure schematically shown in Figure 4a. The device performance of various PEA-modified CsPbBr3 QLEDs is plotted in Figure 4b−f, and the corresponding parameters are summarized in Table 1. It is worth noting in Figure 4b that the PEABr ligand-exchange treatment causes the further decrease in the driving voltage as compared to the J−V characteristics of CsPbBr3 QLEDs without the EX (Figure S3a). Correspondingly, the turn-on voltage (Von) of the devices treated with PEABr was reduced from ∼4.5 V (Table S2) to ∼3.0 V (Table 1), indicating the improved conductivity of the CsPbBr3 QD films after the ligand-exchange process (as shown in Figure S9). More importantly, the peak luminance of the QLEDs treated with PEABr (Figure 4b) is an order of magnitude higher than that with only PEA modification (Figure S3a), leading to the maximum luminance up to 21470 cd m−2 for the case of 0.2 PEA content. However, CsPbBr3 QLEDs treated with PEABr exhibit a lower efficiency than that without the ligand exchange, which is ascribed to the stronger nonradiative recombination induced by the more unbalance of injected holes and electrons in the CsPbBr3 QDs with the EX.43 To optimize the efficiency of CsPbBr3 QLEDs treated with PEABr, the poly-TPD-based HTL was replaced with poly(9,9-dioctylfluorene-co-N-(4-butylphenyl)diphenylamine) (TFB) and poly(9-vinylcarbazole) (PVK) to balance the hole F


Article

Chemistry of Materials presented in Figure S9.39−43 At the same time, the luminance and EQE of 0.2 PEA CsPbI3 QLED are increased to 1444 cd ̀ m−2 and 14.08%, which is the best performance of CsPbI3based red-emission LEDs. And the treated CsPbI3 QLED shows a bright red luminescence with a narrow fwhm of 32 nm (Figure 5c) and a high color saturation with the CIE coordinates of (0.7132, 0.2695) (Figure 5d). The abovementioned facts indicate that the surface ligand engineering presented here is an efficient method to fabricate highperformance CsPbX3 (X = Br or I) QLEDs, which also has a well reproducibility as shown in Figure S13.

Technology of China in National University of Defense Technology (SKL 2015 KF04), and the Key Research and Development Project of Anhui Province of China (Grant 1704a0902023).

■

■

CONCLUSION In summary, PEA-based conjugation molecular ligands have been introduced on the surface of CsPbX3 QDs by a two-step surface ligand engineering method to replace the long insulating ligands: (a) the addition of the PEA as the synthetic ligand for QDs and (b) post-ligand exchange with PEABr or PEAI. The PEA modification can effectively passivate the trap states in CsPbX3 QDs, resulting in slow nonradiative recombination rate and the near-unity PLQYs of 93% and 95% for CsPbBr3 and CsPbI3 QDs. The gentle ligand-exchange process could preserve the quantum confinement, optical properties, and structure integrity of CsPbX3 QDs. The treated QLEDs shows a better electrical conductivity due to the remarkable enhancement of carrier injection and transport. The luminance of CsPbBr3 and CsPbI3 QDs-based LEDs can be improved to 21470 and 1444 cd m−2 for emissions at 516 and 694 nm, respectively. Most importantly, the EQE of CsPbI3 QLEDs reaches 14.08%, which is among the highest efficiency of red perovskite LEDs. We anticipate that the conjugation molecular ligand modification provides a facile route to improving the quality of CsPbX3 QDs as well as the QLED performance.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b02544. Experimental section, Figures S1−S13 of QDs and QLEDs characterization, and Tables S1 and S2 of the CsPbBr3 QDs optical properties and CsPbBr3 QLEDs performance (PDF)

■

REFERENCES

(1) 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 Perovskites (CsPbX3, X = Cl, Br, and I): Novel Optoelectronic Materials Showing Bright Emission with Wide Color Gamut. Nano Lett. 2015, 15, 3692−3696. (2) 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. (3) Swarnkar, A.; Chulliyil, R.; Ravi, V. K.; Irfanullah, M.; Chowdhury, A.; Nag, A. Colloidal CsPbBr3 Perovskite Nanocrystals: Luminescence beyond Traditional Quantum Dots. Angew. Chem. 2015, 127, 15644−15648. (4) Sanehira, E. M.; Marshall, A. R.; Christians, J. A.; Harvey, S. P.; Ciesielski, P. N.; Wheeler, L. M.; Schulz, P.; Lin, L. Y.; Beard, M. C.; Luther, J. M. Enhanced mobility CsPbI3 quantum dot arrays for record-efficiency, high-voltage photovoltaic cells. Sci. Adv. 2017, 3, eaao4204. (5) 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. (6) Ramasamy, P.; Lim, D.-H.; Kim, B.; Lee, S.-H.; Lee, M.-S.; Lee, J.-S. All-inorganic cesium lead halide perovskite nanocrystals for photodetector applications. Chem. Commun. 2016, 52, 2067−2070. (7) Kovalenko, M. V.; Protesescu, L.; Bodnarchuk, M. I. Properties and potential optoelectronic applications of lead halide perovskite nanocrystals. Science 2017, 358, 745−750. (8) Huang, H.; Bodnarchuk, M. I.; Kershaw, S. V.; Kovalenko, M. V.; Rogach, A. L. Lead Halide Perovskite Nanocrystals in the Research Spotlight: Stability and Defect Tolerance. ACS Energy Lett. 2017, 2, 2071−2083. (9) Wang, Y.; Li, X.; Song, J.; Xiao, L.; Zeng, H.; Sun, H. AllInorganic Colloidal Perovskite Quantum Dots: A New Class of Lasing Materials with Favorable Characteristics. Adv. Mater. 2015, 27, 7101− 7108. (10) Bekenstein, Y.; Koscher, B. A.; Eaton, S. W.; Yang, P.; Alivisatos, A. P. Highly Luminescent Colloidal Nanoplates of Perovskite Cesium Lead Halide and Their Oriented Assemblies. J. Am. Chem. Soc. 2015, 137, 16008−16011. (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) Li, G.; Wang, H.; Zhang, T.; Mi, L.; Zhang, Y.; Zhang, Z.; Zhang, W.; Jiang, Y. Solvent-Polarity-Engineered Controllable Synthesis of Highly Fluorescent Cesium Lead Halide Perovskite Quantum Dots and Their Use in White Light-Emitting Diodes. Adv. Funct. Mater. 2016, 26, 8478−8486. (13) Zhang, D.; Eaton, S. W.; Yu, Y.; Dou, L.; Yang, P. SolutionPhase Synthesis of Cesium Lead Halide Perovskite Nanowires. J. Am. Chem. Soc. 2015, 137, 9230−9233. (14) Begum, R.; Parida, M. R.; Abdelhady, A. L.; Murali, B.; Alyami, N. M.; Ahmed, G. H.; Hedhili, M. N.; Bakr, O. M.; Mohammed, O. F. Engineering Interfacial Charge Transfer in CsPbBr3 Perovskite Nanocrystals by Heterovalent Doping. J. Am. Chem. Soc. 2017, 139, 731−737. (15) Hu, Q.; Li, Z.; Tan, Z.; Song, H.; Ge, C.; Niu, G.; Han, J.; Tang, J. Rare Earth Ion-Doped CsPbBr3 Nanocrystals. Adv. Opt. Mater. 2018, 6, 1700864−170868. (16) Liu, W.; Lin, Q.; Li, H.; Wu, K.; Robel, I.; Pietryga, J. M.; Klimov, V. I. Mn(2+)-Doped Lead Halide Perovskite Nanocrystals

AUTHOR INFORMATION

Corresponding Authors

*(Y.J.) E-mail: [email protected]. *(Y.-Q.L.) E-mail: [email protected]. *(J.-X.T.) E-mail: [email protected]. ORCID

Jian-Xin Tang: 0000-0002-6813-0448 Yang Jiang: 0000-0002-5364-1421 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grants U1632151, 61076040, 61520106012, 61522505, and 61722404), the Open Research Fund of State Key Laboratory of Pulsed Power Laser G


Article

Chemistry of Materials with Dual-Color Emission Controlled by Halide Content. J. Am. Chem. Soc. 2016, 138, 14954−14961. (17) Pan, G.; Bai, X.; Yang, D.; Chen, X.; Jing, P.; Qu, S.; Zhang, L.; Zhou, D.; Zhu, J.; Xu, W.; Dong, B.; Song, H. Doping Lanthanide into Perovskite Nanocrystals: Highly Improved and Expanded Optical Properties. Nano Lett. 2017, 17, 8005−8011. (18) Wang, H. C.; Wang, W.; Tang, A. C.; Tsai, H. Y.; Bao, Z.; Ihara, T.; Yarita, N.; Tahara, H.; Kanemitsu, Y.; Chen, S.; Liu, R. S. HighPerformance CsPb1‑x Snx Br3 Perovskite Quantum Dots for LightEmitting Diodes. Angew. Chem., Int. Ed. 2017, 56, 13650−13654. (19) Zou, S.; Liu, Y.; Li, J.; Liu, C.; Feng, R.; Jiang, F.; Li, Y.; Song, J.; Zeng, H.; Hong, M.; Chen, X. Stabilizing Cesium Lead Halide Perovskite Lattice through Mn(II) Substitution for Air-Stable LightEmitting Diodes. J. Am. Chem. Soc. 2017, 139, 11443−11450. (20) Yao, J. S.; Ge, J.; Han, B. N.; Wang, K. H.; Yao, H. B.; Yu, H. L.; Li, J. H.; Zhu, B. S.; Song, J. Z.; Chen, C.; Zhang, Q.; Zeng, H. B.; Luo, Y.; Yu, S. H. Ce(3+)-Doping to Modulate Photoluminescence Kinetics for Efficient CsPbBr3 Nanocrystals Based Light-Emitting Diodes. J. Am. Chem. Soc. 2018, 140, 3626−3624. (21) Parobek, D.; Dong, Y.; Qiao, T.; Son, D. H. Direct HotInjection Synthesis of Mn-Doped CsPbBr3 Nanocrystals. Chem. Mater. 2018, 30, 2939−2944. (22) Udayabhaskararao, T.; Kazes, M.; Houben, L.; Lin, H.; Oron, D. Nucleation, Growth, and Structural Transformations of Perovskite Nanocrystals. Chem. Mater. 2017, 29, 1302−1308. (23) Nedelcu, G.; Protesescu, L.; Yakunin, S.; Bodnarchuk, M. I.; Grotevent, M.; Kovalenko, M. V. Fast anion-exchange in highly luminescent nanocrystals of cesium lead halide perovskites (CsPbX3, X = Cl, Br, I). Nano Lett. 2015, 15, 5635−5640. (24) Tong, Y.; Bladt, E.; Aygüler, M. F.; Manzi, A.; Milowska, K. Z.; Hintermayr, V. A.; Docampo, P.; Bals, S.; Urban, A. S.; Polavarapu, L.; Feldmann, J. Highly Luminescent Cesium Lead Halide Perovskite Nanocrystals with Tunable Composition and Thickness by Ultrasonication. Angew. Chem., Int. Ed. 2016, 55, 13887−13892. (25) Zhang, T.; Li, G.; Chang, Y.; Wang, X.; Zhang, B.; Mou, H.; Jiang, Y. Full-spectra hyperfluorescence cesium lead halide perovskite nanocrystals obtained by efficient halogen anion exchange using zinc halogenide salts. CrystEngComm 2017, 19, 1165−1171. (26) Li, X.; Wu, Y.; Zhang, S.; Cai, B.; Gu, Y.; Song, J.; Zeng, H. CsPbX3 Quantum Dots for Lighting and Displays: Room-Temperature Synthesis, Photoluminescence Superiorities, Underlying Origins and White Light-Emitting Diodes. Adv. Funct. Mater. 2016, 26, 2435− 2445. (27) Akkerman, Q. A.; D’Innocenzo, V.; Accornero, S.; Scarpellini, A.; Petrozza, A.; Prato, M.; Manna, L. Tuning the Optical Properties of Cesium Lead Halide Perovskite Nanocrystals by Anion Exchange Reactions. J. Am. Chem. Soc. 2015, 137, 10276−10281. (28) Liu, F.; Zhang, Y.; Ding, C.; Kobayashi, S.; Izuishi, T.; Nakazawa, N.; Toyoda, T.; Ohta, T.; Hayase, S.; Minemoto, T.; Yoshino, K.; Dai, S.; Shen, Q. Highly Luminescent Phase-Stable CsPbI3 Perovskite Quantum Dots Achieving Near 100% Absolute Photoluminescence Quantum Yield. ACS Nano 2017, 11, 10373− 10383. (29) Wei, S.; Yang, Y.; Kang, X.; Wang, L.; Huang, L.; Pan, D. Homogeneous Synthesis and Electroluminescence Device of Highly Luminescent CsPbBr3 Perovskite Nanocrystals. Inorg. Chem. 2017, 56, 2596−2601. (30) 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. (31) Imran, M.; Caligiuri, V.; Wang, M.; Goldoni, L.; Prato, M.; Krahne, R.; De Trizio, L.; Manna, L. Benzoyl Halides as Alternative Precursors for the Colloidal Synthesis of Lead-Based Halide Perovskite Nanocrystals. J. Am. Chem. Soc. 2018, 140, 2656−2664. (32) Liu, P.; Chen, W.; Wang, W.; Xu, B.; Wu, D.; Hao, J.; Cao, W.; Fang, F.; Li, Y.; Zeng, Y.; Pan, R.; Chen, S.; Cao, W.; Sun, X. W.;

Wang, K. Halide-Rich Synthesized Cesium Lead Bromide Perovskite Nanocrystals for Light-Emitting Diodes with Improved Performance. Chem. Mater. 2017, 29, 5168−5173. (33) Liu, W.; Zheng, J.; Cao, S.; Wang, L.; Gao, F.; Chou, K. C.; Hou, X.; Yang, W. General Strategy for Rapid Production of LowDimensional All-Inorganic CsPbBr3 Perovskite Nanocrystals with Controlled Dimensionalities and Sizes. Inorg. Chem. 2018, 57, 1598− 1603. (34) Akkerman, Q. A.; Raino, G.; Kovalenko, M. V.; Manna, L. Genesis, challenges and opportunities for colloidal lead halide perovskite nanocrystals. Nat. Mater. 2018, 17, 394−405. (35) Voznyy, O.; Sutherland, B. R.; Ip, A. H.; Zhitomirsky, D.; Sargent, E. H. Engineering charge transport by heterostructuring solution-processed semiconductors. Nat. Rev. Mater. 2017, 2, 17026− 17035. (36) Dai, X.; Deng, Y.; Peng, X.; Jin, Y. Quantum-Dot LightEmitting Diodes for Large-Area Displays: Towards the Dawn of Commercialization. Adv. Mater. 2017, 29, 1607022−1607043. (37) Boles, M. A.; Ling, D.; Hyeon, T.; Talapin, D. V. The surface science of nanocrystals. Nat. Mater. 2016, 15, 141−153. (38) 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− 1603893. (39) 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. (40) 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. (41) Wu, H.; Zhang, Y.; Lu, M.; Zhang, X.; Sun, C.; Zhang, T.; Colvin, V. L.; Yu, W. W. Surface ligand modification of cesium lead bromide nanocrystals for improved light-emitting performance. Nanoscale 2018, 10, 4173−4178. (42) Pan, J.; Shang, Y.; Yin, J.; De Bastiani, M.; Peng, W.; Dursun, I.; Sinatra, L.; El-Zohry, A. M.; Hedhili, M. N.; Emwas, A. H.; Mohammed, O. F.; Ning, Z.; Bakr, O. M. Bidentate Ligand-Passivated CsPbI3 Perovskite Nanocrystals for Stable Near-Unity Photoluminescence Quantum Yield and Efficient Red Light-Emitting Diodes. J. Am. Chem. Soc. 2018, 140, 562−565. (43) Krieg, F.; Ochsenbein, S. T.; Yakunin, S.; ten Brinck, S.; Aellen, P.; Süess, A.; Clerc, B.; Guggisberg, D.; Nazarenko, O.; Shynkarenko, Y.; Kumar, S.; Shih, C.-J.; Infante, I.; Kovalenko, M. V. Colloidal CsPbX3 (X = Cl, Br, I) Nanocrystals 2.0: Zwitterionic Capping Ligands for Improved Durability and Stability. ACS Energy Lett. 2018, 3, 641−646. (44) Quan, L. N.; Zhao, Y.; García 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. (45) Ng, Y. F.; Kulkarni, S. A.; Parida, S.; Jamaludin, N. F.; Yantara, N.; Bruno, A.; Soci, C.; Mhaisalkar, S.; Mathews, N. Highly efficient Cs-based perovskite light-emitting diodes enabled by energy funnelling. Chem. Commun. 2017, 53, 12004−12007. (46) Yuan, M.; Quan, L. N.; Comin, R.; Walters, G.; Sabatini, R.; Voznyy, O.; Hoogland, S.; Zhao, Y.; Beauregard, E. M.; Kanjanaboos, P.; Lu, Z.; Sargent, E. H. Perovskite energy funnels for efficient lightemitting diodes. Nat. Nanotechnol. 2016, 11, 872−877. (47) Yang, X.; Zhang, X.; Deng, J.; Chu, Z.; Jiang, Q.; Meng, J.; Wang, P.; Zhang, L.; Yin, Z.; You, J. Efficient green light-emitting diodes based on quasi-two-dimensional composition and phase H


Article

Chemistry of Materials engineered perovskite with surface passivation. Nat. Commun. 2018, 9, 570−577. (48) Fu, Y.; Rea, M. T.; Chen, J.; Morrow, D. J.; Hautzinger, M. P.; Zhao, Y.; Pan, D.; Manger, L. H.; Wright, J. C.; Goldsmith, R. H.; Jin, S. Selective Stabilization and Photophysical Properties of Metastable Perovskite Polymorphs of CsPbI3 in Thin Films. Chem. Mater. 2017, 29, 8385−8394. (49) Li, F.; Pei, Y.; Xiao, F.; Zeng, T.; Yang, Z.; Xu, J.; Sun, J.; Peng, B.; Liu, M. Tailored dimensionality to regulate the phase stability of inorganic cesium lead iodide perovskites. Nanoscale 2018, 10, 6318− 6322. (50) Zhang, L.; Liang, W. How the Structures and Properties of Two-Dimensional Layered Perovskites MAPbI3 and CsPbI3 Vary with the Number of Layers. J. Phys. Chem. Lett. 2017, 8, 1517−1523. (51) Li, N.; Zhu, Z.; Dong, Q.; Li, J.; Yang, Z.; Chueh, C.-C.; Jen, A. K.-Y.; Wang, L. Enhanced Moisture Stability of Cesium-Containing Compositional Perovskites by a Feasible Interfacial Engineering. Adv. Mater. Interfaces 2017, 4, 1700598−1700605. (52) Zou, W.; Li, R.; Zhang, S.; Liu, Y.; Wang, N.; Cao, Y.; Miao, Y.; Xu, M.; Guo, Q.; Di, D.; Zhang, L.; Yi, C.; Gao, F.; Friend, R. H.; Wang, J.; Huang, W. Minimising efficiency roll-off in high-brightness perovskite light-emitting diodes. Nat. Commun. 2018, 9, 608−614. (53) 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. Photonics 2016, 10, 699−705. (54) Si, J.; Liu, Y.; He, Z.; Du, H.; Du, K.; Chen, D.; Li, J.; Xu, M.; Tian, H.; He, H.; Di, D.; Lin, C.; Cheng, Y.; Wang, J.; Jin, Y. Efficient and High-Color-Purity Light-Emitting Diodes Based on In Situ Grown Films of CsPbX3 (X = Br, I) Nanoplates with Controlled Thicknesses. ACS Nano 2017, 11, 11100−11107. (55) 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. (56) Bertolotti, F.; Protesescu, L.; Kovalenko, M. V.; Yakunin, S.; Cervellino, A.; Billinge, S. J. L.; Terban, M. W.; Pedersen, J. S.; Masciocchi, N.; Guagliardi, A. Coherent Nanotwins and Dynamic Disorder in Cesium Lead Halide Perovskite Nanocrystals. ACS Nano 2017, 11, 3819−3831. (57) Shi, Z.; Li, S.; Li, Y.; Ji, H.; Li, X.; Wu, D.; Xu, T.; Chen, Y.; Tian, Y.; Zhang, Y.; Shan, C.; Du, G. Strategy of Solution-Processed All-Inorganic Heterostructure for Humidity/Temperature-Stable Perovskite Quantum Dot Light-Emitting Diodes. ACS Nano 2018, 12, 1462−1472. (58) Kim, Y.-H.; Wolf, C.; Kim, Y.-T.; Cho, H.; Kwon, W.; Do, S.; Sadhanala, A.; Park, C. G.; Rhee, S.-W.; Im, S. H.; Friend, R. H.; Lee, T.-W. Highly Efficient Light-Emitting Diodes of Colloidal Metal− Halide Perovskite Nanocrystals beyond Quantum Size. ACS Nano 2017, 11, 6586−6593. (59) Kim, Y.; Yassitepe, E.; Voznyy, O.; Comin, R.; Walters, G.; Gong, X.; Kanjanaboos, P.; Nogueira, A. F.; Sargent, E. H. Efficient Luminescence from Perovskite Quantum Dot Solids. ACS Appl. Mater. Interfaces 2015, 7, 25007−25013. (60) Dai, X.; Zhang, Z.; Jin, Y.; Niu, Y.; Cao, H.; Liang, X.; Chen, L.; Wang, J.; Peng, X. Solution-processed, high-performance lightemitting diodes based on quantum dots. Nature 2014, 515, 96−99.

I


Surface Ligand Engineering for Near-Unity Quantum Yield Inorganic

Recommend Documents