In Situ Passivation of PbBr64â Octahedra toward Blue Luminescent

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Letter

In-situ Passivation of PbBr64- Octahedrons towards Blue Luminescent CsPbBr3 Nanoplatelets with Near 100% Absolute Quantum Yield Ye Wu, Changting Wei, Xiaoming Li, Yuelei Li, Shuangchen Qiu, Wei Shen, Bo Cai, Zhiguo Sun, Dandan Yang, Zhengtao Deng, and Haibo Zeng ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b01025 • Publication Date (Web): 31 Jul 2018 Downloaded from http://pubs.acs.org on July 31, 2018

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In-situ Passivation of PbBr64- Octahedrons towards Blue Luminescent CsPbBr3 Nanoplatelets with Near 100% Absolute Quantum Yield ⊥

⊥

Ye Wu, †,⊥ Changting Wei, †,⊥ Xiaoming Li, †,* Yuelei Li, † Shuangchen Qiu, † Wei Shen, ‡ Bo Cai, †

†

Zhiguo Sun, † Dandan Yang, † Zhengtao Deng, ‡ Haibo Zeng†,*

MIIT Key Laboratory of Advanced Display Materials and Devices, Institute of Optoelectronics

& Nanomaterials, College of Materials Science and Engineering, Nanjing University of Science and Technology. Nanjing 210094, China ‡

Department of Biomedical Engineering, College of Engineering and Applied Sciences, Nanjing

National Laboratory of Microstructures, Nanjing University. Nanjing 210093, China

AUTHOR INFORMATION Corresponding Author *(X.L.) Email: [email protected], *(H.Z.) Email: [email protected]. Author Contributions ⊥

Y.W. and C.W. contributed equally to this work.


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ABSTRACT Recently, the pursuit of high photoluminescence quantum yields (PLQYs) for blue emission in perovskite nanocrystals (NCs) has attracted enhancive attention because the QY of blue NCs lag behind those of green and red ones severely, which is fatal for three-primary-color displays. Here, we propose an in-situ PbBr64- octahedrons passivation strategy to achieve 96% absolute QY for the ultrapure (line width=12 nm) blue emission from CsPbBr3 nanoplatelets (NPLs) and both the values rank the first among perovskite NCs with blue emission. From the aspect of constructing intact PbBr64- octahedrons, additional Br- was introduced to drive the ionic equilibrium to form intact Pb-Br octahedrons. The reduced Br vacancy and inhibited nonradiative recombination processes are well proved by reduced Urbach energy, increased Pb-Br bonds and slower transient absorption delay. Blue light-emitting diodes (LEDs) using NPLs were fabricated and a high EQE of 0.124% with emission line width of ~12 nm was realized. This work will provide good references to break the ‘blue-wall’ in perovskite NCs.


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Achieving excellent optical properties with low cost and scalable fabrication in quantum confined luminescent nanocrystals (NCs) is a long-standing challenge for their industrial applications. In the past few years, inorganic halide perovskite NCs, have achieved worldwide attentions due to excellent optoelectronic properties.1-3 These NCs are proved to possess narrow emissions in the whole visible region and high photoluminescence quantum yield (PL QY) up to near one unit. More importantly, the ease of synthesis at room temperature without inert gas protection endows them with great potentials in application for wide color gamut and highdefinition displays.4-9 For realizing their application in high-definition displays, the balance development in three-primary-color is important. However, similar to many other NCs and organic luminescent materials, the development of blue band emission in perovskite falls behind severely, while both the green and red bands were reported to achieve near 100% QYs via post surface passivation and modified fabrication procedures, respectively.10-12 Thus, much work is needed to devote for further enhancement in blue band emission. In general, there are three strategies to achieve blue emission for inorganic perovskite NCs, for examples, varying halide composition, decreasing size, or constructing quantum wells (nanoplatelets, NPLs).13-15 However, it is worth to note CsPbBr3 NCs with small size usually exhibit large emission width due to enormous defects, which also results in a low QY.14 Besides, unstable morphology with time will also influence the peak position and emission line widths, which means specific morphology treatments are in demand to avoid aggregation or ripening.16 Although NCs with hybrid halogens (Cl and Br) can also exhibit narrow blue emissions, the unavoidable phase separation induced peak shift and its associated formation of traps are also the limitation for achieving high QY with long-term stability.

17

It is well accepted that quantum

wells with large exciton binding energy (Eb) resulting from the strong quantum confinement


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effect is preferred for emission.18 However, constructing pure CsPbBr3 NPLs with low defect density for standard blue emission (~460 nm) is still difficult. The complexed nucleation mechanism and lattice constructing process always intrduce large amount of surface defect, giving rise to obvious band tails, wide full width at the half maxmium (FWHM) and relatively low QY.14, 19-20 Therefore, we propose an in-situ passivation strategy here by adding excess Brto form intact Pb-Br octahedrons before nucleation to reduce the surface defect density in final NPLs. As a result, ultrapure blue ~460 nm emission (FWHM=12 nm) with absolute QY (96%) approaching unit were achieved and associated high-performance blue light-emitting diodes (LEDs) were successfully demonstrated. For cubic CsPbBr3, Cs ions occupy the vertexes while Pb and Br ions assemble into octahedrons (PbBr64-) in the center of the framework. According to the theoretical calculation, first, Br 4p- and Pb 6p obtials predominantly contribute to upper valence band and the conduction band, respectively.21 Then, the excitation and recombination processes occur within the octahedrons.5, 22 Second, the Br vacancy (VBr) type defect form energy levels in the band gap, which might affect the recombination dynamics of carriers.23-24 Roo et al. found that CsPbBr3 NCs were terminated by oleylammonium bromide. These highly dynamic matters are the origins of the decrease of stability and QY during purification.25 Besides, previous reports indicated that passivation of surface VBr made great contributions to the improvement of QY.26-28 Hence, decreasing VBr density is one of the most important issues that should be taken into consideration for highly luminescent blue perovskite NCs.29 As we know, halide perovskites belong to ionic materials. There exist stronger interactions between ions, leading to an ultrafast nucleation rate, which finishes within 1 ms.30 Then, the coordination states before crystallization have great influence on the final structural defects in


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ACS Energy Letters

perovskite NCs.29 In solution, Pb ions will combine with bromide ions, coordinated solvent molecules and ligands rather than being isolated as shown in Figure 1a.31 The coordinated solvent molecules or ligands will compete with halide ions to form distorted octahedrons, forming VBr before nucleation. However, the nucleation rate is too fast to compensate for all the missing bromide ions in octahedrons, leading to a large amount of VBr defects (Figure 1c)32. Therefore, more halide ions in octahedrons during nucleation will result into lower VBr and this might be one of the reasons that higher PbBr2/Cs+ ratio or halide rich circumstance usually result in higher QY in many cases.33 According to the proposed passivation mechanism, the amount of HBr is very important in the fabrication process. When PbBr2 precursor was first injected, a broad exciton peak at 393 nm was observed in absorption spectrum, indicating the formation of single-layered hybrid perovskites capped with oleylammonium bromide (Figure 1e). However, after the addition of HBr complex solution, the loose connection between PbBr2 and ligands was broken up and the ionic equilibrium moved to the side of forming isolated PbBr64- octahedrons due to the increased Br- concentration. This was proved well by the blue-shift of absorption spectra of the mixture before reaction took place. Additionally, an obvious red-shift was observed with the increase of HBr, indicating the formation of more complete Pb-Br octahedrons. 29


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Figure 1. Schematic demonstration of PbBr64- octahedrons (a) with and (b) without VBr. Via introducing HBr complex solution, the ionic equilibrium was driven to the intact octahedrons side. (c) and (d) show the diagram of final NPLs synthesized by precursor (a) and (b), respectively. (e) The absorption evolution changing with the mass ratio of HBr aqueous solution. (f) XPS spectra of Pb 4f in the pristine and in-situ passivated NPLs. To further confirm the hypothesis discussed above, X-ray photoelectron spectroscopy (XPS) measurements were performed (Figure 1f). The pristine NPLs were synthesized based on a previously reported method by Shamsi19. In Pb 4f XPS core level spectra, two peaks of Pb 4f5/2 (142.93 eV) and Pb 4f7/2 (138.08eV) are defined28. Both the peaks of pristine NPLs can be fitted with two peaks that the signals with high and low binding energy (BE) are assigned to Pb-Br (143.05 and 138.2 eV) and Pb-oleate (142.61 and 137.78 eV) species, respectively. The Pb-


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ACS Energy Letters

oleate signal indicates the existence of VBr defect. On the contrary, the Pb 4f peaks of in-situ passivated NPLs can hardly be fitted into two peaks, which blue shift to higher BEs of 143.05 and 138.2 eV, corresponding with the Pb-Br bonding well. These results imply that with such insitu passivation strategy, the VBr is eliminated succcessfully,28 resulting into less nonradiative recombination defects. As a result, 7-fold improvement of QY is achieved, which change from 16% to 96% (Figure S2). It should be noted that to obtain high performance CsPbBr3 NPLs, the amount of HBr needs careful regulation. The growth kinetics of NPLs was adjusted by the amount of protonated long chain ammonium (OAmBr) 19, 34 Then, small size NCs with wide FWHM formed when HBr was diluted ( mass fraction < 31wt%) (Figure S3), which was also observed by Dutta et al..35 However, when the amount of HBr aqueous solution was excessive, the acidity and water content were enhanced, which would destroy the PbBr64- octahedrons and even the final CsPbBr3 NPLs, resulting in decreased PL QY (Figure S3). The precise crystal structure of the NPLs is confirmed by the small angle X-ray diffraction measurement as shown in Figure 2a. The intense diffraction peaks correspond to the (0 0 2l) reflections. The periodic peaks indicate a stacking distance of 4.5 nm for NPLs. An effective distance of organic ligand layers with ~2.7 nm can be calculated by minusing three inorganic layers, which agrees well with Bekenstein’ report.20 The detailed data from 10 to 45 degree could been found in Figure S4, which is in coincidence with the range containing main peaks of CsPb2Br5 or Cs4PbBr636-37. No obvious peaks of bulk nanoplatelets or other crystal phases could be identified. Hence, we can conclude that the as-prepared nanoplatelets are phase-pure. Figure 2b shows the typical morphology of as synthesized NPLs. The NPLs exhibit a homogeneous thickness distribution of 3±0.2 nm, which is much smaller than the Bohr diameter (7 nm) and


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just corresponds to blue emission around 460 nm.1, 13 It should be noted that the thickness of inorganic layer is 1.8 nm. As the nanoplatelets may not be perpendicular to substrate, a small rotation between nanoplatelets and substrate will lead to visual elongation. The precise interlayer spacing should be calculated from XRD data. Large-area TEM images can be found in Figure S5 and the geometrical relationship of projection is described in Figure S5c.

Figure 2. (a) Small angle XRD data and (b) TEM image of in-situ passivated CsPbBr3 NPLs, the inset of (a) shows the assemble form of NPLs. (c) Absorption and (d) PL spectra of pristine and in-situ passivated NPLs. Summarization of (e) PL QY and (f) FWHM based on inorganic halide perovskite NCs with blue emission.


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ACS Energy Letters

Figure 2c shows the logarithmic absorption spectra of NPLs synthesized by our strategy and Shamsi method. Both of the absorption spectra exhibits obvious exitonic peaks at ~455 nm, indicating the strong quantum confinement effect. Theoretically, the strong confinement effect should result in high Eb and then high QY.20 Eb of both samples were extracted from the absorption spectra according to the following equation18：   ∞   π eπ x  Eb  4π α (ω ) = A ⋅ θ ( hω − E g ) ⋅  ⋅ δ  hω − E g +  + A ⋅ Eg ∑ 1 1  i =1  cosh(π x )  (i + ) 3  (i + ) 2   2 2 

(1)

where A is a constant related to the transition matrix element and sample thickness. ω is the frequency of light, θ is the step function, Eg is the bandgap, x is defined as Eb1/2/(hw-Eg)1/2, i is the principal quantum number and

δ

denotes the delta function. The extracted binding energies

are 90 and 88 meV for pristine and in-situ passivated NPLs, respectively, which are much higher than green NCs (~40 meV, Figure S6). To confirm the accuracy of Eb, temperature-dependent PL measurement of the in-situ passivated sample was conducted and the spectrum is plotted in Figure S6. The fitted Eb is about 89 meV, which is approximate to the analyzed value. It seems that both the samples possess high and similar Eb while the absolute PL QY of pristine sample is only 16% and that of in-situ passivated NPLs is about 96%. Such fascinating value proves our presumption well that in-situ passivation is indeed advantageous. Meanwhile, the low QY in spite of the high Eb for pristine NPLs indicates the great influences of enormous VBr defects, which is also proved by the larger FWHM as shown in Figure 2d. The pristine and passivated NPLs exhibit peak PL wavelengths of 463 and 462 nm, respectively. Obvious larger tails at long wavelength side were observed in absorption and PL spectra of pristine NPLs, corresponding to the higher band edge state density resulted from defects.


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To demonstrate the advantages of our strategy, PL QY and FWHM of perovskite NCs with blue emissions from 440 to 490 nm (via halide doping or quantum confinement effect) up to date are compared in Figure 2e and f.2, 14, 20, 30, 34, 38-45 First of all, it is worth noting that the PL QY is on the uptrend when the emission light shifts to longer wavelength and samples with strong quantum confined effect show higher QY. However, QYs of NCs with PL peak < 480 nm are still below 70%, which are inferior to the green and red ones. In this work, the QY of blue NPLs jumps to a high value of 96%, which is the highest among all the blue samples and comparable to those of green and red perovskite NCs. Generally, samples with short emission wavelength exhibit small FWHM. As shown in Figure 2f, most of the samples show FWHM below 20 nm and some of them reach to ~15 nm. Recently, an ultrapure green emission (FWHM=15.5 nm) was reported, which put forward great challenges to the blue NCs.17 Here, ultranarrow FHWM of 12 nm was achieved, which is also the best result among blue emitting materials. Both the QY and FWHM values manifest the decrease of VBr defects indeed with our in-situ passivation strategy. Additionally, the mechanism is versatile that when the demulsified agent was replaced with other short-chain alcohols, such as propanol, isopropanol, isobutanol, and so on, high PL QYs are also achieved as shown in Figure S7. So far, previous CsPbBr3 NCs with strong quantum confinement effect usually exhibit obvious band-tail emission, which can be assigned to defects.14, 46 Here, the asymmetric behavior at the long wavelength region is suppressed efficiently for the in-situ passivated NPLs, indicating the reduced VBr defect density. To further clarify this point, Urbach energies (EU) of the samples were calculated by plotting the absorption coefficient as a function of photon energy. For semiconductors, the absorption edge is known as the Urbach tail, which is related to the degree of crystal disorder. In general, samples with little content of impurities, defects and electron-


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ACS Energy Letters

phonon interactions exhibit a small EU, as well as a negligible absorption tail. EU can be extracted by fitting the exponential part of the Urbach tail according to the following equation: α ( E ) = α 0 exp[σ (T )

E − E0 ] K BT

(2)

Where kB is the Boltzmann constant, and T is the absolute temperature and the EU is defined as EU=݇஻ ܶ/σ(ܶ). Figure 3a shows the fitting curves of pristine and in-situ passivated NPLs. The calculated EU of pristine NPLs is 23 meV, while the value of in-situ passivated ones is as low as 14 meV, which is smaller than that of red perovskite NCs (18 meV) with 100% QY.11 These results indicate that CsPbBr3 NPLs prepared here possess a very low level of VBr defect density. Then, this difference will influence the exciton dynamics greatly in spite of the similar and high Eb .

Figure 3. (a) Urbach energy diagram, (b) PL decay and (c) transient absorption comparisons between pristine and in-situ passivated CsPbBr3 NPLs. TA spectra of the (d) pristine and (e) insitu passivated NPLs recorded at different delay times. (f) The diagram of possible excition recombination processes. To further clarify the influences of reduced VBr defect density with the in-situ passivation mechanism, PL decay measurements on both samples were carried out as shown in Figure 3b


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where the pump laser wavelength was 405 nm. It is clearly observed that in-situ passivated NPLs exhibit a slower decay. The lifetime of the pristine NPLs is 3.96 ns while that of the in-situ passivated sample exhibits a longer lifetime of 6.46 ns (Table 1). It means that there exist defect related fast exciton trapping process for the pristine sample while the in-situ passivation strategy eliminates the nonradiative recombination pathways effectively. This is in good agreement with the ultrahigh PL QY achieved in the in-situ passivated NPLs. To further reveal the ultrafast exciton trapping processes, transient absorption (TA) measurements were conducted (Figure 3c), and the fitted kinetic were summarized in Table S1. The fit of the kinetics for in-situ passivated sample shows two time constants, 357.2 ps (τ1, 19.6%) and 3733.8 ps (τ2, 80.4%). On the contrary, the pristine samples show much faster decay kinetic processes, with double-exponential time constant of 93.5 ps (τ1, 22.3%) and 1479.8 ps (τ2, 77.7%), respectively. The difference between decay rates are obviously shown in the TA spectra (Figure 3d and e) with a pump intensity of 1.25 µJ cm-2, in which bleach signals with peaks at ~455 nm can be observed. The ∆A decreases to 28.3% at 1189 ps for pristine NPLs while a high ratio of 56.6% is maintained at 1030.2 ps for in-situ passivated NPLs. It is known that ∆A is proportional to the exciton density at the lowest excited states and above difference indicate the faster elimination of excitons in pristine NPLs. Table 1. Lifetime and fractional contribution of different decay channels for pristine and in-situ passivated CsPbBr3 NPLs.

Generally, the excitation and recombination processes associated with excitons can be simplized in Figure 3f. Upon the excitation by absorbing a high energy photon, an electron transits to the conduction band (step 1), which then decaies to the band edge quickly and form a


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free exciton (step 2). The formed exciton will recombine directly (step 3) or trapped by trap states resulted from defects or doping (step 4). Step 3 leads to radiative recombination completely and emit a photon while the trapped exciton might recombine without emission (step 5). Since the cooling of hot exciton is very fast that the time scale is hundreds of femtoseconds or several picoseconds, the fitted short and long lifetime can be assigned to trapping of excitons and band edge exciton recombination processes, respectively.47 The much smaller τ1 for the pristine sample indicates the fast trapping of excitons by VBr defects, resulting in lower density of band edge excitons and increased possibility of nonradiative recombination processes. These behaviors lead to low QY unquentionablely. Besides, though the trapped excitons can recombine in a low probability, the emission energy is lower than that of band edge excitons, resulting in tail and then wider FWHM. On the contrary, the much slower τ1 for the optimized sample indicates the low possibility of exciton trapping process. Higher exciton density near the band edge and longer τ2 contribute to effective radiative recombination, leading to much higher QY and narrower FWHM. All these results are consistent with the excellent optical properties of in-situ passivated NPLs discussed above. Hence, by constructing intact PbBr64- octahedrons, the in-situ passivation strategy eliminates the VBr defects and weakens nonradiative recombination pathway effectively. Interestingly, the NPLs with low VBr defect density exhibit a better stability that after continuous illumination under a strong laser light (10 mW/cm2, 405 nm, 23 oC and 35% humidity), they maintain highly efficient photoluminescence for more than 400 minutes that no additional PL peaks are observed. In contrast, the pristine sample starts to aggregate and new PL peaks at 510 nm and 480 nm appear under irradiation after 150 minutes (Figure S8). Such better stability ensures the fabrication of light-emitting diodes (LEDs) based on these NPLs. We note


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the PL intensity was actually increasing after long irradiation time. It is an interesting phenomenon, which was also discovered by several researchers.48 This can be associated with the smooth of nanoplatelet surface states. We can not give a precise explanation and further investigations are needed. It is widely recognized that chloride-doped perovskites are not ideal candidates for blue LEDs because of low PLQY and phase separation under bias voltage49. The successful fabrication of confined NPLs indicates an alternative method to achieve pure blue color for wide color gamut display. Very recently, Yang and co-workers reported the first blue LED based on quantum confined CsPbBr3 NPLs.50 Their electroluminescence (EL) peak was about 480 nm, exhibiting a 20 nm redshift in comparison with the PL peak. Hence, stable blue emission with λEL< 480 nm is still limited for NPLs. Taking advantages of the optical properties and the better stability of NPLs synthesized here, ultrapure blue (FWHM=~12 nm) LEDs with EL peak at 463 nm were fabricated. The structure of LED device is shown in Figure 4a. Poly-TPD was severed as hole transport layer and TPBi was acted as electron transport layer. Compared to the PL spectrum, the EL spectrum peaks at 463 nm, which barely shift and no additional peak appears (Figure 4b). It means that the NPLs remain their original structural and electronic structure under high current density.51-52 Figure 4c shows the external quantum efficiency (EQE) and luminance of the device as a function of voltage. A maximum luminance of 62 cd/m2 and a maximum EQE of 0.124% are obtained. Besides, the maximum current efficiency was 0.117 cd/A as shown in Figure S9. The results are even better than most of the blue perovskite LEDs based on polycrystalline film or

NCs (Table S2).53 And we believe further improvement in the

performance can be realized by optimized device structure and modifying surface ligands. Figure 4d shows the blue part of Commission Internationale de l’Eclairage (CIE) chromaticity


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diagram of above device. The red star represents the CIE color coordinate of our LED emission (0.157, 0.045), which is closer to the edge and can contribute to a wider color gamut due to the ultra narrow FWHM and standard emission wavelength. Meanwhile, the pure blue emitting with relatively longer wavelength of ~460 nm shows negligible damage to eyes by excluding harmful light from 430 to 450 nm.54 The performance of LEDs can be determined by two factors: the optical properties of active e mitters, and the architecture design of LEDs. This article focused on optical properties of emittin g layers. The structure of LEDs has not yet been considered, which maybe the reason for worse LED performance compared to green perovskite congeners. To further improve the performance of blue LEDs (λEL

In Situ Passivation of PbBr64â Octahedra toward Blue Luminescent

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In Situ Passivation of PbBr64â Octahedra toward Blue Luminescent