Morphology Evolution and Degradation of CsPbBr3 Nanocrystals

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Morphology Evolution and Degradation of CsPbBr Nanocrystals under Blue Light Emitting Diode Illumination Shouqiang Huang, Zhichun Li, Bo Wang, Nanwen Zhu, Congyang Zhang, Long Kong, Qi Zhang, Aidang Shan, and Liang Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14423 • Publication Date (Web): 09 Feb 2017 Downloaded from http://pubs.acs.org on February 10, 2017

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ACS Applied Materials & Interfaces

Morphology Evolution and Degradation of CsPbBr3 Nanocrystals under Blue Light Emitting Diode Illumination Shouqiang Huang, Zhichun Li, Bo Wang, Nanwen Zhu, Congyang Zhang, Long Kong, Qi Zhang, Aidang Shan, and Liang Li* School of Environmental Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, 200240, Shanghai, China.

*Corresponding to: [email protected].

ACS Paragon Plus Environment

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ABSTRACT Under illumination of light emitting diode (LED) or sunlight, the green color of all-inorganic CsPbBr3 perovskite nanocrystals (CPB-NCs) often quickly changes to yellow, and follows by large photoluminescence (PL) loss. To figure out what is happening on CPB-NCs during the color change process, the morphology, structure and PL evolutions are systematically investigated by varying the influence factors of illumination, moisture, oxygen and temperature. We find that the yellow color is mainly originated from the large CPB crystals formed in the illumination process. With maximized isolation of oxygen for the sandwiched film or the uncovered film stored in nitrogen, the color change can be dramatically slowed down no matter there is water vapor or not. In dark condition, the PL emissions are not significantly influenced by the varied relative humidity (RH) levels and the temperatures up to 60 °C. Under the precondition of oxygen or air, color change and PL loss become more obvious when increasing the illumination power or RH level, and the large-sized cubic CPB crystals are further evolved into the oval-shaped crystals. We confirm that oxygen is the crucial factor to drive the color change, which has the strong synergistic effect with the illumination and moisture for the degradation of the CPB film. Meanwhile, the surface decomposition and the increased charge trap states occurred in the formed large CPB crystals play important roles for the PL loss.

KEYWORDS: CsPbBr3 nanocrystals, illumination, color change, photoluminescence, morphology


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1. INTRODUCTION All-inorganic cesium lead halide (CsPbX3, X = I, Br, Cl) perovskite nanocrystals (NCs) are currently garnered tremendous interest due to their higher stability compared to the methylammonium lead halide (MAPbX3, X = I, Br, Cl) perovskites in the applications of solar cells and light emitting diodes (LEDs).1-6 MAPbX3 perovskites (X = I, Br, Cl) are easily subject to autodegradation due to their intrinsic instabilities.7,8 The substitution of the MA cation by the less active cation of Cs enables the CsPbX3 NCs (X = I, Br, Cl) to obtain the striking optical properties and enhanced stabilities.9-11 CsPbBr3 (CPB), for example, exhibits higher thermal and air stabilities compared to MAPbBr3,7,12 and the photoluminescence (PL) of CPB could maintain a stable level under UV light illumination.13 The comparatively high PL stabilities were also found for the CsPb(BrχI1−χ)3 perovskites by using 1 sun illumination (100 mW/cm2).14 However, in comparison to the traditional quantum dots with core/shell structures, e.g., CdSe/CdS,15 CdSe/CdZnS16 and ZnSe/CdSe,17 CsPbX3 NCs (X = I, Br, Cl) have even more serious stability problems under the sustained operational conditions.18,19 To solve the stability issues of CsPbX3 NCs (X = I, Br, Cl), the surface passivation methods by inducing organic capping ligands or inorganic materials have been reported.18-20 The formation of the mixed halide perovskites also brings improved stabilities for them compared to their pure phases.21,22 It is worth noting that the color change of CsPbX3 NCs (X = I, Br, Cl) can be commonly observed with light illumination, accompanied by serious PL loss.18-22 The PL intensity of the CPB-NC hexane solution was reported to decrease to 40% after long time illumination with UV light.20 A large PL loss of 90% could be found for the CPB-NC toluene solution with the laser illumination, despite it can be slowed down with the octadecene (ODE) solution.22 In addition to the poor photostability, CPB-NCs also exhibit pitiful thermal stability,20


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and the green color of the CPB-NC film could be easily changed to yellow by high-temperature heat treatment (≥ 170 °C).23 Although the color change phenomenon and PL loss of CPB present in these studies, the reasons responsible for them remain unclear. Furthermore, current attention for the degradation of CsPbX3 NCs (X = I, Br, Cl) mainly focuses on the influence factors of moisture, illumination and heat, but the role of oxygen is usually neglected, which still needs clarification. In this work, the pristine CPB-NC film easily undergoes a color change from green to yellow under high power LED illumination, and a large PL reduction occurs in the film. According to the evolution of phase structure, morphology and chemical composition as a function of the illumination time, the yellow-colored phases are mainly attributed to the formed large CPB crystals. To restrain the crystal growth of CPB-NCs and maintain their high PL emission, the sandwiched film in two quartz coverslips and the uncovered film protected in nitrogen (N2) have been tested. It is important to find that large PL loss cannot be caused by illumination without the contribution of oxygen during the 8 h of illumination. Considering the fast decreased PL of the CPB film drove by oxygen, in the illumination condition, the crucial factor accounted for the degradation of CPB-NCs should be oxygen rather than moisture. For the moisture, it can facilitate the crystal growth of CPB, which may be caused by its service role as the ion transport channel. Along with the crystal growth of CPB, the surface trap states are increased because of the surface decomposition as well as the desorption of the surface bonding ligands. To well understand the morphology evolution and PL loss of the CPB film, a possible degradation pathway of CPB-NCs under LED illumination is proposed. 2. METHODS 2.1. Chemicals and materials.


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PbBr2 (99%, Aladdin), oleic acid (OA, 90%, Aldrich), ODE (90%, Aldrich), oleylamine (OLA, 80-90%, Aldrich), cesium stearate (98.0%, Shanghai Flute Cypress Chemical Technology Co. Ltd., China) and toluene (analytical grade, Sinopharm Chemical Reagent Co. Ltd, China) were used as received without further purification. 2.2. Preparation of CPB-NCs. 10 mL ODE, 1 mL OA, 1 mL OLA and 0.4 mmol PbBr2 were added into a 150 mL three-neck flask and degassed under vacuum for 30 min at 120 °C to completely dissolve the PbBr2 salt. The temperature was raised to 180 °C under N2 flow, and 0.2 mL of cesium stearate solution (0.5 mmol/mL in ODE) was quickly injected. After 5 s, the resultant mixture was cooled through an ice-water bath. To remove the excess organic residues and facilitate the subsequent CPB-NC film formation,3 the CPB-NC precipitates were separated by adding 20 mL of acetone and 10 mL of toluene via centrifugation at 10000 rpm for 10 min. Then, the CPB-NC precipitates were redispersed into 30 mL of toluene, and the colloidal CPB toluene solution (0.9 mg/mL) was collected from the supernatant after centrifugation at 10000 rpm for 10 min. 2.3. Preparation of the CPB film. A clean circular quartz coverslip (13 mm in diameter and 1 mm in thickness) was added in a 20 mL bottle containing 2 mL of the colloidal CPB toluene solution (0.9 mg/mL). Then, the bottle without sealing was placed in a cooled vacuum space for freeze drying of 3 h (Labconco FreeZone 4.5), and the CPB film was formed on the quartz coverslip (the film on the other side was removed). 2.4. Characterization.


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X-ray power diﬀraction (XRD) patterns of the CPB films were detected by the Bruker D8 Advance X-ray Diffractometer using Cu Kα radiation (λ=1.5406 Å). The JEOL JSM-7800F field emission scanning electron microscope (SEM) and the JEM-ARM200F transmission electron microscope (TEM) instruments were used to analyze the morphology and elemental distributions. The surface composition measurement was performed with X-ray photoelectron spectroscopy (XPS, Kratos Axis UltraDLD), and all the spectra were calibrated to the C 1s peak at 284.8 eV. PL spectra were recorded on a Hitachi F-7000 fluorescence spectrophotometer, and the excitationemission matrix (EEM) spectra were also measured to characterize the total luminescence spectra. The UV-vis absorption spectra of the CPB films were detected on the Lambda 750 UVvis-NIR spectrophotometer. The absolute PL quantum yield (PLQY) of the CPB film was detected using a fluorescence spectrometer with an integrated sphere excited with 450 nm LED light. Fluorescence lifetimes were performed on a time-resolved fluorescence spectrofluorometer (QM/TM/IM, PTI, USA). 2.5. Photostability measurements. The photostability measurements of the colloidal CPB solution (0.9 mg/mL) and the CPB films were performed in a temperature & humidity chamber using the 450 nm LED illumination (RXN-605D DC, China). Every periodic interval, the PL emission and UV-vis absorption spectra were recorded. The photostability measurements for the LED chips. 2 mL of the colloidal CPB solution was added into an agate mortar. After freeze drying, a green film of CBP-NCs was left in the agate mortar, and then the Norland optical adhesive (NOA) was added with the mass ratio of 200: 1 for NOA: CPB-NCs. Following the mechanical vibration and vacuuming (remove bubbles) process, the well mixed CPBs/NOA composite was dropped on the 5050 LED chip with an emission peak


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wavelength of 450 nm, and the CPB/NOA film was formed after 10 s of UV curing. The PL spectra of the CPB/NOA film with different operation times (25 °C, RH 60% in ambient air) were measured by a calibrated spectrophotometer with an integrating sphere. 3. RESULTS AND DISCUSSION 3.1. Degradation of the colloidal CPB toluene solution. The photostability of the colloidal CPB toluene solution is evaluated with 450 nm LED illumination, and the test instrument is illustrated in Figure 1a. The initial PL peak of the CPB solution is centered at 514 nm with a full-width at half-maximum of 19 nm (Figure 1b). After 1 h of illumination, the remnant PL is apparently reduced to 31.3% (Figure S1), accompanied with a peak redshift of 9 nm. Further illumination to 2 h, the remnant PL (32.8%) and peak position (524 nm) stay at the same level. The optical images of the cuvette containing the CPB solution at different illumination times are shown in Figure 1c. The initial CPB solution exhibits bright green color, and then it turns to yellow in less than 1 h. The TEM and high-resolution TEM (HRTEM) images of the pristine CPB-NCs (Figure 1d,e and S2) taken from the green solution reveal their highly monodispersed NCs with the edge lengths of 5-10 nm. For the yellow solution obtained after 2 h, serious aggregate growth occurs in the CPB-NCs (Figure 1f,g), and the formed crystals reach the size range of 26-50 nm. 3.2. Degradation of the CPB film. The CPB-NC thin film deposited on a quartz coverslip is fabricated by a vacuum freeze-drying method (Figure 2a). Because the CPB film has a slow ion migration rate compared to its solution form, the illumination time is increased to 8 h with the power density of 175 mW/cm 2 and a relative humidity (RH) of 60%. The initial CPB film exhibits green color, and then it gradually


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Figure 1. (a) Schematic device of the illumination system. (b) PL emission spectra of the colloidal CPB toluene solution and (c) the optical images of the cuvette as a function of the illumination time (175 mW/cm2). (d) TEM and (e) HRTEM images of CPB-NCs. (f) TEM and (g) HRTEM images of the CPB sample after 2 h of illumination. changes to the yellow film as the illumination extended (Figure 2a). Figure 2b shows the XRD patterns of the CPB film at different illumination times. Three preferential diffraction peaks along the (100), (110) and (200) planes of CsPbBr3 are formed for the pristine CPB film (Figure S3-S6), and similar feature was performed through the centrifugal casting process.24 The (110) broad peak is contributed by the substrate quartz coverslip (SiO2), which almost remains the same during illumination, whereas the (100) and (200) peaks are first broadened and then


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Figure 2. Characterizations of the CPB film treated in the temperature & humidity chamber. (a) Schematics of the fabrication of the CPB film on a quartz coverslip and its illumination system, as well as the optical images of the CPB film as a function of the illumination time (175 mW/cm2, RH 60%). (b) XRD patterns and (c) PL emission spectra of the CPB film as a function of the illumination time (175 mW/cm2, RH 60%). (d) Remnant PL emissions of the CPB films as a function of the illumination time. enhanced with sharp shape. The slightly broadened peak at 0.5 h might be caused by the smoothing of the CPB-NC surfaces during the photoactivation process, leading to the PL enhancement (Figure 2c). Notably, there is no blueshift for the PL peak at 0.5 h, and the photoactivation here may be generated by photo-annealing, where the NC surface atoms are reconstructed by removing the dangling bonds and unsaturated atoms.25 As the illumination time increases, the diffraction peaks become sharp and strong, suggesting the crystal growth of CPB, which results in the PL redshift (Figure 2c). The UV-vis absorption spectra of the CPB film are shown in Figure S7, and the first-excitonic absorption peak redshifts from 501 to 512 nm after 4


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h. Upon further illumination (6-8h), the first-excitonic absorption peak is not changed, since the large sizes of the CPB crystals have exceeded the exciton Bohr diameter (ca. 7 nm),1 and the quantum confinement effect is disappeared.10,26 The time-dependent EEM spectra of the CPB film are shown in Figure S8, and a sharp reduction in PL intensity can be observed, with a remnant PL of 10.6% after 8 h of illumination (Figure 2d and Table S1), which substantially decreases to 0.7% when the illumination power increases to 350 mW/cm2. Some previous reports claimed the CPB-NCs could exhibit good stability under the illumination of X-ray,18,19 UV20 or sun light,14 but we find that these light sources used are not really powerful to drive the degradation of CPB-NCs in a short time. In our experiments, the slow decay of PL intensity can also be observed upon the weak illumination of 88 and 17 mW/cm2. Indeed, in the case of high power illumination, the PL loss mainly occurs during the first 1-2 h, and then it maintains a stable level, where the exciton peak positions are not changed (Figure S7). The TEM images of the CPB crystals scraped from the films as a function of the illumination time are investigated (Figure 3a). After 1 h, some large cubic CPB crystals with the edge lengths of 15-60 nm are generated (Figure 3b), which are different from the monodisperse CPB-NCs (Figure 1d). When the illumination time reaches 4 h, most CPB-NCs have been grown up to form large CPB crystals with long lengths of 40-150 nm (Figure 3c,d). To further increase the illumination to 6 and 8 h, there is no significant difference in the length of the CPB crystals (Figure 3e-h), while their thicknesses are gradually increased, which can also be demonstrated by the SEM morphologies (Figure S9). As for the higher power illumination of 350 mW/cm2, a more serious aggregate growth occurs in CPB (Figure 4a and b), where the corners of the cubic crystals have been melted to form the oval-shaped crystals. In addition, other different


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Figure 3. Morphology and composition analysis of the CPB film treated in the temperature & humidity chamber. (a) Schematic of the CPB crystals scraped from the film. TEM images of the CPB crystals scraped from the films as a function of the illumination time (175 mW/cm2, RH 60%): (b) 1 h, (c,d) 4 h, (e,f) 6 h and (g-i) 8 h; inset of i: FFT pattern. (j,k) HAADF-STEM images of the CPB crystals scraped from the film after 8 h and the corresponding Pb, Cs and Br elemental maps and an overlay of them. (l) Survey scan XPS spectra and (m) the high-resolution Pb 4f spectra of the CPB films as a function of the illumination time (175 mW/cm2, RH 60%). morphologies can be observed for the CPB film by varying the illumination conditions (Figure S10). These enlarged CPB crystals can cause redshift in the PL spectra (Figure 2c), but it is still


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unclear how much the disappeared quantum confinement contributes to their PL loss.27,28 On the other hand, it should be noted that the surface trap states are increased with the crystal growth of CPB, because the CPB films after illumination possess the shorter surface trap-associated lifetimes (τ2) compared to the pristine film (Figure S11), and this trap formation plays an important role for their PL loss.3,29 Regarding the existence of moisture (RH 60%) and oxygen in the illumination system, the surface decomposition of CPB may occur. Figure 3l shows the survey scan XPS spectra of the CPB films, and the spectroscopic signatures of Pb, Cs, Br, O and C are detected. According to the atomic ratios of Pb, O and C elements (Table S2), the O and C contents after illumination are subsequently increased compared to their initial values. With respect to the Cs and Br elements, their contents are first located in the same level and then decreased after 4 h. The high-resolution XPS spectra of C 1s are shown in Figure S12a, and the enhanced C 1s peak at 8 h is resolved into six peaks: the C=C, C-C, C-N, C-O and C=O groups are possibly arising from the residual OA, OED, OLA or the adventitious carbon; and the peak at 289.01 eV is attributed to the carbonate.30,31 The oxygen-related peak for carbonate can be found in the corresponding O 1s spectrum at 531.53 eV (Figure S12b), and another peak at 530.86 eV is assigned to the Pb-O bond.32 The Pb 4f spectra are shown in Figure 3m, and the Pb 4f peaks of the pristine CPB-NCs at 138.21 and 143.31 eV correspond to Pb 4f7/2 and Pb 4f5/2, respectively. After 8 h of illumination, the Pb 4f peaks are broadened obviously, which can be fitted using three different peaks: the lowest peak at 137.97 eV is associated with the Pb-O bond; the peak at 138.60 eV is assigned to the Pb-Br bond or Pb(OH)2; 30,32 and the peak at 139.02 eV is resulted from the lead carbonate (PbCO3).30 Conceivably, the presence of PbO and PbCO3 on the outermost surface layer weakens the Cs and Br signals (Figure S12c,d), due to the limited detection depth of XPS.


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However, the PbO and PbCO3 phases are not found in the XRD patterns (Figure 2b), suggesting their amorphous forms or their very low contents (below the detection limit).30 Furthermore, it is also difficult to identify the PbO and PbCO3 phases in the HRTEM image of the large CPB crystals with high crystalline quality (Figure 3i). The high-angle annular dark field scanning transmission electron microscope (HAADF-STEM) images in Figure 3j,k reveal the large CPB crystals with cubic shape. The corresponding STEMenergy dispersive spectroscopy (STEM-EDS) maps are selected from the red frame of Figure 3k. It can be found the Pb, Cs and Br elements are homogeneously distributed in the inner part of the crystal (overlay map). On the surface edges of the crystals (white frame in the overlay map), however, the Pb clusters (red dots) seem to be more clearly present compared to the Cs and Br elements. To deepen this understanding, the STEM-EDS maps from the higher power (350 mW/cm2) illuminated CPB crystals are measured and shown in Figure 4c-g, and some coarse parts (white circles) with Pb clusters can be observed in the corners of the smooth crystals. Further evidence can be found from the enlarged part as shown in Figure 4h-l, where the Pb clusters are clearly appeared. In the corresponding XRD pattern (Figure S6c), there is a weak board peak at 31.67ºafter illumination of 8 h, which is probably originated from PbO (JCPDS Card No. 46-1211). Considering the XPS, XRD and STEM-EDS mapping results, the PbCO3 and PbO phases may be generated in these coarse places of the large CPB crystals. On a basis of the degradation of CH3NH3PbI3,30 the formation of PbCO3 and PbO from CsPbBr3 might be caused by the erosion of H2O, oxygen and CO2 through the hydration and dehydration process (Figure S12).


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Figure 4. (a) TEM and (b) HRTEM images of the CPB crystals scraped from the film after 8 h of illumination (350 mW/cm2, RH 60% in the temperature & humidity chamber); Insets: FFT pattern. (c,h) HAADF-STEM images of the corresponding CPB crystals, and the (d,i) Pb, (e,k) Cs, and (f,j) Br elemental maps and (g,l) their overlays.


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3.3. Factors influenced the degradation of the CPB film. According to the above analysis, the degradation of the CPB film refers to the large PL loss (89.4%) during the aggregate growth process (175 mW/cm2, RH 60%), which is associated with the increased charge trap states due to the surface decomposition or the removal of surface bonding ligands.28 With the higher power illumination of 350 mW/cm2, the more serious aggregate growth and PL loss (99.3%) occur in the CPB film. Thus, the degradation of the CPB film is evidently influenced by illumination. In addition to illumination, there are other factors in the illumination system, such as moisture and temperature. In view of moisture, higher RH of 80% (Figure 2d) also results in a larger PL loss of 95.6% compared to that (89.4%) with RH 60%. In the dark environment (Figure 5a), there is almost no PL loss for the CPB films exposed to the RH levels of 60% and 80% (25 or 30 °C, 8 h). When the temperatures increase to 40 and 60 °C under the same conditions (RH 60% and dark environment), the remnant PL emissions are decreased to 89.3% and 72.6% (Table S1), respectively. We observe that the temperature of the CPB film itself is just 30 °C with the illumination of 175 mW/cm2, and thus the effect from higher temperature (> 60 °C) is not investigated. In this illumination condition, the temperature doesn’t make big contribution to the degradation of the CPB film, and the effect of moisture depends on the illumination power. To investigate the single factor of illumination for the degradation, the CPB film is sandwiched between two quartz coverslips to maximize the isolation from the air condition (moisture and oxygen), which is distinguished from the uncovered film on one quartz coverslip. The optical images of the sandwiched CPB film as a function of the illumination time are shown in Figure 5c. Surprisingly, the green color of the CPB film is not changed even after 8 h of illumination, and a high remnant PL of 86.5% is maintained (Figure 5b), exhibiting much difference from that of the uncovered CPB film (Figure


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Figure 5. Factors influenced the photostabilities of the CPB film treated in the temperature & humidity chamber. (a) Remnant PL emissions of the CPB films stored in dark with different RH levels and temperatures. (b) Remnant PL emissions and (c) optical images of the CPB film sandwiched between two quartz coverslips as a function of the illumination time (175 mW/cm 2, RH 60%). Optical images of the CPB films exposed to (d) pure N2, (e) N2 + 0.5 μL H2O, (f) oxygen and (g) oxygen + 0.5 μL H2O, as well as (h) their remnant PL emissions as a function of the illumination time (175 mW/cm2). (i) Optical images of the CPB film exposed to oxygen in the dark environment.


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2a). Therefore, the illumination driven degradation can be largely slowed down with the isolation of moisture and oxygen. 3.4. Degradation of the CPB/NOA film on blue LED chip. In the practical LED application, phosphors are often encapsulated with optical adhesive to minimize the erosion from moisture and oxygen. Here, the CPB-NCs are mixed with the introduced NOA in a mass ratio of 1: 200, and then the CPB/NOA film is coated on a 450 nm blue LED chip with UV curing. The initial optical image of the CPB/NOA LED operating at 100 mA is shown in Figure 6a, and the strong green glow can be observed. The absolute PLQY of the CPB/NOA LED is about 38.1%, which is slightly higher than that (33.0%) of the pure CPB LED film (Figure 6b). The photoactivation is apparently appeared with the low operation current of 20 mA. When the currents increase to 100 and 200 mA, the PLQYs are declined to 10.9% and 3.0%, respectively. Although there is large PL loss for the CPB/NOA LED, it still maintains higher emissions compared to that (2.7%) of the pure CPB LED without NOA sealing. The corresponding TEM images of the CPB/NOA composite after 8 h of lighting are given in Figure 6c. The aggregate growth of CPB-NCs in NOA is observed, but it is not so serious compared to those from the pure CPB LED film (Figure 6d) and the uncovered CPB films (Figure 3), and some CPB-NCs are still retained owing to the fixation and protection of NOA. 3.5. The roles of moisture and oxygen on the degradation of the CPB film. It is important to note that the PL intensity of the sandwiched CPB film is decreased very slowly, and the yellow phase is not present during the 8 h of illumination. In the CPB/NOA LED film, the degradation of the CPB crystals are probably caused by the penetration of moisture or oxygen, since there are residual pinholes in the NOA film. To distinguish the effects of moisture


17


a

b

80

60

PLQY (%)

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CPB/NOA film, 20 mA CPB/NOA film, 100 mA CPB/NOA film, 200 mA pure CPB film, 100 mA

40

20

0

c

0

1


7

8

d

Figure 6. Photostabilities of the CPB/NOA LED (RH 60% in ambient air). (a) The initial optical image of the CPB/NOA LED operating at 100 mA. (b) PLQYs of the pure CPB and CPB/NOA LEDs after operating different times. TEM images of (c) the CPB/NOA composite from the CPB/NOA LED film and (d) the CPB crystals from the pure CPB LED film after 8 h of lighting (100 mA). and oxygen on the stabilities of CPB-NCs, four controlled experiments performed in the sealed quartz vials filled with pure N2 (99.99%, RH ≤ 0.04%, oxygen ≤ 0.001%), N2 + 50 μL H2O, pure oxygen (99.99%, RH ≤ 0.02%) and oxygen + 50 μL H2O are investigated. The green color of the CPB film in N2 is strengthened after 1 h of illumination (Figure 5d), along with a large PL enhancement of 2.8 times of the initial PL intensity (Figure 5h), which is originated from the photoactivation process. With the introduction of 50 μL H2O into the N2 contained vial (Figure 5e), the RH level is increased because of the illumination-driven water vapor. Although the PL


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enhancement still exceeds 2 times (Figure 5h), the CPB film is influenced by the raised moisture. As mentioned above, the higher RH of 80% results in a larger PL reduction compared to RH 60% (Figure 2d), and serious decomposition of CPB-NCs can also be observed in the liquid water condition. As shown in Figure S13a, the exposed part of the CPB film is almost disappeared after 4 h of immersion in water, because the hydrated CPB species can be easily formed and dissolved into water.33,34 The dissolution rate of CPB-NCs can be further accelerated by illumination due to the fast ion migration resulted from the light energy (Figure S13b). Therefore, it is conceivable that some hydrated CPB species could be generated when the CPB film exposed to the humid air, but please notice that the hydration of CPB doesn’t turn its color into yellow. For the CPB film exposed to pure oxygen, the green color is quickly changed to yellow after 1 h of illumination (Figure 5f), resulting in a low remnant PL of 32.6% (Figure 5h). It is apparent that oxygen plays a crucial role for the degradation of CPB-NCs, which is correlated with the formed large CPB crystals (Figure S14a,b). Several literatures reported that oxygen could facilitate the crystallization of perovskites, because the defects at the crystal interfaces or within the crystals could be reduced by oxygen.35-37 Oxygen could also etch off unstable nuclei and enable large crystal growth.38 According to the size distribution (5-10 nm) of CPB-NCs, oxygen may display as the etching reagent to erode some of the unstable CPB-NCs with minimum size, and then the dissociated ions from the damaged CPB-NCs migrate to the large CPB crystals and contribute to the crystal growth, exhibiting the Ostwald ripening process. It has been demonstrated that the moisture-assisted crystal growth is an effective way to generate large perovskite crystals,35,39 because the moisture absorbed within grain boundaries might induce grain boundary creep and merge the adjacent grains together, as well as provide the aqueous environment to enhance the ion diffusion length. Although the oxygen-enriched quartz vial is


19


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pretreated through a vacuum procedure, the hydrated CPB species in the CPB film are hard to remove. When the unstable CPB-NCs are etched off by oxygen, the hydrated CPB species may support the ion transport and facilitate the subsequent crystal growth, which can be proved from the CPB film exposed to oxygen with high RH level (Figure 5g), where a deep yellow film with less transparent is present. The corresponding TEM images (Figure S14) and XRD patterns (Figure S15) show that the induced water vapor produces a higher crystalline quality for the CPB crystals, while the resulted PL is decreased to a lower value of 8.3% compared to that (32.6%) only exposed to oxygen (Table S1). In the air condition, higher RH of 80% also results in a more serious aggregate growth for CPB-NCs (Figure S16) compared to RH 60% (Figure S9). There is even no yellow phase appeared for the CPB film in the liquid water no matter there is illumination or not (Figure S13), because all the dissolved ions are transferred to water instead of contributing for crystal growth. Thus, all these changes associated with moisture probably suggest its role as a media to support ion migration for the crystal growth of CPB, despite some dehydration reaction may be present (Figure S12). Furthermore, under the conditions of oxygen and darkness, there is no yellow phase (Figure 5i) and PL loss (Figure 5h) appeared in the CPB film, indicating the degradation is resulted from the synergistic effect among illumination, oxygen and moisture. 3.6. Degradation pathway of the CPB film. The degradation pathway of the CPB film can be comprehensively imagined based on the crystal growth of CPB and the roles of oxygen and moisture, and the schematic illustration is shown in Figure 7. Despite the pristine CPB-NCs are encapsulated by surface bonding ligands, moisture and oxygen can reach to the surface and the crystal interface of CPB-NCs due to their hydrophilic property,4,12 and thus some hydrated CPB species may be formed in the well


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dispersed CPB-NCs. When performing the illumination, the surface bonding ligands are easily removed by the absorption of photons,28 and the CPB-NCs tend to be aggregated due to the strong van der Waals attraction forces. Meanwhile, the hydration reaction can be increased on account of the removal of surface ligands, and the adjacent CPB-NCs are also preferred to be merged together due to the decreased energy barrier among them.11 Subsequently, the unstable CPB-NCs may be quickly eroded by oxygen, and the ion migration is facilitated by moisture for the crystal growth.35 Except to the crystal growth of CPB, the surface decomposition is resulted from the dehydration reaction among the hydrated CPB species, O2 and CO2, and then the oxide phases of PbCO3 or PbO are formed (Figure S12). The decomposition of CPB can explain the decreased light absorption after 8 h of illumination (Figure S7), where the preliminary enhanced absorption may be caused by light scattering. Furthermore, the surface decomposition makes a big contribution to the generation of surface trap states. Throughout these changes, the large cubic CPB crystals are produced after 8 h of illumination with the power density of 175 mW/cm2, and the oval shaped CPB crystals with increased surface trap states (Figure S11) can be further formed with the higher power illumination of 350 mW/cm2. Although the degradation of CPB-NCs is proposed above, their stability can be improved by the encapsulation method, such as the mentioned film sandwiched in two quartz coverslips. It should be pointed out that there is a small PL loss (13.5%) for the sandwiched film, which may be caused by the slow penetration of oxygen and moisture. Generally, it is convenient to induce organic binders to mix with quantum dots for LED application. Unfortunately, the CPB/NOA composite film fails to maintain their excellent PL properties under high current operation. According to the factors influencing the stability of CPB-NCs, the chief culprits are still attributed to oxygen and moisture. Therefore, developing encapsulation technologies with


21


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stronger oxygen barrier and water resistance properties could provide much better protection for CPB-NCs and further boost their stability. On the other hand, the improvement of the quantum dots themselves is another choice to further increase their service life. Our group has reported the tetramethyl orthosilicate driven SiO2 encapsulation method for the perovskite QDs in “waterless” environment.40 In further research, we will use the similar methods to improve the stability of CPB for LED application.

Oxygen Moisture

CPB-NCs Surface bonding ligands

Oxygen speeds up the ripening of CPB-NCs

Monodispersed CPB-NCs

Oxygen Moisture facilitates ion migration

Oxygen+moisture PbCO3 or PbO: Reaction among CsPbBr3, H2O, O2 and CO2

175 mW/cm2

350 mW/cm2

Large cubic CPB crystal

Oval shaped CPB crystal

Figure 7. Schematics of the possible degradation pathways of the CPB film on the quartz coverslip with different illumination power densities. 4. CONCLUSION


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In summary, the degradation of CPB-NCs under different conditions is investigated to understand the stability of CPB-NCs. The color of the CPB film exposed to air is easily changed from green to yellow under the illumination of 175 mW/cm2 (RH 60%), and a large PL reduction of 89.4% is observed after 8 h. According to the structure, morphology, composition and PL decay results of the yellow colored CPB crystals, the surface decomposition and the increased traps states are the critical factors for their PL loss, which becomes more serious with higher power illumination (350 mW/cm2). In contrast, high remnant PL emissions can be maintained for the sandwiched CPB film and the uncovered film exposed to N2 upon the same power illumination, where the yellow phase is not appeared. The degradation of the CPB film is mainly caused by the synergistic effect among illumination, moisture and oxygen. Conflict of Interest The authors declare no competing financial interest. Acknowledgments We thank Dr. Bing Chen for TEM observation. This work is finally supported by the National Natural Science Foundation of China (21271179 and 21607101), Shanghai Municipal Science and Technology Commission Project (16DZ1204802) and China Postdoctoral Science Foundation (2016M590363). Supporting Information UV-vis absorption spectra, EEM spectra, time-resolved PL decay curves, SEM images, tables, and additional TEM images and XPS spectra as mentioned in the text. This material is available free of charge via the Internet at http://pubs.acs.org.


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Corresponding Author *Corresponding Author: Prof. Liang Li; E-mail: [email protected] 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) Yakunin, S.; Protesescu, L.; Krieg, F.; Bodnarchuk, M. I.; Nedelcu, G.; Humer, M.; Luca, G. D.; Fiebig, M.; Heiss W.; Kovalenko, M. V. Low-Threshold Amplified Spontaneous Emission and Lasing from Colloidal Nanocrystals of Caesium Lead Halide Perovskites. Nat. Commun. 2015, 6, 8056. (3) Wang, Y.; Li, X.; Song, J.; Xiao, L.; Zeng, H.; Sun, H. All-Inorganic Colloidal Perovskite Quantum Dots: A New Class of Lasing Materials with Favorable Characteristics. Adv Mater. 2015, 27, 7101-7108. (4) Kulbak, M.; Cahen, D.; Hodes, G. How Important Is the Organic Part of Lead Halide Perovskite Photovoltaic Cells? Efficient CsPbBr3 Cells. J. Phys. Chem. Lett. 2015, 6, 2452-2456. (5) Klein-Kedem, N.; Cahen, D.; Hodes, G. Effects of Light and Electron Beam Irradiation on Halide Perovskites and Their Solar Cells. Acc. Chem. Res. 2016, 49, 347-354. (6) Yang, G. L.; Zhong, H. Z. Organometal Halide Perovskite Quantum Dots: Synthesis, Optical Properties, and Display Applications. Chin. Chem. Lett. 2016, 27, 1124-1130.


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Morphology Evolution and Degradation of CsPbBr3 Nanocrystals

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