Photodegradation of Organometal Hybrid Perovskite Nanocrystals

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Photodegradation of Organometal Hybrid Perovskite Nanocrystals: Clarifying the Role of Oxygen by Single-dot Photoluminescence Lige Liu, Luogen Deng, Sheng Huang, Pei Zhang, Jan Linnros, Hai-Zheng Zhong, and Ilya Sychugov J. Phys. Chem. Lett., Just Accepted Manuscript • Publication Date (Web): 07 Feb 2019 Downloaded from http://pubs.acs.org on February 7, 2019

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The Journal of Physical Chemistry Letters

Photodegradation of Organometal Hybrid Perovskite Nanocrystals: Clarifying the Role of Oxygen by Single-dot Photoluminescence Lige Liu, †, ‡ Luogen Deng, † Sheng Huang, § Pei Zhang,∥ Jan Linnros, ‡ Haizheng Zhong, *,§ and Ilya Sychugov*, ‡ †

Nanophotonics and ultrafine optoelectronic system Beijing Key Laboratory, School of Physics,

Beijing Institute of Technology, 5 South street of zhongguancun, 100081, Beijing, China ‡

Department of Applied Physics, KTH-Royal Institute of Technology, Electrum 229, 16440,

Kista, Sweden §

School of Materials Science & Engineering, Beijing Institute of Technology, 5 South street of

zhongguancun, 100081, Beijing, China ∥

Henan Key Lab of Information-based Electrical Appliances, School of Electrical and

Information Engineering, Zhengzhou University of Light Industry, 450002, Zhengzhou, China AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]

ACS Paragon Plus Environment

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* E-mail: [email protected]

ABSTRACT. Photostability has been a major issue for perovskites materials. Understanding the photodegradation mechanism and suppressing it are of central importance for applications. By investigating single-dot photoluminescence (PL) spectra and lifetime of MAPbX3 (MA=CH3NH3+, X=Br, I) nanocrystals (NCs) with quantum confinement under different conditions, we identified two separate pathways in the photodegradation process. The first is the oxygen-assisted lightinduced etching process (photochemistry). The second is the light-driven slow charge trapping process (photophysics), taking place even in oxygen-free environment. We clarified the role of oxygen in the photodegradation process and show how the photoinduced etching can be successfully suppressed by OSTE polymer, preventing an oxygen-assisted reaction.

TOC GRAPHICS

KEYWORDS Photodegradation, Perovskite nanocrystal, Single-dot, Photoluminescence


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Halide perovskites are emerging as a new generation of functional materials in solution processed optoelectronics due to the combination of unique physical properties, excellent device performance as well as low cost scale-up fabrication.1-3 However, in spite of recent advances, these materials still suffer from poor stability, which limits the application in functional systems. Moreover, the stability issue is greatly pronounced for perovskite nanocrystals (PNCs), which are now considered to be potential candidates as light emitters for display technology.4-6 Therefore, numerous studies have recently been dedicated to address this issue and great progress have been made in developing chemical and physical strategies for stability improvement.7-11 PL quenching under light excitation has been the most obvious feature of degradation. It has been also illustrated that the photodegradation process is strongly correlated to environmental factors12 including oxygen,13,14 light,15,16 moisture,17,18 and heat.19,20 By monitoring the evolution of optical spectra and surface properties during the degradation process, Haque et al. illustrated that oxygen play an important role to accelerate the degradation process.21 Due to the coexistence of different processes and less control of environmental factors,22,23 rational experimental design and advanced techniques are much aspired. Single-dot PL spectroscopy has been a very successful tool to clarify the photophysics of quantum dots.24-28 By combining the environmental control and structural evolution, it has been also applied to illustrate the photochemical process under light excitation.29 This technique at the level of a single particle is highly sensitive to monitor the degradation process. For example, Scheblykin et al. observed that the degradation of MAPbI3 PNC starts locally and then spreads over the whole crystal.30 A recent work by Mulvaney’s group found that the photoinduced decomposition of single inorganic perovskite CsPbI3 NC take place under the combined effects of oxygen and light or moisture.31 In this work, we systematically investigated the photostability of


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MAPbBr3 NCs with quantum confinement at single-dot level at different environments and temperatures. The quantum size effect utilized here makes this measurement scheme very sensitive to possible size modifications. By comparing the photodegradation of MAPbBr3 NCs with and without polymeric coating, we clarify the photochemical (etching) and photophysical (charge trapping) processes during the photodegradation of MAPbX3 PNCs. In particular, we demonstrate how an oxygen-scavenging polymer protective layer can successfully suppress the photo-induced etching. Polymeric encapsulation has been one of the effective and simple methods to protect the perovskite material from detrimental effects of oxygen. Poly (methyl methacrylate) (PMMA), Polyvinylidene fluoride (PVDF), Polydimethysiloxane (PDMS) and polystyrene (PS) etc. are all explored to be used as a polymer protector.32-38 Here the off-stoichiometry thiol-ene (OSTE) monomers were coated around the PNCs, as shown in figure 1, by spin-coating with subsequent polymerization. This polymer features a fast polymerization rate and strong oxygen-scavenging properties due to active radicals.39,40 To provide access to single particles in a wide-field microscope a diluted solution of PNCs was spin-coated on the silicon substrate. The samples without and with OSTE polymer protection were excited by the 405 nm diode laser as shown in figure 1. A schematic diagram of the single-dot experimental set-up has been shown in Figure S1. To verify successful probing of individual nanocrystals we measured numerous PL spectra of single MAPbX3 dots excited by 405 nm laser. Narrowing of the emission linewidth compared to the ensemble and varying peak positions from dot-to-dot clearly indicate single-dot origin of the emission. Mono-exponential decays indicative of a single emitter, as opposed to stretchedexponential ensemble transients were also recorded.41


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Figure 1. Single particle detection of PNCs in the environment with and without oxygen. Figure 2 shows the evolution of the PL center peak of MAPbBr3 single NCs (size ~5 nm) without and with encapsulating polymers under prolonged laser exposure. It can be seen in figure 2a that the PL peak position of individual NCs without polymer protection changed rapidly from 507 to 482 nm: it blue-shifted by ~25 nm after exposing the NC to the laser for only 15 min. Continuing exposure results in a prolonged shift and eventually leads to a complete PL quenching. Note that the spectrum looks noisier with prolonged laser exposure because of the photophysical degradation, resulting in PL intensity drop, as discussed later. However, the PL peak position of a NC with OSTE protection reveals almost no change after exposing the NC to the laser for 30 min (Figure 2b). The phenomenon was statistically confirmed on many other individual MAPbBr3 dots with green PL (green-MAPbBr3) (Figure S2a). We also monitored the PL spectra of the MAPbBr3 with blue PL (blue-MAPbBr3, smaller size of 2-3 nm) and MAPbI3 with red PL (red-MAPbI3). The spectra of all the organometal perovskite nanodots show the same behavior (Figure S2b,c). In short, for the OSTE protected NCs the PL peak position have no measurable change after long laser exposure, while it rapidly blue-shifts with increasing laser exposure time for the NCs without polymer protection. So, as a first observation, here we note that OSTE features a strong stabilization effect for the all types of perovskite MAPbX3 NCs.


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Figure 2. The PL spectra evolution of typical green-MAPbBr3 NC single dots (a) without polymer coating and (b) coated by OSTE film with laser exposure time of 15 min and prolonged to 30 min, respectively. (c) Top: PL peak position of the single green-MAPbBr3 NCs with and without OSTE protection under laser exposure for different time at room temperature. Bottom: The evolution of the PL integrated intensity of the single green-MAPbBr3 NCs without polymer measured at 295 K and with polymer measured at 295 K and 70 K. The acquisition time for each spectrum is 60 s. The excitation wavelength is 405 nm with a power density of ~ 1 W/cm2. Figure 2c (top) summarizes the effect of laser exposure on PL peak position for the greenMAPbBr3 single dots at room temperature (295 K). It can be seen that the PL peak position of the NC without polymer protection blue-shifts rapidly by ~25 nm after 15 min (pink spheres), while using OSTE as a protection layer, the stability improved greatly and the PL peak position of the NC nearly exhibits no shift (blue spheres). The PL center peak position of single green-MAPbBr3 NCs also have no obvious shift at lower temperature of 70 K (liquid nitrogen) and 5 K (liquid


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helium) (figure S3a). With the protection of the OSTE polymer, the blue-MAPbBr3 and redMAPbI3 NCs also keep the peak position unchanged at 70 K or 5 K with laser exposure time increasing (Figure S3b, c). Hence, the stabilizing effect of the OSTE polymer is confirmed to be generic for different material compositions of organolead trihalides at different temperatures. Although the PL peak position of the PNCs protected by OSTE polymer exhibit no noticeable shift under laser light exposure, the PL intensity reduced with longer laser exposure time. The blue circles in figure 2c (bottom) represent the PL intensity of a single green-MAPbBr3 NC with OSTE protection at different laser exposure times at room temperature. It clearly shows that the PL intensity of this green nanodot reduced by 80 % after 30 min. Also at lower temperatures of 70 K (yellow circles in figure 2d) and 5 K (figure S4aiv), the PL shows intensity reduction. Similar intensity degradation for the blue-MAPbBr3 and red-MAPbI3 NCs was also found both at room and at lower temperatures of 70 and 5 K (Figure S4). OSTE protection layer only delays observed intensity degradation: from few minutes for unprotected to tens of minutes for polymer covered NCs (Figure S4). The above results show that for MAPbX3 NCs with OSTE polymer protection the PL demonstrates an intensity reduction without peak position changing. It implies that, in general, two photodegradation pathways take place in organometal trihalides. We clarify their contributions as follows, respectively. The blue-shift of the PL peak position is related to the size reduction of the NCs under laser exposure based on the quantum size effect. In order to verify the decrease of the NC size, a control TEM investigation was carried out. A dilute MAPbBr3 NC solution was dropped on a copper mesh and then left to allow the solvent evaporate. Then the PNCs on the copper mesh were exposed to the UV/blue light (same wavelength, 405 nm) for different times. Figure 3a-e show the TEM images of the green-NCs after different laser exposure time. It can be seen that with the exposure


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time increasing, the average size of the NCs has indeed decreased from 4.8±0.9 to 1.3±0.5 nm after 20 min, and eventually, the size decreased to 0.9±0.3 nm after 30 min exposure. Figure 3f clearly shows the size decrease effect with prolonged exposure time. The original histograms of the size distribution of the green-MAPbBr3 NCs as a result of this treatment has been shown in figure S5. So the photoetching-induced size decrease for unprotected nanodots at room temperature is clearly confirmed, while the OSTE layer has been proven to provide a strong stabilization effect. On the other hand, at room temperature under vacuum condition (inside the evacuated cryostat), the PL peak position of the NC without a polymer protection also shows no obvious shift under laser light exposure (Figure S6). Therefore, the photo-induced size decrease of the MAPbX3 NCs is most likely triggered by its combination with oxygen. The presence of oxygen indeed appears to be central to the photoetching process. Haque et al.21 outlined such degradation process in MAPbX3 NCs as the reaction of excited electron with oxygen molecule to produce a superoxide O2-, which could further react with CH3NH3+ to form volatile CH3NH2. Then the MAPbBr3 framework gradually breaks down due to the removal of the CH3NH3+ from the crystal lattice. Scheblykin et al. further revealed that the structural collapse is primarily due to the migration of methylammonium ions (MA+).30 The break-down of the crystal structure also induced the reduction of the PL intensity. Here we clarify the “oxygen-assisted lightinduced etching” as the photochemical degradation pathway for the organometal perovskites. We have also shown improved stability of the perovskite NCs using OSTE polymer as protector. The OSTE polymer acts as the scavenger of oxygen radicals to strongly suppress oxygen diffusion40 with subsequent erosion of the perovskite crystal framework. In the environment with oxygen the PL intensity of MAPbBr3 NCs reduces slower (PL can be detected after 15-20 min) than that of MAPbI3 NCs (PL completely vanishes after 10 min). This is demonstrated in Figure S4 (ai) and


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(ci), respectively. The better stability of MAPbBr3 than MAPbI3 in this environment may be because of the stronger and shorter Pb-Br bond than Pb-I bond, and the stronger H-Br bond than H-I bond (both due to larger Br electronegativity). In addition, cubic MAPbBr3 structure is denser than tetragonal MAPbI3 crystal structure. Altogether these effects make MAPbBr3 less prone to the attacks by external oxygen, moisture, etc.42,16

Figure 3. TEM images of the green-MAPbBr3 NCs exposed to 405 nm laser light for different time of (a) 0min, (b) 5 min, (c) 10 min, (d) 20 min and (e) 30 min. (f) The plot of the size vs. time revealing PNC size reduction with prolonging laser exposure time. In the environment without oxygen, we found that the UV/blue laser irradiation plays a different role. In order to explore another photodegradation pathway, the time-resolved PL decays on single dots with OSTE polymer protection were measured. The PL decay of MAPbBr3 single dot with green and blue PL show two components (figure S7), which means that there are two recombination pathways to emit PL. However, the PL decay of red-MAPbI3 dot shows a monoexponential recombination. Considering the consistent PL evolution under laser irradiation of the


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red PNCs with green and blue PNCs as discussed above, we focus the time-resolved PL investigation on the red-MAPbI3 dot due to its less complex recombination dynamics. Although the PL intensity reduced under laser exposure without obvious shift of the PL peak position, the PL lifetime of the single MAPbI3 dot exhibits nearly no change after exposing the NC to laser for 30 min. Figure 4a shows the time-resolved PL spectra at room temperature, by fitting the data using a single-exponential function of 𝐼(𝑡) = 𝐼0exp ( ― 𝑡 𝜏), where τ represents the decay time, yielding a decay time of 7.3 ± 0.9 ns for the original NC. After exposing the NC to laser for 30 min, the PL decay time is 7.6 ± 1.0 ns, which is the same as the original value within a fitting error. Figure 4b shows that the PL intensity at the same time has reduced by 30 % after laser exposure for 30 min. So it is clear that while the emission intensity decreases (Figure 4b) or fluctuates (Figure S8) the lifetime remains unchanged under continuous light exposure. Here the PL intensity reduction rate both of the MAPbBr3 and MAPbI3 dots looks similar, and this is valid for all temperatures: at room temperature, at 70 K and also at 5 K (Figure S4). The reduction of PL intensity for MAPbBr3 and MAPbI3 PNCs in this case is related to the light-induced photophysical process, as discussed below, and, therefore, is less dependent on the material composition. The reduction of the PL intensity could be then attributed to the decrease of the number of recombination events, as opposite to the change in dynamics of individual recombination. Due to the unchanged size of the OSTE-protected NC and persistent lifetime, we ascribe this decrease of the number of recombination events to the influence of the polymer film and/or the organic ligands. Under laser exposure, the photoexcited electrons can be trapped by the surface organic ligands and/or polymer film. Charged NCs then lose subsequent excitation energy through efficient Auger process. After removing the laser and leaving the NC without continuous laser exposure for 60


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min, the PL decay still has no obvious change with the PL lifetime of 7.5 ± 0.4 ns, while the PL intensity recovered by 50 % compared with the intensity after continuous laser exposure for 30 min. It indicates that the trapped charges may return back from the trap states and enhance the number of recombination events in the NC core. We call this degradation mechanism the “lightdriven charge trapping”, which is similar to the “blinking” effect.43 Indeed, we observe emission intermittence in these NCs on a long (seconds) time scale (Figure S8). The detailed blinking dynamics may reflect different mechanisms and is typically material system-dependent.44 While this effect itself requires a separate study, here we briefly focus on its influence on the NC photostability. The slow photoluminescent intermittency process (“slow” compared to the lifetime) was also observed in large (no quantum confinement) MAPbI3 single crystal and MAPbBr3 NC ensemble films by Scheblykin and Green groups, respectively, and they attributed the blinking to the photoinduced activation and deactivation of nonradiative recombination sites.45,46 It was also independently reported for MAPbBr3 NCs in a weak confinement regime (> 10 nm in size) by Kobori group.47 In detail (Fig. 4c), under laser exposure the excited electrons in the NC are trapped by the states far away from the NC core (ON-OFF transition). This is a relatively slow process, and it is not reflected in decay curves as a fast trapping would do by stretching decays in so called “delayed” luminescence.48 However, the trapped charges may return back from the trap state to the core (OFF-ON transition), restoring the PL emission. The reversibility implies that there is no loss of volatile components to atmosphere, which is in agreement with the non-reduction of the NC size. Comparing with the literature, Peng’s group e.g. reported recently that individual CdSe/CdZnS core-shell quantum dots maintain PL blinking after exposed to an intense laser beam with barely any photochemical degradation.49 The above results indicate that under the excitation


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of the UV/blue light, the slow trapping of the excited carrier by defects in polymer/ligands contributes to the PL intensity reduction. Importantly, it is not a radiative or a non-radiative center, which is at play here, but rather a trapping site. By manipulating the ligand/matrix material and the structural composition this unwanted effect can be substantially reduced, as was successfully shown for II-VI colloidal NCs.50

Figure 4. (a) Time-resolved and (b) the corresponding steady-state PL spectra of a single redMAPbI3 NC measured without laser exposure (fresh), after exposing the NC to 405 nm laser for 30 min and after keeping the laser off for 60 min measured at room temperature. Weighted residuals from each of the time-resolved PL spectra fits are also presented by orange solid circles. (c) A schematic diagram describing the light-driven slow charge trapping process.


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In summary, we have investigated the degradation of single MAPbX3 NCs exhibiting quantum confinement effects by comparing the PL spectra of the single MAPbX3 NCs without a polymer and with an OSTE polymer as encapsulation. By single-dot photoluminescence, we found that the photoinduced degradation is not only related to photochemistry, but also linked to photophysical processes. We clarified the role of oxygen during the photodegradation process as an oxygenassisted light-induced etching, which results in a blue-shift of the PL peak position due to NC size reduction and eventually complete PL vanishing. The results also demonstrate that OSTE polymers perform well in protecting NCs from this oxygen-assisted erosion. An oxygen-scavenging thiolbased polymer offered significantly improved photostability and has completely suppressed any blue-shift for the investigated organolead trihalide NCs. On the other hand, benefiting from the single-dot technique, we found a photophysical process that manifests in a PL intensity reduction. In this process the PL peak position and the decay lifetime do not noticeably change, yet the light output reduces indicating ON/OFF blinking due to fluctuations. Based on these observations we conclude that under UV/blue light illumination, there exists a slow (compared to the lifetime) charge trapping process in the polymer/ligand layer, reducing the light output, yet recoverable. ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. The fabrication of green-MAPbBr3 NCs, blue-MAPbBr3 NCs and red-MAPbI3 NCs. The details of single-dot measurement. Single-dot PL of additional MAPbX3 PNCs at different environment and temperature, and PL blinking trace of single MAPbI3 NC. (PDF) AUTHOR INFORMATION


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Notes The authors declare no competing financial interests. ACKNOWLEDGMENT We would like to thank Federico Pevere for the help with microscopy operation, and Tommy Haraldsson for providing us with the methodology of synthesizing the OSTE polymer. We also would like to thank Changtao Xiao and Feng Zhang for providing us the blue-MAPbBr3 and redMAPbI3 nanocrystals. We would like to acknowledge the supports from the National Natural Science Foundation of China (No. 61722502 and No. 11474021), the Swedish Research Council (ADOPT), the Swedish Foundation for International Cooperation in Research and Higher Education (STINT), ÅForsk Foundation, and the International Collaborative Program of Henan Province Grant (N 182102410093). Lige Liu would also like to acknowledge the financial support from China Scholarship Council (CSC). REFERENCES (1) Han, D.; Imran, M.; Zhang, M.; Chang, S.; Wu, X. G.; Zhang, X.; Tang, J.; Wang, M.; Ali, S.; Li, X.; Yu, G.; Han, J.; Wang, L.; Zou, B.; Zhong, H. Efficient Light–Emitting Diodes Based on in situ Fabricated FAPbBr3 Nanocrystals: The Enhancing Role of the Ligand– Assisted Reprecipitation Process. ACS Nano 2018, 12, 8808–8816. (2) Kovalenko, M. V.; Protesescu, L.; Bodnarchuk, M. I. Properties and Potential Optoelectronic Applications of Lead Halide Perovskite Nanocrystals. Science 2017, 358, 745–750.


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Figure 1. Single particle detection of PNCs in the environment with and without oxygen. 128x51mm (150 x 150 DPI)



Figure 2. The PL spectra evolution of typical green-MAPbBr3 NC single dots (a) without polymer coating and (b) coated by OSTE film with laser exposure time of 15 min and prolonged to 30 min, respectively. (c) Top: PL peak position of the single green-MAPbBr3 NCs with and without OSTE protection under laser exposure for different time at room temperature. Bottom: The evolution of the PL integrated intensity of the single green-MAPbBr3 NCs without polymer measured at 295 K and with polymer measured at 295 K and 70 K. The acquisition time for each spectrum is 60 s. The excitation wavelength is 405 nm with a power density of ~ 1 W/cm2.


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Figure 3. TEM images of the green-MAPbBr3 NCs exposed to 405 nm laser light for different time of (a) 0min, (b) 5 min, (c) 10 min, (d) 20 min and (e) 30 min. (f) The plot of the size vs. time revealing PNC size reduction with prolonging laser exposure time. 250x119mm (150 x 150 DPI)



Figure 4. (a) Time-resolved and (b) the corresponding steady-state PL spectra of a single red-MAPbI3 NC measured without laser exposure (fresh), after exposing the NC to 405 nm laser for 30 min and after keeping the laser off for 60 min measured at room temperature. Weighted residuals from each of the timeresolved PL spectra fits are also presented by orange solid circles. (c) A schematic diagram describing the light-driven slow charge trapping process. 339x209mm (120 x 120 DPI)


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Table of Contents graphics 139x86mm (150 x 150 DPI)


Photodegradation of Organometal Hybrid Perovskite Nanocrystals

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