1. Introduction

Organo-metal halide perovskites (OMHPs) have attracted extensive interest due to their potential applications in solar cells [1–3], light-emitting diodes [4], photodetectors [5], and optically pumped lasers [6,7], benefiting from their excellent optoelectronic properties, such as high absorption coefficient, good charge carrier mobility, long carrier diffusion length, and high photoluminescence quantum yield. However, the practical application of OMHPs in optoelectronic devices always suffers from the stability of the materials, especially under the influence of light [8–10], heat [11,12], moisture [12,13], and oxygen [14,15]. For instance, the content of humidity in the environment during sample preparation has a great influence on the performance of perovskite-based devices. Moisture is also considered to be one of the most suspected factors which can result in the degradation of as-prepared perovskites. The water tends to bind Pb cations on the perovskite surface, resulting in structure degradation [16]. In addition, it has been argued that light will lead to the degradation of perovskite, which is usually manifested as the quenching of photoluminescence (PL). The light-induced degradation is also believed strongly correlated to moisture [12] and oxygen [14].

Recently, great efforts have been made to improve the long-term stability of perovskite materials, based on either chemical or physical strategy. The core idea of both strategies is to prevent the materials from the influence of the environment, especially moisture in most cases. For example, cross-linking of ligands [17–19] has been proven an effective way to protect the perovskite material from the harmful effect of the ambient environment. Coating with protective matrices, including either inorganic materials [20–22] or organic materials [23–26], has also been widely reported. Particularly, the perovskite/polymer composite film formed during the preparation process maintains good stability against humidity and light, providing impressive improvement of the performance of perovskite-based devices [23,27]. Besides the simplicity and efficiency to prevent perovskite from the influence of water and even oxygen, polymer also provides high flexibility of device fabrication, good mechanical performance, and improved PL properties [24,25]. On the other hand, the polymer films with outstanding insulation properties are also used for the packaging of devices. However, the majority of investigations to date focus on the stability improvement of the final device by polymer coating/encapsulation, little attention has been paid to the influence of the coating/encapsulation procedure on the PL properties of perovskite materials. An important question arises as to whether the improvement of photostability is achieved at the expense of other optoelectronic properties of the material, and thus the eventual performance of perovskite-based devices.

To achieve this goal, an in-situ investigation of the properties of perovskite during the polymer encapsulation process is required. Instead of ensemble measurements, study at the single-particle level provides a powerful tool for demonstrating the photophysical properties of perovskite materials at the micro/nano scale [28,29] and shedding light on the fundamental mechanisms behind the influence of external environment on the photostability of perovskite materials [30,31]. Generally, OMHPs can be achieved by simply solution processible method, offers a promising route towards low-cost devices. However, the solution processing is always accompanied with an abundance of defects, or traps inside the perovskite materials. Though studies have showed strong tolerance of perovskite to large number of defects and traps, some deep traps still exist, resulting in non-radiative recombination pathways for photogenerated electron-hole pairs [32]. At the single-particle level, a slight fluctuation of defects or traps would result in a considerable change of PL properties, for example PL blinking, which makes it more sensitive to the change induced by the external environment, such as light, moisture, and atmospheres. Thus, the fluctuation of PL properties at the single-particle level provides us a sensitive tool to monitor the dynamics of perovskite at nano-scale and to reveal the fundamental mechanism behind the effect of polymer encapsulation.

In this work, we studied the influence of polymer coating/encapsulation procedure on the PL properties of OMHP materials at the single-particle level. The drop-casting processing of poly(methyl methacrylate) (PMMA)/toluene solution on individual CH₃NH₃PbI₃ (MAPbI₃, MA = CH₃NH₃⁺) crystals was chosen as a test-bed system. The goal here is to provide mechanistic insight into the possible degradation or passivation of OMHP materials during polymer encapsulation process. By in-situ analyzing the fluctuation of PL properties of individual MAPbI₃ crystals, such as PL blinking, PL intensity, spectra, and lifetime, we clarify the role of different types of defects in the passivation effect of PMMA polymer and the underestimated negative effect of toluene solvent. The research provides an important basis for further improving the photostability of perovskite materials, especially in devices using polymer as packaging materials.

2. Experimental section

2.1 Sample preparation

MAI (98%), PbI₂ (99%), PMMA (MW15000, powder), and toluene (analytical-grade, 99.5%) were purchased from Sigma-Aldrich Co. Ltd. The gamma-butyrolactone (GBL, 99%) was purchased from TCI Co. Ltd. To prepare the MAPbI₃ crystals, a solution was got firstly by dissolving 159 mg of MAI and 461 mg of PbI₂ in GBL of 1.25 mL. The mixtures were stirred at 60 ${^\circ }\textrm{C}$ for 2 h at the speed of 500 r/min. The precursor solution was then diluted to 0.05%. About 20 $\mathrm{\mu}\textrm{L}$ of the diluted solution was drop-casted onto precleaned glass coverslips, and then annealed for 20 min at 80 ${^\circ }\textrm{C}$. All the preparation processes were performed under ambient condition. In order to perform the in-situ coating/encapsulation experiment, 8.7 mg of PMMA was dissolved in 1 mL of toluene to get a solution of 1 wt%.

2.2 Sample characterization

The size distribution of the individual MAPbI₃ crystals was characterized with the scanning electron microscopy (SEM) on a SU8010 (Hitachi) with an accelerating voltage of 3 kV, and an atomic force microscopy (AFM, Nanosurf C3000). Fourier transform infrared spectroscopy (FTIR) of pristine PMMA polymer and perovskite crystals coated with PMMA was performed on a Nocolet iS50 FT-IR (Thermofisher).

2.3 Experimental setup

The experiment was performed on a home-built wide-field microscope based on a commercial Nikon Ti-U microscopy. A CW laser with wavelength of 532 nm was used for excitation. The laser firstly passed a beam expansion system and then was focused by a long focal lens to the back focal plane of an objective lens (Nikon, CFI Apo 100×, NA=1.49, WD=0.12 mm) to achieve a wide-field illumination with power density of about 2 kW/m². The PL of the sample was recorded by an electron-multiplying charge-coupled devices (EMCCD) (Princeton, ProEM 512B) by which an area of about 34 µm ${\times}$ 34 µm can be imaged. To analyze the PL properties of the crystals, the PL trajectory of each crystal can be extracted from the movie recorded with integration time of 100 ms per frame. The PL spectra of individual crystals can be measured simultaneously by inserting a transmission diffraction grating before EMCCD. In order to measure the lifetime of specific crystal, a pulsed laser of 488 nm was used for excitation. In this case, the PL was separated by a 30/70 beam splitter to a single photon detector (Perkin Elmer, SPCM-AQR-15) and analyzed by a single photon counting system (TCSPC, PicoQuant, Hydraharp 400). A confocal scanning image can be achieved by scanning the sample via a nano-positioning stage and detecting the PL signal by the single photon detector.

3. Results and discussion

3.1 Correlated wide-field and confocal scanning PL image analysis of individual MAPbI₃ crystals

Figures 1(a) and 1(b) show a correlated wide-field PL image and confocal scanning PL image of individual MAPbI₃ crystals in the same area, respectively. The crystals were separated individually and had a random distribution on the glass [see also Appendix A (Fig. 6) for the SEM and AFM images]. Due to the frequent PL blinking of perovskite crystals, it’s hard to show all the crystals in the PL image of one frame. However, we can still get the exact correlation of specific crystal between the two images from the distribution of the crystals and their relative positions. Figure 1(c) shows the typical PL spectrum of the crystal marked with the circle in the images. The PL spectrum gives a peak of 760 nm, which is consistent with the emission of MAPbI₃ perovskite material reported in literature [33,34]. Figure 1(d) shows the PL decay curve of the same crystal marked with the circle. To analyze the PL lifetime, the decay curves were fitted to a tri-exponential function:$I(t) = {I_0} + {A_1}{exp}( - t/{{\tau }_1}) + {A_2}{exp}( - t/{{\tau }_2}) + {A_3}{exp}( - t/{{\tau }_3})$, where τ₁, τ₂ and τ₃ are the lifetimes, A₁, A₂ and A₃ are the relative amplitudes of each component, respectively. The typical PL decay of the crystal marked with the circle in the image reveals the lifetime of τ₁ = 120.40 ns, τ₂ = 32.26 ns, and τ₃ = 9.52 ns. Then, an amplitude-weighted average lifetime of 69.79 ns can be obtained by ${\bar{\tau }}\textrm{ = }\frac{{{A_1}{{\tau }_1} + {A_2}{{\tau }_2} + {A_3}{{\tau }_3}}}{{{A_1} + {A_2} + {A_3}}}$.

 

Fig. 1. (a) Wide-field imaging of individual MAPbI₃ crystals. (b) Confocal imaging of the same area in image (a). (c) Spectrum of the crystal marked with the white circle in (a). (d) Lifetime of the crystal marked with the white circle in (a) in ambient condition.

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3.2 PL response of individual MAPbI₃ crystals under PMMA/toluene encapsulation procedure

In order to in-situ investigate the effect of polymer encapsulation procedure on the PL properties of individual MAPbI₃ crystals, we performed the experiment under the general processing sequence outlined into several stages as illustrated in Fig. 2(a). At the first stage, the PMMA/toluene solution was drop-casted onto as-prepared MAPbI₃ crystals on the glass coverslip. With the evaporation of the toluene solvent, the PMMA was polymerizing gradually during stage II. After a long period of polymerization, pure toluene solution was then added onto the encapsulated PMMA film, as shown in stage III, followed by solvent evaporation again which may result in re-polymerization of PMMA. The PL response of individual MAPbI₃ crystals were monitored during the entire processing sequence.

 

Fig. 2. (a) Schematic illustration of the processing sequence for in-situ analyzing the effect of the polymer matrix and the solvent on individual MAPbI₃ crystals. Left: The as-prepared MAPbI₃ crystals are processed by covering 20 $\mathrm{\mu}\textrm{L}$ of PMMA/toluene solution; Middle: The PMMA polymerizes gradually with the evaporation of toluene; Right: 20 $\mathrm{\mu}\textrm{L}$ of pristine toluene solution is re-dripped on the polymerized PMMA matrix. (b) Typical PL response of a MAPbI₃ crystal under the processing sequence shown in (a).

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Figure 2(b) shows the typical evolution of the PL intensity of individual MAPbI₃ crystal under constant laser illumination. It can be found that the as-prepared MAPbI₃ crystal showed obvious PL blinking in the ambient air. Continuing illumination and exposure in ambient air will not induce clear PL quenching or change of blinking behavior within the time range of experimental measurement, under the laser power density of about 2 kW/m² used in this work. However, after covering with the PMMA/toluene solution, the PL intensity dropped suddenly to a lower level, but the crystal still kept blinking (Stage I). With the evaporation of the toluene solvent, the PL intensity further decreased gradually, and finally maintained at a stable level (Stage II). However, we can also note that, with the evaporation of the solvent (or polymerization of PMMA matrix), the PL blinking events decreased significantly compared to the blinking behavior in ambient condition and the blinking at the very beginning of PMMA/toluene coating. In contrast, when pure toluene solvent was dropped onto the polymerized PMMA film, the MAPbI₃ crystal suddenly recovered to blinking, accompanied by slight enhancement of PL intensity, as shown in Stage III in Fig. 2(b). The PL intensity showed a further decrease after the transient recovery with the treatment of toluene.

Figure 3(a) summarizes the effect of the treatment processes on the PL intensity and blinking. Basically, we can classify the PL responses into two categories. One is the PL decline and enhancement, and the other is the PL blinking suppression and recovery. Here, the PL decline can be characterized by simply comparing the PL intensity of the crystal. In order to evaluate the change of blinking behavior, we use here the threshold method commonly used for PL blinking statistics [35]. Briefly, Gaussian distribution is fitted on the intensity histogram of each crystal. A threshold is set as 2 standard deviations lower than the fitted higher intensity level (considered as the ON state). For simplicity, all states below the threshold are considered as blinking events in the PL trajectory. Instead of statistic the probability distribution of ON and OFF time duration, we simply count the blinking events per second to mark the change of blinking rate. As illustrated in Fig. 3(a), the PL intensity decreases immediately upon coating of PMMA/toluene solution and further goes to a relative stable level upon evaporation of the solvent. During this period, the blinking of the crystal becomes less pronounced [see also Appendix B (Fig. 7) for more evidence]. A further typical observation is the recovery of PL intensity and blinking rate of the crystal upon possible re-dissolvement of PMMA film in toluene in Stage III.

 

Fig. 3. (a) Summary of the evolution of PL intensity and PL blinking events shown in Fig. 2. (b) PL spectrum of individual MAPbI₃ crystal before and after PMMA/toluene coating. (c) PL intensity trajectory and corresponding PL decay curves of individual crystal under the encapsulation procedure. In the lower panels, dark gray indicates the lifetime of the pristine perovskite crystal at ambient condition; red shows the lifetime of the crystal during the polymerization process of PMMA; blue shows the lifetime after toluene was re-dripped.

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Fig. 4. Two typical PL trajectories of individual crystals processed with pristine toluene solvent at ambient condition. (a) The PL intensity of the crystal decreased but the blinking remained. (b) The PL of the crystal was quenched completely. The lower panels show the corresponding PL spectrum of each crystal.

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Fig. 5. Schematic view of the role of PMMA and toluene in the encapsulation procedure. PMMA shows a passivation effect via bond interactions between the C = O group and Pb²⁺ ions on the surface of individual MAPbI₃ crystals, whereas the toluene will induce quenchers (highlighted by the dark stars) that result in the PL decline.

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We also measured the evolution of PL spectrum and lifetime of individual MAPbI₃ crystals. Although the PL image of multiple crystals can be recorded simultaneously by wide-field imaging, the PL spectrum and lifetime can only be achieved by measuring the crystals one by one. Thus, it is hard to monitor the change of spectrum and lifetime of all the crystals at the same time. Instead, we show here the representative change of PL spectrum and lifetime of crystals at each separate stage and gives statistic of studied crystals. The typical PL decay for individual perovskite crystal is shown in Fig. 3(c), revealing a much shorter average lifetime after processing with PMMA/toluene solution (τ = 26.00 ns) than the original state (τ = 96.56 ns). Furthermore, with the transient recovery of PL intensity after processing with pure toluene solution in Stage III, the average PL lifetime has a slight increase (τ = 56.66 ns). Detailed information for the lifetime analysis can be found in Appendix C (Fig. 8 and Table 1). The statistical results of the change of PL lifetime for several individual crystals can be also found in Appendix C (Fig. 9). The results show a coincident shortening of PL lifetime during the encapsulation process. Generally, the longer PL lifetime is used as an indicator of better-performing materials, while the shorter lifetime suggests the higher trap density and therefore greater non-radiative loss [36]. Hence the shorter lifetime indicates a higher defect or trap concentration for the PMMA/toluene-coated perovskite crystals. In stark contrast, the PL spectrum of the crystal remain unchanged throughout the treatment processes [Fig. 3(b)]. More information about the correlation of PL trajectory, PL lifetime and PL spectrum of another typical crystal can be found in Appendix D (Fig. 10).

3.3 Mechanistic view on the observed PL decline and PL blinking suppression

It has been suggested that both the fluctuation of PL intensity and the PL blinking are highly correlated to the fluctuation of quenching sites (or quenchers for simplicity) in the perovskite materials. In this work, it is highly recommended that, as will be discussed later, the quenchers that account for the fluctuation of PL intensity and PL blinking reported here should be different in the mechanism.

In order to account for the fluctuation of PL intensity, a classical model [37] is used, in which the fluctuation of PL intensity is correlated to the PL quantum yield that is determined by radiative and non-radiative recombination rates of the crystal. The PL quantum yield thus can be expressed as:

In the framework of this simple model, it is highly suggested that the PL decline observed in the MAPbI₃ crystals when coating with PMMA/toluene solution is due to the increase of active quenchers in the crystals. This is unexpected, however, considering the fact that encapsulation of PMMA film has been reported to improve the stability of perovskite materials [23,26]. Another unexpected phenomenon is the further PL quenching of the crystal in Stage III after initial “blowout” of PL intensity and blinking events when pure toluene solution was re-dripped. This prompted us to take the effect of toluene into account.

In order to clarify the role of toluene, we processed the pristine perovskite crystals with pure toluene directly, at ambient condition. As illustrated in Fig. 4, the PL of the crystals shows a fast quenching under the treatment of pure toluene. In the in-depth analysis of all the PL trajectories of individual crystals, two main types can be found. The PL of some crystals was quenched to a lower level [Fig. 4(a)] as that showed in Stage I of Fig. 3(b), while the PL of others was quenched completely to the background level [Fig. 4(b)]. For the partially quenched ones, the PL kept blinking and PL intensity will slightly increase after a period of holding in the dark (see Appendix E for more details). Though some crystals were quenched to background level, they didn’t disappear. Instead, it seems that these crystals tend to keep in a long period dark state, accompanied by a sudden rise of PL at some point.

Since toluene was usually chosen as a non-coordinating solvent (or “poor” solvent) which promotes the crystallization process of perovskites [38], the idea that toluene will lead to the degradation of perovskite is not recommended. This can be partly supported by the slightly recovery of PL intensity after keeping dark for a while and the abrupt PL recovery from bottom to up in the trajectory [see Appendix E (Fig. 11)], since the degradation is normally irreversible. Evidence can also be found in the transient PL recovery when toluene was re-dripped in Stage III of Fig. 2(b). Furthermore, no measurable change was presented in the PL spectra of the crystals [see lower panels in Figs. 4(a, b)]. Generally, the shift of PL spectra might happen in the nanocrystals with a smaller size that quantum confinement dominants, due to the decrease of size by decomposition. The stable of PL spectrum, together with the PL trajectories, indicate the absence of decomposition during the toluene-treating procedure.

Based on the discussion above, we can basically attribute the PL quenching observed here to the toluene induced accumulation of the number of active quenchers in Eq. (2), which increases the total non-radiative recombination rate via formation of extra non-radiative recombination channels in the materials. This can also explain the PL decline of the MAPbI₃ crystals when immersion in PMMA/toluene solution as observed in Stage I [Fig. 2(b)]. With the drop-casting of PMMA/toluene, the contact of perovskite crystals with toluene will result in a creation of active quenchers. Or, in other word, fast activation of already existed quenchers in the crystals from their passive states.

However, different from the fast decay of as-prepared crystals when processed with pure toluene, the PL of the crystals covered with PMMA/toluene showed a smooth PL decline after a fast decay, as shown in Stage II of Fig. 2(b), which highly suggests the protective effect of PMMA polymer. With the evaporation of solvent, the gradual polymerization of PMMA protected the crystals from the influence of toluene, as shown in Stage II of Fig. 2(b) where the PL intensity keeps unchanged eventually.

In addition to the protective effect on PL intensity, the suppression of blinking makes PMMA more attractive. As can be found in Figs. 2(b) and 3(a), with the evaporation of toluene (or polymerization of PMMA), the PL blinking of the crystal is suppressed significantly. As will be discussed later, blinking suppression by PMMA encapsulation reported here suggests the role of surface defects on the blinking mechanism.

3.4 Discussion on the chemical nature of the quenchers behind the encapsulation-induced PL decline and PL blinking suppression

It has been argued for long in the blinking mechanism that the charge trapped by surface defects can activate quenchers that effectively quench PL, or a trion formed by the opposite charge left in the crystal that quenches photo-generated excitons via Auger recombination. In both cases, the surface trap states are suspected to play a significant role [39]. These traps are likely relevant to surface defects such as CH₃NH₃⁺ vacancies, Pb²⁺ ions, or halide vacancies [33,39,40]. The importance of surface trap states in the blinking mechanism is also consistent with the validity of passivation method for PL improvement and sensitivity of PL properties to surrounding atmosphere. In our experiment, strong evidence for the correlation between blinking and PMMA passivation can be found both in Stage II and Stage III of Fig. 2(b).

As a high-performance insulating material with good flexibility and transparency, PMMA has been widely used as host matrix for a variety of functional materials and a moisture protection barrier in perovskite solar cells. It is believed that the validity of protective effect of PMMA relies on the compact capping of polymer chains on the surface of perovskite. Hence, the passivation of surface defects via PMMA capping is expected. Literature has reported that the under-coordinated Pb²⁺ ions at the surface of perovskite are highly recommended to act as traps which impair new generated carriers via non-radiative recombination [33,39]. As shown in Fig. 5, upon polymerizing of PMMA, the carbonyl group (C = O) in the ester group of PMMA is expected to bond the Pb cations of perovskite crystal, which brings about annihilation of Pb cation-related traps on surface. This in turn suppresses the blinking dynamics in perovskite crystals. The bonding of the PMMA with perovskite was confirmed by FTIR measurements [see Appendix F (Fig. 12)]. In Appendix F, we can find a spectral region at 1730cm⁻¹, corresponding to the C = O groups, both in pristine PMMA film and perovskite-embedded PMMA film. We can also find an absorption peak near 1590 cm⁻¹ corresponding to the symmetric stretch of COO- groups, which is consistent with the report of Tannenbaum et al. [41]. Compared with the FTIR of pristine PMMA, the absorption band in perovskite-embedded PMMA film showed a slight movement to 1571 cm⁻¹, indicating the influence of Pb²⁺ on the stretch of COO- groups via boning of C = O with Pb²⁺ on the surface of perovskite. Similar bonding effects of PMMA with Pb²⁺ has also been presented recently in the work of Li et al. [26] in PMMA/MAPbBr₃ composite films and Chen et al. [42] in PMMA/CsPbI₃ composite films, respectively. Most probably, the bonding of C = O with Pb cation stopped the possibility of the formation of Pb-related interstitials with deep energy level. This is consistent with the literature where activation of Pb_i²⁺ state to Pb_i⁰ state is believed to be responsible for new quenchers in bulk crystal or film [43,44], which can be somehow regarded as the ensemble of considerable individual crystals. On the other hand, the capping of PMMA also prevent the interaction of water molecules with Pb cations which was reported to account for the decomposition of crystals structure [16]. The cooperation of these two effects results in stabilization of PL intensity as shown in Stage II of Fig. 2(b). However, the PMMA-crystal interaction in the case of coating process in this work is rather poor and the encapsulation is therefore easily broken. The re-dripping of toluene solution will definitely break the bonding of PMMA carbonyl group with Pb cations, which in turn results in the recovery of blinking as shown in Stage III of Fig. 2(b).

In comparison to the quenchers which account for PL blinking, the quenchers that lead to PL decline appear to be different. Although considerable evidence showed above establishes a strong correlation between these quenchers and toluene, we cannot, unfortunately, exclude the influence of atmosphere on the PL dynamics. It is noteworthy that, in addition to the recovery of blinking dynamics in Stage III after re-dripping of pure toluene, an abrupt increase of average PL intensity also happened [see Fig. 2(b)]. This implies a reasonable correspondence of PL enhancement to the passivation effect of components in atmosphere, for example, the oxygen. Recently, similar PL decline effect of perovskite nanocrystals in toluene has been reported by Chen et al. [42]. They found an initial increase of PL quantum yield of the suspension in toluene and subsequent decrease. They assumed that toluene might passivate the surface defects at the very beginning which results in the initial increase of PL quantum yield, whereas the subsequent decrease of PL quantum yield is due to the influence of the external environment. The results at the single-particle level shown here may provide exceptional insight into the influence of toluene and the external environment. On the contrary, based on our experiments at the single-particle level, we propose that the toluene will induce defects (at surface, or in bulk, or both) that will quench the PL (as proposed in Fig. 5), whereas the external environment will show more positive effect. In the case of our model, we can explain the initial increase and then decrease of PL quantum yield in the ensemble observed in Ref. [42] to the balance of negative effect of toluene solvent and the positive effect of the external environment, in which the effect of toluene eventually plays a leading role. In our case, it is likely that the opening of the compact polymer layer by re-dripping of toluene solution provides an opportunity for the interaction of oxygen with perovskite crystals, which results in annihilation of quenchers that responsible for PL decline. This is consistent with previous studies of PL enhancement effect via oxygen [45,46]. However, the enhancement effect shown in this work is weak and transient, far less than the negative effect of toluene. The re-polymerization of PMMA polymer would stop the contact of ambient atmosphere with the crystal and the effect of toluene dominated the quenching of PL intensity after the transient “blowout” of PL intensity. Furthermore, we noticed that the “blowout” of PL intensity in crystals was only found during the re-dripping of the toluene on encapsulated PMMA film [Fig. 2(b)]. The “blowout” of PL can reappear for some crystals if further dripping toluene on re-polymerized PMMA after stage III. However, not any recovery of PL intensity was found in pure toluene processed crystals, even after a long period of solvent evaporation [Fig. 4 and Appendix E (Fig. 11)]. It seems that the toluene-induced quenchers are relatively robust against smooth interaction of ambient atmosphere but will be partially passivated due to, most probably, the transient diffusion of atmosphere into the crystals.

Though an amount of evidence shown here indicates the role of toluene in inducing the quenchers, establishing precise corresponding relationship with specific defects remains a difficult task. Nonetheless, we may be able to see through the fog based on the experimental facts that, firstly, they should be relatively low efficient traps (compared to quenchers that account for PL blinking) with lower active energy, which can be easily induced by toluene or passivated by the external environment; secondly, they cannot be easily passivated by PMMA at the surface; thirdly, keep in dark may partially lead to the annihilation of these defects while light radiation would, conversely, partially promote their formation. The results presented here are reminiscent of the role of ion migration in the creation and annihilation of these quenchers, and most probably, the migration of halide ions with activation energy of around 0.6 eV [47]. In order to clarify the chemical nature of the quenchers, it’s interesting here to discuss the similarity between the PL decline and the PL enhancement, either as shown in this work or as reported in literature, which suggests the fluctuation of quenchers for both processes. In the widely reported cases of light induced PL enhancement, the role of ion migration has been highly suggested [45,48–50]. PL decline and the mechanism behind, on the other hand, were seldom reported and discussed, since most reports concerned only on the degradation of perovskite, accompanying the disappear of PL intensity. Recently, Chen et al. [50] reported light-induced PL quenching in MAPbI₃, which was attributed to the increase of non-radiative recombination pathways due to ion migration, rather than the simple photo-degradation. In the work of Hong et al. [43], a specific correlation between PL decline and Pb interstitial in bulk crystal was proposed. However, rather than to explain the PL decline, we tend to believe that the Pb interstitial could be the reason of PL blinking in individual crystal, as discussed above. Based on the previous theoretical calculations [51–54] and considering the conditions listed above, we found that the iodine-related vacancy (V_I) [51,55] could be the most probable candidate for toluene induced PL decline found in individual MAPbI₃ crystals, because of their proper energy level and available formation energy. Here, we propose that toluene induced ion migration features the creation of the iodide related quenchers. These quenchers cannot be easily passivated by PMMA. Another evidence comes from the annihilation of these quenchers by the external environment when toluene was re-dripped onto PMMA protected perovskite crystals, which results in an abrupt increase of PL intensity. It is worth noting that the PL lifetime has a significant change during these processes. It has reported by deQuilettes et al. [36] that the trap density can be extracted from the PL decay of the perovskite. Here, we estimated the change of trap densities of the crystal shown in Fig. 3(c) as an example. As shown in Appendix G (Fig. 13), the trap density of individual MAPbI₃ crystal shows an increase by order-of-magnitude when processed with PMMA/toluene solution, indicating the role of traps in the PL decline.

4. Conclusions

In conclusion, we have demonstrated that, behind the widely reported improvement of stability of perovskite by polymer coating/encapsulation, the species in the encapsulation would have significant influence on optical behavior of the perovskite crystals at the single-particle level. PMMA polymer has an affinity to passivate the surface defects, and thus suppress the quenchers that accounts for the PL blinking of individual perovskite crystals. Unfortunately, the toluene solution used for polymer encapsulation would induce quenchers that result in the PL decline of as-prepared perovskite crystals. Preliminary evidence shown in this work suggests the difference between the quenchers that lead to PL blinking and PL decline. The PL blinking is strongly related to under-coordinated Pb²⁺ ions, which can be passivated by PMMA via bonding of carbonyl group (C = O) with Pb²⁺. Whereas PL decline is attributed the toluene induced large amount of quenchers which are highly recommended to be halide-ion-related complexes. The results suggest that considerable scope remains for improving the PL efficiency and photostability of hybrid perovskite.

Appendix A: SEM and AFM images of individual MAPbI₃ crystals

Figure 6 shows the SEM and AFM images of individual MAPbI₃ crystals. The crystals have a size distribution around 200–500 nm with height of around 20 nm.

 

Fig. 6. (a) and (b) are the SEM and AFM image of randomly distributed individual MAPbI₃ crystals, respectively.

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Appendix B: more evidence showing the PL decline and PL blinking suppression of individual MAPbI₃ crystals with the encapsulation of PMMA/toluene solution

Here, we listed the change of PL properties of several individual MAPbI₃ crystals before and after encapsulation with PMMA/toluene solution. The PL intensity decreased immediately with the encapsulation and stabilized to a lower level. The PL blinking rate (insets) shows a coincident decrease upon encapsulation.

 

Fig. 7. Decrease of PL intensity and suppression of PL blinking upon processing with PMMA/toluene solution. Insets show the change of blinking rate upon polymerization of PMMA.

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Appendix C: PL lifetime analysis for individual perovskite crystals

Here, the PL lifetime can be well fitted by a tri-exponential function as discussed in the main text. Table 1 shows the fitting parameters for the crystal in the different stages of encapsulation procedure.

 

Fig. 8. Change of PL lifetime for the crystal shown in Fig. 3(c) in the different stage of encapsulation procedure.

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Fig. 9. Statistics results showing the evolution of the PL lifetime of individual crystals in three main stages of encapsulation process (a) and more detailed evolution during the process of PMMA polymerization (b).

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Table 1. Fitting parameters for the PL decays shown in Fig. 8.

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Appendix D: correlation of the change of PL intensity, PL lifetime and PL spectrum of individual MAPbI₃ crystal

 

Fig. 10. In situ analysis of the change of PL intensity, PL lifetime and PL spectra of individual MAPbI₃ crystal during encapsulation procedure. The PL lifetime shows a continuous decrease with the encapsulation of PMMA/toluene solution and subsequent evaporation of the solvent. With the re-dripping of toluene on encapsulated PMMA film, the lifetime shows a slight increase. However, the PL spectrum doesn’t show measurable change during the whole processing sequence.

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Appendix E: toluene induced PL decline

We showed in Fig. 11 a group of crystals which showed PL decline under the process of pure toluene solution. For all the crystals, the PL intensity showed an immediate decrease upon drop-casting of pure toluene. For the crystals in group (a), the PL blinking maintained with a high rate. For some of them, the PL intensity seems slightly increased in the subsequent measurement, after keeping the crystals in dark for a while. For the crystals in group (b), the processing of toluene not only results in the decrease of PL intensity, but also leads to the tendency that crystals kept in dark state for long. The PL will appear suddenly at some time, ruling out the possibility of permanent degradation of the crystals.

 

Fig. 11. PL trajectories of individual crystals processed with pure toluene solution at ambient condition.

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Appendix F: FTIR spectra of pristine PMMA film and perovskite embedded PMMA film

 

Fig. 12. FTIR spectra of pristine PMMA film and perovskite embedded PMMA film showing the fingerprint of the carbonyl group vibrations.

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Appendix G: estimation of trap density in perovskite crystal

We can estimate the change of trap density based on the report of deQuilettes et al. [36]. Briefly, the PL decay curve of the crystal can be simulated by solving the set of coupled differential equations [36]:

(3)$$\frac{{d{N_c}(t)}}{{dt}} ={-} {k_{dt}}{N_{dt}}(t){N_c}(t) - {k_m}{N_c}(t) - {k_b}{N_c}{(t)^2},$$

(4)$$\frac{{d{N_{dt}}(t)}}{{dt}} ={-} {k_{dt}}{N_{dt}}(t){N_c}(t)\textrm{ },$$

where N_c(t) is the time dependent carrier density, N_dt(t) is the deep trap density, k_m is the monomolecular radiative recombination rate, k_b is the bimolecular rate, and k_dt is the non-radiative recombination rate induced by available traps.

Then we simulate the PL decay with $I(t) = {k_m}{N_c}(t) + {k_b}{N_c}{(t)^2}$ using the solution from Eqs. (3) and (4).

Figure 13 shows the simulation of PL decays of the crystal shown in Fig. 3 and Fig. 8. We extracted the trap densities of pristine perovskite crystal, processed perovskite crystal with PMMA/toluene, and after re-dripping of toluene to be 0.5 ${\times}$ 10¹⁵ cm⁻³, 6.5 ${\times}$ 10¹⁶ cm⁻³, and 2 ${\times}$ 10¹⁵ cm⁻³. This indicates that the encapsulation of PMMA/toluene will induce the increase of trap density over one magnitude. Although the trap density varies from different crystals, the trap density processed with PMMA/toluene is always higher than the pristine perovskite crystals, while the slight recovery of PL intensity when re-dripping of toluene is accompanied by decrease of traps.

 

Fig. 13. Simulated PL decays of individual MAPbI₃ crystal show evidence of the role of traps in PL decay.

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Funding

National Key Research and Development Program of China (2017YFA0304203); National Natural Science Foundation of China (62075122, 62075120, 61675119, 61875109, 91950109, 61527824); PCSIRT (IRT_17R70); 1331KSC; 111 project (D18001); Shanxi Scholarship Council of China (HGKY2019002); NSFC-STINT (62011530133).

Disclosures

The authors declare no conflicts of interest.

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