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Giant Photoluminescence Blinking of Perovskite Nanocrystals Reveals Single-Trap Control of Luminescence Yuxi Tian, Aboma Merdasa, Maximilian Peter, Mohamed Abdellah, Kaibo Zheng, Carlito S. Ponseca, Tonu Pullerits, Arkady Yartsev, Villy Sundstrom, and Ivan G. Scheblykin Nano Lett., Just Accepted Manuscript • DOI: 10.1021/nl5041397 • Publication Date (Web): 23 Feb 2015 Downloaded from http://pubs.acs.org on February 24, 2015

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Giant Photoluminescence Blinking of Perovskite Nanocrystals Reveals SingleTrap Control of Luminescence Yuxi Tian,1 Aboma Merdasa,1 Maximilian Peter,1 Mohamed Abdellah,1,2 Kaibo Zheng,1 Carlito S. Ponseca Jr.,1 Tõnu Pullerits,1 Arkady Yartsev,1 Villy Sundström,1 and Ivan G. Scheblykin1,* 1

Chemical Physics, Lund University, Box 124, SE-22100, Lund, Sweden

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Department of Chemistry, Qena Faculty of Science, South Valley University, Qena 83523,

Egypt *Corresponding author: [email protected] Tel: +46462224848 Fax: +46462224119

ACS Paragon Plus Environment

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ABSTRACT

Fluorescence super-resolution microscopy showed correlated fluctuations of photoluminescence intensity and spatial localization of individual perovskite (CH3NH3PbI3) nanocrystals of size 2003030 nm3. The photoluminescence blinking amplitude caused by a single quencher was hundred thousand times larger than that of a typical dye molecule at the same excitation power density. The quencher is proposed to be a chemical or structural defect that traps free charges leading to non-radiative recombination. These trapping sites can be activated and de-activated by light.

KEYWORDS: Perovskite; Photoluminescence; Blinking; Single molecule spectroscopy; Superresolution microscopy; Charge trapping


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Organometal halide perovskites (OMHP) have become very promising materials for highperformance, low-cost solar cells during the past three years1 with a rapid increase of power conversion efficiencies up to 19.3%.2–6 To a large extent these materials possess properties of traditional inorganic semiconductors combined with a great advantage of solution processing. Some reasons for the high efficiency discussed in literature are broad light absorption range, fast charge generation combined with slow recombination7,8 and long-range diffusion of electrons and holes.7–10 Photo-excitation is proposed to predominantly lead to the formation of free charge carriers.7,11 However, many aspects of fundamental photo-physical properties of these materials still remain largely unknown. Recently, OMHPs have also shown promising applications as light emitting materials due to the high photoluminescence (PL) quantum yield and transport of both electrons and holes.8,12,13 Charge recombination was proposed as the main mechanism for PL emission.13,14 PL provides important information about the excited state7–10 and charge dynamics which have also been investigated by transient absorption spectroscopy, time-resolved terahertz and microwave conductivity.7–10,15 All these measurements however were performed on OMHP bulk films which are very inhomogeneous. Under a microscope, one can see that the films actually consist of many islands as shown in Fig. S2 in Supporting Information (SI). Adding spatial resolution to spectroscopy studies of OMHP is therefore a necessary step to further understand these materials. Using the single-molecule spectroscopy (SMS) method, fluorescence from individual molecules can be investigated giving information beyond the ensemble average.16–18 SMS has been successfully applied to rationalize excited state migration processes in multichromophoric systems like conjugated polymers, nanoparticles and aggregates.19–21 In the past decade, super-


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resolution optical microscopy methods have been developed and broadly applied in biological science, where a spatial resolution below the light diffraction limit can be reached.22–25 In material science, super-resolution techniques have been used for example to detect active catalytic sites in nanocrystals26–28 and to study energy migration processes in organic nanomaterials.29–32 In this work we prepared CH3NH3PbI3 (MAPbI3) films using the reported deposition procedure.3 MAPbI3 nanocrystals were obtained in the same way by using diluted solutions. We used fluorescence microscopy to observe both bulk films and individual nanocrystals. Surprisingly, the PL intensity of the films was very inhomogeneous in space and, moreover, possessed strong local temporal fluctuations which were also observed for individual MAPbI3 nanocrystals. Such PL intensity fluctuations are very similar to those of individual dye molecules but with five orders of magnitude larger absolute intensity amplitude (if measured at the same excitation power density). PL blinking of a MAPbI3 nanocrystal is illustrated in Fig. 1a. In the early days of singlemolecule spectroscopy, fluorescence blinking was thought to be an exclusive effect of single quantum systems, e.g. a single dye molecule. However, as it turns out the effect is not limited to such systems. A multichromophoric object (e.g. a long conjugated polymer chain or an aggregate) can also blink19–21 under one of the two following conditions: (1) if the quenching radius of a single photo-induced quencher is close to the size of the multichromophoric system;33 or (2) if there is an efficient transport of the photoexcitations to a single emitting site which can easily be temporally quenched (energy funneling21). In both cases excitation migration either towards the emitting site or towards the quenching site is essential. Therefore, such “on/off” blinking of MAPbI3 nanocrystals with size as large as 200×30×30 nm3 (SEM data) is a direct


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proof of the efficient electron-hole diffusion over similar distances since the longest single-step quenching mechanism is Förster resonance energy transfer (FRET) with a typical range of a few nanometers only. These results are in good agreement with the reported long-range electron-hole transport in OMHP.7–10

Figure 1 PL transients in the units of brightness showing blinking of (a) a MAPbI3 nanocrystal and (b) a bright dot located on the top of a large MAPbI3 crystal. The locations of the objects are indicated on the images in right column. Excitation power densities were 0.02 and 0.5 W/cm2 (corresponding to 5.2×1016 photons/(s·cm2) and 1.3×1018 photons/(s·cm2)) for (a) and (b), respectively. The intensity traces were measured with an exposure time of 100 ms/frame.

Blinking is not only inherent to individual MAPbI3 nanocrystals smaller than the light diffraction limit, but also to localized emitting sites on large micrometer-size crystals (Fig. 1b). Regardless of the crystal size, the blinking PL still comes from an area smaller than the resolution of the optical microscope. Besides the blinking, a clear increase of the average PL intensity can be seen in Fig. 1b. This is because the PL quantum yield of the bulk crystal (which is the background of the blinking PL in this case) increases with the time of light illumination – an effect which has been reported recently.34 A detailed study of the light-induced PL


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enhancement will be published elsewhere.35 Note that the MAPbI3 nanocrystals did not show any significant PL intensity increase upon light illumination. Here we use a method for quantitative measurement of fluorescent ability of individual objects reported by some of us previously.36 Fluorescence brightness (B) is defined as the ratio of the number of detected photons per second to the excitation power density. This parameter allows quantitative comparison of the PL excitation cross-section of different objects. It is proportional to PL intensity measured at 1 W/cm2 excitation power. For example, the brightness of a single dye molecule, like a perylene diimide derivative, with almost unity fluorescence quantum yield, is ~80 cm2W-1s-1 when measured with our microscope.36 Thus, a cluster of 10 such noninteracting dye molecules would give a B value of 800 cm2W-1s-1. We estimated the absolute PL blinking amplitude of MAPbI3 nanocrystals in the units of brightness shown in Fig. 1 to be ~107 cm2W-1s-1, which is more than five orders of magnitude larger than that of an organic dye molecule.36 Therefore, the blinking amplitude of the MAPbI3 nanocrystal is equivalent to that of 100 000 such dye molecules switching “on” and “off” simultaneously. To the best of our knowledge, such enormous PL blinking amplitude of an object has never been reported. The largest amplitudes reported so far have been observed in individual Jaggregates (brightness corresponding to approximately 100 dye molecules),19 in polymer particles containing J-aggregates (approximately 500 dye molecules reported per blinking particle, although the absolute brightness was not measured by the authors),37 and in conjugated polymer nanoparticles (brightness corresponding to less than 100 dye molecules).38,39 To investigate the nature of the PL blinking of individual MAPbI3 nanocrystals, we applied a super-resolution localization microscopy technique to resolve the spatial location of the emission with nanometer precision.40,41 By fitting a 2D Gaussian surface to the emission profile, the center


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position of the emission (the coordinates of maximum of the Gaussian function) is determined. This position will hereafter be called localization position. In general, the method relies on having well-separated, independently emitting objects. Any simultaneously emitting objects separated by a distance shorter than the optical diffraction limit cannot be resolved. In this case, the localization position reflects the average position of all emitting objects. Fig. 2a illustrates how two emitters in close spatial proximity together generate an emission profile with the localization position (white cross) that falls between the locations of the two emitters. In order to determine the position of each emitter, they need to be measured independently, which can be accomplished if they are blinking independently (see Fig. 2b and 2c).

Figure 2. (a) Two simultaneously active emitters (red and green dots) give an emission profile (black circle) with a localization position (white cross) which is the average location of the two emitters. (b-c) When either one of the emitters blinks off, the localization position shifts to the location of the other one.


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The figures on the right show a cross-sectional view of the emission profiles along the horizontal dashed line.

PL intensity transient of a MAPbI3 nanocrystals with time resolution of 100 ms/frame is shown in Fig. 3a and the corresponding localization positions of the emission in each frame are plotted as dots in Fig. 3b (different colors representing the intensity). The most obvious observation is that the localization positions appear to form clusters in space hereafter referred as localization clusters. We found three localization clusters for 23 out of the 64 studied nanocrystals (one example is shown in Fig. 3b, more examples are given in SI). There are also nanocrystals showing two (22 nanocrystals) or one (5 nanocrystals) localization clusters (examples are shown in Fig. S5 in SI). For the other 14 nanocrystals, we did not observe well-defined localization clusters or good correlation between the PL intensity levels and the localization clusters. For all the nanocrystals that show 3 clusters, the clusters are always linearly aligned as shown in Fig. 3b, which is in agreement with the elongated shape of the nanocrystals as measured by SEM (See Fig. S3 in SI). The distance between the end-clusters (on average 250 nm, see Fig. S8 for the distribution) is comparable with the length of the nanocrystals visible on SEM images. In addition, each localization cluster appears to represent a distinct PL intensity level (Fig. 3a). Furthermore, the middle localization cluster always represents the highest PL intensity level, and the localization cluster for lower PL intensity level is further away from the middle cluster. This correlation is observed for all the 23 nanocrystals that possessed three localization clusters. A good linear correlation between the blinking amplitude and the shifting distance is also obtained and shown in Fig. S7 in SI. In this report we concentrate on the distinct PL intensity levels which are correlated with the localization clusters. There are indications of even more complicated fluctuation dynamics in each intensity level which will be studied in the future.


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Figure 3 (a) PL intensity transients (100 ms/data point) of a typical MAPbI3 nanocrystal showing blinking and (b) the corresponding emission localization positions (the PL intensity is shown by the same color scale).

Another interesting observation is the anomalous excitation power density dependence of the blinking behavior. Fig. 4a shows normalized PL transients of the same MAPbI3 nanocrystal taken at different excitation power densities ranging from 0.01 W/cm2 to 0.5 W/cm2 (from  2 to 100 ×1016 photons/(s·cm2)). As clearly observed from the transients and the corresponding PL histograms (Fig. 4b), the blinking behavior is more pronounced at low excitation power density compared to high excitation power density. Indeed, the relative blinking amplitude (the intensity jump relative to the maximum PL intensity, as described in SI) becomes smaller and the “off” state duration becomes shorter upon increasing of the excitation power density (Fig. 4a and Fig. S9). Usually for most organic and inorganic single objects, higher excitation power density leads to enhanced fluctuations of PL intensity; we will discuss this matter in details below.


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Figure 4 (a) PL transients (100 ms/data point), (b) histograms of PL intensity and (c) emission localization positions of the same MAPbI3 nanocrystal measured at different excitation power densities from bottom to top: 0.01 W/cm2, 0.05 W/cm2, 0.2 W/cm2 and 0.5 W/cm2 (corresponding to 2.6×1016, 1.3×1017, 5.2×1017 and 1.3×1018 photons/(s·cm2), respectively). The slight decrease of the average PL intensity over time at the highest excitation power is due to photodegradation.

From the observations of the three localization clusters in correlation with three intensity levels, we can conclude that there are two independent active sites, which are crucial for the PL process, on each individual nanocrystal. In principle, the correlation between PL intensity levels and localization clusters can also be explained by two nanocrystals separated by a distance smaller than the diffraction limit. We think this situation is unlikely as discussed in SI. Considering the rod-like shape of the nanocrystals as shown in the SEM images in SI, these two active sites most


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likely locate at the ends of the nanocrystals. Below we propose two plausible models to explain the observations. Model 1 (quenching sites model, Fig. 5a): The whole nanocrystal is emissive like an ideal semiconductor. In this case the localization position of the emission coincides with the center of the nanocrystal. Once a photo-generated quencher is formed at either end of the nanocrystal, it works as a trap leading to deactivation of all electrons/holes able to reach it during their diffusion, which causes a PL drop and a shift of the localization position. The blinking amplitude is determined by the diffusion length of the charges and the capacity of the quencher. Non-radiative recombination at the quenching site takes time, during which the quencher is not able to accept yet another charge. This is a well-known phenomenon in natural photosynthetic systems where the reaction centers (quenchers, in our language) have limited capacity of accepting excitons and become blocked at high-light conditions.42 A similar effect called exciton blockade has been discussed in relation to organic donor-acceptor systems.43 It means that at high excitation power (high concentration of charges) the quencher cannot quench the same fraction of charges as it is able to do at the low excitation power limit (low concentration of charges). That is why the relative PL blinking amplitude at high power densities should be smaller than at low power as observed in Fig. 4a and Fig. S9 in SI. At the same time, shifts of the localization position in the course of blinking become smaller at elevated excitation power as shown in Fig. 4c where one cannot see clearly separated localization clusters at the highest excitation power density. We estimate the quenching capacity of a single quencher to be around 108 excitations per second (see SI for the details). A similar mechanism, called defect-/trapfilling has also been proposed to explain the increase of PL quantum yield at elevated excitation power density.13,34


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Figure 5 Cartoon of the PL blinking models. (a) The quenching sites model where a quencher (white skull) is formed at either end of the nanocrystal leading to non-radiative recombination in the surrounding volume determined by the efficiency of the quencher. The emission localization position (white cross) shifts from the center where it is located when the entire nanocrystal emits to either side of the rod depending on where the quencher forms. (b) The emitting sites model where the emitting sites are located at the ends of the nanocrystal and the emission localization position falls between them, but closer to the brighter site. When either site is “off”, the localization position shifts to the location of the other one. These models give 3 intensity levels and 3 localization clusters. If only one of the ends contains the quenching site then 2 intensity levels and 2 localization clusters should be observed.

Model 2 (emitting sites model, Fig. 5b): Alternatively, the experimental results can also be explained by assuming that the PL only comes from a few specific positions acting as energy funnels or, in other words, emitting sites. In this model, all the charge carriers in the crystal migrate efficiently to these sites and recombine by emitting light. The emitting sites are located


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at the ends of the nanocrystal. When both ends are emitting, the emission localization position stays close to the center of the nanocrystal. If one of the emitting sites is “off”, either by deactivation or quenching, the PL intensity drops (“blinks down”) and the localization position shifts to the other end. If the emitting sites can be photogenerated, then more emitting sites are generated under high excitation power density. The observed suppression of the blinking at high excitation power would be simply due to PL intensity averaging over many emitting sites. The formation of new sites could be related to photochemical processes involving oxygen from the air as will be discussed in our forthcoming publications.35 Note that similar to model 1, it is possible that the PL blinking is caused by formation of quenchers located close to the emitting sites or by converting the emitting site to a quencher. In this case, the mechanism of the blinking amplitude suppression at high power can be the same as in model 1 (saturation of the quenchers). As mentioned above, the quenchers discussed in the current work are most probably of the same physical nature as the charge traps proposed in recent publications.13,34,44–46 From the blinking amplitude (107 cm2W-1s-1), assuming 100% PL quantum yield and using the published absorption depth of perovskites,10,36,47,48 we can estimate the volume quenched by a single quencher to be about 7×104 nm3 (see SI for details), which is very close to the trap density suggested in the literature (1 trap per 4×104 nm3).10,34 This value is of the same order as the average crystal size measured by SEM (200×30×30 nm3 = 18×104 nm3). Note that the quenched volume estimated from the blinking amplitude is a lower limit because 100% PL quantum efficiency was assumed. Taking into account that all these numbers are estimates and crystal sizes are widely distributed, it shows a nice agreement between our model and the proposed


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charge trapping. Our results therefore strongly suggest that the activation/deactivation of a single charge trap can control the PL of an entire nanocrystal. Slow changes of perovskite photophysical properties on the time-scale of seconds, minutes and longer have been recently reported by several groups.34,35,45 These phenomena are also believed to be related to charge trapping. For example a slow increase of PL intensity upon light illumination correlated with an increase of open-circuit voltage was proposed to result from slow filling of these traps or stabilization of the trapped charges.34 We observed the same lightinduced enhancement of PL of large crystals as shown Fig. 1b on the timescale of minutes. We believe that such slow increase of the PL quantum yield is due to chemical reactions deactivating the charge traps. For example chemical passivation of defects/traps has been successfully achieved by Lewis base treatment, giving rise to enhancement of both PL and device performance.44 Other processes could also be involved, such as ionic migration or modification of the inorganic scaffold.34,45 Our detailed investigation of the enhancement phenomenon will be reported elsewhere.35 At the moment we are not able to identify the nature of the quenching/emitting sites. Trapping sites are usually associated with chemical or crystal defects.47 It is well known that chemical defects are often formed on the edges/corners or other places with great geometrical distortions exposed to the environment.49 For elongated rod-like crystals, the ends are the locations where defect formation should be expected. Depending on the particular chemical structure, the defect site can lead to different local energy level structure and correspondingly act as a PL quencher or as an emitting site.50,51 Photo-induced activation/de-activations of such defects must be the reason for the PL blinking of the nanocrystals.


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Some hints on the nature of the traps and the chemical reaction modifying/creating them can be hidden in the anomalous excitation power dependence of the blinking. Besides the suppression of the PL relative blinking amplitude which we discussed previously, a clear decrease of the “off” state duration was observed at elevated excitation power. This is different from the classical blinking phenomenon observed in most of the organic and inorganic individual objects (dyes, conjugated polymers and multichromophoric systems, quantum dots).18,21 Traditionally blinking is assigned to photogeneration of long-lived non-emissive states with lifetime (the duration of the “off” state) independent (or only weakly dependent) on the excitation power.52 Deviation from such behavior is observed at very high excitation power when more than one excitation is formed in a small quantum dot.52–54 Shortening of the “off” state duration or the trap lifetime in perovskites can be an indication that not only formation, but also de-activation of the trap is photo-induced.

In conclusion, thin films of the organometal halide perovskite CH3NH3PbI3 possess highly spatially inhomogeneous and temporally fluctuating photoluminescence as observed by fluorescence microscopy. Local areas of large CH3NH3PbI3 conglomerates, as well as individual CH3NH3PbI3 nanocrystals, showed photoluminescence blinking behavior with exceptionally large amplitude. Photoluminescence blinking unambiguously shows that charge transport is efficient in CH3NH3PbI3 nanocrystals and demonstrates the presence of very few either emitting or quenching sites per nanocrystal (one site per 104-105 nm3). Photo-induced activation and deactivation of these sites is the course of blinking. The quenching sites leading to PL fluctuations can have the same nature as the charge traps recently discussed by Snaith et al.13,34,47 With the


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optical super-resolution technique, we infer that PL emitting or quenching sites (depending on the model used) are likely formed at the ends of the nanocrystals where the geometrical and chemical defect formation is most probable. The ultra-large amplitude of PL blinking makes the CH3NH3PbI3 nanocrystals promising candidates for application as labels in super-resolution optical imaging.

Supporting Information Experimental; Inhomogeneity of CH3NH3PbI3 thin films; SEM images of the CH3NH3PbI3 nanocrystals; Photoluminescence spectrum; Estimation of the quenched volume in CH3NH3PbI3 nanocrystals; Nanocrystals showing 3, 2 and 1 clusters of emission localization; Correlation between the blinking amplitude and the localization shift distance; Distribution of the distance between the two end-clusters; Excitation power density dependence of the relative blinking amplitude; Estimation of the capacity of a quencher. These materials are available free of charge via the Internet at http://pubs.acs.org. Corresponding Author Email: [email protected] ACKNOWLEDGMENT This study was financially supported by the Swedish Research Council, the Knut & Alice Wallenberg Foundation, the Crafoord Foundation, and the Carl Trygger Foundation, the Swedish Energy Agency. Technical support from the Nano Characterization lab of the Lund University (nmC@LU) is acknowledged.


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