Stark Effect and Environment Induced Modulation of Emission in

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Stark Effect and Environment Induced Modulation of Emission in Single Halide Perovskite Nanocrystals Dharmendar Kumar Sharma, Shuzo Hirata, Vasudevanpillai Biju, and Martin Vacha ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b07677 • Publication Date (Web): 07 Jan 2019 Downloaded from http://pubs.acs.org on January 8, 2019

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Stark Effect and Environment Induced Modulation of Emission in Single Halide Perovskite Nanocrystals Dharmendar Kumar Sharma*1, Shuzo Hirata2, Vasudevanpillai Biju3, Martin Vacha*1

1Department

of Materials Science and Engineering, Tokyo Institute of Technology, Ookayama 2-12-1-S8-44, Meguro-ku, Tokyo 152-8552, Japan 2Department

of Engineering Science and Engineering, The University of Electro Communications, 1-5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japan 3Research

Institute for Electronic Science, Hokkaido University, N20W10, Kita Ward, Sapporo 001-0020, Japan *Correspondence to: [email protected]; [email protected]

ABSTRACT Organic-inorganic halide perovskites have emerged as promising materials for next generation solar cells. In nanostructured form also, these materials are excellent candidates for optoelectronic applications such as lasers and light emitting diodes for displays and lighting. While great progress has been achieved so far in optimizing the intrinsic photophysical properties of perovskite nanocrystals (NCs), in working opto-electronic devices external factors, such as the effects of conducting environment and of the applied electric field on exciton generation and photon emission have been largely unexplored. Here, we use NCs of the allinorganic perovskite CsPbBr3 dispersed polyvinyl carbazole, a hole-conductor, and in polymethyl methacrylate, an insulator, to examine the effects of applied electric field and conductivity of the matrix on the perovskite photophysics at single-particle level. We found that the conducting environment causes a significant decrease of photoluminescence (PL) brightness of individual NCs due the appearance of intermediate-intensity emitting states with significantly shortened lifetime. Applied electric field has a similar effect and, in addition, causes a non-linear spectral-shift of the PL maxima, a combination of linear and quadratic Stark effect caused by environment-induced polarity and field-related polarizability. The environment and electric field effects are explained by ionization of the NCs through hole transfer and emission of the resulting negatively-charged excitons.

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KEYWORDS:

halide perovskite nanocrystals, single-particle spectroscopy, Stark effect,

blinking; ionization

Over the past few years, organic-inorganic perovskites have emerged as one of the most promising material for applications in next generation solar cells.1,2 Later, this class of materials has been re-discovered as an outstanding candidate for light-emitting applications, such as light emitting diodes (LED) for displays and lighting, lasers and memory devices.1,3-8 Generally, the small exciton binding energy and high charge mobility, which are beneficial for solar cells, result in very low quantum yields (QY) of photoluminescence (PL). This obstacle has been overcome by confining the charges into nanoscale structures, such as nanocrystals (NCs). Perovskites NCs are excellent materials not only in terms of opto-electronic properties such as large absorption cross-section, high PLQY, color purity (narrow PL spectral width) and wide-range emission tunability,9,10 but also for their processability, feasibility for scale up synthesis and costeffectiveness.11 Also, perovskite materials are compatible with existing organic charge carrier materials used in fabrication of devices for various applications.4,12 All-inorganic halide perovskites, such CsPbX3, are highly regarded for their stability under ambient conditions.9 Very recently, it has been found that triplet excitons of the CsPbX3 NCs have radiative lifetime on the order of nanoseconds, resulting in high brightness compared even to conventional quantum dots such as CdSe. This finding adds an additional advantage for applications of these materials to LEDs.13 The efficiency of a LED is contributed to by the many factors such as charge injection and transport efficiencies, thermal ionization (low exciton binding energy), charge balance, uniformity of the electron and hole transporting layers and the active material itself. In the last few years, many efforts have been made to enhance electrically driven emission capabilities of 2 ACS Paragon Plus Environment

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perovskite-based devices. For example, external quantum efficiency of green emitting perovskites devices has been improved to ~14% via composition and phase engineering, and surface passivation.14 Current efficiency of perovskites LEDs has also been enhanced from 0.43 cd/A to 45.4 cd/A.15 Perfluorinated ionomer interlayer between hole transporting layer and perovskite NC layer has been shown to smooth hole injection in low lying valence band (V.B.) of CsPbBr3.16 While these studies showed great progress in optimizing the intrinsic properties of the perovskite materials, in working LED devices other factors, such as the effect of the local environment (band structure of the charge-carrier materials, presence of charges, the externally applied electric field) on exciton generation and photon emission in CsPbBr3 NCs are not yet fully explored. These factors are nonetheless crucial to understand the many variables affecting the LED efficiency. Single-particle imaging and spectroscopy are excellent tools to probe the effects of local environment on PL properties of molecules or nanoparticles.17-19 It has been successfully used to uncover the effects of externally applied electric field in organic semiconductors,18,20 as well as to study the mechanism of electroluminescence (EL) on the level of single molecules.18,21,22 Single-particle spectroscopy has been also applied to perovskite materials, including CsPbBr3 to understand their photophysics, degradation mechanism and photoblinking behavior.5,23-25 Temporal fluctuations in PL intensity or photoblinking has been observed not only for perovskite quantum dots (QDs)25,26 but also their aggregates27 and microcrystals28,29 where at times spatially correlated blinking over the entire crystal is visible.30 The magnitude of the blinking phenomenon is also directly correlated with efficiency of LED devices. Different explanations of blinking have been provided in different systems as a single hypothesis has so far failed to explain every aspect of blinking in perovskite NCs. Though it is commonly accepted that


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charging and discharging (possibly associated with Auger recombination) results in suppressed PLQY via fast non-radiative transitions through surface or traps in cadmium and lead chalcogenide QDs, there is so far no clear consensus on the concrete physical mechanism leading to the appearance of dark or intermediate-intensity periods in PL emission of perovskite NCs. In this paper we investigate the impact of the environment and of externally applied electric field on PL characteristics of single CsPbBr3 NCs by performing steady-state and time-resolved single-particle PL imaging and spectroscopy in inert and conductive environments. We use a hole-conducting polymer polyvinyl carbazole (PVK) as the conductive matrix and polymethyl methacrylate (PMMA) as the insulating matrix, respectively, and apply electric field in a capacitance-like device. We observed significant modulations in PL blinking and excited state lifetimes of CsPbBr3 in PVK as compared to PMMA which we attributed to the enhanced ionization via hole transfer to the matrix. Further, under an applied electric field the exciton ionization is significantly enhanced. We observed field-dependent changes in PL peak position, a combination of linear and quadratic Stark effect, which is attributed to polymer matrix-induced polarization. This study will enhance our knowledge on the behavior of perovskite NCs under conditions emulating a working LED device, and will contribute to improvement of LED performance and possibly to development of local electric field sensors using inorganic perovskite materials.

RESULTS AND DISCUSSION Single particle imaging and time and energy resolved spectroscopy CsPbBr3 NCs have been synthesized by the method developed by Protesescu et al9 with slight modifications. Details on the synthesis and experimental methods are provided in supporting


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information. The as synthesized CsPbBr3 NCs have approximately cube-like shapes and size of 16 ± 5 nm (Figure S1). Given the exciton Bohr radius of bulk CsPbBr3 of 7 ~ 11 nm, the NCs size falls into a weak quantum confinement regime and size effects in their optical properties might be expected. Spectral characteristics of CsPbBr3 NCs in solutions are also provided in the supporting information. Single particle PL imaging and spectroscopy of CsPbBr3 NCs were performed in the polymer matrices of PMMA and PVK. In the time-averaged single-particle images, individual CsPbBr3 appear brighter in PMMA as compared to PVK (Figure 1a and 1b). Intensity time trajectories of individual particles were monitored to understand this apparent decrease of PL efficiency in the conductive PVK as compared to the inert PMMA. The intensity fluctuations in PMMA mostly shows discrete two state photo-blinking (Figure 1c, upper trace), i.e., transitions between ON states (high-intensity) and OFF states (at the background level). The intensity traces in PVK, on contrary, show frequent intermediate intensity levels in addition to the ON and OFF states. Hereafter, these intermediate states are collectively referred to as GRAY states. The three levels (ON, GRAY, OFF) have been discriminated as shown in Figure 1d. GRAY states have been often detected before in semiconductor group II-IV NCs at high density excitation and correlated with emission from charged states of the NCs.31,32 On the other hand, for formamidinium lead bromide perovskite NCs, these GRAY emissive states were observed even at low excitation densities.33 The GRAY states have been proposed to originate from fast mixing of neutral and charged NC states, where the GRAY state is a time-average of the emissive (ON) and completely quenched (OFF) states. An alternative explanation includes emission from a charged excited state, so called trion, which relaxes partly non-radiatively, resulting in the appearance of the GRAY states in the PL intensity trajectories. Regardless of its origin, we note that the difference in average intensity (brightness) of individual CsPbBr3 in the


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two matrices occurs due to frequent population of the GRAY states in PVK as compared to PMMA upon identical conditions of excitation. Further, single-particle PL spectroscopy shows blue shifted PL peaks in PVK (ca 510 nm) as compared to PMMA (ca 515 nm) (Figure 1e). Conventional two-state model (with ON and OFF states determined using a single intensity threshold) for blinking analysis is not suitable for quantitative analysis of multi-step blinking behavior. To account for the GRAY states and to quantify the multi-level emission intermittency, each intensity trace is divided into three intensity levels using two different thresholds Ith1 and Ith2 (Figure S2). Using the thresholds, we define background-fluorescence OFF level (≤ Ith1), an intermediate intensity GRAY level (> Ith1 and ≤ Ith2), and a bright intensity ON level (> Ith2). Using such criteria, intensity traces of individual CsPbBr3 in PMMA and PVK were analyzed in terms probability density distributions34 of individual levels 2𝑁𝑡, 𝑖

(𝑃(𝑡𝑖) = (𝑡𝑖 + 1 ― 𝑡𝑖) + (𝑡𝑖 ― 𝑡𝑖 ― 1))

(1)

and their respective fractional times (T𝐹𝑅 𝑂𝑁(𝑜𝑟 𝑂𝐹𝐹 𝑜𝑟 𝐺𝑅𝐴𝑌)) = ∑

∑𝑁𝑡𝑂𝑁(𝑜𝑟 𝑂𝐹𝐹 𝑜𝑟 𝐺𝑅𝐴𝑌) 𝑁

𝑡𝑂𝑁 + ∑𝑁𝑡𝑂𝐹𝐹 + ∑𝑁𝑡𝐺𝑅𝐴𝑌

× 100)

(2)

where N and t are the occurrence and period for any given event. In concrete terms, we used (normalized) intensity levels below Ith1 of 0.25 for the OFF states, between Ith1 of 0.25 and Ith2 of 0.70 for the GRAY states and above Ith2 of 0.70 for the ON states. By this setting, the ranges covered are approximately twice the standard deviation of the noise. The analysis shows that blinking statistics in terms of probability densities (P(t)) for the ON, GRAY and OFF times cannot be described by a simple power law, exhibiting exponential probability at long time durations (Figure 2 a-c). To account for the truncation at long-time


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events, the probability densities P(t) were fitted with a truncated power-law function 𝑃(𝑡) = 𝐴𝑡 ―𝛼 𝑒 ―𝑡/𝑡𝑐 , where  is the exponent and tc is a critical time.34,35 Decrease in  indicates an increasing

period of the respective event.35,36 It is worth mentioning that the truncation time can be different for the individual particles as compared to sum over many particles used in the construction of 𝑃 (𝑡) in Figure 2.36 The difference in the blinking behavior between PMMA and PVK (Figure 2a-c)

is expressed in the values of the fitting parameters. For ON times, the fitting provides  = 1.01 and tc = 1.26 s for PMMA, and  = 1.39 and tc = 0.46 s for PVK. In contrast, for GRAY states the order of the values between the two matrices is reversed, with  = 1.29 and tc = 0.99 s for PMMA and  = 0.99 and tc = 0.87 s for PVK. The OFF states kinetics of CsPbBr3 in both matrices remained comparable, especially at shorter times. Figure 2d-f illustrates distributions of the fractions of ON, GRAY, and OFF times, respectively. The distributions clearly show pronounced ON states in PMMA and GRAY states in PVK. Besides, we note that most of the 𝐹𝑅 GRAY states in PVK switched from ON states in PMMA, since 𝑇𝐹𝑅 𝑂𝑁 and 𝑇𝐺𝑅𝐴𝑌 are reversed

between the two matrices, while the OFF periods (𝑇𝐹𝑅 𝑂𝐹𝐹) are only slightly longer in PVK as compared to PMMA. To account for the above observations, we propose a model shown in Figure 3a. Due to the proximity of the energy level of HOMO of PVK (~5.6 eV with respect to vacuum37,38) and that of the V.B. of CsPbBr3 (~6.2 eV)16, electron transfer can occur from the HOMO of PVK to the V.B. of an excited CsPbBr3 NC (or, equivalently, hole transfer from the excited CsPbBr3 to PVK). This process leads to enhanced photoionization of the CsPbBr3 in the conductive matrix (Figure 3a). The hole in the PVK matrix can migrate away from the NC due to its relatively large mobility in this material.39 After the electron injection to the CsPbBr3 (or the hole migration to the PVK) net positive charge accumulates in PVK which induces a reaction field and stabilizes 7 ACS Paragon Plus Environment

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the negative charge on the CsPbBr3 NC. In addition to the energetic considerations, higher polarizability of the PVK matrix (dielectric constant, δ = 3.0)40 may result in augmented photoionization of CsPbBr3 due to stabilization of the charged species. In the PMMA matrix, in contrast, due to the larger energy barrier between the CsPbBr3 V.B. and the HOMO of PMMA (>7.2 eV), the above described processes of charge transfer are energetically more restricted, resulting in decreased probability of CsPbBr3 ionization. An additional factor is the lower dielectric constant of PMMA (δ = 2.3).41 The above observations were further complemented by excited-state lifetime measurements on single-particle level. An average excited state lifetime of CsPbBr3 (amplitude weighted for several NCs) shortened from 13.1 ns in PMMA to 4.8 ns in PVK. An example of the decay curves for ensembles of CsPbBr3 in the two matrices is shown in Figure 3b, where the curves were fitted using two-exponential functions, with the fitting parameters of 4.9 ns (0.61) and 25.8 ns (0.38) for PMMA, and of 3.11 ns (0.82) and 12.8 ns (0.18) for PVK, respectively (the values in the parentheses represent intensity weighted fractions). These results correspond well with absolute PLQY measurements of CsPbBr3 dispersed in polymer films at ensemble concentrations (Figure S3, measured with an integrating sphere) which show that in PVK the values are about half of those in PMMA. Collectively, the time resolved and PLQY measurements suggest additional non radiative pathways for CsPbBr3 in the PVK matrix (Figure 3b) contributing to the GRAY states. Further, using time resolved time tagged imaging (TTTR), it was possible to separate the photon arrival times associated with the high intensity ON states from the GRAY states (Figure 3c). Single-particle decay curves corresponding to the ON, GRAY and OFF states are constructed from the TTTR data as shown in Figure 3c for the PMMA matrix (the same data are shown normalized to their initial intensities in Figure S4). Further, correlation plots between


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the PL intensity and lifetime for an individual particle can be constructed from the same TTTR data as shown in Figure 3d for both matrices. In the PMMA matrix, the faster deactivation of the GRAY states as compared to the ON states and the good correlation between the PL intensity and lifetime (Figure 3d) indicate that the shorter lifetime component (area 2 in the Figure 3d, top) is related to the intermediate GRAY states and the longer component (area 1 in the Figure 3d, top) to the unperturbed ON emission. Similar correlation was found for the PVK matrix (Figure 3d, bottom). Even though the plot is not clearly separated into two areas, the positive correlation between the PL intensity and lifetime is clearly distinguishable. Thus, the shorter component reflects partial quenching due to enhanced non-radiative recombination, likely caused by charging of the NCs. In the PVK matrix, transfer of the holes into the conducting environment which is accelerated due to energetic reasons leads to overall shortening of the lifetimes.42

Electric field dependence: PL intensity modulations and Stark effect To complement the above model of enhanced charging of CsPbBr3 NCs in PVK, we examined electric field effect on the NC emission.43 Under applied electric field in a capacitance-like device, the CsPbBr3 NC blinking pattern as well as the average PL intensity changed significantly (Figure 4). Under a dc field (of +8 V or +0.4 MV/cm) in the PMMA matrix, the intensity of nearly 45 % of the particles partially quenched to intermediate (GRAY) levels, with the remaining particles irreversibly quenching to a background OFF level after a few seconds. A very small fraction of particles also showed an opposite effect, i.e., PL enhancement. The fractions of particles showing such behavior in both PMMA and PVK are illustrated in the pie charts in the insets in Figure 4d and 4e. An example of the PL intensity modulation trace in PMMA is shown in Figure 4a. The ON intensity of this particle at zero applied field (upper


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panel) was either suppressed to about 50 % or appeared at same ON level but with frequent OFF times under the constant applied dc field of +8 V (middle panel). This behavior was reversible and the initial state recovered after removal of the electric field, as illustrated in the bottom panel of the Figure 4a. More examples of the reversible nature of the electric field effect are shown in Figure S5. This behavior was quantified by analyzing the probability density distributions and fractional times (Eqn. 1 and 2). The results shown in Figure 4b-c indicate a decrease of ON times (change of  from 1.01 to 1.16 and of c from 1.43 to 0.62) and an increase of GRAY times upon the applied external field. We note that nearly half (~ 45 %) of the CsPbBr3 particles in PMMA under the influence of the electric field show blinking dynamics comparable to that of CsPbBr3 in PVK under no electric field. This result might reflect electric field-enhanced chargingdischarging of CsPbBr3 NCs in PMMA, which has also been observed in II-VI semiconductors and other perovskite materials.11 We note that a charge can also accumulate on the NCs during continuous excitation and can result in observations similar to those under the applied electric field.44 The fact that the electric-field induced phenomena observed here are fully reversible (Figure 4a) indicates that any such effect of charging due to prolonged excitation are minimal. Apart from the effect of electric field on PL intensity we also investigated possible PL energy modulations. Similar to an earlier study on MAPbBr3, no clear change in PL energy was observed for dc voltage within a range of ± 5 V.45 Interestingly, however, at higher electric field (for applied voltage of > ± 8 V, or 0.4 MV/cm) a significant and consistent shift in PL peak energy, a Stark shift, was observed, as shown in Figure 5. The PL peak under increasing bias from -1.0 MV/cm to +1 MV/cm is plotted in Figure 5a-b for both matrices. Importantly, the effects are fully reversible, as also seen in the Figure 5a-b. Removal of the electric field causes the PL peak to reverse to energy observed at 0 MV/cm during the sweep. Such recovery of the PL 10 ACS Paragon Plus Environment

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peak position to the zero field state shows that the observed effect is indeed a reversible effect due to the electric field and that the phenomenon is not affected by degradation of CsPbBr3 NCs during the acquisition of the data. Such degradation has been reported extensively and is also accompanied by an apparent but permanent blue spectral shift.26,46 We note that the observed PL peak energy changes from 2.405 eV to 2.430 eV (Figure 5a) and 2.442 eV to 2.490 eV (Figure 5b), which is 2-4 times larger than the uncertainty (experimental and analytical) of our measurement (standard deviation σavg = ± 0.011 eV for zero applied field, Figure 5a-b) for the same particles. Thus, the fielddependent PL emission energy in Figures 5a-b confirms a dc field-induced, non-linear Stark effect in both matrices. Both blue (Figure 5a,b) and red shifts have been observed on different particles, with the blue shift accounting for about 90% of the observations. The sign of the shift with applied voltage is uncorrelated with the respective field-induced change in intensity (enhancement or inhibition). Fitting of the experimental data was done with a combination of a linear and quadratic equation47 1

=0 (PLEmax = PLEmax +ρE + 2βE2)

(3)

where ρ is a measure of a difference in inherent dipole moments between the ground and excited states, and β of a polarizability difference. The values estimated from the fitting for the particular NC in PMMA (in Figure 5a) are ρ = 1.61 ± 0.64 × 10 ―6 eV/(kV/cm) and  = 2.84 ± 0.29 × 10 ―8 eV/(kV/cm)2, and those for the NC in PVK (in Figure 5b) are ρ = 3.20 ± 1.0 × 10 ―6 eV/(kV/cm) and β = 3.36 ± 0.4 × 10 ―8 eV/(kV/cm)2. These fitting values show that both the polar and polarizable characteristics are slightly larger in PVK as compared to PMMA. Appearance of a linear component of the Stark effect in inorganic halides nanocrystals such as CsPbX3 is generally not expected because of symmetry considerations and the presumed non-


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existence of a permanent dipole. We note, however, that a well-defined polarization of a single NC emission can indicate the presence of the intrinsic electrical dipole, and that fine splitting of two orthogonally polarized emission components has recently been reported for CsPbI3 NCs at cryogenic temperatures.48 Such fine splitting would likely be overwhelmed by line broadening at room temperatures, but we did check for the presence of polarized emission by defocused images from individual NCs. In the images (shown in Figure S6) all the NCs appear identical and do not shown signatures of single transition dipoles. Thus, though we cannot completely exclude the existence of an intrinsic permanent dipole, we do not have experimental evidence to support it. On the other hand, in Stark shift experiments on single non-polar molecules in polymer matrices an exclusively linear shift due to symmetry breaking by the molecule-matrix interactions has been observed.49 In a similar way, interactions between CsPbBr3 NCs and PMMA or PVK may result in anisotropic distributions of charges in CsPbBr3 NCs, resulting in a dipole character. Indeed, such behavior was reported for TiO2-perovskite interface where locally ordered interfacial region and dipole induction were observed in mixed-halide CH3NH3PbI3−xClx.50 Recently, a variation in ground and excited state absorption was also observed at CsPbBr3/TiO2 interface under influence of electric field42 which supports the presence of dipole like characteristics of CsPbBr3 in polymer matrices. Further, to understand the origin of the dipole like characteristic and differences in these systems, we compare the extent of the Stark effect in PMMA and PVK by computing the extent of energy shift PLmax at 0.9 MV/cm with respect to zero field. Distributions of absolute values of PLmax in Figure 5c show that the extent of PLmax is up to 45 meV in PMMA and up to 55 meV in PVK. Such large distributions observed at a particular strength of electric field can be attributed to the fact that the both the polar and polarizable characteristics depend on the orientation of a particular NC (and its transition dipole)


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with respect to the electric field. Statistical analysis of the fitting parameters in both polymers shows distributions of both the linear  and quadratic  terms of the electric field response (Figure S7). There is a broad distribution of a similar extent of the linear term  in both polymers, which would support the origin of the permanent dipole as due to the NC – polymer matrix interactions. The quadratic term , on the other hand, is more dispersed in the PVK polymer. This, together with the observed larger extent of the Stark shift in this matrix can be attributed to the enhanced NC ionization in PVK. We note that recently a large electrostrictive effect has been observed for the halide perovskite MAPbI3 for applied voltages comparable to our experiments. The measured 1% strain has been explained as originating from lattice deformation due to formation of additional defects.51 Such lattice deformation, if present in our NCs, could lead to changes in the energy bandgap and to observable spectral blue shifts, thus providing an alternative explanation of the electric-field induced spectral changes. The observation of a Stark shift may have also implications for the mechanism of blinking and origin of GRAY states. As mentioned above, the mechanisms proposed include fast-mixed states in which the emission originates from a neutral NC (ON-state) and a charged NC corresponds to dark (OFF) state. Fast ionization – deionization (via hole transport) causes averaging of the PL intensity during the acquisition time and leading to the apparent intermediate PL intensity levels (GRAY states). In the other mechanism, the ionized NC itself is still emissive but the resulting trion state (charged exciton) has lower PLQY resulting in the appearance of the GRAY states. It is important to note that in the former mechanism the emission originates at all times from a neutral NC whereas in the latter from an ionized NC. Given the fact that the Stark effect itself is being explained by ionization (i.e., the shift of the PL energy is due to a presence of a charge 13 ACS Paragon Plus Environment

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during the process of emission), our results point to a negatively charged trion as the origin of the GRAY states in CsPbBr3 nanocrystals. This assignment is supported by the lifetime analysis. Observation of same lifetime values for the GRAY and ON levels would undoubtedly verify the fast-mixing mechanism. However, the results presented in Figure 3d show significantly shorter lifetime component for the GRAY states, thus supporting the charged exciton (negative trion) emission mechanism. The OFF states are likely caused by different charged species, such as multiply charged NCs. It is illustrative to compare the effects of environment and applied electric field observed here for the halide perovskite NCs with other prototypical optoelectronic nanomaterials, such as semiconductor QDs or conjugated polymer chains. Colloidal QDs such as CdSe have been studied extensively over the past decades and have been known to undergo photoinduced hole transfer into a surrounding matrix, the rate of which depends on numerous parameters such as the QD structure, nature of ligands, the matrix energy levels, etc. This type of ionization (similar to the one proposed here) also strongly affects the blinking behavior, as shown for example for the case of CdSe/ZnS QDs in a conjugated polymer matrix.52 Quantum-confined Stark effect on single CdSe QDs has been first observed at cryogenic temperatures53 and has since been one of key features of quantum-confined materials. At room temperature, the extent of the Stark shift depends on the QD shape and composition and can be as much as 13 nm, with both predominantly linear or predominantly quadratic effects being observed.54 In conjugated polymers, on the other hand, the electric field applied on single chains causes mainly PL quenching which can be due to generation of polarons (in MEH-PPV)17,55 or dissociation of excitons (in polythiophene).20 Stark effect on single chains of MEH-PPV and related conjugated


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polymers was demonstrated at cryogenic temperatures. The shift is mainly linear in nature and its extent is on the order of several nanometers.56

CONCLUSIONS In conclusion, we presented single-particle spectroscopic study of PL properties of perovskite CsPbBr3 NCs. In particular, we systematically examined different external factors that can influence the emission characteristics of this material in working LED devices. We found that both the presence of conductive environment and the application of external bias can result in considerable decrease of brightness of individual CsPbBr3 NCs, due to the population of partially quenched intermediate (GRAY) PL intensity states. In addition, the applied bias can cause a nonlinear Stark shift of up to 55 meV. The experimental results can be consistently explained by environment- or field-induced ionization of the CsPbBr3 NCs, and emission of charged excitons (negative trions) as the origin of the GRAY states.

MATERIALS AND METHODS Synthesis of CsPbBr3: CsPbBr3 NCs have been synthesized by the method developed by Protesescu et al.9 with slight modifications. In brief, cesium precursor was synthesized using dried and degassed (3 cycles of vacuum and purging N2 for 30 min for removal of moisture and O2) Cs2CO3 (107.6 mg), oleic acid (0.30 mL) and 1-octadecene (ODE, 5 mL) in a three neck -RB flask. A clear solution of Cs-oleate in ODE was obtained at 140 oC and kept at 100 oC. In another RB flask, PbBr2 precursor was prepared by mixing dried and degassed PbBr2 in 7.5 ml ODE (107.3 mg), 0.5 ml oleic acid, 0.5 ml oleylamine (technical grade 70%) at 120 oC under N2 environment. This solution was heated upto 225 oC and 0.6 ml of Cs-oleate, pre-heated at 100 oC, was quickly injected to this reaction mixture. After few seconds, the reaction mixture turned to 15 ACS Paragon Plus Environment

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yellow-green and at that time the reaction was stopped by dipping the reaction flask into an ice bath. The synthesized CsPbBr3 NCs were precipitated by adding a mixture of equal amount of tert-butanol and ODE (5 ml each) at room temperature and then centrifuged at 8000 rpm for 15 min. Finally, the synthesized NCs were re-dispersed in toluene and stored at 40C in dark for further experiments. Bulk characterization: Bulk spectroscopic characterization was performed on CsPbB3 dispersed in toluene using absorption (V760, Jasco) and photoluminescence (Quantaurus-QY, Hamamatsu Photonics) spectrometers. Time-resolved PL lifetime measurements were performed on a compact fluorescence lifetime spectrometer (Quantaurus-Tau, Hamamatsu Photonics). Absolute PL quantum yields for the CsPbBr3 NCs dispersed in solid polymer films were measured using an absolute PL quantum yield measurement system (Quantaurus-QY, Hamamatsu Photonics). The system is equipped with a xenon lamp as the excitation source, a monochromator, an integrating sphere and a multichannel detector capable of simultaneous multi-wavelength measurement. Sample preparation for single-particle PL imaging and spectroscopy: For single particle PL imaging and spectroscopy, ~nM suspensions of CsPbBr3 in toluene were prepared either in a conductive polymer of poly (9-vinylcarbazole), PVK (M n~25000-50000, MERCK /Sigma Aldrich), or in an inert poly (methyl methacrylate), PMMA (Mw~350000, MERCK/Sigma Aldrich) by sonicating for 5-10 min. This solution was spin cast at ~1500 rpm (60s) onto a freshly cleaned micro cover glass (24×24 mm, Matsunami). The conditions were optimized to obtain randomly distributed (separated by ≥ 500 nm) diffraction limited emission spots in polymer matrices of 50-200 nm thickness (thickness monitored using AFM). All the samples of CsPbBr3 were prepared in ambient environment and dried under vacuum before mounting on the 16 ACS Paragon Plus Environment

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microscope. For electric field experiments, multi-layered capacitor model device was prepared on pre-cleaned (2 cycles of sonicating with acetone and 1-propanol) indium tin oxide (ITO) coated (56.2 nm) microscope cover glass. CsPbBr3 in PMMA (4 Wt %) or PVK (4 Wt %) was spin coated (~200 nm) on the ITO anode, dried under vacuum and an Al cathode (≥ 150 nm) was vapor deposited on the top. Single-particle PL imaging and spectroscopy: Imaging and spectroscopy have been performed on a multimodal fluorescence microscope in epifluorescence or confocal mode.56 In summary, Olympus (IX71) inverted optical microscope with oil immersion objective (Olympus, UPlan FLN 100x/ 1.3 NA, Oil) was used for PL imaging. The samples were excited with a pulsed 485 nm diode laser (LDH-PC-375/485, PicoQuant) and the PL emission collected using electronmultiplying (EM) CCD camera (iXon, Andor Technology) after passing an imaging spectrograph (Bunkou Keiki, CLP-50, 0.5 nm resolution) and appropriate optical filters. The acquisition times were typically varied between 10-50 ms/frame for the blinking and 100-200 ms/frame for PL spectra. Time-correlated single photon counting (TCSPC) module (TimeHarp 100, PicoQuant) together with an avalanche photodiode detector and the same 485 nm laser were used to measure PL decays of individual CsPbBr3 NCs, as well as time-tagged time-resolved (TTTR) format data. The TTTR data allowed reconstructing intensity trajectories, fluorescence lifetime decay curves (with an accuracy of ~ 1 ns) and correlation plots between PL intensity and time interval between the excitation and emission photons. All measurements were performed in air at room temperature. Electric field modulations. Constant and waveform electric field across CsPbBr3 particles embedded in PMMA or PVK was generated using dc voltage provided and controlled by a multimeter (Keithley, Tektronics) and a function generator. Care has been taken that CsPbBr3 is 17 ACS Paragon Plus Environment

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electrically isolated and no direct transport of charge occurred in the devices. High sensitivity I-V measurements confirmed currents of I ≤ 4×10-10 A/cm2 for the PMMA and I ≤ 10-7 A/cm2 for the PVK devices for applied voltages of > 8V. Data analyses: Single particle imaging and spectroscopy data were analyzed using ImageJ2 (NIH), OriginPro 2016 and MATLAB 2017. PL images were background corrected for excitation field modulations using a rolling ball algorithm. The intensity time trajectories from well-separated emission spot (3×3 pixel) were obtained using ‘z-profile’ in ImageJ and data were exported in ASCII format for further analysis. PL spectral profiles from spectrally resolved images were obtained by integrating 3 pixels along vertical (position) direction and an average background of the same dimension was subtracted to obtain the PL spectrum. The spectrograph was calibrated using several laser lines. The spectral information such as peak positions and full width at half maximum (FWHM) for each particle was obtained by fitting with a Gaussian function. The time evolution of the spectra is presented by making a montage of the movie frames of the individual particle in the same sequences of their recording.

ASSOCIATED CONTENT Supporting Information is available free of charge on the ACS Publications website at DOI: Steady state and time resolved measurements in bulk solution; Criteria for determining ON, GRAY and OFF states; Absolute values of PLQY; Normalized decay curves; Blinking traces with and without applied electric field; Defocused imaging; Correlation plots between distributions of linear and quadratic fitting parameters of the Stark effect


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AUTHOR INFORMATION Corresponding Authors: *E-mail: [email protected]; [email protected]

ACKNOWLEDGMENTS This research was supported by JSPS KAKENHI Grant Number JP26107014 (MV) and 17H05243 (VB) in Scientific Research on Innovative Areas ‘Photosynergetics’.


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(25) Park, Y. S.; Guo, S.; Makarov, N. S.; Klimov, V. I. Room Temperature Single-Photon Emission from Individual Perovskite Quantum Dots. ACS Nano 2015, 9, 10386–10393. (26) Yuan, G.; Ritchie, C.; Ritter, M.; Murphy, S.; Gomez, D. E.; Mulvaney, P. The Degradation and Blinking of Single Perovskite Quantum Dots. J. Phys. Chem. C 2017, 122, 13407–13415. (27) Wen, X.; Ho-Baillie, A.; Huang, S.; Sheng, R.; Chen, S.; Ko, H. C.; Green, M. A. Mobile Charge-Induced Fluorescence Intermittency in Methylammonium Lead Bromide Perovskite. Nano Lett. 2015, 15, 4644–4649. (28) Halder, A.; Pathoor, N.; Chowdhury, A.; Sarkar, S. K. Photoluminescence Flickering of Micron-Sized Crystals of Methylammonium Lead Bromide: Effect of Ambience and Light Exposure. J. Phys. Chem. C 2018, 122, 15133–15139. (29) Tian, Y.; Merdasa, A.; Peter, M.; Abdellah, M.; Zheng, K.; Ponseca, C. S.; Pullerits, T.; Yartsev, A.; Sundström, V.; Scheblykin, I. G. Giant Photoluminescence Blinking of Perovskite Nanocrystals Reveals Single-Trap Control of Luminescence. Nano Lett. 2015, 15, 1603–1608. (30) Pathoor, N.; Halder, A.; Mukherjee, A.; Mahato, J.; Sarkar, S. K.; Chowdhury, A. Fluorescence Blinking beyond Nano-Confinement: Spatially Synchronous Intermittency of Entire Perovskite Micro-Crystals. Angew. Chem. Int. Ed. 2018, 57, 11603–11607. (31) Jha, P. P.; Guyot-Sionnest, P. Trion Decay in Colloidal Quantum Dots. ACS Nano 2009, 3, 1011–1015. (32) Gomez, D. E.; Van Embden, J.; Mulvaney, P.; Fernee, M. J.; Rubinsztein-Dunlop, H. Exciton-Trion Transitions in Single CdSe–CdS Core–Shell Nanocrystals. ACS Nano 2009, 3, 2281–2287. (33) Yarita, N.; Tahara, H.; Saruyama, M.; Kawawaki, T.; Sato, R.; Teranishi, T.; Kanemitsu, Y. Impact of Postsynthetic Surface Modification on Photoluminescence Intermittency in Formamidinium Lead Bromide Perovskite Nanocrystals. J. Phys. Chem. Lett. 2017, 8, 6041– 6047. (34) Peterson, J. J.; Nesbitt, D. J. Modified Power Law Behavior in Quantum Dot Blinking: A Novel Role for Biexcitons and Auger Ionization. Nano Lett. 2009, 9, 338–345. (35) Kuno, M.; Fromm, D. P.; Hamann, H. F.; Gallagher, A.; Nesbitt, D. J. Nonexponential “Blinking” Kinetics of Single CdSe Quantum Dots: A Universal Power Law Behavior. J. Chem. Phys. 2000, 112, 3117. (36) Shimizu, K. T.; Neuhauser, R. G.; Leatherdale, C. A.; Empedocles, S. A.; Woo, W. K.; Bawendi, M. G. Blinking Statistics in Single Semiconductor Nanocrystal Quantum Dots. Phys. Rev. B 2001, 63, 1–5. (37) Glowacki, I.; Szamel, Z. The Nature of Trapping Sites and Recombination Centres in PVK and PVK-PBD Electroluminescent Matrices Seen by Spectrally Resolved Thermoluminescence. J. Phys. D. Appl. Phys. 2010, 43, 295101(1-9). 22 ACS Paragon Plus Environment

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Figure 1. Single particle time averaged PL images in PMMA (a), and PVK (b). Characteristic PL intensity traces (c) and respective intensity histograms (d) from two different single CsPbBr3 NCs in each matrix. In (d), three different regions are highlighted as discussed in the text. (e) Distributions of PL maxima (PLmax) of CsPbBr3 in PMMA (blue) and PVK (green) evaluated from Gaussian fits of 88 and 118 single particle spectra, respectively.


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Figure 2. Probability density distributions (a-c) and distribution of fractional times (d-f) for ON (a, d), GRAY (b, e) and OFF states (c, f) for CsPbBr3 NCs in PMMA (green) and PVK (blue) matrices. All the probability density curves are fitted with a truncated power law equation (P(t) = At ―αe ―t/tc)34 and the fitting coefficient values are shown in the individual plots.


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Figure 3. (a) Schematic diagram showing relative energy levels of HOMO and LUMO of PMMA and PVK, and the levels of V.B. and C.B. of CsPbBr3, respectively. The solid and dashed straight arrows symbolize radiative and non-radiative relaxations while the curved arrows show electron (or hole) transfers. (b) PL decay curves for ensembles of CsPbBr3 NCs in PMMA (green) and PVK (blue), and corresponding two-exponential fits. The gray line represents the instrument response function. (c) PL decay curves constructed separately for the ON (green), GRAY (gray) and OFF (red) intensity levels for a single CsPbBr3 NC in PMMA. (d) Correlation plots between PL intensity and lifetimes for two CsPbBr3 NCs in PMMA (top) and PVK (bottom) matrices. The numbered areas correspond to the decay curves marked with the same numbers in (c). The color scale (inset of the bottom) represents relative frequency (occurrence).


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Figure 4. (a) Single-particle intensity traces with and without an applied electric field of + 8 V (+ 0.4 MV/cm) for a single CsPbBr3 NC in PMMA. Other experimental conditions were identical. (b) Probability density distributions of ON states with (full squares) and without (open circles) applied electric field for CsPbBr3 NCs in PMMA. (c) Distributions of fractional times of ON, GRAY and OFF durations with (full bars) and without (open bars) applied 2

18V 0V electric field in PMMA. Relative PL intensity modulations ∆I = {(I0V int ― Iint )/(Iint)} at

applied field of +18 V (+ 0.9 MV/cm) in PMMA (d) and PVK (e). The percentage of CsPbBr3 NCs for which PL intensity decreased (gray), increased (wine), irreversibly quenched (yellow) and remained unaffected (cyan) at the field of + 0.9 MV/cm are represented by pie-charts. 28 ACS Paragon Plus Environment

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Figure 5. PL peak maxima (PLmax) of a single CsPbBr3 NC upon applied electric field continuously increasing from – 0.9 MV/cm to + 0.9 MV/cm, and recovery of PLmax at zero field. Data were evaluated by Gaussian fits of single NC spectra in PMMA (a) and PVK (b) from respective spectral images shown on the top of each plot. Red lines in (a) and (b) represent the 1

=0 least squared fit with PLEmax = PLEmax +ρE + 2βE2.47 Distributions of absolute values of electric 0V 2 field-induced modulation of PL energy |∆PLmax| = (PL18V max ― PLmax) in PMMA (c) and PVK

(d). The dotted line is a guide for the spectral resolution limit. Typical spectra at 0 MV/cm and at +0.9 MV/cm together with their Gaussian fits are shown in the insets in (c) and (d).


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TOC Graphics

Blinking traces of single CsPbBr3 nanocrystals in inert (PMMA) and conductive (PVK) matrices


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Stark Effect and Environment Induced Modulation of Emission in

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