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Visualizing Carrier Transport in Metal Halide Perovskite Nanoplates via Electric Field Modulated Photoluminescence Imaging Xuelu Hu, Xiao Wang, Peng Fan, Yunyun Li, Xuehong Zhang, Qingbo Liu, Weihao Zheng, Gengzhao Xu, Xiaoxia Wang, Xiaoli Zhu, and Anlian Pan Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b00486 • Publication Date (Web): 26 Apr 2018 Downloaded from http://pubs.acs.org on April 26, 2018

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Nano Letters

Visualizing Carrier Transport in Metal Halide Perovskite Nanoplates via Electric Field Modulated Photoluminescence Imaging Xuelu Hu†, Xiao Wang*,†, Peng Fan†, Yunyun Li†, Xuehong Zhang†, Qingbo Liu†, Weihao Zheng†, Gengzhao Xu‡, Xiaoxia Wang†, Xiaoli Zhu†, Anlian Pan*,†

†

Key Laboratory for Micro-Nano Physics and Technology of Hunan Province, State Key

Laboratory of Chemo/Biosensing and Chemometrics, School of Physics and Electronics, Hunan University, Changsha 410082, People’s Republic of China

‡

Suzhou Institute of Nano-tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou

215123, People’s Republic of China

Email: [email protected], Phone: 0086-731-88822332 E-mail: [email protected], Phone: 0086-731-88820932

ABSTRACT: Metal halide perovskite nanostructures have recently been the focus of intense research due to their exceptional optoelectronic properties and potential applications in integrated photonics devices. Charge transport in perovskite nanostructure is a crucial process

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that defines efficiency for optoelectronic devices but still requires a deep understanding. Herein, we report the study of the charge transport, particularly the drift of minority carrier in both allinorganic CsPbBr3 and organic-inorganic hybrid CH3NH3PbBr3 perovskite nanoplates by electric field modulated photoluminescence (PL) imaging. Bias voltage dependent elongated PL emission patterns were observed due to the carrier drift at external electric fields. By fitting the drift length as a function of electric field, we obtained the carrier mobility of about 28 cm2V-1S-1 in the CsPbBr3 perovskite nanoplate. The result is consistent with the spatially resolved PL dynamics measurement, confirming the feasibility of the method. Furthermore, the electric field modulated PL imaging is successfully applied to the study of temperature depending carrier mobility in CsPbBr3 nanoplates. This work not only offers insights for the mobile carrier in metal halide perovskite nanostructures, which is essential for optimizing device design and performance prediction, but also provides a novel and simple method to investigate charge transport in many other optoelectronic materials.

KEYWORDS: lead halide perovskites, nanoplates, carrier drift, photoluminescence, time resolved PL measurement

Carrier transport and the recombination decay pathways are of paramount importance in determining the performance of semiconductor optoelectronic devices such as solar cells, photodetectors and light emitting diodes (LED). Initially motivated largely by the fast increase in


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efficiency of solar cells over 20 %,1-5 metal halide perovskites with exceptional properties such as high absorption coefficients, long electron−hole pair diffusion lengths, and high carrier mobility, have emerged as a promising class of materials for a wide range of applications.6-10 Recently, perovskite single crystal nanostructures have attracted more attention, due to the extraordinary optoelectronic properties and ultracompact sizes for promising applications in nanophotonics and nanoscale optoelectronics.11-23 For example, nanoscale photodetectors, solar cells and electroluminescence devices based on perovskite nanostructures have been demonstrated.9,13,22-26 To optimize the perovskite nanostructures device and achieve better performance, a deep and comprehensive understanding of the carrier behavior in the perovskite nanostructure is essential. However, it still remains a challenge by using traditional electronic and transient techniques to reveal the carrier properties in individual nanoscale perovskite devices, due to the small device sizes and solvent sensitivity of the single perovskite crystal in preparing the optoelectric devices.3,27-29 Optical spectroscopic techniques have been used to reveal important information on the populations of photoexcited carriers and to monitor the different decay pathways in perovskites nanostrutures.17,19,30-37 By ultrafast transient absorption microscopy with simultaneous high spatial and temporal imaging, long-range charge carrier diffusion in perovskite was studied.38 Beside transient spectroscopy, spatially resolved photoluminescence (PL) imaging make it possible to ascertain the spatial distribution of the luminescent, and further obtain experimental evidence of the long-distance transport of charge carriers in nanostructures.39-43 It is noted that


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quantitative characterization of the carrier diffusion or funneling process in individual single crystal perovskite has been realized through time-resolved and PL-scanned imaging microscopy.35,39,43 However, on one hand, these studies require sophisticated instruments and experimental approaches; on the other hand, since these experiments focus mainly on optical investigations, only charge carrier diffusion and recombination dynamics were studied. The lack of investigation of these materials to external electric field makes it challenging to understand the important role of charge transport in the optoelectronic properties. In this work, we have investigated the charge transport in perovskite nanoplates by a simple electric field modulated PL imaging method. We observed that under a focused laser illumination the PL pattern of the CsPbBr3 nanoplate is elongated to the positively biased electrode, showing a bias dependent long-distance drift length up to several micrometers. Timeresolved PL measurement demonstrated that the drift of photo-generated carriers at external electric fields accounts for the observed elongated PL emission. In addition, the drift length at the same bias shows obvious decrease when temperature decreases from 270 k to 180 k, indicating the reduced mobility of CsPbBr3 nanoplate. Finally, we apply the method to CH3NH3PbBr3 (MaPbBr3) perovskite, demonstrating a wide feasibility of the method. The electric field modulated PL imaging method provides valuable information on the charge transport of the perovskite nanostructures at external electric field and can be applied to many other semiconductor optoelectrical devices.


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We first consider general processes regarding carriers in semiconductors upon illumination. Figure 1a shows a schematic of carrier generation within the excitation laser volume, carrier diffusion due to concentration gradient and carrier drift due to an external electric field though applied bias Vsd. Assuming one-dimensional case, carrier density dynamics can be expressed as

= + + − , where D is the diffusion coefficient, µn is the carrier mobility, E

is the electric field, G is the photogeneration rate and U is the recombination rate of the

carriers.39,44 With the consideration of a Gaussian laser beam ( = / ), Figure 1b shows an illustration of the carrier density dynamics after numerically simulation. At t=0, the laser illumination generates a Gaussian distribution of free carriers. Upon time the initial Gaussian distribution of free carriers shows a broadened peak width due to diffusion, a continuous shift of peak position due to drift and a decrease of distribution area due to recombination. For p-type semiconductor with the assumption of charge neutrality, the light illumination introduces a large influence on the density of minority carriers (electrons) which decreases exponentially in the steady state case under a uniform electric field. The radiative recombination of these minority carriers leads to an elongated shape of PL emission. Therefore, by monitoring the electric field dependent PL emission patterns of the material, we can obtain the information on the carrier mobility in the device. The CsPbBr3 nanoplate device were prepared by a direct chemical vapor deposition on the top of pre-patterned indium tin oxide (ITO) electrodes as detailed described in our previous work.24 By adjusting the growth parameters, rectangular CsPbBr3 nanoplates with lateral dimensions of a


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few tens of micrometers bridged across two ITO electrodes. Figure S1 illustrates the growth mechanism and the formation of the contacts between the CsPbBr3 nanoplates and electrodes during chemical vapor deposition. Figure 1c shows the configuration of the electro optical characteristic measurement of the CsPbBr3 nanoplate device. The inset shows a typical real color image of the device. Both the width of the electrodes and the channel length between the electrodes are ~20 µm. Scanning electron microscopy (SEM) reveals that the nanoplate has well defined facets and smooth and clean surfaces, and the chemical compositions for Cs, Pb and Br were clearly identified from the energy dispersive X-ray spectroscopy spectrum (Figure S2a, b in Supporting Information). X-ray diffraction (XRD) patterns of typical CsPbX3 nanoplates (Figure S2c, d) show sharp peaks correspond to the cubic phase, and the high intensity with a narrow full width at half maximum (FWHM) indicates a good crystallinity of the samples. High crystallinity of the perovskites with smooth surface is a prerequisite for a good electronic performance, as large surface roughness or grain boundaries can cause mobility degradation due to the effects of surface/interface traps and phonon scattering. We identified the predominant photo-generated carriers of the CsPbBr3 nanoplate by performing power-dependent PL measurement under zero bias voltage. Figure 1d shows PL spectra measured at different laser powers (488 nm continuous wave laser). The spectra exhibit a single emission band with a peak center at 529 nm and a narrow full width at half maximum of 21 nm, while the intensity of the spectra significantly increases with increased excitation power. The PL intensity as a function of excitation power is plotted in Figure 1e. A power law


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dependence (IPL = IEXβ, where β denotes the nonlinear component) has been observed with β ~1.8. The observed close quadratic dependence indicates that the PL emission is dominated by the bimolecular recombination of free carriers.32,33 Electrical characteristics of the CsPbBr3 devices were measured as well. The bias voltage was applied and photocurrent was recorded by using Keithley 4200 source-measure unit. Figure 1f shows excitation (488 nm laser) power dependent photocurrent at the bias of 3 V. The insert shows typical current−voltage (I-V) curves of the device in dark and with laser illumination at the power of 1 µW. Although the I-V are exponential which reveals the existence of Schottky barriers, the device has a relatively small resistance with observed electric current of 10-7 A at 3 V bias in dark, and an increase to 10-5 A under illumination. The small resistance can be ascribed to the good contacts between the CsPbBr3 nanoplate and the ITO electrodes which are formed during vapor growth. As a result, external electric field can be effectively applied on the nanoplate, making the device to be a good platform to investigate the transport and dynamics of charge carriers. The linear fit of the experimental data in a double logarithmic scale in Figure 1f shows a close square root power law IPL = IEX0.52 in the low excitation power regime, which is consistent with the observed free carriers by the power dependent PL measurement. It is clear that when the laser power is over about 1 µW, a photocurrent saturation (dashed black line) at which the photocurrent begins to deviate from the square root power relationship is observed. The saturation indicates that the recombination of the photo-generated charges is sufficiently high compared to the transported charges collected by the circuit.45 As a result, in the presence of


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electric field, the modulation of PL emission which originates from the radiative recombination of free carrier can reveal the charge transport in the perovskite materials. Figure 2a shows the real-color PL images of a typical CsPbBr3 nanoplate device with different positive (left panel) and negative (right panel) bias voltages. The power of the excitation laser is ~ 2 µW. Bright and round PL emission spot is observed at zero bias. When positive bias is applied, the emission spot is elongated towards the top electrode, opposite to the direction of applied electric field (the red arrows in Figure 2 a). The elongated length of the spot increases with increasing applied bias. It is also noted that the emission spot is elongated to the other electrode when negative bias applied, always in opposite direction to the applied electric field. PL intensity profiles of the emission images recorded at different positive bias are analyzed and shown in Figure 2b. While the intensity profile at zero bias is symmetric (close to a Gaussian distribution due the Gaussian beam of the excitation laser), the PL intensity distributions at applied bias are non-symmetric with an exponential decay on one side, the whole profile of which can be well fitted by exponentially modified Gaussian functions (solid lines in Figure 2b). After the fitting, the decay length as a function of bias voltage is plotted in Figure 2c, showing a linear law dependence with the slope 0.42 µm/V. The results show that PL emission of the CsPbBr3 nanoplate can be strongly modulated by an external electric field. To elucidate the electric field dependent modulation of the CsPbBr3 nanoplate emission, insitu PL and time resolved PL (TRPL) measurement at different bias voltages were performed. Figure 2d shows in-situ PL decay curves collected from the whole CsPbBr3 nanoplate at different


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bias voltages, which can be well fitted with the same decay profile. The free carrier recombination process with the fitted decay time constant (τ=3.06 ns) dominates (78% composition) the whole decay process. The inset of Figure 2d shows the normalized PL spectra of the CsPbBr3 nanoplate at different applied bias, exhibiting almost identical spectral features with the same emission peak and line width. In addition, PL spectral profiles collected from different locations of the elongated emission spot are similar to that recorded at excitation spot (Figure S3 in Supporting Information), demonstrating the elongated emission pattern is not originated from the optical waveguide, which would show a red shifted PL emission peak.21 Therefore, the transport of the photo-generated minority carriers at external electric fields can be account for the observed elongated PL image. Consistent with the elongated pattern related to the polarity of the applied field observed in Figure 2a, the minority carriers are electrons in the ptype CsPbBr3 nanoplate used in this work, which is also confirmed by the transistor experiments discussed later. As we discuss above, the PL intensity distribution reflects the carrier density. Under stationary condition, the decay length can purely originate from diffusion and has the expression of = √Dτ with the absence of the external electric field or be dominated by the drift of the carriers under large electric field with the expression of = μ τ V/d, where µn is the carrier mobility, τ lifetime, V applied voltage and d channel distance.44 The linear relation of the decay length and the applied voltage (Figure 2c) clearly demonstrates that the elongated PL pattern is due to the drift and radiative recombination of minority carriers. Therefore, by the electric field dependent PL imaging, one can obtain the information on the minority carrier and


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estimate its mobility in a semiconductor device. Considering the lifetime of 3.06 ns, after fitting the carrier mobility expression, we obtain the µn of ~ 28 cm2V-1S-1. The modulation of PL emission can also be demonstrated by acquiring emission intensity time trajectories under periodically applied bias. Bias voltage between 0 V and + 5 V with a triangle waveform and a frequency of 5 Hz was applied to the device. The applied frequency is limited by the CCD framerate (∼20 frames per second). Figure 2e shows the typical results that the emission intensity (open red circles) follows the applied bias (dashed blue line). The PL intensity was collected 2 µm away from the laser excitation spot. Within the limitation of the speed of the CCD, the dynamic response character of the modulation shows a response time less than 0.1 second. These observations exclude the influence of ion migration on the nanoplate emission modulation, as the time scale of ion migration is on the order of seconds.33 We also recorded electric field depended PL images under different laser illumination powers (Figure S4 in Supporting Information), and obtain rather similar results based on the fitting of the intensity distribution profile, which can rule out possible derivations of the PL patterns introduced by the variation of laser powers. The carrier transport in semiconductors can be also studied by the spatially resolved PL dynamics measurement, such that long-distance excitons diffusion was previously revealed.39,42 To further confirm the validity of the electric field dependent PL imaging method, we performed TRPL experiments with spatially separated excitation and detection method on the CsPbBr3 nanoplate. Given the fact that the elongated PL emission patterns are due to the drift and


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radiative recombination of minority carriers, we can characterize the carrier drift process by comparing PL kinetics collected by a streak camera at the excitation site (in situ) and a position away from the excitation site (Detection A), as shown in Figure 3a. With the information of the transport length (L), time (τ) and the applied electric field, the carrier mobility can be estimated. Figures 3b and 3c show TRPL spectra recoded at ‘in situ’ and position ‘detection A’ by the streak camera and the corresponding PL decay kinetics, respectively. Compared to the in-situ situation, the PL kinetics at position ‘Detection A’ (blue squares) includes a fast decay component right after the excitation and a slow decay component. The fast decay component in the early time is the same as the PL kinetics at the ‘in situ’ (green circles) and can be ascribed to scattering of the ‘in situ’ light, which is caused by the collection from the same objective. The configuration of two objectives performing the excitation and collection in transmission mode can optimize the measurements (Figure S5). Nevertheless, the PL kinetics that is purely due to the charge carrier drift and recombination at position ‘Detection A’ can be obtained by the subtraction of the ‘in situ’ component, which shows a clear slow rising and decay component (red circles). The rising time ~ 1.5 ns indicates the transit time (τ) of drifted carriers over the distance (L~ 2 µm) from the excitation to the detection positions under the electric field of 4 kV/cm. The PL kinetics with the formula L = µnτE then yields the mobility µn = 33 cm2V-1S-1, which is comparable with the mobility 28 cm2V-1S-1 measured via the electric field dependent PL imaging method.


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Based on the electric field-dependent PL emissions of the perovskite nanoplate, we further investigated the carrier transport at different temperatures. Figure 4a shows PL image of the CsPbBr3 at 270 k, 210 k and 180 k under 0 V (left panel) and 5 V (right panel) bias. It can be seen that the PL pattern becomes brighter as the temperature decreases, while the observed elongated emission spot at 5 V is much less prominent at lower temperatures. Corresponding PL spectra at these temperatures (Figure 4b) show only one consistent emission peak (dashed line), indicating the absence of structural phase transition in the investigated temperature region. This is consistent with Huo et al.’s work that no phase change of the all-inorganic perovskite would be observed even when the temperature decreases to 77 K.46 Intensity profiles of the PL patterns at these three temperatures under 5 V bias (Figure 4c) show that the exponential decay significantly decreases at lower temperatures. The decay length as a function of bias voltage at different temperatures are plotted in Figure 4d. Linear dependences were observed with slopes decrease from 0.25 to ~ 0.05 µm/V. At temperatures lower than 180 k, the bias-dependent decay is almost not observable in our experiments. As discuss above, the bias-dependent decay is attributed to the carrier drift under an external field. A reduced carrier mobility in the CsPbBr3 nanoplate with deceased temperature can be obtained from the deceasing decay length as shown in insert of Figure 4d. Further experiments of the temperature dependent carrier mobility were performed through CsPbBr3 field effect transistors (FET) (Figure S6 in Supporting Information). The transfer characteristic of the FET shows a p-type conductivity of the device. Its corresponding hole


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mobility at temperatures between 150 and 300 K in Figure S6 exhibits a dramatical decrease when temperature is lower than 240 k. This observation is in general consistent with the minority carrier behaviors that were measured by PL imaging above, as we assume the perovskite nanoplate have a similar mobility for both electrons and holes.2,39 Most likely, the accumulation of ions near semiconductor−dielectric interface in the perovskite at lower temperature, which introduces impurity scattering, is the explanation for the dramatically decreased mobility.46 The mobility measured by FET shows lower values than that measured by PL imaging, which can be due to the charge scattering near the semiconductor−dielectric interface and the high contact resistance between perovskites and electrode. We also study the carrier mobility of organic-inorganic hybrid perovskite CH3NH3PbBr3 (MaPbBr3) nanoplate through the same approach. The MaPbBr3 device has a similar configuration as illustrated in Figure 1c. Figure 5a shows electric field dependent PL images of the MaPbBr3 nanoplate under different bias voltages. We observe similar phenomena compared with those of the CsPbBr3 nanoplate. Due to electrons being minority carriers,47 the emission spots are also elongated to the positively biased electrode, and the elongated length of the spot increased with higher applied bias. It is noted that MaPbBr3 nanoplate have longer elongated emission length than that of CsPbBr3 nanoplate, which is quantitatively shown in corresponding intensity distribution of the emission (Figure 5b). After fitting the experimentally obtained intensity profiles with exponentially modified Gaussian functions, the decay length is plotted as a function of bias voltage in Figure 5c. A linear dependence was observed with the slope of ~ 2.0


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µm/V (solid line in Figure 5c). As discussed previously, the obtained slope represents the production of the carrier mobility (µn) and lifetime (τ). The longer elongated length could be ascribed to the much longer PL decay lifetime of the MaPbBr3 nanoplate. According to the TRPL spectrum measured by streak camera (Figure S7 in Supporting Information), the MaPbBr3 nanoplate shows about 50 ns PL decay lifetime.48 On the basis of the above discussion, we obtain the mobility µn of the MaPbBr3 nanoplate to be 7.8 cm2V-1S-1. To give a comprehensive analysis of the electric field dependent PL imaging method, measurements from several nanoplate devices are summarized. We find that the mobility is mostly in the range of about 20-30 cm2V-1S-1 for CsPbBr3 and 5-10 cm2V-1S-1 for MaPbBr3 (Figure S8 in Supporting Information). The quality of the nanoplates and the device performance, which probably highly depends on their local environments during their growth, give rise to the obtained mobility variation. As the contact resistance of the devices are not taken in account in our measurement, the measured charge mobility of nanoplates by this electric field dependent PL image are relatively small due to the existence of Schottky barriers in the devices. Nevertheless, the measured charge mobilities are within the range of reported values measured in perovskite single crystals by PL quenching, space charge limited current, Hall effect and so on (Table S1. in Supporting Information),2,3,9,27,33,46,49 demonstrating the feasibility of the PL imaging method for investigating the carriers drift. More importantly, compared to the above sophisticated optical spectroscopic methods and electronic techniques, our method provides a direct visualization of carrier transport and the information on the conduction type and the


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mobility of individual perovskite nanoplates through one simple experiment. In addition, as this method is based on the variation of PL emission patterns, we believe that the method can be applied to many other luminescent semiconductor nanostructures with a relative long carrier lifetime and long diffusion length. In conclusion, we present a simple electric field modulated PL imaging method to study the charge transport in individual perovskite nanoplates. Under local laser illumination and applied bias, the PL of perovskite nanoplates shows bias dependent elongated emission patterns with directions opposite to the applied electric field. Long decay length up to micrometers was observed in CsPbBr3 nanoplate. Combined with the time-resolved PL measurement, we conclude that the drift of photo-generated minority carriers under the external electric field leads to the observed elongated PL emissions. After fitting the PL intensity distribution profiles, we find a linear relation of the decay length and the applied voltages, from the slope of which the carrier mobility can be estimated. Consistent with the spatially resolved PL dynamics measurements, we find that the investigated CsPbBr3 nanoplate is a p-type material with an electron mobility over 28 cm2V-1S-1. Temperature dependent PL imaging show that the decay length of the elongated PL emissions decreases obviously at lower temperatures, showing a reduced carrier mobility in the CsPbBr3 nanoplate with deceased temperature, which is further confined by FET measurements. Carrier drift behaviors in CH3NH3PbBr3 (MaPbBr3) perovskite nanoplates were also revealed by imaging PL, showing electron mobility in the range of 5-10 cm2V-1S-1. For both the CsPbBr3 and MaPbBr3 nanoplates, the measured charge mobility is within the range of


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reported values in the literature, demonstrating the feasibility of the electric field modulated PL imaging method. Our work not only provides important insights for understanding the mobile carrier in the perovskite nanoplate, and also offers a new tool to study the photo-generated carrier transport in many semiconductor nanostructures.


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Methods. Optical measurements. PL and PL dynamics measurements were performed with a confocal µPL system (WITec, alpha-300). The 488 nm CW laser was used as the excitation source. The laser was introduced to the confocal system and focused onto the samples with a 100× objective. The PL images were recorded through the same objective and detected by a CCD camera (DFK 23GV024). The PL spectra were collected by a spectrometer with 300 g/mm grating. Spectra Physics Ti:Sapphire laser at 400 nm (100 fs, 80 MHz) and a streak camera system (Hamamatsu, C10910) with a temporal resolution of about 18 ps were used for PL dynamics measurements. During the spatially resolved PL dynamics experiments, a 0.5 mm pinhole is parked at a specific position of the optical path. Changing position of the pinhole ensures to collect photons emission from different positions of the nanoplate. Synthesis of CsPbBr3 nanoplate device. An ITO coated glass substrate with patterned electrodes was firstly prepared by a conventional lithograph and etching method. After that, patterned ITO electrodes glasses (15mm×15mm) were used as the substrates for the CsPbBr3 perovskite nanoplates by vapor-phase approach. We use a home-built CVD system. An alumina boat loaded with mixed powders of PbBr2 and CsBr was put inside the heating center of a quartz tube with the molar ratio 1:2 of PbBr2 and CsBr. At the downstream of the quartz tube, the patterned ITO electrodes glasses are mounted in the furnace (OTF-1200X). Before heating, the quartz tube was pumped down, which was followed by a 60 sccm flow of high purity Ar (99.999%) and maintained the pressure at 300 Torr. Then the furnace was heated to a setting temperature at


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570−600 °C in 30 minutes and maintained at this temperature for 15 min. After the growth, the tube was naturally cooled down to room temperature. Synthesis of MaPbBr3 nanoplate device. Patterned indium tin oxide (ITO) electrodes with channel width ~ 20 µm were prepared as the substrates. Single-crystalline square crystal of CH3NH3PbBr3 perovskite were synthesized using a one-step solution-precipitation method. Basically, mixed CH3NH3Br and PbBr2 with a 1:1 mole ratio was dissolved in N, N dimethylformamide (DMF) to form a CH3NH3Br·PbBr2 stock solution (0.03M). Then 2.5 µL of stock solution was dip-coated over a patterned ITO electrodes glasses substrate (15mm×15 mm), which was placed on a teflon stage in a weighing bottle. Subsequently, 2 mL of dichloromethane (DCM = CH2Cl2) was pour into the weighing bottle and the bottle was sealed at room temperature. For DCM is a poor solvent for CH3NH3PbBr3 and is miscible with DMF, the antisolvent (DCM) vapor within the sealed bottle would gradually diffuse into the stock solution and induce the nucleation and subsequent growth of square CH3NH3PbBr3 nanoplate.


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Figure 1. (a) Schematic of carrier generation, diffusion and drift under an external electric field. The recombination of transported free carrier causes an elongated shape of PL emission spot. (b) Illustration of one-dimensional photo-generated carrier density dynamics in space and time. (c) Sketch of the electro optical characteristic measurement of the CsPbBr3 nanoplate device. Inset shows a typical real color image of the device. (d) PL spectra measured at different laser (488 nm continues wave) powers from 0.5 µW to 3.1 µW. Insert is the PL image of the nanoplate. (e) PL intensity plotted as a function of excitation power in a double logarithmic scale. Red line shows a linear fit with a power law of 1.8. (f) Excitation power dependent photocurrent at Vsd=3 V. Inset shows current-voltage curves under dark and light illumination. Red line is a linear fit, indicating the power law of 0.52.


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Figure 2. (a) Real-color PL images of a typical CsPbBr3 nanoplate device with different positive (left panel) and negative (right panel) bias voltages applied at the upper electrode. Due to the direction of the applied electric field, the PL emission shows distinct modified patterns under positive and negative bias. (b) Intensity distribution of the emission pattern at positive bias. The solid lines are fittings of the experimental data with exponentially modified Gaussian functions. (c) Decay length plotted as a function of bias voltage. Solid line shows a linear fit of the experimental data (L=µτV/d) with a slope of 0.42. (d) In-situ time resolved PL measurement at different bias voltages and the fitting. Inset shows normalized in-situ PL spectra of the CsPbBr3 nanoplate at different bias voltages. (e) PL Emission intensity (red dot) time trajectories under an applied bias cycled periodically between 0 V and + 5 V. A triangle waveform (blue line) is used with a frequency of f = 5 Hz.


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Figure 3. (a) Schematic illustration of carrier drift measurement by comparing PL kinetics collected at the excitation site and a position away from the excitation site (Detection A). (b) Time resolved PL spectra recoded at ‘in situ’ and position ‘detection A’ by streak camera. (c) PL kinetics collect from in situ excitation position (green dot), position detection A (blue dot). Red circles represent the kinetics at position A after subtracting the ‘in situ’ component.


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Figure 4. (a) Real-color PL images of the CsPbBr3 at 0 V (left panel) and 5 V bias (right panel) at 270 K, 210 K and 180 K. (b) Temperature dependent PL spectra at 0 V bias. The dashed line indicates that the peak of the PL remains unchanged at different temperatures. (c) Intensity distribution of the temperature dependent PL emission at 5 V applied bias. The solid lines are fittings of the experimental data with exponentially modified Gaussian functions. (d) Decay length plotted as a function of bias voltage at different temperatures. Inset shows the calculated temperature dependent electron mobility.


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Figure 5. (a) Real-color PL images of a typical MaPbBr3 nanoplate device with different bias voltages. (b) Intensity distribution of the emission pattern at various bias. (c) Decay length plotted as a function of bias voltage and the linear fitting.


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ASSOCIATED CONTENT

Supporting Information.

This material is available free of charge via the Internet at http://pubs.acs.org. EDS spectrum and mapping of the as-grown CsPbBr3 nanoplates, PL spectra collected from different positions of the emission pattern, Laser power dependent PL imaging at applied bias, CsPbBr3 nanoplate transistors and the temperature dependent mobility, PL kinetics of MaPbBr3 nanoplate, and statistics of the mobility in the CsPbBr3 nanoplate and MaPbBr3 nanoplate.

AUTHOR INFORMATION Corresponding Author Xiao Wang*, Email: [email protected] Anlian Pan*, E-mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT

The authors are grateful to the National Natural Science Foundation of China (Nos. 51772084, 51525202, 61574054, 61505051), the Aid Program for Science and Technology Innovative Research Team in Higher Educational Institutions of Hunan Province, Joint Research Fund for


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Overseas Chinese, Hong Kong and Macau Scholars of the National Natural Science Foundation of China (No. 61528403), and The Foundation for Innovative Research Groups of NSFC (Grant 21521063)


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