Direct Observation of Charge Collection at Nanometer-Scale Iodide

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Letter

Direct Observation of Charge Collection at Nanometer-Scale IodideRich Perovskites During Halide Exchange Reaction on CHNHPbBr 3

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Izuru Karimata, Yasuhiro Kobori, and Takashi Tachikawa J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b00482 • Publication Date (Web): 31 Mar 2017 Downloaded from http://pubs.acs.org on April 2, 2017

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The Journal of Physical Chemistry Letters

Direct Observation of Charge Collection at Nanometer-Scale Iodide-Rich Perovskites during Halide Exchange Reaction on CH3NH3PbBr3 Izuru Karimata,1 Yasuhiro Kobori,1,2 and Takashi Tachikawa1,2,* 1

Department of Chemistry, Graduate School of Science, Kobe University, 1-1 Rokkodai-cho,

Nada-ku, Kobe 657-8501, Japan. 2

Molecular Photoscience Research Center, Kobe University, 1-1 Rokkodai-cho, Nada-ku, Kobe

657-8501, Japan.

AUTHOR INFORMATION Corresponding Author *[email protected] TEL: +81-78-803-5736

ACS Paragon Plus Environment

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ABSTRACT: Organolead halide perovskites MAPbX3 (MA = CH3NH3+, X = Cl−, Br−, or I−) are known to undergo reversible halide exchange reactions, enabling bandgap tuning over the visible light region. Using single-particle photoluminescence (PL) imaging for in situ observation, we have studied the structure-dependent charge dynamics during halide exchange with iodide ions on an MAPbBr3 crystal. In particular, we optically detected nanometer-scale iodide-rich domains (i.e., MAPbBrI2) and found that their lifetimes of several tens of milliseconds are limited by reaction with diffusing vacancies. Furthermore, it was discovered that these domains effectively collect the charge carriers from the bulk crystal, thus resulting in amplified spontaneous emission (ASE) under continuous-wave laser irradiation. Our findings will provide direction for development of perovskite heterostructures with enhanced charge utilization.

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MAPbX3 (MA = CH3NH3+, X = Cl−, Br−, or I−) perovskites have emerged as a promising class of materials for solar cell applications as well as optoelectronic devices such as light emitting diodes (LEDs) and lasers owing to their high optical absorption coefficients and long-range diffusion lengths of charge carriers.1–5 Their variable halide compositions also provide full-range bandgap tunability, being promising for colorful and efficient devices.6–9 Previous studies have demonstrated reversible halide exchange reactions of MAPbX3 at room temperature.10–13 Jang et al. have synthesized MAPbCl3−xBrx and MAPbBr3−xIx nanocrystals at any given value of x by reacting MAPbBr3 with MACl or MAI in liquid phase.11 These results are consistent with recent theoretical studies pointing out that halide anions can readily diffuse through the crystal lattice due to the low corresponding activation energies (e.g., 0.08 eV for iodide, 0.09 eV for bromide).14,15 Recently, Kim et al. demonstrated that the cell performance is improved by introduction of bromide concentration gradient near the surface of MAPbBr3−xIx film because of efficient hole extraction.16 Indeed, the ion exchange is a promising strategy for altering material properties and functions. From another standpoint, however, spontaneous ion diffusion causes serious problems, such as current-voltage hysteresis in solar cells and photoinduced structural transformations or degradation, making it difficult to accurately assess the device performance.17,18 A deeper mechanistic understanding of the halide exchange reaction is thus strongly required to optimize the halide composition and control the nanostructure-related dynamics of photogenerated charges and ion species. So far, in situ observations have been conducted using transmission electron microscope (TEM) to follow the real-time crystal growth at the nanometer scale.19,20 However, the sample was often damaged by the electron beam itself.21,22 Fortunately, MAPbX3 perovskites exhibit strong photoluminescence (PL) emissions in full visible-light regions, with a peak position


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depending on the halide composition. Therefore, the single-particle or single-domain PL method23–25 is a powerful to the temporally and spatially resolved investigations of the charge dynamics during the ion-exchange processes. In this work, a single-particle PL spectromicroscopy was employed to ensure that nanometerscale dynamical ion exchange modulates the surface photo-carrier dynamics, which have never been identified by the conventional bulk measurements. We identify single iodide-rich domains with lifetimes of tens of milliseconds that are closely related to migrations of vacancies during the halide exchange, and further demonstrate that they act as a source of amplified spontaneous emission (ASE) under continuous-wave (CW) laser irradiation.

Figure 1A shows a schematic illustration of the in situ observation of halide exchange using a wide-field fluorescence microscope. Micrometer-sized MAPbBr3 crystals were synthesized by recrystallization (Figure S1-S5) and were spin-cast on a clean cover glass that was set in an uncovered sample holder to perform the exchange reaction in the liquid phase. Heptane was chosen as a reaction medium due to the low perovskite solubility therein. The exchange reaction was initiated by adding 2 µL of a reaction solution of [MAI] = 0.22 mM in a 20:1 v/v mixture of heptane and 2-propanol, as determined by a quantitative NMR, into 2 mL of heptane (Figure S6). Visible PLs from the crystals were observed under CW laser excitations at 405 nm as shown in Figure 1. The PL emission peak wavelength is known to be sensitive to x in MAPbBr3−xIx, e.g., 539 nm for the pure MAPbBr3 (x = 0) and 775 nm for the pure MAPbI3 (x = 3), allowing us to follow the reaction and obtain the structural information.13


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Figure 1. (A) Experimental setup for PL imaging and spectroscopy of the single crystal. A cover glass loaded with the MAPbBr3 crystals was set in a sample holder filled with heptane, and the halide exchange was initiated by adding the solution of MAI. The PL of individual crystals was observed using wide-field microscopy configuration. (B) Optical image of the MAPbBr3 crystal before the reaction. (C–G) PL images of the MAPbBr3 crystal reacting with iodine ions in solution. The frame rate of the acquisition was 30 frames per second.

Initially, to get an overview of the halide exchange reaction, the emission images were recorded using a color CCD camera (see Figure 1 and Movie S1). Prior to the reaction, intense green emission was observed for the entire MAPbBr3 crystal (Figure 1B, C). However, the overall PL intensities significantly decreased within a second after adding the MAI solution (20 µL, Figure 1D), while an addition of the corresponding MAI-free solution caused negligible PL variation. After the initial decrease, a number of red emission spots emerged on the crystal (Figure 1E, F). Interestingly, the red emission was burst-like, indicating a generation of shortlived species, the origin of which is discussed later. The red emission completely spread out to cover the crystal within ~1 min (Figure 1G). Because of the very low concentration of MAI and


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the short reaction time, we could focus on the early stage of the exchange reaction at the surface of MAPbBr3 crystals (Figure S4). As verified by scanning electron microscope (SEM) and atomic force microscope (AFM), no morphological changes or nanoparticle formation on the crystal surface occurred after the MAI addition (Figure S7). Furthermore, the green emission could be readily recovered to ~40% of its initial intensity when a solution of MABr was added into the medium at the stage of Figure 1G, indicating a good reversibility of halide exchange (Figure S8). To identify the observed chemical species, PL spectral measurements were coupled with the microscopy imaging (Figure 2). Prior to the reaction, a representative region of the MAPbBr3 crystal showed a narrow PL peak at ~540 nm. Immediately after the MAI addition, the 540-nm peak rapidly decreased in intensity without being significantly shifted (see spectra 1 and 2 in Figure 2B). Subsequently, a new PL peak appeared at 644 nm (spectrum 3 in Figure 2B), gaining intensity over time and eventually red-shifting to 700 nm (spectrum 4 in Figure 2B). Similar experiments were conducted for bulk samples (Figure S5B), where the addition of the reaction solution caused > 99% quenching of green PL and a subsequent peak red shift to 689 nm, which is consistent with the results of single crystal experiments. Steady-state UV-visible diffuse reflectance spectra also showed a subtle change, suggesting the partial replacement of bromide with iodide at the crystal surfaces (Figure S5A).


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Figure 2. (A) Temporal change of PL peak intensity, with peak wavelengths indicated by three different colors. The MAI solution was added at the dropping point. (B) PL spectra measured at each point indicated in panel A. (C) PL peak position of MAPbBr3−xIx as a function of x, with the black line representing literature data (see main text for details). (D) Schematic energy diagram and proposed mechanism of charge carrier transfer, with “h” and “e” denoting holes and electrons, respectively.

Jang et al. reported that the optical bandgap (Eg) of MAPbBr3−xIx could be expressed by the following quadratic equation; Eg = (1 − y)Eg(MAPbBr3) + yEg(MAPbI3) – by(1 – y), where b is the bowing constant (0.57 eV) and y equals x/3.11 Note that this equation might be not valid for nanoscale domains. As demonstrated by several groups,26,27 the PL peak wavelength of the


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perovskites becomes shorter by a few tens of nanometers with decreasing their size down to single-digit-nanometer scale because of quantum confinement effect or structural relaxation, hence leading to an underestimate of x. Using Eg values of 1.6 eV for MAPbI3 and 2.29 eV for MAPbBr3,7,11 we tentatively predicted the compositions of the perovskites formed during the reaction, as plotted in Figure 2C. It is noteworthy that the perovskites with x = 0–1, which correspond to the peak positions between 541 and 640 nm (blue zone in Figure 2C), showed negligible PL intensity. According to a literature,28 the energy difference in the conduction band (CB) levels between MAPbBr3 (x = 0) and MAPbI3 (x = 3) is approximately 0.1 eV. The aforementioned size effect may result in a slight decrease in the difference. For significantly smaller x (= 0–1), the energy difference is comparable with the thermal energy (0.025 eV) at room temperature, implying that free electrons migrate to be delocalized between the MAPbBr3 and MAPbBr3−xIx domains and thus suppressing the radiative recombination with holes trapped in the MAPbBr3−xIx domains with higher valence band (VB) levels than that of pure MAPbBr3.16 As the x values increase with the progress of the ion-exchange, the electrons are readily captured by the MAPbBr3−xIx domains, resulting in red-shifted PL due to radiative recombination with the stationary holes. It should be mentioned here that the PL of MAPbBr3−xIx with x = 0–1 would be detectable if the sample composition is homogeneous over the whole crystal. In fact, Jang et al. reported that completely reacted MAPbBr2.5I0.5 (x = 0.5) exhibited a PL peak at 570 nm.11 We propose a mechanism of interfacial charge dynamics in this paragraph (Figure 2D). The VB level energy difference between MAPbBr3 and MAPbI3 is ~0.5 eV28,29 which is much larger than the thermal energy. Therefore, it can be suggested that the spatially delocalized electrons cannot efficiently recombine with the trapped holes in MAPbBr3−xIx localized on the crystal surface for the smaller x, preventing the radiative recombination both on the green and red lights.


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Moreover, considering the oxidation potential of I− (+1.2 to +1.3 V vs. NHE),30 the photogenerated holes (+1.9 V vs. NHE) in pure MAPbBr3 are effectively captured by iodide ions adsorbed on the surface, resulting in the complete quenching of green emission. However, once the values of x exceed unity in Figure 2C, both electrons and holes are captured at the MAPbBr3−xIx domains and recombine radiatively, because of the larger CB level difference. This is a plausible reason for the absence of PL for MAPbBr3−xIx with x < 1, while MAPbBr3−xIx with x ≥ 1 showed a PL maximum above 644 nm. A similar vectorial charge transfer process was recently observed for all-inorganic cesium lead mixed halides (CsPbBr3−xIx).31 The charge transfer from the MAPbBr3 domain to the MAPbBr3−xIx domain is proven by the fact that the photoinduced degradation of the resulting MAPbBr3−xIx crystal led to the disappearance of the red emission and the appearance of the green emission (Figure S10). As observed using the color CCD camera (Figure 1E), red emissive spots appeared with a duration of several dozen milliseconds (henceforth referred to as “bursts”). To investigate the burst phenomenon in more detail, we monitored the exchange reaction using an ultrasensitive electron multiplying CCD camera and a bandpass filter (663–800 nm) to remove green emission from MAPbBr3. As demonstrated in Figure 3A, B and Movie S2, a number of the PL bursts with signals much higher than the background were observed over the entire surface of the crystal. The burst PL spectra exhibited a peak at ~720 nm with a FWHM of 40 nm (Figure 3C), indicating that x of the short-lived species is approximately 2 (i.e., MAPbBrI2).


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Figure 3. (A) Typical image of burst emission spots (marked by red circles) on a single crystal during the halide exchange reaction. (B) Typical intensity trajectory showing the bursts. (C) PL spectrum (black) of a burst with a Gaussian fit (red). The inset shows an example of ASE (see the red arrows). (D) Histogram of the burst duration time (τon) obtained from one crystal with a single-exponential fit (red). The laser intensity equaled 31 W cm−2. The inset shows the dependence of on excitation intensity (Iex) and the corresponding linear fit (red).

Here immediately arise questions as to why such species have limited lifetimes and, more specifically, how the emitting times are determined. The most possible reason is the dissociation of an iodine-rich domain. Considering the small activation energy (~0.1 V) of halide vacancy diffusion,15 the domain formation/dissociation should be mediated by diffusing vacancies in the crystal (Figure 4). If the vacancy encounters the MAPbBrI2 domain, its PL should be quenched since the photoinduced charge is instantaneously trapped by the diffusing vacancy, resulting in the non-radiative recombination. In this case, the emitting time should be governed by the


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diffusion rate of the iodide vacancy located near the MAPbBrI2 domain but not by the excitation photon flux. Another possible reason is the photoinduced degradation (Figure S10) or a structural change of the domain under intense later excitation. For instance, Hoke et al. reported that a successive photoirradiation of MAPbBr3−xIx with 0.6 < x < 3 exhibited 738 nm PL peak due to segregation into iodide-rich minority and bromide-enriched majority domains.18 It was also proposed that millisecond-to-second time scale diffusion of halide ions is induced by localized charges generated by photoirradiation.32

Figure 4. Illustration of vacancy-mediated domain dissociation and PL quenching.

To characterize the kinetics of individual domains, a statistical analysis of the characteristic time (τon) during which persistent emission is exhibited was examined. As shown in Figure 3D and S11, the histogram of τon could be fitted with a single exponential decay function (red solid line) to obtain an average value (). For example, = 43 ± 4 ms at an excitation intensity (Iex) of 13.5 mW cm−2. Importantly, the reciprocal of increased linearly with increasing Iex, implying the involvement of a photochemical process as mentioned above. The intrinsic lifetime of the domains in the absence of photo-assisted processes, 0, was calculated as 56 ms from the intercept of the plot of −1 vs. Iex (inset of Figure 3D).


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The relocation of the iodide ion to the next neighboring vacancy was reported to occur at a rate of 1.7 × 1012 s−1,15 which is clearly greater than 0−1 (18 s−1). Thus, in the present system, one can assume that the rate of the dissociation (kd) of the iodide vacancy is treated as the bimolecular reaction between the MAPbBrI2 domain and diffusing vacancies as

kd = k V [MAPbBrI2 ][V]

(1)

where kV is the diffusion-limited bimolecular reaction rate constant, [MAPbBrI2] is the local density of the MAPbBrI2 domain, and [V] is the density of the vacancies at the crystal surface. Since [V] is obviously higher than [MAPbBrI2], the reaction can be viewed as being pseudofirst-order with respect to [V] (i.e., kd´ = kV[V]). Neglecting the diffusion of MAPbBrI2, kV is given by

kV = 4πDV R

(2)

where DV is the diffusion coefficient of vacancies (10−12–10−11 cm2 s−1 for an iodide vacancy) that serve as non-radiative recombination centers,33–35 R is the reaction radius, which is assumed to equal the exciton Bohr radius (2.2 nm for MAPbI3)

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. Note that the effect of lead or MA+

vacancy diffusion was ignored due to their smaller diffusion coefficients.33 Adopting a surface defect density of 2 × 1018 cm−3 as [V],26 kd´ can be estimated as 6–60 s−1. This value is in agreement with 0−1 (18 s−1), which is independent from the influence of the photoinduced processes, thus supporting our hypothesis that the limited emitting times are determined by the vacancy diffusion. To understand the overall mechanism, however, further studies of the photodependent kinetics of the bursts are required.


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Notably, the bursts were occasionally accompanied by strong emission with a narrow line width (FWHM = 7.6 nm) and a slightly red-shifted peak below the main peak (CW laser irradiation of 10 W cm−2 at the sample surface, inset of Figure 3C). These characteristics are indicative of amplified spontaneous emission (ASE).37,38 Usually, ASE in perovskites originates from the high density of charges generated by excitation with an intense pulsed laser, with reported threshold ASE fluence rates of 1022–1026 photons cm−2 s−1.39,40 Under the present experimental conditions, the fluence rate was measured as ~1019 photons cm−2 s−1, being much smaller than the threshold. However, if the charges are efficiently transferred from MAPbBr3 to MAPbBr3−xIx over a range of 100 nm2, the local fluence rate reaches ~1023 photons cm−2 s−1. Furthermore, ASE was found to exhibit a weak satellite peak, implying the occurrence of the wave-guiding effect. The mode spacing (∆λ) in a Fabry–Pérot cavity can be estimated by ∆λ = λ2/2neffL, where neff is the effective refractive index (neff = 2.51) of MAPbBr3 and L is the length of the crystal.41,42 Taking L = 4.5 µm for the crystal examined herein (Figure S12), ∆λ is calculated as 25 nm, which is good agreement with the difference (24 nm) measured between the main and satellite peaks. In conclusion, we elucidated the charge dynamics during halide exchange with iodide ions on individual MAPbBr3 crystals using single-particle spectroscopy. It was shown that MAPbBr3−xIx crystals with 0 < x < 1 showed negligible PL due to the electrons being spatially delocalized over the inner MAPbBr3 and the outer MAPbBr3−xIx domains with similar CB levels, while crystals with x ≥ 1 showed strong PL above ~644 nm. Moreover, we could detect burst-like PL from single iodide-rich domains (i.e., MAPbBrI2) with lifetimes of several tens of milliseconds, which are primary limited by the diffusion-controlled reaction with vacancies in the crystal. Interestingly, the domains exhibit ASE under CW laser excitation, indicating efficient charge


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transfer between two perovskites with different compositions. These results suggest that efficient charge extraction in the perovskite solar cell is achievable by introduction of iodide-rich domains (MAPbBr3−xIx, x > 1) near the surface of MAPbBr3 film. Our findings will help to understand the operating principle of mixed halide perovskite-based devices and facilitate their applications in light-to-energy conversion system.

Corresponding Author *Email: [email protected].

ACKNOWLEDGMENT This work has been partially supported by Advanced Characterization Nanotechnology Platform, Nanotechnology Platform Program of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan at the Research Center for Ultra-High Voltage Electron Microscopy (Nanotechnology Open Facilities) in Osaka University and a Grant-in-Aid for Scientific Research (Project 15H03771 and others) from the MEXT, Japan.

Supporting

Information.

Sample

preparation

and

characterization,

PL

microscopy

measurements, photograph and SEM image of MAPbBr3 crystals, TEM images of MAPbBr3 crystals, powder XRD patterns of MAPbBr3 crystals, XPS spectra of MAPbBr3 crystals, Steadystate UV–visible diffuse reflectance spectra of MAPbBr3 crystals, 1H NMR spectrum of the reaction solution, AFM images of MAPbBr3 crystals, PL images of an MAPbBr3 crystal before


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and after exchange reaction, PL images of an MAPbIxBr3−x after intense laser irradiation, histograms of τon values, photograph of MAPbBr3 crystal.

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Direct Observation of Charge Collection at Nanometer-Scale Iodide

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