Photoresponse of CsPbBr3 and Cs4PbBr6 Perovskite Single Crystals

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Photoresponse of CsPbBr and CsPbBr Perovskite Single Crystals Ji-Hyun Cha, Jae Hoon Han, Wenping Yin, Cheolwoo Park, Yongmin Park, Tae Kyu Ahn, Jeong Ho Cho, and Duk-Young Jung J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b02763 • Publication Date (Web): 09 Jan 2017 Downloaded from http://pubs.acs.org on January 9, 2017

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The Journal of Physical Chemistry Letters

Photoresponse of CsPbBr3 and Cs4PbBr6 Perovskite Single Crystals Ji-Hyun Cha,† Jae Hoon Han,‡ Wenping Yin,§ Cheolwoo Park,§ Yongmin Park,† Tae Kyu Ahn,§ Jeong Ho Cho‡* and Duk-Young Jung†* AUTHOR ADDRESS †

Department of Chemistry, Sungkyunkwan University, 16419, Korea

‡

School of Chemical Engineering, Sungkyunkwan University, 16419, Korea

§

Department of Energy Science, Sungkyunkwan University, 16419, Korea

AUTHOR INFORMATION Corresponding Authors *Duk-Young Jung, E-mail: [email protected]. *Jeong Ho Cho, E-mail: [email protected].

ACS Paragon Plus Environment

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ABSTRACT

High-quality and millimeter-sized perovskite single crystals of CsPbBr3 and Cs4PbBr6 were prepared in organic solvents and studied for correlation between photocurrent generation and photoluminescence (PL) emission. The CsPbBr3 crystals, which have a 3D perovskite structure, showed a highly sensitive photoresponse and poor PL signal. In contrast, Cs4PbBr6 crystals, which have a 0D perovskite structure, exhibited more than one order of magnitude higher PL intensity than CsPbBr3, which generated an ultralow photoresponse under illumination. Their contrasting optoelectrical characteristics were attributed to different exciton binding energies, induced by coordination geometry of the [PbBr6]4− octahedron sublattices. This work correlated the local structures of lead in the primitive perovskite and its derivatives to PL spectra as well as photoconductivity.

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Halide materials with the perovskite crystal structure have been widely employed as functional materials in various devices, such as the absorption layer of solar cells,1 light emitting layer of quantum-dot light emitting diodes (QD-LED),2 resistance changing layer of resistive random access memory (ReRAM),3 and light sensing layer of photodetectors,4 due to their good electronic properties.5 Moreover, they have received an enormous amount of attention because of unique properties, including facile deposition processing due to high solubility in polar organic solvents and tunability of band gap energy through manipulation of the halide composition, which is useful in solution-based fabrication of photonic and optoelectronic thin film devices.6,7 Nevertheless, electrical and chemical stability under ambient atmosphere hinders the use of hybrid perovskite materials, the most common being MAPbX3 (methylammonium lead halide), in typical optoelectronic devices.8,9 Significant effort has been applied to overcome these issues, including encapsulation by moisture-resistant layers,10,11 mixing and changing the ‘A’ site cation,12 and enhancing the morphology of thin films by coating engineering.13 As part of the effort to increase the stability of perovskite materials against stress, replacement of the organic cation by cesium cation, Cs+, to form an all inorganic perovskite, which is stable at ambient temperature and pressure, has been studied.5,12 The cesium lead bromide perovskites deliver outstanding optoelectrical properties and are significantly more stable than organicinorganic hybrid perovskite compounds. CsPbBr3 inorganic perovskite has recently been applied to the fabrication of strong luminescent colloidal quantum dots (QD)2, air-stable perovskite solar cells,12 highly sensitive visible light detectors,14 high-energy detectors,15 and a color convertor for a visible light communication system.16 Because of the explosion of developments in perovskite materials, research on their application has rapidly expanded to various fields.


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To understand the intrinsic optoelectrical properties of CsPbBr3, it is necessary to obtain highquality inorganic perovskite samples, with a single phase, high purity, and a macroscopic size over one millimeter, known as crystals. However, the solution-based growth of all inorganic perovskite has proven unsuitable as an efficient and facile method compared with organicinorganic hybrid perovskite.17,18,19 Recently, the growth of CsPbBr3 single and bulk crystals without related byproducts was investigated with melt-grown crystallization at high temperature (above 500 °C)15 and anti-solvent vapor-assisted crystallization (AVC) at low temperature.20 Crystal growth of single-phase perovskite with sufficient dimension is an important issue, because large crystals enable the investigation of detailed optical and electrical properties. Although crystallization through solubility control may provide high-quality CsPbBr3 crystals, preparation of high-purity single crystals is still challenging. Moreover, although the synthesis and optical properties of powder Cs4PbBr6 were investigated,21 as far as we know there has been no report on the synthesis and photoresponse of Cs4PbBr6 perovskite crystals with dimensions of several millimeters prepared in solution. In this study, in order to investigate correlations between crystal structure and optoelectrical properties of CsPbBr3 and Cs4PbBr6, we determined growth procedures to fabricate CsPbBr3 and Cs4PbBr6 crystals of millimeter size by the AVC method at room temperature in polar solvents. We found a narrow window of growth conditions that allow the crystals to be efficiently produced. Photocurrent and PL emission spectra of CsPbBr3 and Cs4PbBr6 single crystals were measured, indicating that the dimensionality of [PbBr6]4− units strongly influences the dissociation and recombination of excitons in the two perovskite crystal structures.


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Figure 1. Optical microscopy images and crystal structure diagrams of (a) CsPbBr3 and (b) Cs4PbBr6 crystals (orange octahedron, [PbBr6]4−; green dots, cesium atoms; blue dots, bromine atoms). (c) XRD patterns and (d) Raman spectra of CsPbBr3 and Cs4PbBr6 crystals.

Figure 1a shows an orange CsPbBr3 crystal, with a rectangular shape and dimensions of 0.67×2.0×0.51 mm3. Figure 1b shows a green Cs4PbBr6 crystal with a parallelepiped shape and dimensions of 0.56×0.39×0.32 mm3, which was the largest solution grown crystal. The precursor stoichiometry and growth time play important roles in the synthesis of the single crystal, where the CsPbBr3 was obtained from PbBr2-rich precursor solution for 3 days, and the Cs4PbBr6 crystal was crystalized from HBr-added CsBr-rich precursor solution for 4 days. More detailed results for crystal growth under various conditions are presented in Figure S1. Quantitative analyses by energy dispersive X-ray spectroscopy (EDS) were performed to characterize the


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purity of the prepared perovskite crystals. CsPbBr3 and Cs4PbBr6 had Cs:Pb:Br compositions of 1.01(2):1.03(1):2.95(1) and 4.02(4):1.09(1):5.88(4), respectively, in accordance with the ideal stoichiometries. The crystal structures and phase purity of the as-grown crystals were confirmed by XRD. Because characteristic optical and electrical properties of perovskite crystals correlate with their crystal structure, understanding the structural distinction between CsPbBr3 and Cs4PbBr6 is one of the most important preliminary characterizations. As shown in Figure 1c, the crystal structure of CsPbBr3 was confirmed to be orthorhombic (Pnma, 3D perovskite structure) with lattice parameters a = 8.26(3), b = 8.20(3), and c = 11.75(3) Å, calculated by least-squares refinements from d-values of XRD peaks. The Cs4PbBr6 crystallized with a trigonal structure (R-3c, 0D perovskite structure) with a = b =13.73(1) and c = 17.31(1) Å. The XRD results demonstrate that CsPbBr3 and Cs4PbBr6 crystals obtained by the AVC method had the same structures as those prepared by the conventional Bridgman method.15,22 XRD patterns of the crystals including simulated patterns and characteristic Bragg’s 2θ positions are shown in Figure S2. Raman spectra of the crystals provided the vibrational modes of the metal-halide sublattice (Figure 1d). When the perovskite crystals were excited by 633 nm laser light at room temperature, sharp and well resolved Raman signals were collected. Three Raman active modes for CsPbBr3 were observed, with a strong peak at 72 cm−1 and broad peaks at 127 and 310 cm−1. According to a previous Raman study of the CsPbCl3 crystal with Pnma phase,23 peaks at 72 and 127 cm−1 are assigned to the vibrational mode of [PbBr6]4− octahedron and motion of Cs+ cations. A weak, broad band at 310 cm−1 is related with the second-order phonon mode of the octahedron.23 Raman active modes of Cs4PbBr6 were detected, with strong peaks at 86.4 and 126.9 cm−1 and a shoulder peak at 70.8 cm−1, which are similar to those previously reported for


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melt-grown Cs4PbBr6 crystals.22 Strong peaks arise from the vibrational mode of the [PbBr6]4− octahedron.24 The Cs-Pb-Br ternary system has three polymorphs, CsPbBr3, CsPb2Br5, and Cs4PbBr6, based on a phase diagram.25 The dimensionality of perovskite structure could be varied from three to zero, 3D consisting of corner-sharing [PbBr6]4− octahedron, and 0D consisting of isolated [PbBr6]4−, which is related with its stoichiometry. CsPbBr3 possesses the orthorhombic phase at room temperature as a 3D perovskite structure,14 where the [PbBr6]4− octahedron are interconnected and tilted with respect to the orthogonal geometry of the ideal perovskite structure (Figure 1a). Cs4PbBr6 consisted of the isolated [PbBr6]4− octahedron as 0D perovskite structure,21 where [PbBr6]4− are surrounded by Cs+ cations (Figure 1b). In order to investigate the photoresponse of the perovskite single crystals, 50-nm-thick Au electrodes were thermally deposited onto the surfaces of both the CsPbBr3 and Cs4PbBr6 crystals through a shadow mask. The channel length and width were 120 and 500 µm, respectively. The photocurrent was measured by a two-probe semiconductor analyzing system equipped with an optical power-controllable light source (Figure 2a). Figure 2b shows the current-voltage characteristics of the crystals upon light illumination at a wavelength of 520 nm and optical power of 500 µW. The CsPbBr3 crystals exhibited a remarkable increase in photocurrent under light illumination, and the photo-switching ratio was 4.6×102 at 6 V. Photon absorption in the perovskite single crystal generated electron-hole pairs. The photo-generated charge carriers (electrons and holes) drifted to each electrode by the potential difference between electrodes. The photoresponse of the CsPbBr3 crystals was confirmed by impedance analysis before and after light illumination (Figure S3). The resistance of the crystal decreased from 840 MΩ·cm to 62


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MΩ·cm after light illumination. In contrast, an ultralow photocurrent of 1.0 (±0.2) nA was observed for the Cs4PbBr6 crystals.

Figure 2. (a) Schematic illustration of photoresponse measurements of cesium lead bromide crystals. The inset shows the optical top-view image of CsPbBr3 crystal with Au electrodes. (b) Current-voltage curves under dark and light illumination of CsPbBr3 and Cs4PbBr6 crystals. (c) Current-voltage curves under various illumination power of the CsPbBr3 crystal. The wavelength of light was 520 nm. (d) Responsivity and photocurrent of CsPbBr3 crystals as a function of the optical power. (e) Temporal photoresponse of CsPbBr3 crystals under alternating dark and light illumination. The right panel shows an enlarged view of both rise and fall regions. (f) Temporal photoresponse of Cs4PbBr6 crystals.

Figure 2c shows the illumination power-dependent current-voltage characteristics of the CsPbBr3 single crystals. The measurements were conducted at a fixed illumination wavelength of 520 nm. The photocurrent increased gradually as the illumination power increased from 1 to 500 µW. This result indicates that a larger number of charge carriers were generated under light illumination at higher optical powers which contributed the enhanced photocurrent. The


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responsivity (R), defined as Iph/P, where Iph is the photocurrent and P is the optical power of illuminated light, was found to be 2.1 A/W at 1 µW. Notably, this value is much higher than that of photodetectors with CsPbBr3 nanosheets (not single-crystal) and ITO contact (~0.6 A/W) due to an absence of long-chain organic insulating species applied during the nanocrystal synthesis.26 Note that such insulating species in crystals may cause poor electrical conductivity, which hinders the extraction of photo-generated charge carriers at each electrode. Figure 2d shows the photocurrent and responsivity of the device as a function of the optical power of incident light. As the optical power increased, the photocurrent increased but the responsivity decreased. Based on the relationship of R α P−1 in our measurements, R is expected to exceed 6.2×103 A/W at an illumination power of 1 pW if the relationship between R and light power keeps down to that value. Temporal photoresponse is one of the critical performance parameters of optoelectronic devices. Figure 2e shows the dynamic photoresponse behavior of the CsPbBr3 crystals over multiple cycles of illumination at 520 nm with a power of 10 µW. The laser pulse bandwidth was kept at 10 s per cycle. The curves exhibited the prompt and reproducible photocurrent response with good cycling stability. The current increased sharply as soon as light illumination began but returned to the original value when the light was blocked. An enlarged view of the temporal photoresponse is shown in the right panel of Figure 2e. The average rise and fall times were 0.3 (±0.04) and 5 (±0.3) s, respectively, which were evaluated by nonlinear curve fitting. The rise time was much shorter than the decay time due to the delayed extraction of the photogenerated carriers trapped in trap sites of the crystal, which was typically observed in typical perovskitebased optoelectronic devices.27 Figure 2f shows the temporal photoresponse characteristics of the Cs4PbBr6 crystals. The curve shape was totally different from that of the CsPbBr3 crystals with


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ultralow photocurrent. Their poor photoresponse of the Cs4PbBr6 crystals can be understood by dimensionality of crystal and the large exciton binding energy. (vide infra)

Figure 3. (a) Tauc plots, (b) PL spectra and (c) TRPL decays of CsPbBr3 and Cs4PbBr6 crystals. The IRF is shown as a gray line. The probe wavelengths were set at 520 nm and 550 nm, which correspond to PL maxima.

Figure 3a shows Tauc plots for CsPbBr3 and Cs4PbBr6 crystals obtained by converting the reflectance spectra using the Kubelka-Munk equation.28 CsPbBr3 and Cs4PbBr6 crystals exhibited a different absorption edge at 540 nm and 527 nm, from which the optical band gaps were determined to be 2.29 and 2.35 eV, respectively, at room temperature. The band gap energy values of CsPbBr3 and Cs4PbBr6 crystals are similar to those of bulk crystal and powder.15,20,29 The PL spectra of CsPbBr3 and Cs4PbBr6 (Figure 3b) were centered at approximately 550 nm and 517 nm, respectively, similar to previously reported results.20,21 The PL intensity of Cs4PbBr6 was almost 20 times higher than that of CsPbBr3, even though they have the same absorbance at an excitation wavelength of 405 nm. The time-resolved (TR) PL decays for


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CsPbBr3 and Cs4PbBr6 have much slower dynamics than the instrumental response function (IRF), which can be easily discriminated from IRF (Figure 3c). The average PL lifetime for CsPbBr3 (2.43 ns) was much shorter than that (19.58 ns) for Cs4PbBr6, which may come from the geometry different. The PL decay curves of each sample were convoluted using multiple exponential functions.

Table 1. Parameters of fitted time-resolved PL of the samples. Crystal

τavg (ns)

A1 (%)

τ1 (ns)

A2 (%)

τ2 (ns)

Cs4PbBr6

19.58

54.83

3.52

45.17

39.08

CsPbBr3

2.43

83.50

1.20

16.50

8.65

where, I(t)=A1exp(-t/τ1)+ A2exp(-t/τ2) and τavg=(A1τ1+A2τ2)/(A1+A2).

After bi–exponential fitting, we estimated the PL decay for CsPbBr3 to get two components of 1.20 ns (83.5 %) and 8.65 ns (16.5 %) (Table 1); the former originates from the power-dependent process in the perovskite active layer, e.g. exciton-exciton annihilation, and the latter is from free carrier recombination in the radiative channel.30,31 The PL decay for Cs4PbBr6 can be convoluted as 3.52 ns (54.8 %), 39.08 ns (45.2 %). In recent reports, an explanation for the outstanding emissive properties of Cs4PbBr6 is caused by the high exciton binding energy compared to that of CsPbBr3 compounds.21 The difference of PL properties is closely related to the electrical photoresponse in perovskite crystals, as described below.


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Figure 4. Schematic illustration of photocurrent generation and the PL emission process in (a) CsPbBr3 and (b) Cs4PbBr6 crystals.

Through structural, optical, and optoelectrical investigations, we found a correlation between photoresponse and crystal structure for perovskite crystals. The CsPbBr3 crystal generates a high photocurrent and emits low level PL (Figure 4a). The Cs4PbBr6 crystal exhibits low photocurrent generation and strong PL emission (Figure 4b). This negative correlation is ascribed to the exciton binding energy of the crystals, originating from their unique crystal structure. In the CsPbBr3 crystal, excitons are easily dissociated into free charge carriers (electron and hole), and then separated carriers are transported by accessing the connected octahedral lattice due to the low exciton binding energy in the 3D perovskite structure (19~62 meV).32,33 Thus, radiative recombination is unfavorable because the carriers rapidly flow via the external circuit through the electrodes. However, opposite photoresponse occurs in the Cs4PbBr6 crystal. The exciton binding energy of Cs4PbBr6 (353 ± 40 meV)21 is larger than that of CsPbBr3 by over one order of magnitude, because the Cs4PbBr6 crystal has the 0D perovskite structure that consists of the disconnected [PbBr6]4− octahedral isolated by Cs+ cations. Thus, it is difficult to the excitons to flow across the internal crystals, and they can be quenched because of the dramatic increase in exciton binding energy. From this correlation, we can predict the


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photoelectric and PL properties of inorganic halide materials from the perovskite structure. For fabrication of optoelectronic devices, our findings may be the key to achieving a desired functionality. Recently, the optical properties of CsPbBr3 QD, including sharp and strong emissions with high photo-luminescence quantum yield (PLQY) values, were investigated.2 However, there have been no reports of the synthesis and application of 0D perovskite QD. According to our results, we expect that 0D perovskite QD will exhibit excellent emissive PL properties compared with higher-dimensional perovskite QD. Large pure CsPbBr3 and Cs4PbBr6 perovskite single crystals were grown by the AVC method in order to investigate their detailed optical and electrical properties, which showed diametrical optical and electrical properties. We suggest that photocurrent and PL have a negative relationship, which is ascribed to the recombination or dissociative process of excitons due to the difference of exciton binding energies. The dissociation of excitons into electrons and holes generates the photocurrent across the crystal, which dominates the photoresponse of CsPbBr3 3D perovskite crystals. In contrast, the radiative recombination of exciton, which induce a strong PL response, is favorable in the Cs4PbBr6 0D perovskite crystal due to the high exciton binding energy. Our findings indicate the importance of the crystal structure of perovskite materials in understanding their superior optoelectrical performance.

AUTHOR INFORMATION Notes The authors declare no competing financial interests.


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ACKNOWLEDGMENT This work was supported by Woo Jang Chun Special Project (PJ009106022013) by Rural Development Association and a grant from the Center for Advanced Soft Electronics (CASE) under the Global Frontier Research Program (2013M3A6A5073177).

ASSOCIATED CONTENT Supporting Information Results of experimental methods, photographs, XRD patterns and impedance spectra. The Supporting Information is available free of charge on ACS Publication website.

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Photoresponse of CsPbBr3 and Cs4PbBr6 Perovskite Single Crystals

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