Inorganic Lead Halide Perovskite Single Crystals

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Makhsud I. Saidaminov, Md Azimul Haque, Jawaher Almutlaq, Smritakshi Sarmah, Xiao-He Miao, Raihana Begum, Ayan A. Zhumekenov, Ibrahim Dursun, Namchul Cho, Banavoth Murali, Omar F. Mohammed, Tom Wu,* and Osman M. Bakr* Semiconductors that are easy to fabricate, stable in neat and in operating devices, and comprised of abundant elements are always sought after as materials for next-generation optoelectronics. Solution-processed hybrid perovskites (ABX3, where A = CH3NH3+ or MA, HC(NH2)2+ or FA; B = Pb2+, Sn2+; X = Cl−, Br−, I−) are the freshest examples of promising semiconductors, illustrating a fast increase of light-to-electricity conversion efficiency through judicious optimizations to over 20% within the past 7 years.[1–7] However, decomposition and volatilization of the organic component in hybrid perovskites are considered to be major issues responsible for their poor long-term stability.[8–12] In contrast, replacing the organic cation with an inorganic one at the A-site to form all-inorganic perovskites (IPs) overcomes the problem of stability, does not deteriorate the transport properties, and maintains the efficiency of the final device.[13–17] Therefore, all-inorganic lead halide perovskites, particularly CsPbX3, have received considerable attention.[18–23] Macroscopic single crystals are critical for both fundamental studies and efficient devices.[24–29] Large hybrid perovskite crystals can be prepared through a variety of solution-based approaches, such as the conventional cooling technique,[30] antisolvent vapor-assisted crystallization,[24] top-seeded growth,[25] inverse temperature crystallization (ITC),[31–34] and solvent acidolysis crystallization.[35] In contrast, the current methods

Dr. M. I. Saidaminov, J. Almutlaq, Dr. S. Sarmah, Dr. R. Begum, A. A. Zhumekenov, I. Dursun, Dr. N. Cho, Dr. B. Murali, Prof. O. F. Mohammed, Prof. O. M. Bakr KAUST Solar Center (KSC) Division of Physical Sciences and Engineering (PSE) King Abdullah University of Science and Technology (KAUST) Thuwal 23955-6900, Saudi Arabia E-mail: [email protected] Md A. Haque, Prof. T. Wu Materials Science and Engineering King Abdullah University of Science and Technology (KAUST) Thuwal 23955-6900, Saudi Arabia E-mail: [email protected] Dr. X.-H. Miao Imaging and Characterization Core Lab King Abdullah University of Science and Technology (KAUST) Thuwal 23955-6900, Saudi Arabia

DOI: 10.1002/adom.201600704

Adv. Optical Mater. 2017, 1600704

to synthesize large IP single crystals are a major hindrance for their broad appeal in research and practical applicability. For instance, synthesis through the Bridgman technique[13] requires high temperatures (600 °C), high vacuum (10−4 mbar), and exceptionally pure precursors, while the conventional crystallization method, through cooling of a saturated solution, is slow and is practical for producing only small size crystals suitable for single-crystal X-ray diffraction (SC-XRD) analysis.[36] There is thus an urgent need to make high-purity macroscopic single crystals accessible to the broader research community, which would enable the investigation of their fundamental properties and applications in optoelectronics. Here we develop a rapid, low-temperature, solution-based phase-selective synthesis of millimeter-sized cesium lead halide IP single crystals, elucidating some of their key optical and transport properties, and demonstrating their potential for selfpowered optoelectronics. In particular, we show exceptionally low minority (hole) carrier concentrations in these low-temperature-synthesized single crystals, which we take advantage of to design self-powered photodetectors with a high ON/OFF ratio that is one of the highest reported for photodetectors of this class. While normally several days or weeks are required to grow large hybrid perovskite single crystals by the classical cooling method,[30] recently a new strategy—ITC—was developed to synthesize them in few hours.[31–34] The ITC of perovskites was found to be selective to the solvent (e.g., I− and Br− require gamma-butyrolactone (GBL) and dimethylformamide (DMF), respectively).[37] For instance, to grow MAPbBr3, MABr and PbBr2 in a 1:1 molar ratio are dissolved in DMF to form a concentrated solution at room temperature, then heated to grow single crystals as large as 5–7 mm in 3 h.[31] The process of ITC is relatively straightforward for hybrid perovskites. In contrast, the situation is much more complex for IPs such as CsPbBr3. Typical solvents for ITC like DMF are unbalanced solvents for CsPbBr3 precursors (i.e., poor solvents for CsBr and good solvents for PbBr2). Moreover, unlike MABr and PbBr2 that form a single compound—MAPbBr3, the phase diagram of CsBr and PbBr2 is more complex and favors the formation of three compositions— Cs4PbBr6, CsPb2Br5, and CsPbBr3.[16,38] Therefore, careful design of the solvent and stoichiometry is required to enable the exclusive formation of CsPbBr3, while suppressing all other phases. The schematic shown in Figure 1a summarizes some of the parameters we explored in order to arrive at the pure phase of

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Inorganic Lead Halide Perovskite Single Crystals: Phase-Selective Low-Temperature Growth, Carrier Transport Properties, and Self-Powered Photodetection


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Figure 1. Synthesis of CsPbBr3 single crystals. a) Schematic representing the optimization of the crystallization (percentages represent the ratios of XRD peak intensities); b) photograph of the crystal; c) powder XRD of the ground crystal along with the calculated spectrum of orthorhombic CsPbBr3; d) crystal structure obtained by SC-XRD.

CsPbBr3. We started with an equimolar solution of CsBr:PbBr2 (1:1) in DMSO, which was selected as a solvent instead of DMF due its overall higher solubility limit for the precursors. While such a solution demonstrated inverse solubility upon heating until 120 °C, yielding a yellow precipitate, the precipitate, unfortunately, contained a mixture of undesired Cs4PbBr6 with only a small amount of the desired CsPbBr3 (Figure S1, Supporting Information) due to the lower solubility of CsBr than that of PbBr2 in dimethyl sulfoxide (DMSO), which causes the faster precipitation of Cs-rich Cs4PbBr6 phase. We reasoned that a solution with higher Pb concentration could dominantly favor the formation of CsPbBr3, which is richer in Pb than Cs4PbBr6. Indeed, CsBr:PbBr2 (1:1.5) showed an increase of the desired orange CsPbBr3, but the Cs4PbBr6 phase was still dominant (Figure S2, Supporting Information). We further increased the Pb-containing component: CsBr:PbBr2 (1:2) gave mainly CsPbBr3, but in this instance a mixture with Pb-rich CsPb2Br5 phase formed. Beyond precursor stoichiometry, we explored temperature, as a lever for favoring the formation of one phase over another. Fortunately, CsPb2Br5 was observed to form at a lower temperature than CsPbBr3. Therefore, we filtered the solution at 100 °C and gradually heated it until 120 °C, at which point we obtained isolated, millimeter-sized, rectangularshaped, pure orange crystals. It should be mentioned that a relevant CsPbBr3 crystallization method has been published online[39] during the revision of this article. Figure 1b shows a semitransparent ≈3 × 2 × 1 mm3 crystal grown in three hours. No reflections were detected in powder XRD spectrum of the ground crystal other those belonging to orthorhombic CsPbBr3 (Figure 1c), which confirms the purity of the CsPbBr3 phase of the crystals. We also performed SC-XRD and resolved the structure (Figure 1d). The space 1600704 (2 of 7)


group (Pnma) and the parameters of orthorhombic unit cells (Supporting Information) are in good agreement with previous reports.[13] Next, we studied the optical properties of CsPbBr3 crystals (Figure 2a). They show a sharp absorption edge at 560 nm, and the Tauc plot indicates a bandgap of 2.21 eV. The steadystate photoluminescence (PL) spectrum peaks at 556 nm. These values are consistent with previous characterizations of Bridgman-grown CsPbBr3 crystals.[13] We then performed PL lifetime measurements.[24,28,40,41] The PL lifetime data were fitted to two-exponential decays with characteristic time constants of 23 and 233 ns, which likely correspond to surface and bulk carrier recombination, respectively[24] (Figure 2b). As shown in Figure 2c,d, different transport mechanisms were observed in the space charge limited current (SCLC) measurements carried out on single-carrier devices of CsPbBr3 crystals sandwiched between two metal contacts. In the current–voltage (I ≈V n) curves, Ohmic (n = 1), trap-filling (n > 3) and Child (n = 2) regions were observed, similar to the previous reports.[42–46] The trap densities (nt) were calculated using the equation nt = (2VTFLεε0)/(eL2), where VTFL is the trap-filled limit voltage, L is the thickness of the crystal, ε0 is the vacuum permittivity, e is the electron charge, ε is the relative dielectric constant for CsPbBr3 single crystals, measured to be ≈22. The mobility (µ) was calculated from the Child region according to Mott–Gurney’s equation μ = (8JDL3)/ (9εε0V2), where JD is the current density. The results of SCLC measurements are summarized in Table 1. The resistivity of CsPbBr3 single crystals in the current study is ≈0.1 GΩ cm, comparable to high-quality MAPbBr3 and FAPbBr3 single crystals,[24,47] implying a high level of purity. We compared also the carrier transport and dynamics in CsPbBr3 with those





Communication Figure 2. CsPbBr3 single crystal characterizations. a) Absorption property of the CsPbBr3 single crystal. Also shown is the Tauc plot (lower inset) and steady-state PL data (upper inset). b) PL lifetime decay upon two-photon excitation. The circles represent experimental data and the solid curve is the best fit with a double-exponential function. c) SCLC characteristics for electron-only and d) hole-only devices.

in MAPbBr3 (also measured by SCLC)[31] and concluded that the nature of the A-site cation does not significantly affect the carrier mobility, but does influence the carrier recombination kinetics. In hybrid perovskites, organic cations form local dipoles that assist in exciton dissociation into free carriers, thus extending the carrier lifetime.[48] Such lower carrier lifetimes of CsPbX3 in the reference of hybrid counterparts were observed in thin films as well.[16,49] Further investigation of CsPbBr3 single crystals reveals an electron mobility that is greater that of holes (Table 1), consistent with recent theoretical calculations.[50] Moreover, we found that CsPbBr3 crystals possess an exceptionally low hole concentration (≈108 cm−3), considerably lower than MAPbBr3 and FAPbBr3 as well as crystalline silicon.[24,51,47] Although Table 1. Electronic properties of CsPbBr3 single crystals.

Mobility [cm2 V−1 s−1] Carrier concentration Trap density

[cm−3]

[cm−3]

Diffusion length [µm]


Electrons

Holes

52

11

1.1 ×

109

1.4 × 108

1.1 ×

1010

4.2 × 1010

5.5

2.5

the conductivity is high and carrier concentration is low in our crystals, their mobility (measured by SCLC) is lower than the previous reports on Bridgman-grown single crystals and colloidal CsPbBr3 nanocrystals, whose mobilities were estimated from photoconductivity and terahertz conductivity measurements, respectively.[13,52] Discrepancies in carrier mobility of perovskites were reported before[53,54] and can be attributed to the different carrier transport processes measured by particular techniques and details of material synthesis. Therefore, in this study, we focus on comparing carrier mobilities of perovskites measured by the SCLC method. The potential of CsPbBr3 to possess a very low dark current drew our attention and inspired us to explore their response under the light as self-powered optical sensors. A self-biased Schottky photodetector was realized by sandwiching the CsPbBr3 crystal between Pt and Au contacts. First, we studied the I–V characteristics of the device in the dark; at 0 V, a very low dark current of 10 pA was observed (Figure 3a), which is a direct result of the ultralow intrinsic carrier concentration. Low dark current is vital for photodetectors to achieve high signal-to-noise ratio and operational reliability.[55] Under white light illumination of 10 mW cm−2 (Figure 3a), the zero-bias photocurrent increases by five orders of magnitude to ≈1 µA.



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Figure 3. Device characterization. a) Dark and light (10 mW cm−2) I–V curves; b) photoresponse under light pulses measured under zero bias; c) transient response and d) current stability measured under continuous illumination.

The high photoresponse of the present CsPbBr3 photodetector enabled an ON/OFF ratio as high as 105 at zero bias, which is one of the highest for solution-processed self-biased photodetectors with the metal–semiconductor–metal structure.[56–58] Our photodetectors exhibit a reasonably good external quantum efficiency (EQE) of over 6% (Figure S5, Supporting Information) and a peak responsivity of 28 mA W−1 at 550 nm. The onset wavelength in the EQE spectrum at 560 nm matches the absorption edge (Figure 2a), verifying the accuracy of the EQE results. The decrease in EQE at the shorter wavelength region was observed as a result of the thick absorber layer.[27,59,60] The noise measurements revealed that the dominant noise is white noise (Figure S6, Supporting Information).[27] The noise current was found to be ≈29 fAHz−1/2, corresponding to a noise equivalent power (NEP) of ≈1 pW.[61] As a result of the very low noise current, a specific detectivity of 1.7 × 1011 Jones was obtained at zero bias, which is comparable to some of the best perovskite photodetectors reported to date.[60,62,63] Sequential switching of the device under zero bias does not display any noticeable performance degradation (Figure 3b). Noteworthy, all cycles consistently showed the same ON/OFF ratio of 105, and after each cycle, the dark current recovers to the order of 10 pA, thus enabling efficient and reliable 1600704 (4 of 7)


photodetection. A rise time (230 ms) and decay time (60 ms) at zero bias (Figure 3c) is comparable to some of the state-of-theart perovskite photodetectors under bias.[64,65] Constant photocurrent over 3000 s (Figure 3d) demonstrates the photostability of the operational device. Also, unlike typical hybrid perovskite devices, there was no hysteresis observed under illumination for these devices (Figure S4, Supporting Information), likely due to the negligible ion migration in IPs. We note that the needle-like, yellow phase of CsPbI3 can also be prepared by this technique (Figure S7, Supporting Information). Powder XRD matches well with the calculated XRD of the yellow nonperovskite phase of CsPbI3.[17] The applicability of ITC for CsPbX3 IPs improves our understanding of the ITC phenomenon. First, it demonstrates that the presence of organic cation is not essential to induce inverse crystallization. Second, it shows that ITC is not limited to only perovskite structures; it works for nonperovskite phase of CsPbI3 as well. Third, since all so far ITC grown perovskites share one common character, i.e., they are lead based, we tend to relate this phenomenon to the chemistry of plumbate complexes,[11] while the organic cation or Cs+ play a role of counter ions. However, the exact mechanism and kinetics of the ITC remain to be studied.





Experimental Section Chemicals: Lead bromide (≥98%), lead iodide (99.999% trace metals basis), cesium bromide (99.9% trace metals basis), cesium iodide (99.9% trace metals basis), DMF (anhydrous, 99.8%), and DMSO (anhydrous, ≥99.9%) were purchased from Sigma-Aldrich and used as received. CsPbBr3 Single Crystals Growth: CsBr (3 mmol) and PbBr2 (6 mmol) were dissolved in DMSO (3 mL) and stirred for one hour. 1.5 mL of the solution was transferred into a vial, and the vial was placed in silicon oil bath at 60 °C. Temperature was then gradually increased to 100 °C with a heating rate of 5 °C per 30 min. Yellow and orange crystals appeared as the temperature was increased. At 100 °C, the filtrate was transferred into new vials that were preheated to 100 °C, and kept for 3 h. To make the crystals larger, the vials can be further heated with a rate of 5 °C per 30 min until 120 °C. Then the crystals were rinsed with hot DMSO to get rid of the leftover solution from the surface and dried in a vacuum chamber at 100 °C for one hour. CsPbI3 Single Crystals Growth: CsI (1 mmol) and PbI2 (1 mmol) were dissolved in DMF (2 mL) at room temperature and stirred for one hour. The solution was filtered with polytetrafluoroethylene filter and placed in a metallic holder. Temperature was increased to 110 °C directly, after 3 h the formation of yellow, needle like crystals were observed. The steady-state absorption was recorded using a Cary 6000i Spectrophotometer with an integrating sphere in diffuse-reflectance mode. The Steady-State PL and PL Decay: All experiments were conducted at room temperature. Steady-state PL was performed using a femtosecond laser system operated at a wavelength 800 nm with 35 fs pulses and a repetition rate of 1 kHz. The 2PA were carried out by directly using the fundamental beam at 800 nm. Time correlated single photon counting for lifetime measurements was performed using a Halcyone MC multichannel fluorescence up-conversion spectrometer (Ultrafast Systems) at an excitation wavelength of 800 nm. Space Charge Limited Current Measurements: Crystals with similar dimensions 1.75 mm × 1.75 mm × 1.25 mm were used to make single carrier devices for SCLC measurements. Hole-only and electrononly devices were realized by sputter deposition of gold and titanium respectively on both sides of the single crystal. We performed photoelectron spectroscopy in air measurements (Riken Photoelectron Spectrometer, Model AC-2) to extract the valence band maximum (VBM) of CsPbBr3 crystals, while conduction band minimum (CBM) can be obtained from the relationship CBM = VBM + Eg, where Eg is the optical bandgap (Figure S9, Supporting Information).[66] Hole- and electroninjecting devices were realized using gold (Au, a deep-work-function


metal) and titanium (Ti, a shallow-work-function metal), respectively.[45] The SCLC measurement was performed using Keithley 2635A sourcemeter in dark, under vacuum at room temperature. Photodetector Fabrication and Measurement: For the photodetector, 80 nm gold and 25 nm platinum were deposited on opposite faces of the crystal. All photoresponse characteristics of the self-biased photodetector were measured on a probe station connected to a Keithley 4200 semiconductor analyzer under dark and light conditions. To measure the wavelength-dependent photoresponse, a Newport quantum efficiency system was used as the light source and a Keithley 2400 source meter was used to measure the photocurrent. The responsivity (R) was calculated as R = ΔI/(PS), where ΔI is the difference between the photocurrent and the dark current, P is the incident power density, and S is the effective illuminated area. EQE was calculated from the responsivity, EQE = Rhc/(eλ) where h is Plank’s constant, c is the speed of light, e is the electronic charge and λ is the incident light wavelength.[67] Specific detectivity was calculated using the equation D* = √(AB)/NEP,[68,69] where A is the device area, B is the bandwidth and NEP is noise equivalent power. Noise current was directly measured using a lock-in amplifier (Stanford Research SR830) in the current measurement mode. Powder XRD was performed on a Bruker AXS D8 diffractometer using Cu-Ka radiation. Single Crystal XRD: X-ray diffraction measurement was collected at 298 K, performed on a Bruker D8 Venture diffractometer with PHOTON 100 complementary metal oxide semiconductor detector with a microfocus source (Cu Ka radiation, λ = 1.54178 A). The computing cell refinement and data reduction was processed using APEX2 software. [SAINT-Plus; APEX2; SADABS, Bruker-AXS Inc.:Madison, Wisconsin, 2004] Crystal data, data collection parameters, and structure refinement details are given in Table S1 (Supporting Information). The structure was solved by direct methods with the structure solution programs SHELXT to deconvolute the single crystal data and determine the structure.[70] Subsequent difference Fourier calculations and full-matrix least-squares refinement against F2 were performed with the programs SHELXL for refinement of crystal structure model.[70] All nonhydrogen atoms were refined with anisotropic displacement parameter. Computer programs: APEX2 software package, APEX2 Version 2014.9-0, SAINT Version 8.34A and SADABS Version 2014/4, Bruker Analytical X-ray Systems, Inc., Madison, Wisconsin, USA, 2014; SHELXT,[71] SHELXL,[70] Olex2.[72]

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements M.I.S. and M.A.H. contributed equally to this work. This work was supported by KAUST. Received: August 27, 2016 Revised: October 9, 2016 Published online:

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In conclusion, we made large and high-quality macroscopic cesium lead halide single crystals readily available to the broad research community by presenting an easily adoptable low-temperature synthesis method. Through phase and solvent engineering of the reaction system we realized high-purity IP single crystals, whose optical and transport properties we elucidated. All-inorganic, APbX3 perovskites possess long charge-carrier lifetimes and diffusion lengths (albeit longer for electrons than holes), which shows that having an inorganic species at the A-site considerably improves material stability (Figure S8, Supporting Information) with little adverse effects on the transport properties. A surprising finding of our study is the discovery of ultralow minority carrier concentrations in CsPbBr3 single crystals, which makes the material particularly useful for designing efficient optoelectronic devices. The simplicity of our method and its highly pure single crystals will permit extensive use of these crystals for in-depth fundamental studies and broader practical applications.

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