Incorporation of Cesium Ions into MA1–x Cs x PbI3 Single Crystals

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Cite This: J. Phys. Chem. Lett. 2018, 9, 5833−5839

Incorporation of Cesium Ions into MA1−xCsxPbI3 Single Crystals: Crystal Growth, Enhancement of Stability, and Optoelectronic Properties Songjie Du,† Lin Jing,† Xiaohua Cheng,† Ye Yuan,† Jianxu Ding,*,† Tianliang Zhou,*,‡ Xiaoyuan Zhan,† and Hongzhi Cui*,† †

College of Materials Science and Engineering, Shandong University of Science and Technology, Qingdao 266590, China College of Materials, Xiamen University, Xiamen 361005, China

J. Phys. Chem. Lett. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 09/25/18. For personal use only.

‡

S Supporting Information *

ABSTRACT: Organic−inorganic hybrid methylammonium lead iodide perovskite (MAPbI3) has attracted extensive attention in a series of optoelectronic devices. The photoelectric properties of the MAPbI3 single crystal have been revealed to be much better than those of it polycrystalline counterparts. However, its poor moisture and heat resistance severely limited further development. The introduction of Cs+ into polycrystalline films has shown to be an effective way to enhance its moisture resistance through a passivation effect. However, the entrance abilities of Cs+ into a MAPbI3 crystal lattice and the influence on photoelectric properties of a single crystal were not clear until now. Therefore, we attempted to grow large MA1−xCsxPbI3 single crystals to introduce Cs+ into the crystal lattice. The existence of Cs+ brought lattice shrinkage and enhanced stability of the MAPbI3 single crystal. A moderate quantity of Cs+ (2%) proved to heighten the photoelectric properties, whereas an excess quantity of Cs+ (5%) brought more shallow defects, which ultimately deteriorated the photoelectric properties.

A

large size organic cations, such as ethylammonium (EA) and formamidinium (FA) partially instead of MA, is an effective way to improve both photoelectric properties and stabilities of perovskites.30−32 Introducing inorganic cations into perovskite films, such as Cs+, can also improve the stability and enhance the power conversion efficiency through a passivation effect at grain boundaries or suppression of atomic vacancies.33−36 However, there is still a lack of sufficient understanding about the influence of Cs+ on the stability and photoelectric properties of MAPbI3 single crystals. To deeply understand the entrance ability of Cs+ into the MAPbI3 single-crystal lattice and how Cs+ affects the stability and photoelectric properties, obtaining large size MA1−xCsxPbI3 single crystals is highly demanded. In the present work, by adjusting the concentrations of Cs+ in growth solutions, a series of large-scale MA1−xCsxPbI3 single crystals with various Cs+ contents were grown. The influence of Cs+ on the single-crystal growth process, crystal structures, and optical properties is discussed in detail. Moreover, in terms of planar photodetectors, the photoelectric properties of MA1−xCsxPbI3 single crystals are compared. The growth of MA1−xCsxPbI3 single crystals is different from the growth of a pure MAPbI3 single crystal in two aspects. One is that the solubility of CsI is not as high as that MAI in a GBL

s one of the most promising perovskite materials, organic−inorganic hybrid methylammonium lead halide (MAPbX3, X = Cl, Br, I) has been widely studied for solar cells,1 photodetectors,2 lasing,3 light-emitting diodes,4 and hydrogen production,5 benefiting from their diverse forms (films, nanocrystal, 2D crystal, single crystal, etc.) as well as their excellent properties, including direct band gap,6 large absorption coefficient,7 long-range balanced electron- and hole-transport lengths,8 and high charge carrier mobility.9−12 MAPbI3, a typical representative of the MAPbX3 family, has attracted extensive attention in the above-mentioned fields. However, so far, the application of MAPbI3 has mostly concentrated on polycrystalline films. Compared with single crystals, the high density of defects and grain boundaries in films reduce the performance of MAPbI3.13 Therefore, the MAPbI3 single crystal is deemed to be more suitable for the development of photodetectors.14−18 Although MAPbI3 exhibits excellent photoelectric properties, its poorer long-term stability than a fully inorganic perovskite, such as CsPbBr3, greatly hinders its further largescale applications as photoelectric devices.19 For instance, the photoelectric properties of MAPbI3 will suffer from rapid deterioration when exposed in ambient.20 The low formation energy and the hydroscopic nature of MAPbI3 result in the poor stability.21,22 Therefore, many adjustments are proposed to improve stabilities and performances of MAPbI3.23−27 Among these proposals, it is pointed out that mixed anions can improve stabilities and photoelectric properties.28,29 Using © XXXX American Chemical Society

Received: August 4, 2018 Accepted: September 21, 2018

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Figure 1. MA1−xCsxPbI3 single crystals with various Cs+ concentrations.

Figure 2. (a,b) XRD patterns of the MA1−xCsxPbI3 powders with various Cs+ contents. (c,d) XRD patterns of the MA1−xCsxPbI3 powders with various Cs+ contents after 40 days.

the XRD patterns seem similar to each other. However, the local enlarged graph from 27.5 to 32.5° in Figure 2b shows that the strongest peaks of the (004) and (220) planes shift to large angles as the Cs+ contents increase, suggesting that the cell parameters decrease. After fitting the powder XRD data, it reveals that the MA1−xCsxPbI3 are tetrahedral perovskites and the space group is assigned to I4/mcm. The unit cell parameters of a and b slightly decrease, and the cell parameter of c is steady, as shown in the inset in Figure 2b. The shrinage of the unit cell parameters is attributed to the entrance of smaller Cs+ into the crystal lattice because the ionic sizes of CH3NH3+ and Cs+ are 2.70 and 1.81 Å, respectively.33,37 On the other hand, to reveal the crystallinity of MA1−xCsxPbI3 single crystals, (100) facet XRD results are depicted in Figure S3. The positions of the diffraction peaks are basically the same, and no other peaks are detected, indicating that the (100) facets are smooth. The narrow similar full width at half-

solution. Therefore, to obtain high proportions of CsI to MAI, the concentration of MAPbI3 should be lowered below 1 M. However, the concentration of MAPbI3 in GBL can reach 1.5 M without CsI in solution. The lower concentration of MAPbI3 causes a longer induction period during the MA1−xCsxPbI3 single-crystal growth process. Another aspect is that CsI dissolved in GBL brings more I− ions, which are ultimately transferred to I2, and the yellow transparent solutions change to brown after single-crystal growth for a period, which is shown in Figure S1. Despite these two phenomena, MA1−xCsxPbI3 (x = 0, 2, 5%) single crystals with various Cs+ contents can be easily grown to a large size, as displayed in Figure 1. Here, it is necessary to note that we failed to grow large-size MA1−xCsxPbI3 single crystals with 10% Cs+ content in solution after trying several times. Only small single crystals were obtained, which is displayed in Figure S2. Figure 2a presents the fresh powder XRD patterns of MA1−xCsxPbI3 single crystals. From the whole point of view, 5834

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Figure 3. XPS of Pb-4f, I-3d, and Cs-3d levels in cleaved MA1−xCsxPbI3 single crystals for (a) 2 and (b) 5% Cs+ contents.

Figure 4. Optical properties of MA1−xCsxPbI3 single crystals: (a) absorption; (b) band gaps; (c) PL intensity; (d) time-resolved PL.

(220) plane at 28.5° appears, implying that some decomposition occurs in pure MAPbI3. We do not notice such phenomena happening for the Cs+-doped MA1−xCsxPbI3. The XRD data prove that the entrance of Cs+ into the crystal lattice enhances the stabilities of MAPbI3 single crystals. Another topic related to Cs+-doped MAPbI3 single-crystal growth is the entrance ability of Cs+ into a crystal lattice. The Cs+ contents in MA1−xCsxPbI3 (x = 2, 5%) single crystals were characterized by means of high-resolution XPS of I-3d, Pb-4f, and Cs-3d, as displayed in Figure 3a,b, respectively. The full

maximum of the (400) peak indicates that the crystalline perfection of MA1−xCsxPbI3 crystals is excellent. On the other hand, the stabilities of MA1−xCsxPbI3 are depicted by comparing the powder XRD after 40 days, which is displayed in Figure 2c,d. Apparently, the intensities of almost all diffraction peaks of the pure MAPbI3 powder decrease, suggesting that the crystallinity of pure MAPbI3 decreases. However, the XRD patterns of Cs+-doped MA1−xCsxPbI3 are nearly the same as the fresh powders. The enlarged graph in Figure 2d gives clear evidence that a satellite peak near the 5835

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Cs+) are adverse toward photoelectric performance, which will be discussed below. Figure 5a−c displays the dark currents and photocurrents of MA1−xCsxPbI3 single-crystal-based planar photodetectors

XPS spectra are provided in the Figure S4. It is amazing that the real contents of Cs+ in MA1−xCsxPbI3 single crystals are higher than those in growth solutions. The Cs+ contents are about 6 and 11 mole % for 2 and 5% Cs+ concentrations in growth solutions. We believe that this is attributed to the smaller spherical ionic size of Cs+ than that of CH3NH3+, which makes Cs+ prone to enter into the crystal lattice compared to CH3NH3+. The entrance of Cs+ into the crystal causes lattice shrinkage, which in return impedes CH3NH3+ from entering into the crystal lattice. Therefore, the Cs+ contents in single crystals are higher than those in the growth solution. Figure 4a,b presents the optical absorption and the calculated band gap curves of MA1−xCsxPbI3 single crystals. The absorption edges of MA1−x Csx PbI3 , which seem independent of Cs+ contents, are located at around 840 nm. However, their corresponding band gaps are slightly different. The band gap of a pure MAPbI3 single crystal is 1.44 eV, whereas it reaches 1.46 eV for a MA0.95Cs0.05PbI3 single crystal; it can be seen that Cs+ does increase the band gap after entering the lattice.28,38 On the other hand, the absorptions centered at 590 and 625 nm are slightly lowered. We believe that these local decreased absorptions are ascribed to the replacement of larger CH3NH3+ with smaller Cs+ in the crystal lattice and further modify the electron transition process between Cs+ and [PbI6]4−, which is similar to previous reports.33,39 Figure 4c shows the photoluminescence (PL) spectra of MA1−xCsxPbI3 single crystals. Both the emission intensities and positions of the PL peaks (located at ∼757 nm) for pure MAPbI3 and MA0.98Cs0.02PbI3 are similar. It should be noted that a blue shift happens for the PL peaks compared with the absorption edges, suggesting that the photon energy measured via PL is slightly smaller than the optical band gap measured by absorption. This phenomenon was revealed in previous reports and is ascribed to photon recycling in the thick MA1−xCsxPbI3 single crystals by reabsorbing the emission.40,41 However, the PL position of MA0.95Cs0.05PbI3 has an obvious blue shift compared with pure MAPbI3 and MA0.98Cs0.02PbI3, and the PL intensity is reduced. This is might be attributed to the broader band gap of the MA0.95Cs0.05PbI3 single crystal. Moreover, a broad weak emission between 900 and 1400 nm occurs in PL of the MA0.95Cs0.05PbI3 single crystal. This emission is ascribed to the recombination caused by defect levels that probably originate from vacancies defects.21,42 As revealed previously, the crystal lattice shrinks after Cs+ replacing CH3NH3+, which brings difficulties for CH3NH3+ or I− entering into the crystal lattice and increases the probabilities of CH3NH3+ and I− vacancies. The crystal lattice shrinkage and vacancy formation are illustrated in Figure 4d. To reveal the recombination dynamics of photoexcited species, the time-resolved PL spectra of MA1−xCsxPbI3 single crystals were obtained using an excitation wavelength of 443 nm, which is shown in Figure 4e. After biexponential fittings, the fast (τ1) and slow (τ2) time components related to surface and bulk recombination mechanisms are obtained; see the inset of Figure 4e. Once Cs+ replaces CH3NH3+ in the crystal lattice, the surface defect-related recombination time (τ1) is increased and the bulk defect-related recombination time (τ2) is decreased, showing evidence of a Cs+-passivated surface and bring more defects into the bulk MAPbI3 single crystal.38 The passivation effect is beneficial to enhancing the stability, which is in agreement with the XRD data. Too high defects (e.g., 5%

Figure 5. Dark currents and photocurrents of MA1−xCsxPbI3 singlecrystal-based photodetectors: (a−c) 0, 2, and 5% Cs+ contents, respectively. (d) Comparison of dark currents of MA1−xCsxPbI3 single-crystal-based photodetectors.

under various illumination powers using a 405 nm laser. The commonality of the three diagrams is that the photocurrents increase with rises of voltages and illumination powers. On the other hand, the photocurrents exhibit inflection points before which the photocurrents increase rapidly (low-voltage region) and after which they increase slowly (high-voltage region). For example, the inflection point of pure MAPbI3 in Figure 5a appears at about 1 V at 1 mW illumination, while it occurs at 3 V when a power of 2 mW is provided. In Figure 5b,c, the inflection points of both MA0.98Cs0.02PbI3 and MA0.95Cs0.05PbI3 photodetectors are 2 and 4 V at 1 and 2 mW illuminations, respectively. The larger inflection points for both MA0.98Cs0.02PbI3 and MA0.95Cs0.05PbI3 are attributed to the scatter effect between carriers and ions located on the crystal lattice.43 The incorporation of Cs+ brings crystal lattice shrinkage and then increases the scatter probability of carriers and ions, which shortens the carrier migration path. Therefore, to overcome the scatter effect and reach carrier equilibrium, a larger electric field (larger voltage) should be applied. On the other hand, compared with the pure MAPbI3 single crystals, the MA0.98Cs0.02PbI3 single crystals have a larger photocurrent than MAPbI3 single crystals under the same conditions. We believe that the photocurrent enhancement is on account of the surface passivation to restrict carrier recombination by introducing moderate Cs+. On the contrary, excessive Cs+ (e.g., 5%) causes more defects and promotes carrier recombination and consequently deteriorates the photoelectric properties. This can be also seen from the comparison of the dark currents of MA1−xCsxPbI3 single-crystal devices in Figure 5d. It is obvious that the dark current of the MA0.98Cs0.02PbI3 single crystal is lower, whereas the dark current of the 5836

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Figure 6. (a) I−V characteristic for hole-only devices. (b−d) Five continuous on/off cycles under various applied voltages of the MA1−xCsxPbI3 single crystals, x = 0, 2, and 5%, respectively.

content of Cs+ induces crystal surface passivation, such as MA0.98Cs0.02PbI3 single crystals. It inhibits the carrier recombination caused by defects. Therefore, the MA0.98Cs0.02PbI3 single crystal possesses better photoelectric properties: a lower dark current, higher photocurrents, and higher on/off ratios. Excess content of Cs+ (MA0.95Cs0.05PbI3) causes a slight increase of the energy band, brings vacancy defects, and results in shallow energy level absorption. The photoelectric properties of MA0.95Cs0.05PbI3 show that excessive Cs+ leads to deterioration of the photoelectric properties.

MA0.95Cs0.05PbI3 single crystal is greater than that of pure MAPbI3. Therefore, we conclude that the photoelectric properties of crystals after introducing a spot of Cs+ are improved, and they change worse in the case of introducing excessive Cs+. The space-charge-limited current (SCLC) measurements also prove that excessive Cs+ causes more defects, which is related to traps states and expressed by formula:VTFL =

eNtl 2 2εε0

(Figure 6a),41 where Nt is the trap

density, l is the single-crystal depth, e is the elementary charge, ε is the relative dielectric constant, and ε0 is the vacuum permittivity. Here, the single crystals were manufactured with quite similar depths (∼1.3 mm), and we presumed that the incorporation of Cs+ had little effect on the relative dielectric constant of MAPbI3. As can be seen, with an increase of Cs+ in the MAPbI3 single crystal, the VTFL values increase, suggesting that more Cs+ causes more trap states. Figure 6b−d represents five continuous on/off cycles under various applied voltages of MA1−xCsxPbI3 single-crystal devices under 1 mW power illumination, and the time interval between on and off is 10 s. The maximum on/off ratios of MA1−xCsxPbI3 (x = 0, 2, 5%) single crystals at 5 V are ∼14, ∼30, and ∼13, respectively. Compared with the pure MAPbI3 single crystal, the MA0.98Cs0.02PbI3 single crystal possesses a higher on/off ratio, including higher photocurrents and lower dark currents, which is consistent with the I−V data. However, the on/off ratio of the MA0.95Cs0.05PbI3 single-crystal device is lower than that of the MAPbI3 single-crystal device under a 5 V bias. This also validates that excessive Cs+ may lead to the increase of defects, thus affecting the photoelectric properties of the crystal. In conclusion, we successfully grew large-sized MA1−xCsxPbI3 (x = 0, 2, 5%) single crystals. The XRD results show that long-term stability of MAPbI3 is improved and the crystal lattice shrinks after introducing Cs+. An appropriate

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EXPERIMENTAL SECTION High-purity MAI were synthesized according to our previous reports.44,45 Other raw materials, PbI2 (≥99.5%), CsI (≥99.5%), and butyrolactone (GBL, ≥99.5%), were purchased from Aldrich. All raw solutes and solvents were purchased without any further purification. MA1−xCsxPbI3 single crystals were grown from GBL solutions by reacting a stoichiometric mixture of MAI, CsI, and PbI2 under continuous stirring at 60 °C. In this work, MAI and CsI were considered as a whole and were dissolved with PbI2 with a 1:1 molar ratio in 5 mL of GBL solution. The variables of Cs+ were controlled by changing the CsI contents. At the original time, a turbid system was obtained. Then, it was continuously stirred at 60 °C for 2 h until a clear yellow solution appeared. Seed crystals were added into the yellow transparent solutions, which were sealed and heated at 90 °C. It should be noted that the yellow transparent solutions gradually turned brown after growth for a period. After growing over 2 days, MA1−xCsxPbI3 single crystals with various Cs+ contents were obtained. The (100) facets of the MA1−xCsxPbI3 single crystals were polished to eliminate surface decomposition or pollutants. The 5837

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The Journal of Physical Chemistry Letters alloy hollow contact pattern masks were laid flat on the polished facets, and then the sputtering processes were carried out. Au interdigital electrodes (150 μm width) were formed on the blank area of the hollowed mask. X-ray diffraction of both MA1−xCsxPbI3 crystal planes and powders was carried out with a D/Max2500PC X-ray diffractometer in the range of 10−60° with Cu KαI irradiation at 40 kV and 100 mA. The step-scan mode was used during the XRD analysis with a step size of 0.02° and a dwell time of 5 s for each step. The UV−vis absorption spectra of MA1−xCsxPbI3 were obtained by a UV-2550 spectrometer with an integrating sphere over the spectral range of 500−1000 nm. The PL spectra were acquired by FLS-920 fluorescence spectroscopy, using a 470 nm excitation wavelength. The timeresolved measurements were carried out on an Edinburgh Instruments FLS980 with a nanosecond fluorescence spectrometer under a 443 nm excitation wavelength. The XPS data were collected by X-ray photoelectron spectroscopy (ESCALAB 250). The photoelectric properties of devices were carried out in a confined space at room temperature by using a photoelectric detection device Keithly 2450 and probe table. The SCLC measurements were carried out using a Au− crystal−Au structure, in which the single crystals were manufactured and polished to about 1.3 mm in depth. The photocurrent and the time-dependent on/off cycle measurements were obtained by using a laser source of 405 nm.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.8b02390. Detail description of the MA1−xCsxPbI3 single-crystal growth process, growth of higher Cs+ contents of MA0.9Cs0.1PbI3 single crystals, XRD of planar (100) facets and fwhm of the (400) peak of MA1−xCsxPbI3 single crystals, and XPS profiles of MA0.98Cs0.02PbI3 and MA0.95Cs0.05PbI3 single crystals (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.D.). *E-mail: [email protected] (T.Z.). *E-mail: [email protected] (H.C.). ORCID

Jianxu Ding: 0000-0002-3002-7023 Hongzhi Cui: 0000-0002-2212-0295 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Natural Science Foundation of Shandong Province (ZR2016EMQ10), the National Natural Science Foundation of China (No. 51202131), and the Distinguished Taishan Scholars in Climbing Plan (No. tspd20161006).

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DOI: 10.1021/acs.jpclett.8b02390 J. Phys. Chem. Lett. 2018, 9, 5833−5839