Article pubs.acs.org/JPCC
High Detectivity and Rapid Response in Perovskite CsPbBr3 SingleCrystal Photodetector Jianxu Ding,*,† Songjie Du,† Zhiyuan Zuo,*,‡ Ying Zhao,† Hongzhi Cui,*,† and Xiaoyuan Zhan† †
College of Materials Science and Engineering, Shandong University of Science and Technology, Qingdao 266590, China Advanced Research Center for Optics, Shandong University, Jinan 250100, China
‡
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S Supporting Information *
ABSTRACT: Band gap tunable hybrid organic−inorganic lead halide perovskites (APbX3, A = CH3NH3+ and NH2CHNH2+, and X = Cl, Br, or I) have attracted significant attention in optoelectronic- and photovoltaic-related fields on account of their outstanding optoelectronic properties. Single crystals of hybrid perovskites, such as CH3NH3PbI3 and CH3NH3PbBr3, were certified to be advantageous over thin films as photodetectors. However, their resistance toward heat and moisture hinders their future development. Fully inorganic perovskite CsPbBr3 stands a chance to fill the gap as a novel photodetector with perovskite structure. We revealed the growth of CsPbBr3 single crystal was of a 2D nucleation mechanism. Similarities of d values and octahedra arrangements along [101] and [020] orientations restricted single-crystal growth. Under optimized conditions, orthorhombic CsPbBr3 single crystals with (101) crystallographic facets were grown by using methyl alcohol as antisolvent from saturated DMSO solution. The optoelectronic properties of the single crystal were explored through a metal− semiconductor−metal photodetector device. Meanwhile, their steady and transient performances were also investigated. A highest responsivity of 0.028 A/W and a response time of 24 h with continuous stirring. The precipitation then turned yellow−green (Figure S1b). The saturated transparent solution was filtered and transferred to another container. CsPbBr3 single crystals were grown using methyl alcohol and ethanol as antisolvent. By controlling diffusion of the antisolvent vapor and growth temperature, the crystal growth process would differ. Device Fabrication. Planar photodetector was fabricated on (101) facet of CsPbBr3 single crystal. The surface was wellpolished and dust was cleared under nitrogen flow before device fabrication. Through conventional lithography (MABA6, SUSS) and wet etching, hollow aluminum contact pattern mask was fabricated and laid flat on the smooth (101) surface of CsPbBr3 single crystal. Au interdigitated electrodes were formed on the blank area of the hollowed mask during sputtering process (Discovery-550, Denton). The aluminum mask area was the corresponding light absorption area of the photodetector. Characterizations and Measurements. Both powder and planar of CsPbBr3 single-crystal X-ray diffraction patterns were collected in the range of 10−50° on X-ray diffractometers (D/ Max2500PC) with Cu KαI irradiation using tube voltage at 40 kV and 40 mA. The UV−vis spectra of CsPbBr3 powders were carried out on a UV-2550 spectrometer with an integrating sphere over the spectral range of 400−700 nm. A FLS-920 fluorescence spectroscopy had been used to collect photoluminescence (PL) spectra, and the excitation wavelength was 405 nm. The time-resolved measurement was carried out on an Edinburgh Instruments FLS980 with a nanosecond fluorescence spectrometer under 445 nm excitation wavelength. To depict the growth mechanism of CsPbBr3 single crystal through surface evolution, atomic force microscopy (AFM, Veeco Dimension Icon) in a tapping mode in an ambient atmosphere at room temperature was chosen to observe the surface structure. After the single crystal finished growth, the residual solutions on surfaces were cleared using filter papers at constant temperature. The optoelectronic properties of the planar photodetector were investigated inside of a copper shielding box in air at room temperature using an electrical properties measurement system (B1500A with preamplifier, Agilent) and a probe station (PM5, SUSS). Semiconductor laser diodes (LDs, 450 nm, 20 mW, InGaN-based) were chosen as the excitation light sources to collect photocurrents, and a high-frequency InGaN-based LED
3. RESULTS AND DISCUSSION We are aware of hybrid organic−inorganic perovskites being resolved in a variety of organic solvents with temperature reverse solubility, enabling them to be grown from organic solvents by increasing temperature,25,26 antisolvent method,27 and so on. Compared with hybrid perovskites, fully inorganic perovskite CsPbBr3 have a similar temperature reverse solubility in DMSO. It noted that the concentration could reach to 0.5 M in DMSO by dissolving equivalent CsBr and PbBr2 simultaneously at room temperature. However, a toohigh-concentration solution caused a fast growth process, resulting in simultaneous crystals. On the contrary, in the Cs− Pb−Br system, Cs4PbBr6 and CsPb2Br5 are unexpected compounds, which are prone to emerging with slight element deviation of 1:1 ratio of PbBr/CsBr.24 The titrating of methyl alcohol process as well as increasing temperature (to 40 °C, for example) was effective to eliminate the appearance of Cs4PbBr6, an unexpected yellow−green material with strong fluorescence under UV light (Figure S1c). Figure 1a illustrates the growth
Figure 1. Descriptions of CsPbBr3 single crystal. (a) Schematic representation of the single crystal growth process via antisolvent method. (b,c) Photos of CsPbBr3 crystals grown at room temperature (polycrystalline) and 40 °C (single crystal), respectively. (d) Powder and planar XRD patterns of orthogonal CsPbBr3 grown at room temperature and 40 °C, respectively.
process of CsPbBr3 single crystal using methyl alcohol as antisolution. The growth of CsPbBr3 single crystal was divided into two stages: nucleation and facet evolution, both of which were affected by diffusion velocity of methyl alcohol and growth temperature. For example, at a growth temperature of 60 °C, plenty of single crystals appeared separately and can hardly grow beyond 2 mm in length, while tiny single crystals tend to 4918
DOI: 10.1021/acs.jpcc.7b01171 J. Phys. Chem. C 2017, 121, 4917−4923
Article
The Journal of Physical Chemistry C
Figure 2. Growth mechanism analysis of CsPbBr3 crystal. (a,b) Surface morphologies of the bulk polycrystalline of CsPbBr3 observed via optical microscope. (c,d) Comparison of [PbBr6]− and Cs+ arrays along [101] and [020] orientations. (e) AFM topologies of 2D nucleation on (101) facet of CsPbBr3 single crystal. (f) Height files of the 2D nucleation and the derivative steps.
means of energy-dispersive X-ray spectroscopy (EDS) (Figure S2). The growth of CsPbBr3 single crystal depends on facet evolution, which is associated with the incorporation of growth units onto crystal surface. Therefore, to deeply understand the growth mechanism of CsPbBr3 single crystal, the surface structure should be resolved. The macroscopic surface structures of polycrystalline CsPbBr3 crystals grown at room temperature were observed under optical microscope, as shown in Figure 2a. Such polycrystalline was constructed by various tiny single crystals, each of which presented respective growth orientations. For example, the tiny single crystals marked with red rectangles exhibited layer-by-layer growth model with two vertical growth directions, which coincided the crystal morphologies displayed in Figure 1b. On the contrary, from Figure 2b, each single crystal performed layer-by-layer growth, whereas the connection of the individual single crystals seemed like grain boundaries. We believe that grain boundaries were [101] and [020] orientations. Two reasons could account for the claim. One is that the d values along the [101] and [020] orientations were close enough to form coherent boundaries. The other is that the connections between anionic polyhedra
be aggregate together and form opaque bulk polycrystalline at room temperature, as shown in Figure 1b, suggesting that nuclei velocity was too high. Accordingly, by controlling the diffusion velocity of methyl alcohol vapor into the growth solution (adjusting the pore number) and holding temperature at 40 °C as well, CsPbBr3 single crystals with various shapes were successfully grown (Figure 1c). The powder XRD patterns are displayed in Figure 1d. On the basis of the XRD pattern, the crystal structure is assigned to orthorhombic perovskite structure, and the space group is assigned to Pnma with unit-cell parameters a = 8.24 Å, b = 11.75 Å, and c = 8.20 Å. The XRD analysis shows that the crystal parameters are in agreement with the previous reported data.28 The planar XRD pattern was collected by scanning the exposed facet of CsPbBr3 single crystal, in which both (101) and (020) diffraction peaks appeared. The synchronized appearance of (101) and (020) is attributed to their close d values (5.81 Å for (101) and 5.86 Å for (020)), and common X-ray diffraction could not distinguish such difference. According to the detailed analysis in ref 24 the facet was determined and assigned to (101) facet. The stoichiometric ratio of the elements was characterized by 4919
DOI: 10.1021/acs.jpcc.7b01171 J. Phys. Chem. C 2017, 121, 4917−4923
Article
The Journal of Physical Chemistry C
Figure 3. Descriptions optical properties of CsPbBr3 single crystal. (a) Absorption and photoluminescence spectra of CsPbBr3 single crystal. (b) Time-resolved PL (λexc = 445 nm) for CsPbBr3 single crystal.
Figure 4. Photocurrents, responsivities, EQEs, and detectivities of the devices of CsPbBr3 single crystal. (a) Photoresponses. (b) Responsivities and EQEs. (c) Detectivities of the devices, respectively.
[PbBr6]− along both the [101] and [020] orientations were similar, and both were connected with alternating [PbBr6]− and Cs+ arrays, as illustrated in Figure 2c,d. Once growths of both (101) and (020) become entangled, bulk polycrystalline easily occurred and therefore the growth of a single crystal became more difficult. As for the (101) facet of CsPbBr3 single crystal, the surface structures were identified through AFM, as displayed in Figure 2e. Apparently, the growth of (101) facet of CsPbBr3 single crystal was controlled by the 2D nucleation mechanism and the motion of their derivative steps. By measuring the step heights in the white rectangle marked section, as shown in Figure 2f, the elementary step heights were determined to be 0.56 to 0.73 nm, equivalent to the d value of (101) facet of CsPbBr3. The results suggest that the minimum growth layer was one interplanar space. The 1.21 nm step height indicated that two elementary step could bunch together. The coexistence of elementary and bunched steps could incorporate with volume-varied growth units, which was beneficial to single-crystal growth.
Figure 3a present the optical absorption and PL spectra of CsPbBr3 single crystal. The absorption edge was located around 560 nm. By fitting the Tauc plot, the energy band gap (Eg) was calculated to be 2.16 ± 0.02 eV, lower than ∼2.32 eV of thin film29 or ∼2.25 eV of high-temperature grown single crystal.20 Our lower Eg value suggests that CsPbBr3 single crystal is more suitable in application as photodetector than polycrystalline film. Figure S3 displays the calculated band structure. The magnitude of the band gap is estimated to be 2.23 eV, which fits well with the experiment result. In addition, it shows a direct band gap with the highest point in valence band and the lowest point in conduction bands both occurring at the G point. From the PL spectrum with a 405 nm excitation, the PL peak centered at 540 nm was detected, with a red shift relative to nanowires (519 nm)19 and nanocrystals (511 nm),30 which also provide evidence that such CsPbBr3 single crystal could hold great promise for applications in photodetector. Moreover, the broad PL emission width was from 510 to 570 nm, implying that near-edge defect levels related to surface states played roles 4920
DOI: 10.1021/acs.jpcc.7b01171 J. Phys. Chem. C 2017, 121, 4917−4923
Article
The Journal of Physical Chemistry C
Figure 5. Time-dependent photocurrents and the switching times. (a) Five continuous on−off circles under various applied voltages of the devices. (b) Detailed response times under various applied voltages of the devices.
in the emission process.31 Figure 3b depicts the time-resolved PL decay, which is fit to biexponential decay. The faster decay time (τ1) and slower decay time (τ2) correspond to trapassisted recombination on surface and free carrier recombination in bulk, respectively. Both faster and slower decay times were shorter than other reported results of CsPbBr3 (τ1 = 4.4 ns and τ2 = 30 ns).24 Figure 4 shows the photocurrent, responsivity, external quantum efficiency (EQE), and detectivities of the CsPbBr3 single-crystal photodetector. The dark and photocurrents (Figure 4a) were obtained by using a 450 nm LD. At an applied voltage of 5 V, the dark current is as low as 7.98 × 10−8 A. Positive correlation can be found between photocurrents and applied voltages as well as illumination intensities. The tendency of dark current shows a mainly linear feature and is in accordance with the ohmic phenomenon. The responsivities R and EQEs can be calculated by using R=
IPC − Idark Pirra·S
(1)
R ·hc eλ
(2)
D* =
(3)
where R and Idark are responsivities and dark current, respectively.33 Transient measurements were performed to obtain switching characteristics of the device at different applied voltages (Figure 5a). The switch features of the devices were measured by employing a high-frequency LED with a dominant wavelength of 462 nm. A highest on/off ratio of ∼100 was achieved under 5.00 V bias. Significant overshoot can be seen while the illustration is on due to the sudden excited and departed carriers.21 Similar current values of subsequent cycles under different bias voltages reveal a weak built-in potential34 induced by ions and charged vacancies.35 Figure 5b shows the rise and decay properties of the device. With increasing times, a positive correlation with applied voltages could be observed, and no similar phenomenon was found during the decay processes. Compared with previous reports on the rise and decay time of polycrystalline CsPbBr3 film,21 the time scales
