High-performance CH3NH3PbBr3 Polycrystalline Wafer Prepared by Hot-pressing Method

Considering the drawbacks of the common methods for CH3NH3PbBr3 single crystal growth, such as the complicated and uncontrollable process, a simple hot-pressing (HP) method was introduced to fabricate CH3NH3PbBr3 polycrystalline wafers. The effect of hot-pressing temperature on the crystalline quality and corresponding optical and electrical properties of the CH3NH3PbBr3 polycrystalline wafers was investigated. The hot-pressing temperature for CH3NH3PbBr3 was optimized at 150°C, and the optimized CH3NH3PbBr3 wafer exhibited a low defect density (1.55×1010 cm−3), long carrier lifetime (1734 ns), and high carrier mobility (51.24 cm2V−1s−1) as a photoconductive detector. Furthermore, the detector showed a highly sensitive weak light response under 525 nm LED illumination with an optical power density of 84 nWcm−2, exhibiting a high responsivity of 63 AW−1, EQE of 1.5×104 %, and detectivity of 2.33×1013 Jones, and a fast response speed with a rise time of 17.7 μs and a fall time of 57.4 μs has been achieved.


Introduction
In recent years, there has been a notable focus on halide perovskites owing to their remarkable optoelectronic characteristics, including a substantial absorption coefficient, small exciton binding energy, elevated carrier mobility, and extensive carrier diffusion length [1] .These attributes confer upon them considerable potential in the realm of optoelectronic devices, encompassing applications such as solar cells, photodetectors, and high-energy radiation detectors.CH3NH3PbBr3 is a typical organicinorganic hybrid perovskite ABX3-type material, which has excellent water and oxygen stability, high photoelectric conversion efficiency, high intrinsic resistivity, good defect tolerance, and a wide working temperature range [2] , and has therefore received widespread attention from researchers.Currently, many methods for preparing CH3NH3PbBr3 single crystals have been studied, including Solution Temperature-Lowering (STL), Inverse Temperature Crystallization (ITC), Top-Seeded Solution Growth (TSSG) [3] , etc.However, the process of growing single crystals by these methods is usually complicated and slow.Meanwhile, the area of the single crystals is difficult to enlarge and the thickness and shape of the grown single crystals are uncontrollable, thus further cutting, polishing, and other operations are required to prepare the CH3NH3PbBr3 single crystals into photoelectric detectors.
Recently, the hot-pressing (HP) method has been increasingly used to prepare polycrystalline perovskite wafers, whose photoelectric properties and X-ray detection performance can approach or even exceed those of single crystals.Shrestha et al first proposed the use of mechanical sintering for the production of expansive-area and superior-quality perovskite thick films [4] .Since then, due to its advantages of simple operation, short processing time and controllable area, thickness and shape, the HP method has been used to prepare various organic, inorganic, 2D and 3D perovskites.Considered about the issues of high cost and poor shape controllability of traditional single crystal growth methods for CsPbBr3, Zhao et al utilized the HP method to produce phase-pure, regularly-shaped and dense CsPbBr3 wafer [5] .The resulting planar photodetector based on this wafer exhibited ultra-sensitive weak light response.Similarly, Li et al employed the HP method to prepare large area (1.33 cm 2 ) 2D perovskite (F-PEA)3BiI6 with a simpler preparation process and larger area than single crystal growth, while displaying excellent photodetection performance and stability comparable to 2D perovskite single crystals [6] .In addition, during the hot-pressing process of perovskite materials, there are many factors that can influence the properties of the resulting polycrystalline wafer.Hu et al investigated the effects of the thermal treatment time on the chemical composition of MAPbI3 wafers.Xiao et al discovered that the grain size in MAPbI3 polycrystalline wafers was influenced by both the hot-pressing temperature and pressure [7] .Witt et al found that a combination of pressure and temperature was beneficial for achieving high-density and enhanced crystallinity of MAPbI3 wafer [8] .According to the literatures, researchers have delved deeply into the optimization of CH3NH3PbI3 through the hot-pressing method.However, the hot-pressing conditions of different materials vary, and the impact of various parameters of HP on the internal grain size, density, and optoelectronic properties of CH3NH3PbBr3 wafers remains unclear.
In this work, we have prepared CH3NH3PbBr3 polycrystalline wafers by hot-pressing method.By adjusting the hot-pressing temperature, enabled the effective grain growth inside CH3NH3PbBr3 polycrystalline wafers, thus reducing the grain boundaries and the defects inside the wafers, and achieving the improvement of the photoelectric detection performance of CH3NH3PbBr3 polycrystalline wafers.

Experiment Details
CH3NH3PbBr3 powder was obtained through solution method according to previous literature [9] .30 mmol of CH3NH3Br and PbBr2 powder were weighed and dissolved in 30 mL DMF solution (i.e. the concentration of CH3NH3Br was 1 mol/L) to obtain a precursor solution.The solution was filtered through a 0.22 휇m polytetrafluoroethylene membrane filter to remove any precipitate, and then placed on a hotplate stirrer at 80 ℃ for 3 hours to precipitate CH3NH3PbBr3 microcrystals.The microcrystals were subsequently dried in a vacuum drying oven for 12 hours and then ground in an agate mortar to obtain the CH3NH3PbBr3 powder needed for hot pressing.0.176g CH3NH3PbBr3 powder was then weighed and placed into a cylindrical mould with a diameter of 10 mm.The mould was then placed in the hot-pressing machine, then heated and pressed for several hours.The hot-pressing pressure was set to 200 MPa, the heating time was set to 60 min, and the temperature was set to 50 ℃ ~ 180 ℃, lower than the melting point of CH3NH3PbBr3 [10] .The final step of the hot-pressing process was to cool the entire device naturally to room temperature.
The crystal structure analysis of the CH3NH3PbBr3 wafers was conducted using an X-ray diffractometer (Bruker D2 Phaser).Examination of the sample morphology was performed utilizing a scanning electron microscope (Quanta 200 FEI).Optical absorption spectra were acquired via a UV-Vis-NIR spectrometer (Lambda 950).Photoluminescence (PL) patterns and PL lifetimes were measured employing an Edinburgh Instruments FS5 (UK).Deposition of Au thin films was accomplished through vacuum thermal evaporation utilizing a ZZS500-3/G system.The I-V characteristics were assessed utilizing a Keithley 2400 parameter analyzer in conjunction with an LED (525 nm).Mobility measurements were carried out using a 520nm pulsed laser (Q-Spark) and an oscilloscope (Agilent 7.5G, USA).Noise current analysis involved extraction from the recorded dark current utilizing an Agilent B1500A with a current amplifier, followed by the application of a fast Fourier transform to compute the noise current.Response time evaluation was executed using an oscilloscope (Agilent 7.5G, USA) paired with a Keithley 2400 voltage source.

Temperature regulation of hot-pressed CH3NH3PbBr3
The effect of temperatures on the performance of CH3NH3PbBr3 polycrystalline wafers was investigated by setting the hot-pressing temperature at 50℃, 70℃, 150℃, and 180℃.To better observe the effect of temperature on the growth process of the grains, the powder was first compacted with pressure of 10 MPa and then placed in the hot-pressing machine for 60 minutes at four temperatures, during which the internal grain morphology of the samples was observed by scanning electron microscopy.As shown in figure 1(a), numerous original small grains with a size of approximately less than 1 휇m are present in the samples at 50℃ and 70℃.When the temperature is increased to 150℃ and 180℃, the number of dispersed small grains decreases, and most of them grow and fuse into large grains, with the largest grain size reaching approximately 50 휇m.The large grain size ensures a long diffusion length of the charge carriers, which is conducive to the effective transmission and collection of photogenerated carriers [11] .The XRD patterns of the CH3NH3PbBr3 wafers are shown in figure 1(b), and the diffraction peaks are consistent with those of the cubic CH3NH3PbBr3 crystal structure [12] .In addition, the relative peak intensities of the diffraction peaks in other directions, except for the diffraction peak in the (001) direction, are also relatively high in the spectra, indicating that four samples are all polycrystalline CH3NH3PbBr3.Furthermore, the grain sizes of the samples hot-pressed at different temperatures were compared by analysing the full width at half maximum (FWHM) of the first major XRD peak using high-resolution X-ray rocking curve analysis (shown in figure 1(c)).The FWHM of the (001) diffraction peaks for the samples hot-pressed at 50℃, 70℃, 150℃, and 180℃ were calculated to be 0.1163°, 0.1247°, 0.0967°, and 0.0980°, respectively.According to the Scherrer equation, the grain size is inversely proportional to the FWHM, and thus, we can conclude that the crystal size of the 150℃ and 180℃ samples is larger and has better crystalline quality than that of the 50℃ and 70℃ samples.The photoluminescence (PL) spectra of the CH3NH3PbBr3 wafers (hot-pressed at 200Mpa and 60min) are shown in figure 1(d), and the PL peaks of all four samples are located around 582 nm, which is close to the PL peak position of the single crystal CH3NH3PbBr3 reported before [13] .It is worth noting that the PL peak intensity of the 150℃ sample is the highest, which is attributed to the fewer defects in the 150℃ sample, resulting in less non-radiative recombination.Subsequently, Au-CH3NH3PbBr3-Au photoconductive devices with Au electrodes deposited on both the top and bottom surfaces of the wafers were prepared for electrical characterization.figure 2(a) shows the dark current curves of CH3NH3PbBr3 wafers with different HP temperatures, where the pressure and time were fixed at 200 MPa and 60 min, respectively.As the temperature increased from 50°C to 150°C, the dark current gradually decreased.Specifically, the dark current of the 150°C sample decreased by nearly two orders of magnitude compared to that of the 50°C sample, which can be attributed to the promotion of grain growth at higher temperatures.The photocurrent response curves shown in figure 2(b) were obtained under illumination from a 525 nm LED with a light power density of 2.5 mWcm -2 .The large photo current of CH3NH3PbBr3 samples were obtained by hot-pressing at 50°C, 70°C and 150°C.By calculating the on/off ratio of each sample at 30 V bias voltage (all samples with uniform thicknesses of 600 µm), based on both the dark and photo current, the on/off ratios of the 50 ℃ and 70 ℃ samples were found to be 7.706 and 5.770, respectively.However, the 150 ℃ sample was able to maintain a low dark current of 10 -6 magnitude while still obtaining a high photocurrent of 10 -4 magnitude, resulting in an on/off ratio of 174.712.The photocurrent of the 180 ℃ sample was the lowest among the four samples, and due to the defects caused by high temperature, its dark current was relatively high, resulting in an on/off ratio of only 11.260.In summary, the polycrystalline CH3NH3PbBr3 wafer hot-pressed at 150 ℃ had the largest grain size inside, and therefore the fewest grain boundaries, greatly reducing defects at the grain boundaries and resulting in a significant reduction in dark current.In addition, it exhibited the highest photoluminescence intensity and relatively large photo response current among the four samples, so the optimal hot-pressing temperature for CH3NH3PbBr3 was finally determined to be 150 ℃.The sample pressed under 150 ℃, 200 MPa and 60 min was later named Opt-CH3NH3PbBr3 to further investigate its photoelectric detection properties.

Photoelectric performance of CH3NH3PbBr3
Wafer under optimal hot-pressing parameters figure 3(a) displays the steady-state absorption spectrum of the Opt-CH3NH3PbBr3 polycrystalline wafer, exhibiting a sharp absorption edge at 555nm.The bandgap was calculated to be 2.18 eV using the Tauc plot, which is close to the reported value of 2.15 eV for the bandgap of CH3NH3PbBr3 single crystal [14] .Furthermore, the photoluminescence (PL) lifetime of Opt-CH3NH3PbBr3 was investigated using timeresolved photoluminescence (TRPL) spectroscopy.The bi-exponential decay model was fitted to the TRPL results to obtain the bi-exponential carrier lifetime.The fast and slow decay lifetimes correspond to the surface and bulk carrier recombination, respectively.As shown in figure 3(b), Opt-CH3NH3PbBr3 exhibits a fast decay lifetime of 317ns and a slow decay lifetime of 3100ns, yielding an average carrier lifetime of 1734ns, which is comparable to or even exceeds some of the performance of CH3NH3PbBr3 single crystal [15] .This indicates the suppression of defects within Opt-CH3NH3PbBr3 polycrystalline wafer and its excellent performance.The trap density (휂 ) of Opt-CH3NH3PbBr3 was studied using the space-charge-limited current (SCLC) method.Au/CH3NH3PbBr3/Au sandwich devices were fabricated for testing, and figure 4(a) shows the dark current I-V characteristics of Opt-CH3NH3PbBr3.
According to the SCLC model, 휂 was determined to be 1.55× 10 10 cm -3 .The hole and electron mobilities of Opt-CH3NH3PbBr3 were measured using the time-of-flight (TOF) method, as shown in figure 4(b, c).The hole mobility was found to be 51.24 cm 2 V -1 s -1 , and the electron mobility was 32.29 cm 2 V -1 s -1 , which is slightly higher than the reported mobility of CH3NH3PbBr3 single crystal [15] .The mobility-lifetime product (휇휏) of Opt-CH3NH3PbBr3 was measured using the photoconductivity method.
The measured photocurrent curves (and fitting curves) are shown in figure 4(d).The 휇휏 product of Opt-CH3NH3PbBr3 is 2.214×10 -4 cm 2 V -1 .According to the previously measured hole mobility and minority carrier lifetime, the calculated value of 휇휏 is approximately 8.8×10 -5 cm 2 V -1 , which is close to the 휇휏 value obtained from the Hecht equation.The light response performance of the Opt-CH3NH3PbBr3 polycrystalline wafer was further measured using a Keithley 2400 parameter analyser.Illumination was provided by a 525 nm LED light source with a range of light power densities from 84 nWcm -2 to 15 mWcm -2 , and the device bias voltage was set at 30V. figure 5(a) shows the variation of device response current and responsivity (R) with light power.The detector exhibited a response over a wide range of light intensities.Responsivity decreased almost linearly with increasing light power, consistent with previous reports [16] , as more charge carrier recombination occurs at high light intensities.It is worth noting that the responsivity (R) could reach 63 A W -1 under weak light irradiation of 84 nWcm -2 , indicating ultra-sensitive weak light photoelectric properties of the device.At the same time, the external quantum efficiency (EQE) and detectivity (퐷 * ) were calculated to be 1.5×10 4 % and 2.33×10 13 Jones, respectively, under weak illumination of 84 nWcm -2 (shown in figure 5(b) and figure 5(c)).Noise is a fundamental parameter of photodetectors.To further determine the resolution of weak signals, Fourier transforms were performed on the dark current at a bias voltage of 30 V. As shown in figure 5(d), the noise current (in) of the device at 1 Hz was frequency-independent, with a value of 3.364×10 -10 AHz -1/2 .This result indicates that the current noise of the device is mainly 1/f noise in the low-frequency range and thermal noise in the high-frequency range.The low-noise current of the device provides a foundation for the subsequent construction of high-performance detection units.To further investigate the response speed of the photoelectric detector, the instantaneous light response was measured using a 2 kHz square wave to control the 525 nm LED.As shown in figure 6(a) and figure 6(b), the rise time (trise) from 10% to 90% and fall time (tfall) from 90% to 10% were 17.7 휇푠 and 57.4 휇푠, respectively, which is comparable to those of CH3NH3PbBr3 single crystals.Table 1 summarizes the performance parameters of reported CH3NH3PbBr3 single-crystal and polycrystalline thin films, as well as their corresponding photoelectric detectors.It can be observed that the performance of our device is not inferior to that of CH3NH3PbBr3 single-crystal detectors, and some of the performance even surpasses that of single crystals.The polycrystalline wafer in this study has a long PL lifetime and a relatively high hole mobility, which ensures effective carrier transport.Therefore, the Opt-CH3NH3PbBr3 wafer exhibits excellent responsivity, surpassing most single crystals listed in the table, and has relatively high EQE and D * .In addition, the rise and fall time of the device are also lower than that of most CH3NH3PbBr3 materials.Based on the comparison in Table 1, it can be concluded that CH3NH3PbBr3 polycrystalline wafers hot-pressed at 150°C have better crystal quality and excellent photo-response characteristics.

Figure 1 .
Figure 1.(a) Cross-sectional SEM images of CH3NH3PbBr3 wafers hot-pressed at different temperatures; (b)XRD patterns of CH3NH3PbBr3 wafers hot-pressed at different temperatures; (c) Local magnification of the (001) diffraction peak; (d) Photoluminescence (PL) spectra of CH3NH3PbBr3 wafers hot-pressed at different temperatures.

Figure 2 .
Figure 2. (a) I-V curves in the dark of CH3NH3PbBr3 wafers hot-pressed at different temperatures; (b) I-V curves under the illumination of 525nm LED of CH3NH3PbBr3 wafers hot-pressed at different temperatures.

Figure 3 .
Figure 3. (a) UV-Vis absorption spectrum of Opt-CH3NH3PbBr3, inset is a Tauc plot; (b) PL lifetime curve of Opt-CH3NH3PbBr3 and its fitted curve.

Figure 4 .
Figure 4. (a) The dark I-V curve of the Au/Opt-CH3NH3PbBr3 wafer/Au device; (b, c) Normalized transient current curves of Opt-CH3NH3PbBr3 devices at different bias voltages.Inset: Inverse of bias voltage versus charge transfer time.Fig(b) calculates the hole mobility and Fig(c) calculates the electron mobility; (d)Photoconductivity of Opt-CH3NH3PbBr3 device.Fitting lines are also shown.

Figure 5 .
Figure 5. (a) Response current and responsivity of Opt-CH3NH3PbBr3 devices under 525nm LED illumination of different power desity; (b) EQE of Opt-CH3NH3PbBr3 devices under 525nm LED illumination of different power desity; (c) Detectivity of Opt-CH3NH3PbBr3 devices under 525nm LED illumination of different power desity; (d) Dark current noise in Opt-CH3NH3PbBr3 devices under bias voltage of 30V.

Figure 6 .
Figure 6.(a,b) Rise and fall times of the optical response of Opt-CH3NH3PbBr3 devices at 30V bias.

Table 1 .
Performance Comparison of CH3NH3PbBr3 Based Photodetectors.In summary, this work successfully controlled the internal grain size and photoelectric properties of CH3NH3PbBr3 polycrystalline wafers by regulating the hot-pressing temperature, resulting in highquality CH3NH3PbBr3 polycrystalline wafers.The wafer hot-pressed at 150°C had the largest grain size, ICCSS-2023 Journal of Physics: Conference Series 2613 (2023) 012003 single crystals via low-temperature inverting solubility: enhancement of mobility and trap density for photodetector applications Nanoscale 13(17) 8275-82 [18] Su J, Bai Y, Huang Y, Wang D, Kuang W and Xu L 2019 Morphology, optical and photoelectric properties of CH3NH3PbBr3 single crystal Physica B. Condensed Matter 571 307-11 [19] Liu H, Wei X, Zhang Z, Lei X, Xu W, Luo L and et al 2019 Microconcave MAPbBr3 Single Crystal for High-Performance Photodetector J Phys Chem Lett 10(4) 786-92 [20] Ye F, Wu H, Qin M, Yang S, Niu G, Lu X and et al 2020 High-Quality MAPbBr(3) Cuboid Film with Promising Optoelectronic Properties Prepared by a Hot Methylamine Precursor Approach ACS Appl Mater Interfaces 12(21) 24498-504 6.Acknowledgments This work is financially supported by National Key Research and Development Program of China(2021YFE0105900), the South Africa/China Joint Research Program (UID: 132796).