MAPbBr3−xIx Crystals Improved by Accurate Solution-Grown Procedure for Alpha Particle Detection

High-quality organic–inorganic halide perovskite brings an opportunity for ideal nuclear radiation detection due to its large carrier mobility, lifetime, and heavy atoms. However, the relatively low bulk resistivity restricts its development for alpha particles and X/γ-ray detectors. Here, we report on the MAPbBr3−xIx (MPB:I) perovskite crystals grown using the modified inverse temperature crystallization method. Based on the measured supersolubility and solubility curves, the heating procedure was accurately designed to control the nucleation and growth rate. The resulting MPB:I single crystals exhibit higher bulk resistivity of 1.4 × 109 Ω cm, which is 10 times that obtained from the traditional inverse temperature crystallization method. Finally, an uncollimated 0.8 μCi 241Am @5.48 MeV alpha particles source was adopted to evaluate the MPB:I single crystals at room temperature condition. The electron mobility (μe) and mobility-lifetime product (μτ)e of (264.6 ± 6.5) cm2 V−1 s−1 and (2.6 ± 0.3) × 10−3 cm2/V, respectively, were obtained.

Although organic-inorganic halide perovskites MAPbBr 3 (MPB) single crystals (SCs) has been proven to be a sensitive material for X-ray radiation detectors, the detection performance is still restricted by its large leakage current [16,17]. Low bulk resistivity is significantly affected by the types and concentrations of defects in MPB SCs, which is mainly determined by the crystal growth. Some modified methods have been reported by different research groups, such as the modified antisolvent vapor-assisted crystallization methods [18], settled temperature and controlled antisolvent diffusion system [19], and seed-crystal inverse temperature crystallization (ITC) method [20]. The obtained resistivity of MPB SCs is mostly in the range of 1 × 10 7 -5 × 10 8 cm, which is still limited for the development of nuclear detectors. @ Therefore, currently, further increasing the resistivity is imperative. Huang's group [21] reported that the charge carrier concentration was reduced in the MPB bulk crystal by dopant compensation of chlorine so that the carrier mobility and lifetime have been improved. For I − -doped MPB:I, there are few literatures on the photoelectric performance, especially radiation detection [22,23].
Here, we report the MPB:I perovskite crystals grown by a modified ITC (MITC) method. The heating rate is controlled accurately based on the obtained solubility and supersolubility curves. Then, the X-ray diffraction (XRD), scanning electron microscopy, and UV-visible-near infrared spectrum measurements are employed for the characterization of the crystal quality. Finally, the photoresponse and charge carrier transport behaviors are evaluated by light-emitting diode light and alpha particles source, respectively.

Synthesis of MABr
The hydrobromide acid was added into the CH 3 NH 2 with the molar ratio of 1:1.2. After chemical reaction and full cooling, the well-mixed solution was evaporated at ∼70 • C in the evaporator for ∼6 h to obtain crude white MABr (MA = CH 3 NH 3 Br) powders. After washing by absolute ethanol (99.8%) and recrystallization in anhydrous diethyl ether (99.0%) for several times, the purified MABr crystalline powders were finally dried in a vacuum oven at 60 • C for 24 h.

Crystallization of MPB:I
A MITC method was employed to grow MPB:I single crystals. The MABr, PbBr 2 , and PbI 2 with the molar ratios of 7:6:1 were dissolved in single solvent DMF for ∼60 min in a magnetic stirring apparatus. The solution concentration was controlled at 1.6, 1.7, 1.8, 1.9, 2.0, and 2.1 M in DMF. After filtering using a 0.22-µm pore size filter membrane, the solution was for standby. To reduce the crystal growth rate of MPB:I, the controlling temperature was increased very slowly at 1-2 • C/day and the holding time (∼48 h) for 42, 45, 50, 55, 60, and 65 • C, respectively.

Characterization
XRD patterns of MPB:I single crystals were collected using D/Max2500PC with Cu K α1 from 10 to 60 • (2θ ) under 40 kV tube voltage and 40 mA tube current. Scanning electron microscopy and energy-dispersive spectral maps were measured using a JEOL (JSM-7500F) scanning electron microscope with an Oxford (X-Max50) energy-dispersive spectrometer. UV-VIS-NIR spectrum measurements were performed using a UVvisible spectrophotometry (UV-3150, Shimadzu, Japan). ZHD-300 high-vacuum resistance evaporation coating machine was used for Au electrodes preparation. Current-voltage (I-V) and current-time (I-t) measurements were evaluated using a Keithley 6517b electrometer/high resistance system. The charge transport properties of the MPB:I crystals were evaluated by an uncollimated 241 Am 5.48 MeV particle source at room temperature in air. Moreover, an ORTEC 710 high voltage supply, a charge sensitive preamplifier (ORTEC 142), and a shaping amplifier (ORTEC 570) with an optimized shaping time of 2 µs were used during the measurement. Finally, a standard Imdetek AMCA-01 multichannel analyzer was employed for the pulse height spectra acquisition.

Crystal Growth and Structure
Because the priorities of halogen solubility in precursor solution DMF are shown below: I > Br > Cl, the replacement of Br by I can maintain the driving force in nucleation and growth processes under higher supersaturation [22]. The supersolubility was measured by a series of solution under the initial concentrations from 1.6 to 2.1 M (2.1 M cannot be completely dissolved at room temperature). After being sealed completely, two batches of solution were placed in a temperature-controlled water tank, in which the temperature could be controlled with a maximum fluctuation of 0.1 • C.
For supersolubility measurement, the solution was heated at a rather low speed (1-2 • C/day). In the solution growth process, once we observed tiny crystals the first time, the concentration at this temperature was recognized as the supersolubility. Then, a series of the same concentration solution was chosen for solubility measurement. The solution was kept preserved at a certain constant temperature for a long holding time (∼48 h) to ensure that the crystal growth process is completed. Based on the solute mass (the raw material weight), the initial solution volume, and the weight of crystals, the concentration of solubility at a temperature range from 40 to 65 • C was obtained.
Three processing zones (nucleation zone, growth zone, and dissolution zone) were divided by the two curves (supersolubility and solubility curves), as shown in Figure 1A. First, in the dissolution zone (Part III), all the raw materials were dissolved in the solution. With the temperature increasing, the solubility of precursors decreases, and the solution enters the nucleation zone (Part I); the tiny crystals began to occur. Then, to avoid the new crystal nucleation, the solution is maintained in the growth zone (Part II).
During the traditional ITC growth, the rapid heating usually leads to higher nucleation rate; thus, the grains are in small sizes because of lots of nucleation. Besides, the larger supersaturation of MPB:I in the precursor solution also results in the fast nucleation. Therefore, the crystal size is limited, and the crystals are usually overlapped, as shown in Figure 1B.
To reduce the nucleation and growth rates, the heating process was taken into account. As a result, fewer numbers of MPB:I tiny crystals were observed at the beginning of the crystal growth process by the MITC method. After ∼10 days, less than four single crystals were observed in the precursor solution, with the average dimension over 5 × 5 × 2 mm 3 ( Figure 1C).
The color of MPB:I single crystals is dark red when iodide was introduced, from Figures 1B,C. Comparing Figures 1B,C, surface roughness and crystal morphology are improved by the MITC method, and the stacking-free MPB:I SCs are obtained.
Furthermore, the I content was characterized by energy dispersive spectroscopy, as shown in Figure 2A. The obtained ratio of Pb/Br/I is 13.45:36.96:0.57. The resulting value of x in MAPbBr 3−x I x is 0.04. According to the molar ratio of the raw materials MABr, PbBr 2 , and PbI 2 (7:6:1), the nominal doping ratio of x in MAPbBr 3−x I x is 0.286. The actual I content is lower than the theory value, which may be attributed to the larger ionic radius of iodine [ABX 3 ; ionic radius r X , 0.22 nm (r I ) > 0.196 nm (r Br )], making it hard to enter into the crystal lattice in MAPbBr 3 [24]. Figure 2B shows the XRD patterns of the MPB:I and MPB crystals. The sharp peaks in each curve suggest the pure phase of single crystals, without any impurity phase peaks. The (100) peaks of the MPB and MPB:I crystals appeared at 2θ of 15.22 and 15.51 • , respectively. Using the equation d = λ/(2sinθ ), the corresponding lattice constants are obtained. Therefore, it clearly suggested that the lattice can be adjusted by the compensation of the doped iodine. Furthermore, the UV-VIS-NIR spectra of the MPB (thickness of 4.64 mm) and MPB:I (thickness of 3.62 mm) were measured from 200 to 800 nm, as shown in Figure 2C. Based on the spectrum and Tauc law [25], the bandgaps of the direct-gap semiconductor were obtained, which decreased from 2.21 to ∼2.19 eV, as seen in Figure 2D.
The I-V curves of the above MPB:I single crystals are shown in Figure 3A. The resistivity of MPB:I-1 is 1.4 × 10 8 cm. As comparison, the reported resistivity of solution-processed Pbbased perovskite (MPB and MPI) single crystals exhibits a large range from 10 7 to 10 8 cm [26][27][28]. The resistivity is enhanced by the MITC method, which reaches (1.4 ± 0.02) × 10 9 cm in as-grown MPB:I-2 (as shown in Figure 3B).
To investigate the photoresponse performance, the MPB:I-2 crystal was illuminated by a 375-nm light-emitting diode UV light with the power of 14.09 mW/cm 2 at bias range from 1 to 50 V (as shown in Figure 3C). In Figure 3C, we can find that the values of dark current of 1 and 50 V are 2.4 and 315 nA,  respectively. There are more than 50 times difference between 1 and 50 V. The baseline shift phenomenon appeared as the bias increases. In our past research, the baseline shift phenomenon often happens on current-time curve measurements of 3D Pbbased perovskites samples. Maybe, Br ion migration leads to this problem.
According to the ion-beam-induced charge [29] and photoconductivity [10], the charge collection efficiency is not linear and reaches saturation gradually as the bias voltage increases. However, in this work, the I (I on -I off ) is linear and not saturated even at bias of 50 V (as shown in Figure 3D).

α Particle Detection Characterization
Charge carrier mobility (µ) and mobility-lifetime product (µτ ) of the MPB:I-2 sample are evaluated by the alpha particles. The measurements were carried out at room temperature and air condition. Generally, the incident depth of the α particles is near the surface of the crystal detector, with the distance of dozens of micrometers (<50 µm) [18,30]. Therefore, we use the simplified single-carrier approximation Hecht equation (Equation 1) [29] to estimate the electron mobility-lifetime product (µτ ) e .
where U is the applied voltage, and d is the device thickness. Simultaneously, we used time-of-flight technique to analyze pulse shape rise time and extract the carrier mobility (µ) of MPB:I detector. Figure 4A shows the transient pulse for the MPB:Ibased detector under 50 V. Generally, a 10-90% amplitude rise time τ 10−90% was selected for further processing. Through analyzing the pulse height rise time distribution, the carrier drift time (t dr ) was acquired. Under the bias from 10 to 50 V, the t dr were 5.01, 4.13, 3.16, 2.98, and 2.65 µs, respectively. The mobility µ is given by where V is the bias, t dr is carrier drift time, and d is the thickness of the MPB:I detectors.
The estimated electron mobility (µ e ) for the MPB:I detector is (264.6 ± 6.5) cm 2 V −1 s −1 , seen in Figure 4B, which is higher than the reported µ e for MPB and MPI single crystals with the value in the range of 24-167 cm 2 V −1 s −1 [2,4,[31][32][33][34][35] using the transient photocurrent response measurements. The high mobility of MPB:I may be attributed to the reduction in trapstate density and the charge carrier concentration and inhibition of ionic migration by I-doped compensation. Figure 4C shows the pulse height spectra vs. applied bias for MPB:I-3 detector, which were illuminated by 241 Am alpha particles source. With the applied bias increases, the cutoff edges of the curves reasonably shift to the high channel direction. The electron mobility-lifetime product (µτ ) e is estimated to be (2.6 ± 0.3) × 10 −3 cm 2 /V ( Figure 4D) by fitting the channel (cutoff edges of the curves) vs. the applied bias using the single-carrier Hecht equation.

CONCLUSIONS
High-resistivity MPB:I bulk crystals have been grown by the MITC method. Both supersolubility and solubility curves were acquired to tailor the growth rate by controlling the heating process. The Au/MPB:I/Au structure device exhibits a higher resistivity of 1.4 × 10 9 cm. The electron mobility-lifetime (µτ e ) products are (2.6 ± 0.3) × 10 −3 cm 2 /V by fitting the corresponding pulse height spectra as a function of the applied bias using the Hecht equation. The electron mobility is estimated to be (264.6 ± 6.5) cm 2 V −1 s −1 , using a 0.8 µCi 241 Am α particle source 5.48 MeV measured at room temperature under air environment.

DATA AVAILABILITY STATEMENT
All datasets generated for this study are included in the article/supplementary material.

AUTHOR CONTRIBUTIONS
XL and YX conceived the idea. XL, BZ, and YX designed the experiments. JF and YH synthesized MABr, the perovskite precursors, and crystals growth. MX carried out scanning electron microscopy and EDS measurements. XL and DZ performed the UV-VIS-NIR spectrum measurements, I-V measurements, and I-t measurements. XL and HZ performed alpha particle detection measurements. XL and YX analyzed the data. XL, DZ, and YX drafted the manuscript. BZ, WJ, and YX advised on the overall experiments with critical comments, and finalized the manuscript. All the authors contributed to interpretation of the results and revision of the manuscript.