Charge Transport Behavior in Solution-Grown Methylammonium Lead

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C: Energy Conversion and Storage; Energy and Charge Transport

Charge Transport Behavior in Solution-Grown Methylammonium Lead Tribromide Perovskite Single Crystal Using Alpha Particles Xin Liu, Hongjian Zhang, Binbin Zhang, Jiangpeng Dong, Wanqi Jie, and Yadong Xu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03512 • Publication Date (Web): 05 Jun 2018 Downloaded from http://pubs.acs.org on June 5, 2018

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The Journal of Physical Chemistry

Charge transport behavior in solution-grown methylammonium lead tribromide perovskite single crystal using alpha particles Xin Liu1,3, Hongjian Zhang1,3, Binbin Zhang1,3*, Jiangpeng Dong1,3, Wanqi Jie1,2,3, Yadong Xu1,2,3* 1

Key Laboratory of Radiation Detection Materials and Devices, 2State Key Laboratory of

Solidification Processing, & 3School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, China

ABSTRACT Methylammonium (MA) lead hybrid perovskite single crystal recently received attention as a potential radiation detection material. Here we report the MAPbBr3 bulk crystals grown by the modified anti-solvent vapor-assisted crystallization method. The growth rate is determined by diluting the anti-solvent, which results in the average size of MAPbBr3 crystals significantly increase from 2×2×1 mm3 to 15×15×5 mm3. The morphology evolution of MAPbBr3 crystals which is contributed by the growth anisotropy has been discussed according to the molar ratios of precursors and the bond theory. The resulting centimeter-sized MAPbBr3 crystals exhibit high resistivity (5.6×108 Ω·cm) at room temperature. The mobility-lifetime (µτ) products are measured under

241

Am (5.48 MeV) alpha particles irradiation, with the values of

2.2×10-4 and 4.2×10-4 cm2/V for electrons and holes, respectively. Simultaneously, the electrons and holes mobility are estimated to be 24.6 and 59.7 cm2·V-1·s-1, respectively, using the alpha source induced transient waveforms.

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1. Introduction Rapid development on X-ray and γ-ray spectrometers and imaging arrays result in tremendous opportunities in the field of astronomy, high energy physics, nuclear medicine, non-destructive inspection, and national security.1-5 To achieve a higher spatial and energy resolution, semiconductor detectors have exhibited significant advantages, due to the direct photoelectric conversion.6-7 Recently, there has been considerable interest in identifying new low-cost, heavy element, chemically robust compound materials for room-temperature radiation detection. Among the various semiconductors, the methylammonium lead hybrid perovskites (MAPbX3, where MA=CH3NH3 and X=Cl, Br or I)8-11 and full inorganic halide perovskites12-14 are receiving attention as potential radiation detection materials. The organometallic MAPbX3 has demonstrated tunable resistivity and superior carrier transport behaviors (carrier mobility and lifetime), large X-ray attenuation coefficient, and low-cost solution growth of single crystals.15-16 The past several years has witnessed the booming development of MAPbX3 thin film triggered by solar cells.17-20 However, to achieve a higher stopping power for hard X-ray or γ-ray, larger size MAPbX3 single crystal with high resistivity and mobility lifetime products are required. Since the pioneer work on the growth of centimeter-sized MAPbI3 bulk crystal by Dang et al.21 There are many techniques have been adopted to grow MAPbX3 perovskite single crystals from solution.22-26 To realize MAPbX3 detectors operating with low leakage current under large bias voltage, high resistivity at room temperature is an essential parameter, which can be evaluated by the current-voltage (I-V) curve on metal-perovskite-metal structure. For carrier transport properties, Shi et al.26 observed the charge carrier diffusion lengths exceeding 10 micrometers in MAPbX3 single crystals. Dong et al.22 reported that the diffusion lengths in MAPbI3 single crystals could exceed 175 µm under 1 sun illumination and exceed 3 mm under weak light for both electrons and holes. These results are mainly obtained by optical methods. However, the reaction theories of optical light and high energy particles or photons on matters are different. To evaluate the carrier transport properties of the nuclear detector grade MAPbX3 single crystals, consequently, it is critically necessary to understand the carrier mobility (µ) and lifetime (τ) by high energy photons or particles. The carrier mobility and lifetime product (µτ) is the primary parameter which can 2


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reflect both the charge collection efficiency (CCE) and the energy resolution. Generally, the µτ value is derived from the analysis of the maximal CCE as a function of applied bias voltage using the single polarity Hecht relation, illuminated by X-/γ-ray photons, or alpha particles.27-28 A time-of-flight (TOF) method is usually employed to measure the carrier drift mobility at room temperature by a pulsed electron beam, alpha-particle source, or fast laser pulse. These methods are proven effective for the wide band-gap semiconductor X-/γ-ray detectors, such as CdZnTe, HgI2, TlBr and so on.29-33 In this article, we report on the charge transport behaviors of MAPbBr3 single crystal using alpha particles. Centimeter-sized MAPbBr3 bulk crystals have been grown by anti-solvent vapor-assisted crystallization (AVC) method. The morphology of as-grown MAPbBr3 single crystal is discussed based on the nucleation and growth conditions. Then the MAPbBr3 single crystals were cut and processed into size-appropriate wafers for optical and electrical property studies, as well as alpha particles response measurements. Both the electron and hole mobility-lifetime product are obtained, and the mobility is calculated by analyzing the rise time distribution.

2. Experimental Section 2.1. Materials Methylamine (CH3NH2) (40 wt.% in water), hydrobromide acid (HBr) (48 wt.% in water), Lead bromide (PbBr2, 98%), N,N-dimethylformamide (DMF, 99.8%), dichloromethane (DCM, 99.8%), and other basic materials were purchased from Aladdin Reagent Ltd., China. All these materials were used as received without any further purification. 2.2. Synthesis of MABr (MA=CH3NH3Br) MABr was synthesized by chemical reaction between CH3NH2 and HBr with the molar ratio of 1.2:1. The HBr was added drop by drop into the CH3NH2 in a flask under ambient atmosphere in an iced bath for 3 hours, the resulting solution was evaporated at 60 °C in the evaporator for about 6 hours to remove the solvent. The white MABr crystalline powders were washed by absolute ethanol (99.8%) for 3 times and recrystallized in anhydrous diethyl ether (99.0%), finally dried in a vacuum oven at 60 °C for 12 hours. 3



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2.3. Crystallization of MAPbBr3 An anti-solvent vapor-assisted crystallization (AVC) method (as shown in Figure 1a) was employed to grow MAPbBr3 single crystals. The solution concentration was controlled at 1 M in DMF. Moreover, different molar ratios mixtures of MABr and PbBr2 from 1:0.66 to 1:1.5 were dissolved in DMF for 30 minutes in a magnetic stirring apparatus. After that, the solution was filtered by using a filter membrane with the pore sizes of 0.25 µm. To reduce the growth rate of MAPbBr3 perovskite, the anti-solvent DCM was attenuated by the DMF. 2.4. Characterization X-ray diffraction (XRD) patterns of both powder and single crystals of materials were collected using a D/Max2500PC with Cu Kα1 in the range of 10-60° (2θ) under the tube voltage and current of 40 kV and 40 mA, respectively. Au electrodes were thermally evaporated on two sides of the crystal sample by using a ZHD300 high vacuum resistance evaporation coating machine. Current-voltage (I-V) measurements were performed on the planar devices using a Keithley 487 Picoammeter and stabilized bias supply. The charge transport properties with alpha particles of the MAPbBr3 crystals were evaluated at room temperature. Pulse height spectra as a function of bias voltages were obtained, illuminated by an un-collimated

241

Am 5.48

MeV particle source. The pulse shape information was simultaneously recorded by connecting the pre-amplifier output to a high-speed waveform digitizer card. An ORTEC 710 bias supply, an ORTEC 142 charge sensitive pre-amplifier and an ORTEC 570 shaping amplifier with an optimized shaping time of 2 µs were used during the measurement. Finally, a standard multi-channel analyzer (Imdetek AMCA-01) was used for the pulse height spectra acquisition.

3. Results and discussion 3.1. Crystal Growth and Structure MAPbBr3 single crystals were grown by the AVC method, in Figure 1a. The anti-solvent DCM was volatilized into the precursor solution to reduce the solubility of MAPbBr3 in the solution. As a result, MAPbBr3 crystals gradually grew from the 4


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solution. When the pure DCM anti-solvent is employed for MAPbBr3 crystal growth, the growth usually ended after 1-2 days and tens of millimeter-sized cubic grains appeared (the inset in Figure S1). The grain sizes are limited because of too many nucleated seeds. The supersaturation of MAPbBr3 in the precursor is very large since the higher DCM anti-solvent pressure, which stimulates the fast nucleation. Therefore, to effectively reduce the growth rate and the number of nuclei, DCM and DMF mixture solution with the volume ratio from 1:0 to 1:2 was adopted. Fewer numbers of MAPbBr3 crystal seeds were observed in the beginning of crystal growth. Only 1~3 single crystals presented in the precursor solutions after ~30 days, with the mean length of side over 10 mm (Figure S2). Typical MAPbBr3 crystal with the size of 1.5×1.5×0.5 cm3 is shown in Figure 1c. Generally, well-defined cubic MAPbBr3 crystals with six {100} facets were obtained in Figure 1b and Figure S3. However, polyhedral MAPbBr3 crystals tend to be appeared when changing the growth situations, with eight {110} and two {100} facets identified by the single crystal and powder XRD patterns in Figure 1d. To elucidate the morphology evolution, the mole ratios of MABr/PbBr2 and volume ratio of DCM/DMF, were taken into account, in Figure 1e. Only the cubic MAPbBr3 crystals were obtained when the pure DCM is employed, which is independent on the MABr/PbBr2 ratio. These are mainly attributed to the anisotropic growth rates along and directions. To slow the growth rate, the mixture DCM+DMF anti-solvent was used. It is suggested that cubic MAPbBr3 crystals prefer to be formed under the MABr-rich and stoichiometric conditions (MABr: PbBr2=1:0.66, 1:0.75 and 1:1). When excess PbBr2 (MABr: PbBr2=1:1.5) adopted in the precursors, polyhedron MAPbBr3 crystals with extra {110} faces were appeared. The crystal growth anisotropy is mainly determined by the interfacial energies and formation energies of different crystal facets, when grown under the equilibrium condition. The resulting crystal morphology is determined by the relative growth rates of the crystal faces. The faces with faster growth rate will disappear, while develop the faces with lower growth rate. In general, the higher crystal growth rate resulted in 5



the annihilation of {110} faces and emergence of {100} faces for the well-defined cubic shape crystals. The arrangement of the atomic layers in MAPbBr3 crystal are (MABr)+(PbBr) and (MAPbBr)+(Br), respectively, along and directions, as shown in Figure S4. Because the ammonium cationic head group of the surfactants could strongly coordinate to Br ions by hydrogen bonds, the position of the Br ions in PbBr6 octahedral on the surfaces of MAPbBr3 crystals is very critical for the selective interaction.34 Thus, the MABr tends to locate on the surfaces of {100} faces. For grown under excessive PbBr2, the lead halide (PbBr2) colloids would be dissolved and produce a higher concentration of the Pb and Br ions in the precursor solutions.35 This situation promotes the chemical bonding along , which in turn weakens the growth anisotropy. Finally, both {100} and {110} facets were survived after growth, exhibiting the polyhedral shape. In addition, the non-uniform mass transfer conditions during crystal growth also play an important role in the morphology evolution to equilibrium. Moreover, there are other factors could be taken into accounted. The organic solvent can generate the organic-inorganic complexes which may affect the growth anisotropy. The as-grown centimeter-sized MAPbBr3 crystals with higher optical light transmittance were evaluated by the optical transmission spectra. Figure 1f reveals the MAPbBr3 crystal grown by the modified AVC method with a mean transmittance over 70% in the range of 600-1000 nm, indicating a high crystallize quality. The resulting band gap is examined to be ~2.21 eV by fitting the absorption edge according to the Tauc law,36 which is similar to the previous reports.26

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Figure 1. (a) The schematic diagram of MAPbBr3 crystals grown from the solution by AVC method. (b) and (c) The photograph of as-grown cubic and polyhedral MAPbBr3 crystals with the mixed anti-solvent of DCM:DMF=1:2. (d) {100} and {110} faces are identified by the single crystal XRD patterns. The powder XRD pattern of MAPbBr3 shows the space group Pm-3m. (e) The changes of the morphologies for single anti-solvent DCM and mixed anti-solvent DCM+DMF. (f) The UV-VIS-NIR spectrum of as-grown MAPbBr3 crystal using modified AVC method. The obtained band gap is about 2.21 eV.

3.2. Resistivity and photoresponse The electrical properties of MAPbBr3 single crystal were estimated by current-voltage (I-V) and current-time (I-t) measurements. Usually, a linear I-V curve for MAPbBr3 single crystal under higher bias is hard to achieve due to the low bulk resistivity and ionic polarization effect.37-38 In this work, Au/MPB/Au devices were fabricated by thermally evaporating Au contacts on MAPbBr3. The thickness of Au electrode is about 70 nm. Two representative MAPbBr3 single crystal samples grown by the modified and traditional anti-solvent vapor-assisted crystallization method are 7



named MPB1 and MPB2, respectively. The sizes of MPB1 and MPB2 single crystals (7.3×6.5×1.5 mm3 and 4.5×3.5×1.5 mm3). The linear relationship between the current and voltage from -1 V to 1 V was observed as shown by the inset of Figure 2a and Figure 2c. The corresponding bulk resistivity of MPB1 was estimated to be ~5.6×108 Ω·cm, which is comparable to that of the detector-grade CdTe crystals.39 However, the bulk resistivity of MPB2 was only ~2.6×107 Ω·cm. To increase the charge collection efficiency, higher operation voltage is usually adopted. Figure 2a reveals the linear I-V curve from -100 V to 100 V at room temperature. The leakage current is 400 nA at 100 V. Based on the I-V curve from -100 V to 100 V given in Figure 2a, we fitted the Ohmic coefficient using equation I=aVb. The obtained Ohmic coefficient b = 1.06 is close to 1, the contact performance is more consistent with the Ohmic contact. According to the I-V test results of Figure 2a, the nonlinear change of the current did not be observed due to the potential barrier at 100 V. The contact barrier between Au and MPB is very low. Through tunneling, carriers can pass through the barrier and Ohmic contact is formed. As a result, the most of the voltage is applied to the bulk crystal instead of the interface. So it can be assumed that the electric field inside the wafer is constant. In addition, the optical photoresponse of MAPbBr3 device, illuminated by a 365 nm LED light with the power of ~1 mW, was measured under 1 V bias. The ratio of photocurrent to dark current is ~2.5 from the I-t curve. The sharply rise and decline curves without exponential decay indicates few carrier trapping and de-trapping centers within the materials. For the low resistivity MPB2, the I-V curve only maintains good linearity at low voltage range. However, there is a noticeable electrical injection into the device under high voltages, in Figure 2c, which is probably attributed to the shallow level traps. The significant current decay (when the light is OFF) and the declination of saturated current (when the light is ON) indicate the carrier trapping and de-trapping centers within the materials, in Figure 2d. These phenomena indicate the fast crystal growth rate lead to more Br vacancies within the materials and then the carrier trapping and de-trapping enhance. In addition, more bromide ions can easily migrate to the interface between the electrode and the sample when the high bias voltage is applied, which may play a role in the charge 8


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recombination. Therefore, the photocurrent exhibits exponential decay.

Figure 2. I-V curve of MPB1 (a) and MPB2 (c) from -100V to 100V. Inset is I-V curve from -1V to 1V. Photoresponse of MPB1 (b) and MPB2 (d) under light pulses with 365nm and 1 mW measured under 1 V bias.

3.3. The principle of detecting charged α particles Figure 3a shows the diagram of the charge carrier transport process in MAPbBr3 crystal irradiated by alpha particles (He2+). Immediately, electrons and holes drift towards cathode and anode, respectively, driven by the external electric field. Due to the shallow penetration depth (about 18 µm28), amounts of electron-hole pairs were produced near the cathode region. Therefore, the resulting induced charge signal is predominantly due to the drift of a single carrier polarity.40 Generally, the CCE is the sum of two parts due to the separate electron and hole contributions to the total induced charge, which is dependent on the drift length (λ) of the charge carrier and the thickness (d) of the sample. For the non-saturated situation, drift length is related to mobility (µ), lifetime (τ) and applied electrical field strength (E) in the form of λ= µ·τ·E (1). According to the Hecht equation,40 9



−x

λh 

 λ CCE ( x ) = ⋅ 1 − e  + e  d   d λh

x−d  ⋅ 1 − e λe  

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   

(1)

where x is the distance from the position of electron-hole pair generation to the illuminated electrode, and λe and λh are the drift lengths of electron and hole, respectively. Due to the short range (x

Charge Transport Behavior in Solution-Grown Methylammonium Lead

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