Pressure Dependence of Mixed Conduction and Photo

Jun 14, 2017 - The pressure dependent photo responsiveness of MAPbBr3 was also studied by in situ photocurrent measurements up to 3.8 GPa. The mixed ...
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Pressure Dependence of Mixed Conduction and Photo Responsiveness in Organolead Tribromide Perovskites Huacai Yan,†,⊥ Tianji Ou,†,⊥ Hui Jiao,† Tianyi Wang,† Qinglin Wang,‡ Cailong Liu,*,† Xizhe Liu,*,† Yonghao Han,*,† Yanzhang Ma,§ and Chunxiao Gao† †

Institute of Atomic and Molecular Physics and State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, China ‡ Shandong Key Laboratory of Optical Communication Science and Technology, School of Physical Science & Information Technology of Liaocheng University, Liaocheng 252059, China § Department of Mechanical Engineering, Texas Tech University, Lubbock, Texas 79409, United States S Supporting Information *

ABSTRACT: The electrical transport properties of CH3NH3PbBr3 (MAPbBr3) polycrystals were in situ investigated by alternating-current impedance spectroscopy under high pressures up to 5.6 GPa. It is confirmed that ionic and electronic conductions coexist in MAPbBr3. As pressure below 3.3 GPa ions migration is the predominant process, while above 3.3 GPa electronic conduction becomes the main process. An obvious ionic-electronic transition can be observed. The pressure dependent photo responsiveness of MAPbBr3 was also studied by in situ photocurrent measurements up to 3.8 GPa. The mixed conduction can be clearly seen in photocurrent measurement. Additionally, the photocurrents remain robust below 2.4 GPa, while they are suppressed with pressure-induced partial amorphization. Interestingly, the photoelectric response of MAPbBr3 can be enhanced by high pressure, and the strongest photocurrent value appears in the high-pressure phase II at 0.7 GPa, which is similar to previous results in both MAPbI3 and MASnI3.

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Moreover, the ionic and electronic conductivities was changed considerably in MAPbI3 as MA was partially replaced by FA (HN = CHNH3+) to form MA1−xFAxPbI3.31 The above observations of changes in migrating ion species and ionic conductivities indicated that the ionic transport properties of OPTs are sensitive to external conditions in different ways. Pressure, like temperature and chemical constituent, is the most primary factor that is capable of radically tuning the physical properties and then endowing materials with new ones. However, how pressure affects the OPTs is still rarely understood so far, and what kind of novel properties can be induced by pressure is highly expected to be revealed, which motivates us to conduct further high-pressure studies. Very recently, OPTs have gained intensive studies on various physical properties under high pressures. Swainson et al. observed a phase transformation from cubic Pm3̅m to cubic Im3̅ phase around 0.9 GPa before the amorphization of MAPbBr3.32 Zhao et al. studied the structure and visible light response of MAPbBr3 upon compression up to 34 GPa. Besides the phase transformation from Pm3̅m to Im3̅ phase at 0.4 GPa, they also found a new phase transformation to orthorhombic Pnma phase at 1.8 GPa, and obvious visible-light response during the entire measured pressure range.33 However,

rganometal trihalide perovskites (OTPs) have set off a “OTPs rush” in the scientific community due to their excellent properties such as large absorption coefficient, high carrier mobility, long carrier diffusion length and surprisingly high tolerance to defects, etc.1−9 and have been widely used in the fields of photovoltaics, photonics, and optoelectronics.10−18 During the past few years, great efforts have been made to improve the power conversion efficiency (PCE) spectacularly from 3.8%19 to a record of 22.1%.20 Despite rapid improvement in PCE, two basic predominate issues, relatively fragile stability of OTPs and ambiguity in accurate determination of the PCE for OTPs based solar cells due to strong current−voltage (I−V) hysteresis, have never been resolved. Pioneering studies has revealed that ion-migration behavior in OTPs has important implications in terms of the long-term stability and performance of perovskite solar cell devices.21,22 Related perovskites such as CsPbI3, CsPbBr3 or MAGeCl3, MASnCl3 are acknowledged as halide ion conductors.23−25 In MAPbI3, any constituting ions (MA+, Pb2+, I−) may migrate even including the H+.26 Theoretical calculations using different computational models gave very different activation energy (EA) values and agreed on that I− ions are more readily mobile than MA+ and Pb2+ ions.22,27,28 Huang’s group directly observed the migration of MA+ in MAPbI3 films at 300 K under electric field by using energy-dispersive X-ray spectroscopy.29 In their later work, the migration of I− ions in MAPbI3 films was observed at 330 K.30 Almost at the same time, Yang et al. identified the migrating species was I− ions at 323 K.31 © XXXX American Chemical Society

Received: April 26, 2017 Accepted: June 14, 2017

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DOI: 10.1021/acs.jpclett.7b01022 J. Phys. Chem. Lett. 2017, 8, 2944−2950

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The Journal of Physical Chemistry Letters systematic studies on the migration behaviors of ions in OPTs are still rare under high pressures, resulting in a very hazy physical picture about electrical transport properties. We have even investigated the electrical transport properties of MAPbI3 under compression, and found a noticeable promotion of photocurrent by high pressures.34 MAPbBr3 belongs to the group of OTPs, but has smaller halogen ion radius and bigger interstitial sites than MAPbI3, and therefore quite different ion transporting properties can be detected with great possibility, due to the high sensibility of ions migration to the changes in crystal structure, ion radius, ion-hopping distance, and ion charge numbers.35 In this paper, the ion migrating mechanism of MAPbBr3 has been studied in detail for the final purpose of drawing a distinct physical picture for the electrical transport properties of OPTs. MAPbBr3 is a typical AMX3-type perovskite in cubic structure with Pm3m ̅ space group. The methylammonium cation (MA+) occupies the central A site surrounded by 12 nearest-neighbor bromine ions in corner-sharing PbBr6 octahedra. It has a direct energy band gap of 2.21 eV under ambient conditions, leading to a good absorption for visible light.36 In Zhao’s experiment, obvious ion migration was not observed in MAPbBr3, which is not consistent with previous theoretical calculations.22,27,28 The reason for discrepancy between experiment and calculations is still unknown. Thus, further detailed investigation is urgently necessary for elucidating the underlying mechanisms. Moreover, pressure can enhance the visible-light response in both MAPbI334 and MASnI3,37 but whether it is a universal law for all the OPTs still needs answering from photocurrent measurement on MAPbBr3. Based on the above issues, we carried out in situ highpressure alternating current (AC) impedance spectroscopy and photocurrent measurements on MAPbBr 3. In situ AC impedance spectroscopy measurement is very effective in distinguishing ionic and electronic conductions, because ions and electrons have quite different responses to varied-frequency AC signals as they pass through grains and grain boundaries. According to the results from impedance spectroscopy and photocurrent measurements, the electrical transport mechanisms and photo responsiveness in MAPbBr3 are discussed in the microscopic levels of ions and electrons, which is helpful for improving the conversion efficiency of MAPbBr3-based photovoltaic devices. The crystal structure and morphology of MAPbBr3 have been characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) before compression, as shown in Figure 1. All peaks in the XRD pattern can be well indexed into cubic structure with the space group Pm3̅m, indicating the sample is of high purity.36 Most of the samples are regular cubic polycrystalline particles with side lengths of 100−150 μm. Figure 2 shows the impedance spectra of MAPbBr3 in Nyquist representation under high pressures up to 5.6 GPa. At lower pressures, the impedance spectra consist of a well-defined semicircle in the high-frequency region and an upward inclined line in the low-frequency region, unlike our previous impedance spectroscopy measurements on MAPbI3, where only one semicircle can be seen.34 For a typical ionic conduction, the low-frequency part in the impedance spectrum is a straight line with an inclination angle of 45°, which can be attributed to the long-distance diffusion of ions.38,39 In MAPbBr3, however, the upward inclined straight line in the low-frequency region shows a slight bending toward the Z′ axis. Such a phenomenon can be

Figure 1. XRD patterns of MAPbBr3 at ambient pressure. The inset is the SEM image of MAPbBr3 particles.

explained as both ionic and electronic conductions existing in MAPbBr3, and they present parallel relationship in the equivalent circuit (see Supporting Information Figure S1). Compared with our previous study on MAPbI3, the semicircle in the impedance Nyquist representation is attributed to the local vibrations of charge carriers in high-frequency AC electric field.40 Ions and electrons transferring through electrodes are not detected by the impedance measurement, indicating that electrodes have very little effect within the experimental frequency range.34 In the process of fitting the impedance spectrum, we found that grain boundary also affects the ions and electrons migration, and it presents a series relationship with grain in an equivalent circuit (see Supporting Information Figure S1). At ambient conditions, we found a significant ion diffusion process in MAPbBr3 rather than in MAPbI3, meaning that ions migrate more easily in MAPbBr3. For both pure MAPbI3 and MAPbBr3, their ion valence states are the same, so two other reasonsmigrating ion radius and hopping distanceare responsible for the ion migration in MAPbBr3. (1) In MAPbBr3, Br− radius is smaller than I−, leading to the capability of Br− ions hopping along ion migration channels. According to recent theoretical calculations on MAPbI3,22 I− migrates along the I−−I− edge of PbBr6 octahedron, and MA+ migrates in the (100) plane along the ⟨100⟩ direction. Due to sharing similar crystal structure and symmetric group, similar migration channels should also exist in MAPbBr3. (2) By comparing the structural parameters between MAPbBr3 and MAPbI3, it can be seen that both MA+ and Br− have a shorter hopping distance.33,34,41,42 The hopping distance becomes shorter, which makes the migration of ions easier.35,43,44 In addition, Meloni et al.45 and Hwang et al.43 have proved both experimentally and theoretically that Br− ions has lower energy barrier for migrating in OPTs than I− ions, which strongly supports our experimental observations. As shown in Figure 3a, the impedance spectrum of MAPbBr3 at 1.0 GPa is selected as an example to demonstrate the process of deducing the electrical parameters of ions and electrons migration with the equivalent circuit in Supporting Information Figure S1. By fitting the impedance spectra at different pressures (Supporting Information Figures S1, S3 and S4), the pressure dependence of Ri, Re, Rb, and Rg were obtained as shown in Figure 3b and Supporting Information Figure S2. Rb is the bulk resistance to characterize the total difficulty of ions and 2945

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Figure 2. Nyquist plots of MAPbBr3 impedance spectra at different pressures: (a) 0−0.4 GPa; (b) 0.6−1.2 GPa; (c) 1.4−1.8 GPa; (d) 1.9−2.4 GPa; (e) 2.9−3.9 GPa; (f) 4.4−5.6 GPa, respectively.

phase II (cubic Im3̅) is mainly due to the shrinkage of PbBr6 octahedra. Additionally, the crystal parameters changed from √2a to a,33 indicating that the hopping distance decreases as pressure is applied in phase I. Thence, for the ion transport process, the decrease of Ri in Phase I can be contributed to a continuous decrease of hopping distance for ions migration as pressure increases. The shorter the hopping distance, the easier it is for ion migration. Nevertheless, although the hopping distance becomes shorter under high pressures, the interstitial spaces are correspondingly smaller, which results in the gradual closure of hopping channels and therefore is responsible for the increase of ionic resistance in Phases II and III. As pressure exceeds 3.3 GPa, Ri becomes extremely large in the fitting results, and Rb equals Re, indicating that the ionic conduction can be negligible, and electronic conduction is the only predominant process in MAPbBr3. From the Nyquist plots in Figure 2, it can also be seen that the typical characteristic of ion migrationthe upward inclined straight linestarts to disappear from 2.4 GPa, and at last, the semicircle, representing the electronic conduction, is the only detectable process. A

electrons migrating in bulk of MAPbBr3 (see Supporting Information eq S4). In the analysis of resistance change under high pressures, previous studies on the structural phase transition of MAPbBr3 were used for reference. The inflection points both in Ri, Re, and Rg at 0.4 and 1.7 GPa as shown in Figure 3b can be attributed to the phase transition sequence of MAPbBr3 from phase I (cubic Pm3m ̅ ) to phase II (cubic Im3)̅ and then to phase III (orthorhombic Pnma) under high pressures.33 Above 2.4 GPa, the changes in resistance reflect the onset of structural disorder and partial amorphization in MAPbBr3.33 From ambient to 2.4 GPa, Re is always about 2 or 3 orders of magnitude larger than Ri and Rb and nearly equal to Ri, meaning that the ionic conduction is the predominant process. Similarly, Rb is always about 1 order of magnitude smaller than Rg, meaning that the bulk conduction is dominant in MAPbBr3. Band-gap narrowing and broadening under high pressures lead to a decrease of Re in Phase I and increase in Phases II and III, respectively.33,46 According to previous studies, pressureinduced phase transition from phase I (cubic Pm3m ̅ ) to 2946

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suggesting that pressure suppresses the ion diffusion in MAPbBr3. As a kind of excellent light-absorbing material, the OTPs are frequently used in the fabrication of photovoltaic devices. Since pressure has brought dramatic change in the electrical transport properties of MAPbBr3, the pressure effect on photo responsiveness becomes naturally another investigated issue in our studies. Therefore, in situ photocurrent measurements on MAPbBr3 were conducted under high pressure. As shown in Figure 4a, MAPbBr3 exhibits obvious response to the light in the pressure range between 0.2 and 2.0 GPa as

Figure 3. (a) The impedance spectrum at 1.0 GPa (green). The red solid curve represents the simulated result. (b) The pressuredependent resistance of MAPbBr3. Re is the electronic resistance, Ri is the ionic resistance, and Rb is the bulk resistance. (c) The frequency of the inflection point at different pressures. Inset: an enlarged view from 1.7 to 3.3 GPa.

Figure 4. (a) The pressure dependence of photocurrents in MAPbBr3. (b) The detailed response of photocurrent at 0.7 GPa.

the switch turns on and off. It can be seen that the photonic responsiveness of MAPbBr3 increases first and then decreases with the increasing pressure, indicating a significant pressure effect on the photoelectric properties of MAPbBr3. Moreover, the response to light in phase II is obviously superior to the other phases, suggesting that pressure can improve the photovoltaic properties of MAPbBr3. From the high-pressure investigations, we can be inspired that the PCE of OPT-based devices can be improved via inducing the artificial stress, such as chemical doping and lattice-mismatched thin film fabricating. As the pressure exceeds 3.0 GPa, the photocurrent of MAPbBr3 could hardly be detected, which is believed from the complete amorphization of the samples. This behavior is similar to our previous study on MAPbI3.34 Figure 4b shows that the most obvious photo responsiveness of MAPbBr3 occurs at 0.7 GPa. The total current is the sum of dark current and photocurrent. We can see a dramatic increase/decrease in photocurrent as the light is turned on/off, reflecting the good

pressure-induced ionic−electronic transition has been found in MAPbBr3. Actually, another hidden parameter can be used to reflect the diffusion speed of ions, namely, the starting frequency at which the ions show obvious long-distance diffusion rather than local vibration under AC electric field. In Nyquist representation it corresponds to the inflection point that connects the semicircle and the inclined line, as shown in Figure 3a. From the definition of the starting frequency, it can be seen that the higher the starting frequency, the faster the ions migrate. The pressure dependence of starting frequency up to 3.3 GPa is shown in Figure 3c. Below 0.4 GPa, the starting frequency increases with the pressure increasing, indicating that pressure enhances the ion migration in MAPbBr3. Above 0.4 GPa, the starting frequency decreases with the increased pressure, 2947

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seen in parallel resistances, where the total resistance is determined by the smaller one. In summary, we have presented a detailed study on the electrical transport properties and photoresponsiveness of MAPbBr3 by in situ AC impedance spectroscopy and photocurrent measurements under high pressures. From the impedance measurement, the mixed conduction of ions and electrons was found in MAPbBr3. An ionic-electronic transition would take place at 3.3 GPa. By comparing the impedance data with previous structural studies on MAPbBr3, the inflection points found in the pressure dependence of ionic and electronic resistances can be attributed to the pressure-induced structural phase transitions. Ionic conduction is not a universal law in OPTs. Due to longer hopping distance and larger radius for MA and halogen ions in MAPbI3, ionic conduction was not observed. In photocurrent measurements it has been proved that the response to light in phase II is obviously superior to the other phases, suggesting that pressure can improve the photovoltaic properties of MAPbBr3. From the high-pressure investigations, we can be inspired that the PCE of OPT-based devices can be improved via inducing the artificial stress, such as chemical doping and lattice-mismatched thin films fabricating.

photoresponsiveness for MAPbBr3. When the light is on, lots of photogenerated electrons and holes appear instantaneously, leading to the sharp increase in photocurrent, and then migrate along the external electric field. The ions mobile in the dark and under the applied field of 5 V have migrated to the contacts to create double layers. Although lots of electrons and holes have been generated under illumination, charge recombination in MAPbBr3 samples also occurred immediately, which causes the sudden drop of the photocurrent. Finally, the photogeneration and recombination reaches a dynamic equilibrium. Another reason that should be responsible for the gradual decrease of photocurrent is related to lattice distortion caused by charge polarization. As a large amount of photogenerated charge carriers are injected into sample, an enhanced polarizability is induced by electronic carriers located in the conduction band and holes in the valence band.47 However, in a highly polar crystal, the electronic carriers become strongly localized by electron−phonon coupling. The self-stabilized electronic charge will drag the lattice distortion,48 and therefore sample resistance has slight increase with the distortion. When the light is off, the photogenerated electrons can be quenched on the spot immediately, which makes a sudden decrease of photocurrent. However, the distorted lattice will take a relatively long time to recover its original state, correspondingly causing the moderate increase of photocurrent. Compared with Zhao’s experimental results,33 ours are quite different in the following aspects: The electrical resistance of MAPbBr3 in our experiment was 5 orders of magnitude larger than Zhao’s, and no obvious photo responsiveness was observed when the pressure exceeded 3.8 GPa. However, in Zhao’s study, more considerable response was observed when the pressure reached 30 GPa. Three possible reasons could be responsible for the differences: (i) the purities of samples are different due to different chemical synthesis; (ii) the trap-state densities in the two samples are different; (iii) the I−V regimes selected for photocurrent measurement are different. In the current−voltage (I−V) curve measurement on MAPbBr3 crystals, three regimes can be found as voltage increase: the first regime has simple ohmic characteristic, taking place at low voltages; the second regime, locating at intermediate voltages, exhibited a rapid nonlinear rise in current and signaled the transition onto the trap-filled limit, where all the available trap states were filled by injected carriers; the third regime showed a quadratic voltage dependence of current in the Child’s regime at high voltages.36 In our experiment, a constant direct-current (DC) voltage of 5 V is applied, making our measurements confined within the first regime, and therefore our I−V response is ohmic. As depicted above, the I−V curve is linear, and the resistance is relatively large in the first regime. By contrast, Zhao’s I−V is believed with great possibility to locate in the third regime, where the current has a quadratic dependence with voltage, and consequently the resistance is smaller by orders of magnitude. Finally, we think the third reason is the most important to explain the differences between Zhao’s and our experiments. Since the third regime was selected for photoresponsiveness measurement, it is easy to find out why the obvious photoelectric response can be seen even at high pressures up to 30 GPa and the electrons conduction is predominant in Zhao’s experiment. It is because more electrons can be photogenerated in the third regime with high electric field and the electronic resistance is small enough to conceal the ionic conduction. Similar concealing effect can always be

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EXPERIMENTAL SECTION

Methylammonium lead bromide (MAPbBr3) with purity of >99% was obtained from Xi’an Polymer Light Technology Corp. and used without further purification. Considering that the samples are air-sensitive, the sample loading processes were carefully conducted in a N2-filled glovebox. In the experiment, a diamond anvil cell with a diameter of 400 μm was used to generate high pressure. In situ AC impedance spectroscopy and photocurrent measurements were measured by using symmetrical electrodes, which was integrated on the surface of one of the diamond anvils. The specific integration process of Mo film electrodes can be found in previous studies.49 One sheet of T301 stainless steel was preindented to a thickness of 70 μm, and a hole of 250 μm in diameter was drilled in the center of the indentation by laser. Then, a mixture of alumina power and epoxy was compressed into the indentation. Subsequently, another hole of 180 μm was drilled and served as the sample chamber. Pressure was calibrated by the R1 fluorescence peak of a ruby ball with a diameter of about 5 μm.50 In order to avoid the introduction of other interferences in AC impedance spectroscopy and photocurrent measurements, no pressuretransmitting medium was used. AC impedance spectroscopy was measured by a Solarton 1296 impedance analyzer equipped with Solarton 1260 dielectric interface. A 1 V sine signal was applied on the sample, and test frequency ranged from 0.1 to 106 Hz. The impedance spectra measurements are conducted in the dark. In order to improve the signal-to-interference-plus-noise ratio, we placed the DAC in a sealed metal box. Photocurrent was recorded by CHI660D electrochemical workstation, the illumination was AM 1.5 simulated sunlight provided by 3A class solar simulator (UHE-16, ScienceTech Inc.), and then a constant DC voltage of 5 V was applied on the sample. The interval time Δt between light on and off was 40 s. The MAPbBr3 samples were characterized by XRD (Rigaku D/max-Ra) at ambient conditions. The SEM images were obtained via a FEI MAGELLAN-400 microscope operating at 20 kV. 2948

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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.7b01022. Additional equivalent circuit fitting details, including the dependency of the parameters of the components in the equivalent circuit on the pressure, and the equation for calculating the impedance of the MAPbBr3 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (C.L.). *E-mail: [email protected] (X.L.). *E-mail: [email protected] (Y.H.). ORCID

Cailong Liu: 0000-0003-0702-7225 Author Contributions ⊥

H.Y. and T.O. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos. 11404133, 11674404, 11374121, 51273079, and 11604133). The Open Project of the State Key Laboratory of Superhard Materials (Jilin University, Grant No. 201612), and the Initial Foundation for the Doctor Programme of Liaocheng University (Grant No. 318051610).

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REFERENCES

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Letter

The Journal of Physical Chemistry Letters

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DOI: 10.1021/acs.jpclett.7b01022 J. Phys. Chem. Lett. 2017, 8, 2944−2950