Electric-Field-Induced Dynamic Electronic Junctions in Hybrid Organic

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Article Cite This: ACS Omega 2018, 3, 1445−1450

Electric-Field-Induced Dynamic Electronic Junctions in Hybrid Organic−Inorganic Perovskites for Optoelectronic Applications Haizhen Wang, Meng Zhou, and Hongmei Luo* Department of Chemical and Materials Engineering, New Mexico State University, Las Cruces, New Mexico 88003, United States ABSTRACT: Organic−inorganic metal halide perovskites have attracted great attention as optoelectronic materials because of their low cost, relative insensitivity to defects, and solutionprocessible properties. However, some of their properties, such as thermal instability, toxicity, and current−voltage hysteresis still remain elusive. Ion migration, which has been proven to be a thermalactivated process, is regarded as one of the major origins of the hysteresis and thus detrimental to the long-term stability of the optoelectronic devices. Nevertheless, by using the external electric field to pole the perovskite, ion migration would be possible to be utilized to create dynamic electronic junctions. In this paper, electric-field-induced dynamic electronic junctions have been manipulated for photodetection and energy harvesting through the ion migration under external electric field. Ionmigration-induced p−n or n−p junction has been successfully created via tuning the polarity of the external applied voltage, which is used for photodetection with a relatively fast response. By freezing out of the nonuniformly distributed ions after migration at low temperature, we demonstrate that the ion-migration-induced dynamic junctions can function as an energy harvesting device with an external quantum efficiency of 20%.

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INTRODUCTION Organic−inorganic metal halide perovskites have been one of the most intensively investigated optoelectronic materials with applications in high-efficiency solar cells because of their low cost, relative insensitivity to defects, and solution-processible properties.1−13 Starting from 3.8%, a certified power conversion efficiency higher than 20% has been achieved just within a few years.14 Besides, the rapid progress in material synthesis and device fabrication as well as the remarkable optical and electronic properties of perovskites also have promoted the development of other optoelectronic applications, including lasers, 15,16 light-emitting diodes (LEDs), 17−19 transistors,17,20−22 and photodetectors.3,23−26 Despite the great advancement of perovskite-based optoelectronic applications, there are still some challenges for their further commercialization, such as thermal instability,27,28 toxicity,29 and current−voltage hysteresis.17,30−32 In particular, the presence of hysteresis not only prevents the precise evaluation of power conversion efficiency, but also leads to the long-term instability of the optoelectronic devices.33,34 Up to date, the origin of hysteresis in perovskites still has not been fully understood. Understanding the underlying mechanism of hysteresis is essential to produce high-efficient and long-term stable optoelectronic devices. Several possible explanations for hysteresis have been proposed, including slow ion migration, charge trapping−detrapping, and ferroelectricity, in which ion migration has been regarded as the most possible factor responsible for the hysteresis at room temperature.35,36 Ion migration has been proven to be a thermal-activated process.30 Therefore, with the decrease of temperature, the ion migration could be gradually frozen out resulting in the reduction of hysteresis at low temperature. © 2018 American Chemical Society

By utilizing the electric-field-induced ion migration, the creation and manipulation of p−n junctions in perovskite materials have been reported by a number of groups. Nevertheless, creating the electric-field-induced dynamic electronic junctions for photodetection and energy harvesting through ion migration under external electric field and symmetrically investigating how the dynamic electronic junctions evolve with temperature have not been fully explored yet. In this paper, we utilize the ion migration induced by an external electric field to create dynamic electronic junctions in hybrid organic−inorganic perovskites for photodetection and energy harvesting. We have successfully created p−n and n−p junctions in MAPbI3 via reversing the polarity of the external applied voltage, which has been used for photodetection with a fast response. Moreover, by freezing out of the accumulated ions at low temperature after migration, we have realized the energy harvesting devices with an external quantum efficiency (EQE) of 20%.

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RESULTS AND DISCUSSION The schematic of a MAPbI3 perovskite microplate device structure is shown in Figure 1a. Two symmetric 10 nm Cr/100 nm Au electrodes with a channel length of 20 μm have been adopted. Perovskite microplates on 300 nm SiO2/Si substrate have been obtained by converting PbI2 microplates, which is dropped onto the substrate, via vapor transport method. As shown in Figure 1b, ions across the MAPbI3 channel are evenly distributed without unbalanced ionic charges before the device Received: December 18, 2017 Accepted: January 23, 2018 Published: February 2, 2018 1445

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After a +4 V poling field has been applied for 60 s, the device behaves similar to a weak p−n junction, whereas by switching the poling field from +4 to −4 V, an opposite I−V behavior n− p junction has appeared, suggesting that the rectification of the I−V relationship is reversible depending on the polarity of the external poling field. This indicates that the ion migration induced by the external field provides an approach to create the dynamic electronic junctions for optoelectronic applications. The dynamic junctions created through the ion migration in MAPbI3 perovskite microplates can be utilized for photodetection. Figure 2a shows the output curves of the perovskite

Figure 1. (a) Schematic of a perovskite microplate device structure on 300 nm SiO2/Si substrate with 10 nm Cr/100 nm Au as electrodes; (b) schematic diagrams of ion distribution of the perovskite devices before (upper panel) and after (bottom panel) poling. Different colors represent the uneven ion distribution; (c) temporal response curves after positive and negative biasing at room temperature under illumination; and (d) output curves of the perovskite device with and without poling under 1.4 mW/cm2 light.

is poled by the external electric field. When an external bias is applied, the positive (I− vacancies) and negative (MA+ vacancies) ions start to drift to opposite directions under the applied electric field and accumulate at the interfaces between perovskite and electrodes, which leads to the formation of an ion-induced internal electric field (denoted as Eion). The direction of the ion-induced electric field is opposite to that of the applied external field and thus partially offsets it, thus resulting in decreasing current. Figure 1c shows the temporal response of current curves of a typical MAPbI3 microplate device after positive (+2 V) and negative (−2 V) bias voltages at room temperature under white light (1.4 mW/cm2) illumination. Completely opposite trends under positive and negative biases have been observed because of the symmetry of the electrodes in the device. When the positive bias is applied, the current jumps to a certain value and decreases gradually to a constant value, which is due to the generation of the ion-induced electric field inside perovskites, contradicting the external electric field. The ion-induced electric field (Eion) gradually increases and finally reaches a constant value, thus leading to the gradual decrease in current and finally a constant nonzero current. Once the bias is removed (after around 220 s), the current with an opposite sign has been recognized, which decreases to zero gradually. The observation of currents with opposite directions can be attributed to the ion-induced electric field, which has an opposite direction to the external applied field. With time, the accumulated ions start to diffuse and finally distribute evenly across MAPbI3. Therefore, the opposite-direction current gradually decreases to zero finally. The migration of ions can be considered as dynamic doping, and thus it is anticipated that dynamic electronic junctions could be created by controlling the migration of ions under the applied external electric field.37 The output (I vs V) curves of the devices exhibit dependence on the history of applied biases (Figure 1d). The I−V curve without poling under white light (1.4 mW/cm2) illumination indicates that the contact between perovskite and Au is not perfectly Ohmic (or Ohmic-like).

Figure 2. (a) Output curves of the perovskite device in dark and under illumination; (b) temporal response curve after 2 V poling at room temperature under illumination; (c) response speed of the perovskite device under an external bias of 2 V; and (d) response speed of the device under the ion migration induced by the external electric field.

device both in dark and under illumination. There is almost no detectable current under dark at room temperature, which might be due to the high crystallinity of the perovskite microplates and thus the lack of intrinsic charge carriers. However, under white light (1.4 mW/cm2) illumination, a nearly linear I−V behavior has been observed, similar to the results previously reported.34 A slight hysteresis for the output curve under illumination is observed, which may mainly be due to the ion migration induced by the external electric field. To further explore the photodetection properties of the dynamic electronic junction, we have measured the temporal response at room temperature under a 532 nm laser illumination with a power density of 0.5 mW/cm2 modulated by a mechanical chopper. First, a positive 2 V biasing was applied to pole the device, and we have examined the time response after the photocurrent reaches a constant value when the ion migration reaches the equilibrium condition. As shown in Figure 2c, which is taken from Figure 2b (510−517 ms portion), the rising time (defined as the time for the photocurrent rising from 10 to 90% of the maximum photocurrent) and the falling time (defined as the time for the photocurrent falling from 90 to 10% of the maximum photocurrent) are 650 and 780 μs, respectively.3 After removing the external electric field, the ion-induced electric field still exists, which causes the device to behave similar to a 1446

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Figure 3. (a) Output curves of the perovskite device in dark and under illumination at 77 K and (b) temporal response curves without poling and under negative and positive poling at 77 K. The device has not been either poled at room temperature or poled during cooling. Thus, there is no electric field induced by ion accumulation and no open-circuit voltage and short-circuit current at 77 K. (c) Output curves of the perovskite device in dark and under illumination at 77 K and (d) temporal response curves without poling and under negative and positive poling at 77 K. The device has been poled at room temperature, and thus there is a small electric field induced by ion accumulation, leading to a small open-circuit voltage and short-circuit current at 77 K. (e) Output curves of the perovskite device in dark and under illumination at 77 K and (f) temporal response curves without poling and under negative and positive poling at 77 K. The device is always poled during cooling; thus, there is a relatively large electric field induced by ion accumulation, giving rise to a larger open-circuit voltage and short-circuit current at 77 K. (g,h) The photocurrent under the ioninduced electric field under light illumination with different power for the device used in (e,f).

For the second case, first, we poled the device at room temperature which indicates that ion migration exists within MAPbI3 and then cooled the device immediately down to 77 K for further measurement. Different from the first case, we observed an open-circuit voltage of 0.13 V and a short-circuit current of 0.038 nA (Figure 3c), which can be attributed to uneven distribution of ions because of poling at room temperature. Although the external electric field has been removed, the migrated ions have not been completely restored to the original uniform distribution state during the cooling process. The residue of the migrated ions induces an electric field, which results in the measurable open-circuit voltage and short-circuit current. In addition, we observed a negative photocurrent of 0.038 nA under a 532 nm laser illumination even without applying any external electric field (Figure 3d). Similar to the first case, the applied external poling electric field cannot exert any influence on the ion-induced electric field, which was supported by the temporal response under illumination (Figure 3d). For the third case, the device was being poled with an external electric field while being cooled simultaneously. For this case, the diffusion of accumulated ions would be prohibited by the applied external electric field during the cooling process until 77 K, by when the ion migration has been totally frozen out. Thus, we expect a great number of ions accumulate near the electrodes at 77 K, leading to a large ion-induced electric field and thus large open-circuit voltage and short-circuit current. Figure 3e,f shows the output curves of the MAPbI3 perovskite device both in dark and under illumination at 77 K and temporal response curves without poling and under negative and positive poling at 77 K, respectively. Compared with the second case, similar results could be observed but the open-circuit voltage and short-circuit current are much larger for the third case, which are 0.95 V and 0.063 nA at 77 K,

photodiode, and thus shows a negative current because the direction of the ion-induced electric field is opposite to that of the external electric field, as discussed previously. The photocurrent would decrease gradually with the diffusion of the ions back to the original uniform distribution in the perovskite. From the enlarged portion of 570−577 ms in Figure 2d, the rising and falling time of the poled device are around 330 and 300 μs, respectively, which is two times faster than that under the external electric field. The faster time response of the device after switching off the external field indicates that the p− n junction is indeed formed after the device was poled. For a photoconductor, the photogenerated carriers might circulate inside the circuit for many times, leading to a higher gain and slower response compared with the photodiode. As the ion migration is a thermal-activated process, we expect that the ion migration can be frozen out at low temperature, and thus we can properly design the cooling process to realize the energy harvesting devices by using the ion-migration effect. For the first case, the as-fabricated device was directly cooled down to 77 K so that we can avoid the influence from ion migration because of the poling effect at room temperature, which usually cannot completely restore to their original uniform distribution state within a short time. When temperature decreased to 77 K, the ion migration is frozen out with uniform distribution, thus leading to zero ion-induced electric field. The electrical measurement at 77 K for this device shows that there is no observable photocurrent because of the zero ion-induced electric field under illumination by a 532 nm laser with a power density of 0.5 mW/cm2, supported by the zero open-circuit voltage and short-circuit current (Figure 3a). Because of the ion migration being frozen out at 77 K, poling with the external electric field would not induce any ion migration within MAPbI3 and thus no ion-induced electric field, leading to zero opposite-direction current (Figure 3b). 1447

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below 240 K, whereas above 240 K, it decreases with the decrease of the temperature. The behavior below 240 K might be because of the phase transition and temperature-dependent change of carrier mobility as previous reported.39 Above 240 K, the ion-migration starts to play a role in the ion-induced electric field, which leads to an external electric-field-dependent Irev. The decrease of Irev with the surrounding temperature might be because of the temperature-dependent electron/hole transport. Previous studies show that both ions and electron/hole transport are thermal-activated processes.34 With the decrease of the temperature, the electronic current exponentially decreases, which leads to the decrease of the observed Irev.

respectively (Figure 3e). The ion-induced reverse current under a 532 nm laser illumination (0.5 mW/cm2) also shows an independence of the external poling electric field, with a constant reverse current of 0.063 nA (Figure 3f), indicating the frozen out ion migration at 77 K. The corresponding EQE is estimated to be 20% for this case. We have also examined the photocurrent because of the ioninduced electric field under light illumination with different powers for the device used in Figure 3e,f. With the increase of the light power, the photocurrent would increase as well because of the increase of the photogenerated carriers (Figure 3g). Figure 3f displays the extracted photocurrent versus the illumination from Figure 3g. The photocurrent shows nearly a linear increase with the light power, indicating the feasibility to use the ion-migration-induced electronic junctions for photodetection. For the third case, when the device is poled during the cooling process, we have also studied the relationship between the cooling temperature and the open-circuit voltage and shortcircuit current (Figure 4a) as well as the photocurrent under

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CONCLUSIONS We have demonstrated the creation of dynamic electronic junctions via poling the MAPbI3 microplates for further photodetection and energy harvesting. By manipulating the polarity of the external electric field, we have successfully created p−n and n−p junctions for photodetection with a faster response speed. Furthermore, energy harvesting has also been achieved by freezing out the nonuniformly distributed accumulated ions after electric-field-induced migration at low temperature. Our studies not only help on the fundamental understanding of the ion migrations in MAPbI3 but also shed light on how to utilize the ion-migration effect for the potential optoelectronic applications.

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EXPERIMENTAL SECTION Preparation of Methylammonium Iodide Powder. Methylammonium iodide (MAI, CH3NH3I) powder, which was used as the intercalation source, was synthesized using a previously reported method.15 Specifically, MAI was synthesized by neutralizing 40 w/w % aqueous methylamine (MA) and 57 w/w % aqueous hydriodic acid with a molar ratio of 1:1, which was stirred at 0 °C for 4 h. The solvent was then evaporated at 60 °C, and the resultant solid was washed three times with diethyl ether followed by drying at 70 °C for 12 h. Fabrication of Perovskite Microplate Devices. PbI2 aqueous solution (0.1 g/100 mL) was prepared by dissolving 0.1 g of PbI2 in deionized water at 80 °C for 2 h and then cooled down to room temperature, resulting in the formation of the suspended PbI2 microplates. The Cr/Au (10 nm/100 nm) electrodes with a channel length of 20 μm were defined by photolithography onto the SiO2/Si substrates and followed by thermal evaporation and liftoff. Then, PbI2 microplate suspension was dropped onto SiO2/Si substrates randomly, and we can find the PbI2 microplates bridging the gap between two electrodes. Finally, the dispersed PbI2 microplates were further converted into CH3NH3PbI3 (denoted as MAPbI3) by vapor-phase intercalation.40 Electrical Measurement. Temperature-dependent electrical measurements were carried out in a probe station (Lakeshore, PS-100) coupled with a computer-controlled analogue-to-digital converter (National Instruments) and an SR570 current preamplifier. For the measurements under illumination, a white LED with a power density of 1.4 mW/cm2 was used.

Figure 4. (a) Temperature-dependent open-circuit voltage and shortcircuit current and (b) temperature-dependent photocurrent induced by ion-induced electric field. The device is always poled during the cooling process, which is the same as the one measured in Figure 3e,f.

the ion-induced electric field (Figure 4b) because the ion migration strongly depends on the surrounding temperature.30,34 As shown in Figure 4a, as the temperature increases, both of the open-circuit voltage and short-circuit current would decrease at first (77−140 K, stage I) until they reach a plateau (140−240 K, stage II) and then decrease (240−300 K, stage III) again. As previously reported, MAPbI3 undergoes a firstorder solid−solid phase transition from tetragonal phase to orthorhombic phase around 160 K depending on the thickness of perovskites.38,39 Within the orthorhombic phase, there are small inclusions of the tetragonal phase, the size of which decreases with the decrease of the temperature.39 As the temperature further decreases, the number of the grain boundaries continuously decreases, leading to the increase of the carrier mobility. Thus, the increase of the open-circuit voltage and short-circuit current might be due to the phase transition at stage I. At stage II, the phase transition has been completed, and the ion migration is still frozen out, thus both open-circuit voltage and short-circuit current show temperature independence. For stage III, when the temperature is above 240 K, the ions start to migrate, which can lead to the decrease of the ion-induced electric field and thus the decrease of both open-circuit voltage and short-circuit current.30,34 Figure 4b shows the temperature-dependent photocurrent induced by ion-induced electric field (denoted as Irev). The absolute value of Irev continuously increases with the decrease of temperature and does not depend on the poling electric field

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (H.L.). 1448

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Hongmei Luo: 0000-0002-9546-761X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS H.L. would like to thank the support from New Mexico EPSCoR with NSF-1301346.

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