Metal Halide Perovskites as Mixed ElectronicâIonic Conductors

Perspective pubs.acs.org/JPCL

Metal Halide Perovskites as Mixed Electronic−Ionic Conductors: Challenges and OpportunitiesFrom Hysteresis to Memristivity Wolfgang Tress Laboratory for Photonics and Interfaces, Institute of Chemical Sciences, Engineering, École Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland ABSTRACT: Metal halide perovskites are promising candidates for many classes of different optoelectronic devices. Apart from being a semiconductor, they additionally show ionic conductivity. It expresses itself in slow response times, reversible degradation, and hysteresis in the current− voltage characteristics of solar cells. This Perspective gives a condensed overview about experiments and theory on ion migration in metal halide perovskites focusing on its effects in solar cells. Apart from being a potential stability concern for photovoltaics, ion migration paired with the excellent optoelectronic properties of this material offers opportunities for novel devices such as optically controlled memristors and switchable diodes.

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milliseconds to seconds, but also for a short-term degradation on the time scale of minutes to hours.15 Both processes are (largely) reversible. However, no information is available on extended cycling, and the question remains whether a system with migrating ions is inherently unstable on the long-term due to some irreversibility coming along with continual compositional changes. This Perspective discusses the challenges but also the opportunities arising from ion migration occurring in metal halide perovskites. It gives a brief review on experimental and modeling studies regarding ion migration and its correlation with hysteresis. It touches upon module-related issues so far less considered by the perovskite research community. The outlook of this Perspective covers the possibilities the mixed ionic-electronic conductivity, if well-controlled, provides for novel devices such as memristors or switchable diodes. We start with a brief introduction to defect ions in perovskite crystals. The perovskite material of interest is an ABX3 compound (Figure 1a), where A is a monovalent cation, B is a divalent cation, and X is an anion. A can be both an organic or inorganic cation such as methylammonium (MA+, CH3NH3+), formamidinium (FA+, HC(NH2)2+), or Cs+. Commonly, B is Pb (also Sn) and X is a halide such as I or Br. These metal halide perovskites can show diverse intrinsic point defects, such as vacancies on each position, antisites, where atoms/ions are interchanged, or interstitials, i.e., atoms/ions located on nonlattice positions (Figure 1b). Many defects appear paired (e.g., an X− moves to an interstitial position and leaves a vacancy behind) and form neutral Frenkel defects (Figure 1c), whereas nonpaired,

etal halide perovskites have become a hot topic in solar cell research in the past few years because of the rapid increase in power-conversion efficiency exceeding 20% and the simple processability of the precursors from solution. Originating from the dye solar cell,1 the device design of perovskite solar cells evolved into a typical thin film architecture with a compact approximately half micron thick nanocrystalline perovskite layer sandwiched between (solid state) charge transport layers and planar electrodes.2 Two years after the development of these solid state devices with an efficiency of approximately 10%,3,4 a hysteresis in the current−voltage curve was discovered5 and described.6 It is observed in cyclic voltamogramms that show a loop instead of current−voltage curves independent of voltage sweep direction. Subsequent studies further characterized the hysteresis and found a slow transient response7,8 and a strong dependence on voltage sweep rate.9 Since the early studies, ion migration has been hypothesized as one possible reason for the slow response; others being ferroelectricity and charge carrier trapping.6 Detailed studies followed, providing a large body of evidence for the ion migration hypothesis. It was “confirmed” by modeling and microscopic simulations,10−12 although the other hypotheses as well. This finding was not all that surprising, as related compounds such as metal oxide perovskites are well-known for their ionic (oxygen) defect conductivity.13 Even inorganic halide perovskites had already been investigated for halide conduction.14

Detailed studies are providing a large body of evidence for the ion migration hypothesis.

Received: April 21, 2017 Accepted: June 12, 2017

Ion migration was not only proposed as a reason for the hysteresis, where processes happen on the time scale of © XXXX American Chemical Society

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DOI: 10.1021/acs.jpclett.7b00975 J. Phys. Chem. Lett. 2017, 8, 3106−3114

The Journal of Physical Chemistry Letters

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Figure 1. Mobile ionic defects in perovskite crystals. (a) Cubic representative of perovskite crystal structure ABX3. (b) Sketch of exemplary intrinsic Schottky point defects: vacancy VI, interstitial Ii, and dislocation IPb. (c) A Frenkel defect (consisting of VI and Ii) and ion drift directions due to an electric field (d) Comparison of activation energy in experiment and simulation. Table reprinted with permission from ref 17. Copyright 2016 American Chemical Society. (e) Migration path of I− along the equatorial-to-equatorial (i) and equatorial-to-axial channels (ii). (Source: ref 12) Image used in accordance with the Creative Commons Attribution license (https://creativecommons.org/licenses/by/4.0/). (f) Proof of ion (I−) conduction in pellet samples under applied voltage (positive at Pb). Reproduced with permission from ref 18. Copyright 2015 WILEY-VCH.

so-called Schottky, defects (e.g., an X− vacancy) are less likely. In principle, defects can move under external forces such as a diffusion gradient or an electric field in the case of charged defects. This has been known for oxide perovskites, where, e.g., mobile oxygen vacancies have been extensively investigated.13,16 Whether a defect is “relevant” here depends on its concentration and its ability to migrate. The first is influenced by processing conditions such as stoichiometry and crystal growth processes. Additionally, the feasibility of creating defects can be described thermodynamically. The lower the so-called formation energy, the more likely it is to obtain a higher equilibrium concentration of thermally generated defects. The ability of defects to migrate, on the other hand, depends on available migration paths and temperature. Density functional and molecular dynamics simulations can give insights into these processes. Activation energies can be calculated that need to be overcome, e.g., for the defect to migrate along a certain trajectory.11,12 For MAPbI3, several modeling studies have been performed yielding considerably varying results regarding the absolute values of the activation energies of ionic defects (Figure 1d).17 However, they consistently predicted rather low formation19,20 (0.1 to 0.2 eV) and migration (0.1 to 0.6 eV) energies for halide vacancies. Figure 1e depicts different potential migration paths of I−, where the activation energies depend on how much lattice distortion is required during movement of the defect.

An unspecific activation energy was the parameter of choice to compare experimental data with simulation data.11,12 Experimentally, this activation energy was obtained by measuring the transient response of an electrical quantity such as the current of a device upon a voltage step. Assuming an Arrhenius-type behavior, the energy is extracted by plotting the inverse of the response time as a function of the inverse temperature and fitting it with a linear function. In line with the simulations, experimental values show a large spread, making this approach problematic (Figure 1d). There are several reasons for this. First, a small temperature range is usually investigated, meaning that the Arrhenius relation cannot be clearly identified. Second, the device response is a convolution of several factors, and it cannot be always assumed that it directly represents the microscopic process of ion migration. It can be influenced by the starting position and source of the defects, their diffusion and drift contributions, an additional temperature activated formation, etc. Third, the systems commonly measured are not single but nanocrystalline. The resulting grain boundaries can potentially either increase or decrease the mobility of ionsa question that seems to not be answered yet. Performing various transient experiments as a function of temperature in our group, we found that it is not possible to assign a value for the activation energy. The device-to-device variation was too large, the value would depend on the exact experimental procedure, and sometimes even the response time would not follow an Arrhenius behavior. Furthermore, often multiexponential fitting had to be applied to reproduce the measured traces. Extracted “activation energies” could be anywhere between 0.05 and 0.5 eV.21 Nevertheless, experimental evidence for iodine migration was obtained by designing special samples of MAPbI3 pellets,

Experimental values of the activation energy show a large spread, making a comparison with simulation problematic. 3107



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Figure 2. Rate dependent current−voltage hysteresis. (a) For selective contacts, fill factor (FF) is affected. (b) For nonselective contacts (TiO2 blocking layer omitted), the open-circuit voltage (Voc) is affected as well due to surface recombination. Reproduced from ref 9 with permission from The Royal Society of Chemistry. (c) Transient response in current (blue) during stepwise voltage sweep (gray) for forward (FS) and backward (RS) scan. Reproduced with permission from ref 8. Copyright 2014 American Chemical Society. (d) Simplest band diagram (under illumination, quasi-Fermi levels dashed, CB = conduction band edge, VB = valence band edge) showing the screening effect of mobile ions (coded with colored + and −) and how charge extraction (electrons e−, holes h+) toward the contact is hindered in the case of a fast forward sweep. Note that, if there was no electronic built-in potential, forward bias would quickly result in the accumulation of the reverse ionic charge at the contacts compared to what is shown here. The effects on the hysteresis would remain similar.

where the ion conductivity was measured and visualized, e.g., by using electron and ion selective contacts. Figure 1f shows PbI2 formation at the Pb/perovskite interface due to I− migration (equivalently MA+ into the opposite direction) upon positively biasing the Pb electrode.18 In that study, the diffusion coefficient was found to be in the range of 10−8−10−7 cm2 s−1. In the following sections, we address mobile ionic defects in nanocrystalline perovskite films employed in solar cells. The interest in defect ion migration was triggered by the observation of a hysteresis in the current−voltage (JV) curve of perovskite solar cells, which made the determination of the efficiency from a JV sweep problematic (Figure 2a). It was found that the hysteresis is due to a slow transient response (Figure 2c) and mainly dependent on scan rate and prebias conditions, meaning governed by the voltage (rather than by light).9 It was observed in all kinds of perovskite compositions and solar-cell architectures, although to a strongly varying extent. In particular, the so-called inverted architecture is less prone to hysteresis compared to devices where perovskite is deposited onto an electron selective oxide layer.22 However, hysteresis still becomes pronounced at lower temperatures,23 and transient experiments reveal that the underlying processes causing hysteresis are still present.24

Initially, ferroelectricity was discussed vividly as the reason for hysteresis, because ABX3 compounds such as BiFeO3 show ferroelectric behavior and spontaneous polarization due to a polar distortion of the crystal structure when transitioning from a high symmetry to lower symmetry state.25 For MAPbI3, the dipole moment of the polar MA cation was hypothesized to contribute to ferroelectricity if the MA is aligned in switchable domains.26 However, more detailed simulations showed that the MA cations are randomly oriented, and MAPbI3 is not ferroelectric at room temperature.12,27 Therefore, polarization switching effects observed experimentally are proposed to be interpreted with ionic motion consistent with their characteristic time scales and large hysteretic charge densities.28,29

A rather broad consensus has been reached that the hysteresis is a result of mobile ions (and their vacancies). Regardless of the remaining debate on the existence of piezoand ferroelectric domains in MAPbI3,30 there is no experimental evidence of their effect on the performance of photovoltaic devices. Therefore, a rather broad consensus has been reached 3108



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role, although slow charge transport and/or extraction probabilities (e.g., because of grain boundaries) paired with ion migration are sufficient to reproduce hysteresis. Apart from being visible in different fill factors between reverse and forward scan, hysteresis can affect the open-circuit voltage as shown in Figure 2b. This effect can be explained by a modified probability of surface recombination.9 In the case of nonselective contacts, the “wrong” charge carrier can recombine at the “wrong” contact (e.g., a hole at the FTO in a device of standard geometry). This process, requiring the wrong charge carrier diffusing to the wrong electrode, is enhanced in the case of a screened electric field as sketched in Figure 2d. Furthermore, energetic barriers may be modified by the displaced ions as well.37 Even an inverted hysteresis is possible, where the forward scan results in a higher fill factor and open-circuit voltage compared to the reverse scan.38 These observations show the complexity of the interplay between the collection and recombination of photogenerated charges and the movement of ions.38

that the hysteresis is a result of mobile ions. In accordance with the results of the pellet samples, evidence for iodine migration (via vacancies) for thin film devices was also reported.31 However, an experimental direct proof of reversible ion migration, e.g., obtained by mapping techniques in devices under operation is still missing. Therefore, though anticipated, it is not entirely proven experimentally that reversible hysteresis effects are due to a displacement of ions from one contact to the other. Such a long-distance ion migration was only experimentally observed in noncycled experiments, where a long-term bias voltage was applied just once and ion distributions were just measured before and after the biasing. On the other hand, device modeling was successful in describing experimental data such as current or voltage transients.32 These modeling approaches are based on the assumption of ionic charge (representing predominantly halide vacancies) moving by drift and diffusion in the perovskite and being blocked at the interfaces to the contacts. All existent models (first formulated qualitatively6,9 and later refined by numerical (drift-diffusion) device modeling24,33−36,32) say essentially the same thing: ionic charge is slowly (with a diffusion coefficient of 10−12 cm2s−1) redistributed upon the application of a voltage and tends to screen the electric field in the perovskite. If the prebiasing voltage was more negative than the measurement voltage (such as is the case for a forward JV scan) the charge collection efficiency is reduced (Figure 2d). Therefore, a larger number of charges recombine instead of being collected, and the fill factor decreases (Figure 2a). All further claims postulated in the literature regarding very detailed conditions that need to be met are not proven by control experiments, and discussion is ongoing among the modeling community whether, e.g., a combination of surface states and mobile ions is required to observe hysteresis. So far, voltage or light-driven formation of ionic defects has not yet been considered in device simulations. Furthermore, it is unclear what the starting situation of distribution of ions is (so far homogeneous distribution assumed) or what the equilibrium distribution in the dark under 0 V is (so far assumed that ions are piled up at contacts to compensate for a presumed electronic built-in potential) (Figure 2d). Traps at the contacts may play a

Complex interplay between ionic and electronic response causes hysteresis. As described so far, the hysteresis is rather a problem than a feature. Consequently, research has focused on avoiding it and putting effort into fabrication of “hysteresis-free” or “hysteresisless” devices. It seems that anything that helps to increase charge carrier extraction probability, such as lager grains and less grain boundaries39 or high-conductivity interface materials, decreases hysteresis.40,41 These measures do not necessarily act on the underlying process but rather on the expression of hysteresis, probed by (photogenerated) charges, as illustrated in Figure 3. The more field-independent charge carrier collection becomes (e.g., diffusion driven), the less effect mobile ions and screened electric fields will have on the photocurrent. On the other hand, it was also reported that a modified morphology influences the ionic response time.39 According to a recent

Figure 3. Interplay between ionic (left) and electronic (right) response upon applying a voltage. The ionic response influences the electronic one through modifications of the electric field or interface dipoles (red). If these parameters are important for the electronic JV curve, their slow ioninduced manipulation results in hysteresis. Consequently, hysteresis is influenced by the parameters (framed boxes) that govern either the ionic or the electronic response or both. Another possible “interaction” between ionic and electronic charge is not visualized here: Mobile ionic defects could constitute recombination centers for electronic charges that directly influence charge recombination and extraction. 3109



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Figure 4. Effects of mobile ions. (a) Hysteretic JV curve under illumination including polarization switching. Reprinted by permission from Macmillan Publishers Ltd.: Nature Materials, ref 46, Copyright 2015. The device architecture is shown on the left in panel b. (b) On the right: selfstabilized Voc. Reproduced with permission from ref 54. Copyright 2015 WILEY-VCH. (c) Indications for nonreversible morphology changes after applying a bias voltage (average field 1 V/μm) for 110 min to a lateral device consisting of Au/perovskite/Au. Note that the electric field close to the contacts might be much larger and increase during the accumulation of ions. Reprinted by permission from Macmillan Publishers Ltd.: Nature Materials, ref 46, Copyright 2015. (d) Suggestion for a band diagram close to a metal contact where accumulating ions modify carrier injection due to a narrow space charge region allowing for tunneling or a dipole layer. (e) Resistive switching of an ITO/PEDOT:PSS/perovskite/Cu device. High positive voltages turn the device off (high resistance), and high negative voltages turn it on (low resistance). Reproduced from ref 55 with permission of The Royal Society of Chemistry. (f) Photomultiplication as a transient phenomenon due to the accumulation of ions facilitating hole injection at the FTO/perovskite interface. The proposed mechanism is analogous to the one depicted for electrons in panel d. Reproduced with permission from ref 56. Copyright 2015 WILEY-VCH.

study, ion migration “dominates through grain boundaries” as more hysteretic microscopic JV curves are detected at grain boundaries by contact-atomic-force-microscopy (c-AFM).42 A Kelvin-probe-force-microscopy (KPFM) study revealed slow charging effects on both grains and grain boundaries, but claimed that ion migration is faster at grain boundaries.43 Here, whether faster ions cause more or less hysteresis is not straightforward to predict because it depends on the regime of the scan rate compared to the migration rate of the ions (in Figure 2a, hysteresis is minimized either for very high or very low scan rates). Further studies are required to understand how ions (concentration, mobility, etc.) are affected by grain boundaries. Figure 3 intends to give an idea of which processes influence hysteresis by modifying either the ionic (left) or the electronic (right) response when they are coupled through the electric field (red); e.g., a changed morphology that enhances charge transport and/or reduces recombination centers would reduce hysteresis without affecting any process related to ion migration. For state-of-the-art high-efficiency solar cells, hysteresis does not seem to be a big practical issue, and reported JV curves are by default complemented with stabilized efficiency traces tracked at maximum power point. However, this does not mean that ionic defect formation and migration become irrelevant. Their effects are still pronounced at longer time scales leading to a reversible degradation of the performance under illumination on the time scales of minutes to hours;15,38 letting the devices rest in the dark results in a full performance recovery. At first glance, this is a positive result making the devices “reset” during the natural day-night cycling. Consequently, the permanent

degradation is less than deduced from a test under constant illumination. In ref 15 it was hypothesized that mobile cations are responsible for the slow effect, whereas in ref 44, it was proposed that deep trap states that are slowly photogenerated are the reason for the reversible effects. The traps are proposed to result from interactions between lattice and photogenerated localized charges (small polarons). Interestingly, in the case of the reversible degradation, it is rather the light that triggers the effect, and full recovery can only be obtained in the dark. This indicates that more than just voltage-driven ion displacement is involved. The interaction of voltage, light, defects, photogenerated charges, and mobile ions is not fully understood and needs to be further studied. In the bigger picture and for future research, mainly three aspects are of uttermost importance. First, does ion migration constitute an intrinsic instability to the perovskite, which can result in catastrophic failure or irreversible degradation of solar modules? Second, what exactly is moving, along which pathways, and from where does it originate? Finally, can we control ion migration and exploit it in novel devices? Regarding voltage-induced migration of ions, 100% reversibility might not be possible and indeed there are reports indicating that once-measured devices do not behave as fresh ones anymore.45 This was explained by ions getting immobilized at a certain location in the device such as the adjacent layers. Long-term tests including voltage and illumination cycling are required to assess the extent of reversibility. Besides the (rather reversible) migration of single defect ions, experiments indicate that even lattice ions move,15 leading to an exchange of ions or the decomposition of the perovskite by formation of lead iodide. 3110



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charged ion accumulates at a respective interface or where other driving forces keep ions close to an interface. Not much is known about a “direct interaction” between mobile ionic defects and electronic charges. Apart from screening the field, charged ionic defects could trap electronic charge and finally lead to enhanced defect-mediated recombination, even if the defects are homogeneously distributed in the perovskite film. Although most of the intrinsic defects are expected to form shallow traps only.19 Furthermore, they could be neutralized by capturing a photogenerated electron or hole and lose their ability of drift due to an electric field. Beyond solar cells there are other electronic devices that can be fabricated with lead-halide perovskites to exploit the effect of mobile ions. One of the most simple is an electrochemical capacitor, where mobile ionic species contribute to the double layer capacitance.57 A switchable diode based on a device with nonselective gold and PEDOT:PSS contacts has been reported as well (Figure 4a).46 Here, a considerably large either positive or negative writing voltage determines the polarity of the diode. Piled-up ions (MA+ according to ref 58) seem to change the energetics at the contacts by introducing an interface dipole that shifts the electrode work function between conduction and valence band of the perovskite. Alternatively, a narrow spacecharge layer could allow for tunneling currents (Figure 4d). Such diodes could be useful for single-circuit-element bidirectional switches. Devices where perovskite is sandwiched between asymmetric contacts such as ITO/PEDOT:PSS and Cu, can be turned on and off by moderate (>1 V) forward (ITO positive) and reverse voltages, respectively (Figure 4e).55 The state can be probed by smaller read voltages which give a rather ohmic response with low (on) or high (off) resistance state dependent on the sign of the preceding write voltage (reported cycle-proof on/off ratio of 104). Therefore, this device is called memory resistor (memristor). The original, stricter definition comes from circuit theory, where the memristor is the fourth basic element among resistor, capacitor, and inductor. In simple words, it is a device whose resistance is a function of the charge that passed the device previously. This allows for simultaneous storage and processing of data, a feature that is supposed to be essential for more powerful and novel (e.g., neuromorph) computing devices. A more general term would be resistive switching random access memory (ReRAM), which is a nonvolatile memory based on changes in resistance. Early proof of concept of “resistive switching” or rather exploiting the hysteresis of dark curves of sandwich devices (FTO-MAPbI3−Au) has been obtained.59 Later, a maximum nonstable on/off ratio of 109 has been reported,60 and flexible memory devices have been fabricated as well.61,62 Strikingly, the data in Figure 4e shows an instantaneous switching considering that the sweep rate was 500 mV/s. We observed a rapid stepwise switching at certain, not always very well-defined, voltages in solar cells as well, where the shunt resistance is reversibly varied by several orders of magnitude. These findings contradict the idea of ions slowly moving through the complete device. Instead, a more localized displacement might introduce a surface dipole at one interface that could lead to the formation of conducting filaments as, e.g., hypothesized in refs 60 and 62. The photoresponse of metal halide perovskites combined with resistive switching allows development of light-controlled electrical memory. A logical OR gate with writing voltage and illumination as the two inputs and the resistance (on, off) as output has been realized.55 There, the resistance change was explained by changes in energy barrier at the contact due to

This is obviously severe and destructive to the device. The formation of lead iodide, e.g., was observed when applying higher voltages to lateral devices (90 V, 75 μm, 2 h, Figure 4d).46,47 Partially irreversible changes in photoluminescence behavior upon biasing have been reported as well.48 For solar cells, we collected preliminary indications that reverse biasing leads to an accelerated irreversible degradation. This might become an issue for modules where solar cells are connected in series and shading of one cell would in the worst case lead to the illuminated cells reverse biasing the shaded one. Thus, besides the reverse break down voltage, which can be higher than 10 V in perovskite solar cells, this degradation effect is an additional parameter to consider and investigate on the way toward a commercialization of perovskite solar modules. Ion migration in solar cells is not an entirely new phenomenon. It is known for, e.g., CdTe that Cu+ diffuses from the p-contact along grain boundaries and accumulates in the photoactive material, where it generates recombination centers and was suspected as a reason for degradation.49 In CIGS, migration of intrinsic defects (Cu) is observed, reversible, and seems to be compensated by metastable states.50 Judging from the successful commercialization of both technologies, these issues related with stability could be overcome. Ion migration remains mainly a potential stability concern for perovskite solar cells. By contrast, it seems that it is a severe obstacle for efficient perovskite transistors, where it is hypothesized that ions accumulate at the interface to the gate insulator and do not allow for a reliable gate-controlled conductive channel.51,52 However, in particular in a solar cell, mobile ions could also be beneficial and one of the reasons for a low rate of surface recombination and, in turn, the extraordinarily high photovoltages (Voc). Accumulated ions could repel the wrong charge carrier from the wrong contact and perform a kind of self-passivating effect by possibly introducing something equivalent to what is known as a back-surface field in silicon solar cells. Indeed, various transient photovoltage experiments show slow response times attributed to ion migration, seen in both the rise and decay of Voc after turning on and off the light, respectively.36,53 A self-stabilized Voc was reported, meaning that Voc gradually increases itself possibly by triggering ion movement, readjusting the equilibrium between drift and diffusion of electrons and holes, and at the same time decreasing recombination (Figure 4b).54 For exploiting such an effect, the challenge is to increase the self-stabilized Voc to values as large as possible in a controlled way. Whether a situation without mobile ions would have been better, i.e., previously accumulated ions in the dark just had a negative effect on the initial Voc,36 remains a valid question. Based on the simplified assumptions so far, the ions are homogeneously distributed in equilibrium, and accumulation at the contacts is solely due to the built-in potential (cf. Figure 2d). In this case, it seems that piled-up ions would not assist a higher Voc, but rather cause higher recombination in the bulk and at the surface of the absorber material, leading to a lower Voc in a forward scan. However, scenarios are imaginable where, e.g., the oppositely

Apart from screening the field, charged ionic defects could trap electronic charge and finally lead to enhanced defect-mediated recombination. 3111



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piled-up trapped charge. This explanation is in line with investigations of photomultiplication observed in solar cells without electron selective layer under reverse bias (Figure 4f), where ions accumulating at the FTO−perovskite interface are hypothesized to modify the injection barrier for holes.56

photovoltaic concepts and technologies. His research focuses on the device physics of perovskite solar cellsmost recently, investigating recombination and hysteresis phenomena in this emerging material system. Previously, he analyzed and modeled performance-limiting processes in organic solar cells.

The photoresponse of metal halide perovskites combined with resistive switching allows development of light-controlled electrical memory.

ACKNOWLEDGMENTS Support by Prof. Michael Graetzel and Prof. Anders Hagfeldt is kindly acknowledged, including financial support by the King Abdulaziz City for Science and Technology (KACST) under a joint research project. Furthermore, I thank Konrad Domanski, Dr. Juan Pablo Correa Baena, and Prof. Sining Yun for valuable comments on the manuscript, which improved both content and style considerably.

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An electronic−ionic conductor could also act as an interfacing element between electronic systems such as semiconductor chips and ionic systems such as neural networks in biology. Early studies reported some properties of synapses in MAPbBr363 and MAPbI3.64 Here, the electrical response that is dependent on the biasing history can be triggered by voltage spikes (similarly to action potentials in neurons) and is interpreted as learning and forgetting. It has to be noted that these interpretations and the memristive devices are at the beginning of their development and far from requirements for practical applications with regard to, e.g., speed or leakage currents. This is in contrast to solar cells, where the desired performance metrics in efficiency and (to some extent) in (intrinsic) stability are already met. In summary, defect ion migration is an important property of metal halide perovskites that influences the optoelectronic response and causes hysteresis in the current−voltage curve of devices. Iodine (halide) vacancies have been identified to be the predominant mobile ionic defect related to hysteresis, whereas displacement of further mobile anions and cations might be observable on longer time scales. Device simulations assuming mobile charges (representing, e.g., halide vacancies) that move slowly from one to the other contact, successfully reproduced experimental current−voltage data. This qualitative agreement needs to be complemented by follow-up studies that are quantitative and more specific regarding the motion of ionic charge and their role at the interfaces. The major experimental challenge regarding both problems and features of ion migration is to control the type, concentration, and possibly the mobility and pathways (from where to where and how?) of ionic species. Furthermore, reversibility and cyclability, though promising for the first memory devices (3000 cycles shown55), need to be demonstrated for solar cells, and eventually stabilization measures need to be (and will be) found and implemented. More research from both the experimental and the theoretical side on the role of defects, interfaces, and grain boundaries, will give plenty of novel insights in the years to come. These findings will be essential for predicting and guaranteeing stable solar cells and for application of the perovskite material in novel devices.

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REFERENCES

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AUTHOR INFORMATION

ORCID

Wolfgang Tress: 0000-0002-4010-239X Notes

The author declares no competing financial interest. Biography Wolfgang Tress is currently working as a scientist at LPI, EPFL in Switzerland, with general interests in developing and studying novel 3112



Perspective

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Metal Halide Perovskites as Mixed ElectronicâIonic Conductors

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