Electronic and Ionic Transport Dynamics in Organolead Halide Perovskites Dehui Li,† Hao Wu,‡ Hung-Chieh Cheng,‡ Gongming Wang,†,§ Yu Huang,‡,§ and Xiangfeng Duan*,†,§ †
Department of Chemistry and Biochemistry, ‡Department of Materials Science and Engineering, and §California Nanosystems Institute, University of California, Los Angeles, California 90095, United States S Supporting Information *
ABSTRACT: Ion migration has been postulated as the underlying mechanism responsible for the hysteresis in organolead halide perovskite devices. However, the electronic and ionic transport dynamics and how they impact each other in organolead halide perovskites remain elusive to date. Here we report a systematic investigation of the electronic and ionic transport dynamics in organolead halide perovskite microplate crystals and thin films using temperature-dependent transient response measurements. Our study reveals that thermally activated ionic and electronic conduction coexist in perovskite devices. The extracted activation energies suggest that the electronic transport is easier, but ions migrate harder in microplates than in thin films, demonstrating that the crystalline quality and grain boundaries can fundamentally modify electronic and ionic transport in perovskites. These findings offer valuable insight on the electronic and ionic transport dynamics in organolead halide perovskites, which is critical for optimizing perovskite devices with reduced hysteresis and improved stability and efficiency. KEYWORDS: ionic transport, electronic transport, activation energy, hysteresis, organolead halide perovskite
O
and resolve the hysteretic behavior, which has important implications in terms of the stability and performance of the perovskite-based solar cells.16 The origin of this phenomenon has not been fully understood, although a number of hypotheses have been suggested to explain this giant hysteresis with the three most invoked ones being ferroelectricity,17,18 charge trapping,19 and ion migration.15,20 Previous studies indicate that vacancies (VI, VPb, and VMA, standing for I−, Pb2+ or CH3NH3+ vacancies, respectively) and interstitials (MAI, PbI, IMA, and IPb) are common defects in organolead halide perovskite materials (CH3NH3PbI3) due to their low formation energy.21,22 These defects can create shallow donor or acceptor levels near the band edge22 and might migrate from sites to sites under an external or built-in electric field. Recent simulation studies suggested the existence of vacancy-mediated ion migration and mixed ionic−electronic conduction in the organolead halide perovskites and identified the migrating ion species and quantitatively calculated the underlying migration activation energies.16,23−28 Eames et al. and Meloni et al. have also recently experimentally extracted the activation energy for a perovskite-based solar cell.23,26 Among three types of possible vacancies (VI, VMA, and VPb), studies to date suggest that VI migration has the smallest activation energy
rganolead halide perovskites have recently emerged as a family of highly promising materials for high efficiency, low-cost solution processed solar cells.1−5 With a number of favorable characteristics including high optical absorption coefficient, moderate mobility, and long diffusion length,5 the perovskite materials have enabled solar cells with a certified power conversion efficiency rapidly soared to 20.1% in just a few years.6 However, the perovskite-based photovoltaic devices today are typically plagued with considerable hysteresis in current−voltage curves that impedes precise evaluation of the power conversion efficiency and is fundamentally associated with the device instability. Beyond solar cells, the organolead halide perovskites have also been explored for many other optoelectronic applications including photodetectors,7 lasers,8−10 and light-emitting devices.11 Despite the rapid advancement of the perovskite-based optoelectronic applications, many of their intrinsic properties remain elusive such as slow photoresponse,12 large hysteresis in current−voltage curves,13,14 and giant switchable photovoltaic effect.15 It is likely that these challenges are associated with a common underlying microscopic mechanism but with different macroscopic manifestations. In particular, the hysteresis issue is of considerable concern since the dependence of the current− voltage curves on the scan direction and speed prevents precise evaluation of power conversion efficiency3 and could lead to other undesired effects including charge building up and premature device failure. Therefore, it is essential to understand © 2016 American Chemical Society
Received: April 27, 2016 Accepted: June 17, 2016 Published: June 17, 2016 6933
DOI: 10.1021/acsnano.6b02795 ACS Nano 2016, 10, 6933−6941
Article
www.acsnano.org
Article
ACS Nano (see Table S1) and is likely the primary migration species in organolead halide perovskite devices.16,23,24,26 However, these studies were conducted in devices with asymmetric contact, and intrinsic built-in electric field could modify the ion distribution and thus affect the extracted activation energies. Additionally, it is important to distinguish ionic and electronic current contribution to the total current and understand how ionic transport affects the electronic transport, which is still in its infancy stage.28 Here we report a systematic investigation of the electronic and ionic transport dynamics in organolead halide perovskite thin-film and microplate devices both in dark and under illumination conditions, extracting the ion-induced electric potential across the device channel and understanding how electronic and ionic transport contributes to the total current. The polycrystalline thin-film devices are obtained through a conventional spin-coating process with grain size typically on the order of 10 nm and abundant grain boundaries,29 while the nearly single-crystalline microplate devices are obtained using a vapor-phase intercalation approach with much larger grain sizes (micrometer scale) and few grain boundaries,30 which is verified by scanning electron microscopy (SEM) images and photoluminescence spectra (Figure S1).31 The thicknesses of both the thin films and microplates are around 150 nm. A simultaneous investigation of both types of devices can offer an excellent platform for probing the fundamental origins of the hysteresis and ionic transport. Importantly, our study reveals that both the ion motion and hysteresis can be greatly suppressed or eliminated in nearly single crystalline microplates due to greatly reduced grain boundaries and better crystalline quality. The extracted activation energy for electronic transport in microplates (70 meV) is much smaller than that in thin films (430 meV), while the activation energy for ionic transport in microplates (210 meV) is considerably larger than that in thin films (90 meV).
Figure 1. Temporal response and schematics of the ion migration in organolead halide perovskite. (a) Schematic of a two-probe halide perovskite film device fabricated on a 300 nm SiO2/Si substrate with 5 nm Cr/50 nm Au as electrodes. (b) Temporal response curves following positive and negative biasing at 296 K. The bias sequences also display as solid lines as well. The applied external bias is ±1.5 V. Stage 1 (①): the external bias is applied, and stage 2 (②): the external bias is removed. (c) Schematic diagrams indicating the dynamics of ionic transport following external bias. The two yellow bars represent Cr/Au electrodes, and organolead halide perovskite film fills between these two electrodes.
suggested in previous studies, and can exclude the charge trapping for the observed phenomena in our study.15,32 The dynamics of the ion migration is described in Figure 1c. In the beginning without the external bias, VI, VPb, and VMA are randomly distributed throughout the device channel. With the application of a positive bias (pointing from left to right in Figure 1c), the electronic current is instantly observed due to a much faster speed of electrons/holes compared with that of ions. Afterward, the ions start to drift under the applied external bias, leading to an ionic current with the same polarity as that of the electronic current. With continued external bias, the oppositely charged ions pile up near to the perovskite− electrode interfaces, resulting in an ion-induced electric field with the direction opposite to the external electric field, which partially cancels the external bias and reduces the electronic current. As more and more ions move toward and accumulate near the perovskite−electrode interfaces, the ion-induced electric field continuously increases, leading to a gradual decrease of total current and finally to a stable value when the ion accumulation reaches the equilibrium conditions (stage 1 in Figure 1b). After removing the applied bias, the previously accumulated ions at the perovskite−electrode interfaces create a net reversal electric field, leading to an instant switching of the polarity of the electronic current. The accumulated ions gradually diffuse away from the perovskite−electrode interfaces with time, resulting in the decrease of the ion-induced electric field and thus the reduction of the electronic current (Figure 1b). With sufficient time, when all ions return back to the initial equilibrium condition, the ion-induced electric field is reduced to zero, leading to a zero total current (stage 2 in Figure 1b). To quantitatively characterize the contributions from electronic transport and ionic conduction and to determine the magnitude of the ion-induced electric potential in our
RESULTS AND DISCUSSION The two-probe symmetric Cr/Au (5/50 nm) electrodes with a channel length of 20 μm (Figure 1a) are utilized to avoid the influence from the intrinsic built-in electric field in the asymmetric electrode structure that was commonly adopted in previous studies. The perovskite thin film was spin-cast on 300 nm SiO2/Si substrate with prefabricated electrodes, following similar protocols developed previously.29 For the individual perovskite microplate devices, we adopted the pattern growth method to directly grow the microplates across the predefined electrode pairs.30 Once the device fabrication is completed, we immediately transfer the devices into vacuum chamber for electrical measurements. All the measurements were carried out in the dark unless otherwise specified. Figure 1b displays the temporal response of current curves of a thin-film perovskite device upon the application and removal of a positive (black) or negative (red) bias voltage, along with the corresponding bias sequences. These processes show a completely opposite trend under positive and negative bias due to the symmetric electrodes used. After switching on the applied external bias, the dark current instantly increases and then slowly decays to a nonzero constant value. After the applied bias is removed (60 s later), a current with the opposite sign is immediately observed, which gradually decays to zero over time. Such switchable behavior can be attributed to the ion migration within the organolead halide perovskites, as 6934
DOI: 10.1021/acsnano.6b02795 ACS Nano 2016, 10, 6933−6941
Article
ACS Nano
Figure 2. External bias-dependent temporal response in organolead halide perovskite. (a) Temporal response curves following +3 V biasing at 296 K for a organolead halide perovskite film device in the dark. The black solid lines are biexponential fittings. I10 is the constant total current after the device reaches equilibrium conditions. I1 is the instant current after switching on the applied external bias, and I2 is the instant current after switching off the applied external bias. (b, c) The applied external bias dependence of temporal response curves for stage 1 (b) and stage 2 (c) for the organolead halide perovskite film device in the dark. (d) The applied external bias dependence of I1, I10, and I1 − I10 extracted from (b) and (c). The solid black line is the linear fitting result. (e) The applied external bias dependence of I1 and I2 extracted from (b) and (c). The solid black line is linear fitting result. (f) The applied external bias dependence of ion decay rate extracted from (b) and (c).
⎛ t − t 20 ⎞ I(t ) = I20 + (Ie − I2) exp⎜ − ⎟ + (I2 − I20) τe ⎠ ⎝
devices, we investigated the temporal response of the current curve when an increasing external bias is applied (stage 1) and removed (stage 2). The transient response at both stages 1 and 2 can be fitted with a biexponential function, from which we can extract the necessary parameters for further determining the current origins and magnitude of the ion-induced electric field. Figure 2a displays an example of the fitting results for a temporal response of total current under an applied external bias of 3 V. The fitting function used for stage 1 can be explicitly expressed as
⎛ t − t 20 − δ ⎞ exp⎜ − ⎟ τ2 ⎠ ⎝
where I20 is the constant total current after the device reaches equilibrium conditions without external bias (essentially zero); Ie and τe relates to equipment response (RC delay); I2 is the instant current right after external bias (t20 + δ) is removed at stage 2 (electronic current due to the ion-induced electric field because of the much faster motion speed of electrons/holes); τ2 is time constant of ion diffusion/restoration at stage 2; t20 is the starting point right after the applied external bias is removed. Therefore, the information on the electronic transport can be extracted from current I10, I1, and I2 since the ionic current is negligibly small compared with electronic current due to the much slower motion speed of ions, while the dynamics of the ionic transport can be reflected by the decay time constants τ1 and τ2 because the current decay is mainly due to the ion motion, as discussed in a previous report.23 Figure 2b,c shows temporal response curves at stages 1 and 2, with varying external bias (Vsd). By fitting the temporal response curves using eq 1, I10, I1, and I2 and the decay time constant τ1 and τ2 are extracted. The bias-induced electronic current I1 shows a linear relationship with applied external bias Vsd, while the constant total current I10 against applied external bias Vsd shows a nonlinear behavior (Figure 2d). The linearity of I1 vs Vsd indicates that ohmic contact is formed between the Au electrodes and the perovskite thin films. The difference between I1 and I10 (I1 − I10) is electronic current induced by the opposite direction ion-induced electric field (Figure 2d). I1 − I10 first increases with Vsd and then reaches a stable value at around ±4.5 pA when the external applied bias Vsd reaches ±3 V, suggesting that the ion-induced electric field first increases with the external bias and then saturates at the external bias
⎛ t − t10 ⎞ I(t ) = I10 + (Ie − I1) exp⎜ − ⎟ + (I1 − I10) τe ⎠ ⎝ ⎛ t − t10 − δ ⎞ exp⎜ − ⎟ τ1 ⎠ ⎝
(1b)
(1a)
where I10 is the constant total current after the device reaches equilibrium conditions under applied external bias (indicated in Figure 2a); Ie and τe relates to equipment response (RC delay, which is supported by the temperature independent τe) and are nearly identical for both stages 1 and 2; I1 is the instant current right after switching on the applied external bias (at t10 + δ) for stage 1 (electronic current due to the applied external electric field) since the motion speed of electrons/holes is much larger than that of ions leading to the negligible ionic current at the instant right after switching on the applied bias; τ1 is the time constant of ion migration under applied bias for stage 1; t10 is the starting point for switching on the applied external bias for stage 1; and δ is the relaxation time of equipment (smaller than 1 s). Similarly, we can fit the temporal response curve of stage 2 using the same biexponential function: 6935
DOI: 10.1021/acsnano.6b02795 ACS Nano 2016, 10, 6933−6941
Article
ACS Nano around 3 V (Figure 2d). Based on I1 vs Vsd plot, we can find that the voltage required to generate ±4.5 pA current is ±1.2 V. Therefore, we could draw the conclusion that the maximum ion-induced electric potential across the device channel (20 μm) is around 1.2 V. The saturation of the ion-induced electric field (potential) might be due to the lack of available mobile vacancies. When the applied external bias Vsd is removed, the ioninduced electric field would produce a current with opposite polarity. Therefore, it is expected that the instant current I2 after removing the external bias Vsd should follow the same trend as I1 − I10 except the polarity if the ionic current is negligibly small compared with the electronic current. Indeed, I2 first increases with Vsd and then saturates with the same saturation current and voltage, which are ±4.5 pA and ±1.2 V, respectively (Figure 2e). Since I10 is the current after the ion migration has reached the equilibrium conditions, it represents the electronic current driven by the sum of the applied bias and ion-induced electric potential. While the instant current I2 is the electronic current driven by pure ion-induced electric potential, the difference between the instant current I1 (electronic current driven by pure applied external bias) and electronic current I10 (I1 − I10) is due to presence of the ion-induced electric potential as well. This completely identical saturation current and voltage between I2 and I1 − I10 further verifies that the contribution of ionic current to the total current is negligible. Otherwise, the value of I1 − I10 should be larger than I2, since a larger applied bias at t10 can induce a larger ionic current than the ionic current driven by ion-induced ionic current at t20, while there is no ionic current after the ion migration has reached the equilibrium conditions due to the limited available mobile ions. The time constant τ1 and τ2 can reflect the dynamics of ionic transport with and without the applied external bias Vsd, respectively. The decay rate k, defined as τ−1 (k1 = τ−1 1 , k2 = ), continuously increases with the applied external bias for τ−1 2 stage 1, and first decreases, then reaches a saturation value for stage 2 (Figure 2f). This is expected for stage 1 since the larger the applied external bias is, the faster the ion migration reaches equilibrium conditions. For the stage 2, the accumulated ions barely rely on the diffusion to reach equilibrium conditions. Therefore, the decay rate k2 depends on the distribution of the ions accumulated near the electrodes. Before the ion-induced electric field reaches saturation (applied external bias 3 V), the distribution of the polarized ions reaches a stable state, resulting in the constant restoration rate. This also supports that the saturation of ion-induced electric field is due to the lack of mobile vacancies. Based on the continuum theory developed in ionic liquids,33 we have studied the evolution of the distribution of the ions under the applied bias with time (see Supporting Information text). The calculations indicate that the positive ions pile up near the cathode, while net negative charges accumulate around the anode when the equilibrium conditions have established. With increasing time, more and more ions accumulate near the perovskite−electrode interfaces, and the distribution of iodine vacancies gradually decays into the channel, which might be
due to the expelling of positive ions by the preaccumulated positive ions near the cathode interface (see Figure S2).33 The accumulated ions could generate an opposite polarity electric field to the applied bias and drive the current to flow after removing the applied bias, which agrees well with the experimental results discussed above. To further explore the nature of electronic and ionic transport, we have further measured temporal response under different temperatures (Figure 3a,b). The external bias was
Figure 3. Temperature-dependent temporal response in organolead halide perovskite. (a, b) The temperature dependence of temporal response curves for stages 1 (a) and 2 (b) under an applied external bias of 3 V for the organolead halide perovskite film device in the dark. (c) Arrhenius plots of the temperature dependence of I1 and I2 extracted from (a) and (b). The solid lines are linear fitting results. (d) Arrhenius plots of the temperature dependence of ion decay rate extracted from (a) and (b). The solid lines are linear fitting results.
selected as 3 V so that the saturation voltage is reached. The temperature-dependent I1 and I2 extracted from temporal response curves all show an exponential decrease with decreasing temperature, suggesting that the electronic transport is thermally activated (Figure 3c). These exponential decrease plots can be well fitted by Arrhenius function: ln(I ) = C − Ea /kBT
(2)
where I can be either current or decay rate k, C is a constant, kB is Boltzmann’s constant, T is the temperature, and Ea is activation energy. The activation energies derived from I1 and I2 are estimated to be 360 and 540 meV, respectively. The larger activation energy for I2 might be partly attributed to the smaller ion-induced electric field at lower temperatures. Electronic current I2 is driven by ion-induced electric field. The decrease of ion-induced electric potential due to the slower ion migration rate at low temperature could lead to an overestimation of the activation energy extracted from I2 (see more discussions later). −1 Since the decay rate k (k1 = τ−1 1 , k2 = τ2 ) represents ionic transport dynamics, the extracted the activation energy for ionic transport can also be derived by fitting temperature-dependent decay rate k1 (or k2) using the Arrhenius equation (Figure 3d). Importantly, the relationship of k1 (or k2) vs T can be well fitted using single exponential function, suggesting that only one type 6936
DOI: 10.1021/acsnano.6b02795 ACS Nano 2016, 10, 6933−6941
Article
ACS Nano
Figure 4. Comparisons of temperature-dependent temporal response in organolead halide perovskite film and microplate devices under illumination. (a) The temperature dependence of temporal response curves under the applied external bias of 3 V in the halide perovskite film device under white light illumination (light power: ∼8 mW/cm2). (b) Arrhenius plot of the temperature dependence of I1 and I2 extracted from (a). The solid lines are linear fitting results. (c) Arrhenius plot of the temperature dependence of ion decay rate k2 extracted from (a). The solid line is linear fitting result. (d) The temperature dependence of temporal response curves under the applied external bias of 3 V in a halide perovskite microplate device under white light illumination (light power: ∼8 mW/cm2). (e) Arrhenius plot of the temperature dependence of I1 and I2 extracted from (d). The solid lines are linear fitting results. (f) Arrhenius plot of the temperature dependence of ion decay rate k2 extracted from (d). The solid line is linear fitting result.
electric field, suggesting the lack of intrinsic charge carriers (electrons, holes) due to the high crystalline quality of the perovskite microplates with few defects (dopants). Nevertheless, under white light (8 mW/cm2) illumination, the photocurrent observed in perovskite microplate devices is more than 2 orders of magnitude larger than that in the thin-film devices, further confirming the high crystalline quality of the perovskite microplates30,36 (Figure 4a,d). Due to the lack of intrinsic charge carriers in microplate devices in the dark, we have to conduct the transient response measurements under light illumination to probe the electronic and ionic transport dynamics in microplates. As a strict control, we have first conducted similar measurements on thin-film devices under light illumination. These studies under light illumination can also allow us to probe how the light illumination alters the electronic and ionic transport, which is critical for the optoelectronic applications such as solar cells, photodetectors, and light-emitting devices. Figure 4a,d displays the temporal response curves of perovskite thin-film and microplate devices upon applying and removing a constant bias of 3 V under white light illumination (8 mW/cm2) at various temperatures. Overall, the temporal responses of both types of devices show exponential decay under light illumination at stage 2, similar to that of the thin-film devices in the dark. It is important to note that stage 1 shows distinct features under light illumination, particularly for the microplate devices (and thin-film devices at low temperatures), where a gradual current increase instead of the exponential decay is observed. Such current increase might be attributed to low intrinsic carrier concentration in microplates (or thin films at low temperature), light-induced carrier release, and much slower ion migration rate. Based on the temperature-dependent instant (electronic) current I1 and I2, we can derive the electronic transport activation energies to be 430 and 410 meV for the thin-film devices under light illumination (Figure 4b), comparable to
of ions (VI) dominates the ion migration in perovskite thin films. Otherwise, bi- or triexponential function should be expected for the k1 and k2 vs T curves if more than one type of ion species contributes to the ion migration. The activation energies for ionic transport both under the external bias Vsd and under ion-induced potential are 260 meV, which is comparable to both the calculated and measured VI migration activation energy in perovskite films (see Table S1)24,26 and the measured activation energy for migration of halide-ion vacancies in other inorganic halide perovskites such as CsPbCI3 (290 meV), CsPbBr3 (250 meV)25,34 and CuPbI3 (290 meV),35 confirming that the VI migration dominates the ion transport in the organolead halide perovskite thin films. The migration of ions is expected to be affected by the perovskite grain size since devices with a larger perovskite grain size have a smaller number of grain boundaries and lower vacancy concentration.15 To further probe how the grain size and grain boundary impact the electronic and ionic transport, we have also studied microplate devices with higher crystalline quality and fewer grain boundaries.30,36 The better crystalline quality of the microplates has been demonstrated by SEM images and photoluminescence spectra (Figure S1) and might be attributed to the larger grain size, fewer grain boundaries, and avoidances of using solvent during the growth process. In general, the evaporation of solvent after spin-coating may introduce pinholes and stoichiometric defects in the spincoated films, while the vapor intercalation approach produces perovskite microplates with less pinholes and grain boundaries.37 Although the perovskite films and microplates are prepared by different methods, we expected that the nature of migration ionic species should be the same since VI migration has the smallest activation energy. Therefore, the different ionic migration activation energies between the films and microplates can be ascribed to the crystalline quality and number of grain boundaries. Notably, there is no detectable dark current either under the applied external bias Vsd or under the ion-induced 6937
DOI: 10.1021/acsnano.6b02795 ACS Nano 2016, 10, 6933−6941
Article
ACS Nano
Table 1. Summary of Activation Energies and Ion-Induced Electric Potential Across the Device Channel for Both Perovskite Film and Microplate Devices in the Dark Under Stage 2 Illumination and Full Illumination electronic transport activation energy film (dark) film (light @ stage 2) film (light @ stages 1 and 2) plate (dark) plate (light @ stage 2) plate (light @ stages 1 and 2)
ionic transport activation energy
from I1 (meV)
from I2 (meV)
from τ1 (meV)
from τ2 (meV)
ion-induced potential (V)
360 ± 50 − 430 ± 80 − − 120 ± 10
540 ± 40 430 ± 50 410 ± 100 − 70 ± 10 160 ± 10
260 ± 40 − − − − −
260 ± 30 90 ± 10 80 ± 10 − 210 ± 20 150 ± 25
1.2 − 0.9 − − 0.3
in organolead halide perovskites, agreeing with previous report in polycrystalline perovskite films.41 It should be noted that the activation energy for electronic transport extracted from I2 could be overestimated due to the smaller ion-induced electric field at lower temperature, as discussed for the thin-film devices in the dark. Without the effect from ion-induced electric field, the electronic transport activation energy extracted from I1 would be more accurate. Indeed, the activation energy for electronic transport extracted from I2 in the perovskite thin-film device in the dark is apparently larger than that extracted from I1 (Figure 3c and Table 1). However, for the thin-film devices under light illumination, the electronic transport activation energy extracted from I2 is rather similar to that extracted from I1 (Table 1). This might be attributed to the light illuminationpromoted ion migration in perovskite thin-film devices such that the ion-induced electric field does not significantly decrease at lower temperatures under light illumination. Nevertheless, in case of the microplate devices with high crystalline quality, the ion migration is still very slow at lower temperatures even with the help of the light illumination, which leads to the slightly larger electronic transport activation energy extracted from I2 (160 meV) than that extracted from I1 (120 meV). Because of the lack of intrinsic carriers in microplates in the dark, its electronic and ionic transport dynamics must be probed under light illumination, which can promote the ion motion and considerably underestimate the intrinsic ionic transport activation energy (e.g., ionic activation energy is only 80 meV under light illumination vs 260 meV in the dark for thin-film devices). To reduce the effect of light illumination, we can also probe the electronic and ionic transport dynamics by only illuminating the device at stage 2, while keeping stage 1 in the dark (see Figure S4). For the thin-film device under stage 2 illumination, the obtained the activation energies for electronic and ionic transport in thin films are 430 and 90 meV, respectively, both comparable to that derived with illumination at both stages. In contrast, for the microplate devices under stage 2 illumination only, the extracted electronic activation energy is only 70 meV, much smaller than that in thin films (430 meV), while the ionic activation energy is 210 meV, much larger than that in thin-film device (80 meV) (see Table 1). Furthermore, the ion migration rate under stage 2 illumination is comparable to that under illumination at both stages in thinfilm devices; while for microplate devices, the ion migration rate under stage 2 illumination is much smaller (2−6 times) than that under illumination at both stages, which again indicates that light illumination may promote the ion motion in microplate devices. If both stages 1 and 2 could be kept in the dark to completely eliminate light-promoted ion migration, it is not unreasonable to expect ionic transport activation energy
those obtained in the dark (360−540 meV). Based on the current decay rate (k2) in stage 2, we can also derive ionic transport activation energy of 80 meV under light illumination (Figure 4c). It is noted that we cannot derive the ion migration activation energy from stage 1 (1/τ1) under illumination, since the temporal response curves in stage 1 do not consistently follow the exponential decay characteristics (Figure 4a) such that we cannot extract the decay time constant τ1. While the difference between the activation energies for electronic transport under illumination and in the dark is relatively small (within the experimental error ranges), the activation energy for ion migration under illumination (80 meV) is much smaller than that in the dark (260 meV), suggesting that light illumination could promote ion motion, consistent with previous studies.38 The much smaller ion transport activation energy under illumination might be attributed to light illumination-induced lowering of the potential barrier for ion transport across the grain boundaries, which has been demonstrated by the scanning Kelvin force microscopy.39,40 However, further investigation will be necessary to fully clarify the underlying mechanism.38 The derived ion-induced electric potential across the device channel for the thin-film devices under light illumination is 0.9 V, slightly smaller than that in the dark (1.2 V), which might be attributed to the screening effect from photogenerated carriers. Similarly, we can extract the electronic transport activation energy (120 meV from I1 and 160 meV from I2) and ionic transport activation energy (150 meV from k2) for the microplate devices under light illumination (Figure 4e,f). It is important to note that the ionic transport rate in microplate devices is only about 10−25% of that in thin films (Figure 4e, f), and the ionic transport activation energy in microplate devices (150 meV) is considerably larger than that in film devices (80 meV) under light illumination (Table 1), which can be attributed to the better crystalline quality with much fewer grain boundaries and greatly reduced vacancy concentration in microplates when compared with polycrystalline thin films. On the contrary, the electronic transport activation energy in microplate devices (120−160 meV) is much smaller than that in thin films (430 meV), suggesting the electron transport in microplate is considerably easier than that in thin films, which can be attributed the reduced grain boundary scattering and charge trapping states in the nearly single crystalline microplates. Importantly, the higher ion motion activation energy and the reduced vacancy concentration also lead to a considerably smaller ion-induced electric potential of around 0.3 V in the microplate devices (vs 0.9−1.2 V in thin films) and negligibly small hysteresis in current−voltage curves (see Figure S3). Together, our studies demonstrate that the improved crystalline quality can greatly suppress the ionic transport, reduce or eliminate hysteresis, and enhance electronic transport 6938
DOI: 10.1021/acsnano.6b02795 ACS Nano 2016, 10, 6933−6941
Article
ACS Nano
Electrical and Photoluminescence Measurements. Temperature-dependent two-probe device measurements were carried out in a probe station (Lakeshore, TTP4) coupled with a computer-controlled analogue-to-digital converter (National Instruments model 6030E) and a SR570 current preamplifier. For the measurements under illumination, a white lamp with a power density of 8 mW/cm2 was used. The photoluminescence measurement was conducted under a confocal mirco-Raman system (Horiba LABHR) equipped with a 600 gr/mm grating in a backscattering configuration excited by a solid-state laser (633 nm) with a power of 4 μW. For the low-temperature measurement, a liquid nitrogen continuous flow cryostat (Cryo Industry of America) was used to control the temperature from 77 to 300 K.
would be even larger (>210 meV), and the electronic transport activation energy would be ever smaller ( 175 μm in Solution Grown CH3NH3PbI3 Single Crystals. Science 2015, 347, 967−970. (45) Shi, D.; Adinolfi, V.; Comin, R.; Yuan, M.; Alarousu, E.; Buin, A.; Chen, Y.; Hoogland, S.; Rothenberger, A.; Katsiev, K. Low TrapState Density and Long Carrier Diffusion in Organolead Trihalide Perovskite Single Crystals. Science 2015, 347, 519−522. 6940
DOI: 10.1021/acsnano.6b02795 ACS Nano 2016, 10, 6933−6941
Article
ACS Nano (46) Nie, W.; Tsai, H.; Asadpour, R.; Blancon, J.-C.; Neukirch, A. J.; Gupta, G.; Crochet, J. J.; Chhowalla, M.; Tretiak, S.; Alam, M. A. HighEfficiency Solution-Processed Perovskite Solar Cells with MillimeterScale Grains. Science 2015, 347, 522−525.
6941
DOI: 10.1021/acsnano.6b02795 ACS Nano 2016, 10, 6933−6941
