Reader TTS

ABSTRACT Ionic transport in organometal halide perovskites is of vital importance because it dominates anomalous phenomena in perovskite solar cells, from hysteresis to switchable

photovoltaic effects. However, excited state ionic transport under illumination has remained elusive, although it is essential for understanding the unusual light-induced effects

(light-induced self-poling, photo-induced halide segregation and slow photoconductivity response) in organometal halide perovskites for optoelectronic applications. Here, we quantitatively

demonstrate light-enhanced ionic transport in CH3NH3PbI3 over a wide temperature range of 17–295 K, which reveals a reduction in ionic transport activation energy by approximately a factor

of five (from 0.82 to 0.15 eV) under illumination. The pure ionic conductance is obtained by separating it from the electronic contribution in cryogenic galvanostatic and voltage-current

measurements. On the basis of these findings, we design a novel light-assisted method of catalyzing ionic interdiffusion between CH3NH3I and PbI2 stacking layers in sequential deposition

perovskite synthesis. X-ray diffraction patterns indicate a significant reduction of PbI2 residue in the optimized CH3NH3PbI3 thin film produced via light-assisted sequential deposition, and

the resulting solar cell efficiency is increased by over 100% (7.5%–15.7%) with little PbI2 residue. This new method enables fine control of the reaction depth in perovskite synthesis and,

in turn, supports light-enhanced ionic transport. SIMILAR CONTENT BEING VIEWED BY OTHERS SUPPRESSION OF PHASE SEGREGATION IN WIDE-BANDGAP PEROVSKITES WITH THIOCYANATE IONS FOR

PEROVSKITE/ORGANIC TANDEMS WITH 25.06% EFFICIENCY Article 29 March 2024 FIRST-PRINCIPLES IDENTIFICATION OF THE CHARGE-SHIFTING MECHANISM AND FERROELECTRICITY IN HYBRID HALIDE PEROVSKITES

Article Open access 12 November 2020 EFFICIENT AND STABLE PEROVSKITE SOLAR CELLS WITH REGULATED DEPLETION REGION Article 12 February 2024 INTRODUCTION Electrical conduction in materials can

be classified into two categories, electronic and ionic, depending on the conducting species. A mixed conductor is a material that possesses both electronic and ionic conductivity, and the

recently emerging family of organometal halide perovskites, ABX3 (A: CH3NH3(MA)/NH2CH=NH(FA)/Cs, B: Pb/Sn, X: I/Cl/Br), has proven to be mixed conductors1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11,

12. Ion migration has been reported to be one of the main mechanisms responsible for anomalous I-V hysteresis and poor stability in perovskite solar cells. For optoelectronic applications of

these organometal halide perovskites, light-induced effects will inevitably occur in optoelectronic devices such as solar cells, light-emitting diodes (LEDs) and lasers13, 14, 15. Under

working conditions and continuous light illumination, the long-term output of perovskite-based solar cells is reported to be unstable. LED applications also exhibit a decrease in efficiency

after some period of operation. It is reasonable to speculate that these perovskites possess different properties in their excited state under photoexcitation, which deserve further

investigation to gain a better understanding of the optoelectronic properties of perovskites, especially with regard to these unusual light-induced effects10, 16, 17, 18, 19, 20, 21, 22, 23.

However, to date, excited state ion migration for perovskites under photoexcitation has rarely been reported in CH3NH3PbI3 (MAPbI3). Herein, we demonstrate a systematic and quantitative

study of light-dependent ionic transport in MAPbI3 film over a wide range of temperatures (17–295 K) and light intensities (0–20 mW cm−2), by means of combined cryogenic galvanostatic6, 24

and voltage-current measurements. Distinct from the band-like nature of the high-mobility electronic transport in this material, the ionic transport exhibits an obvious hopping mechanism

with varying activation energies under different illumination intensities. The activation energy for ionic transport shows a significant decrease by approximately a factor of five (from 0.82

to 0.15 eV) as the light intensity increases from 0 to 20 mW cm−2. To the best of our knowledge, this result is the first demonstration of light-dependent ionic transport in organometal

halide perovskites. This light-enhanced ion migration can explain the photo-induced giant dielectric constant in the low-frequency regime and the photo-instability in perovskite-based

devices as well as the light-induced halide segregation in mixed perovskites. We further demonstrate that this property can be utilized to manipulate the synthesis kinetics of perovskites

through light-assisted ion migration. The results yield an obvious increase in solar cell efficiency due to reduced interfacial PbI2. Our findings provide important fundamental insights into

the excited state properties of perovskite film, which deserves future investigation in inorganic halide perovskites, even for the whole ionic crystal family. MATERIALS AND METHODS MAPbI3

was prepared from an MAI/PbAc2 (Sigma-Aldrich, Shanghai, China) (3:1) precursor solution (215 and 172 mg ml−1 for MAI and PbAc2, respectively) with an additional 4 μl of H3PO2 (Aladdin,

Shanghai, China). Then, the solution was spin coated onto a plasma-cleaned FTO/TiO2 substrate. The film was sintered at 100 °C for 50 min in ambient air (~30% relative humidity). The hole

transport material (HTM) was then spin coated onto the perovskite film at 3000 rpm for 40 s. The spin-coating formulation was prepared by dissolving 72.3 mg of

2,2',7,7'-tetrakis(N,N-p-dimethoxy-phenylamino)-9,9'-spirobifluorene (spiro-MeOTAD) purchased from Yingkou OPV Tech New Energy Co., Ltd. (Shanghai, China), 30 μl of

4-tert-butylpyridine (tBP) and 20 μl of a stock solution of 520 mg ml−1 lithium bis(trifluoromethylsulphonyl)imide (Li-TFSI) in acetonitrile in 1 ml of chlorobenzene. Finally, 90-nm-thick

gold electrodes were deposited via thermal evaporation. The active electrode area was fixed at 9 mm2. For the light-assisted method of sequential deposition used to prepare the perovskite,

PbI2 in DMF at a concentration of 450 mg ml−1 was spin coated onto an FTO/TiO2 substrate (3000 rpm for 30 s), followed by drying at 90 °C for 10 min. An MAI/MACl (45/5 mg ml−1) mixed

solution was then spin coated onto the prepared PbI2 film (4000 rpm for 30 s), followed by annealing at 110 °C for 10 min. Three batches of samples were prepared for comparison: annealing

without light (0 min), annealing with 5 min of light exposure (5 min), and annealing with 10 min of light exposure (10 min). The light intensity was ~40 mW cm−2. All samples were treated

with the same annealing temperature and duration (110 °C/10 min) and were then solvent annealed with DMF at 100 °C for 40 min. For XRD characterizations, these as-prepared samples were

directly mounted on the sample stage. For the preparation of solar cell devices, these samples were covered with an electron-blocking layer of spiro-MeOTAD and a gold layer, as described

above. X-ray powder diagrams were recorded using an X’PertMPD PRO from PANalytical equipped with a ceramic tube (Cu anode, _λ_=1.54060 A), a secondary graphite (002) monochromator and an

RTMS X’Celerator detector, operating in the Bragg-Brentano geometry. For a description of the steady-state photoluminescence (PL) spectra and time-resolved fluorescence spectra, which were

recorded using a high-resolution streak camera system (Hamamatsu C10910), and the current-voltage measurements, please refer to reference 12. X-ray photoelectron spectra were measured using

a PHI Quantera SXM system (Chigasaki, Kanagawa, Japan) under a vacuum of 1.0 × 10−7 Torr, after using an Ar+ gun at 2 kV/30 s to clean the sample surface. For high-field electric poling

experiments, four pairs of electrodes were prepared on a glass substrate using a hard silica template with four pairs of square apertures (with 50-μm gaps and 500-μm widths). The thickness

of the gold electrodes was ~0.8 μm. After 30 s of oxygen plasma treatment to remove residual organic material to enhance the wetting properties of the plates, an MAPbI3 precursor solution

(MAI/PbAc2 (Sigma-Aldrich) (3:1)) was spin coated onto the substrate, followed by annealing at 100 °C for 50 min in ambient air (30% relative humidity). In the high-field poling experiments,

optical images under different illuminations were recorded in ambient air at room temperature (~300 K). _In situ_ observations of the ion drift were performed by placing the samples under

an optical microscope (Olympus BX51) coupled to a colored CMOS digital camera, model GCI-070103 (Daheng New Epoch Technology, Inc., Shanghai, China). The optical microscope was operating in

the reflection mode with the sample illuminated from the bottom. The current–voltage (I–V) and galvanostatic characteristics of the samples were obtained using an Agilent B2900 Series

precision source/measure unit (Beijing, China). The time duration for each galvanostatic measurement was 0.1 s, and the applied currents were 0.02, 0.04, 0.2, 0.8 and 2 nA for 0, 0.05, 1, 5

and 20 mW cm−2, respectively, considering that the gradually increasing conductivity under stronger illumination would have resulted in very weak signals if the same current of 0.02 nA had

been used. After these data were collected, we used the procedures described in the main text to extract the ionic and electronic conductances. Finally, the ideal formula below was used to

obtain the conductivity: , where _G_ is the conductance, _s_ is the cross-sectional area, and _L_ is the gap in the lateral device architecture. For cryogenic electrical experiments, we used

a small silica template to prepare Au electrodes confined to the sample stage in the chamber, which left a gap on the perovskite film of 50 μm in width. The cryogenic experiments were

conducted in a cryostation (Montana model C2) at temperatures ranging from 17 K to room temperature. The lateral device was directly mounted on the He-cooled cryostat with a temperature

controller, in a high-vacuum box at 0.9 μTorr. The device was measured at increments of temperature from 17 to 295 K, with stabilization for more than 10 min at each temperature. The

temperature increase was found to result in an increase of less than 2 °C in the temperature of the device under 20 mW cm−2 illumination (230V MI-150 Fiber Optic Illuminator). RESULTS AND

DISCUSSION HIGH-FIELD POLING BEHAVIOR OF THE AU/MAPBI3/AU LATERAL STRUCTURE First, for macroscopic _in situ_ detection of ionic motion under different illumination conditions, we performed

high-field electrical poling experiments using an Au/MAPbI3/Au lateral device structure with a 50-μm gap filled with MAPbI3. Because a change in the contrast of optical images recorded under

an optical microscope can be observed as a result of mobile ions under high-field poling1, 2, 25, a 100-V bias was applied to this device under three different light intensities (0, 5 and

20 mW cm−2). The dynamic process was recorded on video using a time accelerated mode (Supplementary Movie 1–3), nine snapshots of which are presented in Figure 1. Under dark conditions

(Figure 1a), no contrast change of the perovskite film induced by ion migration was observed, whereas under illumination, a black line formed after 10 s of poling (Figure 1b). Moreover, many

plane-dendrite structures formed under stronger illumination (Figure 1c), implying more severe ionic motion. Similar evidence of light-enhanced ionic motion was also observed in vacuum

under high-field poling (Supplementary Fig. S1). As illustrated in Figure 1d, I− reduction and a subsequent I2 volatilization process occurred at the cathode under high-field poling, and MA+

could also move toward the anode, where it evaporated away in the form of CH3NH2, assisted by moisture and illumination. The gradual transformation25, 26 from MAPbI3 into PbI2

(Supplementary Fig. S2) was accompanied by MA+ migration from the cathode to the anode, which together created the observed threads in the perovskite film. This scenario was further

confirmed by comparing the elemental distributions before and after high-field poling, which revealed no discernible ionic pile-up near the electrodes after high-field poling and the

formation of many voids around these black threads (Supplementary Fig. S2). CHARACTERIZATION OF LIGHT-ENHANCED IONIC TRANSPORT With these macroscopic findings in mind, we proceeded to a

quantitative evaluation of the change in the energy barrier for ion migration under a varying light intensity. Gold was used for the electrodes to guarantee ohmic contact because of the

p-type nature of our perovskite film (Supplementary Fig. S3). The time-resolved PL spectrum confirmed the high intrinsic quality of the perovskite film, with a ~270 ns lifetime

(Supplementary Fig. S3). To extract the ionic conductivity (_σ_ion), a current–voltage (I–V) scan was first performed to obtain the mixed conductivity _σ_total in the perovskite film at a 50

 V s−1 scan rate in the Au/MAPbI3/Au device. Here, the ionic accumulation effect was essentially negligible because the scanning period was much shorter than the relaxation time for ionic

motion (Figure 2a and 2c). Then, we performed galvanostatic characterization with a sufficiently weak current to separate the pure electronic conductivity _σ_e from the mixed conductivity

_σ_total, which is a standard technique in mixed conductor investigations24, 27, 28. When the current was switched on from 0 to 20 pA, the measured resistance immediately reached an initial

value and then gradually increased to a stable value (Figure 2b). The ionic migration and accumulation are the rate-determining processes, whereas the equilibrium value was determined only

by the electronic conductivity _σ_e. As the schematic illustrations (Figure 2c–2e) show, initially, both electrons and ions contributed to the conductance, corresponding to the fast-scan I-V

measurements and region I in Figure 2b. Subsequently, these mobile ions were gradually depleted because of ionic accumulation at the two sides of the film, resulting in fewer ions

contributing to the conductance of the perovskite film (region II and Figure 2d). Finally, the conductance reached a stable value, with all mobile ions blocked at the boundary; then, only

the electronic conductance remained (region III and Figure 2e). The double-layer capacitance effect was very weak2, 5 because of the small bias voltage applied in the measurements and the

high resistance of the perovskite film. An 800-nm short-pass filter was used here to exclude the influence of phonons on the ionic motion under illumination. Finally, the ionic contribution

was extracted from the mixed conductivity by subtracting the electronic conductance from the mixed conductance: _σ_ion =_σ_total−_σ_e. Through the analysis described above, we extracted the

pure electronic and ionic conductivities over a wide range of temperatures (17 to 295 K) and light intensities (0 to 20 mW cm−2) using the apparatus shown in Figure 3a, in which incident

light was shed on the sample through an objective lens (numerical aperture (NA)=0.2). Figure 3b presents the results of our galvanostatic measurements at different temperatures, where the

gradual increase in the resistance corresponds to the slow depletion of the ionic conductivity in the perovskite thin film. In addition, the relaxation times required to reach the

steady-state resistance reflect the kinetic constant for ion migration at different temperatures from 100 to 295 K, which is denoted by _k_=1/_τ_. The ionic relaxation time is inversely

correlated with temperature because of the reduced vibration frequency at lower temperature. The pure electronic resistance decreases exponentially with increasing temperature, and an

obvious linear region corresponding to defect ionization appears above 150 K (Figure 3c). The corresponding ionization energies for these defects in the perovskite film are 92, 84, 73, 44

and 53 meV under illumination intensities of 0, 0.05, 1, 5 and 20 mW cm−2, respectively. The results indicate shallow-level defects in the sample. The reduction in the ionization energy

level under illumination is attributed to a screening effect induced by the photoexcited carriers. We also observe a conductivity flip below 70 K under illumination. This behavior strongly

implies an inverse power dependence of the mobility on the temperature, _μ_∝_T_−3/2, at temperatures ranging from 150 to 17 K10, 29, originating from the acoustic phonon scattering in

perovskites. The corresponding electronic conductivity is described by the semi-empirical formula below: where _N_d is the total defect concentration, _β_ is 1/_k_B_T_, _k_B is the Boltzmann

constant, _E_a is the activation energy for trap defects, _a_ is a constant, _I_0 is the light intensity, and _f_(_I_0) is the carrier concentration generated via photoexcitation at an

excitation power of _I_0. Without photoexcitation (_f_(_I_0)=0), the conductivity flip is very weak because of the extremely low carrier concentration near 100 K. However, under constant

illumination, the second term dominates the conductivity near 100 K, where _μ_(T) manifests itself by photogenerated carriers. Using the above formula, the experimental electronic

conductivity data can be well fitted from 150 to 20 K (solid lines in Figure 3c). The temperature-dependent ionic conductivity is plotted in Figure 3d. Unlike the exponential law that

governs the electronic conductivity with respect to 1000/T, ln(_σ_ion) shows a much less clear behavior in the activation region, especially under photoexcitation. Unlike the band-like

transport observed for electrons, the formula that describes the hopping-like ionic transport depends on the energy barrier _E_a27: Through transformation, we obtain where _Z_i is the ionic

charge, _N_A is Avogadro's constant, _C_v is the concentration of intrinsic defects, _k_B is the Boltzmann constant, _V_m is the molar volume of perovskite, _D_ is the diffusion

coefficient, _G_v is the formation energy for vacancy defects, _E_a is the activation energy for ionic diffusion, and _E_aeff is defined to consider an excess vacancy formation energy in a

vacancy-mediated mechanism. This formula shows many differences from the band-like electronic transport behavior and suggests that ln(_σ_ion_T_), rather than ln(_σ_ion), should be plotted

versus 1000/T (Figure 3e). Linear regions from 140 to 295 K are observed in this plot, and the corresponding slopes markedly decrease with stronger illumination, reflecting the reduction in

the activation energy for ionic transport. A zoomed-in view of the activation region (Figure 3f) further reveals two separate linear regions of the ionic conductance, _E_a1 (T>250 K) and

_E_a2 (180<T<250 K), which are summarized in Table 1 and offer quantitative evidence for light-dependent ion migration. The reported activation energies for I− and MA+ migration range

from 0.1 to 1 eV8, 9, 10, 24. H+ migration is theoretically predicted to have an activation energy of 0.17 eV, which will decrease as a result of nuclear quantum tunneling7. The _E_a1 values

obtained in our measurements range from 0.82 to 0.14 eV and can thus be assigned to I- or MA+, whereas the _E_a2 values are quite small, from 0.13 to 0.06 eV, and therefore should be

assigned to H+. Further experimental evidence of proton migration via quantum tunneling is indicated by the crossover point of the kinetic constant near 100 K, where the effects of quantum

tunneling and thermal hopping meet26. This behavior will be discussed in our future work. The generation of protons should be closely related to MA+. An H+ ion is attracted by the lone pair

of nitrogen atom in CH3NH2, where H+ acts as a Lewis acid, leading to a charge transfer interaction; however, the resulting bond can be broken by thermal perturbation. For both I−/MA+ and

H+, the activation energy shows a marked reduction as the illumination intensity increases (Table 1), consistent with the increased ionic conductivity in perovskites under stronger

illumination. To the best of our knowledge, this result is the first demonstration of light-dependent ionic transport in organometal halide perovskites. Unlike the electronic conductance10,

29, we observe that the ionic motion is influenced by phase transition (Supplementary Fig. S4), as indicated by a jump in the conductivity at ~190 K (Figure 3f). This phenomenon should be

ascribed to the light-induced enhancement of the ion migration behavior caused by the reduced activation energy: the change in the energy barrier due to the phase transition results in a

larger degree of influence on ionic transport with a smaller activation energy. In addition, the disappearance of the linear region corresponding to _E_a1 (T>250 K) under dark condition

may be related to a larger activation energy under dark condition, which should be observed at higher temperatures. A structural transformation of the perovskite, a change in the valence of

the ions or a weakened bond strength of the MA+ may explain the reduction in the migration barrier under illumination2, 7, 20, 21, 22. Theoretical calculations show that differently charged

defects allow ionization-enhanced migration and that photoexcitation may modulate the defects’ charge, thereby influencing ionic transport7. Recent work has also revealed that polar

molecules can greatly influence MA+ motion in perovskites because of the reduced bond strength between MA+ and adjacent I− cations2. Hence, illumination may also weaken the hydrogen bonding

of MA+ by means of photo-induced carriers, thereby influencing the ionic motion in perovskites21, 30. LIGHT-INDUCED PHENOMENA IN METAL HALIDE PEROVSKITES Light-dependent ionic transport has

important implications for the effects of light exposure on perovskite solar cells, such as the photo-induced halide segregation20, 31, the giant dielectric constant23, and the

photo-instability of the long-term output16. We present a schematic diagram of the enhanced ion migration under illumination in Figure 4a. Under the same external field, the ionic

accumulation is accelerated by light exposure because of the increase in the diffusion coefficient, . Therefore, ionic transport becomes easier when the light intensity increases. The

observed halide segregation behavior under illumination in mixed MAPb(IxBr1−x)3 can be understood as an effect of light-enhanced ionic transport. For MAPb(I0.5Br0.5)3, the initial PL peak at

640 nm splits into two peaks (~650 and 750 nm) under illumination (Figure 4b), accompanied by a monotonic increase in the PL intensity near 750 nm over time. These finding indicate that ion

migration and redistribution occur when the sample is illuminated. However, this process is not fully reversible if the samples are stored under dark conditions for 60 min; only a slight

blue shift is observed in the PL (Figure 4c). Phenomenologically, we speculate that light can induce a bonding transformation, thereby changing the Gibbs energy-concentration relation

(Figure 4d), reducing the miscibility under light exposure. Meanwhile, because of the reduced energy barrier, ions move rapidly toward more stable sites to minimize the total Gibbs energy,

causing the MAPb(I0.5Br0.5)3 to separate into MAPb(IxBr1−x)3 with various doping concentrations. Although the miscibility of the mixed perovskites returns to a high level after storage under

dark conditions, _E_a also immediately increases, thereby suppressing ionic motion; consequently, the perovskite tends to remain in a metastable state. The light-induced giant dielectric

constant of this material is also related: since chemical capacitance can be induced by ionic motion in the low-frequency regime, a larger dielectric constant will result from the stronger

ionic motion under illumination, . Finally, we discuss the photo-instability observed in the stabilized power conversion efficiency (PCE) of perovskite solar cells in a glove box, which is

among the greatest challenges hindering the commercialization of perovskite solar cells. It is well known that perovskite solar cells demonstrate worse stability under illumination than

under dark conditions, even in the absence of decomposition induced by moisture or oxygen16. Moreover, this fundamental degradation mechanism is strongly related to the illumination

intensity, as shown in Figure 4e. The steady-state PCE exhibits only a slight decrease under illumination at 0.5 or 4.5 mW cm−2, but it drops to ~70% of its initial value during the first

100 s when the light intensity is increased to 30 or 100 mW cm-2. We speculate that under strong illumination, more ions tend to migrate toward the two sides of the device because of the

reduced energy barrier and that the charge transfer is consequently suppressed by these excess defects2, 8, 10, 13. Therefore, additional concerns regarding the stability of perovskite solar

cells under illumination should be raised by the light-dependent ionic transport in MAPbI3. LIGHT-ASSISTED SEQUENTIAL DEPOSITION Considering that ionic interdiffusion is involved in the

reaction between PbI2 and MAI stacking layers fabricated via sequential deposition, the limited ionic transport in the vertical direction usually results in excess PbI2 residue32.

Microstructured PbI2 has been proposed to solve this problem in previous studies33, 34, 35. Herein, inspired by the observation of light-enhanced ion migration, we developed a novel

light-assisted sequential deposition method to reduce excess PbI2. The procedures are illustrated in Figure 5a. After MAI is spin coated onto a prepared uniform film of PbI2, the subsequent

annealing process is assisted by 40 mW cm−2 light exposure. Three batches of samples were produced for comparison, corresponding to 0, 5 and 10 min of light exposure during the annealing

process (110 °C/10 min). The XRD patterns show evidence of a significant reduction in PbI2 residue (2_θ_=12.6°) in the case of light-assisted sequential deposition (Figure 5b). As a result,

the corresponding device efficiency is increased from 7.5 to 15.7% with the reduction in PbI2 (Figure 5c). The series resistances _R_s of the cells were derived from the intercepts of the

linear fitting results for plots of −d_V_/d_J_ vs (_J_SC−_J_)−1, where _J_ and _J_sc are the current density and short-circuit current density, respectively36. The derived series resistance

was found to decrease from 25.5 to 3.8 Ω cm2 (Figure 5d) as a result of light-assisted synthesis, indicating that the reduction in interfacial PbI2 enhances the charge transfer process. The

steady-state performance at the maximum power point reveals an extremely slow response of the control device (0 min) when the light is switched on (Figure 5e). This slow process is

consistent with the gradual improvement in the I–V measurements with increasing illumination duration observed for the control device (Figure 5f), in which stability is reached only after 4 

min of light exposure. To detect the location of the residual PbI2, we used time-of-flight secondary-ion-mass spectrometry (ToF-SIMS) to obtain the chemical depth profiles for lead and

iodide (Supplementary Fig. S5), which revealed that these substances are mainly located at the bottom of the film. Therefore, excess PbI2 at the bottom of the film will hinder charge

transfer, and we speculate that, facilitated or activated by light, MA+/I− will slowly move and react with PbI2 under a chemical potential gradient. This slow reaction leads to the gradual

improvement in charge transfer behavior observed here. Therefore, the results presented above further support the observation of light-enhanced ionic transport. CONCLUSIONS Light-dependent

ionic transport has been quantitatively demonstrated in perovskite film over a wide range of illumination intensities by separating the ionic conductivity from the mixed conductivity. Light

exerts a significant influence on the ionic transport in MAPbI3 by reducing the activation energies for I−/MA+ and H+. This property of MAPbI3 has important implications for the

photo-induced halide segregation, giant dielectric constant, and unstable behavior observed in perovskite films and devices. A light-assisted synthesis method is proposed to control the

formation of PbI2 residues in perovskites to ensure better film quality by means of the light-enhanced ionic interdiffusion process. Further attention should be paid to the entire perovskite

family, such as in NH2CH=NHPbI3 and CsPbI3, especially for inorganic system, to determine the role of organic component in light-induced effects. Our findings also imply a complicated

interplay between light conditions and optoelectronic properties in excited state perovskite materials as a result of ion redistribution, thus making it urgent to investigate the microscopic

mechanisms of these behaviors. REFERENCES * Xiao ZG, Yuan YB, Shao YC, Wang Q, Dong QF _et al_. Giant switchable photovoltaic effect in organometal trihalide perovskite devices. _Nat Mater_

2015; 14: 193–198. Article  ADS  Google Scholar  * Leijtens T, Hoke ET, Grancini G, Slotcavage DJ, Eperon GE _et al_. Mapping electric field-induced switchable poling and structural

degradation in hybrid lead halide perovskite thin films. _Adv Energy Mater_ 2015; 5: 1500962. Article  Google Scholar  * Azpiroz JM, Mosconi E, Bisquert J, de Angelis F . Defect migration in

methylammonium lead iodide and its role in perovskite solar cell operation. _Energy Environ Sci_ 2015; 8: 2118–2127. Article  Google Scholar  * Leijtens T, Eperon GE, Noel NK,

Habisreutinger SN, Petrozza A _et al_. Stability of metal halide perovskite solar cells. _Adv Energy Mater_ 2015; 5: 1500963. Article  Google Scholar  * Leijtens T, Srimath Kandada AR,

Eperon GE, Grancini G, D'Innocenzo V _et al_. Modulating the electron-hole interaction in a hybrid lead halide perovskite with an electric field. _J Am Chem Soc_ 2015; 137: 15451–15459.

Article  Google Scholar  * Yang TY, Gregori G, Pellet N, Grätzel M, Maier J . The significance of ion conduction in a hybrid organic-inorganic lead-iodide-based perovskite photosensitizer.

_Angew Chem Int Ed Engl_ 2015; 54: 7905–7910. Article  Google Scholar  * Egger DA, Kronik L, Rappe AM . Theory of hydrogen migration in organic-inorganic halide perovskites. _Angew Chem Int

Ed Engl_ 2015; 54: 12437–12441. Article  Google Scholar  * Eames C, Frost JM, Barnes PRF, O'Regan BC, Walsh A _et al_. Ionic transport in hybrid lead iodide perovskite solar cells. _Nat

Commun_ 2015; 6: 7497. Article  ADS  Google Scholar  * Meloni S, Moehl T, Tress W, Franckevičius M, Saliba M _et al_. Ionic polarization-induced current-voltage hysteresis in CH3NH3PbX3

perovskite solar cells. _Nat Commun_ 2016; 7: 10334. Article  ADS  Google Scholar  * Brenner TM, Egger DA, Kronik L, Hodes G, Cahen D . Hybrid organic—inorganic perovskites: low-cost

semiconductors with intriguing charge-transport properties. _Nat Rev Mater_ 2016; 1: 15007. Article  ADS  Google Scholar  * Xu JX, Buin A, Ip AH, Li W, Voznyy O _et al_. Perovskite-fullerene

hybrid materials suppress hysteresis in planar diodes. _Nat Commun_ 2015; 6: 7081. Article  Google Scholar  * Zhao YC, Zhou WK, Ma W, Meng S, Li H _et al_. Correlations between immobilizing

ions and suppressing hysteresis in perovskite solar cells. _ACS Energy Lett_ 2016; 1: 266–272. Article  Google Scholar  * Cho H, Jeong SH, Park MH, Kim YH, Wolf C _et al_. Overcoming the

electroluminescence efficiency limitations of perovskite light-emitting diodes. _Science_ 2015; 350: 1222–1225. Article  ADS  Google Scholar  * Zhu HM, Fu YP, Meng F, Wu XX, Gong ZZ _et al_.

Lead halide perovskite nanowire lasers with low lasing thresholds and high quality factors. _Nat Mater_ 2015; 14: 636–642. Article  ADS  Google Scholar  * Wei HT, Fang YJ, Mulligan P,

Chuirazzi W, Fang HH _et al_. Sensitive X-ray detectors made of methylammonium lead tribromide perovskite single crystals. _Nat Photonics_ 2016; 10: 333–339. Article  ADS  Google Scholar  *

Zhao YC, Wei J, Li H, Yan Y, Zhou WK _et al_. A polymer scaffold for self-healing perovskite solar cells. _Nat Commun_ 2016; 7: 10228. Article  ADS  Google Scholar  * Kim HS, Mora-Sero I,

Gonzalez-Pedro V, Fabregat-Santiago F, Juarez-Perez EJ _et al_. Mechanism of carrier accumulation in perovskite thin-absorber solar cells. _Nat Commun_ 2013; 4: 2242. Article  Google Scholar

  * Deng YH, Xiao ZG, Huang JS . Light-induced self-poling effect on organometal trihalide perovskite solar cells for increased device efficiency and stability. _Adv Energy Mater_ 2015; 5:

1500721. Article  Google Scholar  * Lee JW, Kim DH, Kim HS, Seo SW, Cho SM _et al_. Formamidinium and cesium hybridization for photo- and moisture-stable perovskite solar cell. _Adv Energy

Mater_ 2015; 5: 1501310. Article  Google Scholar  * Hoke ET, Slotcavage DJ, Dohner ER, Bowring AR, Karunadasa HI _et al_. Reversible photo-induced trap formation in mixed-halide hybrid

perovskites for photovoltaics. _Chem Sci_ 2015; 6: 613–617. Article  Google Scholar  * Gottesman R, Gouda L, Kalanoor BS, Haltzi E, Tirosh S _et al_. Photoinduced reversible structural

transformations in free-standing CH3NH3PbI3 perovskite films. _J Phys Chem Lett_ 2015; 6: 2332–2338. Article  Google Scholar  * Gottesman R, Haltzi E, Gouda L, Tirosh S, Bouhadana Y _et al_.

Extremely slow photoconductivity response of CH3NH3PbI3 perovskites suggesting structural changes under working conditions. _J Phys Chem Lett_ 2014; 5: 2662–2669. Article  Google Scholar  *

Juarez-Perez EJ, Sanchez RS, Badia L, Garcia-Belmonte G, Kang YS _et al_. Photoinduced giant dielectric constant in lead halide perovskite solar cells. _J Phys Chem Lett_ 2014; 5:

2390–2394. Article  Google Scholar  * Yokota I . On the theory of mixed conduction with special reference to the conduction in silver sulfide group semiconductors. _J Physical Soc Japan_

1961; 16: 2213–2223. Article  ADS  Google Scholar  * Yuan YB, Chae J, Shao YC, Wang Q, Xiao ZG _et al_. Photovoltaic switching mechanism in lateral structure hybrid perovskite solar cells.

_Adv Energy Mater_ 2015; 5: 1500615. Article  Google Scholar  * Yuan YB, Wang Q, Shao YC, Lu HD, Li T _et al_. Electric-field-driven reversible conversion between methylammonium lead

triiodide perovskites and lead iodide at elevated temperatures. _Adv Energy Mater_ 2016; 6: 1501803. Article  Google Scholar  * Mizusaki J, Arai K, Fueki K . Ionic conduction of the

perovskite-type halides. _Solid State Ion_ 1983; 11: 203–211. Article  Google Scholar  * Yokota I . On the electrical conductivity of cuprous sulfide: a diffusion theory. _J Physical Soc

Japan_ 1953; 8: 595–602. Article  ADS  Google Scholar  * Oga H, Saeki A, Ogomi Y, Hayase S, Seki S . Improved understanding of the electronic and energetic landscapes of perovskite solar

cells: high local charge carrier mobility, reduced recombination, and extremely shallow traps. _J Am Chem Soc_ 2014; 136: 13818–13825. Article  Google Scholar  * Wu XJ, Yu H, Li LK, Wang F,

Xu HH _et al_. Composition-dependent light-induced dipole moment change in organometal halide perovskites. _J Phys Chem C_ 2015; 119: 1253–1259. Article  Google Scholar  * McMeekin DP,

Sadoughi G, Rehman W, Eperon GE, Saliba M _et al_. A mixed-cation lead mixed-halide perovskite absorber for tandem solar cells. _Science_ 2016; 351: 151–155. Article  ADS  Google Scholar  *

Xiao ZG, Bi C, Shao YC, Dong QF, Wang Q _et al_. Efficient, high yield perovskite photovoltaic devices grown by interdiffusion of solution-processed precursor stacking layers. _Energy

Environ Sci_ 2014; 7: 2619–2623. Article  Google Scholar  * Zhang H, Mao J, He HX, Zhang D, Zhu HL _et al_. A smooth CH3NH3PbI3 film via a new approach for forming the PbI2 nanostructure

together with strategically high CH3NH3I concentration for high efficient planar-heterojunction solar cells. _Adv Energy Mater_ 2015; 5: 1501354. Article  Google Scholar  * Liu TH, Hu Q, Wu

J, Chen K, Zhao LC _et al_. Mesoporous PbI2 scaffold for high-performance planar heterojunction perovskite solar cells. _Adv Energy Mater_ 2016; 6: 1501890. Article  Google Scholar  * Wu CG,

Chiang CH, Tseng ZL, Nazeeruddin MK, Hagfeldt A _et al_. High efficiency stable inverted perovskite solar cells without current hysteresis. _Energy Environ Sci_ 2015; 8: 2725–2733. Article

  Google Scholar  * Shi JJ, Dong J, Lv ST, Xu YZ, Zhu LF _et al_. Hole-conductor-free perovskite organic lead iodide heterojunction thin-film solar cells: high efficiency and junction

property. _Appl Phys Lett_ 2014; 104: 063901. Article  ADS  Google Scholar  Download references ACKNOWLEDGEMENTS This work was supported by the National 973 Project (2013CB932602, MOST) of

the Ministry of Science and Technology of China and the National Natural Science Foundation of China (NSFC51272007, 61571015, 11327902, 11234001, 91433102 and 51522201). QZ acknowledges the

Beijing Nova Program (XX2013003) and the Program for New Century Excellent Talents in University of China. AUTHOR INFORMATION AUTHORS AND AFFILIATIONS * State Key Laboratory for Mesoscopic

Physics and Electron Microscopy Laboratory, School of Physics, Peking University, Beijing, 100871, China Yi-Cheng Zhao, Wen-Ke Zhou, Xu Zhou, Kai-Hui Liu, Da-Peng Yu & Qing Zhao *

Collaborative Innovation Center of Quantum Matter, Beijing, 100084, China Kai-Hui Liu, Da-Peng Yu & Qing Zhao Authors * Yi-Cheng Zhao View author publications You can also search for

this author inPubMed Google Scholar * Wen-Ke Zhou View author publications You can also search for this author inPubMed Google Scholar * Xu Zhou View author publications You can also search

for this author inPubMed Google Scholar * Kai-Hui Liu View author publications You can also search for this author inPubMed Google Scholar * Da-Peng Yu View author publications You can also

search for this author inPubMed Google Scholar * Qing Zhao View author publications You can also search for this author inPubMed Google Scholar CORRESPONDING AUTHORS Correspondence to

Kai-Hui Liu or Qing Zhao. ETHICS DECLARATIONS COMPETING INTERESTS The authors declare no conflict of interest. ADDITIONAL INFORMATION Note: Supplementary Information for this article can be

found on the _Light: Science & Applications’_ website. SUPPLEMENTARY INFORMATION SUPPLEMENTARY INFORMATION (DOCX 3211 KB) SUPPLEMENTARY MOVIE 1 (MOV 1043 KB) SUPPLEMENTARY MOVIE 2 (MOV

1668 KB) SUPPLEMENTARY MOVIE 3 (MOV 1474 KB) RIGHTS AND PERMISSIONS This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License. The images or

other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in the credit line; if the material is not included under the

Creative Commons license, users will need to obtain permission from the license holder to reproduce the material. To view a copy of this license, visit

http://creativecommons.org/licenses/by-nc-nd/4.0/ Reprints and permissions ABOUT THIS ARTICLE CITE THIS ARTICLE Zhao, YC., Zhou, WK., Zhou, X. _et al._ Quantification of light-enhanced ionic

transport in lead iodide perovskite thin films and its solar cell applications. _Light Sci Appl_ 6, e16243 (2017). https://doi.org/10.1038/lsa.2016.243 Download citation * Received: 06 July

2016 * Revised: 24 September 2016 * Accepted: 18 October 2016 * Published: 21 October 2016 * Issue Date: May 2017 * DOI: https://doi.org/10.1038/lsa.2016.243 SHARE THIS ARTICLE Anyone you

share the following link with will be able to read this content: Get shareable link Sorry, a shareable link is not currently available for this article. Copy to clipboard Provided by the

Springer Nature SharedIt content-sharing initiative KEYWORDS * activation energy * ion migration * light * perovskite