Electrochemical Doping of Halide Perovskites with Ion Intercalation

Electrochemical Doping of Halide Perovskites with Ion Intercalation Qinglong Jiang,† Mingming Chen,† Junqiang Li,† Mingchao Wang,‡ Xiaoqiao Zeng,⊥ Tiglet Besara,∥ Jun Lu,⊥ Yan Xin,∥ Xin Shan,† Bicai Pan,# Changchun Wang,△ Shangchao Lin,‡ Theo Siegrist,¶ Qiangfeng Xiao,*,§ and Zhibin Yu*,† †

Department of Industrial and Manufacturing Engineering, High-Performance Materials Institute, ‡Department of Mechanical Engineering, Materials Science & Engineering Program, and ¶Chemical and Biomedical Engineering, FAMU-FSU College of Engineering, Florida State University, Tallahassee, Florida 32310, United States § Department of Chemical and Biomolecular Engineering, University of California, Los Angeles, California 90095, United States ⊥ Chemical Sciences and Engineering Division, Argonne National Laboratory, Lemont, Illinois 60439, United States ∥ National High Magnetic Field Laboratory, 1800 E Paul Dirac Drive, Tallahassee, Florida 32310, United States # Key Laboratory of Strongly-Coupled Quantum Matter Physics, Department of Physics, Hefei National Laboratory for Physical Science at Microscale, University of Science and Technology of China, Hefei, Anhui 230026, China △ State Key Laboratory of Molecular Engineering of Polymers, Department of Macromolecular Science, Laboratory of Advanced Materials, Fudan University, Shanghai 200433, China S Supporting Information *

ABSTRACT: Halide perovskites have recently been investigated for various solution-processed optoelectronic devices. The majority of studies have focused on using intrinsic halide perovskites, and the intentional incoporation of dopants has not been well explored. In this work, we discovered that small alkali ions, including lithium and sodium ions, could be electrochemically intercalated into a variety of halide and pseudohalide perovskites. The ion intercalation caused a lattice expansion of the perovskite crystals and resulted in an n-type doping of the perovskites. Such electrochemical doping improved the conductivity and changed the color of the perovskites, leading to an electrochromism with more than 40% reduction of transmittance in the 450−850 nm wavelength range. The doped perovskites exhibited improved electron injection efficiency into the pristine perovskite crystals, resulting in bright light-emitting diodes with a low turn-on voltage. KEYWORDS: halide perovskite, electrochemical, doping, intercalation, density functional theory, light-emitting diodes

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With the recent rapid progress of halide perovskite semiconductors (MPbX3, where M = Cs+, CH3NH3+, etc.; X = Br−, Cl−, I−), practical applications in solar cells, lightemitting diodes (LEDs), and laser diodes are anticipated.10−15 The junction formation in these devices has heavily depended on using dissimilar interfacial layers including electron transportation layers (ETLs) and hole transportation layers (HTLs) that are inserted between the halide perovskite and the contact electrodes.13,16−18 Nonetheless, the effectiveness of doped perovskites to aid electron/hole collection or injection has not been examined largely due to limited studies of doping processes in halide perovskites.

oping is critical for junction formation in semiconducting thin-film electronic devices to direct the flow of charge carriers and improve their transport properties.1 The dopants come in the form of donors or acceptors and are typically added during deposition of the semiconductor thin films or introduced through a subsequent diffusion or implantation process. Doping can also be achieved using an electrochemical approach in semiconducting polymers,2 graphitic materials,3,4 and single-crystalline or polycrystalline metal oxide thin films.5−8 Electrochemical doping utilizes an external bias to reduce (n-dope) or oxidize (p-dope) the semiconductors in an electrolyte environment. Excess charge carriers are formed, accompanied by the insertion of cations or anions to maintain overall charge neutrality in the doped semiconductors.9 © 2017 American Chemical Society

Received: November 29, 2016 Accepted: January 5, 2017 Published: January 5, 2017 1073

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ACS Nano Thus far, only a few publications have reported on the doping behaviors of halide perovskites which involved either the use of nonstoichiometric perovskites19,20 or the incorporation of trivalent cations (Bi3+, Sb3+, etc.)21−24 during perovskite synthesis. For instance, Wang et al. obtained methylammonium lead triiodide (CH3NH3PbI3) films using different ratios of methylammonium iodide (MAI) and lead iodide (PbI2) in the precursor solution. It was observed that MAI-rich composition led to p-type, and a PbI2 -rich composition caused n-type doping in the CH3NH3PbI3 films.19 The ratio of MAI to PbI2 can also be tuned by annealing temperatures. As reported by Song et al., increasing the annealing temperatures from 60 to 160 °C switched the CH3NH3PbI3 film from p-type to n-type.20 Abdelhady et al. achieved Bi3+-doped MAPbBr3 single crystals. The doped perovskites exhibited a reduced band gap (by 300 meV) and an enhanced electrical conductivity.21 Zhang et al. incorporated Sb3+ into polycrystalline CH3NH3PbI3 films and observed ntype doping characteristics.22 In this work, we explored electrochemical doping of halide perovskites. As a proof of concept, cesium lead tribromide (CsPbBr3) was first used for the electrochemical doping study. It was discovered the CsPbBr3 could be reduced in a lithiumsalt-containing electrolyte. Lithium ions were intercalated into the reduced perovskite, resulting in n-type doping of the perovskite, an improved conductivity, and a reduction of transmittance in the wavelength range from 450 to 850 nm. The doped perovskite could be used as an ETL to enhance electron injection efficiency into the perovskite crystals, resulting in bright LEDs with a low turn-on voltage.

RESULTS AND DISCUSSION The electrochemical reduction process of CsPbBr3 was characterized by cyclic voltammetry (CV). A composite film consisting of the CsPbBr3, conductive carbon black, and polyvinylidene fluoride (PVDF) with a weight ratio of 50:25:25 was prepared on a copper foil and connected as the working electrode for the CV measurement. A lithium foil was used as the reference electrode, and a 1.0 M bis(trifluoromethane) sulfonimide lithium salt (LiTFSI) in 1,3-dioxolane/dimethoxyethane (1:1 volume ratio) served as the electrolyte. The potential on the working electrode was scanned from 2.8 to 0.05 V and then back to 2.8 V (vs Li/Li+) to complete one full cycle. A cathodic peak was distinguished at ∼0.93 V, as shown in Figure 1a, suggesting that an electrochemical reduction reaction had occurred. Noticeably, there were no major current changes or peak shifts after 30 cycles of CV scans. Such a result indicates that the electrochemical process that reduces the perovskite is quite reversible in this current testing environment. We speculated that lithium ions in the electrolyte had been intercalated into or expelled from the perovskite during the electrochemical reduction and oxidation processes, respectively. To further confirm this hypothesis, lithium ion coin batteries were assembled with the above CsPbBr3 composite as the working electrode. Figure 1b shows the first charge−discharge cycle of voltage versus specific capacity measurement at a constant 60 μA cm−2 current density. The charge and discharge capacity reached 94.8 and 102.6 mAh/g, respectively. The performance for a total of 32 charge−discharge cycles is summarized in Figure 1c. The charge capacity decreased to 73.8 mAh/g after 13 cycles and then remained fairly constant until the end of the 32 test cycles. The Coulombic efficiency, the

Figure 1. (a) Cyclic voltammetry measurement of CsPbBr3 vs Li+/ Li at a scan rate of 10 mV/s. The arrows indicate the scanning direction. The potential started at 2.8 V for total 30 cycles. (b) First charge−discharge cycle of voltage vs specific capacity measurement at a constant 60 μA cm−2 current density from a lithium ion coin battery with a CsPbBr3 electrode and a lithium counter. (c) Charge capacity evolution of the above coin battery for 32 charge− discharge cycles (black) and its corresponding Coulombic efficiency (blue).

ratio of charge capacity to discharge capacity, reached and stayed at ∼99% after 10 cycles. Note that our findings agree well with the recent work by Xia et al., who reported lithium ion batteries based on methylammonium lead tribromide and CH3NH3PbI3.25 In this regard, the electrochemical intercalation of lithium ions into MPbX3-type halide perovskites can be universal. However, the electrical and optical properties of the ion-intercalated perovskites were not investigated in the previous study. Figure 2a−d shows the surface morphologies of the CsPbBr3 crystals. The starting perovskite had smooth surfaces with a faceted crystal feature (Figure 2a,b), which evolved into a rough surface topology after the lithium ion intercalation (Figure 2c,d). Figure 2e shows the X-ray diffractiton (XRD) patterns of CsPbBr3 before and after lithium ion intercalation. The orignal CsPbBr3 had an orthorhombic crystal structure.26 All the characteristic peaks had remained after the film had been reduced at −5 or −10 V in the lithium salt electrolyte for 1 min. 1074

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Figure S1b, and the bubble-like shadows are indicated by arrows. Interestingly, Liu et al. also observed a similar bubbling phenomenon for their lithium-intercalated anode materials under TEM observation.27 Although the underlying mechanism of this bubbling phenomena requires a more detailed analysis, it serves as the supplementary evidence to support the lithiation process of CsPbBr3 in our study. The lithium-ion-intercalated CsPbBr3 exhibited a distinct color change, as shown in Figure 3a,b. In our experiment, a film

Figure 2. (a−d) Scanning electron microscopy images showing the surface morphologies of the CsPbBr3 crystals before (a,b) and after (c,d) lithium ion intercalation. (e) XRD patterns of CsPbBr3 before (black) and after lithium ion intercalation at −5 V (red) and −10 V (blue). (f) Normalized XRD patterns show (220) peak position shift toward lower diffraction angles after the lithium ion intercalation.

Figure 3. (a,b) Photos of CsPbBr3 samples before (a) and after (b) lithium ion intercalation, showing the electrochromism of the perovskite film. Photos by Florida State University use with permission. (c) Transmittance spectra for films before (black) and after (red) lithium ion intercalation. (d) Transient response and cyclic testing of CsPbBr3 electrochromic devices a square wave bias between −5 and +5 V.

Such a result indicates the CsPbBr3 crystal structure had not been modified after the lithium ion intercalation. A close-up look at the (220) diffraction peak (Figure 2f) shows a peak position shift from 30.78 to 30.76 and 30.74° in the original, the −5 and −10 V reduced films, respectively. These findings suggest a slight increase of the lattice contant as a consequnce of lithiation, acounting for the rough surface morphology due to inhomogeneous volume expansions as caused by surface and interior defects in the CsPbBr3 crystals. Figure S1a shows the high-resolution transmission electron microscopy (TEM) image of the original CsPbBr3 crystal along the [100] direction. However, for the samples with lithium ion intercalation, the crystal became very unstable under the electron beam irradiation during TEM imaging. The perovksite appeared to produce bubble-like shadows that moved and coalesced inside the lattice. A snapshot image is presented in

of CsPbBr3 was formed on a transparent indium tin oxide (ITO)/glass substrate. A drop of a lithium salt electrolyte was added on top of the perovskite film, followed by placing a second ITO/glass substrate directly onto the liquid electrolyte. A bias was applied between the two ITO electrodes with the perovskite side connected to the negative potential. The color of the CsPbBr3 film changed from the original light yellow to black when a −5 V bias was applied and returned to light yellow at a +5 V bias. Transmittance spectra were collected from 450 to 850 nm wavelength, as shown in Figure 3c. The original film had a noticeable transmittance change within the 520−540 nm wavelength range, consistent with the direct band 1075

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ACS Nano gap of the CsPbBr3 (∼2.3 eV).28 Such a transition became negligible in the lithium-ion-intercalated film, and the overall transmittance was reduced by more than 40% from 450 to 850 nm compared to that in the original film. The transient response of such an electrochromic phenomenon was evaluated with a square wave bias between −5 and +5 V. Transmitted photons were monitored by a silicon photodiode placed under the perovskite film, while a xenon lamp was irradiating the opposite side of the film. As shown in Figure 3d, it took about 2 s to switch from light yellow to black and vice versa when the polarity of the applied bias was alternated. Similar peformance was seen for repeated cycles with a slight decreasing trend of transmittance. The results again indicate that the lithium ion intercalation and deintercalation processes are both reversible in CsPbBr3, in accordance with the CV and battery measurements in Figure 1. We speculated that the dark color in the reduced CsPbBr3 film was caused by the formation of metallic Pb (Pb0). Figure S2a shows the X-ray photoelectron spectroscopy (XPS) results of the Pb 4f core level in the CsPbBr3 film. Two peaks at around 138.5 and 143.4 eV can be assigned to Pb 4f 7/2 and 4f 5/2, respectively, corresponding to the Pb2+ in the pristine CsPbBr3. After −5 V biasing, the film showed two new peaks located at around 136.5 and 141.4 eV (Figure S2b), confirming the existence of Pb0 in the electrochemically reduced film.29 It is worth mentioning the source of lithium ions can be versatile. We tested LiClO4 and LiCF3SO3 as lithium ion sources, and they both produced similar electrochromic phenomena. The intercalation was extended to other perovskites including cesium lead perovskites with mixed halide anions (CsPbBr 2 Cl), methylammonium lead halides (CH3NH3PbCl3), and perovskites with pseudohalide thiocyanate anions (CsPb(SCN)2Br),30 as shown by the first row in Table S1. It was also discovered that electrochromism occurred in a sodium-ion-containing electrolyte (NaClO4), as shown in Table S1, second row. In contrast, no color change was obtained using a potassium- or cesium-based electrolyte, likely due to the large ion sizes that exceeded the intercalation limit of the perovskite crystals. It was also observed that the reduced perovskites were fairly stable: a sample maintained its dark colored state without a bias for 6 months inside a nitrogen-filled glovebox (oxygen and moisture levels below 0.1 ppm). A study is still ongoing to evaluate stability on a longer time scale. Density of states (DOS) calculations based on density functional theory (DFT) were used to investigate how the lithium ion intercalation changed the electronic properties of the perovskites. Due to its lowest formation energy, we assumed lithium ions preferentially occupied the octahedral interstitial sites over the substitutional and tetrahedral interstitial sites in the CsPbBr3 lattice (CsPbBr3:Li). In our calculation, we adopted a configuration of four unit cells (2 × 2 × 1) with one lithium ion. As shown in Figure 4a, the CsPbBr3 and CsPbBr3:Li exhibit very similar DOS spectra with an energy band gap of 2.15 eV. The calculated band gap is close to the reported experimental value.28 However, the Fermi level (Ef) in CsPbBr3:Li has moved into the conduction band. The result suggests that lithium is a donor-type impurity for CsPbBr3 and can potentially cause degenerate doping. The lithium ion doping effect was further verified by current−voltage (I−V) measurements. For such experiments, isolated CsPbBr3 crystals were formed on ITO/glass sbustrates. Two Au wires with a diameter of 50 μm each were used to make electrical contacts on the top surface of the perovskite

Figure 4. (a) DOS calculation for the intrinsic (black) and lithium ion intercalated (red) CsPbBr3. (b) I−V measurements of isolated CsPbBr3 microcrystals before (black) and after (red and blue) lithium ion intercalation. The CsPbBr3:Li samples were electrochemically reduced at −5 V for 1 or 2 min in a lithium salt electrolyte. Inset in (b) shows the experimental setup for the I−V measurements. Two Au probes were used to make electrical contacts on the CsPbBr3 microcrystals.

crystal (Figure 4b, inset). The distance between the two contacts was about 200 μm. The undoped CsPbBr3 crystal showed a background current at ∼10 pA from −0.15 to 0.15 V (Figure 4b, black line). In contrast, the current from the two lithium-ion-doped CsPbBr3 samples appeared much larger (Figure 4b, red and blue lines). The improved current can be attributed to a reduced energy barrier between the Au and perovskite and a higher electrical conductivity in the perovskite after the doping. We continued investigating the transport characteristic of the lithium-ion-doped CsPbBr3 for perovskite LED devices. The doped material was employed as an ETL between a high work function gold cathode and an intrinsic CsPbBr3 emissive layer. The fabrication details can be found in the Experimental Section. In brief, CsPbBr3 microcrystals were formed on an ITO/glass substrate (Figure 5a, left). The thickness of the crystals was 10−20 μm. The device structure is illustrated in Figure 5a (right): ITO was connected as the anode, and a gold probe in contact with the microcrystal top surface was connected as the cathode of the LEDs. In our experiments, the doped perovskite layer was formed by partially reducing the top surface of the microcrystals in a lithium salt electrolyte. An example of such a treatment is shown in Figure S3. Figure 5b,c shows schematics of energy band alignment along the gold and perovskite interface at zero bias without and with a doped perovskite ETL. There exists an energy barrier as large as 1.2 eV between the Au and the conduction band minimum that prohibits efficient electron injection through thermionic emission.31,32 By introducing a doped layer, severe band 1076

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Figure 5. (a) Microscopic optical image of a LED device with a CsPbBr3:Li ETL (left) and the device structure schematic (right). The LED was fabricated on a CsPbBr3 microcrystal. (b,c) Energy band diagram at the Au and perovskite interface at zero bias without (b) and with (c) a doped perovskite ETL. (d) I−V characteristics of LEDs without (black) and with (red) a doped perovskite ETL. (e) Electroluminescence spectrum and a microscopic optical image from a lit LED with the doped perovskite ETL. Both were obtained at 3.5 V.

tronics including LEDs, solar cells, and electrically driven laser diodes.

bending can be introduced, thus facilitating electron injection through a thermionic field emission mechanism. The I−V characteristics of the LEDs are shown in Figure 5d. Notably, all LED measurements were carried out in ambient air at 50−65% relative humidity. As expected, the current increased much more rapidly and attained a value of about 20 mA at 4 V in the device with a doped CsPbBr3 ETL. Visible light emission was observed at 3 V in a dark room, proving the effectiveness of the lithium-doped perovskite in aiding electron injection. Figure 5e shows the emission spectrum of the device at 3.5 V. The spectrum has a peak intensity at 545 nm and a full width at half-maximum of 17.5 nm. The inset in Figure 5e shows a photo of a lit LED at 3.5 V. The emission was intense enough to be clearly visible even in a well-illuminated laboratory. In comparison, the device without the doped CsPbBr3 ETL had a current of 0.48 μA at 4 V, occasionally emitting very dim light at 20−25 V in some CsPbBr3 crystals that were able to sustain such a high bias.

EXPERIMENTAL SECTION Materials. Lead(II) bromide (99.999%), lead(II) chloride (99.999%), cesium bromide (99.999%), cesium chloride (99.999%), bis(trifluoromethane)sulfonimide lithium (LiTFSI, 99.95%), lithium perchlorate (LiClO4, 99.95%), lithium bromide (LiBr, 99.999%), lithium trifluoromethanesulfonate (LiCF3SO3, 99.995%), sodium perchlorate (NaClO4, 98%), potassium trifluoromethanesulfonate (KCF3SO3, 98%), cesium perchlorate (CsClO4, 99.995%), sodium thiocyanate (NaSCN, 99.99%), Pb(BF4)2 (50 wt % in H2O), dimethylsulfoxide (DMSO, anhydrous, 99.9%), 1,3-dioxolane (anhydrous, 99.8%), dimethoxyethane (anhydrous, 99.5%), dichloroethane (anhydrous, 99.8%), and diethyl carbonate (anhydrous, 99%) were purchased from Sigma-Aldrich. Methylammonium chloride was purchased from 1-Material Inc. All materials were used as received. Perovskite Film Fabrication for Electrochromic Devices. PbBr2 (200 mg) and CsBr (116 mg) were dissolved in 2 mL of DMSO at 100 °C. The precursor was spin-coated at 1500 rpm (Chemat Technology) onto an ITO/glass substrate and annealed at 110 °C for 2 min. CsPbBr2Cl, CH3NH3PbCl3, and CsPb(SCN)2Br films were prepared using a similar procedure with precursors of PbBr2 + CsCl, CH3NH3Cl + PbCl2, and Pb(SCN)2 + CsBr, respectively. Pb(SCN)2 was synthesized in-house according to literature procedures.30 Material Characterizations. High-resolution field emission SEM images were taken with JEOL 7401F. TEM was carried out with a JEOL JEM-ARM200cF at 200 kV. The samples for TEM were dispersed on a carbon/formvar grid after grinding in anhydrous alcohol. XRD measurements were collected using a Scintag PAD-V θ− 2θ diffractometer using Cu Kα radiation. Transmittance spectra were measured with a UV−vis−NIR spectrometer (Varian Cary 5000). XPS was tested on a Kratos Axis Ultra DLD surface analysis instrument by Argonne National Lab. The base pressure of the analysis chamber during these experiments was 3 × 10−10 Torr, with operating pressures around 1 × 10−9 Torr. Spectra were collected with a monochromatic Al Kα source (1486.7 eV) and a 300 × 700 μm spot size. Fitting was done using the program CasaXPS.

CONCLUSION We have presented experimental and theoretical evidence suggesting the electrochemical n-type doping of CsPbBr3. Small cations including lithium and sodium ions were intercalated into the perovskite accompanying the electrochemical doping process. The doping changed the color of the perovskites, leading to more than 40% reduction of transmittance at the 450−850 nm wavelength range. The doping also improved the transport properties of the perovskites, facilitating efficient electron injection in halide perovskite LEDs. The LEDs with a high work function Au cathode exhibited a low turn-on voltage and a high luminance, neither of which was possible in control devices without the Li-doped perovskite as an ETL. We hope the discovery of ion intercalation and electrochemical doping of halide perovskites in this work may encourage the exploration of doped perovskites for more versatile perovskite optoelec1077

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ACS Nano Density Functional Theory Calculations. Calculations were based on DFT within Perdew−Burke−Ernzerhof generalized gradient approximation and the projected-augmented wave method performed using Vienna ab initio simulation packages. The structure models were constructed by using a 80-atom supercell consisting of 2 × 2 × 1 unit cells of orthorhombic CsPbBr3. The cutoff energy for the plane-wave basis set was 400 eV. In the first Brillouin zone, the k-points were sampled with 2 × 2 × 2 Γ-centered Monkhorst−Pack meshes. By computing the quantum mechanical forces and stress tensor, the special quasi-random structures were relaxed with respect to both the volume and shape of the unit cell until their residual forces were less than 0.02 eV/Å. The atomic configurations of CsPbBr3 structures before and after lithium ion intercalation were visualized using the VESTA software package.33 Cyclic Voltammetry and Battery Measurements. The CsPbBr3, super P carbon black (General Motors Research and Development Center), and PVDF with a weight ratio of 50:25:25 were dispersed in N-methylpyrrolidone. The slurry was cast on a copper foil and dried at room temperature in a glovebox (O2 < 0.1 ppm and H2O < 0.1 ppm). The electrode loading (including binder and carbon) is about 3 mg/cm2. Afterward, the electrodes were punched into a circular disc with a diameter of 0.5 in. The coin cells, composed of the electrode, a microporous polypropylene separator, and a lithium counter electrode were assembled in an argon-filled glovebox. The electrolyte was a 1.0 M LiTFSI solution in 1,3-dioxolane/dimethoxyethane (1/1 vol %). Cyclic voltammetry was obtained at a scan rate of 10 mV/s with voltage cutoff of 0.05−2.8 V using the BioLogic (VPM3) workstation. Microcrystalline CsPbBr3 for I−V Measurements and LEDs. The CsPbBr3 microcrystals were obtained by drop-casting the precursor solution on an ITO/glass substrate, followed by slowly evaporating the solvent at 60 °C. Scattered CsPbBr3 crystals were formed with varied lateral dimensions and thicknesses. For the convenience of operation, we chose crystals with a width of 200 μm and a length of 500 μm for the I−V and LED measurements. Lithiumion-doped CsPbBr3 was obtained by electrochemically reducing the perovskite crystals in LiClO4/diethyl carbonate electrolyte. A −5 V was applied for 1 min (unless specified otherwise) to ensure that the top surface of the crystal was nearly covered by a dark coating of doped perovskite. The crystals were thoroughly rinsed with diethyl carbonate and dichloroethane, dried in air, and then ready for the I−V and LED tests. I−V characteristics were measured with a Keithley 2410 source meter. An electroluminescence spectrum was taken using a Renishaw confocal research Raman microscopy.

ACKNOWLEDGMENTS The authors are thankful for the financial support from Air Force Office of Scientific Research under Award FA9550-16-10124 (program manager Dr. Charles Lee), and the support from National Science Foundation under Award ECCS1609032 (program manager Dr. Nadia El-Masry). TEM work was performed at the National High Magnetic Field Laboratory, which is supported by NSF DMR-1157490 and the State of Florida. The FSU Research Computing Center (RCC) and the computational center of USTC are acknowledged for computational support. We thank Mr. Thomas Geske for critically reviewing the manuscript. REFERENCES (1) Sze, S. M.; Ng, K. K. Physics of Semiconductor Devices, 3rd ed.; John Wiley & Sons Inc.: Hoboken, NJ, 2008. (2) MacInnes, D.; Druy, M. A.; Nigrey, P. J.; Nairns, D. P.; MacDiarmid, A. G.; Heeger, A. J. Organic Batteries: Reversible N- and P- Type Electrochemical Doping of Polyacetylene, (CH)x. J. Chem. Soc., Chem. Commun. 1981, 317−319. (3) Bao, W.; Wan, J.; Han, X.; Cai, X.; Zhu, H.; Kim, D.; Ma, D.; Xu, Y.; Munday, J. N.; Drew, H. D.; Fuhrer, M. S.; Hu, L. Approaching the Limits of Transparency and Conductivity in Graphitic Materials through Lithium Intercalation. Nat. Commun. 2014, 5, 4224. (4) Wan, J.; Gu, F.; Bao, W.; Dai, J.; Shen, F.; Luo, W.; Han, X.; Urban, D.; Hu, L. Sodium-Ion Intercalated Transparent Conductors with Printed Reduced Graphene Oxide Networks. Nano Lett. 2015, 15, 3763−3769. (5) van de Krol, R.; Goossens, A.; Schoonman, J. Spatial Extent of Lithium Intercalation in Anatase TiO2. J. Phys. Chem. B 1999, 103, 7151−7159. (6) Pelouchova, H.; Janda, P.; Weber, J.; Kavan, L. Charge Transfer Reductive Doping of Single Crystal TiO2 Anatase. J. Electroanal. Chem. 2004, 566, 73−83. (7) Tada, H.; Nojima, T.; Nakamura, S.; Shimotani, H.; Iwasa, Y.; Kobayashi, N. Preparation of N -Type YBa2Cu3OY Films by an Electrochemical Reaction Method. J. Phys.: Conf. Ser. 2009, 150, 052255. (8) Ueno, K.; Shimotani, H.; Iwasa, Y.; Kawasaki, M. Electrostatic Charge Accumulation versus Electrochemical Doping in SrTiO3 Electric Double Layer Transistors. Appl. Phys. Lett. 2010, 96, 252107. (9) Smela, E. Conjugated Polymer Actuators for Biomedical Applications. Adv. Mater. 2003, 15, 481−494. (10) Green, M. A.; Ho-Baillie, A.; Snaith, H. J. The Emergence of Perovskite Solar Cells. Nat. Photonics 2014, 8, 506−514. (11) Kazim, S.; Nazeeruddin, M. K.; Grätzel, M.; Ahmad, S. Perovskite as Light Harvester: A Game Changer in Photovoltaics. Angew. Chem., Int. Ed. 2014, 53, 2812−2824. (12) Sutherland, B. R.; Sargent, E. H. Perovskite Photonic Sources. Nat. Photonics 2016, 10, 295−302. (13) Tan, Z.-K.; Moghaddam, R. S.; Lai, M. L.; Docampo, P.; Higler, R.; Deschler, F.; Price, M.; Sadhanala, A.; Pazos, L. M.; Credgington, D.; Hanusch, F.; Bein, T.; Snaith, H. J.; Friend, R. H. Bright LightEmitting Diodes Based on Organometal Halide Perovskite. Nat. Nanotechnol. 2014, 9, 687−692. (14) Xing, G.; Mathews, N.; Lim, S. S.; Yantara, N.; Liu, X.; Sabba, D.; Grätzel, M.; Mhaisalkar, S.; Sum, T. C. Low-Temperature Solution-Processed Wavelength-Tunable Perovskites for Lasing. Nat. Mater. 2014, 13, 476−480. (15) Cho, H.; Jeong, S.-H.; Park, M.-H.; Kim, Y.-H.; Wolf, C.; Lee, C.-L.; Heo, J. H.; Sadhanala, A.; Myoung, N.; Yoo, S.; Im, S. H.; Friend, R. H.; Lee, T.-W. Overcoming the Electroluminescence Efficiency Limitations of Perovskite Light-Emitting Diodes. Science 2015, 350, 1222−1225. (16) Zhou, H.; Chen, Q.; Li, G.; Luo, S.; Song, T.; Duan, H.-S.; Hong, Z.; You, J.; Liu, Y.; Yang, Y. Interface Engineering of Highly Efficient Perovskite Solar Cells. Science 2014, 345, 542−546.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b08004. Supplemental figures and a table summarizing the electrochromic results for a group of halide and pseudohalide perovskites in different electrolytes (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Jun Lu: 0000-0003-0858-8577 Shangchao Lin: 0000-0002-6810-1380 Zhibin Yu: 0000-0002-4630-4363 Notes

The authors declare no competing financial interest. 1078

DOI: 10.1021/acsnano.6b08004 ACS Nano 2017, 11, 1073−1079

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DOI: 10.1021/acsnano.6b08004 ACS Nano 2017, 11, 1073−1079