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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 ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.6b08004 • Publication Date (Web): 05 Jan 2017 Downloaded from http://pubs.acs.org on January 7, 2017
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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*, Zhibin Yu* Dr. Q. Jiang, M. Chen, J. Li, Ms. X. Shan, Prof. Z. Yu Department of Industrial and Manufacturing Engineering, High-Performance Materials Institute, FAMU-FSU College of Engineering, Florida State University, Tallahassee FL 32310, USA Dr. Q. Xiao Department of Chemical and Biomolecular Engineering, University of California, Los Angeles, California 90095, USA. Dr. X. Zeng, J. Lu Chemical Sciences and Engineering Division, Argonne National Laboratory, Lemont, IL 60439. Dr. T. Besara, Y. Xin, National High Magnetic Field Laboratory, 1800 E Paul Dirac Dr., Tallahassee, FL 32310. Prof. T. Siegrist Chemical and Biomedical Engineering, FAMU-FSU College of Engineering, Tallahassee, FL 32310 Dr. M. Wang, Prof. S. Lin Department of Mechanical Engineering, Materials Science & Engineering Program, FAMU-FSU College of Engineering, Florida State University, Tallahassee FL 32310, USA Prof. B. Pan 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. Prof. C. Wang State Key Laboratory of Molecular Engineering of Polymers, Department of Macromolecular Science, Laboratory of Advanced Materials, Fudan University Shanghai, 200433, China
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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; lightemitting diodes
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Doping 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 With the recent rapid progress of halide perovskite semiconductors (MPbX3 where M= Cs+, CH3NH3+ etc.; X=Br-, Cl-, I-), practical applications in solar cells, light-emitting 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. Thus far, only a few publications have reported on the doping behaviors of halide perovskites which involved either the use of non-stoichiometric 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 a 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 ºC to 160 ºC 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 bandgap (by 300 meV) and an enhanced electrical conductivity.21 Zhang et al. incorporated Sb3+ into polycrystalline CH3NH3PbI3 films, and observed n-type doping characteristics.22
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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 lithium salt 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 1,3dioxolane/dimethoxyethane (1:1 volume ratio) served as the electrolyte. The potential on the working electrode was scanned from 2.8 V 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 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 vs. specific capacity measurement at a constant 60 µA cm-2 current density. The charge and discharge capacity reached 94.8 mAh/g 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, then remained fairly constant until the end of the 32 test cycles. 4
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The coulombic efficiency, the ratio of charge capacity to discharge capacity, reached and stayed at ~99% after 10 cycles. To be noted, 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 2 a-d shows the surface morphologies of the CsPbBr3 crystals. The starting perovskite had smooth surfaces with a faceted crystal feature (Figure 2 a and b), which evolved into a rough surface topology after the lithium ion intercalation (Figure 2 c and 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 remaind after the film had been reduced at -5V or -10V in the lithium salt electrolyte for one minute, respectively. 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 degrees in the original, the -5 V 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 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 3 a and b. In our experiment, a film of CsPbBr3 was formed on a transparent indium tin oxide 5
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(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 520-540 nm wavelength range, consistent with the direct bandgap 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 the original film. The transient response of such an electrochromic phenomenon was evaluated with a square wave bias between -5 V 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 seconds 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 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 eV and 143.4 eV can be assigned to Pb4f 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 eV 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 (CsPbBr2Cl), 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 6
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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 six months inside a nitrogen filled glove box (oxygen and moisture levels below 0.1ppm). A study is still ongoing to evaluate stability on a longer timescale. Density of state (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 bandgap of 2.15 eV. The calculated bandgap 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 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 gold (Au) wires with a diameter of 50 µm each were used to make electrical contacts on the top surface of the perovskite crystal (Figure 4b inset). The distance between the two contacts were about 200 µm. The undoped CsPbBr3 crystal showed a background current at ~10 pA from -0.15 V 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 micro crystals were formed on an ITO/glass substrate (Figure 5a, left). The thickness of the crystals was 10-20 µm. The device structure was illustrated in Figure 5a (right): ITO was connected as the anode, and a gold probe in contact with 7
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the micro crystal 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 micro crystals in a lithium salt electrolyte. An example of such a treatment is shown in Figure S3. Figure 5b and c show 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 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 (FWHM) 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, ocasionally emitting very dim light at 20-25 V in some CsPbBr3 crystals that were able to sustain such a high bias.
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 with the electrochemical doping process. The doping changed the color of the perovskites, leading to more than 40% reduction of transmittance at 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 8
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ion intercalation and electrochemical doping of halide perovskites in this work may encourage the exploration of doped perovskites for more versatile perovskite optoelectronics including LEDs, solar cells and electrically driven laser diodes.
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%), and 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), dimethyl sulfoxide (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. 200 mg PbBr2 and 116 mg CsBr were dissolved in 2 ml DMSO at 100 ℃. The precursor was spin coated at 1500 rpm (Chemat Technology) onto an ITO/glass substrate and annealed at 110 ℃ for 2 minutes. 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 200kV. The samples for TEM were dispersed on carbon/formva 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
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operating pressures around 1 × 10−9 torr. Spectra were collected with a monochromatic Al Kα source (1,486.7 eV) and a 300 × 700µm spot size. Fitting was done using the program CasaXPS. Density Functional Theory (DFT) Calculations. Calculations were based on DFT within Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation and the projected augmented wave (PAW) method performed using Vienna ab initio simulation packages (VASP). The structure models were constructed by using a 80-atom super-cell 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 quasirandom structures (SQSs) 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 polyvinylidene fluoride (PVDF) with a weight ratio of 50:25:25 were dispersed in N-methylpyrrolidone (NMP). The slurry was cast on a copper foil and dried at room temperature in glovebox (O2
