Interstitial Mn2+-Driven High-Aspect ... - ACS Publications

Mar 21, 2018 - value from 0 to 0.07° was observed for Mn content of the precursor increasing ... SEM images, magnified SEM images, contact angle resu...
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Interstitial Mn2+-Driven High-Aspect-Ratio Grain Growth for Low-Trap-Density Microcrystalline Films for Record Efficiency CsPbI2Br Solar Cells

Dongliang Bai,†,# Jingru Zhang,†,# Zhiwen Jin,*,† Hui Bian,† Kang Wang,† Haoran Wang,† Lei Liang,† Qian Wang,*,† and Shengzhong Frank Liu*,†,‡ †

Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education, Shaanxi Key Laboratory for Advanced Energy Devices, Shaanxi Engineering Lab for Advanced Energy Technology, and School of Materials Science & Engineering, Shaanxi Normal University, Xi’an 710119, People’s Republic of China ‡ Dalian National Laboratory for Clean Energy, and iChEM, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, People’s Republic of China S Supporting Information *

ABSTRACT: It is imperative to develop a large-aspect-ratio grainbased thin film with low trap density for high-performance inorganic perovskite CsPbI2Br solar cells. Herein, by using Mn2+ ion doping to modulate film growth, we achieved CsPbI2Br grains with aspect ratios as high as 8. It is found that Mn2+ ions insert into the interstices of the CsPbI2Br lattice during the growth process, leading to suppressed nucleation and a decreased growth rate. The combination aids in the achievement of larger CsPbI2Br crystalline grains for increased JSC values as high as 14.37 mA/cm2 and FFs as large as 80.0%. Moreover, excess Mn2+ ions passivate the grain boundary and surface defects, resulting in effectively decreased recombination loss with improved hole extraction efficiency, which enhances the built-in electric field and hence increases VOC to 1.172 V. As a result, the champion device achieves stabilized efficiency as high as 13.47%, improved by 13% compared with only 11.88% for the reference device.

H

deficiencies of the single single-halide perovskites were quickly exposed. Even though the perovskite CsPbBr3 shows very good stability, its bandgap is as high as 2.3 eV, too large to absorb light beyond 540 nm in the long-wavelength spectrum.25−27 Although CsPbI3 has a more suitable bandgap of 1.73 eV, it suffers from the notorious phase instability.28−30 The dualhalide perovskites were, therefore, developed. In particular, the perovskite CsPbI2Br shows a reasonable bandgap of 1.91 eV with much improved ambient stability.31,32 Hence, everincreasing efforts are being directed to the development of high-performance all-inorganic CsPbI2Br PSCs.33−36 In this regard, larger microcrystalline grains with fewer boundaries are found to be key factors to increase the solar cell efficiency.37−39 It appears that grain boundaries in the perovskite film cause charge recombination at their associated charge trap states.40 Meanwhile, the grain boundaries may

ybrid organic−inorganic perovskites are spotlighted as very promising next-generation photovoltaic materials.1−4 With their superior low exciton binding energy and ambipolar charge-transport characteristics, perovskitebased solar cells (PSCs) have displayed a power conversion efficiency (PCE) as high as 22.7%, attracting remarkable attention from both academic and industrial communities.5−8 Despite the rapid progress, the intrinsic volatility of organic components in the hybrid perovskites remains a major limitation to survival in harsh operating environments.9−12 Although cation-exchange13,14 and encapsulation15,16 have been developed to produce more stable perovskite devices against some stress conditions, challenges remain in overcoming degradation related to all environmental parameters, including moisture corrosion,17 electric-field-induced degradation,18 thermal aging,19 UV irradiation, and photo-oxidation.20 More recently, all-inorganic cesium lead halide [CsPbX3 (X = halide)] perovskite has been developed for its intrinsic stability against the above-mentioned stress conditions and for its attractive optoelectronic performance.21−24 Advantages and © XXXX American Chemical Society

Received: February 14, 2018 Accepted: March 21, 2018 Published: March 21, 2018 970

DOI: 10.1021/acsenergylett.8b00270 ACS Energy Lett. 2018, 3, 970−978

Letter

Cite This: ACS Energy Lett. 2018, 3, 970−978

Letter

ACS Energy Letters

Figure 1. (a) Schematic structure of the device and illustration of the Mn2+ doping modes: interstitial and substituting; (b) cross-sectional SEM image of the completed device; (c) XRD patterns of the CsPbBrI2 films doped using different MnCl2 concentrations; (d) magnification of a portion of the XRD patterns.

the doped CsPbBrI2 film achieved a stabilized record efficiency as high as 13.47%. Figure 1a illustrates the cell design used for the inorganic CsPbI2Br PSCs including consisting of FTO glass/TiO2/ CsPbBrI2/CsPbI2Br QDs/PTAA/Au, as illustrated in Figure 1a, wherein the TiO2 film deposited on the FTO glass is employed as the electron-transport layer (ETL); the CsPbI2Br film fabricated thereupon acts as the active absorbing layer; the CsPbI2Br quantum dots (QDs) were introduced to form a graded structure to enhance hole extraction; the PTAA film is the hole-transport layer (HTL); and the Au coating is the top anode. Figure 1b is a cross-sectional scanning electron microscopy (SEM) image of the completed device. The mixed perovskite precursor contains equivalent molar amounts of PbI2 and CsBr, with additional MnCl2 in molar ratios of 0, 0.5, 1, and 2% with respect to PbI2. It has been reported that the impurity ions usually substitute for Pb ions or Cs ions in the lattice.44,45 Because Mn2+ is smaller, there is less resistant to being located in the interstices of the CsPbI2Br lattice.46 X-ray diffraction (XRD) of the CsPbI2Br films was measured to identify a change in the crystal structure, and the results are shown in Figure 1c. In agreement with there being no overall structural change, the Mn2+ ions doped in CsPbI2Br film formed a cubic perovskite structure.48 This indicates that Mn in the CsPbI2Br formed an excellent solid solution, retaining the structure of the perovskite host. The peaks at 14.6 and 29.5° are assigned to the (100) and (200) planes of CsPbI2Br, respectively.32 In the case of substitution, the XRD peak shifts to higher 2θ, indicating the incorporation of smaller Mn into the lattice sites of Pb.49,50 However, in our work, a monotonic shift of the XRD peaks to lower angles with increasing Mn content indicates an expansion of the crystalline lattice.51 The phenomenon indicates that Mn2+ ions were inserted into the interstices of the CsPbI2Br lattice, resulting in expansion of the

induce shallow states near the valence band edge that will hinder hole diffusion.41 Hence, it is desired to develop a CsPbI2Br film with a large grain size and low density of charge traps.42 For this purpose, impurity doping by incorporating different ions into the host lattice has been extensively explored to modulate film performance.43 For instance, it has been proposed to partially substitute A-site cations in the ABX3 perovskite to increase the grain size. By potassium incorporation into the CsPbI2Br, larger CsPbI2Br crystallites were obtained for improved charge carrier formation and transport, leading to increased PCE.44 More recently, it was reported that by substituting on the B-site with strontium cations the Srenriched CsPbBrI2 surface formed exhibits effective passivation, leading to the PCE being further enhanced to 11.3%.45 Despite the rapid progress, the highest PCE achieved to date for the allinorganic CsPbBrI2 solar cells is still much lower that of the organic−inorganic hybrid perovskite solar cells. One effective route is to further maximize the aspect ratio of the microcrystalline grains to minimize the trap density to achieve optimum device performance. In this Letter, we report CsPbBrI2 films with an aspect ratio as high as 8 achieved by incorporating manganese cations (Mn2+) into the perovskite lattice. Differing from previous reports in which Mn2+ ions substituted for Pb ions in the lattice without changing the original octahedral structure with Pb cations coordinated by six halide atoms,46,47 the Mn2+ ions in this work were found to lie in the interstices of the CsPbI2Br lattice structure during the thin film growth process, showing advantages in terms of suppressed nucleation and decreased deposition rate, leading to improved microcrystalline thin films. Furthermore, the excess Mn2+ ions chemisorbed at the grain boundary surfaces provide effective passivation, leading to enhanced hole extraction efficiency due to decreased recombination loss. As a result, the optimized device using 971

DOI: 10.1021/acsenergylett.8b00270 ACS Energy Lett. 2018, 3, 970−978

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ACS Energy Letters

Figure 2. SEM images, magnified SEM images, contact angle results, and grain size distribution histograms for the CsPbBrI2 films doped using different MnCl2 concentrations: (a−c) without MnCl2, (d−f) 0.5% MnCl2, (g−i) 1% MnCl2, (j−l) 2% MnCl2.

CsPbI2Br films could prevent water entry and thus slow the degradation of perovskite solar cells in air.53 For the purpose of identifying the chemical states of the fabricated films, X-ray photoelectron spectra (XPS) were investigated. Figure S3 is a plot of the XPS results for the 2% MnCl2-doped CsPbI2Br film. As expected, there are small binding energy peaks at 655 eV, corresponding to Mn 2p core levels.54 Figure 3a shows the XPS spectra for all relevant elements including Cs, Pb, Br, and I. Notably, in the Mn-doped CsPbI2Br films, the peaks for all of the elements are seen slightly shifted compared to the reference film without Mn. As previous research by Xu et al. has concluded that simple physical mixing could not cause any remarkable chemical state change,55 nor by a little perturbation on the stoichiometric ratio between I and Br in the mixed-halide perovskites,56 we attribute the shifts in the XPS peaks to Mn2+ doping. Together with the XRD results, we conclude that Mn is doped into the interstice of the CsPbI2Br lattice. Energy-dispersive X-ray (EDX) analysis was conducted to detect the distribution of Mn. Figure S4 shows the SEM image of the CsPbBrI2 film with six red dots identifying the locations where corresponding EDX spectra were taken. The table shows the EDX results. It is clear that dots 9, 10, and 14 from the top surface of three microcrystalline grains show a very small amount of Mn ranging from 0 to 0.2%. In comparison, dots 11,

crystalline lattice. As in Figure 1d, a negative shift of the 2θ value from 0 to 0.07° was observed for Mn content of the precursor increasing from 0 to 1%. With further enhancement of the Mn content to 2%, the 2θ value did not change. The compact and homogeneous CsPbI2Br layers are illustrated in Figure 2, and it is obvious that the grains gradually grew from 500 to 1200 nm (average value) with increased MnCl2 content. Such enlarged grain size is attributed to the introduction of MnCl2. Most grains in CsPbI2Br films with MnCl2 doping were much larger than the film thickness, with the average grain size/film thickness aspect ratio reaching 3.3, 4.6, 5.3, and 8 for 0, 0.5, 1, and 2% MnCl2, respectively. However, when the MnCl2 content exceeds 2%, the MnCl2 is difficult to dissolve. The presence of chlorine on the surface makes it more hydrophobic.52 Figure S1 shows SEM images of CsPbI2Br layers with PbCl2 doping. There is essentially no change with PbCl2 content, which indicates that for the films with Mn, the enhanced grain size is caused by the Mn. Meanwhile, the J−V characteristics for CsPbBrI2 PSCs doped with PbCl2 at different concentrations (shown in Figure S2 and Table S1) were also conducted, which reported on changes in the device performance. The contact angles of water on these CsPbI2Br films are 44.9, 66.2, 66.8, and 69.7° for 0, 0.5, 1, and 2% MnCl2, respectively. The enhanced hydrophobicity of the 972

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ACS Energy Letters

Figure 3. (a) XPS spectra for Cs 3d5, Pb 4f, Br 3d, and I 3d5 for pristine and MnCl2-doped CsPbBrI2 films; (b) vertical compositional analysis from the surface (deduced via etching and XPS characterization) for MnCl2-doped CsPbBrI2 films; (c) EDS mapping images for a 2% MnCl2doped CsPbBrI2 film.

contributes to improving the photoinduced charge carrier generation and transport. Photophysical characteristics of Mn-containing CsPbI2Br were also examined. Figure 4b shows the absorbance spectra of fabricated films, whose thicknesses were carefully controlled to be the same. The films show nearly the same absorption, indicating that the Mn in the samples caused at most a negligible loss due to parasitic absorption. The electron extraction dynamics were investigated via photoluminescence (PL) and time-resolved PL (TRPL) of samples with the structure glass/TiO2/perovskite film, and results are shown in Figure 4c,d. The 510 nm light was incident from the glass side of the film as the photon excitation light. The CsPbI2Br films with or without Mn doping exhibited a broad emission band centered at 647 nm originating from CsPbI2Br. However, the Mn-doped CsPbI2Br film had strong PL quenching. The TRPL decay curves were fitted using a biexponential decay function [times (τi) and amplitudes (Ai)], with the relevant key parameters listed in Table 1. The average recombination lifetime (τave) was estimated using58

12, and 13 in the grain boundaries give much higher Mn concentration, ranging from 2.8 to 4.0. It is evident that the Mn concentration in grain boundaries is much higher than that on the grain surface, demonstrating that the excess amount of Mn2+ mainly exists in grain boundaries. Shown in Figure 3b, the Mn/Pb and Mn/Cl atomic ratios at the surface for the MnCl2-doped CsPbI2Br film are greater than approximately ∼0.2 and ∼0.5, respectively. At a depth of 2 nm, the Mn/Pb atomic ratio drops dramatically to ∼0.075 while the Mn/Cl atomic ratio remains at ∼0.5. Clearly, Cl− was believed to be distributed accompanied by the excessive Mn2+. However, their ratios are higher than the molar ratios used in the synthesis of the film. This shows that the film surface is strongly enriched with Mn and Cl. At etching depths larger than 10 nm, photoemission of Mn 2p and Cl 2p could not be resolved because the low atomic percentage was below the detection limits. Energy-dispersive X-ray spectroscopy (EDS) mapping was carried out on the 2% MnCl2-doped CsPbI2Br film, with results shown in Figure 3c, which shows that all of the elements, including Mn and Cl, were distributed homogeneously throughout the film. A summary of the mechanisms of CsPbI2Br formation with and without Mn is outlined in Figure 4a: After film fabrication from the solution, the Mn was distributed uniformly throughout the precursor film. During the annealing process, the CsPbI2Br crystal formed and enlarged. With added Mn, a small amount of the Mn was inserted into the interstices of the CsPbI2Br lattice, and the increase of the lattice constant caused a slight structure change. Meanwhile, the excess Mn2+ ions were situated around the formed CsPbI2Br crystal. Such a distorted structure and Mn2+-enriched environment slows down the perovskite crystallization process, forming highly crystalline, uniform perovskite layers.57 Ultimately, the excess Mn2+ aggregated in the grain boundaries, especially on the surface of the CsPbBrI2 film providing a passivating effect, which may further improve the perovskite grain quality. This surely

τave =

∑ Ai τi 2 ∑ Ai τi

(1)

The PL lifetime shortens from 1.42 to 0.56 ns for MnCl2 from 0 to 2%. Such dramatic PL quenching and lifetime shortening of the Mn-doped CsPbI2Br film indicates faster electron extraction than that for the CsPbI2Br film. This suggests better grain boundaries and surface passivation provided by the Mnenriched surface, as is evident in the XPS results. In order to confirm the improved performance mainly due to bigger grain sizes and exclude the Mn2+-doped effect on electrical properties, the I−V curves of CsPbI2Br film-based CsPbI2Br QDs with and without Mn2+ doping are shown in Figure S5. Clearly, there are no changes in the conductivity of the Mn2+ doping system compared to the pristine CsPbI2Br. Such a phenomenon has also been verified by the calculation, as 973

DOI: 10.1021/acsenergylett.8b00270 ACS Energy Lett. 2018, 3, 970−978

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ACS Energy Letters

Figure 4. (a) Schematic diagram for MnCl2-driven crystalline grain growth with surface passivation; (b) absorption spectra; (c) PL spectra; (d) decay curves; (e) dark I−V measurements of the devices with VTFL kink point behavior; (f) Nyquist plots; (g) Mott−Schottky fitting to the CV data.

Table 1. Comparison of the Performance Parameters of the PSCs Based on Different CsPbBrI2 Filmsa

a

film

JSC (mA/cm2)

VOC (V)

FF (%)

PCE (%)

JSC(EQE) (mA/cm2)

τave (ns)

Vb (V)

VTFL (V)

pristine MnCl2 0.5% MnCl2 1% MnCl2 2%

14.15 14.21 14.29 14.37

1.115 1.133 1.144 1.172

75.3 76.8 79.9 80.0

11.88 12.36 13.07 13.47

13.82 13.86 13.93 14.09

1.42 0.97 0.76 0.56

1.01 1.03 1.05 1.08

1.98 1.58 1.34 0.92

nt (cm−3)

τn (ms)

× × × ×

0.11 0.68 1.07 1.34

2.3 1.8 1.5 1.1

1016 1016 1016 1016

Extracted from Figure 4 and Figure 5.

reported: the Mn introduces localized electronic states at ∼6.5 eV below the valence band maximum (VBM) and between 4 and 6 eV above the VBM. The result shows negligible contributions of the Mn orbitals to the band edges.47 The limited variation of the electronic structures at the band edges is in good agreement with our experiments. We have also investigated the influence of aggregated Mn2+ on the trap state density in the CsPbI2Br films. We collected dark current voltage (I−V) characteristics for devices with an FTO/TiO2/perovskite/PCBM/Ag architecture to measure the trap densities. The dark I−V characteristics of representative devices are presented in Figure 4e. The trap state density was determined from the trap-filled limit voltage (VTFL) using59

VTFL =

entrapL2 2ε0ε

(2)

where e is the electron charge, L is the thickness of the electrononly device, ε is the relative dielectric constant for CsPbI2Br, and ε0 is the vacuum permittivity. For Mn-doped CsPbI2Br films, the Mn2+ aggregated in the grain boundaries, especially on the surface of the CsPbBrI2 film, appears to show significantly reduced electron trap density. The results are in good agreement with the steady-state PL data in Figure 4c. From electrical impedance spectroscopy (EIS, Figure 4f), the effective lifetime, τn, can be extracted from the peak of the large semicircle in the Cole−Cole plot and is related to the reciprocal of the frequency at the peak60 974

DOI: 10.1021/acsenergylett.8b00270 ACS Energy Lett. 2018, 3, 970−978

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ACS Energy Letters

Figure 5. Comparison of the related PSCs’ performance for CsPbBrI2 films doped with MnCl2 at different concentrations: (a) J−V characteristics and (b) EQEs and the integrated products of the EQE curves with the AM1.5G photon flux; (c) PCE distribution histograms of 30 devices. Performance of the champion device: (d) J−V characteristics under both the reverse and forward scan directions. (e) PCE and JSC measured as a function of time for the cells biased at 1 V. (f) Long-term stability of the best-performing device stored without encapsulation (25 °C and RH = 25−35%).

τn =

1 2πf

PCE, respectively. When the MnCl2 concentration was further increased to the saturated 2%, the PCE reached a maximum of 13.47% (as well as a VOC of 1.172 V, a JSC of 14.37 mA/cm2, and a FF of 80.0%). Figure 5b shows the corresponding external quantum efficiency (EQE), which agrees with the J−V measurements: the highest EQE was achieved using the highest Mn concentration, while the reference device without Mn doping and those using less Mn gave lower EQE responses. It should be noted that the JSC values calculated using the EQE curves are consistent with the J−V measurements (errors less than 5%), indicating that the latter is well calibrated. To determine the reproducibility of the devices, 30 individual devices of each type were fabricated, and histograms of their PCE distributions are shown in Figure 5c. It appears that the PCE has a fairly narrow distribution, meaning that the devices display very good reproducibility. Figure 5d presents typical J−V curves of the best-performing device measured using both the reverse and forward scan directions, with the key parameters are summarized in the inset. It should be noted that the device shows very little hysteresis, as evidenced by the very similar J−V curves measured using the reverse and forward scan directions. To ensure the reliability of the J−V measurements, the PCEs of the best-performing devices were recorded as a function of time, during which the cells were biased at their respective VMP values (1 V), as presented in Figure 5e. When measured for over 100 s, the PCE of the best-performing reference device was measured at 13.37% with a stable JSC (13.37 mA/cm2). The PCE values were very close to those obtained by the direct J−V measurements. We also examined the long-term stability of low-temperature inorganic perovskite solar cells under dark storage conditions as well as under operating conditions [25 °C and relative humidity (RH) = 25−35%], and the results are shown in Figure 5f. The PCE of the device dropped only slightly to 97% of the initial value. Such long-term stability of PSCs is closely related to the quality of the perovskite active layers, which themselves must have excellent long-term stability.

(3)

The τn values for the devices with Mn (1.34, 1.07, and 0.68 ms) are longer than that of the reference (0.11 ms), also indicating reduced carrier recombination and more efficient carrier transport. Capacitance−voltage measurements were also performed to further understand the effect of the Mn passivation. Figure 4g shows the capacitance−voltage (C−2−V) plots for the devices, from which the built-in potentials can be extracted based on the Mott−Schottky equation61,62 2(V − V ) 1 = 2 bi C2 A eεε0NA

(4)

where Vbi is the built-in potential, V is the applied voltage, A is the device area, NA is the doping concentration, ε is the relative permittivity, and ε0 is the vacuum permittivity. The built-in potential of the optimized device with Mn doping is the highest at approximately 1.08 V, which is much higher than that of the reference. A larger built-in potential means an enhanced driving force for the separation of photogenerated carriers as well as an extended depletion region for efficient suppression of electron− hole recombination. Therefore, the introduction of the Mn2+ directly contributes to the output voltage of corresponding solar cells. The prepared film was used as the photoactive layer to construct PSCs for photovoltaic performance evaluation. The current density−voltage (J−V) curve and photovoltaic parameters are presented in Figure 5a and Table 1. The reference PSC had inferior performance with a PCE of 11.88%, a VOC of 1.115 V, a short-circuit current density (JSC) of 14.15 mA/cm2, and a fill factor (FF) of 75.3%. Even with the use of a small amount of MnCl2 (0.5%), the device performance is improved significantly to a PCE of 12.36%, with the corresponding VOC, JSC, and FF improved to 1.133 V, 14.21 mA/cm2, and 76.8% respectively. When 1% MnCl2 was used, all of the device performance metrics increased markedly to 1.144 V, 14.29 mA/cm2, 79.9%, and 13.07% for VOC, JSC, FF, and 975

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In summary, interstitial Mn2+ ions doped in a CsPbI2Br film were demonstrated to be effective in enhancing the aspect ratio of the crystalline grains. In addition, the excess Mn2+ ions were found to aggregate in the grain boundaries, especially on the surface of the CsPbBrI2 film, providing a passivating effect. The champion device shows an increased VOC of 1.172 V, JSC of 14.37 mA/cm2, and FF of 80.0%, with an overall PCE of 13.47%, which is a nearly 13% increase compared with 11.88% for the reference device. This work provided a simple compositional engineering technique for achieving a highaspect, low-defect density, high-quality inorganic perovskite polycrystalline thin film, which possesses much improved optoelectronic properties, such as fewer bulk and surface traps and higher carrier mobilities. We believe that such improved properties have potential applications in other types of devices, such as high mobility for transistors, higher responsivity and lower noise for photodetectors, lower driving voltage for lightemitting diodes, and higher efficiency for lasers.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.8b00270. Experimental section, SEM images of used films, and properties of the fabricated PSCs (PDF)

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Letter

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Z.J.). *E-mail: [email protected] (Q.W.). *E-mail: [email protected] (S.L.). ORCID

Jingru Zhang: 0000-0002-4923-0208 Zhiwen Jin: 0000-0002-5256-9106 Hui Bian: 0000-0002-5761-2601 Qian Wang: 0000-0003-2286-9164 Shengzhong Frank Liu: 0000-0002-6338-852X Author Contributions #

D.B. and J.Z. contributed equally to this work. D.B., Z.J., and Q.W. performed the experiments, data analysis, and experimental planning. The project was conceived of, planned, and supervised by Z.J., Q.W., and S.L. The manuscript was written by Z.J., Q.W., and S.L. All authors reviewed the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by the National Key Research and Development Program of China MOST (2017YFA0204800), National Natural Science Foundation of China (61704099/ 61674098), Fundamental Research Funds for the Central Universities (GK201703026/GK201603054), the China Postdoctoral Science Foundation (2016M602759/2017M613052), the Shaanxi Province Postdoctoral Science Foundation (2017BSHYDZZ04), Natural Science Basic Research Plan in Shaanxi Province of China (Program No. 2017JQ2038), Changjiang Scholar and Innovative Research Team (IRT_14R33), and the Chinese National 1000-talent-plan program (1110010341). 976

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DOI: 10.1021/acsenergylett.8b00270 ACS Energy Lett. 2018, 3, 970−978