Letter Cite This: J. Phys. Chem. Lett. 2018, 9, 3646−3653
pubs.acs.org/JPCL
High-Voltage-Efficiency Inorganic Perovskite Solar Cells in a Wide Solution-Processing Window Linxing Zhang,† Bo Li,† Jifeng Yuan,† Mengru Wang,† Ting Shen,† Fei Huang,† Wen Wen,† Guozhong Cao,†,‡ and Jianjun Tian*,† †
Institute for Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, China Department of Materials and Engineering, University of Washington, Seattle, Washington 98195-2120, United States
‡
J. Phys. Chem. Lett. 2018.9:3646-3653. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 08/14/18. For personal use only.
S Supporting Information *
ABSTRACT: Inorganic halide perovskites exhibit significant photovoltaic performance due to their structural stability and high open-circuit voltage (Voc). Herein, a general strategy of solution engineering has been implemented to enable a wide solutionprocessing window for high Voc (∼1.3 V) and power conversion efficiency (PCE, ∼12.5%). We introduce a nontoxic solvent of dimethyl sulfoxide (DMSO) and an assisted heating process in the fabrication of CsPbI2Br (CPI2) to control the improved crystallization. A wide solution-processing window including a wide range of solvent components and solute concentrations has been realized. The CPI2-based inorganic perovskite solar cells (IPSCs) exhibit a high PCE up to 12.52%. More importantly, these devices demonstrate a remarkable Voc of 1.315 V. The performance has possessed such a region with high Voc and PCE in all Cs-based IPSCs, unveiling wide solution-processing windows with enhanced solution processability facilitating potential industrial application especially for tandem solar cells.
H
leads to high density of the uniform nucleation site, which is available to the flat surfaces and complete surface coverage. The controllable fast crystallization methodologies such as the antisolvent process, air flow, and hot casting, which quickly reach a supersaturated concentration and then experience outbreak of nucleation, can be used for preparing high-quality perovskite films. On the other hand, the solvent engineering process uses the viscosity of the solution or the appearance of intermediate phases to retard the rapid reaction during grain growth, resulting in a dense and uniform perovskite,17−21 especially for organic−inorganic ones such as MAPbI3 or FAPbI3. The all-inorganic perovskites of CsPbI3−xBrx have recently attracted vast research by introducing many methods to stabilize black phases and improve the quality of films,12,22−38 such as the additive of HI, the employment of coevaporation, and the formation of quantum dots and two dimensions. Nam et al. investigated the crystal formation behavior of CPI2 by precisely controlling the annealing temperature, which revealed the complexity of the crystal formation process of inorganic perovskites and its profound influence on both phase stability and solar cell performance.29 By using Mn2+ ion doping, the nucleation and growth rate could be modulated for different sizes of grains in CPI2 films.39 However, such a general strategy of solution engineering by controlling the improved crystallization is still lacking for highquality all-inorganic perovskites due to the high phase
ybrid organic−inorganic halide perovskites have been considered competitive photovoltaic materials for nextgeneration solar cells. These perovskites have an extensive formula of ABX 3 (A is an organic cation, such as methylammonium (MA) or formamidinium (FA), B is typically Pb, and X is a halogen element). They exhibit excellent photovoltaic performance, such as an appropriate (narrow and direct) band gap, large absorption coefficient, high carrier mobility, and long carrier diffusion, resulting in a rapidly increasing PCE from 3.8 to 22%.1−10 Recently, another promising halide perovskite system, which is known as a purely inorganic version (CsPbX3), has received great attention for employing in luminescence and solar cells with a power conversion efficiency (PCE) higher than 11%.11−15 These inorganic perovskite systems, particularly those with the a bromine substitute, will not generate organic volatile decomposition products and exhibit significantly enhanced structural stability under thermal and environmental stresses, such as oxygen, heat, and ultraviolet (UV) light, comparable to the organic−inorganic hybrid species.13,16 In particular, CsPbI2Br (CPI2) possesses a band gap of 1.91 eV, which is available as a photovoltaic light absorber, and can generate high Voc important for tandem solar cells. The theoretical maximum Voc from Shockley−Queisser analysis for the given band gap is as high as 1.615 V. This makes them suitable candidates for the top cell in tandem solar cells combined with the relatively low Voc of the bottom cells,7−9 such as organic− inorganic perovskites or Si-based cells. The formation process of the perovskite crystal involves crystal nucleation and grain growth. The high nucleation rate © 2018 American Chemical Society
Received: May 17, 2018 Accepted: June 19, 2018 Published: June 19, 2018 3646
DOI: 10.1021/acs.jpclett.8b01553 J. Phys. Chem. Lett. 2018, 9, 3646−3653
Letter
The Journal of Physical Chemistry Letters
Figure 1. (a) Heat-assisted process of HAF and optical images of the initial wet film, the processed film after HAF, and the annealed film. (b) Vapor pressure for mixed solvent and solubility for CPI2 as a function of DMSO content. (c) PCE and (d) Voc of the wide solution-processing window dependence on the DMSO content and CPI2 solubility.
transition temperature and low solubility of cesium halide, especially CsBr for CPI2. Herein, coupled with an additional heating process, we introduced DMSO that serves as a capping agent for controlling the uniform growth of crystals to realize the wide solution-processing window, which includes the large range of solvent components and solute concentrations. These present CPI2 inorganic perovskite solar cells (IPSCs) feature a high Voc (∼1.3 V) and outstanding PCE (∼12.5%) performance. In order to control the nucleation and crystallization of allinorganic perovskites, a heat-assisted reaction of the hot air flow (HAF) method is used for high-quality perovskites with high PCEs (Figures 1a and S1). During film formation, the wet films first become light brown immediately after HAF and then turn dark brown by annealing (Figure 1a), as displayed in the absorption spectra (Figure S2). Compared with the solar cells preheating on hot plates, the cells with the HAF method feature an enhanced PCE performance (Figure S3a). This indicates that the HAF can exhibit uniform rapid heating and would undergo a molecular self-assembly process between the DMSO molecule and precursor molecule.40 The process facilitates the nucleation rate and improved crystallization for high-quality films. In addition, the whole process proceeds in ambient air under controlled relative humidity (RH 15−25%), which also demonstrates higher performance than that in the nitrogen or oxygen environments (Figure S3b). This would verify that moderate moisture accumulation at the grain boundaries avails the improved crystallization for high-quality morphology.41,42
For both scientific research and industrial applications, a wide solution-processing window is of great significance to the repeatable preparation of thin films. For the present system of inorganic CPI2, the precursor properties and film thickness are still the restrictive link for preparation of high-quality IPSCs.29,30 Incorporated with the preliminary findings, we introduce a nontoxic solvent of DMSO partially substituting for the toxic N,N-dimethylformamide (DMF) to realize the wide solution-processing window for high-voltage-efficiency all-inorganic CPI2 solar cells (Figure 1b). The DMSO-based solvents exhibit lower toxicity, which is suitable for industrial manufacturing of perovskites (Table S1). Both the vapor pressure and the solubility are key parameters to be controlled for a wide solution processing window. The DMSO solvent demonstrates a higher boiling point (∼189 °C) and lower vapor pressure (∼126 Pa at 25 °C) than those (∼153 °C and ∼418 Pa at 25 °C) of DMF. The vapor pressure of the mixed solvent decreases with the increasing fraction of DMSO, as shown in Figure 1b. Hence, the solvent evaporation rate associated with the vapor pressure could be effectively controlled by tuning the ratio of the DMF and DMSO solvents, which is closely associated with the nucleation and crystal growth of the perovskite. On the other hand, the introduced DMSO facilitates dissolution of the precursors such as CsBr. The maximum solubility of CPI2 in the mixed solvent increases with the increasing fraction of DMSO (Figure 1b). The high solubility of precursors with the introduced DMSO (∼35%) could be up to more than 1 M, while the solubility is only ∼0.4 M in the pure DMF solvent. The solubility of precursors is important to control the thickness of films, as 3647
DOI: 10.1021/acs.jpclett.8b01553 J. Phys. Chem. Lett. 2018, 9, 3646−3653
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The Journal of Physical Chemistry Letters
Figure 2. (a) XRD patterns, (b) UV−vis absorption spectra, (c) PL decay profiles, and (d−f) SEM images of 0.7 M D25-, D50-, D75-based films, respectively. The insets of (b) exhibit the optical images. (g) J−V characteristics and (h) IPCE spectra and integrated current densities for 0.7 M D25-, D50-, D75-based IPSCs, respectively. The inset table of (g) shows the detailed photovoltaic parameters.
fixed CPI2 concentration of 0.7 M. We classified the three representative systems in the present work as D25, D50, and D75, corresponding to volume percentages of DMSO of 25, 50, and 75%, respectively, in the mixture of DMF and DMSO (Figure 2). Figure 2a features the crystal structures of the general X-ray diffraction (XRD) patterns, which are indexed to the cubic perovskite phase with an apparent (100) orientation for all three inorganic CPI2 perovskite films, indicating a highquality perovskite. The intensities of (100) and (200) peaks display a weakening trend with increasing DMSO contents. Simultaneously, the full width at half-maximum (fwhm) of the (200) peaks shows an enhanced trend especially for an obvious increase of D75 (Figure S4), revealing the poor crystallinity with excess DMSO. The UV−vis absorption spectrum of CPI2 with the accepted 650 nm absorption cutoff shows a decreasing absorbance value dependent on the DMSO content, also especially for an obvious decrease of D75 (Figure 2b), which is consistent with the photographs for the sample colors (insets of Figure 2b). As shown in Figure 2c, the time-resolved photoluminescence (PL) decay profiles were measured for all
discussed below. Hence, the introduced DMSO permits the optimized suitable thickness for light absorption. Here, a large range of fractions of DMSO from 15 to 55% and solute concentrations from 0.7 to 1.2 M are used out to verify the feasibility of the wide solution-processing window. As shown in Figure 1c, and 1d, the Voc and PCE near the maximum solubility line demonstrate higher values. This is because the high initial solution concentration can more easily reach the supersaturated concentration, which is one of the key conditions for promoting nucleation and crystallization. Hence, both the initial solution concentration and precursor properties such as vapor pressure are significant factors for controlled crystallization films. Furthermore, the PCE is higher than ∼11% and the Voc is also higher than ∼1.2 V in such a wide solution range. This strategy with a wide solutionprocessing window shows excellent tolerance of the precursor solution composition so that is very well suited for large-scale manufacture. In order to further explore the window range, we designed the comparison experiment with different DMSO contents at a 3648
DOI: 10.1021/acs.jpclett.8b01553 J. Phys. Chem. Lett. 2018, 9, 3646−3653
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The Journal of Physical Chemistry Letters
Figure 3. (a) Representative cross-sectional SEM image and (b) schematic view of the IPSC with the configuration of FTO/c-TiO2/mp-TiO2/ CPI2/spiro-OMeTAD/Ag. (c) J−V characteristic and (d) IPCE spectra and integrated current densities of the optimal CPI2-based IPSCs. The inset of (c) shows the J−t plots measured at a fixed voltage of 0.92 V for the optimal steady-state output.
three films to understand the kinetics of excitons and free carriers (Figure 2c). The D25- and D50-based films possess a similar A2 of τ2, which is a long lifetime corresponding to the radiative recombination, while that of the D75 film shows a strong decrease, indicating that excess DMSO would cause strong nonradiative recombination (Table S2). Scanning electron microscopy (SEM) images of the films unveil that the introduction of appropriate DMSO contents can significantly increase the grain size of CPI2 perovskites (Figure 2d−f). The D50-based films achieve larger grain sizes of about 700 nm compared to 500 nm for D25-based films, while excess DMSO (D75) causes small grain sizes (∼400 nm). The D75-based films also exhibit some pinholes from the surface to the FTO substrate. These pinholes would result in low orientation, poor crystallinity, and weak absorbance, as shown in the XRD and absorption spectra (Figure 2a and 2b). This variation on morphology could be attributed to the strong bonding ability of DMSO, which can serve as a capping agent and/or facilitate a molecular self-assembly process between the precursor molecules.40,42,43 Apart from the lower vapor pressure and higher viscosity than DMF associated with the crystal reaction rate, as discussed above, DMSO might also coordinate a precursor to form intermediates of the colloid cluster, resulting in a variational grain size dependence on DMSO contents. However, the solution with excess DMSO contents exhibits too low of a vapor pressure, which is detrimental to the crystallization, as shown in D75-based films. A similar phenomenon has been verified in Sn-based organic− inorganic perovskites by using DMSO as an additive solvent.43 To underpin the above basic performance characterization, the photocurrent density−voltage (J−V) measurements with difference DMSO were carried out on the solar cell structure of FTO/c-TiO2/mp-TiO2/CPI2/spiro-OMeTAD/Ag (Figure 2g). The D25- and D50-based IPSCs exhibit similar relatively high PCEs of 10.6 and 10.4%, respectively, while the D75based IPSCs show a smaller one of 8.78%. The detailed
photovoltaic parameters are listed in the inset table of Figure 2g. The incident photon-to-electron conversion efficiency (IPCE) spectra and integrated short-current densities (Jsc) over an AM 1.5G spectrum for IPSCs are displayed in Figure 2h. The integrated Jsc values from the IPCE spectrum are calculated to be 12.1, 11.7, and 10.7 mA cm−2 for D25-, D50-, and D75-based IPSCs, respectively, which match well with the values derived from the corresponding J−V measurements (inset of Figure 2g). Throughout the above results, we found that the IPSC performance can be stable at relatively high values when the DMSO contents are up to 50%, indicating a wide processing window of DMSO. Furthermore, apart from the factors of vapor pressure and viscosity properties of solvents, the D25-based IPSCs featuring the best performance compared to D50 and D75 can be ascribed to the fact that the control concentration (∼0.7 M) is close to the maximum solubility in the D25 precursor (∼0.8 M) (Figure 1). This relatively high concentration of the D25 precursor can easily reach the supersaturated concentration, promoting the nucleation and improving the crystallization. These results facilitate exploring the optimized solute concentration and solvent ratio for improved PCE and increased Voc in this wide solution-processing window. The formation of high-quality and sufficiently thick inorganic perovskite films is indispensable to achieve superior efficiency. To obtain the optimal thickness for light absorption, the solubility can be controlled up to 1.2 M in D50 precursor. The total thicknesses of both CPI2 (∼175 nm) and mesoporous TiO2 (∼50 nm) layers are ∼225 nm for 0.7 M D50-based solar cells, while that of the 0.4 M DMF-based film is only about 100 nm.29,30 When the solute concentration is up to ∼1 M for the D50 precursor, the total thickness can reach ∼300 nm (including ∼250 nm of CP2 and ∼50 nm of mpTiO2). This is consistent with the fact that the 1 M D50-based films exhibit stronger intensity of XRD and absorption than the 0.7 M D50-based films (Figure S5). This thickness would be 3649
DOI: 10.1021/acs.jpclett.8b01553 J. Phys. Chem. Lett. 2018, 9, 3646−3653
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The Journal of Physical Chemistry Letters
Figure 4. Statistical histogram of (a) Voc and (b) PCEs obtained from 34 individual CPI2-based IPSCs. (c) Voc and PCE distributions of different IPSCs based on this work and reported ones. (d) Long-term stability for Voc and PCEs of the IPSC device stored without encapsulation (25 °C and RH < 25%).
which is higher than that (11.7 mA cm−2) of 0.7 M D50-based IPSCs, as shown above, revealing the suitably thick and highquality perovskite for superior efficiency. To ensure the repeatability of fabrication with high Voc and excellent PCE, the photovoltaic performance of 34 individual CPI2-based IPSCs on FTO/c-TiO2 (mp-TiO2) substrates has been drawn to the histograms, which exhibit a high average Voc of 1.234 V and an excellent average PCE of 11.05% (Figure 4a, 4b and Table S3). It is noted that the highest Voc for the present CPI2 can be up to 1.315 V (Figure S8a), higher than that of all reported CPI2-based solar cells, which is ∼81.4% of the maximum voltage (1.615 V) from the Shockley−Queisser analysis for the given band gap. We also replaced the c-TiO2/ mp-TiO2 layers with the compact SnO2 nanolayers as the electron transfer layer (Figure S8b). These SnO2-based solar cells also show a high Voc of 1.275 V and good PCE of 12.0%. Furthermore, the low-cost carbon layer has been used to replace both the organic hole transport layer (HTL) of spiroOMeTAD and metal electrodes of Ag (Figure S8c), which also display a high Voc of 1.283 V. We compared the Voc and PCE of the present 10 individual CPI2-based IPSCs to those of both the reported CPI2-based and the other CsPb-based IPSCs in Figure 4c (Table S4).44 Apparently, the performance of our present fabricated devices distributes the region with both high voltage and efficiency for all Cs-based IPSCs. Hence, the present high-quality perovskite films with high Voc and PCE performance fabricated in the wide solution-processing windows provide a possibility for application of tandem solar cells in simple planar heterojunctions or stable all-inorganics.
enough to guarantee full light harvesting at around 650 nm for CPI2, compared with that (∼400 nm) of a typical hybrid perovskite layer with an absorption cutoff at ∼800 nm. The control concentration of ∼1 M in the D50 precursor avails to improve crystallization due to the fact that it is close to the maximum solubility (∼1.2 M), as discussed above. The representative cross-sectional SEM image of the present solar cell has been fabricated to unveil the uniform stack of functional layers, as shown in Figure 3a, which consists of FTO/c-TiO2/mp-TiO2/CPI2/spiro-OMeTAD/Ag (Figure 3b). After the above optimization, we obtained the best PCE performance of 12.52% for CPI2 at the 1 M D50-based solar cells (Figure 3c), which is comparable with the recently reported value of 12.39%.37 The key parameters are summarized in the inset of Figure 3c, including the high Voc (1.243 V), Jsc (13.56 mA cm−2), and fill factor (FF, 0.743). The PCE performance dependence on the thickness of mpTiO2 layers can also be optimized (Figure S6), which indicates that a very thin layer (∼50 nm) of mesoporous can improve the efficiency and maintain a high Voc. The J−V curves of the CPI2-based IPSCs were measured using both the reverse and forward scan directions (Figure S7), revealing a little hysteresis. The J−t plots measured at a fixed voltage of 0.92 V exhibit a stable Jsc (12.10 mA cm−2) and stable high PCE of 11.13%, verifying the good output stability (inset of Figure 3c). Figure 3d demonstrates the IPCE spectrum and integrated Jsc of the present IPSCs. The IPCE from 400 to 500 nm is higher than ∼80%, indicating high-quality perovskite films. The integrated Jsc values are calculated to be 13.29 mA cm−2, 3650
DOI: 10.1021/acs.jpclett.8b01553 J. Phys. Chem. Lett. 2018, 9, 3646−3653
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The Journal of Physical Chemistry Letters
Xuan). The spiro-OMeTAD solution was spin-coated on perovskite film at 4000 rpm for 30 s. Finally, the Ag electrode was deposited by thermal evaporation. The active area of the device was 0.12 cm−2. All of the fabrication steps were performed in a dry atmosphere (∼25 °C, RH < 25%), except that of the spiro-OMeTAD layer. Characterization. The crystal structure of the perovskite film was determined by the XRD, which was performed on the diffractometer (PW3040/60, PANalytical, Holland) with Cu Kα radiation. The morphologies were investigated by SEM measurement, which was performed using a cold field emission scanning electron microscope (SU4800, Hitachi). The absorption spectra were measured using a UV−vis spectrophotometer (T10, Persee). The PL decay spectra were measured using a time-resolved fluorescence spectrometer (FLS900, Edinburg), and the excitation wavelength was 450 nm. The photocurrent J−V characteristics were measured using a digital source meter (2400, Keithley Instruments Inc.) under AM 1.5 G illumination simulated sunlight (100 mW cm−2) (7SS1503A, 7 Star Optical Instruments Co., Beijing, China). The incident light intensity was calibrated with a standard Si solar cell for 1 sun. The IPCE as a function of wavelength was measured in direct current (DC) mode using a custom measurement system consisting of a 150 W xenon lamp (7ILX150A, 7 Star Optical Instruments Co., Beijing, China), a monochromator (7ISW30, 7 Star Optical Instruments Co., Beijing, China), and a digital source meter (2400, Keithley Instruments Inc.).
The long-term stability of the present CPI2-based IPSCs were evaluated under dark storage conditions at ambient atmosphere (∼25 °C and RH ≤ ∼25%), as shown in Figures 4d and S9. Both the PCE and Voc feature no detectable degradation for more than 500 h, unveiling the excellent long-term air stability. In conclusion, we introduced a solvent of dimethyl sulfoxide accompanied by an additional heating process of HAF for allinorganic perovskite CPI2. It realized a wide solutionprocessing window for excellent performance along with the highest Voc (∼1.315 V) and optimal PCE (∼12.52%). Controlled crystallization can be implemented by incorporation of the solution properties, the control solute concentration, and HAF. We need to stress some functions of the present additional solvent, which could create a distinct pathway for repeatable extensive preparation: (i) It could coordinate precursor to form a molecular self-assembly process for intermediates of the colloid cluster and serve as a capping agent due to its strong bonding ability, which is related to the grain size of perovskite films; (ii) the low vapor pressure and high viscosity of DMSO facilitate effective control of the solvent evaporation rate, which is associated with improved crystallization and growth for high-quality films; (iii) the good solubility for precursors permits control of suitable thickness to guarantee full light harvesting. The present wide solutionprocessing window with high-voltage performance will promote research and industrial applications of inorganic perovskites such as tandem solar cells and large-scale manufacture.
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EXPERIMENTAL METHODS Device Fabrication. The etched FTO glasses were cleaned in sequence with deionized water, acetone, and ethanol and then further treated under ultraviolet ozone for 10 min. A compact TiO2 layer (c-TiO2) was deposited at 450 °C on the FTO glass by spray pyrolysis deposition using a solution diluting titanium diisopropoxide bis(acetylacetonate) (75 wt % in isopropanol) in ethanol with a volume ratio of 1/25 and annealed at the same temperature for 30 min. After cooling, a mesoporous TiO2 (mp-TiO2) layer was deposited by spin-coating for 30 s at 4000 rpm using a Dyesol 18NRT paste diluted in ethanol with a weight ratio of 1/14 and annealed at 450 °C for 30 min. Afterward, the substrate was immersed in a 20 mM TiCl4 (99.0%, Aladdin) aqueous solution at 80 °C for 30 min and washed with distilled water and ethanol, followed by annealing again at 450 °C for 30 min. CsPbI2Br precursor solution at different concentrations was prepared by dissolving equimolar PbI2 (99.9%, Yingkou, You Xuan Trade Co., Ltd.) and CsBr (99.9%, Aladdin) in the mixed solvents of DMF (N,Ndimethylformamide, 99.8%, Sigma-Aldrich) and DMSO (dimethyl sulfoxide, Sigma-Aldrich) and stirring at 120 °C for 30 min. The wet perovskite film was formed by spincoating the perovskite precursor solution at 5000 rpm for 30 s on glass/FTO/c-TiO2/mp-TiO2. Then, the wet transparent film was blown into a semitransparent light brown film by HAF, as shown in Figure S1. Right after that, the film was annealed at 280 °C for 6 min. To prepare the HTL precursor solution, 72.3 mg of spiro-MeOTAD (Yingkou, You Xuan Trade Co., Ltd.) was dissolved in 1 mL of chlorobenzene (99.8%, Sigma-Aldrich) and mixed with 29 μL of 4-tertbutylpyridine (Yingkou, You Xuan Trade Co., Ltd.) and 17.5 μL of Li-TFSI solution (lithium bis(trifluoromethanesulfonyl)imide salt solution in acetonitrile (520 mg mL−1), both You
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.8b01553.
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Typical physical parameters and chemical properties for the DMF and DMSO solvents, detailed parameters of the PL decay curves for the D25- and D50-based films fitted, photovoltaic parameters of 34 individual CPI2based IPSCs, photovoltaic performance comparison of CsPb-based IPSCs, device fabrication details, and some properties’ characterization details (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Guozhong Cao: 0000-0003-1498-4517 Jianjun Tian: 0000-0002-4008-0469 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (51774034, 51772026, and 51611130063), Beijing Natural Science Foundation (2182039), Fundamental Research Funds for the Central Universities (FRF-TP-17-030A1, FRF-TP-17-083A1, FRF-TP-17-082A1, TW2018010), and Project funded by China Postdoctoral Science Foundation (2017M620611, 2018M630068). 3651
DOI: 10.1021/acs.jpclett.8b01553 J. Phys. Chem. Lett. 2018, 9, 3646−3653
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The Journal of Physical Chemistry Letters
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