Impact of Peripheral Groups on Phenothiazine-Based Hole-Transporting Materials for Perovskite Solar Cells Fei Zhang,*,§,‡,†,⊥ Shirong Wang,*,§,† Hongwei Zhu,§,† Xicheng Liu,§,† Hongli Liu,§,† Xianggao Li,§,† Yin Xiao,§,† Shaik Mohammed Zakeeruddin,*,‡ and Michael Graẗ zel*,‡ Downloaded via IOWA STATE UNIV on January 26, 2019 at 21:25:51 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
§
School of Chemical Engineering and Technology, Tianjin University, 300072 Tianjin, China Laboratory of Photonics and Interfaces, Institute of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne (EPFL), Station 6, CH-1015 Lausanne, Switzerland † Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), 300072 Tianjin, China ‡
S Supporting Information *
ABSTRACT: Three hole-transporting materials (HTMs) based on the phenothiazine core containing 4,4-dimethyltriphenylamine (Z28), N-ethylcarbazole (Z29), and 4,4-dimethoxytriphenylamine (Z30) as the peripheral groups connected by double bonds were designed and synthesized. The HTMs were tested in mixed cation/anion perovskite solar cells (PSCs) of the composition [(FAPbI3)0.85(MAPbBr3)0.15]. A power conversion efficiency (PCE) of 19.17% under 100 Mw cm−2 standard AM 1.5G solar illumination was obtained using Z30. Importantly, the devices based on Z30 show better stability compared to those using Z28 and Z29 when aged under ambient air of 40% relative humidity in the dark for 1008 h and under continuous sunlight soaking without encapsulation for 600 h. These results indicate that the 4,4-dimethoxytriphenylamine is a promising peripheral group in combination with the phenothiazine core, providing an alternative to develop small molecular HTMs for efficient and stable PSCs.
T
unit, which provides access to a wide library of HTMs by relatively cheap synthetic procedures. Phenothiazine-based materials have been reported in organic photovoltaics (PVs) as well as organic transistors.33−35 S-based heterocycles have been found to strengthen the interaction between the perovskite and HTM.36−38 Herein, we report the facile synthesis of three phenothiazinebased HTMs, shown in Figure 1a. Phenothiazine has a butterfly symmetrical nonplanar structure.39 We substitute its 3- and 7positions by 4,4-dimehtyltriphenylamine, N-ethylcarbazole, and 4,4-dimethoxytriphenylamine as the peripheral groups affording HTMs coded Z28, Z29, and Z30, respectively. These peripheral groups are introduced to tune the energy levels, hole mobility, and thermal properties. We investigate here the impact of these peripheral groups on the physicochemical properties and PV performance of phenothiazine-based HTMs. These three HTMs show high glass transition temperatures, hole mobility, and suitable valence band levels. The cost of the new HTMs is
he power conversion efficiency (PCE) of perovskite solar cells (PSCs) has increased within a few years from 3.81 to over 22%1−4 due to the favorable optoelectronic features of metal halide perovskites. PSCs comprise, apart from the perovskite light harvester, electronand hole- transporting materials (HTMs) and a metallic counter electrode. The HTM plays an important role in facilitating hole migration from the perovskite layer to back contact and preventing charge recombination.5 The frequently used HTM, spiro-OMeTAD, has several caveats, in particular, its tendency to crystallize at 85 °C. There is a need to develop alternative HTMs, which should have suitable energy levels, good solubility in organic solvents, smooth and stable film formation, low-cost production, high hole mobility, and elevated glass transition temperature.6−9 Various small molecules, inorganic and polymer HTMs, have been developed to replace spiro-OMeTAD,10−19 and several materials have emerged that approach the performance of spiro-OMeTAD, especially in terms of PCE, cost, and stability.20−25 Small molecules have the advantage of ease of purification and smaller batch-to-batch variation.26−32 Phenothiazine is a low-cost and versatile electron-rich heteroaromatic © 2018 American Chemical Society
Received: March 12, 2018 Accepted: April 17, 2018 Published: April 17, 2018 1145
DOI: 10.1021/acsenergylett.8b00395 ACS Energy Lett. 2018, 3, 1145−1152
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Cite This: ACS Energy Lett. 2018, 3, 1145−1152
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ACS Energy Letters
Figure 1. (a) Chemical structures of three HTMs. (b) Synthetic routes for three HTMs.
Figure 2. (a)Normalized UV−vis absorption spectra and PL spectra of three HTMs in THF solution (c = 1.0 × 10−5 mol L−1). (b) Calculated frontier molecular orbitals of three HTMs. (c) Energy level diagram of the corresponding materials used in PSCs. (d) Cross-sectional SEM image of the representative Z30-based device; the scale bar is 200 nm.
estimated to be ca. 60 US$/g. The devices employing Z30 show PCEs up to 19.17%. Furthermore, the devices based on the
three HTMs show long shelf life when aged in ambient air of 40% relative humidity in the dark for 1008 h and good 1146
DOI: 10.1021/acsenergylett.8b00395 ACS Energy Lett. 2018, 3, 1145−1152
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slightly higher than that of the double-cation perovskite (FAI)0.85(PbI2)0.85(MABr)0.15(PbBr2)0.15 (−5.5 eV).18 The LUMO levels of HTMs are calculated to be −2.83, −2.82, and −2.73 eV, which are above that of the double-cation perovskite (−3.9 eV).24 These results agree well with the trend derived from DFT calculations. Figure 2a shows the normalized UV−vis absorption and photoluminescence (PL) spectra of three HTMs in THF solution (1.0 × 10−5 mol L−1). The absorption band at 400− 450 nm is attributed to the intramolecular charge transfer (ICT) of the π−π* transition.44 Due to the smaller degree of conjugation,45 the ICT peak (λabs/max) of Z29 shows a blue shift compared with Z28 and Z30. In addition, the PL spectra of Z28, Z29, and Z30 show maximum emission at 495, 484, and 497 nm, respectively. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) show high decomposition temperatures, that is, Td = 413.1, 406.9, and 421.1 °C and glass transition temperatures (Tg) of 113.8, 111.4, and 125.7 °C for Z28, Z29, and Z30, respectively (Figure S10b,c). The higher Tg of Z30 is due to the larger molecular weight and lager molecular stiffness.46,47 We determined the hole mobilities of three HTMs by using the space charge-limited current (SCLC) method. The obtained intrinsic hole mobilities (Figure S10d) of Z28, Z29, and Z30 are 6.18 × 10−5, 6.82 × 10−6, and 6.70 × 10−5 cm2 V−1 s−1, respectively, which are comparable with that of spiro-OMeTAD (8.09 × 10−5 cm2 V−1 s−1).22 The trend of hole mobility is consistent with the extent of π-conjugation after introduction of different peripheral groups, as seen from Figure 2b. As shown in Table S3, Z30 has a lower calculated reorganization energy than Z28 and Z29, which agrees with the result of the TOF measurement.26 The steady-state PL spectra and time-resolved photoluminescence (TRPL) decay spectra are shown in Figure S11. After the HTM materials were coated on top of perovskite films, the PL intensity (Figure S11a) was reduced to roughly 11, 18, 6, and 3% of that obtained from pristine films for Z28, Z29, Z30, and spiro-OMeTAD, respectively, suggesting that fast interface charge separation in Z30-based devices can contribute to a better Jsc and FF in the PSCs compared to Z28and Z29-based devices but lag behind the hole capture rate by spiro-OMeTAD.24 Figure S11b presents the measured PL decay spectra. The PL decay lifetimes for the devices with Z28, Z29, Z30, and spiro-OMeTAD are significantly shorter than those for the devices without a HTM layer, confirming the trends that emerge from the steady-state luminescence measurements shown in Figure S11a. Clearly, spiro-OMeTAD shows the fastest extraction time followed by Z30. We fabricate double-cation PSCs to investigate the performance of doped Z28, Z29, and Z30 as HTMs in state of the art devices. Figure 2d shows a typical cross section SEM image of the PSC. Figure 3a−d illustrates the current−voltage (J−V) curves collected under simulated solar illumination (AM 1.5G, 100 mW cm−2) for the best PSC among 20 devices, and the PV parameters are summarized in Table 2. The best devices based on Z28, Z29, and Z30 produce open-circuit voltages (Voc) of 1.124, 1.087, and 1.114 V, short-circuit current densities (Jsc) of 23.01, 22.35, and 23.53 mA cm−2, and fill factors (FF) of 0.69, 0.60, and 0.73, leading to PCEs of 17.77, 14.65, and 19.17%, respectively, while the spiro-OMeTAD -based PSCs present the best performance with a PCE of 19.66%, Jsc of 23.76 mA cm−2, Voc of 1.108 V, and FF of 0.74 under AM 1.5G (100 mW cm−2) illumination. The lower performance of Z29 is mainly related to
operational stability when subjected to continuous light soaking under N2 for 600 h at their maximum power point and ambient temperature. The three HTMs were synthesized in three steps involving electrophilic substitution, followed by the Vilsmeier and Wittig reactions. Synthetic routes for Z28, Z29, and Z30 are depicted in Figure 1b, and experimental details are given in the Experimental Section of the Supporting Information. The new HTMs were characterized by nuclear magnetic resonance (NMR) and mass spectrum (MS) technologies. All of the analytical data are consistent with the proposed structures (Figures S1−S9). It is worth noting that the synthesis costs of Z28, Z29, and Z30 are 51.50, 48.54, and 45.44 $/g, respectively. To investigate the potential application of these new HTMs in PSCs, the optimized molecular geometries, the highest occupied molecular orbital (HOMO) levels, and the lowest unoccupied molecular orbital (LUMO) energy levels were calculated by density functional theory (DFT) calculations, and results are shown in Figure 2b. The optimized molecular structures of the compounds in Figure 2b reveal that after introducing the double bonds and different peripheral groups, the angles between the phenothiazine core ring and the attached benzene ring of the peripheral groups are 13.81, 14.29, and 3.83° for Z28, Z29, and Z30, respectively. Also, the length of conjugation in the compounds increases by the introduction of 4,4-dimethoxytriphenylamine over that using 4,4-dimethyltriphenylamine or N-ethylcarbazole as substituents. We can conclude that the Z30 molecule shows more extended πconjugation than Z28 and Z29, assisting in the hole transport, which follows a hopping mechanism.24,40 The LUMO of the three HTMs is located mainly on the central core part, while the HOMO orbitals spread over the entire molecular skeleton. This substantial overlapping between HOMO and LUMO orbitals benefits the formation of excitons and hole migration.41 The calculated HOMO levels of Z28, Z29, and Z30 are −4.40, −4.45, and −4.27 eV, while the calculated LUMO levels are −1.31, −1.16, and −1.20 eV, respectively. With the increase of conjugation area, the HOMO energy level will also increase.42 The wider band gap of Z29 (2.62 eV) compared to those of Z28 (2.56 eV) and Z30 (2.54 eV) is also ascribed to the more extended π-conjugation system for the latter two HTMs, which decreases the energy gap between the HOMO and LUMO.43 Thus, the three HTMs act as hole-selective charge transport layers. We confirmed that their energy levels were experimentally determined by performing cyclic voltammetry (CV) measurements (Figure S10a). The data are shown in Figure 2c and are summarized in Table 1. The HOMO energy levels of Z28, Z29, and Z30 are −5.39, −5.44, and −5.27 eV, which are Table 1. Photophysical and Electrochemical Data and Thermal Characteristics of Three HTMs HTM Z28 Z29 Z30
Eg (eV)
HOMO (eV)
LUMO (eV)
Td (°C)
Tg (°C)
μ (cm2 V−1 s−1)
a
−5.39 −4.40b −5.44a −4.45b −5.27a −4.27b
−2.83 −1.31b −2.82a −1.16b −2.73a −1.20b
413.1
113.8
6.18 × 10−5
406.9
111.4
6.82 × 10−6
421.1
125.7
6.70 × 10−5
2.56 3.09b 2.62a 3.29b 2.54a 3.07b
a
a
a
Values determined experimentally from CV measurements. bTheoretically derived values. 1147
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Figure 3. (a−d) Current−voltage hysteresis curves of PSCs comprising champion devices with HTMs measured starting with backward scan and continuing with forward scan. The inset is the stabilized power output of three HTM-based devices. (e) Stabilized power output of the corresponding devices. (f) IPCE spectra and integrated current curves of the corresponding devices.
Table 2. J−V Curves of HTM-Based Devices under Different Scan Directions with a Bias Step of 5 mV HTM
Jsc (mA cm−2)
Voc (V)
FF
PCE (%)
spiro-OMeTAD (backward) spiro-OMeTAD (forward) Z28 (backward) Z28 (forward) Z29 (backward) Z29 (forward) Z30 (backward) Z30 (forward)
23.76 23.74 23.01 23.04 22.35 22.39 23.53 23.49
1.108 1.104 1.124 1.123 1.087 1.076 1.114 1.105
0.74 0.73 0.69 0.65 0.60 0.56 0.73 0.73
19.66 19.21 17.77 16.81 14.65 13.42 19.17 19.02
current densities estimated from the IPCE spectra (Figure 3f) are 22.18, 21.37, 22.61, and 23.02 mA cm−2 for Z28, Z29, Z30, and spiro-OMeTAD, respectively, which are in good agreement with the Jsc values obtained from the J−V curves. Moreover, the similar shape of the IPCE spectra for all devices indicates that light absorption by the HTM does not compete with the light harvesting by the perovskite. Figure 5a shows the stability tests of corresponding PSCs in an ambient environment of 40% relatively humidity without encapsulation. As shown in Figure 5a, the PCE maintained 67.1, 50.1, and 85.1% of the initial value in the Z28, Z29, and Z30-based PSC after 1008 h. The photostability of corresponding PSCs was also studied by keeping the devices under continuous simulated AM 1.5G sunlight soaking and under N2 flow without encapsulation at their maximum power point and at ambient temperature. As shown in Figure 5b, the PCE maintained 59.1, 45.2, and 80.2% of the initial value in the Z28, Z29, and Z30 PSCs after 600 h of full sunlight light exposure. We derived further information on the surface energetics of the hole conductors by measuring the contact angle formed by a water droplet. This depends on the surface tension of the 3 interface according to Young’s equation, Figure S12, where the γLV, γSV, and γSL are the molar free energies (surface tensions) of the liquid and solid against their saturated vapor and of the interface between the liquid and solid, respectively.51−54 According to the Young’s equation, the observed 7° difference in the contact angles of Z30 and Z28 could arise from either an increase in the difference between the surface tension prevailing at the HTM/gas and HTM/water interface or by an increase in the surface tension between the water droplet and the gas phase or by both. Given the small magnitude of the effect, it is difficult to attribute the observed trend with certainty. Nevertheless, we speculate that the molar interfacial entropy of the Z30 is larger
an insufficient driving force for hole injections related to its deeper HOMO energy level (−5.44 eV) as compared to the valence band of perovskite (−5.49 eV) and lower hole mobility.19,20 Batches of 20 cells each using these HTMs were fabricated, and excellent reproducibility was demonstrated, as shown in Figure 4. The trends of Jsc and FF are consistent with that of hole mobility from other reports.24,48,49 This is likely the reason why Z30-based devices obtain a better Jsc and FF compared to those for Z28 and Z29 and a smaller Jsc and FF compared to those for spiro-OMeTAD. A small hysteresis was observed in the J−V curves. According to a previous report,50 charge carrier trapping at the interfaces with the electron and hole-selective contact materials is a likely reason for the occurrence of hysteresis. The hysteresis of the device based on Z30 is less pronounced, which we attribute to the faster hole extraction by this material compared to that of the other phenothiazine derivatives. The stabilized power outputs from devices based on Z28, Z29, Z30, and spiroOMeTAD are 17.29, 13.74, 19.12, and 19.44%, respectively (Figure 3e), consistent with the obtained PCE. The integrated 1148
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Figure 4. Reproducibility of 20 devices using different HTMs: (a) Jsc; (b) Voc; (c) FF; (d) PCE.
Figure 5. (a) Stability of corresponding devices in ambient air of 40% relatively humidity without encapsulation. (b) Stability of corresponding devices during stability tests under continuous light irradiation in N2 without encapsulation at their maximum power point and ambient temperature. (c) Contact angles between perovskite/HTMs and water. (d) Surface view SEM images of corresponding perovskite/HTM films. The scale bar is 2 μm.
at the HTM/water interface than that of Z28 due to decreased ordering by hydrogen bonding with water via the oxygen atoms of the Z30 alkoxy groups. This would lead to the observed increase in the contact angle. From the SEM images in Figure 5d, the film based on Z30 is more uniform and has fewer pinholes than that based on Z28 and Z29, which is consistent with results of the contact angle
test (Figure 5c). We ascribed the improvement of the devices’ stability mainly due to the smaller number of pinholes in the HTM layer and its increased hydrophobicity.17−20,23,55 As a result, the Z30-based perovskite device showed a much stronger resistance to degradation under ambient conditions over longer time periods than the other two corresponding HTM-based devices. 1149
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(2) Zhang, F.; Wang, S. R.; Li, X. G.; Xiao, Y. Recent Progress Of Perovskite Solar Cells. Curr. Nanosci. 2016, 12, 137−156. (3) Zhang, F.; Wang, S. R.; Li, X. G.; Xiao, Y. Improvement In Photovoltaic Performance Of Perovskite Solar Cells By Interface Modification And Co-Sensitization With Novel Asymmetry 7Coumarinoxy-4-Methyltetrasubstituted Metallophthalocyanines. Synth. Met. 2016, 220, 187−193. (4) Yang, W. S.; Park, B. W.; Jung, E. H.; Jeon, N. J.; Kim, Y. C.; Lee, D. U.; Shin, S. S.; Seo, J.; Kim, E. K.; Noh, J. H.; et al. Iodide Management In Formamidinium-Lead-Halide−Based Perovskite Layers For Efficient Solar Cells. Science 2017, 356, 1376−1379. (5) Sun, M. N.; Liu, X. Y.; Zhang, F.; Liu, H. L.; Liu, X. C.; Wang, S. R.; Xiao, Y.; Li, D. M.; Meng, Q. B.; Li, X. G. Simple Dopant-Free Hole-Transporting Materials With P-π Conjugated Structure For Stable Perovskite Solar Cells. Appl. Surf. Sci. 2017, 416, 124−132. (6) Krishna, A.; Grimsdale, A. C. Hole Transporting Materials For Mesoscopic Perovskite Solar Cells − Towards A Rational Design? J. Mater. Chem. A 2017, 5, 16446−16466. (7) Zhu, H. W.; Zhang, F.; Liu, X. C.; Sun, M. N.; Han, J. L.; You, J.; Wang, S. R.; Xiao, Y.; Li, X. G. A Dopant-Free Hole Transporting Material With A Tetra- Phenylethene Core For Efficient Perovskite Solar Cells. Energy Technol. 2017, 5, 1257−1264. (8) Guo, J. J.; Meng, X. F.; Niu, J.; Yin, Y.; Han, M. M.; Ma, X. H.; Song, G. S.; Zhang, F. A Novel Asymmetric Phthalocyanine-Based Hole Transporting Material For Perovskite Solar Cells With An OpenCircuit Voltage Above 1.0 V. Synth. Met. 2016, 220, 462−468. (9) Qi, P.; Zhang, F.; Zhao, X.; Bi, X.; Wei, P.; Xiao, Y.; Li, X.; Wang, S.; Liu, X. Efficient, Stable, Dopant-Free Hole-Transport Material With A Triphenylamine Core For CH3NH3PbI3 Perovskite Solar Cells. Energy Technol. 2017, 5, 1173−1178. (10) Rakstys, K.; Saliba, M.; Gao, P.; Gratia, P.; Kamarauskas, E.; Paek, S.; Jankauskas, V.; Nazeeruddin, M. K. Highly Efficient Perovskite Solar Cells Employing An Easily Attainable Bifluorenylidene-Based Hole-Transporting Material. Angew. Chem., Int. Ed. 2016, 55, 7464−7468. (11) Nishimura, H.; Ishida, N.; Shimazaki, A.; Wakamiya, A.; Saeki, A.; Scott, L. T.; Murata, Y. Hole-Transporting Materials With A TwoDimensionally Expanded π-System Around An Azulene Core For Efficient Perovskite Solar Cells. J. Am. Chem. Soc. 2015, 137, 15656− 15659. (12) Bi, D.; Mishra, A.; Gao, P.; Franckevičius, M.; Steck, C.; Zakeeruddin, S. M.; Nazeeruddin, M. K.; Bäuerle, P.; Grätzel, M.; Hagfeldt, A. High-Efficiency Perovskite Solar Cells Employing A S,Nheteropentacene-Based D-A Hole-Transport Material. ChemSusChem 2016, 9, 433−438. (13) Huang, C.; Fu, W.; Li, C. Z.; Zhang, Z.; Qiu, W.; Shi, M.; Heremans, P.; Jen, A. K.-Y.; Chen, H. Dopant-Free Hole-Transporting Material With A C3h Symmetrical Truxene Core For Highly Efficient Perovskite Solar Cells. J. Am. Chem. Soc. 2016, 138, 2528−2531. (14) Hu, Z.; Fu, W.; Yan, L.; Miao, J.; Yu, H.; He, Y.; Goto, O.; Meng, H.; Chen, H.; Huang, W. Effects Of Heteroatom Substitution In Spiro-Bifluorene Hole Transport Materials. Chem. Sci. 2016, 7, 5007−5012. (15) Molina-Ontoria, A.; Zimmermann, I.; Garcia-Benito, I.; Gratia, P.; Roldán-Carmona, C.; Aghazada, S.; Grätzel, M.; Nazeeruddin, M. K.; Martín, N. Benzotrithiophene-Based Hole-Transporting Materials For 18.2% Perovskite Solar Cells. Angew. Chem., Int. Ed. 2016, 55, 6270−6274. (16) Zhang, F.; Yi, C.; Wei, P.; Bi, X.; Luo, J.; Jacopin, G.; Wang, S.; Li, X.; Xiao, Y.; Zakeeruddin, S. M.; et al. A Novel Dopant-Free Triphenylamine Based Molecular “Butterfly” Hole-Transport Material For Highly Efficient And Stable Perovskite Solar Cells. Adv. Energy. Mater. 2016, 6, 1600401. (17) Liu, X. C.; Zhu, L. F.; Zhang, F.; You, J.; Xiao, Y.; Li, D. M.; Wang, S. R.; Meng, Q. B.; Li, X. G. Stable Perovskite Solar Cells Based On Hydrophobic Triphenylamine Hole-Transport Materials. Energy Technol. 2017, 5, 312−320. (18) Zhao, X. M.; Zhang, F.; Yi, C. Y.; Bi, D. Q.; Bi, X. D.; Wei, P.; Luo, J. S.; Liu, X. C.; Wang, S. R.; Li, X. G.; et al. Novel One-Step
In conclusion, we synthesized three novel phenothiazinebased HTMs using a simple low-cost process. 4,4-Dimethyltriphenylamine (Z28), N-ethylcarbazole (Z29), and 4,4dimethoxytriphenylamine (Z30) were introduced into a phenothiazine core structure as the peripheral groups connected by double bonds. They were successfully applied in PSCs based on their high solubility, sufficiently high hole mobility, and appropriate HOMO energy level alignment. The PSC based on Z30 as the HTM affords an impressive PCE of 19.17%. The devices based on Z30 obtained a higher stability than that of Z28 and Z29 at room temperature aged under ambient air of 40% relative humidity in the dark without encapsulation after 1008 h and under continuous sunlight soaking without encapsulation after 600 h. These results open a new direction for designing HTMs by using 4,4-dimethoxytriphenylamine as the peripheral groups in the future deployment of efficient and stable PSCs.
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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.8b00395. Materials measurements, synthesis of HTMs, solar cell fabrication, analytical data of the structures, CV curves, DSC, TGA curves, current density versus voltage for the HTMs, PL spectra and TRPL spectra, the contact angle of a liquid droplet wetted to a solid surface and the Young’s equation, synthesis cost estimation of 1 g of Z28, Z29, and Z30, and reorganization energies of Z28, Z29, and Z30 (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: *E-mail: *E-mail: *E-mail:
[email protected] (F.Z).
[email protected] (S.W.). shaik.zakeer@epfl.ch (S.M.Z). michael.graetzel@epfl.ch (M.G.).
ORCID
Fei Zhang: 0000-0002-3774-9520 Michael Grätzel: 0000-0002-0068-0195 Present Address ⊥
F.Z.: Chemistry and Nanoscience Center, National Renewable Energy Laboratory, Golden, Colorado 80401, United States. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Key R&D Program of China (2016YFB0401303), the National Natural Science Foundation of China (21676188), and Key Projects in Natural Science Foundation of Tianjin (16JCZDJC37100). F.Z. thanks the Collaborative Innovation Center of Chemical Science and Engineering (Tianjin) for funding. M.G. and S.M.Z. thank the King Abdulaziz City for Science and Technology (KACST) for financial support.
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REFERENCES
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DOI: 10.1021/acsenergylett.8b00395 ACS Energy Lett. 2018, 3, 1145−1152
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DOI: 10.1021/acsenergylett.8b00395 ACS Energy Lett. 2018, 3, 1145−1152
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DOI: 10.1021/acsenergylett.8b00395 ACS Energy Lett. 2018, 3, 1145−1152
