Zinc Porphyrin–Ethynylaniline Conjugates as Novel Hole-Transporting

Zinc Porphyrin−Ethynylaniline Conjugates as Novel Hole-Transporting Materials for Perovskite Solar Cells with Power Conversion Efficiency of 16.6% Hsien-Hsin Chou,†,§ Yu-Hsien Chiang,‡,§ Ming-Hsien Li,‡ Po-Shen Shen,‡ Hsiang-Jung Wei,† Chi-Lun Mai,† Peter Chen,*,‡ and Chen-Yu Yeh*,† †

Department of Chemistry and Research Center for Sustainable Energy and Nanotechnology, National Chung Hsing University, Taichung 402, Taiwan ‡ Department of Photonics and Research Center for Energy Technology and Strategy (RCETS), National Cheng Kung University, Tainan 701, Taiwan S Supporting Information *

ABSTRACT: New zinc porphyrins Y2 and Y2A2 have been utilized in perovskite solar cells specifically as hole-transporting materials (HTMs) rather than photosensitizers. The combination of MAPbI3 as photosensitizer and porphyrins as HTMs is a potential alternative to well-known MAPbI3/Spiro-OMeTAD hybrids owing to high performance and versatility toward molecular engineering of porphyrin families. A high efficiency of 16.60% is achieved by n-butyl tethered Y2 HTM (VOC = 0.99 V; JSC = 22.82 mA cm−2) which is comparable to that of Spiro-OMeTAD of 18.03% (VOC = 1.06 V; JSC = 22.79 mA cm−2). Both materials possess similar highest occupied molecular orbital level and the same order of magnitude of hole mobility at 10−4 cm2 V−1 s−1. The slightly poorer performance of 10.55% (VOC = 1.01 V; JSC = 17.80 mA cm−2) is obtained for n-dodecyl tethered Y2A2 HTM. This is believed to stem from more surface pinholes when deposited on perovskite leading to an order of magnitude slower mobility.

H

While generalized p−i−n or p−n junctions of PSCs employ perovskite as n-type conductor, the choice of a hole-conductor requires suitable energy level and good hole mobility for rapid transfer of holes from perovskite and suppressed charge recombination. It is a great advantage to graft the concept of arylamine-based hole-transporting materials (HTMs) designed for organic light-emitting diodes and organic field-effect transistors12−14 to PSCs because of their good thermal properties and hole mobility. 2,2′,7,7′-Tetrakis(N,N-di-pmethoxyphenylamine)-9,9′-spirobifluorene (Spiro-OMeTAD)15−17 is a well-known HTM characterized by spiro framework and multiple electron-rich arylamine substituents. Introducing Spiro-OMeTAD as an alternative solid-state HTM to I−/I3− electrolytes18 unlocks the possibility of PSCs toward valid candidates for next-generation solar cells.11 Careful engineering of PSC devices reaches high conversion efficiencies of 19.3% and 20.2% using Spiro-OMeTAD and its cyclopentadithiophene analogue, respectively.19,20 Addition of Li-TFSI is

arvesting solar energy as a promising solution to the growing energy crisis of the community is undoubtedly one of the attractive objectives in the new century. Perovskite-based solar cells (PSCs) have shown great potential over other organic and hybrid solar cells as the power conversion efficiency of PSCs has been dramatically improved from 3.8% to over 20% in the past few years.1,2 With ABX3 type tetragonal crystal structure where A, B, and X typically represent small molecular cation, lead or tin, and halide, respectively, the organometal halide perovskite, e.g., CH3NH3PbI3 or MAPbI3, is fascinating for several features like lower manufacturing costs, long electron−hole diffusion length,3,4 tunable band gap,5−7 tolerance toward solution processing,8 extremely low exciton binding energy,9,10 suppressed charge recombination, and hybrid nature to be operated as p- or n-type materials.3,4 The broad absorption in the ultraviolet−visible (UV−vis) region with band gap typically of 1.5 eV for CH3NH3PbI3 and highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels at −5.43 and −3.93 eV, respectively,11 make perovskite materials promising photosensitizers for semiconductor solar cells. © 2016 American Chemical Society

Received: September 12, 2016 Accepted: October 13, 2016 Published: October 13, 2016 956

DOI: 10.1021/acsenergylett.6b00432 ACS Energy Lett. 2016, 1, 956−962

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ACS Energy Letters

Figure 1. Molecular structures of hole-transporting materials in this study.

Figure 2. Absorption spectra of (a) Y2 and Y2A2 in THF and as thin films and (b) TiO2/perovskite films with and without Y2.

bulk-heterojunction photovoltaics with hole mobility in the range between 10−4 and 10−6 cm2 V−1 s−1.50−55 Although these porphyrin derivatives present good ability to carry positive charges in both types of solar cells, they work basically as efficient light-harvesters, which leads to the high performance of the devices. Nevertheless, reports of the application of porphyrins exclusively as a hole-transporter rather than a lightharvester in photovoltaic devices are absent. Herein we have devised novel zinc porphyrins Y2 and Y2A2 (see Figure 1) and proved their potential application as hole-transporting materials for high-performance perovskite-based solar cells. The UV−vis region of incident light is found to be nicely absorbed by the perovskite layer, and only a limited amount of visible light approaches the hole-transporting layer. As a result, the porphyrin-based hole-transporters can be regarded as working in the dark without harvesting light. During the period of our study on these new porphyrin HTMs, a similar system using natural chlorophyll derivatives has recently shown a power conversion efficiency (PCE) of 11.44%.56 To the best of our knowledge, however, this is the best-performing porphyrinbased solar cell with PCE > 16%. Both molecular structures of Y2 and Y2A2 consist of a meso5,15-bis(ethynylaniline)porphyrin backbone bearing bilateral alkyl chains. Bridging of ethynyl groups between the porphyrin core and anilines ensures sufficient electronic communication between each other. The lateral alkyl chains are found to increase the solubility and affect the film morphology of porphyrin materials. These materials are designed to be structurally simple and symmetric such that the synthesis of the final compounds involves only simple condensation, bromination, and Sonogashira coupling57 steps starting from commercially available chemicals and reagents (detailed in the Supporting Information). Thermogravimetric analysis (TGA) shows only 5% weight loss of Y2 and Y2A2 at temperatures as

compulsory for Spiro-OMeTAD because of its poor hole mobility in pristine form. However, this chemical oxidation process eventually leads to erosion of the perovskite layer.16,21 In addition, the lower amenability toward functionalization,22,23 lengthy and tedious synthetic route originating from the spiro backbone, as well as the involvement of hazardous nbutyllithium during traditional synthesis bring about certain obstacles to practical application of Spiro-OMeTAD. Considerable efforts therefore focus on developing alternative holetransporters with optimized synthetic protocols. Judicious molecular engineering has also successfully produced several high-performance HTMs with high carrier mobility and stability via either modification of spiro-based materials such as fluorine-9,9′-xanthenes,24,25 bifluorenylidene,26 or extensive integration of alternative core skeletons like triphenylamines,27 carbazoles, 28−31 triptycenes, 32 biphenyls, 29,33−35 thiophenes,36−40 triazines,41 pyrenes,42 and heteroacenes.43,44 As an advantage, the HTM layer is supposed to be colorless because any UV−vis light absorption of HTM would affect the light-harvesting efficiency of the perovskite layer. In this regard, porphyrin derivatives would definitely be removed from the wish list for its superior light-harvesting properties. Questions such as the following are raised: Do the light-harvesting properties of porphyrins deteriorate the efficiency of PSCs? Can porphyrins carry holes efficiently in PSCs? In fact, synthetic zinc porphyrin dyes have experienced fruitful achievements as highly efficient photosensitizers for dyesensitized solar cells (DSCs).45−47 Zinc porphyrins such as GY5048 and SM31549 with proper design of donor and acceptor/anchoring groups have shown record high conversion efficiency of 12.75% and 13.0%. While as small-molecule holeconducting media, the zinc porphyrin−EWG conjugates,50−54 where EWG denotes electron-withdrawing groups like rhodanine,53 diketopyrrolopyrrole,50,54 and dicyanovinyl groups,52 also show promising performance (η = 4−8%) for 957

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ACS Energy Letters Table 1. Photophysical and Electrochemical Data for HTM Materials Y2 Y2A2

λabsa (nm)

fwhma,b (cm−1)

λabsc (nm)

fwhmb,c (cm−1)

E1/2(ox)d (V vs Fc/Fc+)

EHOMOe (eV)

472, 684 473, 685

1181 1316

487, 654, 694, 722 487, 658, 720

2967 1579

0.12 0.11

−5.22 −5.21

Absorption spectra of sample solution were measured in THF at 25 °C. bfwhm is the full width at half-maximum height. Values indicated are for Soret bands. cThin-film absorption of the samples on FTO glass. dE1/2(ox) denotes first oxidation potentials of 1 × 10−3 M sample solution in THF containing 0.1 M [(n-Bu)4N]PF6 as supporting electrolyte. Potentials are repoted versus ferrocene/ferrocenium (Fc/Fc+). eEHOMO denotes HOMO energy in electronvolts calculated based on the formula EHOMO = −(5.1 + E1/2(ox)). a

Figure 3. (a) Cross-sectional SEM image of perovskite solar cell device with Y2 HTM. (b) Photoluminescence spectra of perovskite thin films with and without HTM.

high as 384 and 366 °C, respectively, indicating fairly stable structure of porphyrins (Figure S1). UV−vis absorption spectroscopy has revealed the difference in intermolecular interactions and competitive light-harvesting with perovskite for Y2-series porphyrins (Figure 2 and Table 1). Both Y2 and Y2A2 show almost identical absorption in THF owing to the same structural backbone (see also Figure S2 and Table S1). Two absorption bands were observed at ca. 472 nm and ca. 684 nm and were characterized as Soret and Qband, respectively (Figure 2a). Nevertheless, Y2 as thin-film shows both Soret and Q bands red-shifted and significantly broadened (with full width at half-maximum (fwhm) of ca. 3000 cm−1), indicating stronger intermolecular interaction probably originating from aggregation of porphyrin molecules. For the thin film of long-chain tethered Y2A2, only moderate broadening (with fwhm of ca. 1500 cm−1) of the absorption bands and a shoulder at ca. 650 nm accompanied by Q-band were observed. The discrepancy between two porphyrins in the condensed phase might be indicative of different surface morphology while deposited on perovskite layer. Compared with the Spiro-OMeTAD that absorbs photons only in the UV region, porphyrin-based HTMs may participate in the lightharvesting process inside the solar cells. To address this issue, we measured the UV−vis absorption spectra for perovskite film with and without Y2, which are shown in Figure 2b. It is clear that there is no significant difference for the absorption from 350 to 640 nm due to the saturated absorption by the perovskite film, whereas longer wavelength absorption (640− 750 nm) has minor contribution from the absorption of Y2. Therefore, the porphyrin-based HTM is regarded as having limited effect in terms of the light absorption competition with perovskite. The fluorescence spectra for both porphyrins in principle follow Kasha’s rule with small Stokes shift both in solution and condensed phase (337−534 cm−1), showing relatively smaller energy required for geometrical reorganization at the excited state (Figure S2 and Table S1). Cyclic voltammograms reveal a reversible first oxidation potential

E1/2(ox) at ca. +0.12 V referenced to Fc/Fc+ for both porphyrins, which is in the proximity of that for SpiroOMeTAD (+0.118 V vs Fc/Fc+)58 (see Figure S3 and Table S2). Consequently, Y2-series materials share a comparable HOMO level with Spiro-OMeTAD at −5.22 eV,59,60 leading to a large energy difference with the LUMO of perovskite at −3.93 eV.11 Figure 3a shows a typical device configuration of perovskite solar cells with porphyrin HTMs as imaged by cross-sectional scanning electron microscopy (SEM). The perovskite capping layer/mesoporous TiO2 layer with 550 nm thickness is deposited with 70 nm of Y2 HTM and 60 nm of gold electrode via spin-coating and thermal evaporation, respectively. The effective hole-extraction ability of porphyrin HTMs is evident in their photoluminance (PL) spectra (Figure 3b). All the PL for the perovskite is quenched very efficiently in the presence of HTMs, either Spiro-OMeTAD or Y2-series porphyrins. This phenomenon indicates prominent ability of charge separation at perovskite interface for Y2-series porphyrins. The energy diagram of the device with different HTMs is illustrated in Figure 4. The HOMO of Y2 and Y2A2 are −5.25 and −5.10 eV, respectively, according to ultraviolet photoelectron spectroscopy measurements (Figure S4). These values are very close to that for Spiro-OMeTAD (−5.22 eV);

Figure 4. Energy levels of each layer in perovskite solar cells. 958

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Figure 5. (a) J−V and (b) IPCE plots for PSCs with HTMs.

× 10−5 cm2 V−1 s−1), which is in good relationship with the FF of the devices. Note also the comparable hole mobility of Y2 with reported spiro-analogous X6024 (1.9 × 10−4 cm2 V−1 s−1). When Y2 with Y2A2 are compared, the difference of the hole mobility may be attributed to the length of bilateral alkyl chains which increase the intermolecular distance and hampers the hole hopping. SEM images show that the morphology of HTMs (Y2 and Y2A2) on perovskite film apparently includes some pinholes on the surface (Figure S8). The existence of pinholes suggests possible exposure of the perovskite layer toward the electrode without proper hole-transporting layer. Minor pinholes on the surface are found for Y2 as compared to Y2A2, indicating increased recombination for the later between electrons from the perovskite and holes from the gold electrode. Also, the pinholes in HTMs retard charge-transport, leading to the low photocurrent and FF.23,64 This also explains statistically lower VOC and JSC for Y2A2 (see Table S3). Figure 5b shows comparable incident photon-to-electron conversion efficiency (IPCE) spectra for Y2 and SpiroOMeTAD over the entire UV−vis region (350−800 nm). This indicates that both HTMs exhibit superior charge extraction and collection as well as reduced recombination of accumulated charge within the perovskite layer. Again, much worse performance in IPCE for Y2A2 results from the lower hole mobility and hindered charge transport. Although Y2A2 shows steady-state PL quenching ability that is slightly higher than that of the others (Figure 3b), we cannot exclude the possible nonradiative process during the charge extraction between the perovskite and Y2A2 interface, which contributes the lower PCE. Another interesting observation is that the additional absorption by porphyrin Y2 at around 700 nm (Figure 2b) does not lower the IPCE and photocurrent because the photons of around 700 nm that are not fully absorbed and penetrate through the perovskite layer could possibly be reflected from the metal back contact and reharvested by the perovskite layer again. Another possibility is the slightly inefficient process for photon-induced electron−hole pairs to separate and reach the perovskite/HTM interface due to the low dielectric constant typically occurring in organic semiconductors.63,65 Finally, preliminary stability tests indicate that the perovskite cell made with Y2 has better moisture resistance than that made with Spiro-OMeTAD. The photovoltaic performances of Y2device under ambient storage decays much slower as compared to the Spiro-OMeTAD device (Figure S9). This substantiates another merit for the use of porphyrin-based HTM. In summary, we have designed and prepared new symmetric ethynylaniline-substituted porphyrins Y2 and Y2A2 and tested

therefore, they would be expected to exert high open-circuit voltage (V OC) of ca. 1 V like Spiro-OMeTAD does. Additionally, the LUMO levels for Y2 and Y2A2 are at −3.39 and −3.42 eV, respectively. These are also higher than that of perovskite layer (−3.93 eV) such that the possible current leakage from the active layer toward gold electrode can be largely retarded (see Table S2). The current density−voltage (J−V) characteristics of devices with different HTMs are measured under AM 1.5G 1 Sun (100 mW cm−2) intensity with mask aperture size of 0.2 cm2 (Figure 5a). In fact, as shown in Table 2 for their best performing parameters, the VOC of Table 2. Photophysical and Electrochemical Data for HTM Materials

Y2 Y2A2 Spiro-OMeTAD

VOC (V)

JSC (mA cm−2)

FF (%)

PCE (%)

Rs (Ω)

0.99 1.01 1.06

22.82 17.80 22.79

73.34 58.69 74.39

16.60 10.55 18.03

18.93 74.78 23.34

devices using Y2 and Y2A2 are 0.99 and 1.01 V, respectively, only slightly smaller than that using Spiro-OMeTAD (1.06 V). In addition, the Y2 device has almost identical short-circuit current (JSC) of 22.82 mA cm−2 and fill factor (FF) of 73.34%, comparable to Spiro-OMeTAD (JSC = 22.79 mA cm−2; FF = 74.39%), leading to PCE of 16.60% for the former and 18.03% for the later. To the best of our knowledge, our result records the best-performing porphyrin-based hybrid/organic solar cells with PCE > 16%, not to mention the first verification of porphyrin HTM for PSCs. It is noted that a more pronounced hysteresis effect was observed for Y2-based PSC rather than that for Spiro-OMeTAD (Figure S5). The Y2A2-based device, on the other hand, has lower PCE of 10.55% with JSC of 17.80 mA cm−2 and FF of 58.69%. The histograms of PCE and statistical average of parameters for 50 porphyrin HTM-based devices are given in Figure S6 and Table S3, respectively. On average, the efficiency of the device made of Y2 (14.00%) is higher than that of Y2A2 (9.56%) with notable differences in photocurrent. To understand the effect of hole mobility on the device performance, we performed the space charge limited current (SCLC) measurement,61 a common and useful method for evaluating the mobility of a hole in organic semiconductors,62,63 following the Mott−Gurney law, J(V) = (9/8)εε0μd−3V2. The fitting data of the J−V curve for HTMs sandwiched between gold electrode and FTO substrate is shown in Figure S7. The trend in hole mobility is Spiro-OMeTAD (9.46 × 10−4 cm2 V−1 s−1) > Y2 (2.04 × 10−4 cm2 V−1 s−1) > Y2A2 (1.53 959

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(4) Xing, G.; Mathews, N.; Sun, S.; Lim, S. S.; Lam, Y. M.; Grätzel, M.; Mhaisalkar, S.; Sum, T. C. Long-range Balanced Electron- and Hole-transport Lengths in Organic-inorganic CH3NH3PbI3. Science 2013, 342, 344−347. (5) Stoumpos, C. C.; Malliakas, C. D.; Kanatzidis, M. G. Semiconducting Tin and Lead Iodide Perovskites with Organic Cations: Phase Transitions, High Mobilities, and Near-infrared Photoluminescent Properties. Inorg. Chem. 2013, 52, 9019−9038. (6) Mosconi, E.; Amat, A.; Nazeeruddin, M. K.; Grätzel, M.; De Angelis, F. First-Principles Modeling of Mixed Halide Organometal Perovskites for Photovoltaic Applications. J. Phys. Chem. C 2013, 117, 13902−13913. (7) Hao, F.; Stoumpos, C. C.; Chang, R. P.; Kanatzidis, M. G. Anomalous Band Gap Behavior in Mixed Sn and Pb Perovskites Enables Broadening of Absorption Spectrum in Solar Cells. J. Am. Chem. Soc. 2014, 136, 8094−8099. (8) Zhao, Y.; Zhu, K. Solution Chemistry Engineering Toward HighEfficiency Perovskite Solar Cells. J. Phys. Chem. Lett. 2014, 5, 4175− 4186. (9) D’Innocenzo, V.; Grancini, G.; Alcocer, M. J.; Kandada, A. R.; Stranks, S. D.; Lee, M. M.; Lanzani, G.; Snaith, H. J.; Petrozza, A. Excitons Versus Free Charges in Organo-lead Tri-halide Perovskites. Nat. Commun. 2014, 5, 3586 DOI: 10.1038/ncomms4586. (10) Lin, Q.; Armin, A.; Nagiri, R. C. R.; Burn, P. L.; Meredith, P. Electro-optics of Perovskite Solar Cells. Nat. Photonics 2014, 9, 106− 112. (11) Kim, H. S.; Lee, C. R.; Im, J. H.; Lee, K. B.; Moehl, T.; Marchioro, A.; Moon, S. J.; Humphry-Baker, R.; Yum, J. H.; Moser, J. E. Lead Iodide Perovskite Sensitized All-solid-state Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%. Sci. Rep. 2012, 2, 591 DOI: 10.1038/srep00591. (12) Thelakkat, M. Star-Shaped, Dendrimeric and Polymeric Triarylamines as Photoconductors and Hole Transport Materials for Electro-Optical Applications. Macromol. Mater. Eng. 2002, 287, 442− 461. (13) Shirota, Y. Photo- and Electroactive Amorphous Molecular Materials - Molecular Design, Syntheses, Reactions, Properties, and Applications. J. Mater. Chem. 2005, 15, 75−93. (14) Jou, J.-H.; Kumar, S.; Agrawal, A.; Li, T.-H.; Sahoo, S. Approaches for Fabricating High Efficiency Organic Light Emitting Diodes. J. Mater. Chem. C 2015, 3, 2974−3002. (15) Malinauskas, T.; Tomkute-Luksiene, D.; Sens, R.; Daskeviciene, M.; Send, R.; Wonneberger, H.; Jankauskas, V.; Bruder, I.; Getautis, V. Enhancing Thermal Stability and Lifetime of Solid-State DyeSensitized Solar Cells via Molecular Engineering of the HoleTransporting Material Spiro-OMeTAD. ACS Appl. Mater. Interfaces 2015, 7, 11107−11116. (16) Nguyen, W. H.; Bailie, C. D.; Unger, E. L.; McGehee, M. D. Enhancing the Hole-Conductivity of Spiro-OMeTAD without Oxygen or Lithium Salts by Using Spiro(TFSI)2 in Perovskite and DyeSensitized Solar Cells. J. Am. Chem. Soc. 2014, 136, 10996−11001. (17) Jeon, N. J.; Lee, H. G.; Kim, Y. C.; Seo, J.; Noh, J. H.; Lee, J.; Seok, S. I. o-Methoxy Substituents in Spiro-OMeTAD for Efficient Inorganic-Organic Hybrid Perovskite Solar Cells. J. Am. Chem. Soc. 2014, 136, 7837−7840. (18) Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. J. Am. Chem. Soc. 2009, 131, 6050−6051. (19) Zhou, H.; Chen, Q.; Li, G.; Luo, S.; Song, T.-b.; Duan, H.-S.; Hong, Z.; You, J.; Liu, Y.; Yang, Y. Interface Engineering of Highly Efficient Perovskite Solar Cells. Science 2014, 345, 542−546. (20) Saliba, M.; Orlandi, S.; Matsui, T.; Aghazada, S.; Cavazzini, M.; Correa-Baena, J.-P.; Gao, P.; Scopelliti, R.; Mosconi, E.; Dahmen, K.H.; et al. A Molecularly Engineered Hole-transporting Material for Efficient Perovskite Solar Cells. Nature Energy 2016, 1, 15017. (21) Zhao, X.; Park, N.-G. Stability Issues on Perovskite Solar Cells. Photonics 2015, 2, 1139−1151. (22) Reddy, S. S.; Gunasekar, K.; Heo, J. H.; Im, S. H.; Kim, C. S.; Kim, D. H.; Moon, J. H.; Lee, J. Y.; Song, M.; Jin, S. H. Highly

their application as hole-transporting materials for perovskite solar cells. Unlike those generally used in dye-sensitized solar cells as porphyrin sensitizers, our porphyrins Y2 and Y2A2 were utilized not to harvest incident light but to function as hole-transporters. These materials are structurally simple and easy to synthesize. The power conversion efficiencies of PSCs with Y2, Y2A2, and Spiro-OMeTAD as HTMs are 16.60%, 10.55%, and 18.03%, respectively. The films of porphyrin HTMs presented in this work exhibit good hole-transporting ability, especially in the case of Y2. The low HOMO level, high hole mobility, and fewer observed surface pinholes while deposited on perovskite lead to open-circuit voltage and shortcircuit current of Y2 comparable to those of Spiro-OMeTAD. We cannot yet clarify if there exists any other beneficial effects for porphyrin-based HTMs beyond their efficient holetransporting property. Nevertheless, unlike spiro and most organic frameworks, the total of 13 positions on the peripheral (4 meso and 8 beta positions) and coordination center of a porphyrin indicates high versatility of structural modification such that future fine-tuning of photophysical, electrochemical, and charge-transporting properties become feasible. Considering the absence of porphyrin-based HTMs for use in PSCs, this work opens a new paradigm of hole-transporters based on porphyrin families.

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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.6b00432. Experimental details; photophysical, electrochemical, thermal analysis, and SCLC measurements; device measurement data; and 1H and 13C NMR spectra (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions §

H.-H.C. and Y.-H.C. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support for this work from the Ministry of Science and Technology (MOST) in Taiwan with Grants MOST 104-2119-M-005-005-MY3, MOST 103-2221-E-006-029-MY3, and MOST 105-2623-E-006-002ET.

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REFERENCES

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DOI: 10.1021/acsenergylett.6b00432 ACS Energy Lett. 2016, 1, 956−962

Letter

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DOI: 10.1021/acsenergylett.6b00432 ACS Energy Lett. 2016, 1, 956−962