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DOI: 10.1021/acs.chemmater.8b01250. Publication Date (Web): June 22, 2018. Copyright © 2018 American Chemical Society. Cite this:Chem. Mater. XXXX ...
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Cite This: Chem. Mater. 2018, 30, 4639−4645

Thiazole-Induced Quinoid Polymers for Efficient Solar Cells: Influence of Molecular Skeleton, Regioselectivity, and Regioregularity Dangqiang Zhu,† Qian Wang,† Yingying Wang, Xichang Bao, Meng Qiu, Bilal Shahid, Yonghai Li, and Renqiang Yang*

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CAS Key Laboratory of Bio-based Materials, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, China S Supporting Information *

ABSTRACT: Fluorination strategy has been regarded as a promising approach to improve the photovoltaic performance in polymer solar cells. However, the synthesis is relatively tedious and costly for most fluorinated momomers. In this work, which is different from works where fluorine atoms are usually incorporated on the side chain, we successfully developed a thiazole-induced strategy to construct efficient photovoltaic materials via inserting one thiazole unit into the backbone of a nonfluorinated quinoid polymer, which would enhance the intermolecular interactions and decrease the ionization potential (IP) of the resulted polymers, benefiting from the desirable molecular skeleton. And then, considering the asymmetry nature of acceptor segments, the influence of molecular regioselectivity with different orientations of the thiazole unit on optoelectronic properties was systematically investigated. Encouragingly, a superior power conversion efficiency (PCE) of 9.36% for PBTzT-4-based photovoltaic device was obtained, higher than that of the isomer polymer PBTzT-6 (PCE = 8.52%) and a signficant increase of 50% compared to the widely reported analogue polymer PBDTTT-E-T (PCE = 6.21%) just without a thiazole unit, which can be ascribed to more planar molecular conformation, stronger crystallinity, and excellent phase separation. More interestingly, compared with random polymers, the regioregular copolymers exhibit enhanced red-shifted absorption and better crystallinity and compatibility with PC71BM, leading to more desirable efficicency for PBTzT-4R-based devices (PCE = 9.63%) with higher JSC of 17.56 mA/cm2, which can be comparable to the typical polymer PTB7-Th. This work not only provides a new strategy to improve the intermolecular interaction through backbone design but also reveals that the orientations of the asymmetric unit (that is, regioselectivity and regioregularity) along the polymer backbone play a crucial role and should be taken into account in future molecule design.

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INTRODUCTION Conjugated polymers have attracted considerable attention in organic electronics due to their distinct characteristics of the electronic properties, structure diversity, lightweight property, and large-area fabrication of flexible devices via ease of solution processing. Nowadays, power conversion efficiency (PCE) of over 11% in polymer/fullerene solar cells (PSCs) has been realized with continuous efforts in design of new building blocks, backbone modification, side-chain engineering, and device optimization.1,2 Hereinto, a molecular skeleton, such as configuration, conformation, and planarity, has been put forward to regulate the energy levels, intermolecular interactions, and carrier mobility, as well as the photovoltaic performance.3−9 Andersson tailored the polymer backbone conformation from zigzagged to linear type by the change of πconjugated spacer from thiophene to thieno[3,2-b]thiophene, which exhibited stronger structural rigidity and planarity.10 Consequently, the corresponding polymer presented highly © 2018 American Chemical Society

ordered intermolecular stacking, leading to superior hole mobility and photovoltaic performance. Similarly, the widely reported polymer PBDTTT system based thieno[3,4-b]thiophene (TT) and benzo[1,2-b:4,5-b′]dithiophene (BDT) also presents a zigzagged backbone with the big included angle (36°), likely due to the linear BDT and one five-membered thiophene (a twisted angle of 150° between chemical bonds 2 and 5). Hou introduced two thiophene units as π bridges to obtain the polymers with a more straight conformation.4,11 The aforementioned examples indicate that it could be an efficient strategy to improve molecular packing and photovoltaic performance through rational backbone modulation. However, the electron-donating thiophene or thieno[3,2b]thiophene unit could make the ionization potential (IP) Received: March 26, 2018 Revised: June 22, 2018 Published: June 22, 2018 4639

DOI: 10.1021/acs.chemmater.8b01250 Chem. Mater. 2018, 30, 4639−4645

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Figure 1. Molecular design diagram and chemical structures of the four polymers synthesized in this work.

high-performance photovoltaic materials by introducing fluorine atoms on the side chain, this new strategy employing thiazole to construct highly efficient light-harvesting polymers would be more suitable for commercialization in terms of cost in the future.

upshift, which is unfavorable to the open circuit voltage (VOC). By contrast, thiazole can significantly decrease the frontier orbital energy levels and reduce the steric hindrance with the neighboring units, in favor of enhancing backbone planarity.12−14 Moreover, considering the asymmetry nature of the TT segment, molecular regioselectivity and regioregularity should also be the critical factors for the molecular design.15 For example, Bazan found that regioselectivity significantly influenced the intermolecular contacts, charge transport, and photovoltaic performance via the investigation of isomeric polymers based another asymmetric monofluorination 2,1,3benzothiadiazole (BT).16 Meanwhile, some works also reveal that the regioregular copolymers usually exhibit enhanced redshifted absorption, better crystallinity, and higher photovoltaic performance in comparison with random polymers.17−20 Here, we propose a new strategy to reduce the twisted backbone skeleton through the introduction of five-membered heterocycle thiazole into the backbone of PBDTTT system, and four polymers, i.e., PBTzT-4, PBTzT-6, PBTzT-4R, and PBTzT-6R (R represents regioregularity) were designed and synthesized, to systemically investigate the influence of molecular skeleton, orientation of TT, and regioregularity (Figure 1). Encouragingly, just as expected, the introduction of thiazole significantly enhances the intermolecular interactions with smaller π−π stacking distance (3.65 Å), as well as decreases IP value, leading to an obvious increase of VOC (∼0.1 V). As a result, the polymer PBTzT-4 exhibits more planar molecular conformation, stronger crystallinity, and higher hole mobility in comparison with those of PBTzT-6. Combined with excellent phase separation with PC71BM, a superior photovoltaic performance (PCE = 9.36%) was obtained in the corresponding devices with a short-circuit current density (JSC) of 16.92 mA/cm2, higher than that of PBTzT-6 (PCE = 8.52%), and a signficant increase of 50% compared to PBDTTT-E-T (PCE = 6.21%) just without thiazole unit. In addition, compared with the random polymers, the regioregular copolymers exhibit red-shifted absorption, better crystallinity, and hole mobility. Finally, PBTzT-4R-based devices gave more desirable efficiency of 9.63% with a remarkable JSC of 17.56 mA/cm2, which can be comparable to the classical polymer PTB7-Th and among the highest values for the TT-based solar cell. In general, this work not only provides a method to improve the crystallinity and π−π stacking through rational molecular design but also reveals the crucial role of the orientations of the asymmetric unit along the polymer backbone. More importantly, different from most

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RESULTS AND DISCUSSION The synthetic routes of monomers are shown in Scheme 1. The compounds 1 and 3 were synthesized according to the Scheme 1. Synthetic Routes of the Monomers and Polymersa

a

Reagents and conditions: (i) 2-(tributylstannyl)thiazole, Pd(PPh3)4, PhMe, 110 °C, 12 h; (ii) NBS, CHCl3, rt, 30 min; (iii) NIS, CHCl3, rt, 1 h; (iv) 2D-BDT-Sn, Pd(PPh3)4, PhMe, 100 °C, 24h; (v) Pd(PPh3)4, PhMe/DMF, 110 °C, 15 h.

previous literatures.21 To construct 4-position linked polymer, the key monomer M1 was obtained in two steps of Stille coupling reaction and bromination by N-bromosuccinimide (NBS) starting from compound 1. For the 1HNMR spectra of the isomer monomers, the chemical shift of CH (3-position of TT) for M1 (4-position thiazole) was significantly shifted to 7.97 ppm from 7.60 ppm for M2 (6-position thiazole), which could be due to the inductive effect of NC in thiazole. Meanwhile, from the ultraviolet−visible (UV−vis) spectra (Figure S1, Supporting Information), M1 shows a significant red-shift of 25 nm for the maximum absorption peak (λmax) in comparison with that of M2, which would be ascribed to the longer conjugated length for M1 (5 conjugated double bonds including the CO unit). Furthermore, to realize the complete regioregularity, we first synthesized the compound 5 with monoiodo and monobromo substituents and then selectively coupled with BDT to give a chemical regioregular 4640

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spacing was mainly determined by the long alkyl side chains, the difference of the (100) peaks (0.263 Å−1) for both polymer films was inconspicuous. Additionally, the strong (010) π−π stacking peaks at about 1.72 Å−1 were observed, corresponding to a π−π stacking distance of 3.65 Å, obviously smaller than that of the PBDTTT system, such as PTB7-Th (3.85 Å),24 PBDTTT-S-T (3.88 Å),5 and PBDTTT-C-T (3.93 Å).19 The aforementioned results suggest the formation of more ordered structures and more compact π−π stacking when introducing an additional five-membered heterocycle into the backbone, which confirms our initial expectation to modulate a molecular skeleton by this strategy. The influence of regioselectivity on physiochemical properties was further investigated. Owing to the polymer backbone composed of the same building blocks, both polymers show excellent thermal stability with the similar decomposition temperatures (Td, 5% weight loss) at 332 and 323 °C for PBTzT-4 and PBTzT-6, respectively (Figure S3), as well as the almost same redox peaks according to the cyclic voltammetry (CV) measurement (Table 1 and Figure S4). The IP values for PBTzT-4 and PBTzT-6 from the CV curves are estimated to be −5.35 eV and −5.31 eV, respectively, much lower than that of PBDTTT-E-T (−5.22 eV), which verified the feasibility of the strategy that introducing thiazole as the π bridge can decrease the energy level. Interestingly, some great differences were observed in optical absorption, crystallinity, and hole mobility of the two polymers. From the ultraviolet−visible (UV−vis) spectra in Figure 3, Table 1, and Figure S5, both polymers PBTzT-4 and PBTzT-6 cover the broad absorption ranging from 300 to 750 nm and show similar λonset at ∼750 nm as films, slightly red-shifted by 10−15 nm in comparison with those in solution. Notably, the maximum absorption peak of PBTzT-4 has a tiny bathochromic-shift of ∼10 nm compared to that of PBTzT-6, and the two polymers exhibit significantly different absorption profiles in solution and as film. In detail, PBTzT-4 shows a defined vibronic structure with absorption peak at ∼680 nm even in dilute solution (10−5 M), which is absent in the case of PBTzT-6, implying there should have strong intermolecular aggregation in solution. Meanwhile, the photoluminescence (PL) spectra of the polymer/PC71BM blend films were carried out to investigate the charge transfer. It can be observed from Figure S5b that the high PL intensity for the neat films can be completely quenched when blended with PC71BM, implying the efficient charge transfer between polymer donor and electron acceptor. To further verify this, spectra of the polymers in dilute solutions are recorded to estimate the aggregation degree as shown in Figure S6.25 One can observe that the defined vibronic structures are gradually weakened, accompanying with a blue-shift of several nanometers for the main absorption at ∼625 nm, and the intensity ratio of I0−0/I0−1 between λ0−0 and λ0−1 almost linearly decreased as the temperature increased (Table S1), implying the formation of isolated polymer chain

M3 due to the higher activity of the I atom in the coupling reaction. The M2 and M4 were obtained through the similar synthetic routes. For the polymerizations, Stille coupling reaction and Soxhlet extraction purification were adopted to yield the target polymers. These polymers exhibited similar number-average molecular weights (Mn) ranging from 50.0 to 60.5 kDa with polydispersity index (PDI) < 2.6 (Figure S2, Supporting Information), implying the polymerization shows negligible influece on the molecular weight. Moreover, the resulted polymers can be dissolved very slowly in chlorobenzene (CB) and o-dichlorobenzene (o-DCB) at room temperature; however, once heated to 50−60 °C, they can well dissolve in the CB solution in several minutes, which is suitable for the solution-processed device fabrication. In addition, there is almost no obvious difference in the solubility for these four polymers even at the concentration of 10 mg/mL. To explore the influence of the introduction of the thiazole unit on molecular skeleton, grazing-incidence wide-angle X-ray scattering (GIWAXS) was employed to probe the molecular packing characteristics of PBTzT-4 and PBTzT-6 in neat film.22 The polymer PBDTTT-E-T only exhibited a weak reflection at 0.274 Å−1, corresponding to a lamellar d-spacing of 22.9 Å.23 By contrast, both of the new polymer films showed clear (100) reflection in the in-plane direction and (010) diffraction signal in the out-of-plane direction, which indicated a favorable face-on orientation (Figure 2). Since the d100-

Figure 2. GIWAXS profiles of PBTzT-4 and PBTzT-6 in neat films.

Table 1. Optical and Electrochemical Properties of the Polymers polymers PBTzT-4 PBTzT-6 PBTzT-4R PBTzT-6R

solution λmax (nm) 334, 321, 336, 316,

622, 627, 625, 631,

672 662 674 673

εmax (M−1 cm−1) 5.4 4.8 5.6 5.1

× × × ×

104@672 104@627 104@674 104@631

nm nm nm nm

film λmax (nm) 340, 330, 341, 328,

624, 628, 630, 633,

film λonset (nm)

Egopta (eV)

IP (eV)

751 747 759 757

1.65 1.66 1.63 1.64

−5.35 −5.31 −5.35 −5.32

681 671 689 683

μh (cm2 V−1 S−1) 1.09 8.42 1.99 1.26

× × × ×

10−3 10−4 10−3 10−3

a

Estimated from the onset wavelength of the optical absorption: Egopt = 1240/λonset. 4641

DOI: 10.1021/acs.chemmater.8b01250 Chem. Mater. 2018, 30, 4639−4645

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Figure 3. UV−vis absorption spectra of four polymers in solution (a) and intensity ratio values of λ0−0 and λ0−1 of the polymers in solution at different temperature (b).

Figure 4. J−V curves of devices based on polymer PC71BM (a) and EQE curves at the optimal conditions (b).

when continuously heated (Figure 3b). More importantly, the I0−0/I0−1 values for PBTzT-4 are greatly higher than those of PBTzT-6 at the same temperature, indicating it shows stronger crystallinity. Combined stronger and sharper diffraction signal was observed in the GIWAXS image of PBTzT-4 film (Figure 2), and it can be concluded that PBTzT-4 possesses more ordered structure and higher crystalline compared to PBTzT-6. Accordingly, PBTzT-4 neat film exhibits higher hole mobility than that of PBTzT-6 film (Table 1). This phenomenon may be explained by the backbone planarity for the two polymers. As we know, the steric hindrance caused by the C−H bond (3 position of TT) will yield a bigger dihedral angle at the connection 4 position of TT and BDT moiety, leading to a twisted configuration.26,27 Considering the thiazole unit can efficiently decrease the dihedral angle, it could make the polymer PBTzT-4 present a more planar struture. The “planar” polymer backbone will yield strong driving force to facilitate intermolecular stacking, which would be the main reason for different behaviors in UV−vis absorption spectra and GIWAXS images. Furthermore, compared to the random polymers, the corresponding regioregular polymers show a red-shifted absorption peak of 8−10 nm and slightly enhanced molar extinction coefficient, which could be in favor of sunlight utilization in PSCs. On the other hand, the obviously enhanced I0−0/I0−1 values (Figure 3b), as well as the stronger diffraction signal presented in the GIWAXS images (Figure S7), also confirms that the regioregular polymers possess stronger intermolecular interaction and higher ordered structures than those of the random polymer. As a result, higher hole mobilities of the regioregular polymers are obtained (Table 1, Figure S8), which can be in favor of charge transport in PSCs. The bulk heterojunction PSCs were fabricated with the structure of ITO/PEDOT:PSS/polymer:PC71BM/PFN-Br/Al. The photovoltaic performance was optimized by various conditions (different D/A weight ratios and additive amount) as shown in Figure 4, Table 2, and Table S2. The current

density−voltage (J−V) characteristics and photovoltaic parameters under the optimized conditions are shown in Figure 4a and Table 2, and the average efficiencies are obtained from over 10 separated devices. All the devices exhibit desirable VOC of ∼0.78 V, obviously higher than that of PBDTTT-E-T (0.68 V), which can be comparable to that of the fluorinated TT-based polymer PTB7-Th. The results also confirm that the introduction of thiazole as π bridge is in favor of higher VOC, which is as efficient as the fluorine strategy. Without any additive, the solar cells exhibit very poor performance due to the unfavorable phase separation which will be discussed later. When DIO added, the JSC and FF are simultaneously increased, which result in the PCEs significantly enhanced. Noticeably, the photovoltaic devices exhibit high FF of >70%, which are superior to most analogue polymers (Table S3). In detail, the best device based on PBTzT-4 exhibits a maximum PCE of 9.36%, with a JSC of 16.92 mA/cm2, an increase of 50% in comparison with the widely reported polymer PBDTTT-E-T (PCE = 6.21%). In contrast, the isomeric polymer PBTzT-6-based solar cell exhibits a slightly lower PCE of 8.52% and a decrease of JSC (15.72 mA/cm2), which may be ascribed to the slightly blueshifted absorption and a little low hole mobility. The significant improvements verify that the thiazole-induced strategy in the main chain is effective. However, the regioregularity does not have great influence on its molecular energy levels, leading to almost same VOC of ∼0.77 V. On the contrary, the JSC values are obviously increased due to the redshifted absorption, enhanced crystallinity, and higher hole mobility. Consequently, a superior efficiency of 9.63% is obtained for PBTzT-4R with a desirable JSC value of 17.56 mA/cm2, which is one of the highest values among the TTbased solar cells, and obviously higher than that of nonfluorinated polymers (generall PCE < 8%) (Figure 5 and Table S3). More importantly, it can be expected that the photovoltaic 4642

DOI: 10.1021/acs.chemmater.8b01250 Chem. Mater. 2018, 30, 4639−4645

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Chemistry of Materials Table 2. Photovoltaic Parameters of PSCs Based on Polymer/PC71BM PCEa (%) VOC (V)

polymer PBDTTT-E-T30 PTB7-Th12 PBTzT-4 PBTzT-6 PBTzT-4R PBTzT-6R

0.68 0.78 0.78 0.77 0.77 0.77

(0.77 (0.77 (0.77 (0.77

± ± ± ±

0.01) 0.01) 0.01) 0.01)

2

JSC (mA/cm ) 14.59 16.76 16.92 15.72 17.56 16.84

(16.48 (15.45 (17.31 (16.38

± ± ± ±

0.53) 0.47) 0.40) 0.56)

62.6 64.5 70.9 70.4 71.1 70.3

FF (%)

max.

ave.

± ± ± ±

6.21 8.43 9.36 8.52 9.63 9.12

----9.19 8.28 9.48 9.01

(69.7 (69.1 (70.1 (69.2

1.6) 1.7) 1.5) 1.8)

μh (cm2 V−1 S−1) ----2.08 1.16 4.25 2.44

× × × ×

10−4 10−4 10−4 10−4

a

Data are obtained from 10 separate devices.

From external quantum efficiency (EQE) curves of the optimal devices (Figure 4b), four devices show good photoresponse in the 300−750 nm regions, and the polymers PBTzT-4(R) exhibit slightly higher spectra than that of PBTzT-6(R) in the range of 500−700 nm, resulting in a higher JSC. As shown in Figure S9 and Table 2, the hole mobility of the PBTzT-4/PC71BM blend film is 2.08 × 10−4 cm2 V−1 S−1, nearly two times higher than that of PBTzT-6/ PC71BM film (1.16 × 10−4 cm2 V−1 S−1). Meanwhile, the regioregular polymer/PC71BM blend films further enhance the hole mobility twice compared to their random counterparts, which is one main reason for higher JSC in regioregular polymer-based devices. In addition, we further evaluated the charge recombination of the isomeric polymer-based solar cells through the relationship of JSC or VOC under different light intensities (P) with the formulas of log JSC ∝ α log(P) and VOC ∝ nkT/q ln(P), respectively.31,32 As shown in Figure S10a, the fitting slope α is closer to 1, indicating weaker bimolecular recombination for PBTzT-4-based devices. Meanwhile, according to VOC−P curves (Figure S10b), in theory, the value of n equal to 1 indicates free bimolecular recombination

Figure 5. Maximum PCE vs VOC for reported TT-based polymer solar cells (the details are shown in Table S3).

performance would be further increased through precise optimization of the donor segment, side chain, and device engineering referring to the development of PTB7-Th (see the green triangles in Figure 5).2,28,29

Figure 6. AFM and TEM images of polymer:PC71BM blend films. (a, b) for PBTzT-4, (c, d) for PBTzT-6, (e, f) for PBTzT-4R, (g, h) for PBTzT6R. a-1 to h-1 for AFM height image, a-2 to h-2 for AFM phase image, a-3 to h-3 for TEM image. (a, c, e, g) without DIO, (b, d, f, h) with DIO. The size is 4 × 4 μm for AFM images. 4643

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Chemistry of Materials while n = 2 means strong trap-assisted recombination. Additionally, the relatively smaller value for the PBTzT-4based device further verified lower recombination. The higher hole mobility and weaker bimolecular recombination for PBTzT-4-based devices would be the main reason to give superior photovoltaic performance in compariosn with that of the isomeric polymer PBTzT-6. The morphology of active layer with and without solvent additive was investigated by atom force microscopy (AFM) and bright field transmission electron microscopy (TEM) (Figure 6). It can be seen that all blend films exhibit relatively rough surface with a root-mean-squared (RMS) roughness > 4 nm without any additives from AFM images. Meanwhile, some PC71BM show strong aggregations from TEM images, especially for PBTzT-6/PC71BM film with a domain size of over 200 nm, which is very unfavorable for exciton separation. Surprisingly, when solvent additive DIO is added, the morphologies of all blend films demonstrate a dramatic change to relatively desirable nanophase separation, which is in favor of charge transfer, leading to the significant increase of JSC and FF in the photovoltaic devices (Table S2). Meanwhile, the PBTzT-4/PC71BM film also exhibits more uniform bicontinuous interpenetrating network compared to PBTzT-6/ PC71BM film (which shows a little nutty structures from AFM and TEM amplified images shown in Figures S11 and S12). Finally, compared to random polymers, the regioregular polymers exhibit more desirable fibril-like nanophase separation, especially for PBTzT-4R/PC71BM film, which would be one important reason why the PBTzT-4R polymer enables high photovoltaic performance. The significant difference in morphology is closely related to the backbone planarity and regioregularity.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (R.Y.). ORCID

Xichang Bao: 0000-0001-7325-7550 Bilal Shahid: 0000-0003-1194-7702 Yonghai Li: 0000-0002-5748-0258 Renqiang Yang: 0000-0001-6794-7416 Author Contributions †

D. Zhu and Q. Wang contributed equally to this work. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are deeply grateful to the National Natural Science Foundation of China (21604092, 51573205, and 51773220), the Ministry of Science and Technology of China (2014CB643501), China Postdoctoral Science Foundation (2017M610453), and the Youth Innovation Promotion Association CAS (2016194). The authors thank beamline BL16B1 (Shanghai Synchrotron Radiation Facility) for providing beam time.

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CONCLUSION In this work, the thiazole unit was introduced into the backbone of the quinoid polymer as the π bridge to modulate the molecular skeleton. As a result, the intermolecular interactions of the resulted polymers were enhanced and their ionization potentials were decreased. It can be seen that the thiazole unit in the main chain functions exactly as the fluorine atoms, which usually is incorporated on the side chain. Interestingly, the orientation of the thiazole unit and regioregularity of the polymers show significant influence on the physiochemical and photovoltaic performance. The device based on PBTzT-4R gave a superior PCE of 9.63% with a remarkable JSC of 17.56 mA/cm2, which can be ascribed to more planar molecular conformation, red-shifted absorption, stronger crystallinity, and excellent phase separation compared to the random analogue polymer PBTzT-4 and its isomer PBTzT-6R. This work not only provides a promising and simple strategy to regulate the energy levels, the crystallinity, and π−π stacking through employing the thiazole-induced effect, which functions as efficiently as the widely recognized F strategy, but also reveals that the orientations of the asymmetric unit along the polymer backbone play a crucial role and should be taken into account in future molecular design.

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The synthesis procedure and characterization of materials, fabrication of PSC devices, TGA curves, temperature-dependent UV−vis absorption spectra, electrochemical properties, PL spectra, hole mobility of the polymers, photovoltaic performance of the polymer under different fabrication conditions, and AFM and TEM images (PDF)

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REFERENCES

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b01250. 4644

DOI: 10.1021/acs.chemmater.8b01250 Chem. Mater. 2018, 30, 4639−4645

Article

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DOI: 10.1021/acs.chemmater.8b01250 Chem. Mater. 2018, 30, 4639−4645