Polystyrenesulfonate Dispersed Dopamine with Unexpected Stable

Finally, DA·HCl (1 g) was fed at room temperature and the reaction lasted for at least 12 h .... (83, 84) The broad peak signal between 0 and 2.3 ppm...
2 downloads 0 Views 3MB Size
Download PDF
Research Article pubs.acs.org/journal/ascecg

Polystyrenesulfonate Dispersed Dopamine with Unexpected Stable Semiquinone Radical and Electrochemical Behavior: A Potential Alternative to PEDOT:PSS Wanshan Liang,†,‡ Lijia Xu,§ Sheng Sun,∥ Linfeng Lan,∥ Xueqing Qiu,*,†,‡ Runfeng Chen,*,§ and Yuan Li*,†,‡

Downloaded via UNIV OF SOUTH DAKOTA on July 1, 2018 at 12:18:17 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

School of Chemistry and Chemical Engineering, South China University of Technology, Wushan Road 381, Tianhe District, Guangzhou 510641, China ‡ State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Wushan Road 381, Tianhe District, Guangzhou 510641, China § Key Laboratory for Organic Electronics and Information Displays & Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing University of Posts & Telecommunications, 9 Wenyuan Road, Nanjing 210023, China ∥ State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Wushan Road 381, Tianhe District, Guangzhou 510641, China S Supporting Information *

ABSTRACT: Inspired by the p-doped PEDOT:PSS, a traditional anode modifier, we proposed to prepare polydopamine:polystyrenesulfonate (PDA:PSS) via the self-polymerization of dopamine in aqueous PSS initially. However, DA and its semiquinone radical were dispersed by PSS to form DA:PSS successfully. Interestingly, a strong electron spin resonance signal was detected in DA:PSS, suggesting the stable semiquinone radical was formed. More importantly, water-soluble DA:PSS exhibited stable and quasireversible electrochemical oxidation behavior, and excellent filmformation capability. Consequently, as an indium tin oxide (ITO) anode modifier, solution processed DA:PSS film showed hole injection property in organic light emitting diodes. Our results open a new avenue for the design of semiconductor and organic electronic application inspired by the electron transfer of phenol derivatives such as DA. Phenol-based organic electronic material has showed potential and it should be taken into consideration in the future. KEYWORDS: PEDOT:PSS, Hole transport material, Organic light-emitting diode, Organic electronic, Phenol

■

INTRODUCTION Over the past more than 30 years, the dramatic development of organic electronic (OE) devices, including organic lightemitting diodes (OLEDs),1−9 organic field effect transistors (OFETs),10−14 organic photovoltaics (OPVs)15−20 has attracted worldwide attention of chemists and material scientists. For all of devices mentioned, the organic charge transport materials are usually divided into p-type21−26 and n-type27−30 materials, as well as ambipolar transport31−33 materials. It is well-known that p-type materials refer to hole-transport materials (HTMs), and they play an indispensable role in organic electronic devices. As acknowledged, it has been challenging and important, as well as a hot topic, to develop high performance n-type semiconductors in the past 10 years. Compared with the n-type materials, numerous efforts have been focused on the research of p-type ones. Indeed, there have been many excellent candidates for p-type materials, especially for vacuum-deposited small molecules in ongoing commercializing OLED.34−37 However, solution processable polymeric p© 2016 American Chemical Society

type material is still in high demand for solution-processed organic electronic. Poly(3,4-theylene dioxythiophene):poly(styrene sulfonic acid) (PEDOT:PSS) is one of the most successful and excellent water-soluble p-type materials as it has advantages including tunable high conductivity, high transparency in UV−visible region, the capability of smoothing the ITO morphology and so on.38−44 There are still some drawbacks including poor device lifetime induced by its acidity, structural and electronic inhomogeneity. Considering the weakness of PEDOT:PSS, many alternatives of PEDOT have been widely explored in previous work.45−48 Some work related to hydroquinone−quinone complexes on molecular electronics has been reported.49,50 Motivated by our curiosity on the potential of electron-rich aromatic phenolic compounds to act as hole-transport materials, recently we further applied phenolReceived: August 3, 2016 Revised: October 22, 2016 Published: October 24, 2016 460

DOI: 10.1021/acssuschemeng.6b01845 ACS Sustainable Chem. Eng. 2017, 5, 460−468

Research Article

ACS Sustainable Chemistry & Engineering

8.5 with the addition of diluted HCl. PSS (2 g, Mw = 70 kDa) was dissolved in the buffer solution, and the solution was stirred for 5 min subsequently. Finally, DA·HCl (1 g) was fed at room temperature and the reaction lasted for at least 12 h. The DA·HCl monomer can be partially oxidized under alkaline conditions in the presence of O2 as the oxidant. With the dispersion of PSS, the color of the solution changed from colorless to pale brown, and finally to deep brown. The product was dialyzed by a dialysis membrane (Special Products Laboratory, USA, MWCO = 1000 Da) to remove inorganic salt, and the purified products was then freeze-dried to obtain solid power samples. The pure product has a good solubility in water (>10 mg/ mL), and it also can be dissolved in DMSO. Oxidation of Catechol. Catechol (200 mg) was dissolved in 10 mL of ethanol. Catechol was oxidized easily, and the color changed from colorless to brown gradually in air, followed by the addition of 5 mL of ammonia. After 1 h of stirring at ambient condition, solid oxidized catechol (OC) with a color of pale-violet-red was obtained by means of evaporation. UV−vis Absorption Spectra. The UV−vis absorption spectra of DA:PSS, DA·HCl, and PSS (dialyzed by the same method as DA:PSS) aqueous dispersion with a concentration of 0.1 mg/mL were measured using a Shimadzu UV-3600 spectrophotometer (Japan). The spectra were recorded between 190 and 700 nm. NMR Measurement. The 1H NMR, 13C NMR, and 1H-15N HSQC spectra were recorded with DA:PSS dissolved in 0.5 mL of deuterium DMSO (DMSO6) at room temperature by a DRX-400 spectrometer (400 MHz 1H NMR frequency, 600 MHz 13C NMR frequency, 600 MHz 1H-15N HSQC frequency, Bruker Co., Ettlingen, Germany). Electron Spin Resonance (ESR). Solid state electron spin resonance (ESR) spectroscopy was used to determine the presence of free radicals of DA:PSS and OC at room temperature. It was conducted by a Bruker A300. A Bruker E580 frequency counter was provided to calibrate the microwave frequencies. The Bruker Company provides a g-factor marker of S3/2, and its g-value is supposed to be 1.9800 ± 0.0006. However, no teslameter was equipped in our device; a g-factor marker at 1.9850 (see Figure 4) was detected, which was 0.0050 higher than that provided by Bruker Company. Therefore, the accurate g value was the result of g value (experimental data) minus 0.0050. Fourier Transform Infrared Spectra (FTIR). The infrared spectra of DA:PSS and DA·HCl were recorded using Fourier transform infrared spectrometry with an Auto system XL/I-series/Spectrum 2000 instrument (Thermo Nicolet Co., Madison, WI, USA). The samples were dried under vacuum and mixed with KBr. Then the mixtures were tableted for infrared spectrum analysis. The spectra were recorded between 4000 and 500 cm−1. Cyclic Voltammetry (CV). Cyclic voltammetry measurement was conducted using CH760D Electrochemical Workstation, CH Instruments (Austin, Texas, USA). A glassy carbon electrode was first polished carefully with alumina powder and rinsed with distilled water repetitively. The concentrated DA:PSS solution was deposited on the surface of the clean glassy carbon electrode. The resulting electrode was immersed in anhydrous dichloromethane using 0.1 M Bu4NPF6 as the electrolyte. The scanning potential was between −0.2 and +1.6 V at a scan rate of 100 mV·s−1. Ultroviolet Photoelectron Spectrometry (UPS). DA:PSS solution was spin-coated on TIO to obtain the DA:PSS film. The sample was stored in a vacuum desiccator and exposed only briefly to the air before introduced into an UHV chamber. UPS was carried out by an ESCALAB 250Xi. Atomic Force Microscopy (AFM). The preparation of PDA:PSS film was as follows: ITO-coated glass substrates of area 2.0 × 1.5 cm were cleaned ultrasonically in acetone for 15 min and then in deionized water for another 15 min, followed by drying in nitrogen atmosphere before use. The films of DA:PSS were deposited from a solution filtered through a 0.22 μm syringe filter via spin-casting on the precleaned ITO-coated glass substrates with rates at 500 rpm for 5 s, then 2000 rpm for 1 min, and finally 900 rpm for 15 s. AFM images of

based material such as lignin-based derivatives as p-type transport materials in OPVs and OLEDs for the first time.51−56 In general, aromatic phenolic derivatives (PDs) are well-known as unstable compounds as they can be readily converted into fragile phenolic radicals (PRs); thereby, studies of their potential as p-type material are rarely reported. In fact, PDs and PRs are not as vulnerable as we postulate based on a traditional viewpoint. To give an example, catechol can be oxidized into a semiquinone radical intermediate or quinone under mild conditions,57,58 along with an electron transport process.51−56,59 Generally, a semiquinone radical is unstable because of its relatively high thermodynamic potential.60,61 But in fact, for many biopolymers related to catechol, such as lignin, melanin, humic acid and tannin, stable PRs and persistent semiquinone radical were widely detected.62−64 To sum up, there are several strategies to stabilize semiquinone radical: (1) protection of the bulky and hydrophobic tert-butyl group borne with catechol structure,65 (2) metalloprotein was found to have the capability of stabilizing semiquinone radical,61 (3) intramolecular/intermolecular hydrogen bonding between the aryloxyl radical semiquinone hinders the further loss of one electron to form quinone,65 (4) some heavy metals such as Zn2+ and Pb2+ are known to stabilize semiquinones via formation of radical complexes,66−70 whereas some paramagnetic metal ions such Cu2+, Mn2+, or Fe3+ are opposite, and (5) a tightly bounded three-dimensional micromulecular network of lignin can also stabilize the semiquinone radical.71 Overviewing the interesting fundamental work above, we proposed melanin-like polydopamine (PDA), with electron-rich blocks containing catechol hydroxyl and semiquinone radical,72 might act as either an amorphous organic semiconductor or an electronic-ionic hybrid conductor.73−75 PDA derivatives have been applied in batteries, supercapacitors, and catalysts as reported,76−80 whereas reports about PDA in OE devices directly have not yet been found. Inspired by the traditional PEDOT:PSS,81,82 we intended to use PDA to replace PEDOT. To our surprise, with tris(hydroxymethyl)aminomethane (Tris) as a catalyst and PSS as a dopant, none of self-polymerization occurred though self-polymerization of DA was vivacious and polymerization degree was difficult to control as previous work. Fortunately, well confirmed DA:PSS was obtained. The synthetic PDA had a problem of solubility in water and other common solvents such as DMSO and DMF with high polarity. PSS was introduced to ensure its good water solubility and solution processability to achieve excellent film formation. Moreover, phenol-based materials showed irreversible oxidation behavior.51−56 In contrast, PDA:PSS showed stable and quasi-reversible electrochemical oxidation behavior. Consequently, as an indium tin oxide (ITO) anode modifier, solution-processed DA:PSS film showed enhanced performance in organic light emitting diodes. The mechanism was studied and discussed in detail.

■

EXPERIENMENTAL SECTION

Materials. 3-Hydroxytyramine hydrochloride (DA·HCl), with a purity of 98%, from Energy Chemical Co. Ltd. (Shanghai, China), was kept at 0 °C. Poly(styrene sulfonic acid) sodium salt (PSS, Mw = 70 000 Da) was brought from Alfa. Tris(hydroxymethyl)aminomethane was supplied by Energy Chemical Co. Ltd. (Shanghai, China) with a purity of 99.5%. All other chemicals were of analytical grade, including hydrochloric acid (HCl) of 36.5 wt %. The water used in the laboratory was deionized water. Preparation and Purification of DA:PSS. The pH of tris(hydroxymethyl)aminomethane buffer (Tris) solution was adjusted to 461

DOI: 10.1021/acssuschemeng.6b01845 ACS Sustainable Chem. Eng. 2017, 5, 460−468

Research Article

ACS Sustainable Chemistry & Engineering DA:PSS were observed using a Park XE-100 instrument in tapping mode. Conductivity Test. An organic field-effect transistor (OFET) was fabricated in a top-gate, bottom-contact (TG-BC) architecture with a bare Au source/drain electrode and hydroxyl-free poly(per-fluorobutenylvinylether) commercially known as CYTOP (400 nm) acting as the gate dielectric, which was spun-cast onto the DA:PSS (20 nm). In prepatterned electrodes, the channel width (W) and length (L) are 500 and 70 μm for the device with CYTOP. The Au gate electrode was the deposited by thermal evaporation to complete the field-effect transistors. The OFET characterizations were measured with a semiconductor parameter analyzer (Agilent 4155C) and a probe station at room temperature in air atmosphere. Organic Light-Emitting Devices (OLEDs). The blue OLEDs with configurations of ITO/DA:PSS/TAPC (25 nm)/mCP (8 nm)/ mCP:FIrpic (10 wt %, 22 nm)/TmPyPb (35 nm)/LiF (1 nm)/Al (100 nm) and the control device with bare ITO were fabricated, respectively. The patterned ITO glass substrates were cleaned in sequential ultrasonic baths using detergent solution, deionized water, acetone, alcohol, and then dried at 120 °C in a vacuum oven for 20 min. After ultraviolet-ozone treatment for 8 min, the DA:PSS layer was spin coated on the ITO substrate and annealed using a hot plate at 120 °C for 15 min to remove residual solvents. After that, the samples were transferred to a thermal evaporator chamber. The TAPC (30 nm), mCP (8 nm), mCP:FIrpic (10 wt %, 22 nm), TmPyPb (35 nm), LiF (1 nm), and Al (100 nm) were deposited subsequently by thermal evaporation under a pressure of 5 × 10−4 Pa. The thickness of the organic films was measured using a α-SE spectroscopic ellipsometry. The active area of the device is 9 mm2. The devices without encapsulation were measured immediately after fabrication in ambient atmosphere at room temperature. The current−voltage−luminance characteristics were measured with a PR650 Spectroscan spectrometer and a Keithley 2400 programmable voltage−current source.

Figure 1. UV−vis spectra of 0.1 mg/mL PSS, DA·HCl and DA:PSS in H2O. The partial enlarged UV−vis spectra of DA:PSS is inserted. The inset is DA·HCl solid, DA·HCl in H2O, DA:PSS solid, and DA:PSS in H2O, respectively (from left to right).

To further illustrate the structure of our product, 1H NMR spectra of DA:PSS and DA·HCl are given in Figure 2.

■

RESULTS AND DISCUSSION Synthesis and Purification of DA:PSS. Initially inspired by chemical structure of PEDOT:PSS, we proposed to prepare a water-soluble dopant PSS dispersed PDA with similar chemical doping effect reported in the work of Mostert.74 In previous reports on the synthesis of PDA, the self-polymerization of DA is active under alkaline condition with O2 as oxidation reagent or by means of an enzymatic oxidation as an alternative approach. It is well-known the synthetic PDA has a poor solubility in water, even in the other common solvents such as DMSO and DMF with high polarity. To solve this problem, PSS was added as a template and dispersant under Tris catalyst with pH of 8.5 at room temperature. The color of the reaction solution gradually changed from colorless to pale brown, finally turning to deep brown after 24 h (see Figure 1 inset). Subsequently, the product of the reaction was purified by dialysis to remove catalyst Tris, free DA monomerm and DA-based oligomer with low molecule weight. It is interesting that DA:PSS showed a relatively lower acidity with a pH of 5.3, compared with the pH of 1.9 for PEDOT:PSS.51 Considering the deep brown color freeze-dried product, we proposed that DA was oxidized into new compounds. Chemical Structure Analysis of DA:PSS. To study the chemical structure and the origin of deep brown color of DA:PSS, we tested the UV−vis absorption of DA:PSS, DA· HCl, and PSS in aqueous solution. It is noteworthy that DA:PSS showed obvious absorption that ranged from 300 to 600 nm (Figure 1), which is very different from those of raw material DA·HCl and PSS. The wide absorption spectrum of DA:PSS confirmed the DA was oxidized during the preparation of DA:PSS.

Figure 2. 1H NMR spectra of (a) DA:PSS, (b) DA·HCl, and (c) OC (catechol was oxidized in the presence of alkaline ammonia in ethanol; after spin flash drying, OC was obtained).

Surprisingly, the spectrum of DA:PSS was quite clear, which is very different from the complex 1H NMR spectra as previously reported work on typical PDA.83,84 The broad peak signal between 0 and 2.3 ppm was ascribed to alkyl protons of PSS and the broad signal peaks at around 6.5 and 7.5 ppm belonged to aromatic protons of PSS (Figure 2). The other signals, including two kinds of methane groups and aromatic protons almost exactly overlapped with those of DA·HCl. Moreover, there were two humps at 9.0 ppm, ascribed to phenolic hydroxyl protons, distinct from two sharp peaks of DA·HCl. The signal of amino groups shifted to high field and no other signal appeared. These evidence suggested no self-polymerization occurred between the DA monomers. 462

DOI: 10.1021/acssuschemeng.6b01845 ACS Sustainable Chem. Eng. 2017, 5, 460−468

Research Article

ACS Sustainable Chemistry & Engineering Scheme 1. Structure of PEDOT:PSS, DA:PSS, and Typical Polydopamine (PDA)

Simultaneously, two-dimensional 2D 1H-15N HSQC of DA:PSS was conducted (Figure S1). There was only one intense signal at 33.0 ppm, from the typical nitrogen of amine. The 1H spectrum peak at 7.8 ppm (associated with N) split into triplet peaks. The DA intramolecular cyclization reaction did not occur based on the lack of low-field indole derivatives signals.85,86 In addition, 13C NMR spectra of DA:PSS and DA· HCl monomer were recorded to study whether DA monomer was oxidized to dopaminequinone (Figure S2). The 13C NMR spectrum of DA:PSS matched well with that of DA monomer, except for the broad signal from PSS. More importantly, no signal at around 170 ppm was observed in Figure S2 and this confirmed that no dopaminequinone was generated.83,85,86 All of this evidence confirmed our proposal on the structure of DA:PSS (Scheme 1). The S/N number ratio was 1.46 as shown in element analysis of DA:PSS (Table 1); therefore, we Table 1. Element Analysis of DA:PSS element

N

C

H

S

mass ratio (%) calculated atom number

2.51 1

48.3 22.45

5.018 27.99

8.399 1.46

Figure 3. FTIR spectra of (a) DA:PSS, (b) DA·HCl, (C) OC, and (d) catechol.

ESR Spectra of Semiquinone Radical and Mechanism. The UV and NMR results mentioned above confirmed the oxidization of DA, and the color of DA:PSS turned deep brown. No quinone structure was observed. What is the origin of the deep color of DA:PSS? It is usually acknowledged the color is from the quinone structure. Inspired by the deep color of lignin with phenol radical,52 we further studied the ESR of DA:PSS (Figure 4). An obvious g-factor marker at 1.9850 was detectable. The single-line spectrum of DA:PSS is very different from the refined spectra of amidogen radical.90,91 And the accurate g-factor at 2.0038 (the experimental data was 2.0088)

supposed that there were two DA monomers arranged on every three styrene sulfonic acid units. Based on the NMR and elemental analysis data collected, the structure of DA:PSS was gradually revealed and it was different from that of the synthesis PDA, but has something similar to that of PEDOT:PSS (see Scheme 1). Furthermore, hydrogen bonding in phenol− quinone system was mentioned in many previous literatures.87−89 Thus, the FTIR spectra of DA:PSS, DA·HCl, OC, and catechol are presented in Figure 3. The sharp peaks around 3452 cm−1 were ascribed to the phenol groups of DA· HCl (Figure 3b) and catechol (Figure 3d), respectively. Although relatively broad peaks of DA:PSS and OC were detected in Figure 3a,c, indicating that the new intermolecular hydrogen bonding was produced during the oxidation of DA· HCl and catechol, respectively. These interesting findings motivate us to study the underlying mechanism for the unexpected result. A control experiment with catechol as the starting material was designed and carried out to support the results above. Catechol was oxidized in the presence of ammonia, along with a color change from colorless to dark brown in ethanol. Then the pale-violetred solid product was obtained after drying and it was further studied using 1H NMR (Figure 2c). Only three types of protons, belonging to three kinds of protons of catechol, were observed; however, no o-benzoquinone was detected. Moreover, the round stack-like signals from phenolic hydroxyl groups was similar to that of DA:PSS.

Figure 4. ESR signals of solid DA:PSS and OC at room temperature. 463

DOI: 10.1021/acssuschemeng.6b01845 ACS Sustainable Chem. Eng. 2017, 5, 460−468

Research Article

ACS Sustainable Chemistry & Engineering Scheme 2. Formation Mechanism of Semiquinone Radical in (a) OC and (b) DA:PSS

was consistent with the reported g-factor of semiquinone radical.49 We verified this by testing the ESR spectrum of solid OC. A single-line ESR of OC with the accurate g-factor of 2.0050 (the experimental data was 2.0010) at room temperature was also detected, and this result was in good agreement with the result of DA:PSS. It was convincible that during the weak oxidation condition, semiquinone radical was formed with preformed obenzoquinone as an intermediate, thus no o-benzoquinone proton is observed in Figure 2a,c. As a result, we proposed the synthesis, chemical structure, and mechanism on the semiquinone radical in Scheme 2, which is similar to a report on multi-phenol biopolymer, such as lignin and melanin.61,92 Under mild conditions, DA and catechol can be oxidized to form low bandgap semiquinone radical species without dopaminequinone or o-benzoquinone.61 On the basis of all the results above, we can conclude that the self-polymerization of DA was forbidden due to the electrostatic interaction between PSS and amino groups of DA. The amino group involved cyclization reaction played a key role for the typical self-polymerization of DA in previous work. In our work, amino group involved cyclization was prevented by the addition of PSS. Electrochemical Behavior of DA:PSS and OC, UPS of DA:PSS. Considering the phenolic hydroxyl group and semiquinone radical in DA:PSS, cyclic voltammetry (CV) was used to investigate the oxidation behavior of DA:PSS (Figure 5), and OC was also tested for comparison. The onset potential of DA:PSS and OC was measured to be 0.90 and 0.96 V with respect to the Ag/AgCl reference electrode, thereby their highest occupied molecular orbital energy levels (HOMO) were estimated to be −5.60 and −5.66 eV. The ultraviolet photoelectron spectroscopy (UPS) in ultrahigh vacuum (UHV) was used to calibrate the HOMO level.93−95 The HOMO value of DA:PSS was calculated as 5.64 eV (Figure 5b), slightly different from the result of CV. It is worth noting that a quasireversible redox process of DA:PSS was found and it had a good repeatability in 10 runs CV curves of DA:PSS in 0.1 M Bu4NPF6 solution of dichloromethane (Figure 5b), quite different from that of electron-rich phenol-based hole-transport material.51−56 And the oxidation behavior of DA:PSS in 0.05 M H2SO4 solution is given in Figure S3. The proton provided in the H2SO4 system ensured the pronounced reversibility and repeatability of the CV of DA:PSS. This result indicated the potential of DA:PSS as an anode modifier in organic electronics. Conductivity Test of DA:PSS. OFET with DA:PSS as an organic semiconductor layer was used to estimate the conductivity of DA:PSS. The device architecture and the

Figure 5. (a) Ultraviolet photoelectron spectroscopy (UPS) measurement of DA:PSS film spin-coated on ITO. (b) CV curves of DA:PSS and OC film in anhydrous dichloromethane using 0.1 M Bu4NPF6 as electrolyte, and it was scanned for 10 runs at a scan rate of 100 mV·s−1. The HOMO level was calculated according to HOMO = −(EOX + 4.7) eV.

output performance are shown in Figure 6. A slope of 2.48 × 10−11 was available in the linear fitting of ID−VD curve when gate voltage (VG) equaled to 0 V and the slope represented the reciprocal of resistance R. The conductivity of DA:PSS was determined to be 1.73 × 10−7 S·cm, according to the following: ρ·L R= (1) S

κ=

1 ρ

(2)

where ρ is the resistivity, L is the length, S is the cross-sectional area, and κ is the conductivity. Performance and Morphology of DA:PSS as Anode Modifier. On the basis of the results aforementioned and to evaluate the performance of DA:PSS film in OLED, it was spin 464

DOI: 10.1021/acssuschemeng.6b01845 ACS Sustainable Chem. Eng. 2017, 5, 460−468

Research Article

ACS Sustainable Chemistry & Engineering

Figure 6. (a) Output of an OFET with a top-gate, bottom-contact (TG-BC) architecture (W = 500 μm, L = 70 μm) measured in air atmosphere (the black one is the linear fitting). (b) Architecture with DA:PSS as organic semiconductor layer-based OFETs.

coated on the ITO to modify and smoothen the ITO surface. The device structure is shown in Figure 7. The device without an anode modifier was also prepared as the control device for comparison. The current density−voltage curve, brightness− voltage curve, and detailed performance of OLED are provided in Figure 7 and Table 2, respectively. The DA:PSS modified anode device exhibited a higher turn-on voltage (Von) of 5.1 V than the control bare ITO device and the device with PEDOT:PSS as an anode modifier. It is proposed that the HOMO level of DA:PSS is around −5.6 eV, which is lower than the work function of ITO and PEDOT:PSS, resulting in a large energy barrier for hole injection. However, the device with DA:PSS modified anode showed maximum current efficiency (CEmax) of 22.5 cd/A, which was obviously higher than that of the control bare ITO device, and slightly lower than that of PEDOT:PSS modified device. In addition, the brightness of the DA:PSS device is as high as 16369 cd/cm2. The operation current of the DA:PSS device was obviously lower than that of the control device due to the modification of ITO. The power efficiency of DA:PSS is even higher than that of PEDOT:PSS. The underlying mechanism is that DA:PSS will reduce the roughness and smoothen the surface of ITO by filling the pinhole, further decreasing the leakage current, which is similar to the role of PEDOT:PSS and other anode modifiers in organic electronics.45 However, DA:PSS showed much lower conductivity compared with that of PEDOT:PSS. We will forcast that the performance of DA:PSS can be further enhanced by the optimization of chemical structure, such as the introduction of lager conjugation of semiquinone radical. Considering this, our result might provide a promising scaffold for the design of anode modifiers in the future, which is supported by our previous work.51−56

Figure 7. Current density−voltage, brightness−voltage curves (a), and the current efficiency curves (b) of OLEDs with DA:PSS as anode modifier. (c) Device structure with DA:PSS as anode modifier-based OLEDs.

Table 2. Photovoltaic Performance of OLEDs with DA:PSS and PEDOT:PSS as Anode Modifier and the Control Sample anode modifier

Von (V)

CEmax (cd/A)

PEmax (lm/W)

none DA:PSS PEDOT:PSS56

4.5 5.1 4.5

17.5 22.5 25.09

6.32 8.66 8.25

AFM was used to investigate the film-forming capability, and the results are shown in Figure 8. The root-mean-square (RMS) of bare ITO was 3.6 nm, whereas DA:PSS solution spin-coated ITO became much smoother and the RMS decreased to as low as 1.1 nm. It was confirmed that DA:PSS has great potential to form uniform and smooth film, which meets the requirement of anode modifier in organic electronic devices.

■

CONCLUSION Inspired by the dispersion and doping effect of PSS to PEDOT, we developed a novel water-soluble polymer via PSS dispersing 465

DOI: 10.1021/acssuschemeng.6b01845 ACS Sustainable Chem. Eng. 2017, 5, 460−468

Research Article

ACS Sustainable Chemistry & Engineering

(4) Coe, S.; Woo, W. K.; Bawendi, M.; Bulovic, V. Electroluminescence from single monolayers of nanocrystals in molecular organic devices. Nature 2002, 420, 800−803. (5) Muller, C. D.; Falcou, A.; Reckefuss, N.; Rojahn, M.; Wiederhirn, V.; Rudati, P.; Frohne, H.; Nuyken, O.; Becker, H.; Meerholz, K. Multi-colour organic light-emitting displays by solution processing. Nature 2003, 421, 829−833. (6) Reineke, S.; Lindner, F.; Schwartz, G.; Seidler, N.; Walzer, K.; Lussem, B.; Leo, K. White organic light-emitting diodes with fluorescent tube efficiency. Nature 2009, 459, 234−238. (7) Uoyama, H.; Goushi, K.; Shizu, K.; Nomura, H.; Adachi, C. Highly efficient organic light-emitting diodes from delayed fluorescence. Nature 2012, 492, 234−238. (8) Li, W. J.; Pan, Y. Y.; Xiao, R.; Peng, Q. M.; Zhang, S. T.; Ma, D. G.; Li, F.; Shen, F. Z.; Wang, Y. H.; Yang, B.; Ma, Y. G. Employing ∼ 100% Excitons in OLEDs by Utilizing a Fluorescent Molecule with Hybridized Local and Charg-Transfer Excited State. Adv. Funct. Mater. 2014, 24, 1609−1614. (9) Peng, Q. M.; Obolda, A.; Zhang, M.; Li, F. Organic LightEmitting Diodes Using a neutral π radical as emitter: the emission from a doublet. Angew. Chem., Int. Ed. 2015, 54, 7091−7095. (10) Horowitz, G. Organic field-effect transistors. Adv. Mater. 1998, 10, 365−377. (11) Sirringhaus, H. Device physics of solution-processed organic field-effect transistors. Adv. Mater. 2005, 17, 2411−2425. (12) Zaumseil, J.; Sirringhaus, H. Electron and ambipolar transport in organic field-effect transistors. Chem. Rev. 2007, 107, 1296−1323. (13) Wang, C. L.; Dong, H. L.; Hu, W. P.; Liu, Y. Q.; Zhu, D. B. Semiconducting π-conjugated systems in field-effect transistors: a material odyssey of organic electronics. Chem. Rev. 2012, 112, 2208− 2267. (14) Zhang, C. C.; Chen, P. L.; Hu, W. P. Organic field-effect transistor-based gas sensors. Chem. Soc. Rev. 2015, 44, 2087−2107. (15) Tang, C. W. Two-layer organic photovoltaic cell. Appl. Phys. Lett. 1986, 48, 183−185. (16) O’regan, B.; Grätzel, M. A low-cost, high-efficiency solar cell based on dye-sensitized. Nature 1991, 353, 737−740. (17) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Polymer photovoltiac cells: Enhanced efficiencies via a network of internal donor-acceptor heterojunctions. Science 1995, 270, 1789− 1791. (18) Schmidt-Mende, L.; Fechtenktter, A.; Mullen, K.; Moons, E.; Friend, R. H.; MacKenzie, J. D. Self-organized discotic liquid crystals for high-efficiency organic photovoltaics. Science 2001, 293, 1119− 1122. (19) Thompson, B. C.; Fréchet, J. M. J. Polymer-fullerene composite solar cells. Angew. Chem., Int. Ed. 2008, 47, 58−77. (20) Congreve, D. N.; Lee, J.; Thompson, N. J.; Hontz, E.; Yost, S. R.; Reusswig, P. D.; Bahlke, M. E.; Reineke, S.; Van Voorhis, T.; Baldo, M. A. External quantum efficiency above 100% in a singlet-excitonfission-based organic photovoltaic cell. Science 2013, 340, 334−337. (21) Blochwitz, J.; Pfeiffer, M.; Fritz, T.; Leo, K. Low voltage organic light emitting diodes featuring doped phthalocyanine as hole transport material. Appl. Phys. Lett. 1998, 73, 729−731. (22) Tress, W.; Leo, K.; Riede, M. Influence of hole-transport layers and donor materials on open-circuit voltage and shape of I-V curves of organic solar cells. Adv. Funct. Mater. 2011, 21, 2140−2149. (23) Bi, D. Q.; Yang, L.; Boschloo, G.; Hagfeldt, A.; Johansson, E. M. J. Effect of different hole transport materials on recombination in CH3NH3PbI3 perovskite-sensitized mesoscopic solar cells. J. Phys. Chem. Lett. 2013, 4, 1532−1536. (24) Jeon, N. J.; Lee, J.; Noh, J. H.; Nazeeruddin, M. K.; Gratzel, M.; Seok, S. I. Efficient inorganic-organic hybrid perovskite solar cells based on pyrene arylamine derivatives as hole-transporting materials. J. Am. Chem. Soc. 2013, 135, 19087−19090. (25) Liu, J.; Wu, Y. Z.; Qin, C. J.; Yang, X. D.; Yasuda, T.; Islam, A.; Zhang, K.; Peng, W. Q.; Han, L. Y.; Chen, W. A dopant-free holetransporting material for efficient and stable perovskite solar cells. Energy Environ. Sci. 2014, 7, 2963−2967.

Figure 8. AFM morphology of (a) blank ITO, (b) DA:PSS film spincoated on ITO with the sizes of 3 × 3 μm.

DA and its semiquinone radical. The well-known selfpolymerization of DA was avoided due to the interaction of PSS with amino groups of DA. This result revealed that amino group involved cyclization reaction played a key role for the typical self-polymerization of DA in previous work. Interestingly, DA:PSS has a stable quasi-reversible oxidation behavior, which is also detected in an oxidized catechol system. Moreover, DA:PSS, with a pH of 5.3, has lower acidity than PEDOT:PSS. These results indicate the potential of DA:PSS to act as an anode modifier. The mechanism is based on the electron transfer during the oxidation of DA:PSS, owning to the structure of the phenolic hydroxyl group and semiquinone radical. Our method might open a new avenue to explore the novel hole transport material based on phenol-containing materials. POE materials with conjugated structure have shown great potential, and this should be taken into consideration in the future.52,96,97

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publication website at DOI: 10.1021/acssuschemeng.6b01845 HSQC spectrum, NMR spectra, and CV curve (PDF)

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Yuan Li). *E-mail: [email protected] (Xueqing Qiu). *E-mail: [email protected] (Runfeng Chen). Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors acknowledge the financial support of National Natural Science Foundation of China (21436004, 21402054), Guangdong Province Science Foundation (2014B050505006).

■

REFERENCES

(1) (a) Tang, C. W.; VanSlyke, S. A. Organic electroluminescent diodes. Appl. Phys. Lett. 1987, 51, 913−915. (2) Tang, C. W.; VanSlyke, S. A.; Chen, C. H. Electroluminescence of doped organic thin films. J. Appl. Phys. 1989, 65, 3610−3616. (3) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Burns, P. L.; Holmes, A. B. Light-emitting diodes based on conjugated polymers. Nature 1990, 347, 539−541. 466

DOI: 10.1021/acssuschemeng.6b01845 ACS Sustainable Chem. Eng. 2017, 5, 460−468

Research Article

ACS Sustainable Chemistry & Engineering (26) Zhou, H.; Chen, Q.; Li, G.; Luo, S.; Song, T. B.; Duan, H. S.; Hong, Z. R.; You, J. B.; Liu, Y. S.; Yang, Y. Interface engineering of highly efficient perovskite solar cells. Science 2014, 345, 542−546. (27) Strukelj, M.; Papadimitrakopoulos, F.; Miller, T. M.; Rothberg, L. J. Design and application of electron-transporting organic materials. Science 1995, 267, 1969−1972. (28) Adachi, C.; Baldo, M. A.; Forrest, S. R.; Thompson, M. E. Highefficiency organic electrophosphorescent devices with tris (2-phenylpyridine) iridium doped into electron-transporting materials. Appl. Phys. Lett. 2000, 77, 904−906. (29) Kulkarni, A. P.; Tonzola, C. J.; Babel, A.; Jenekhe, S. A. Electron transport materials for organic light-emitting diodes. Chem. Mater. 2004, 16, 4556−4573. (30) Xiao, L. X.; Su, S. J.; Agata, Y.; Lan, H.; Kido, J. Nearly 100% internal quantum efficiency in an organic Blue-Light electrophosphorescent device using a weak electron transporting material with a wide energy gap. Adv. Mater. 2009, 21, 1271−1274. (31) Liao, Y. L.; Lin, C. Y.; Liu, Y. H.; Wong, K. T.; Hung, W. Y.; Chen, W. J. An unprecedented ambipolar charge transport material exhibiting balanced electron and hole mobilities. Chem. Commun. 2007, 18, 1831−1833. (32) Cornil, J.; Bredas, J. L.; Zaumseil, J.; Sirringhaus, H. Ambipolar transport in organic conjugated materials. Adv. Mater. 2007, 19, 1791− 1799. (33) Kulkarni, A. P.; Zhu, Y.; Babel, A.; Wu, P. T.; Jenekhe, S. A. New ambipolar organic semiconductors. 2. effects of electron acceptor strength on intramolecular charge transfer photophysics, highly efficient electroluminescence, and field-effect charge transport of phenoxazine-based donor-acceptor materials. Chem. Mater. 2008, 20, 4212−4223. (34) Hung, L. S.; Chen, C. H. Recent progress of molecular organic electroluminescent materials and devices. Mater. Sci. Eng., R 2002, 39, 143−222. (35) Shirota, Y.; Kageyama, H. Charge carrier transporting molecular materials and their applications in devices. Chem. Rev. 2007, 107, 953− 1010. (36) So, F.; Kondakov, D. Degradation mechanisms in smallmolecule and polymer organic light-emitting diodes. Adv. Mater. 2010, 22, 3762−3777. (37) Xiao, L. X.; Chen, Z. J.; Qu, B.; Luo, J. X.; Kong, S.; Gong, Q. H.; Kido, J. Recent progresses on materials for electrophosphorescent organic light-emitting devices. Adv. Mater. 2011, 23, 926−952. (38) Groenendaal, L.; Jonas, F.; Freitag, D.; Pielartzik, H.; Reynolds, J. R. Poly (3, 4-ethylenedioxythiophene) and its derivatives: past, present, and future. Adv. Mater. 2000, 12, 481−494. (39) Jonsson, S. K. M.; Birgerson, J.; Crispin, X.; Greczynski, G.; Osikowicz, W.; Van Der Gon, A. W. D.; Salaneck, W. R.; Fahlman, M. The effects of solvents on the morphology and sheet resistance in poly(3,4-ethylenedioxythiophene)-polystyrenesulfonic acid (PEDOTPSS) films. Synth. Met. 2003, 139, 1−10. (40) Kirchmeyer, S.; Reuter, K. Scientific importance, properties and growing applications of poly (3, 4-ethylenedioxythiophene). J. Mater. Chem. 2005, 15, 2077−2088. (41) Crispin, X.; Jakobsson, F. L. E.; Crispin, A.; Grim, P. C. M.; Andersson, P.; Volodin, A.; van Haesendonck, C.; Van Der Auweraer, M.; Salaneck, W. R.; Berggren, M. The origin of the high conductivity of poly(3,4-ethylenedioxythiophene)-poly (styrenesulfonate) (PEDOT-PSS) plastic electrodes. Chem. Mater. 2006, 18, 4354−4360. (42) Nardes, A. M.; Kemerink, M.; Janssen, R. A. J.; Bastiaansen, J. A. M.; Kiggen, N. M. M.; Langeveld, B. M. W.; Van Breemen, A. J. J. M.; de Kok, M. M. Microscopic understanding of the anisotropic conductivity of PEDOT: PSS thin films. Adv. Mater. 2007, 19, 1196−1200. (43) Kim, Y. H.; Sachse, C.; Machala, M. L.; May, C.; MullerMeskamp, L.; Leo, K. Highly conductive PEDOT: PSS electrode with optimized solvent and thermal post-treatment for ITO-free organic solar cells. Adv. Funct. Mater. 2011, 21, 1076−1081.

(44) Alemu, D.; Wei, H. Y.; Ho, K. C.; Chu, C. W. Highly conductive PEDOT: PSS electrode by simple film treatment with methanol for ITO-free polymer solar cells. Energy Environ. Sci. 2012, 5, 9662−9671. (45) Irwin, M. D.; Buchholz, D. B.; Hains, A. W.; Chang, R. P.; Marks, T. J. p-Type semiconducting nickel oxide as an efficiencyenhancing anode interfacial layer in polymer bulk-heterojunction solar cells. Proc. Natl. Acad. Sci. U. S. A. 2008, 105, 2783−2787. (46) Kroon, R.; Lenes, M.; Hummelen, J. C.; Blom, P. W. M.; De Boer, B. Small bandgap polymers for organic solar cells (polymer material development in the last 5 years). Polym. Rev. 2008, 48, 531− 582. (47) Li, S. S.; Tu, K. H.; Lin, C. C.; Chen, C. W.; Chhowalla, M. Solution-processable graphene oxide as an efficient hole transport layer in polymer solar cells. ACS Nano 2010, 4, 3169−3174. (48) Yip, H. L.; Jen, A. K. Y. Recent advances in solution-processed interfacial materials for efficient and stable polymer solar cells. Energy Environ. Sci. 2012, 5, 5994−6011. (49) Aviram, A.; Joachim, C.; Pomerantz, M. Evidence of switching and rectification by a single molecule effected with a scanning tunneling microscope. Chem. Phys. Lett. 1988, 146, 490−495. (50) Aviram, A.; Ratner, M. A. Molecular rectifiers. Chem. Phys. Lett. 1974, 29, 277−283. (51) Xia, L. P.; Xue, Y. Y.; Xiong, K.; Cai, C. S.; Peng, Z. S.; Wu, Y.; Li, Y.; Miao, J. S.; Chen, D. C.; Hu, Z. H.; Wang, J. B.; Peng, X. B.; Mo, Y. Q.; Hou, L. T. Highly improved efficiency of deep-blue fluorescent polymer light-emitting device based on a novel hole interface modifier with 1, 3, 5-triazine core. ACS Appl. Mater. Interfaces 2015, 7, 26405−26413. (52) Li, Y.; Hong, N. L. An efficient hole transport material based on PEDOT dispersed with lignosulfonate: preparation, characterization and performance in polymer solar cells. J. Mater. Chem. A 2015, 3, 21537−21544. (53) Li, Y.; Zeng, W. M. PEDOT dispersed with sulfobutylated phenol formaldehyde resin: a highly-efficient hole transport material in polymer solar cells. Macromol. Mater. Eng. 2016, 301, 133−140. (54) Li, Y.; Xue, Y. Y.; Xia, L. P.; Hou, L. T.; Qiu, X. Q. 1, 3, 5triazine crosslinked 2, 5-dibromohydroquinone as new hole-transport material in polymer light-emitting diodes. Phys. Status Solidi A 2016, 213, 429−435. (55) Wu, Y.; Wang, J. Y.; Qiu, X. Q.; Yang, R. Q.; Lou, H. M.; Bao, X. C.; Li, Y. Highly efficient inverted perovskite solar cells with sulfonated lignin doped PEDOT as hole extract layer. ACS Appl. Mater. Interfaces 2016, 8, 12377−12383. (56) Hong, N. L.; Xiao, J. Y.; Li, Y. D.; Li, Y.; Wu, Y.; Yu, W.; Qiu, X. Q.; Chen, R. F.; Yip, H. L.; Huang, W.; Cao, Y. Unexpected fluorescent emission of graft sulfonated-acetone-formaldehyde lignin and its application as a dopant of PEDOT for high performance photovoltaic and light-emitting devices. J. Mater. Chem. C 2016, 4, 5297−5306. (57) Quideau, S.; Deffieux, D.; Douat Casassus, C.; Pouysegu, L. Plant polyphenols: chemical properties, biological activities, and synthesis. Angew. Chem., Int. Ed. 2011, 50, 586−621. (58) Esguerra, K. V. N.; Fall, Y.; Petitjean, L.; Lumb, J. P. Controlling the Catalytic Aerobic Oxidation of Phenols. J. Am. Chem. Soc. 2014, 136, 7662−7668. (59) Milczarek, G.; Inganas, O. Renewable cathode materials from biopolymer/conjugated polymer interpenetrating networks. Science 2012, 335, 1468−1471. (60) McMillen, D. F.; Golden, D. M. Hydrocarbon bond dissociation energies. Annu. Rev. Phys. Chem. 1982, 33, 493−532. (61) Ulas, G.; Lemmin, T.; Wu, Y. B.; Gassner, G. T.; DeGrado, W. F. Designed metalloprotein stabilizes a semiquinone radical. Nat. Chem. 2016, 8, 354−359. (62) Steelink, C. Stable free radicals in lignin and lignin oxidation products. In Lignin Structure and Reaction; American Chemical Society, 1966; Vol. 5, pp 51−64. (63) Swan, G. A. Structure, chemistry, and biosynthesis of the melanins. In Fortschritte der Chemie Organischer Naturstoffe/Progress in 467

DOI: 10.1021/acssuschemeng.6b01845 ACS Sustainable Chem. Eng. 2017, 5, 460−468

Research Article

ACS Sustainable Chemistry & Engineering the Chemistry of Organic Natural Products; Springer, 1974, Vol. 31, pp 521−582. (64) Page, S. E.; Sander, M.; Arnold, W. A.; McNeill, K. Hydroxyl radical formation upon oxidation of reduced humic acids by oxygen in the dark. Environ. Sci. Technol. 2012, 46, 1590−1597. (65) Xi, F. D.; Barclay, L. R. C. Cooperative antioxidant effects of ascorbate and thiols with di-tert-butylcatechol during inhibited peroxidation in solution and in sodium dodecyl sulfate (SDS) micelles. Can. J. Chem. 1998, 76, 171−182. (66) Chaudhuri, P.; Verani, C. N.; Bill, E.; Bothe, E.; Weyhermuller, T.; Wieghardt, K. Electronic structure of bis (o-iminobenzosemiquinonato) metal complexes (Cu, Ni, Pd). The art of establishing physical oxidation states in transition-metal complexes containing radical ligands. J. Am. Chem. Soc. 2001, 123, 2213−2223. (67) Pierpont, C. G. Ligand redox activity and mixed valency in firstrow transition-metal complexes containing tetrachlorocatecholate and radical tetrachlorosemiquinonate ligands. Inorg. Chem. 2011, 50, 9766−9772. (68) Witwicki, M.; Jerzykiewicz, M.; Jaszewski, A. R.; Jezierska, J.; Ozarowski, A. Influence of Pb (II) ions on the EPR properties of the semiquinone radicals of humic acids and model compounds: High field EPR and relativistic DFT studies. J. Phys. Chem. A 2009, 113, 14115− 14122. (69) Christoforidis, K. C.; Un, S.; Deligiannakis, Y. Effect of metal ions on the indigenous radicals of humic acids: high field electron paramagnetic resonance study. Environ. Sci. Technol. 2010, 44, 7011− 7016. (70) Witwicki, M.; Jerzykiewicz, M.; Ozarowski, A. Understanding natural semiquinone radicals-Multifrequency EPR and relativistic DFT studies of the structure of Hg (II) complexes. Chemosphere 2015, 119, 479−484. (71) Steelink, C.; Reid, T.; Tollin, G. On the nature of the freeradical moiety in lignin. J. Am. Chem. Soc. 1963, 85, 4048−4049. (72) Sealy, R. C.; Hyde, J. S.; Felix, C. C.; Menon, I. A.; Prota, G. Eumelanins and pheomelanins: characterization by electron spin resonance spectroscopy. Science 1982, 217, 545−547. (73) Bothma, J. P.; de Boor, J.; Divakar, U.; Schwenn, P. E.; Meredith, P. Device-Quality Electrically Conducting Melanin Thin Films. Adv. Mater. 2008, 20, 3539−3542. (74) Mostert, A. B.; Powell, B. J.; Pratt, F. L.; Hanson, G. R.; Sarna, T.; Gentle, I. R.; Meredith, P. Role of semiconductivity and ion transport in the electrical conduction of melanin. Proc. Natl. Acad. Sci. U. S. A. 2012, 109, 8943−8947. (75) Kong, J. H.; Yee, W. A.; Yang, L. P.; Wei, Y. F.; Phua, S. L.; Ong, H. G.; Ang, J. M.; Li, X.; Lu, X. H. Highly electrically conductive layered carbon derived from polydopamine and its functions in SnO2based lithium ion battery anodes. Chem. Commun. 2012, 48, 10316− 10318. (76) Ryou, M. H.; Lee, Y. M.; Park, J. K.; Choi, J. W. Mussel-inspired polydopamine-treated polyethylene separators for high-power Li-Ion batteries. Adv. Mater. 2011, 23, 3066−3070. (77) Liu, Y. L.; Ai, K. L.; Lu, L. H. Polydopamine and its derivative materials: synthesis and promising applications in energy, environmental, and biomedical fields. Chem. Rev. 2014, 114, 5057−5115. (78) Zhu, J. X.; Yang, D.; Yin, Z. Y.; Yan, Q. Y.; Zhang, H. Graphene and graphene-based materials for energy storage applications. Small 2014, 10, 3480−3498. (79) Cai, Z. Y.; Xu, L.; Yan, M. Y.; Han, C. H.; He, L.; Hercule, K. M.; Niu, C. J.; Yuan, Z. F.; Xu, W. W.; Qu, L. B.; et al. Manganese oxide/carbon yolk-shell nanorod anodes for high capacity lithium batteries. Nano Lett. 2014, 15, 738−744. (80) Chung, D. Y.; Jun, S. W.; Yoon, G.; Kwon, S. G.; Shin, D. Y.; Seo, P.; Yoo, J. M.; Shin, H.; Chung, Y. H.; Kim, H.; Mun, B. S.; Lee, K. S.; Lee, N. S.; Yoo, S. J.; Lim, D. H.; Kang, K.; Sung, Y. E.; Hyeon, T. Highly durable and active PtFe nanocatalyst for electrochemical oxygen reduction reaction. J. Am. Chem. Soc. 2015, 137, 15478−15485. (81) Pingree, L. S.; MacLeod, B. A.; Ginger, D. S. The changing face of PEDOT: PSS films: substrate, bias, and processing effects on vertical charge transport†. J. Phys. Chem. C 2008, 112, 7922−7927.

(82) Lim, F. J.; Ananthanarayanan, K.; Luther, J.; Ho, G. W. Influence of a novel fluorosurfactant modified PEDOT: PSS hole transport layer on the performance of inverted organic solar cells. J. Mater. Chem. 2012, 22, 25057−25064. (83) Hong, S.; Na, Y. S.; Choi, S.; Song, I. T.; Kim, W. Y.; Lee, H. Non-covalent self-assembly and covalent polymerization co-contribute to polydopamine formation. Adv. Funct. Mater. 2012, 22, 4711−4717. (84) Liebscher, J.; Mrowczynski, R.; Scheidt, H. A.; Filip, C.; Hadade, N. D.; Turcu, R.; Bende, A.; Beck, S. Structure of polydopamine: a never-ending story? Langmuir 2013, 29, 10539−10548. (85) Dreyer, D. R.; Miller, D. J.; Freeman, B. D.; Paul, D. R.; Bielawski, C. W. Elucidating the structure of poly (dopamine). Langmuir 2012, 28, 6428−6435. (86) Della Vecchia, N. F.; Avolio, R.; Alfe, M.; Errico, M. E.; Napolitano, A.; d'Ischia, M. Building-block diversity in polydopamine underpins a multifunctional eumelanin-type platform tunable through a quinone control point. Adv. Funct. Mater. 2013, 23, 1331−1340. (87) Moser, R. E.; Cassidy, H. G. Electron-transfer polymers. XXV. On “hydrophobic bonding.” The effect of solvent on quinhydrone. J. Am. Chem. Soc. 1965, 87, 3463−3467. (88) Lazarev, G. G.; Lebedev, Y. S.; Serdobov, M. V. Mechanism of the formation of weak complexes of the spatially hindered quinones and phenols. Bull. Acad. Sci. USSR, Div. Chem. Sci. 1978, 27, 2249− 2254. (89) Serdobov, M. V.; Maiorov, V. D. Complexes with hydrogen bonding between sterically hindered o-quinones and pyrocatechols. Bull. Acad. Sci. USSR, Div. Chem. Sci. 1988, 37, 2482−2486. (90) Milligan, D. E.; Jacox, M. E. Matrix-Isolation Infrared Spectrum of the Free Radical NH2. J. Chem. Phys. 1965, 43, 4487−4493. (91) Ennis, C. P.; Lane, J. R.; Kjaergaard, H. G.; McKinley, A. J. Identification of the water amidogen radical complex. J. Am. Chem. Soc. 2009, 131, 1358−1359. (92) Fisher, O. Z.; Larson, B. L.; Hill, P. S.; Graupner, D.; Nguyen Kim, M. T.; Kehr, N. S.; De Cola, L.; Langer, R.; Anderson, D. G. Melanin-like gydrogels derived from gallic macromers. Adv. Mater. 2012, 24, 3032−3036. (93) Park, Y.; Choong, V.; Gao, Y.; Hsieh, B. R.; Tang, C. W. Work function of indium tin oxide transparent conductor measured by photoelectron spectroscopy. Appl. Phys. Lett. 1996, 68, 2699−2701. (94) Zhou, Y. H.; Fuentes-Hernandez, C.; Shim, J.; Meyer, J.; Giordano, A. J.; Li, H.; Winget, P.; Papadopoulos, T.; Cheun, H.; Kim, J.; Fenoll, M.; Dindar, A.; Haske, W.; Najafabadi, E.; Khan, T. M.; Sojoudi, H.; Barlow, S.; Graham, S.; Bredas, J.-L.; Marder, S. R.; Kahn, A.; Kippelen, B. A universal method to produce low-work function electrodes for organic electronics. Science 2012, 336, 327−332. (95) Zeng, X. Y.; Zhang, Q. K.; Yu, R. M.; Lu, C. Z. A new transparent conductor: silver nanowire film buried at the surface of a transparent polymer. Adv. Mater. 2010, 22, 4484−4488. (96) Li, Y.; Heng, W. K.; Lee, B. S.; Aratani, N.; Zafra, J. L.; Bao, N.; Lee, R.; Sung, Y. M.; Sun, Z.; Huang, K. W.; Webster, R. D.; Navarrete, J. T. L.; Kim, D.; Osuka, A.; Casado, J.; Ding, J.; Wu, J. S. Kinetically blocked stable heptazethrene and octazethrene: Closed-shell or openshell in the ground state? J. Am. Chem. Soc. 2012, 134, 14913−14922. (97) Aotake, T.; Suzuki, M.; Aratani, N.; Yuasa, J.; Kuzuhara, D.; Hayashi, H.; Nakano, H.; Kawai, T.; Wu, J. S.; Yamada, H. 9, 9′Anthryl-anthroxyl radicals: strategic stabilization of highly reactive phenoxyl radicals. Chem. Commun. 2015, 51, 6734−6737.

468

DOI: 10.1021/acssuschemeng.6b01845 ACS Sustainable Chem. Eng. 2017, 5, 460−468