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PEDOT:PSS nanocomposites as flexible transparent electrodes for OLEDs
Journal Pre-proofs Full Length Article Fabrication of Architectural Structured Polydopamine-Functionalized Reduced Graphene Oxide/Carbon Nanotube/PEDOT:PSS Nanocomposites as Flexible Transparent Electrodes for OLEDs Tao Wang, Li-Chao Jing, Qingxia Zhu, Anita Sagadevan Ethiraj, Ying Tian, Hui Zhao, Xiao-Tong Yuan, Jian-Gong Wen, Lin-Kuo Li, Hong-Zhang Geng PII: DOI: Reference:
S0169-4332(19)32813-2 https://doi.org/10.1016/j.apsusc.2019.143997 APSUSC 143997
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
13 August 2019 11 September 2019 12 September 2019
Please cite this article as: T. Wang, L-C. Jing, Q. Zhu, A. Sagadevan Ethiraj, Y. Tian, H. Zhao, X-T. Yuan, J-G. Wen, L-K. Li, H-Z. Geng, Fabrication of Architectural Structured Polydopamine-Functionalized Reduced Graphene Oxide/Carbon Nanotube/PEDOT:PSS Nanocomposites as Flexible Transparent Electrodes for OLEDs, Applied Surface Science (2019), doi: https://doi.org/10.1016/j.apsusc.2019.143997
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Fabrication of Architectural Structured Polydopamine-Functionalized Reduced Graphene Oxide/Carbon Nanotube/PEDOT:PSS Nanocomposites as Flexible Transparent Electrodes for OLEDs
Tao Wang1, Li-Chao Jing1, Qingxia Zhu*1, Anita Sagadevan Ethiraj2, Ying Tian1, Hui Zhao1, Xiao-Tong Yuan1, Jian-Gong Wen1, Lin-Kuo Li1, Hong-Zhang Geng 1
1. Tianjin Key Laboratory of Advanced Fibers and Energy Storage, School of Material Science and Engineering, Tiangong University, Tianjin 300387, China 2.Department of Physics, VIT-AP University, Amaravati, Andhra Pradesh 522237, India
Corresponding author. E-mail addresses:[email protected] (Q. Zhu); [email protected] (H.-Z. Geng).
Abstract High performance, flexible transparent conductive films with a structure similar to that of reinforced concrete and constructed by sandwiching single-walled carbon nanotubes (SWCNT) between poly (3,4-ethylene dioxythiophene):poly (styrene sulfonate) (PEDOT:PSS) and polydopamine functionalized reduced graphene oxide (PDA-RGO) were fabricated using simple spray coating. Mussel-inspired polydopamine was introduced as a graphene oxide reducing agent and modifier; the obtained PDA-RGO improved the interfacial adhesion between the conductive coating layers and substrate, and an effective post fabrication treatment method was performed on the hybrid film to achieve better conductivity. It was found that the resulting electrode exhibited a low sheet resistance of 52.2 Ω/sq. with a high optical transmittance of 88.7% at 550 nm. Moreover, the transparent film exhibited long-term stability with a relatively low roughness (ca. 2.41 nm), and its architectural structure sustained the flexibility of the film during bending. The organic light emitting diodes which using PDA-RGO/SWCNT/PEDOT:PSS film as anode was successfully fabricated, the luminance of the device was 2032 cd/cm2 at 15 V and the maximum current efficiency was 2.13 cd/A at 14 V, indicating the strong potential of this type of transparent electrode for flexible electroluminescent devices. Key words: Flexible transparent conductive films; Carbon nanotubes; Polydopamine Functionalized Reduced Graphene Oxide; PEDOT:PSS; OLEDs
1. Introduction Over the past several years, optoelectronic devices, including organic light emitting diodes (OLEDs) [1], organic solar cells [2], and field effect transistors, have been extensively developed [3,4]. Transparent conductive films (TCFs) are essential in all of these optoelectronic devices. Indium tin oxide (ITO), the conventional conductive metal oxide thin film, has been widely used for transparent electrodes. However, a few drawbacks of ITO such as its high cost due to indium scarcity, complicated processing requirements, sensitivity to acid and basic environments, and
cracking and delaminating when subjected to bending deem it unsuitable for applications that require flexibility. For these reasons, numerous candidate materials including metal nanowires [5,6], carbon nanotubes (CNTs) [7,8], graphene [9,10], and conducting polymers [11,12] are being actively investigated as replacements for ITO. Among these materials, CNTs present potential advantages in chemical stability, thermal and electrical conductivity, mechanical strength, flexibility, and work function (4.5–5.2 eV) [13]. However, many problems must be overcome. For instance, CNT films suffer from weak interfacial adhesion to substrates resulting from their inherent tube–tube junctions; moreover, these junctions readily open, causing relatively high sheet resistance [14]. Recently, Han et al. [15] introduced graphene oxide (GO) as a multi-purpose p-type dopant for CNT films. Although GO sheets deposited onto the porous nanotube networks reduced the surface roughness, there was no significant effect on improving the conductivity and film adhesion by this method. Polydopamine (PDA), which mimics adhesive proteins [16,17], was used to simultaneously reduce (reduced GO, RGO) and functionalize GO during self-polymerization [18]. Therefore, PDA functionalized reduced graphene oxide (PDA-RGO) was used as a conductive adhesion promoter to improve the interfacial adhesion between the conductive coating layers and substrate. Spray coated poly (3,4-ethylene dioxythiophene):poly (styrene sulfonate) (PEDOT:PSS) was applied for planarization of TCFs [19,20], owing to its intrinsic fairly high conductivity, high transparency in the visible range, and outstanding film-forming properties. However, pristine PEDOT:PSS films exhibit a very low conductivity (<1 S/cm) and cannot be used as a stand-alone electrode. Fortunately, the conductivity of PEDOT:PSS can be increased up to 2 or 3 orders of magnitude by adding some components, such as organic compound salts, zwitterions, carboxylic or inorganic acids, or cosolvents [21,22,23]. Despite ample efforts and success in improving the conductivity, it is still too low to meet the demands of optoelectronic applications. Therefore, We have taken an effective method to significantly enhance
the conductivity and crystallinity of the π-conjugated polymer coating layer by DMSO-EG bath combined with H2SO4 treatment [24]. In this study, we fabricated a high performance, flexible transparent electrode with a structure resembling that of reinforced concrete by sandwiching CNTs between PEDOT:PSS and PDA-RGO by using a simple method, spray coating. The successful functionalization of reduced graphene oxide by PDA was confirmed and characterized by Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and Fourier transform-infrared (FT-IR), and ultraviolet-visible (UV-vis) spectroscopies. The modified RGO was expected to improve the interfacial adhesion between the conductive coating layers and substrate. Moreover, an effective post fabrication treatment method was performed on the hybrid films to obtain better conductivity, and the conductivity mechanism was systematically investigated. To verify the potential applications of this hybrid film, which have excellent optical conductivity, low root mean square (rms) roughness, and ultra-adhesion to substrates, the organic light emitting diodes (OLEDs) which using PDA-RGO/SWCNT/PEDOT:PSS film as anode was successfully fabricated and characterized.
2. Experimental 2.1. Materials The GO nanosheets were prepared via a modified Hummers’ method. High purity single-walled CNTs (SWCNTs, purity of 95 wt.%, diameter less than 2 nm, length >5 Pm) were provided from Carbon Star Technology (Tianjin) Co. Ltd. Sodium
dodecylbenzenesulfonate
(SDBS),
dopamine
hydrochloride
(98%),
tris(hydroxymethyl)aminomethane (Tris) (99.8%), ethylene glycol (EG), and dimethyl sulfoxide (DMSO) were purchased from Aladdin. PEDOT:PSS solution (Clevios pH 1000) with a concentration of 1.3 wt.% was purchased from H. C. Starck. All reagents were used as received. 2.2. Synthesis of PDA-RGO Nanosheets PDA-RGO was prepared as follows: 200 mg of GO and 100 mg of dopamine hydrochloride were added to 400 ml of Tris buffer solution (10 mM, pH = 8.5) and
dispersed by sonication for 10 min while submerged in an ice bath. Afterwards, the solution was vigorously stirred at 60 °C for 24 h; the color of the GO suspension changed from dark brown to black during the stirring process. After the reduction reaction, few-layer and uniformly sized PDA-RGO sheets were obtained by washing via centrifugation. Finally, the PDA-RGO precipitate was repeatedly washed with distilled water several times to remove free PDA particles and thick multilayer flakes, and a clean PDA-RGO solution (0.1 mg/ml) was obtained. 2.3. Preparation of SWCNT solutions The preparation of the SWCNT solutions was reported in our previous work [25]. CNTs were dispersed in deionized water (1 mg/ml) using SDBS as a dispersant. Then, the solution was sonicated in a bath sonicator for 30 min followed by probe ultrasonication at 120 W for 30 min to obtain a suspension solution. The mixture was centrifuged at 8000 rpm for 20 min, and the 80% supernatant was collected. 2.4. Fabrication of PDA-RGO/SWCNT/PEDOT based TCFs Flexible polymer-polyethylene terephthalate (PET) substrates were immersed in ethanol with bath ultrasonication to remove contamination. Then the clean PET substrates were dried in a dryer for later processing. DMSO (5 wt.%) was added into the PEDOT:PSS solution, after which the mixture was diluted ten times with deionized water and magnetic stirring for one hour to ensure good mixing. First, the PET films were fixed on a heating plate, and the PDA-RGO solution was poured into the airbrush and sprayed on the surface of the PET substrates. Then, the SWCNT solution was sprayed onto the PDA-RGO layer. In order to remove the SDBS between the coating layers, the PDA-RGO/SWCNT films were immersed in deionized water for 20 min; after drying, the samples were immersed in 12 M nitric acid for 30 min, and the residual acid was washed away by deionized water several times. The TCFs were dried again at 80 °C. Next, the PEDOT:PSS solution was sprayed
on
the
PDA-RGO/SWCNT
networks
to
form
PDA-RGO/SWCNT/PEDOT:PSS films. These films were immersed in EG for 40 min and washed by deionized water, after which they were annealed in air at 120 Ԩ for 15
min to remove DMSO and EG. A 1 M H2SO4 solution was pipetted onto the dried films at 120 °C and kept on the sample for 5 min. The film was then further washed by deionized water and annealed again. Finally, we obtained the sandwich structured PDA-RGO/SWCNT/PEDOT:PSS based TCFs. In our study, all spray conditions were maintained at a constant spray velocity with a 0.3 MPa spraying pressure and 15 cm distance between the nozzle and the substrate. During the spray coating process, the PET substrates were heated at 120 °C on a hotplate to avoid the coffee-ring effect. 2.5 Fabrication of OLEDs using PDA-RGO/SWCNT/PEDOT TCFs as anode The OLED structures in this study were (PDA-RGO/SWCNT/PEDOT) / NPB (40 nm)/ Alq3 (60 nm)/ LiF (0.8 nm)/ Al (100 nm). All of the organic materials were thermally evaporated onto the patterned anodes using a thermal evaporation coating system at a pressure of approximately 7×10-4 Pa. 2.6. Characterization The transmittance of the films was measured using a UV-vis spectrophotometer at 550 nm. The sheet resistance of the films was measured using the four-point probe method (Keithley 2700 multi-meter). The surface morphology of the films was characterized by field-emission scanning electron microscopy (FE-SEM) (Hitachi S-4800) and atomic force microscopy (AFM, BRUKER). Transmission electron microscopy (TEM) was performed using a Hitachi H-800 electron microscope operating at an accelerating voltage of 120 kV. The changes in the surface functional groups were detected by XPS (ELMER PHI 5600, Al Kα source) and FT-IR (TENSOR37), and the surface wettability was assessed by a contact-angle analysis device (DSA100, KRUSS). The nanostructures were further characterized via a DXR Raman microscope with a laser excitation wavelength of 532 nm. The adhesion was investigated using the 3M tape test, in which the tapes adhere to the films by rubbing the entire surface area using constant pressure and then peeled off from the samples. The performance of the OLEDs was characterized by an optical detection system for OLED display panels developed by Suzhou Fstar Scientific Instrument Co., Ltd. 3. Results and Discussion
The morphology and structure of the GO and PDA-RGO nanosheets were observed by TEM. In Fig. 1a, the large GO sheet (ca. 4 μm) has a planar texture with a smooth surface. After being modified by PDA, the PDA-RGO nanosheet exhibits an obvious rough surface with folds and wrinkles resembling gecko feet, which may increase the contact area and enhance the adhesion to substrates, as shown in Fig. 1b [26].
Fig. 1 TEM images of (a) GO sheet and (b) PDA-RGO sheet.
The synthesized GO and PDA-RGO were further characterized by UV-vis, Raman, FT-IR, and XPS. The simultaneous reduction and surface functionalization of GO took place during the oxidative self-polymerization of dopamine, and GO was converted into PDA-RGO. The successful reduction of GO to PDA-RGO was confirmed by UV-Vis. As shown in Fig. 2a, GO exhibits a characteristic absorption peak at 230 nm, however, the peak shifts to 260 nm in PDA-RGO, which is a characteristic peak of RGO, indicating a refreshed electronic conjugation in graphene sheets due to the reduction of GO. The reduction of GO was further verified by
Raman spectroscopy. As shown in Fig. 2b, GO and PDA-RGO exhibit two characteristic peaks. The spectra of GO and PDA-RGO are characterized by D and G bands. The D and G bands of GO are found at 1358 and 1591 cm−1, respectively. As for PDA-RGO, the curve displays a strong D band at 1353 cm−1 and a G band at 1589 cm−1. The red shifting of the two bands upon dopamine reduction is an indication of graphitic self-healing behavior [27]. The ratio of the intensities of D and G bands (ID/IG) is used to evaluate the degree of graphitization and the defect density in graphene materials. Here, the value of ID/IG increased from 0.84 to 0.94, which is attributed to the formation of smaller graphitic domains during the reduction of GO. Furthermore, the surface functionalization of GO by PDA is also evident in the FT-IR and XPS spectra. As shown in its FT-IR spectrum in Fig. 2c, GO exhibits 3 characteristic peaks at around 1060, 1400, and 1731 cm−1 that correspond to the C–O (epoxy or alkoxy), C–OH (hydroxyl), and C=O (carboxyl) bonds of the GO skeleton, respectively, which are characteristic features of the FT-IR spectrum of GO. When compared with the spectrum of GO, that of PDA-RGO reveals two new peaks at 1386 and 1465 cm−1, corresponding to C–OH bending in the catechol groups and the stretching of C–N bonds, respectively, thus confirming the successful grafting of PDA molecular chains onto the surface of the GO sheets. In addition, the band at 1726 cm−1 for the carbonyl group almost disappeared, indicating that GO was reduced during the self-polymerization [28]. Generally speaking, these results demonstrate the simultaneous reduction and surface functionalization of GO that results in PDA-RGO. The change of the chemical information in GO and PDA-RGO was also investigated by XPS. Fig. 2d shows the XPS survey spectra of GO and PDA-RGO. The O/C ratios of GO and PDA-RGO are 0.38 and 0.27, respectively. It is clear that the peak intensities of the oxygen-containing groups on PDA-RGO have decreased significantly, which is due to the release of electrons that attack the oxygen-containing species such as C=O in GO during the polymerization of dopamine; the process reduces GO to some extent and facilitates the nucleation and growth of PDA film on the RGO sheets. In addition, the N 1s peak at 400.0 eV, which is related to the amine
group of PDA, was only observed in the PDA-RGO spectrum. The C 1s XPS spectra of GO and PDA-RGO are shown in Fig. 2e and 2f, respectively. The XPS C1s core-level spectrum of GO can be divided into four peak components with binding energies at approximately 284.9, 286.9, 288.3, and 289.7 eV, which can be indexed to C–C/C=C, C–O, C=O, and O–C=O species, respectively. For PDA-RGO, five different peaks at 284.7, 285.7, 286.4, 288.6, and 291.16 eV are related to the C=C/C–C, C–N, C–O, C=O, and O–C=O groups, respectively. The emergence of the new C–N peak further indicates the successful functionalization of PDA-RGO by PDA. Moreover, the strong π-π stacking interaction also proven; due to the similar structure of PDA and GO, the PDA coating on GO is highly stable. Thus, functionalizing GO using PDA combined advantages of both covalent and noncovalent functionalization [29].
Fig. 2 (a) UV-vis, (b) Raman, (c) FT-IR, and (d) XPS survey spectra of GO and
PDA-RGO; C 1s spectrum of (e) GO and (f) PDA-RGO. The SEM image in Fig. 3a shows the CNT film was after washing with deionized water and nitric acid sequentially [30]. The random entanglement among tubes may increase the roughness of the film and generate hundreds of nanoscale pores, resulting in poor carrier transport [31]. The SEM image of the PDA-RGO/CNT film in Fig. 3b reveals that the CNTs and PDA-RGO sheets are homogeneously distributed on the substrate. The higher magnification SEM image shown in Fig. 3c indicates that the large-sized PDA-RGO sheets with wrinkles cover the holes in the CNT network and act as a bridge between CNT bundles, which created several new electrical percolation
channels
[32].
In
Fig.
3d,
an
image
of
the
untreated
PDA-RGO/CNT/PEDOT:PSS film is shown; it exhibits a fairly flat surface, likely because the pores in the networks and the wrinkles from the PDA-RGO sheets were covered by PEDOT:PSS [33]. The surface of the film after the DMSO-EG bath and H2SO4 treatments remains relatively flat as shown in Fig. 3e. The surface of the PDA-RGO/CNT/PEDOT:PSS-treated film after the 3M tape adhesion test is shown in Fig. 3f, in which many tubular structures are exposed, but the coating remains intact due to the catechol groups of PDA-RGO serving as an anchor, thus enhancing the adhesion between the coating and the substrate.
Fig. 3 SEM images of different TCFs: (a) CNTs, (b) CNT/PDA-RGO, (c) higher magnification of CNT/PDA-RGO film showing the structure between the CNTs and PDA-RGO, (d) untreated PDA-RGO/CNT/PEDOT:PSS, (e) treated
PDA-RGO/CNT/PEDOT:PSS, and (f) treated PDA-RGO/CNT/PEDOT:PSS after the 3M tape adhesion test. An adhesion factor incorporating transmittance, which can be easily measured from absorption spectroscopy, was applied to quantitatively characterize the adhesion as follows [34]: ܶ െܶ
߂ܶ
݊ Ͳ ൌ ͳ െ ͳͲͲെܶ ݂ܶ ൌ ͳ െ ͳͲͲെܶ Ͳ
,
Ͳ
(1)
where fT is the adhesion factor, and T0 and T1 are the transmittance of hybrid films before and after the trial test, respectively. As shown in Table 1, the adhesion factor of all 10 treated PDA-RGO/CNT/PEDOT:PSS samples (S1–S10) is all higher than 0.92, which indicates ultra-adhesion of the films. However, the CNT films and pure PEDOT:PSS films have different degrees of shedding after trial test.
Table 1 The change in the transmittance after the 3M tape adhesion test and the adhesion factor for treated PDA-RGO/CNT/PEDOT:PSS films (S1–S10) Samples
1
2
3
4
5
6
7
8
9
10
T0 (%)
92.8
91.0
89.6
88.7
86.5
84.7
81.9
81.1
79.9
77.1
T1 (%)
93.2
91.5
90.2
89.4
87.4
85.7
83.2
82.4
81.3
78.8
fT
0.95
0.94
0.94
0.94
0.99
0.93
0.93
0.93
0.93
0.93
Fig. 4 shows AFM images of the CNT, PDA-RGO/CNT, untreated PDA-RGO/CNT/PEDOT:PSS, and treated PDA-RGO/CNT/PEDOT:PSS films, from which the rms roughness of the sample surfaces was determined; the rms is 12.00, 5.12, 1.09, and 2.41 nm, respectively. We suggest that doping PDA-RGO into the CNT network reduced the surface roughness of the CNT films, and the presence of the PEDOT:PSS layer resulted in the desired smooth surface structures. Despite the increase in surface roughness measured on the PDA-RGO/CNT/PEDOT:PSS film after treatment in the DMSO-EG bath and H2SO4, it is still much lower than that of the pure CNT samples. Such a low electrode surface roughness is advantageous for device performance.
Fig. 4 Two- and three-dimensional AFM images of different TCFs: (a–b) CNT, (c–d) PDA-RGO/CNT, (e–f) untreated PDA-RGO/CNT/PEDOT:PSS, and (g–h) treated PDA-RGO/CNT/PEDOT:PSS.
The PDA-RGO/CNT/PEDOT:PSS film's ultra-adhesion and relatively low roughness can likely be attributed to its special architectural structure that also enhances the film's electrical conductivity. The insulating surfactant SDBS was effectively removed, and the formation of the subsequent densified film improved the cross-junction conductivity within the CNT network, enhancing the metallicity of the CNTs. However, the CNTs were entangled by X-type junctions that could easily open. Therefore, PDA-RGO sheets with abundant catechol functional groups were carefully introduced to bind the CNTs firmly together, acting like an adhesive. This is depicted in Fig. 5a. The two-dimensional conductive PDA-RGO sheets act as a p-type dopant
and surface energy modifier of the CNT film, which decreases the Schottky barrier height between the nanotubes to improve the conductivity of hybrid film (For the PDA-RGO film, it has a sheet resistance of 8.26 MΩ/sq. at the transparency of 80% at 550 nm ). In Fig. 5b, the stretchable PEDOT:PSS film coating placed on the PDA-RGO/CNT layer fills up the voids of the CNT network and covers the wrinkled structure of the PDA-RGO sheets, thus smoothing the surface morphology of the hybrid films and significantly decreasing the sheet resistance. The combination of materials in the hybrid film results in a structure similar to that of reinforced concrete in buildings, shown in Fig. 5c. Such a structure effectively improves the stability of film during bending.
Fig. 5 Schematic illustration of TCFs with ultra-adhesion, relatively low roughness, and excellent electrical conductivity: (a) PDA-RGO/CNT and (b) PDA-RGO/CNT/PEDOT:PSS; (c) a picture of reinforced concrete structure used in building construction. For TCFs, high optical transmission in combination with low sheet resistance is key to improving the performance of thin-film optoelectronic devices. The relationship between transmittance and sheet resistance of the pure CNT,
PDA-RGO/CNT/PEDOT:PSS (untreated), PDA-RGO/CNT/PEDOT:PSS-DMSO-EG bath treated (DMSO-EG bath treated), and PDA-RGO/CNT/PEDOT:PSS-DMSO-EG bath and H2SO4 treated (DMSO-EG bath and H2SO4 treated) films is shown in Fig. 6. The pure CNT films exhibit a sheet resistance of 58.8 Ω/sq. with an optical transmittance of 80% at 550 nm. The untreated films show poor conductivity (Rs = 462.2 Ω/sq., T = ca. 80% at 550 nm), which is likely caused by the pristine PEDOT:PSS layer that usually has a conductivity below 1 S/cm. After being treated with organic solvent, the resistance of the hybrid films is reduced to some extent (Rs = 246.3 Ω/sq., T = ca. 80% at 550 nm). In order to further improve the electrical properties of the TCFs, we innovatively used organic solvents combined with inorganic acids to treat the hybrid films; when compared with the pure CNT film, the resistance of the hybrid films decreased significantly (an order of magnitude) to 35.5 Ω/sq., and the film simultaneously exhibited a high optical transmittance of 79.9% at 550 nm. In addition, the hybrid films continue to exhibit a low sheet resistance (52.2 Ω/sq.) at 88.7%. optical transmittance. The changes in the films during the treatment are discussed in detail below.
Fig. 6 The relationship between the transmittance (at 550 nm) and sheet resistance of pure CNT, untreated, DMSO-EG bath treated, and DMSO-EG bath and H2SO4 treated films.
To quantitatively characterize the optoelectrical properties of the treated PDA-RGO/CNT/PEDOT:PSS films, an approximate relationship between the optical
transmittance and sheet resistance, direct current conductivity (σdc), and optical conductivity (σop) is given by following relation [35]: ߪܥܦ ߪܱ
ൌ
ͳͺͺሺߗሻ ܴݏ
ξܶ ൨ ൫ͳെξܶ ൯
,
(2)
where, Rs is the sheet resistance, and T is the transmittance of the TCFs. The values of σop /σdc for the 10 DMSO-EG bath and H2SO4 treated samples are listed in Table 2. It is found that when the transmittance is above 77.12%, the values of σop/σdc are higher than 44.0, which is excellent performance when compared with Zheng’s report (σop/σdc =7.48) [36] and our previous work (σop/σdc = 25.3) [37]. Table 2 Optoelectrical properties of the treated PDA-RGO/CNT/PEDOT:PSS films. Samples
1
2
3
4
5
6
7
8
9
10
T1 (%)
92.9
91.0
89.6
88.7
86.5
84.7
81.9
81.1
79.9
77.1
Rs (Ω/sq)
108.9
75.7
72.8
52.2
46.9
42.0
37.4
37.1
35.5
30.7
σop/σdc
45.8
51.2
45.6
58.2
53.5
51.6
47.8
45.9
44.6
44.1
The
effect
of
the
polar
solvent
and
inorganic
acid
on
the
PDA-RGO/CNT/PEDOT:PSS film was characterized by FT-IR and XPS. As shown in Fig. 7a, the FT-IR spectra of three type films (untreated, DMSO-EG bath treated, and DMSO-EG bath and H2SO4 treated) all show similar characteristic peaks. The vibrations around 1266–1456 cm−1 correspond to C–C and C=C stretching of the quinoidal structure and ring stretching of thiophene rings (similar to benzoidal structure). Further vibrations at approximately 1160 and 1011 cm−1 are both assigned to the stretching modes of the C–O–C bond, and the peak at 941 cm−1 is indexed to the ethylenedioxy group in the molecular deformation mode. Also, the C–S bond vibrations in the thiophene ring are found at 993 and 858 cm−1. No significant differences were observed in the FT-IR spectra of the films, but the peaks shifted to higher wave numbers after treatment, which suggests an intermolecular interaction in the PEDOT:PSS layer [38]. The change of the films before and after the polar solvent and acid treatment was
further confirmed by XPS (Fig. 7b). The two typical S 2p peaks were observed at binding energies below and above 166.0 eV, corresponding to the sulfur atoms in PEDOT and PSS, respectively. The ratio of the peak area of S 2p in PSS to that in PEDOT for the three types of films is 2.96, 2.40, and 1.67, respectively, which suggests the loss of PSS from the PEDOT:PSS films after treatment. The removal of PSS during treatment is caused by polymer chain expansion or the phase separation between the conducting PEDOT-rich grains and the insulating PSS [39]. The change in the transmittance before and after treatment is presented in Fig. 7c. Due to the removal of PSS in the PEDOT:PSS layer, the treated films possess better optical transmission than the untreated films. Fig. 7d shows the contact angle of the CNT (A1), PDA-RGO/CNT (A2), untreated PDA-RGO/CNT/PEDOT:PSS (A3), PDA-RGO (A4), and S1–S10 films. The CNT films exhibit the highest contact angle of 108.0 ± 1.0°, whereas the PDA-RGO films have the lowest contact angle of 70.2 ± 0.1°. The contact angle of the films before and after treatment clearly changed, likely on account of the removal of the hydrophilic PSS, and the contact angle of all of the S1–S10 films was higher than 82.9 ± 0.9°, meaning that the films were rather hydrophobic surface, which may indicate improved the stability of TCF in air.
Fig. 7 (a) FT-IR and (b) XPS S 2p spectra of the untreated, DMSO-EG bath treated, and DMSO-EG bath and H2SO4 treated films; (c) changes in the transmittance before and after treatment and (d) contact angles of different samples.
A schematic illustration of the changes occurring in the PEDOT:PSS layer before and after treatment is shown in Fig. 8. In the pure PEDOT:PSS layer, PEDOT and PSS are held together by ionic attraction and have a coiled or core-shell structure due to repulsion between long PSS chains (Fig. 8a). The addition of DMSO and EG-bath treatment screens the Coulombic interaction between the PEDOT and PSS chains by forming hydrogen bonding with both PEDOT and PSS, thus leading to phase separation between PEDOT and PSS. At the same time, the coil structure of the PEDOT chains transforms to a linear or expanded-coil structure (Fig. 8b). According to Ouyang’s report [23], the catechol groups on the PDA-RGO sheets have two polar phenolic hydroxyl groups that may hydrogen bond to PSS chains and reinforce the screening effect (Fig. 8c). In order to further improve the optoelectronic properties, the films were treated with sulfuric acid. The effect of sulfuric acid can be understood in terms of the interaction between PSS− and H2SO4 ( ʹ Ͷ െ ՜ Ͷ ); the PSSH chains are neutral and do not have any Coulombic interaction with PEDOT, which can lead to more removal of insulating PSS from the films (Fig. 8d). The films were annealed at 120 Ԩ for 15 min after treatment, and crystallization occurred when the water and additive solvent evaporated which enhanced the crystallinity of the film [40]. The phase separated, crystalline, and oriented PEDOT polymer chains allow more inter-chain interaction between the conducting polymers, which results in a lower energy barrier for inter-chain and interdomain charge hopping, thus enabling easier charge transfer among the PEDOT chains. Charge hopping among the polymer chains is believed to be the dominant conduction mechanism in conducting polymers. PEDOT-rich chains with improved crystallinity, preferred orientation, linear structure, larger grain size, and lower intragrain hopping promote charge hopping, which significantly enhances the conductivity.
Fig. 8 Schematic illustration of the changes in the PEDOT:PSS layer: (a) pure PEDOT:PSS; (b), (c) the effect of DMSO/EG and PDA-RGO on PEDOT:PSS ; (d) the effect of further treated with H2SO4. The evolution of the stability of the sheet resistance for the treated PDA-RGO/CNT/PEDOT:PSS film was measured as a function of bending cycle numbers and air exposure time is shown in Fig. 9 [41]. In Fig. 9a, the TCF had constant resistance at a bending radius of 10 mm and bending cycle up to 1000 cycles and no cracking was observed from the surface of films, which indicated that the hybrid film was highly flexible and resistant to bending fatigue. In Fig. 9b, the gradual increase of sheet resistance of the films is most likely caused by aging effect due to exposure to moisture and oxygen in the air. However, this change is very slight, which
indicates
the
ultra-stability
of
the
films.
In
addition,
the
PDA-RGO/CNT/PEDOT:PSS TCF is connected to a light-emitting diode (LED) after an air exposure test as the picture shown in the inset in Fig. 9b, thus demonstrating the excellent conductivity of the electrode [42].
Fig. 9 The sheet resistance of PDA-RGO/CNT/PEDOT:PSS film was measured as a function of bending cycle numbers (a) and air exposure time (b); Inset in Fig 9 (b) is a photograph of the PDA-RGO/CNT/PEDOT:PSS TCF connects to a LED after the air exposure test.
The electroluminescence characteristics of the flexible organic light emission device, which using PDA-RGO/CNT/PEDOT:PSS films before and after 1000 cycles bending tests as anode is shown in Fig. 10. Fig. 10a depicts the schematic structure of the OLED, Fig. 10b and 10c show the luminance and the current efficiency of the OLED device, respectively. The luminance of the device using pristine film as anode was 2032 cd/cm2 at 15 V, and the maximum current efficiency was 2.13 cd/A at 14 V. Moreover, the device which using the film after 1000 cycles bending tests as anode also shows the same performance as the former one, indicating the reliability of the film. We believe that if further optimization is conducted, the general OLED performance could be improved.
Fig. 10 (a) The structure of OLED using PDA-RGO/CNT/PEDOT:PSS film as anode; OLEDs (PDA-RGO/CNT/PEDOT:PSS films before and after 1000 cycles bending text) performances for the (b) luminance versus operating voltage (inset is a picture of a lighted OLED), (c) current efficiency versus operating voltage.
4. Conclusion In conclusion, we fabricated a high performance flexible transparent film by simple spray coating. The PDA-functionalized reduced graphene oxide improved the interfacial adhesion between the conductive coating layers and substrate, which leads to enhanced electrical transport properties of the hybrid films. The obtained hybrid films have a relatively low rms roughness of approximately 2.41 nm. In addition, significant improvement in the conductivity was achieved by implementing a novel treatment method. The mechanism generating the improved adhesion and conductivity was investigated in detail. The best results were obtained from the PDA-RGO/CNT/PEDOT:PSS-DMSO-EG bath and H2SO4 treated electrode that exhibited a sheet resistance of 52.2 Ω/sq. with an optical transmittance of 88.7% at
550 nm. OLEDs which using PDA-RGO/SWCNT/PEDOT:PSS film as anode were successfully fabricated, and the luminance of the device was 2032 cd/cm2 at 15 V, and the maximum current efficiency was 2.13 cd/A at 14 V. It is believed that the developed hybrid TCF and corresponding processing technology can have wide applications for flexible electro-optic and photoelectric devices.
Acknowledgements The authors gratefully acknowledge financial support from the Natural Science Foundation of Tianjin China (Grant No. 19JCZDJC37800), the Science and Technology Plans of Tianjin China (Grant No. 18PTSYJC00180), the Natural Science Foundation of jiangxi province China (Grant No. 20181BAB206008), and the Science and Technology Plans of Jingdezhen China (Grant No. 20182GYZD011-06). References [1] P. Heimel, A. Mondal, F. May, W. Kowalsky, C. Lennartz, D. Andrienko, and R. Lovrincic, Unicolored phosphor-sensitized fluorescence for efficient and stable blue OLEDs, Nat. Commun., 9 (2018) 4990-4998. [2] H. Bi, F. Huang, J. Liang, X. Xie, and M. Jiang, Transparent conductive graphene films synthesized by ambient pressure chemical vapor deposition used as the front electrode of CdTe solar cells, Adv. Mater., 23 (2011) 3202-3206. [3] W. J. Jie and J. H. Hao, Graphene-based hybrid structures combined with functional materials of ferroelectrics and semiconductors, Nanoscale, 6 (2014) 6346-6362. [4] W. Lan, Z. Yang, Y. Zhang, Y. Wei, P. Wang, A. Abas, G. Tang, X. Zhang, J. Wang and E. Xie, Novel transparent high-performance AgNWs/ZnO electrodes prepared on unconventional substrates with 3D structured surfaces, Appl. Surf. Sci., 433 (2013) 821-828. [5] Y.-Y. Choi, S. J. Kang, H.-K. Kim, W. M. Choi, and S.-I. Na, Multilayer graphene films as transparent electrodes for organic photovoltaic devices, Sol. Energ. Mat. Sol. C, 96 (2012) 281-285.
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Graphical Abstract
The PDA-RGO/CNT/PEDOT:PSS film with a structure similar to reinforced concrete has high electrical conductivity and stability during bending.
Highlights:
●
The PDA-RGO /CNT/PEDOT:PSS film with a structure similar to reinforced
concrete was prepared by simple spraying method. ●
PDA-RGO was introduced as a conductive adhesion promoter, and a novel
post treatment method for the film was taken to achieve better conductivity. ●
The film had a low sheet resistance of 52.2 Ω/sq. with a high optical
transmittance of 88.7% at 550 nm, and its roughness was as low as 2.41 nm.
S0169-4332(19)32813-2 https://doi.org/10.1016/j.apsusc.2019.143997 APSUSC 143997
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
13 August 2019 11 September 2019 12 September 2019
Please cite this article as: T. Wang, L-C. Jing, Q. Zhu, A. Sagadevan Ethiraj, Y. Tian, H. Zhao, X-T. Yuan, J-G. Wen, L-K. Li, H-Z. Geng, Fabrication of Architectural Structured Polydopamine-Functionalized Reduced Graphene Oxide/Carbon Nanotube/PEDOT:PSS Nanocomposites as Flexible Transparent Electrodes for OLEDs, Applied Surface Science (2019), doi: https://doi.org/10.1016/j.apsusc.2019.143997
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Fabrication of Architectural Structured Polydopamine-Functionalized Reduced Graphene Oxide/Carbon Nanotube/PEDOT:PSS Nanocomposites as Flexible Transparent Electrodes for OLEDs
Tao Wang1, Li-Chao Jing1, Qingxia Zhu*1, Anita Sagadevan Ethiraj2, Ying Tian1, Hui Zhao1, Xiao-Tong Yuan1, Jian-Gong Wen1, Lin-Kuo Li1, Hong-Zhang Geng 1
1. Tianjin Key Laboratory of Advanced Fibers and Energy Storage, School of Material Science and Engineering, Tiangong University, Tianjin 300387, China 2.Department of Physics, VIT-AP University, Amaravati, Andhra Pradesh 522237, India
Corresponding author. E-mail addresses:[email protected] (Q. Zhu); [email protected] (H.-Z. Geng).
Abstract High performance, flexible transparent conductive films with a structure similar to that of reinforced concrete and constructed by sandwiching single-walled carbon nanotubes (SWCNT) between poly (3,4-ethylene dioxythiophene):poly (styrene sulfonate) (PEDOT:PSS) and polydopamine functionalized reduced graphene oxide (PDA-RGO) were fabricated using simple spray coating. Mussel-inspired polydopamine was introduced as a graphene oxide reducing agent and modifier; the obtained PDA-RGO improved the interfacial adhesion between the conductive coating layers and substrate, and an effective post fabrication treatment method was performed on the hybrid film to achieve better conductivity. It was found that the resulting electrode exhibited a low sheet resistance of 52.2 Ω/sq. with a high optical transmittance of 88.7% at 550 nm. Moreover, the transparent film exhibited long-term stability with a relatively low roughness (ca. 2.41 nm), and its architectural structure sustained the flexibility of the film during bending. The organic light emitting diodes which using PDA-RGO/SWCNT/PEDOT:PSS film as anode was successfully fabricated, the luminance of the device was 2032 cd/cm2 at 15 V and the maximum current efficiency was 2.13 cd/A at 14 V, indicating the strong potential of this type of transparent electrode for flexible electroluminescent devices. Key words: Flexible transparent conductive films; Carbon nanotubes; Polydopamine Functionalized Reduced Graphene Oxide; PEDOT:PSS; OLEDs
1. Introduction Over the past several years, optoelectronic devices, including organic light emitting diodes (OLEDs) [1], organic solar cells [2], and field effect transistors, have been extensively developed [3,4]. Transparent conductive films (TCFs) are essential in all of these optoelectronic devices. Indium tin oxide (ITO), the conventional conductive metal oxide thin film, has been widely used for transparent electrodes. However, a few drawbacks of ITO such as its high cost due to indium scarcity, complicated processing requirements, sensitivity to acid and basic environments, and
cracking and delaminating when subjected to bending deem it unsuitable for applications that require flexibility. For these reasons, numerous candidate materials including metal nanowires [5,6], carbon nanotubes (CNTs) [7,8], graphene [9,10], and conducting polymers [11,12] are being actively investigated as replacements for ITO. Among these materials, CNTs present potential advantages in chemical stability, thermal and electrical conductivity, mechanical strength, flexibility, and work function (4.5–5.2 eV) [13]. However, many problems must be overcome. For instance, CNT films suffer from weak interfacial adhesion to substrates resulting from their inherent tube–tube junctions; moreover, these junctions readily open, causing relatively high sheet resistance [14]. Recently, Han et al. [15] introduced graphene oxide (GO) as a multi-purpose p-type dopant for CNT films. Although GO sheets deposited onto the porous nanotube networks reduced the surface roughness, there was no significant effect on improving the conductivity and film adhesion by this method. Polydopamine (PDA), which mimics adhesive proteins [16,17], was used to simultaneously reduce (reduced GO, RGO) and functionalize GO during self-polymerization [18]. Therefore, PDA functionalized reduced graphene oxide (PDA-RGO) was used as a conductive adhesion promoter to improve the interfacial adhesion between the conductive coating layers and substrate. Spray coated poly (3,4-ethylene dioxythiophene):poly (styrene sulfonate) (PEDOT:PSS) was applied for planarization of TCFs [19,20], owing to its intrinsic fairly high conductivity, high transparency in the visible range, and outstanding film-forming properties. However, pristine PEDOT:PSS films exhibit a very low conductivity (<1 S/cm) and cannot be used as a stand-alone electrode. Fortunately, the conductivity of PEDOT:PSS can be increased up to 2 or 3 orders of magnitude by adding some components, such as organic compound salts, zwitterions, carboxylic or inorganic acids, or cosolvents [21,22,23]. Despite ample efforts and success in improving the conductivity, it is still too low to meet the demands of optoelectronic applications. Therefore, We have taken an effective method to significantly enhance
the conductivity and crystallinity of the π-conjugated polymer coating layer by DMSO-EG bath combined with H2SO4 treatment [24]. In this study, we fabricated a high performance, flexible transparent electrode with a structure resembling that of reinforced concrete by sandwiching CNTs between PEDOT:PSS and PDA-RGO by using a simple method, spray coating. The successful functionalization of reduced graphene oxide by PDA was confirmed and characterized by Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and Fourier transform-infrared (FT-IR), and ultraviolet-visible (UV-vis) spectroscopies. The modified RGO was expected to improve the interfacial adhesion between the conductive coating layers and substrate. Moreover, an effective post fabrication treatment method was performed on the hybrid films to obtain better conductivity, and the conductivity mechanism was systematically investigated. To verify the potential applications of this hybrid film, which have excellent optical conductivity, low root mean square (rms) roughness, and ultra-adhesion to substrates, the organic light emitting diodes (OLEDs) which using PDA-RGO/SWCNT/PEDOT:PSS film as anode was successfully fabricated and characterized.
2. Experimental 2.1. Materials The GO nanosheets were prepared via a modified Hummers’ method. High purity single-walled CNTs (SWCNTs, purity of 95 wt.%, diameter less than 2 nm, length >5 Pm) were provided from Carbon Star Technology (Tianjin) Co. Ltd. Sodium
dodecylbenzenesulfonate
(SDBS),
dopamine
hydrochloride
(98%),
tris(hydroxymethyl)aminomethane (Tris) (99.8%), ethylene glycol (EG), and dimethyl sulfoxide (DMSO) were purchased from Aladdin. PEDOT:PSS solution (Clevios pH 1000) with a concentration of 1.3 wt.% was purchased from H. C. Starck. All reagents were used as received. 2.2. Synthesis of PDA-RGO Nanosheets PDA-RGO was prepared as follows: 200 mg of GO and 100 mg of dopamine hydrochloride were added to 400 ml of Tris buffer solution (10 mM, pH = 8.5) and
dispersed by sonication for 10 min while submerged in an ice bath. Afterwards, the solution was vigorously stirred at 60 °C for 24 h; the color of the GO suspension changed from dark brown to black during the stirring process. After the reduction reaction, few-layer and uniformly sized PDA-RGO sheets were obtained by washing via centrifugation. Finally, the PDA-RGO precipitate was repeatedly washed with distilled water several times to remove free PDA particles and thick multilayer flakes, and a clean PDA-RGO solution (0.1 mg/ml) was obtained. 2.3. Preparation of SWCNT solutions The preparation of the SWCNT solutions was reported in our previous work [25]. CNTs were dispersed in deionized water (1 mg/ml) using SDBS as a dispersant. Then, the solution was sonicated in a bath sonicator for 30 min followed by probe ultrasonication at 120 W for 30 min to obtain a suspension solution. The mixture was centrifuged at 8000 rpm for 20 min, and the 80% supernatant was collected. 2.4. Fabrication of PDA-RGO/SWCNT/PEDOT based TCFs Flexible polymer-polyethylene terephthalate (PET) substrates were immersed in ethanol with bath ultrasonication to remove contamination. Then the clean PET substrates were dried in a dryer for later processing. DMSO (5 wt.%) was added into the PEDOT:PSS solution, after which the mixture was diluted ten times with deionized water and magnetic stirring for one hour to ensure good mixing. First, the PET films were fixed on a heating plate, and the PDA-RGO solution was poured into the airbrush and sprayed on the surface of the PET substrates. Then, the SWCNT solution was sprayed onto the PDA-RGO layer. In order to remove the SDBS between the coating layers, the PDA-RGO/SWCNT films were immersed in deionized water for 20 min; after drying, the samples were immersed in 12 M nitric acid for 30 min, and the residual acid was washed away by deionized water several times. The TCFs were dried again at 80 °C. Next, the PEDOT:PSS solution was sprayed
on
the
PDA-RGO/SWCNT
networks
to
form
PDA-RGO/SWCNT/PEDOT:PSS films. These films were immersed in EG for 40 min and washed by deionized water, after which they were annealed in air at 120 Ԩ for 15
min to remove DMSO and EG. A 1 M H2SO4 solution was pipetted onto the dried films at 120 °C and kept on the sample for 5 min. The film was then further washed by deionized water and annealed again. Finally, we obtained the sandwich structured PDA-RGO/SWCNT/PEDOT:PSS based TCFs. In our study, all spray conditions were maintained at a constant spray velocity with a 0.3 MPa spraying pressure and 15 cm distance between the nozzle and the substrate. During the spray coating process, the PET substrates were heated at 120 °C on a hotplate to avoid the coffee-ring effect. 2.5 Fabrication of OLEDs using PDA-RGO/SWCNT/PEDOT TCFs as anode The OLED structures in this study were (PDA-RGO/SWCNT/PEDOT) / NPB (40 nm)/ Alq3 (60 nm)/ LiF (0.8 nm)/ Al (100 nm). All of the organic materials were thermally evaporated onto the patterned anodes using a thermal evaporation coating system at a pressure of approximately 7×10-4 Pa. 2.6. Characterization The transmittance of the films was measured using a UV-vis spectrophotometer at 550 nm. The sheet resistance of the films was measured using the four-point probe method (Keithley 2700 multi-meter). The surface morphology of the films was characterized by field-emission scanning electron microscopy (FE-SEM) (Hitachi S-4800) and atomic force microscopy (AFM, BRUKER). Transmission electron microscopy (TEM) was performed using a Hitachi H-800 electron microscope operating at an accelerating voltage of 120 kV. The changes in the surface functional groups were detected by XPS (ELMER PHI 5600, Al Kα source) and FT-IR (TENSOR37), and the surface wettability was assessed by a contact-angle analysis device (DSA100, KRUSS). The nanostructures were further characterized via a DXR Raman microscope with a laser excitation wavelength of 532 nm. The adhesion was investigated using the 3M tape test, in which the tapes adhere to the films by rubbing the entire surface area using constant pressure and then peeled off from the samples. The performance of the OLEDs was characterized by an optical detection system for OLED display panels developed by Suzhou Fstar Scientific Instrument Co., Ltd. 3. Results and Discussion
The morphology and structure of the GO and PDA-RGO nanosheets were observed by TEM. In Fig. 1a, the large GO sheet (ca. 4 μm) has a planar texture with a smooth surface. After being modified by PDA, the PDA-RGO nanosheet exhibits an obvious rough surface with folds and wrinkles resembling gecko feet, which may increase the contact area and enhance the adhesion to substrates, as shown in Fig. 1b [26].
Fig. 1 TEM images of (a) GO sheet and (b) PDA-RGO sheet.
The synthesized GO and PDA-RGO were further characterized by UV-vis, Raman, FT-IR, and XPS. The simultaneous reduction and surface functionalization of GO took place during the oxidative self-polymerization of dopamine, and GO was converted into PDA-RGO. The successful reduction of GO to PDA-RGO was confirmed by UV-Vis. As shown in Fig. 2a, GO exhibits a characteristic absorption peak at 230 nm, however, the peak shifts to 260 nm in PDA-RGO, which is a characteristic peak of RGO, indicating a refreshed electronic conjugation in graphene sheets due to the reduction of GO. The reduction of GO was further verified by
Raman spectroscopy. As shown in Fig. 2b, GO and PDA-RGO exhibit two characteristic peaks. The spectra of GO and PDA-RGO are characterized by D and G bands. The D and G bands of GO are found at 1358 and 1591 cm−1, respectively. As for PDA-RGO, the curve displays a strong D band at 1353 cm−1 and a G band at 1589 cm−1. The red shifting of the two bands upon dopamine reduction is an indication of graphitic self-healing behavior [27]. The ratio of the intensities of D and G bands (ID/IG) is used to evaluate the degree of graphitization and the defect density in graphene materials. Here, the value of ID/IG increased from 0.84 to 0.94, which is attributed to the formation of smaller graphitic domains during the reduction of GO. Furthermore, the surface functionalization of GO by PDA is also evident in the FT-IR and XPS spectra. As shown in its FT-IR spectrum in Fig. 2c, GO exhibits 3 characteristic peaks at around 1060, 1400, and 1731 cm−1 that correspond to the C–O (epoxy or alkoxy), C–OH (hydroxyl), and C=O (carboxyl) bonds of the GO skeleton, respectively, which are characteristic features of the FT-IR spectrum of GO. When compared with the spectrum of GO, that of PDA-RGO reveals two new peaks at 1386 and 1465 cm−1, corresponding to C–OH bending in the catechol groups and the stretching of C–N bonds, respectively, thus confirming the successful grafting of PDA molecular chains onto the surface of the GO sheets. In addition, the band at 1726 cm−1 for the carbonyl group almost disappeared, indicating that GO was reduced during the self-polymerization [28]. Generally speaking, these results demonstrate the simultaneous reduction and surface functionalization of GO that results in PDA-RGO. The change of the chemical information in GO and PDA-RGO was also investigated by XPS. Fig. 2d shows the XPS survey spectra of GO and PDA-RGO. The O/C ratios of GO and PDA-RGO are 0.38 and 0.27, respectively. It is clear that the peak intensities of the oxygen-containing groups on PDA-RGO have decreased significantly, which is due to the release of electrons that attack the oxygen-containing species such as C=O in GO during the polymerization of dopamine; the process reduces GO to some extent and facilitates the nucleation and growth of PDA film on the RGO sheets. In addition, the N 1s peak at 400.0 eV, which is related to the amine
group of PDA, was only observed in the PDA-RGO spectrum. The C 1s XPS spectra of GO and PDA-RGO are shown in Fig. 2e and 2f, respectively. The XPS C1s core-level spectrum of GO can be divided into four peak components with binding energies at approximately 284.9, 286.9, 288.3, and 289.7 eV, which can be indexed to C–C/C=C, C–O, C=O, and O–C=O species, respectively. For PDA-RGO, five different peaks at 284.7, 285.7, 286.4, 288.6, and 291.16 eV are related to the C=C/C–C, C–N, C–O, C=O, and O–C=O groups, respectively. The emergence of the new C–N peak further indicates the successful functionalization of PDA-RGO by PDA. Moreover, the strong π-π stacking interaction also proven; due to the similar structure of PDA and GO, the PDA coating on GO is highly stable. Thus, functionalizing GO using PDA combined advantages of both covalent and noncovalent functionalization [29].
Fig. 2 (a) UV-vis, (b) Raman, (c) FT-IR, and (d) XPS survey spectra of GO and
PDA-RGO; C 1s spectrum of (e) GO and (f) PDA-RGO. The SEM image in Fig. 3a shows the CNT film was after washing with deionized water and nitric acid sequentially [30]. The random entanglement among tubes may increase the roughness of the film and generate hundreds of nanoscale pores, resulting in poor carrier transport [31]. The SEM image of the PDA-RGO/CNT film in Fig. 3b reveals that the CNTs and PDA-RGO sheets are homogeneously distributed on the substrate. The higher magnification SEM image shown in Fig. 3c indicates that the large-sized PDA-RGO sheets with wrinkles cover the holes in the CNT network and act as a bridge between CNT bundles, which created several new electrical percolation
channels
[32].
In
Fig.
3d,
an
image
of
the
untreated
PDA-RGO/CNT/PEDOT:PSS film is shown; it exhibits a fairly flat surface, likely because the pores in the networks and the wrinkles from the PDA-RGO sheets were covered by PEDOT:PSS [33]. The surface of the film after the DMSO-EG bath and H2SO4 treatments remains relatively flat as shown in Fig. 3e. The surface of the PDA-RGO/CNT/PEDOT:PSS-treated film after the 3M tape adhesion test is shown in Fig. 3f, in which many tubular structures are exposed, but the coating remains intact due to the catechol groups of PDA-RGO serving as an anchor, thus enhancing the adhesion between the coating and the substrate.
Fig. 3 SEM images of different TCFs: (a) CNTs, (b) CNT/PDA-RGO, (c) higher magnification of CNT/PDA-RGO film showing the structure between the CNTs and PDA-RGO, (d) untreated PDA-RGO/CNT/PEDOT:PSS, (e) treated
PDA-RGO/CNT/PEDOT:PSS, and (f) treated PDA-RGO/CNT/PEDOT:PSS after the 3M tape adhesion test. An adhesion factor incorporating transmittance, which can be easily measured from absorption spectroscopy, was applied to quantitatively characterize the adhesion as follows [34]: ܶ െܶ
߂ܶ
݊ Ͳ ൌ ͳ െ ͳͲͲെܶ ݂ܶ ൌ ͳ െ ͳͲͲെܶ Ͳ
,
Ͳ
(1)
where fT is the adhesion factor, and T0 and T1 are the transmittance of hybrid films before and after the trial test, respectively. As shown in Table 1, the adhesion factor of all 10 treated PDA-RGO/CNT/PEDOT:PSS samples (S1–S10) is all higher than 0.92, which indicates ultra-adhesion of the films. However, the CNT films and pure PEDOT:PSS films have different degrees of shedding after trial test.
Table 1 The change in the transmittance after the 3M tape adhesion test and the adhesion factor for treated PDA-RGO/CNT/PEDOT:PSS films (S1–S10) Samples
1
2
3
4
5
6
7
8
9
10
T0 (%)
92.8
91.0
89.6
88.7
86.5
84.7
81.9
81.1
79.9
77.1
T1 (%)
93.2
91.5
90.2
89.4
87.4
85.7
83.2
82.4
81.3
78.8
fT
0.95
0.94
0.94
0.94
0.99
0.93
0.93
0.93
0.93
0.93
Fig. 4 shows AFM images of the CNT, PDA-RGO/CNT, untreated PDA-RGO/CNT/PEDOT:PSS, and treated PDA-RGO/CNT/PEDOT:PSS films, from which the rms roughness of the sample surfaces was determined; the rms is 12.00, 5.12, 1.09, and 2.41 nm, respectively. We suggest that doping PDA-RGO into the CNT network reduced the surface roughness of the CNT films, and the presence of the PEDOT:PSS layer resulted in the desired smooth surface structures. Despite the increase in surface roughness measured on the PDA-RGO/CNT/PEDOT:PSS film after treatment in the DMSO-EG bath and H2SO4, it is still much lower than that of the pure CNT samples. Such a low electrode surface roughness is advantageous for device performance.
Fig. 4 Two- and three-dimensional AFM images of different TCFs: (a–b) CNT, (c–d) PDA-RGO/CNT, (e–f) untreated PDA-RGO/CNT/PEDOT:PSS, and (g–h) treated PDA-RGO/CNT/PEDOT:PSS.
The PDA-RGO/CNT/PEDOT:PSS film's ultra-adhesion and relatively low roughness can likely be attributed to its special architectural structure that also enhances the film's electrical conductivity. The insulating surfactant SDBS was effectively removed, and the formation of the subsequent densified film improved the cross-junction conductivity within the CNT network, enhancing the metallicity of the CNTs. However, the CNTs were entangled by X-type junctions that could easily open. Therefore, PDA-RGO sheets with abundant catechol functional groups were carefully introduced to bind the CNTs firmly together, acting like an adhesive. This is depicted in Fig. 5a. The two-dimensional conductive PDA-RGO sheets act as a p-type dopant
and surface energy modifier of the CNT film, which decreases the Schottky barrier height between the nanotubes to improve the conductivity of hybrid film (For the PDA-RGO film, it has a sheet resistance of 8.26 MΩ/sq. at the transparency of 80% at 550 nm ). In Fig. 5b, the stretchable PEDOT:PSS film coating placed on the PDA-RGO/CNT layer fills up the voids of the CNT network and covers the wrinkled structure of the PDA-RGO sheets, thus smoothing the surface morphology of the hybrid films and significantly decreasing the sheet resistance. The combination of materials in the hybrid film results in a structure similar to that of reinforced concrete in buildings, shown in Fig. 5c. Such a structure effectively improves the stability of film during bending.
Fig. 5 Schematic illustration of TCFs with ultra-adhesion, relatively low roughness, and excellent electrical conductivity: (a) PDA-RGO/CNT and (b) PDA-RGO/CNT/PEDOT:PSS; (c) a picture of reinforced concrete structure used in building construction. For TCFs, high optical transmission in combination with low sheet resistance is key to improving the performance of thin-film optoelectronic devices. The relationship between transmittance and sheet resistance of the pure CNT,
PDA-RGO/CNT/PEDOT:PSS (untreated), PDA-RGO/CNT/PEDOT:PSS-DMSO-EG bath treated (DMSO-EG bath treated), and PDA-RGO/CNT/PEDOT:PSS-DMSO-EG bath and H2SO4 treated (DMSO-EG bath and H2SO4 treated) films is shown in Fig. 6. The pure CNT films exhibit a sheet resistance of 58.8 Ω/sq. with an optical transmittance of 80% at 550 nm. The untreated films show poor conductivity (Rs = 462.2 Ω/sq., T = ca. 80% at 550 nm), which is likely caused by the pristine PEDOT:PSS layer that usually has a conductivity below 1 S/cm. After being treated with organic solvent, the resistance of the hybrid films is reduced to some extent (Rs = 246.3 Ω/sq., T = ca. 80% at 550 nm). In order to further improve the electrical properties of the TCFs, we innovatively used organic solvents combined with inorganic acids to treat the hybrid films; when compared with the pure CNT film, the resistance of the hybrid films decreased significantly (an order of magnitude) to 35.5 Ω/sq., and the film simultaneously exhibited a high optical transmittance of 79.9% at 550 nm. In addition, the hybrid films continue to exhibit a low sheet resistance (52.2 Ω/sq.) at 88.7%. optical transmittance. The changes in the films during the treatment are discussed in detail below.
Fig. 6 The relationship between the transmittance (at 550 nm) and sheet resistance of pure CNT, untreated, DMSO-EG bath treated, and DMSO-EG bath and H2SO4 treated films.
To quantitatively characterize the optoelectrical properties of the treated PDA-RGO/CNT/PEDOT:PSS films, an approximate relationship between the optical
transmittance and sheet resistance, direct current conductivity (σdc), and optical conductivity (σop) is given by following relation [35]: ߪܥܦ ߪܱ
ൌ
ͳͺͺሺߗሻ ܴݏ
ξܶ ൨ ൫ͳെξܶ ൯
,
(2)
where, Rs is the sheet resistance, and T is the transmittance of the TCFs. The values of σop /σdc for the 10 DMSO-EG bath and H2SO4 treated samples are listed in Table 2. It is found that when the transmittance is above 77.12%, the values of σop/σdc are higher than 44.0, which is excellent performance when compared with Zheng’s report (σop/σdc =7.48) [36] and our previous work (σop/σdc = 25.3) [37]. Table 2 Optoelectrical properties of the treated PDA-RGO/CNT/PEDOT:PSS films. Samples
1
2
3
4
5
6
7
8
9
10
T1 (%)
92.9
91.0
89.6
88.7
86.5
84.7
81.9
81.1
79.9
77.1
Rs (Ω/sq)
108.9
75.7
72.8
52.2
46.9
42.0
37.4
37.1
35.5
30.7
σop/σdc
45.8
51.2
45.6
58.2
53.5
51.6
47.8
45.9
44.6
44.1
The
effect
of
the
polar
solvent
and
inorganic
acid
on
the
PDA-RGO/CNT/PEDOT:PSS film was characterized by FT-IR and XPS. As shown in Fig. 7a, the FT-IR spectra of three type films (untreated, DMSO-EG bath treated, and DMSO-EG bath and H2SO4 treated) all show similar characteristic peaks. The vibrations around 1266–1456 cm−1 correspond to C–C and C=C stretching of the quinoidal structure and ring stretching of thiophene rings (similar to benzoidal structure). Further vibrations at approximately 1160 and 1011 cm−1 are both assigned to the stretching modes of the C–O–C bond, and the peak at 941 cm−1 is indexed to the ethylenedioxy group in the molecular deformation mode. Also, the C–S bond vibrations in the thiophene ring are found at 993 and 858 cm−1. No significant differences were observed in the FT-IR spectra of the films, but the peaks shifted to higher wave numbers after treatment, which suggests an intermolecular interaction in the PEDOT:PSS layer [38]. The change of the films before and after the polar solvent and acid treatment was
further confirmed by XPS (Fig. 7b). The two typical S 2p peaks were observed at binding energies below and above 166.0 eV, corresponding to the sulfur atoms in PEDOT and PSS, respectively. The ratio of the peak area of S 2p in PSS to that in PEDOT for the three types of films is 2.96, 2.40, and 1.67, respectively, which suggests the loss of PSS from the PEDOT:PSS films after treatment. The removal of PSS during treatment is caused by polymer chain expansion or the phase separation between the conducting PEDOT-rich grains and the insulating PSS [39]. The change in the transmittance before and after treatment is presented in Fig. 7c. Due to the removal of PSS in the PEDOT:PSS layer, the treated films possess better optical transmission than the untreated films. Fig. 7d shows the contact angle of the CNT (A1), PDA-RGO/CNT (A2), untreated PDA-RGO/CNT/PEDOT:PSS (A3), PDA-RGO (A4), and S1–S10 films. The CNT films exhibit the highest contact angle of 108.0 ± 1.0°, whereas the PDA-RGO films have the lowest contact angle of 70.2 ± 0.1°. The contact angle of the films before and after treatment clearly changed, likely on account of the removal of the hydrophilic PSS, and the contact angle of all of the S1–S10 films was higher than 82.9 ± 0.9°, meaning that the films were rather hydrophobic surface, which may indicate improved the stability of TCF in air.
Fig. 7 (a) FT-IR and (b) XPS S 2p spectra of the untreated, DMSO-EG bath treated, and DMSO-EG bath and H2SO4 treated films; (c) changes in the transmittance before and after treatment and (d) contact angles of different samples.
A schematic illustration of the changes occurring in the PEDOT:PSS layer before and after treatment is shown in Fig. 8. In the pure PEDOT:PSS layer, PEDOT and PSS are held together by ionic attraction and have a coiled or core-shell structure due to repulsion between long PSS chains (Fig. 8a). The addition of DMSO and EG-bath treatment screens the Coulombic interaction between the PEDOT and PSS chains by forming hydrogen bonding with both PEDOT and PSS, thus leading to phase separation between PEDOT and PSS. At the same time, the coil structure of the PEDOT chains transforms to a linear or expanded-coil structure (Fig. 8b). According to Ouyang’s report [23], the catechol groups on the PDA-RGO sheets have two polar phenolic hydroxyl groups that may hydrogen bond to PSS chains and reinforce the screening effect (Fig. 8c). In order to further improve the optoelectronic properties, the films were treated with sulfuric acid. The effect of sulfuric acid can be understood in terms of the interaction between PSS− and H2SO4 ( ʹ Ͷ െ ՜ Ͷ ); the PSSH chains are neutral and do not have any Coulombic interaction with PEDOT, which can lead to more removal of insulating PSS from the films (Fig. 8d). The films were annealed at 120 Ԩ for 15 min after treatment, and crystallization occurred when the water and additive solvent evaporated which enhanced the crystallinity of the film [40]. The phase separated, crystalline, and oriented PEDOT polymer chains allow more inter-chain interaction between the conducting polymers, which results in a lower energy barrier for inter-chain and interdomain charge hopping, thus enabling easier charge transfer among the PEDOT chains. Charge hopping among the polymer chains is believed to be the dominant conduction mechanism in conducting polymers. PEDOT-rich chains with improved crystallinity, preferred orientation, linear structure, larger grain size, and lower intragrain hopping promote charge hopping, which significantly enhances the conductivity.
Fig. 8 Schematic illustration of the changes in the PEDOT:PSS layer: (a) pure PEDOT:PSS; (b), (c) the effect of DMSO/EG and PDA-RGO on PEDOT:PSS ; (d) the effect of further treated with H2SO4. The evolution of the stability of the sheet resistance for the treated PDA-RGO/CNT/PEDOT:PSS film was measured as a function of bending cycle numbers and air exposure time is shown in Fig. 9 [41]. In Fig. 9a, the TCF had constant resistance at a bending radius of 10 mm and bending cycle up to 1000 cycles and no cracking was observed from the surface of films, which indicated that the hybrid film was highly flexible and resistant to bending fatigue. In Fig. 9b, the gradual increase of sheet resistance of the films is most likely caused by aging effect due to exposure to moisture and oxygen in the air. However, this change is very slight, which
indicates
the
ultra-stability
of
the
films.
In
addition,
the
PDA-RGO/CNT/PEDOT:PSS TCF is connected to a light-emitting diode (LED) after an air exposure test as the picture shown in the inset in Fig. 9b, thus demonstrating the excellent conductivity of the electrode [42].
Fig. 9 The sheet resistance of PDA-RGO/CNT/PEDOT:PSS film was measured as a function of bending cycle numbers (a) and air exposure time (b); Inset in Fig 9 (b) is a photograph of the PDA-RGO/CNT/PEDOT:PSS TCF connects to a LED after the air exposure test.
The electroluminescence characteristics of the flexible organic light emission device, which using PDA-RGO/CNT/PEDOT:PSS films before and after 1000 cycles bending tests as anode is shown in Fig. 10. Fig. 10a depicts the schematic structure of the OLED, Fig. 10b and 10c show the luminance and the current efficiency of the OLED device, respectively. The luminance of the device using pristine film as anode was 2032 cd/cm2 at 15 V, and the maximum current efficiency was 2.13 cd/A at 14 V. Moreover, the device which using the film after 1000 cycles bending tests as anode also shows the same performance as the former one, indicating the reliability of the film. We believe that if further optimization is conducted, the general OLED performance could be improved.
Fig. 10 (a) The structure of OLED using PDA-RGO/CNT/PEDOT:PSS film as anode; OLEDs (PDA-RGO/CNT/PEDOT:PSS films before and after 1000 cycles bending text) performances for the (b) luminance versus operating voltage (inset is a picture of a lighted OLED), (c) current efficiency versus operating voltage.
4. Conclusion In conclusion, we fabricated a high performance flexible transparent film by simple spray coating. The PDA-functionalized reduced graphene oxide improved the interfacial adhesion between the conductive coating layers and substrate, which leads to enhanced electrical transport properties of the hybrid films. The obtained hybrid films have a relatively low rms roughness of approximately 2.41 nm. In addition, significant improvement in the conductivity was achieved by implementing a novel treatment method. The mechanism generating the improved adhesion and conductivity was investigated in detail. The best results were obtained from the PDA-RGO/CNT/PEDOT:PSS-DMSO-EG bath and H2SO4 treated electrode that exhibited a sheet resistance of 52.2 Ω/sq. with an optical transmittance of 88.7% at
550 nm. OLEDs which using PDA-RGO/SWCNT/PEDOT:PSS film as anode were successfully fabricated, and the luminance of the device was 2032 cd/cm2 at 15 V, and the maximum current efficiency was 2.13 cd/A at 14 V. It is believed that the developed hybrid TCF and corresponding processing technology can have wide applications for flexible electro-optic and photoelectric devices.
Acknowledgements The authors gratefully acknowledge financial support from the Natural Science Foundation of Tianjin China (Grant No. 19JCZDJC37800), the Science and Technology Plans of Tianjin China (Grant No. 18PTSYJC00180), the Natural Science Foundation of jiangxi province China (Grant No. 20181BAB206008), and the Science and Technology Plans of Jingdezhen China (Grant No. 20182GYZD011-06). References [1] P. Heimel, A. Mondal, F. May, W. Kowalsky, C. Lennartz, D. Andrienko, and R. Lovrincic, Unicolored phosphor-sensitized fluorescence for efficient and stable blue OLEDs, Nat. Commun., 9 (2018) 4990-4998. [2] H. Bi, F. Huang, J. Liang, X. Xie, and M. Jiang, Transparent conductive graphene films synthesized by ambient pressure chemical vapor deposition used as the front electrode of CdTe solar cells, Adv. Mater., 23 (2011) 3202-3206. [3] W. J. Jie and J. H. Hao, Graphene-based hybrid structures combined with functional materials of ferroelectrics and semiconductors, Nanoscale, 6 (2014) 6346-6362. [4] W. Lan, Z. Yang, Y. Zhang, Y. Wei, P. Wang, A. Abas, G. Tang, X. Zhang, J. Wang and E. Xie, Novel transparent high-performance AgNWs/ZnO electrodes prepared on unconventional substrates with 3D structured surfaces, Appl. Surf. Sci., 433 (2013) 821-828. [5] Y.-Y. Choi, S. J. Kang, H.-K. Kim, W. M. Choi, and S.-I. Na, Multilayer graphene films as transparent electrodes for organic photovoltaic devices, Sol. Energ. Mat. Sol. C, 96 (2012) 281-285.
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of
conductivity
poly(3,4-ethylenedioxythiophene):poly(styrene
enhancement sulfonate)
film
through
in solvent
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Graphical Abstract
The PDA-RGO/CNT/PEDOT:PSS film with a structure similar to reinforced concrete has high electrical conductivity and stability during bending.
Highlights:
●
The PDA-RGO /CNT/PEDOT:PSS film with a structure similar to reinforced
concrete was prepared by simple spraying method. ●
PDA-RGO was introduced as a conductive adhesion promoter, and a novel
post treatment method for the film was taken to achieve better conductivity. ●
The film had a low sheet resistance of 52.2 Ω/sq. with a high optical
transmittance of 88.7% at 550 nm, and its roughness was as low as 2.41 nm.