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C: Surfaces, Interfaces, Porous Materials, and Catalysis

In-Situ Photoelectron Spectroscopy Study on the Buffer Role of Multiwalled Carbon Nanotubes Against Thermal Degradation in Organic Conducting Composite Film With PEDOT:PSS Dong-Jin Yun, Sung-Hoon Park, Jinyoung Hwang, Hyemin Ra, Jung-Min Kim, JaeGwan Chung, Seong Heon Kim, Yong-Su Kim, Sung Heo, and Ki-Hong Kim J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b10345 • Publication Date (Web): 11 Jan 2019 Downloaded from http://pubs.acs.org on January 15, 2019

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The Journal of Physical Chemistry

In-situ Photoelectron Spectroscopy Study on the Buffer Role of Multiwalled Carbon Nanotubes Against Thermal Degradation in Organic Conducting Composite Film with PEDOT:PSS Dong-Jin Yun1,*, Sung-Hoon Park2, Jinyoung Hwang3, Hyemin Ra4, Jung-Min Kim4, JaeGwan Chung1, Seong Heon Kim1, Yong-Su Kim1, Sung Heo1, and Ki-Hong Kim1

1 Analytical

Engineering Laboratory of Samsung Advanced Institute of Technology,

PO Box 111, Suwon 440-600, Korea 2

Department of Mechanical Engineering, Soongsil University, Seoul 06978, Korea

3

School of Electronics and Information Engineering, Korea Aerospace University, Goyang-si,

10540, Korea 4 Department

of Chemical Engineering, Pohang University of Science and Technology

(POSTECH), Pohang 790-784, Korea

Author Information * Corresponding Author. Tel.: +82-10-7471-3501 E-mail

address:

[email protected]

ACS Paragon Plus Environment

(D.

J.

Yun)

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT To ensure stable operation during use, the fundamental characteristics of each component in devices must be maintained. Organic electrodes of poly(3,4-ethylenedioxythiophene) polymerized with poly(4-styrenesulfonate) (PEDOT:PSS) have many deficiencies in this regard despite being widely used in various fields. Here, on the basis of various analytical results, we clearly identify not only significant denaturation of PEDOT:PSS at high temperature but also stability improvement by doping with multiwalled carbon nanotubes (MWNTs). As the thermal annealing temperature increases, the ratio of C–S to S–Ox bonds in PEDOT:PSS films becomes higher owing to chemical changes and decomposition of PEDOT:PSS molecules. Further, obvious declines in work function and electrical conductivity occur. On the other hand, the MWNT chains in a MWNT/PEDOT:PSS composite form conductive charge carrier paths resembling a densely intertwined web. Thanks to its excellent thermal and electrical conductivity, the MWNT/PEDOT:PSS composite can serve as an electrode even after high-temperature annealing, despite denaturation of the PEDOT:PSS molecules.


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1. Introduction Conducting polymer films of poly(3,4-ethylenedioxythiophene) polymerized with poly(4styrenesulfonate) (PEDOT:PSS) have been employed as electrodes in various devices thanks to their unrivaled electrical, mechanical, and optical advantages.1-4 Further, continuous process development has yielded PEDOT:PSS films with an electrical conductivity of more than 1000 S/cm, an optical transmittance of more than 90%, and a work function of more than 5.0 eV without loss of flexibility.1-4 These characteristics of PEDOT:PSS films are evidently

fundamental

to

obtaining

excellent

electrode

performance

in

diverse

organic/inorganic electronics applications, such as organic photovoltaics, organic lightemitting diodes, organic memory devices, organic thin-film transistors, dye-sensitized solar cells (DSSCs), and thermoelectronic devices.1,4-6 As many research groups have reported, the molecular configuration between the conductive PEDOT and insulating PSS chains is a crucial factor determining the properties of PEDOT:PSS films.1-6 In practice, several effective processes for controlling it have become widely known; representative methods are organic compound doping, polar organic solvent treatment, acidic solution treatment, and annealing.1-4,7 These methods produce conformational changes between PEDOT and PSS molecules as well as selective purging of PSS molecules. However, despite these significant advances, certain challenges regarding PEDOT:PSS films must be overcome before they can be applied in industry. Examples include instability against reactive gases, high electrical bias, and high temperature. Combining carbon nanotubes (CNTs) and PEDOT:PSS is another efficient way to not only address these shortcomings but also further improve the strengths of PEDOT:PSS materials.811

The metallic nature of multiwalled carbon nanotubes (MWNTs) has strong potential for enhancing the stability of PEDOT:PSS materials.11-14 MWNTs generally display excellent



electrical conductivity, catalytic activity, and flexibility. Moreover, in comparison with PEDOT:PSS, MWNTs are much more stable against thermal, chemical, or mechanical impact. For these reasons, many researchers have been investigating methods for fabricating composites of PEDOT:PSS and MWNTs. In particular, we found a simple way to disperse MWNTs into a PEDOT:PSS solution using sonication. Through our unique method, MWNTs and PEDOT:PSS can be mixed in various concentrations in MWNT/PEDOT:PSS composite films without significant degradation of their inherent material properties. Furthermore, the electrode performance of MWNT/PEDOT:PSS composite with more specialized processes was also developed using ultraviolet (UV)–ozone or organic/acidic treatment.3,15,16 Although MWNT/PEDOT:PSS composite films are clearly more stable than PEDOT:PSS films, there are few studies that clearly identify how MWNTs affect the stability. A comparative study on the thermal stability is the best way to identify the differences between a PEDOT:PSS film and an MWNT/PEDOT:PSS composite. For these reasons, we investigated the chemical and electronic structure changes after thermal annealing at various temperatures using UV photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). UPS/XPS analyses not only provide accurate information on the chemical and electronic structures but are also suitable for studying both the surface and interior regions of organic films, in combination with specialized in-situ deposition/sputtering processes. This study clearly describes the differences in the behavior of PEDOT:PSS and MWNT/PEDOT:PSS films after high-temperature annealing.

2. Experimental The PEDOT:PSS solution (CLEVIOSTM PH1000, 1–1.3 wt.% in water) and MWNTs were purchased from H.C. Starck and Carbon Nano-material Technology Co., respectively. By using a specialized combination of MWNTs and the PEDOT:PSS solution, different types of


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MWNT/PEDOT:PSS composite films were fabricated. Specifically, in addition to the aspurchased MWNTs, two types of functionalized MWNTs, UV-ozone treated MWNT (UVMW) and acid-treated MWNT (OMW), were prepared by UV–ozone and acidic solution (H2SO4/HNO3 mixture with 3/1 volumetric ratio) treatments, respectively.15 As-purchased MWNTs (MW, 0.2 g) were added to the diluted PEDOT:PSS solution (10 mL of PEDOT:PSS and 10 mL of deionized water), and the solution was sonicated for 30 min to prepare the basic MWNT/PEDOT:PSS solution. Similarly, 0.3 g of OMW and 0.2 g of UVMW were combined with 20 mL of PEDOT:PSS solution and glycerol-doped PEDOT:PSS solution (0.5 mL of glycerol and 20 mL of PEDOT:PSS), respectively.17 Each of the MWNT/PEDOT:PSS films was fabricated on a 300-nm-thick SiO2/N++Si substrate by spin-coating (5000 rpm) at room temperature, followed by annealing at 100 °C in a vacuum oven for 2 h. Subsequently, a PEDOT:PSS film prepared using the PEDOT:PSS solution (denoted as PE) and MWNT/PEDOT:PSS films were fabricated using each type of MWNT/PEDOT:PSS solution, specifically: sample 02 MW, prepared using 0.2 g of OMW in dilute PEDOT:PSS; 02 UVGL, prepared using 0.2 g of UVMW in glycerol-doped PEDOT:PSS; 03 OMW, prepared using 0.3 g of OMW in PEDOT:PSS; and 0.4 Coil, prepared by spin-coating using 0.4 g of coiled CNTs in glycerol-doped PEDOT:PSS). The sheet resistance and thickness changes in the MWNT/PEDOT:PSS films were measured using a four-point probe method (Keithley 2400 sourcemeter) and X-ray reflectometry (XRR), respectively. In accordance with specific thermal annealing process, in-situ UPS analysis with organic semiconductor (OSC) deposition was performed in home-made in-situ analysis equipment. The UPS analysis was carried out using He II (hν: 40.8 eV) photon source under applied voltage of 5 V. Meanwhile, the pentacene OSC was deposited with thermal evaporation process at the rate of 0.2 ~ 0.3 Å/s. In addition, after the sample was moved to commercial



photoelectron spectroscopy (PES) equipment (Versaprobe, ULVAC-PHI), the chemical and electronic structures in the interior of the MWNT/PEDOT:PSS (with or without pentacene deposition) were investigated using UPS (He I of hν: 21.2 eV and beam diameter: ~5 mm) and XPS (Al Kα source of hν: 1486.6 eV beam diameter: 100 μm) depth profiles in conjunction with Ar gas cluster ion beam (GCIB) sputtering. An acceleration voltage of 5 kV and a raster size of 5

5 mm2 were chosen as the default settings for the Ar GCIB

sputtering process; the methods used for determining the electrode work function and the energy level alignment at interface region are summarized in detail elsewhere.15-17 Finally, PE and 02 MW films before and after thermal annealing were used as catalytic counter electrodes (CEs) in DSSCs; the device fabrication and characterization methods are summarized in detail elsewhere.16,17

3. Results and discussion In contrast to inorganic or metallic materials, the characteristics of organic materials are more easily degraded by reactive gas, solvent, ion, or heat treatment.18-20 Further, investigation of the properties of organic materials or organic–organic interfaces by conventional processes presents difficulties.21-23 Therefore, to address these problems, optimization and technical development of analytical methodologies are pursued. As a representative example, UPS/XPS analysis, which is widely used as the most efficient analytical tool to study the chemical and electronic states of organic and inorganic materials, has demonstrated many methodological advances, such as in-situ deposition, low-damage sputtering, time-resolved spectroscopy, the use of hard X-ray sources, and ambient pressure/temperature/operando analysis.23-25 Thus, the use of this analytical method recently became feasible for accurate measurement of both the surface and interior regions of organic materials or organic–organic interfaces, in combination with specialized in-situ


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deposition/sputtering processes.23-26 Our group has built a UPS/XPS analysis system designed for organic materials that employs specialized functionalities: in-situ thermal annealing, organic molecule deposition, and low-damage Ar GCIB sputtering.23,26,27 This system allows us to design an experimental setup

for

studying

the

thermal

degradation

mechanisms

of

PEDOT:PSS

and

MWNT/PEDOT:PSS composite films, as shown in Figure 1a and b. Various types of PEDOT:PSS and MWNT/PEDOT:PSS composite films were prepared using well-known processes, that is, organic compound doping, acidic solution, or polar solvent treatment.1-4,7 Then, each film was loaded into an in-situ home-made PES system. UPS results obtained at specific stages of thermal annealing or organic molecule deposition offer significant information about the process of material degradation. After the in-situ UPS measurements, the chemical and electronic structures in both the surface and inner regions of the samples were explored further using the XPS/UPS depth profile in conjunction with Ar GCIB sputtering. All of the UPS/XPS data were important resources for clarifying the differences between the degradation processes of PEDOT:PSS and MWNT/PEDOT:PSS. In addition to as-purchased PEDOT:PSS films (PH1000: PE and PH500: PE_PH500), different types of PEDOT:PSS films were also prepared by organic compound doping (a glycerol-doped film, PEGL, and a dimethyl-sulfoxide-doped film, PEDM) and hydrochloric acid treatment (HCl_PE).3,16 Then, the changes in their electronic structure after annealing were characterized using in-situ UPS analysis. As shown in Figures 2a–f and S1a–d, the secondary cut-off and valence band regions of the UPS spectra provide information about the work function and electronic structure, respectively, of the PEDOT:PSS films.3,15,26 Their valence band structures, which are composed of σ and π hybrid states among the C 2s, C 2p, O 2s, O 2p, and S 3p orbitals, are directly connected with the energy level alignment, including the work function. Consequently, all the as-deposited PEDOT:PSS films show



similar work functions of approximately 5.0 eV, arising from their similar valence band structures Furthermore, we investigated the changes in work function and valence band structure after thermal annealing in comparison with the reported UPS spectra of poly(styrenesulfonic acid) and PEDOT materials.28,29 As a result, we determined that the electronic structures revealed in the UPS spectra directly indicate the degree of denaturation of the PEDOT:PSS films. Except for HCl_PE, the PEDOT:PSS films exhibited increases in the work function and chemical state at approximately 5 eV after thermal annealing at 150 °C, owing mainly to a slight increase in the PSS/PEDOT ratio. However, their work function, as well as the PSS/PEDOT ratio, decreased rather than increased after annealing at temperatures above 200 °C; ultimately, all the PEDOT:PSS films entirely lost their inherent valence band structure as well as work function after thermal annealing at 350 °C. To identify the cause of these changes, the chemical, electronic, and physical structures were also investigated through XPS, sheet resistance, and XRR measurements. The atomic components of PEDOT:PSS films are carbon, oxygen, sulfur, hydrogen, and sodium atoms, and their chemical combinations, including the composition and chemical bonds, are crucial factors in determining the films’ characteristics. Thus, the XPS results, which were obtained after annealing, provide crucial information on the changes in the PE films depending on the annealing temperature. Figure 3a–c show the XPS C 1s, O 1s, and S 2p core-level structures of the PE films, respectively, which were annealed at 100, 200, and 250 °C for 1 h. The C 1s core-level structures in Figure 3a represent four chemical states: the main asymmetric peak at 284.8 eV is assigned to carbon–carbon sp3 (C–C) and sp2 (C=C) bonds, the peak at 286.2 eV is assigned to the carbon–sulfur (C–S) bond, the peak at 287.4 eV is assigned to the carbon–oxygen (C–O) bond, and the tiny peak at 291.1 eV indicates the π–π* transition.3 As the annealing temperature increases from 100 to 250 °C, there is little


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change in the C 1s core-level structures of PE; only the peak area of the C–S bond gradually decreases. In addition, considering that the C–S bond in the C 1s core-level structure derives from the C–S bond in PEDOT and S–Ox bonds in PSS, the O 1s and S 2p core-level structures allow us to elucidate which of them changes more. After high-temperature annealing, the S–Ox bond in PSS decreases considerably compared to the C–O bond or C–S bond in PEDOT; consequently, the ratio of C–S bonds to S–Ox bonds grows. The chemical composition changes in Figure 3d and e illustrate the decrease in S–Ox bonds in PE as the annealing temperature increases. In addition, the XPS core-level structures in the inner region of the PE films were further studied using the XPS depth profile in conjunction with Ar GCIB sputtering. The Ar GCIB sputtering used in this study makes it possible to expose the inner region without significant chemical distortion by peeling organic material from the surface region at a stable etching rate of approximately 5 nm/min.22,23,26 Therefore, the XPS results in Figs. S2 and S3 allow us to directly compare the chemical information according to the vertical depth in the PE film. In addition to the core-level structures (C 1s, O 1s, and S 2p) of PE films annealed at 250 °C, the S 2p core-level structures of all the PE films annealed at 100, 200, and 300 °C show few differences in the chemical characteristics of the surface and inner regions (see Figure S3). This behavior of the S–Ox/C–S composition ratio provides a good criterion for evaluating the degree of thermal damage to PEDOT:PSS molecules. Therefore, the variations in the S– Ox/C–S ratio indicate that changes in the chemical structure resulting from thermal annealing occur throughout the PE films. Like the XPS results, the XRR results are also excellent indicators of the transition behavior of PEDOT:PSS films after annealing at various temperatures. The X-ray intensities of different PEDOT:PSS films as a function of incident angle were measured using a Cu Kα



beam source (λ = 1.54 Å). The best fit for each XRR spectrum enables us to determine the thickness, roughness, and density of the PEDOT:PSS film.3 As-deposited PE (As_PE) and PE treated with a H2SO4 solution (H2SO4_PE) were subjected to thermal annealing at temperatures of 100, 200, 300, and 400 °C, and XRR measurement was performed. Figure 4a–d reveal that the As_PE and H2SO4_PE films exhibit similar changes in their properties after annealing at each temperature. Specifically, they both exhibit significant changes in thickness, along with roughness variations, at annealing temperatures above 200 °C. In comparison with the thicknesses of PE films annealed at 100 °C (As_PE: 38.1 nm and H2SO4_PE: 14.6 nm), the thicknesses to decrease drastically at annealing temperatures above 200 °C and ultimately become at least three times lower at temperatures above 350 °C (As_PE annealed at 400 °C: 10.56 nm and H2SO4_PE annealed at 400 °C: 2.8 nm). We strongly believe that these thickness changes after thermal annealing are directly connected to the chemical structure changes. In addition, in-situ XRR measurements of PEDM and PEGL were also performed immediately after they were spin-coated on a substrate. The results indicate that the organic-compound-doped PEGL and PEDM both exhibit changes in thickness and roughness as a function of annealing temperature (see Figure S4a–d). The rate at which the thickness decreases indicates that glycerol doping acts as a partial buffer against thermal damage, but this effect appears only at temperatures below 200 °C. On the other hand, the MWNT/PEDOT:PSS composite has higher than several tens of surface roughness so that the atomic force microscopy (AFM) analysis was employed to figure out its morphology change on behalf of XRR measurement. As shown in Figure S5a and b, the highly entangled structure among MWNT chains in 02 MW is more clearly observed after thermal annealing process. We believe that such change originates from the considerable loss of PEDOT:PSS layers surrounding the MWNT chains. Before we present the sheet resistance results, we consider the XPS/UPS results of 02 MW.


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Figure 5 shows significant variations in the XPS C 1s, O 1s, S 2p, and Na 1s core-level structure (Figure 6a–d, respectively), and the atomic composition of 02 MW (Figure 5e) as a function of the thermal annealing temperature. The 02 MW sample, like the PE samples, exhibited significant changes in the chemical structure of PEDOT:PSS molecules at high temperatures despite the presence of MWNTs. As the annealing temperature increased, the chemical states of both C–O and C–S bonds at the C 1s core level decreased significantly; in addition, the C–O, C=O, and S–Ox bonds in the O 1s core level decreased together. In particular, after annealing at 350 °C, the S–Ox bond in the S 2p core level almost disappeared in 02 MW, and the ratio of the C–S bond (in PEDOT) to the S–Ox bond (in PSS) grew greatly as a result. Further, these changes in the chemical bonds are reflected in the atomic composition changes depending on the annealing temperature; the carbon composition gradually increased with decreasing oxygen or sulfur content. The XPS results in the surface regions do not clearly reveal whether PE and 02 MW behaves differently after thermal annealing. However, the XPS depth profiles of 02 MW (Figure S6a–f) are significantly different from those of PE (Figure S2a–e). In contrast to PE annealed at 300 °C, 02 MW annealed at the same temperature has a higher C–S to S–Ox ratio, and the atomic composition varies depending on the depth in the film. Figure S6a and e show that the concentrations of both C=C and C–C bonds are higher in the interior than at the surface of 02 MW. In other words, the concentrations of MWNT and PEDOT:PSS are notably higher in the inner and surface regions of 02 MW, respectively. Furthermore, the effects of the MWNTs on the thermal stability become clearer when the XPS results of PE and 02 MW films annealed at 250 °C are directly compared. As in the 02 MW sample annealed at temperatures above 300 °C, the MWNT concentration exhibits a gradient from the surface to the inner region, as shown in Figure 6a–c. Consequently, the S–Ox to C–S ratio is higher in the interior of 02 MW, where more MWNT chains appear. Further, regardless of



the depth, the numerical ratio remains higher than that of PE annealed 250 °C (Figure 6b and c). Taken together, these differences are not large, but the PEDOT:PSS molecules in 02 MW sustain less thermal damage than those in PE owing to the excellent thermal conductivity of MWNTs. The effects of MWNTs are more clearly observed in the electronic structure changes in the UPS spectra after annealing at each temperature. Specifically, the UPS spectra of different MWNT/PEDOT:PSS composites (Figure 7a and b, 02 MW; Figure 7c and d, 03 OMW; Figure 7e and f, 04 Coil) exhibit variations in the energy level alignment and valence band structure after annealing at each temperature. In a typical MWNT/PEDOT:PSS composite, most of the MWNT chains are covered with several layers of PEDOT:PSS molecules, and the concentration of PEDOT:PSS molecules is higher at the surface. Therefore, the electronic structures of MWNT/PEDOT:PSS composites depend mainly on the properties of PEDOT:PSS molecules below a certain temperature. However, at higher temperatures, the properties of MWNTs become more prominent in the electronic structure as both decomposition and chemical distortion of PEDOT:PSS molecules occur. The valence band structures in Figure 7b, d, and f vary with the type of MWNT, but their behavior after annealing at different temperatures is similar to that of PE films. On the other hand, the MWNT chains in the MWNT/PEDOT:PSS composites form conductive charge carrier paths resembling densely intertwined webs; moreover, their excellent thermal conductivity and stability allow them to maintain those features at high temperatures (above 350 °C). Therefore, the chemical configuration and energy level alignment could be maintained in the UPS spectra of the MWNT/PEDOT:PSS composites even after annealing at 350 °C. In addition, owing to the presence of the MWNT chains, the work functions of 02 MW, 03 OMW, and 04 Coil still remain above ~4.70, 4.88, and 4.40 eV, respectively after annealing at 350 °C, even though the PEDOT:PSS molecules completely lose their original


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characteristics. The numerical discrepancies among those work function values stem from differences in the degrees of surface functionality and entanglement of MWNT chains. Like the work function, as described above, the sheet resistance shows huge differences after annealing at temperatures above 300 °C depending on whether MWNTs are present. PE, PEGL, and PEDM films were prepared by doping with different organic compounds.3,17 The as-deposited PE, PEGL and PEDM films show sheet resistance values of 0.5 ± 0.25 MΩ/sq, 0.75 ± 0.2 kΩ/sq, and 1.0 ± 0.25 kΩ/sq, respectively (Figure 8a). After thermal annealing above 300 °C, the values for all of those samples exceed the measuring range (>20 MΩ/sq) of the equipment. We believe that these changes are due to chemical distortion and decomposition of PEDOT:PSS molecules during thermal annealing above 300 °C. On the other hand, the sheet resistance of MWNT/PEDOT:PSS composites shows completely different patterns of change after thermal annealing, as shown in Figure 8b and Table 1. Asdeposited films of 02 MW, 02 UVGL, and 03 OMW show sheet resistance values of 2.35 ± 0.45 kΩ/sq, 0.625 ± 0.13 kΩ/sq, and 56.9 ± 15 kΩ/sq, respectively. Regarding the electrical conductivity of MWNT/PEDOT:PSS composites, the PEDOT:PSS molecules in 02 UVGL have a significant role owing to glycerol doping, whereas those in 02 MW and 03 OMW do not.15-17 Accordingly, after annealing at 300 °C, the sheet resistance of 02 UVGL (24.7 ± 6 kΩ/sq) increases by approximately 40 times, but those of 0.2 MW (2.54 ± 0.5 kΩ/sq) and 03 OMW (72.3 ± 20 kΩ/sq) increase by 25% or less. Nevertheless, all of the MWNT/PEDOT:PSS composites maintain their electrical conductivity owing to the conductive charge carrier paths consisting of densely tangled MWNT chains. Next, a pentacene layer acting as a p-type semiconductor was deposited on a PEDOT:PSS or MWNT/PEDOT:PSS composite electrode after the electrode was thermally annealed at a specific temperature. Further, the changes in the energy level alignment at the pentacene/PE (or MWNT/PEDOT:PSS composite) interface were also investigated through in-situ UPS



analysis (see the schematic diagram in Figure 1). The UPS spectra, which were obtained at each stage of the pentacene deposition process, provide the position of the highest occupied molecular orbital (HOMO) level and the secondary cut-off. In accordance with the electronic structure of PE or the MWNT/PEDOT:PSS composite, the HOMO level shifts, and these changes correspond to changes in the hole injection barrier (ΦB) against pentacene in the interface region. After 12 min of pentacene deposition, the hole injection barriers were determined, as shown in Figs. 9a–h and S6. Before the denaturation reactions of PEDOT:PSS molecules occur, there is little difference between the ΦB values of the pentacene/PE and pentacene/02 MW structures. For example, the UPS spectra in Figure S7 show that PE and 02 MW annealed at 150 °C have nearly equal ΦB values of 0.50 and 0.52 eV against the pentacene layer. However, an obvious difference in ΦB is observed after PE and 02 MW are thermally annealed at temperatures above 300 °C. As shown in the UPS spectra in Figure 10a–d, the work function of PE decreases by 0.6 eV when we increase the annealing temperature from 325 to 325 °C. The work function of PE annealed at 325 °C (4.2 eV) is lower than that of PE. Along with this work function change, the pentacene layer grown on PE annealed at 325 °C has a significantly higher ΦB value of 1.20 eV compared to that grown on PE annealed at 300 °C. Further, PE annealed at 350 °C has a remarkably low work function of approximately 3.0 eV with ambiguous electronic structure in the valence band owing to significant denaturation of PEDOT:PSS molecules. As a result, the pentacene layer on it cannot be determined, as illustrated in Figure 9e and f. On the other hand, even after thermal annealing at 350 °C, 02 MW still has a work function of 4.7 eV. In addition, its pentacene layer has a ΦB value of 0.57 eV, which is comparable with that of the pentacene on 02 MW annealed at 150 °C. In the study described above, we clarified that adding MWNTs to PEDOT:PSS significantly affects the thermal stability.15-17 We are convinced that this improvement can be a major


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advantage when MWNT/PEDOT:PSS composites are used as CEs in DSSCs. Therefore, CEs of PE/fluorine-doped tin oxide (FTO) and 02 MW (with or without thermal annealing at 300 °C) were prepared with the structures shown in Figure 10a and b, respectively, and their performance in DSSCs was characterized. The cell performance parameters [short-circuit current density (Jsc), open-circuit voltage (Voc), fill factor (FF), and efficiency ()], which were determined using the current–voltage (I–V) characteristics of the DSSCs (Figure 10c and d), were used to compare the electrode performance.17,30,31 The DSSC with the asdeposited PE/FTO CE (Jsc = 6.2 mA/cm-2, FF = 26.7%, and η = 1.3%) displays better cell performance than that with the annealed PE/FTO CE (Jsc = 3.7 mA/cm-2, FF = 26.1%, and η = 0.72%). In contrast, the cell performance of the DSSC with 02 MW was significantly enhanced by thermal annealing. When the 02 MW CE that was thermally annealed at 300 °C was used, Jsc decreased slightly (from 13.7 to 12.4 mA/cm2), whereas slight increases in the FF (from 34.1% to 44.9%) and η (from 3.8% to 4.7%) were observed. Like PE, 02 MW undergoes significant denaturation and loss of PEDOT:PSS molecules during thermal annealing at 300 °C. Thus, MWNT chains with excellent catalytic activity become more exposed to the electrolyte without changes in their characteristics. Consequently, the enhanced charge transfer at the electrolyte/02 MW interface produces superior cell performance. These device characteristics elucidate that the thermal annealing process has the potential to enhance DSSC performance further through process optimization.

4. Conclusion Various types of PEDOT:PSS films and MWNT/PEDOT:PSS composite films were fabricated. Then, the changes in their characteristics after thermal annealing at various temperatures were investigated. Through comprehensive analysis including XPS/UPS analysis, we found that a significant quantity of the PEDOT:PSS molecules disappeared with



chemical and electrical changes at high temperatures above a threshold. As a result, all the PEDOT:PSS films, including those treated with organic compound doping or an acidic solution, completely lost the properties necessary for use as an electrode. On the other hand, even after thermal annealing at high temperatures, the MWNT/PEDOT:PSS composites could serve as an electrode owing to the presence of MWNT chains. The MWNT chains in the MWNT/PEDOT:PSS composites formed conductive charge carrier paths resembling densely intertwined webs. Further, their excellent thermal conductivity and stability prevented denaturation during high-temperature annealing. In summary, this study clearly identified not only the degradation process of PEDOT:PSS molecules but also the buffering effects of MWNT chains during high-temperature annealing.

Association content: Supporting information The Supporting Information is available free of charge on the ACS Publications website at DOI: UPS/XPS spectra, XRR results and AFM results of PEDOT:PSS providing the information on the film property changes of PE and 02 MW according to thermal annealing process.

Acknowledgment: This research was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (No. 2016R1C1B1012710).


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Figure captions Figure 1 (a) Schematic of PE and MWNT/PEDOT:PSS films. PE consists of the molecular arrangements between PEDOT and PSS. MWNT/PEDOT:PSS composite film has highly entangled MWNT chains covered with several layers of PEDOT:PSS shells. OSC: organic semiconductor. (b) Experimental design for examining the behavior of PE and MWNT/PEDOT:PSS films before and after high-temperature annealing. Figure 2 UPS spectra of PE films showing changes in electronic structure after annealing. Secondary cut-off (Sec. cut-off) region of (a) PE, (c) PEGL, and (e) HCl_PE and valence band region of (b) PE, (d) PEGL, and (f) HCl_PE. Figure 3 XPS (a) C 1s, (b) O 1s, and (c) S 2p core-level structures of PEs annealed at 100, 200, and 250 °C for 1 h, (d) composition ratios of carbon-containing chemical states obtained from C 1s core-level structures and (e) atomic composition derived from XPS core-level structures. Figure 4 XRR results for (a) PE and (c) H2SO4_PE samples after annealing at 100, 200, 300, and 400 °C. Changes in density and thickness of (b) PE and (d) H2SO4 PE samples after annealing at various temperatures based on best fit for each XRR spectrum. Figure 5 XPS (a) C 1s (b) O 1s, (c) S 2p, and (d) Na 1s core-level structures of 02 MW after annealing at 100, 200, 300, and 350 °C for 1 h and (e) atomic composition derived from XPS core-level structures. Figure 6 (a) Composition ratios of carbon-containing chemical bonds obtained from C 1s core-level structures of PE and 02 MW samples annealed at 250 °C, (b) S 2p core-level structures of 02 MW sample annealed at 250 °C, and (c) S–Ox/C–S ratio. Results were obtained using XPS depth profiles in conjunction with Ar GCIB sputtering. Figure 7 UPS spectra of various MWNT/PEDOT:PSS composites showing changes in electronic structure after annealing at different temperatures. Secondary cut-off region of (a)



PE, (c) PEGL, and (e) HCl_PE and valence band region of (b) PE, (d) PEGL, and (f) HCl_PE. Figure 8 Sheet resistance of various (a) PE and (b) MWNT/PEDOT:PSS composite films before and after thermal annealing at 300 °C. Figure 9. UPS spectra showing energy level alignment in pentacene/PE annealed at 300 °C in (a) secondary cut-off (Sec. cut-off) and (b) valence band regions and in pentacene/PE annealed at 325 °C in (c) secondary cut-off and (d) valence band regions. UPS spectra of PE in (e) secondary cut-off and (f) valence band regions and 02 MW in (g) secondary cut-off and (h) valence band regions showing changes in electronic structure for various annealing temperatures and pentacene deposition times. Figure 10. Schematic diagrams showing structures of DSSCs with (a) PE/FTO CE and (b) 02 MW CE. I–V characteristics of DSSCs with (c) PE/FTO CE and (d) 02 MW CEs, which show the catalytic activity of the CEs with and without annealing at 300 °C.


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Table 1. Sheet resistances of various kinds of PEDOT:PSS and MWNT/PEDOT:PSS composite. Each of those values was measured before and after thermal annealing process at 300 oC counter electrodes. Sheet resistance (ohm/sq) As-dep.

PEDOT:PSS

Annealed at 300 oC

PE

0.5 ± 0.25

PEGL

0.75 ± 0.2

PEDM

1.0 0.25

02 MW

2.35 ± 0.45 k

2.54 ± 0.45 k

02 UV

0.625 ± 0.13 k

24.7 ± 6 k

03 OMW

56.9 ± 15 k

56.9 ± 15 k

> 20 M

MWNT/PEDOT:PS S composite



Figure 1. Yun et al.


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