Extracting carrier mobility using a photoinduced charge transfer reaction: From conducting polymers to nanocarbon materials

Journal Pre-proof Extracting carrier mobility using a photoinduced charge transfer reaction: From conducting polymers to nanocarbon materials Lingyun Lyu, Kazuhiro Kirihara, Yuki Okigawa, Masataka Hasegawa, Wuxiao Ding, Ying Wang, Masakazu Mukaida, Ying Zhou, Qingshuo Wei PII:

S1566-1199(20)30001-X

DOI:

https://doi.org/10.1016/j.orgel.2020.105615

Reference:

ORGELE 105615

To appear in:

Organic Electronics

Received Date: 11 December 2019 Accepted Date: 1 January 2020

Please cite this article as: L. Lyu, K. Kirihara, Y. Okigawa, M. Hasegawa, W. Ding, Y. Wang, M. Mukaida, Y. Zhou, Q. Wei, Extracting carrier mobility using a photoinduced charge transfer reaction: From conducting polymers to nanocarbon materials, Organic Electronics (2020), doi: https:// doi.org/10.1016/j.orgel.2020.105615. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.

Extracting Carrier Mobility Using a Photoinduced Charge Transfer Reaction: From Conducting Polymers to Nanocarbon materials Lingyun Lyu, a Kazuhiro Kirihara, b Yuki Okigawa, b Masataka Hasegawa, b Wuxiao Ding, b Ying Wang, bd Masakazu Mukaida, *ab Ying Zhou, *ac and Qingshuo Wei*abd a

AIST-UTokyo Advanced Operando-Measurement Technology Open Innovation Laboratory (OPERANDO-OIL), National Institute of Advanced Industrial Science and Technology, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan b

Nanomaterials Research Institute, Department of Materials and Chemistry, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan

c

CNT-Application Research Center, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan d

Precursory Research for Embryonic Science and Technology (PRESTO), Japan

Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan

Abstract Charge-carrier mobility is an important and crucial performance criterion for organic semiconductors and nanocarbon materials such as carbon nanotubes (CNTs). Although high conductivity has been achieved in highly doped organic semiconductors and CNT networks, reliable measurements of charge-carrier mobility in such disordered materials remain a challenge. Here, we developed and extended a new mobility measurement method, referred to as photoinduced charge transfer, from conducting polymers to nanocarbon materials. We apply this method to examine the carrier mobility of graphene, where the carrier mobility can be extracted using a Hall-effect measurement. By comparing the mobility values obtained using these two methods, we can understand the potential errors in the mobility values extracted using photoinduced dedoping reactions. We further extend this approach to disordered CNT networks and determine the relationship among CNT structure, defects, and carrier mobility.

KEYWORDS: Conducting Polymers, Carbon Nanotubes, Graphene, Carrier Mobility, Carrier Density

INTRODUCTION Organic semiconductors and nanocarbon materials, such as graphene and carbon nanotubes (CNTs), have attracted tremendous attention because of their potential applications in flexible and low-cost electronic devices.[1] The electrical conductivity (σ) of p-type materials has been significantly improved in recent years via the doping of high-mobility semiconductors using oxidative agents or protonic acids.[2-6] Even though the electrical conductivity of doped materials can be accurately measured using four-probe methods, the carrier density (n) and carrier mobility (µ) after doping are typically unknown.[7, 8] Traditional measurements, such as thin-film transistors (TFTs), the time-of-flight technique, and space charge limited current methods, are usually only applicable for materials without doping or with a low doping density.[9-12] Hall-effect measurements are limited by structural disorders, which make the interpretation of the Hall coefficient difficult.[2, 13-15] To understand the transport properties of these materials and propose novel approaches toward making better materials, a reliable measurement method of carrier mobility and doping density is necessary at this stage. Recently, we proposed a novel method to control the carrier mobility in a conducting

polymer,

poly(3,4-ethylenedioxythiophene)/polystyrene

sulfonate

(PEDOT/PSS), using a photoinduced charge transfer reaction.[16, 17] The idea is to introduce

a

photobase

generator,

2-(9-oxoxanthen-2-yl)propionic

acid

1,5,7-triazabicyclo[4.4.0]dec-5-ene salt (PBG), on the conducting polymer films. During UV irradiation (Scheme 1), a strong base, 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD), forms at the interface via a photodecarboxylation reaction.[18] TBD molecules subsequently transfer the electrons to the conducting polymers. If we know the number of charges transferred from the photobase generators to the conducting polymers, the

carrier mobility can be extracted from the conductivity change. This consideration is similar to the extraction of carrier mobility from a thin film transistor in the depletion mode.[19] The molecular density at the interface is higher than 1014 cm−2, which is more than two orders of magnitude than the charge density in typical TFTs introduced by using SiO2 as the dielectric layer. It is suggested that the conductivity change may be observable even if the initial doping density is high. Combining the number of TBD formations during irradiation, the carrier mobility can be calculated using the following equation: =(

× ℎ × )/( ×

× (1 − 10

) × Φ ), (1)

where G is the conductance, h is the Planck constant, c is the speed of light, e is the elementary charge, Φλ is the reaction yield, λ is the wavelength of light, I0 is the light intensity, and ABS is the absorbance of the first layer at λ. Using this approach, we successfully demonstrated that the addition of co-solvents increased both the carrier mobility and the carrier concentration in PEDOT/PSS films.[17] In this study, we attempt to characterize the device structure using scanning tunneling electron microscopy and confirm the accuracy of this method using standard samples. By comparing the mobility values extracted using the photoinduced charge transfer reaction and the Hall-effect measurement of standard samples, we can determine the potential errors inherent to this method. Besides different types of conducting polymers, we further extend this approach to disordered CNT networks and determine the relationship among carrier mobility, nanotube structure, and defects.

EXPERIMENTAL SECTION

Chemicals. PEDOT/PSS (Clevios PH1000 and PH500) was purchased from H.C. Starck. Ethylene glycol (EG) and PBG were purchased from TCI Chemicals. Methanol (super dehydrated) and polyacrylic acid (PAA) with MW values of 5000, 25,000, and 1,000,000 g/mol were purchased from Wako Chemicals. Single-walled CNTs (EC1.5-P) and Double-wall CNTs (EC1.5-DP) were obtained from Meijo Nano Carbon. The Tuball single-walled CNT was purchased from OCSiAl. Monolayer Graphene on polyethylene terephthalate (PET) was provided by AirMembrane Corporation.

Film preparation. The PEDOT/PSS film was first spin-coated onto glass pretreated with a UV-ozone cleaner and then annealed at 150°C for 10 min in air. EG rising was used to enhance the conductivity of the PEDOT/PSS film. CNT films were also prepared via spin coating. First, 0.33 mg/mL PAA was added to a mixture of ethanol and isopropanol at a volume ratio of 1:9 and stirred for 1 h. Then, 0.33 mg/mL CNT was added to the PAA solution and stirred vigorously overnight. Next, the dispersion was sonicated (Sonics VCX-750) for 10 min at an amplitude level of 50% cooling in a water bath at 10°C. The CNT–PAA films were prepared on glass substrates via spin coating and then dried at room temperature. The film thickness (transmittance) was controlled by varying the spin speed from 600 to 6000 rpm with a spin time of 120 s. A 1-mm-thick cross-linked PDMS (SILPOT 184, Toray) film was prepared in a polystyrene case and then cut to a size of 1 cm × 2.5 cm. The PDMS film was treated using a UV-ozone cleaner (ASUMI-GIKEN ASM401N) for 10 min. A total of 200 µl of a PBG methanol solution (20 mg/mL) was drop-cast onto the PDMS substrate, which was then heated on a hot plate at 80°C for 5–120 min. Then, the PBG/PDMS film was laminated on a PEDOT/PSS surface by hand.

Characterization. Scanning tunneling electron microscopy (STEM) was carried out using a JEM-ARM200F (JEOL), and EDX analysis was conducted using a JED-2300 detector. The acceleration voltage was 200 kV. Samples for STEM analysis were prepared using a cryo-focused ion beam (cryo-FIB, SMI3200SE SINNT). The film thickness was measured using a surface profilometer (Surfcoder ET 200, Kosaka Laboratory Ltd.). Fourier-transform infrared (FTIR) spectra were acquired using an ALPHA FTIR spectrometer from Bruker. A Hyper Monolight System (Bunko Keiki, SM-250) was used to control the light irradiation. The full width at half maximum (FWHM) was controlled at approximately 3 nm. The setup for the conductance measurement was constructed in-house. The probes were made of gold-plated copper. The conductance change with time was recorded using a source meter (Keithley 2400), and the Hall effect was measured using a Resitest8300 (Toyo Corporation) with a magnetic field of 0.55 Tesla (T). Raman spectroscopy was performed using a RamanStation 400 (PerkinElmer) spectrometer with an excitation laser at 785 nm.

RESULTS AND DISCUSSION

Device structure To estimate the carrier mobility using a photoinduced dedoping reaction, the first step is to form a PBG film on the target samples. A bilayer structure is ideal because we can calculate the molecular density at the interface and build a model to extract the carrier mobility. Making a bilayer structure using orthogonal solvents is possible; however, it should not be the general method, because the solvents need to be optimized

depending on the sample properties. We attempt to construct a bilayer structure using a PDMS stamp. As shown in Figure 1a, a PDMS film with 1 mm thickness was first prepared. This PDMS substrate was treated with a UV-ozone cleaner for 10 min. Then, a 200 µL PBG methanol solution was drop-cast onto the PDMS substrate. After drying on a hot plate at 80°C for 5–120 min, a PDMS/PBG film was laminated on a target film to form a bilayer structure. The setup for the conductance measurement was constructed in-house (Figure 1b). UV light was irradiated from the sample side. The probes were made of gold-plated copper (Figure 1c), and FWHM was controlled at approximately 3 nm (Figure 1d). The key point is to form a uniform and homogeneous PBG film on the PDMS substrates. We studied different solvents and found that methanol is better in terms of film quality than water and other alcohols such as ethanol, isopropanol, or butanol. Drop casting a methanol solution of PBG on PDMS can form an amorphous transparent film. Films cast from other solvents crystallize and accumulate at the substrate corner or form ring-like shapes, which is known as the coffee-ring effect.[20] One reason why a methanol solution suppresses this effect may be related to its low surface tension.[21] This makes the PBG solution spread over the PDMS surface. Another reason is that PBG forms a strong hydrogen bond with methanol, making PBG films difficult to crystallize. As shown in Figure 2, even after we tried the PBG film at 80°C for more than 120 min, a strong and broad O–H stretching vibration was observable at approximately 3500 cm−1 in the PBG films. This suggests that the methanol was not completely volatilized. The amorphous nature of the PBG film is beneficial for film transfer; however, the next issue is whether methanol affects the bilayer structure during the lamination.

To confirm the film structure, a cross-sectional STEM observation was performed. The sample was prepared using a cryo-FIB. As shown in Figures 3a (bright field) and 3b (dark field), individual layers of glass, PEDOT/PSS, and PBG can be clearly distinguished in the STEM images without any physical damage. To obtain a closer view of the film structure, a STEM cross-sectional image with higher magnification is shown in Figure 3c. The STEM energy-dispersive X-ray spectrometry and elemental (S) maps of the cross section are shown in Figure 3d. The interface between the PEDOT/PSS layer and the PBG layer is observable, which suggests that the residual methanol in the PBG layer does not significantly affect the layered structure during lamination. Please note that TBD may diffuse into the target samples along with the dedoping reaction, which could cause interfacial mixing. It should not affect the calculation of carrier mobility because extracting the carrier mobility is from the starting point of UV irradiation. The small effect of residual methanol in PBG is also supported by the mobility measurement results. As shown in Figure 4, the average mobility values (from 5 to 14 samples for each point) for PEDOT/PSS remained nearly constant when the drying time of PBG changed from 10 to 120 min. The carrier mobility of PEDOT is near 10 cm2/V s, which is close to the mobility for trap-free organic single crystals and is reasonable because of the fraction of mobile charges filling the disorder-induced traps.

Comparison with Hall-effect measurements using reference samples The accuracy and absolute value of the carrier mobility extracted using the photoinduced dedoping reaction are affected by the quantum yield for TBD formation and charge transfer efficiency at the interface. Direct measurements of these parameters

at the interface or inside the matrix of conducting polymers are challenging. We used the quantum yield of TBD formation in the bulk state, assuming that the charge transfer efficiency from TBD to p-type materials is 100% because of the high pKa value of TBD. One way to verify this assumption is to choose a standard sample and compare the mobility value extracted using photoinduced dedoping reactions. Graphene is ideal for this purpose. For a single layer of graphite with an ordered structure, the charge-carrier mobility can be accurately extracted using a Hall-effect measurement. The doping mechanism of p-type graphene is also close to that of p-type organic semiconductors and CNTs, which is due to oxidative agents or protons.[22] A comparison between the mobility value of graphene using a Hall-effect measurement and using photoinduced dedoping reactions should conclusively demonstrate the accuracy of the proposed approach. The Hall effect was measured using a Resitest8300 (Toyo Tech) with a magnetic field of 0.55 T. Four-point electrodes with a van der Pauw geometry at the sample corners were utilized. The sample size was 1 cm × 1 cm on PET substrate, which is about the same as the samples used for photoinduced dedoping. Under a DC current of 10 mA, the Hall voltage is approximately 10 mV, which is three orders of magnitude higher than the noise level. The Hall voltage also has an isotropic nature with an anisotropic difference of less than 2%. The calculated Hall mobility of the graphene sheet is approximately 900 cm2/V s, and the sheets are p-type doped with a carrier density of approximately 1013 cm−2. The same graphene sheet was used for the photoinduced dedoping experiments. As shown in Figure 5, for the graphene sheet without PBG, the conductance remains nearly constant despite being exposed to 30 µW/cm2 of 340 nm UV irradiation, suggesting that

graphene is stable under this irradiation. For graphene−PBG films, the conductance starts to decrease once the films are exposed to UV irradiation at the same level of conductance. From the slope of the conductance change, the calculated carrier mobility is approximately 800 cm2/V s. This value is close to the carrier mobility extracted using the Hall-effect measurement. The measured average Hall mobility of graphene was 722 ± 336 cm2/V s (calculated from 12 samples), and the average carrier mobility obtained from the calculated mobility using a photoinduced charge transfer reaction was 742 ± 157 cm2/V s (calculated from five samples), which was expected to be nearly identical to the Hall mobility. This result verified the reliability of mobility measurements using photoinduced dedoping and suggests that the assumptions used for the calculation are reasonable. This approach could also easily be used to compare the carrier mobilities of different samples. For example, graphene sheets and 30 nm PEDOT/PSS have similar values of conductance (Figure 5); however, the slope of the conductance change measured under the same conditions is much larger (approximately 100 times) for graphene, suggesting a 100-magnitude-higher carrier mobility. We have compared the carrier mobility of different types of PEDOT/PSS films. As summarized in Table 1, a wide range of carrier mobility could extract by using this approach.

Extension to disordered carbon nanotube networks CNT networks are highly conductive and porous, which is beneficial for electronic and ionic transportation. PAA-doped CNT films show sheet resistances of 60 Ω/square at 84% optical transmittances and could therefore be used as transparent electrodes

instead of indium tin oxide.[6] CNTs with controlled doping levels could be an ideal material for thermal energy conversions. For these types of applications, understanding the carrier mobility is critically important. Unfortunately, similar to the case of conducting polymers, the charge-carrier mobility is difficult to obtain using a Hall-effect measurement owing to the structural disorder.[23] Here, we extend the mobility measurement method using the photoinduced dedoping reaction to CNT films. Three widely used commercial CNTs are selected in this study: single-walled CNTs (eDIPS SWCNTs), double-walled CNTs (eDPIS DWCNTs) by enhanced Direct Injection Pyrolytic Synthesis method (Meijo eDIPS), and Tuball SWCNTs (OCSiAl). All the films were prepared using the same process. As shown in Figures 6a–6c, all these films show a disordered spaghetti structure and uniform CNT bundles with widths of approximately 10–20 nm. Resonance Raman scattering (excited at 785 nm) in the low-frequency region is assigned to the radial breathing mode vibration of the CNTs (Figure 6d).[24] Tuball nanotubes show a single peak at around 170 cm-1, indicating that the single-wall CNTs with an average diameter of 1.5 nm are dominated.[24] On the other hand, eDIPS SWCNTs and DWCNTs show two main peaks at 170 cm and 270 cm-1, corresponding to DWCNTs with an outer and an inner tube of 1.5 nm and 0.7 nm, respectively. It implies that commercial eDIPS CNTs composes both SWCNTs and DWCNTs. The higher intensity ratio of inner and outer tubes for eDIPS DWCNTs suggests that they have much more DWCNTs. Moreover, Figure 6e shows typical G- and D-bands at around 1600 and 1310 cm-1, respectively. The intensity ratio (G/D) of G- and D-bands is usually utilized to evaluate the quality of CNTs. The G/D ratios for eDIPS SWCNTs and DWCNTs and Tuball SWCNTs are 14, 13, and 6, respectively. Therefore, CNT-PAA film using Tuball

SWCNTs have the largest structural defects, PBG films were laminated on all these films, and the carrier mobility was successfully extracted using photoinduced dedoping reactions. The conductance change during UV irradiation is shown in Figure S1. As summarized in Table 2, eDIPS DWCNT films show the highest carrier mobility of approximately 116 cm2/V s. eDIPS SWCNTs films show an average mobility of 56 cm2/V s, and Tuball CNT films have a mobility of approximately 12 cm2/V s. The carrier mobility in the CNT networks could be affected by many reasons including the difference in the number of CNT-CNT junction, the CNT bundle size, the ratio between semiconducting CNT and metallic CNT, and the defect concentration in CNT.[25] One of the straightforward explanation here is the structure and properties of the CNTs. For SWCNTs, the carrier mobility may be limited by defect concentration. From the Raman spectra shown in Figure 6e, Tuball SWCNTs have the lowest G/D ratio of 6, suggesting the presence of the largest amount of sidewall defects. As a result, they show the lowest mobility than the other samples. The eDIPS SWCNTs show a higher G/D ratio of 14, and also higher carrier mobility. For the DWCNTs, the coaxial structure results in intact inner walls (i.e., a reduced number of sidewall defects on the inner walls), which could contribute to high carrier mobility.[26] Furthermore, DWCNTs could behave as metallic, which will enhance the drift mobility in the CNT networks.[27] Please note that eDIPS SWCNTs also contains a certain amount of DWCNTs, which could make the film higher mobility. The carrier mobility of semiconducting carbon nanotubes could also be extracted using thin film transistors if the doping density is not high. The values from transistor measurement are in the same range as we extracted using photoinduced dedoping reactions.[28, 29]

CONCLUSIONS In conclusion, we developed and extended a new mobility measurement method, referred to as photoinduced charge transfer, from conducting polymers to nanocarbon materials. By studying the carrier mobility of graphene and comparing it with Hall-effect measurements, we verified the validity of this approach. We successfully extended the approach to disordered CNT networks and provided evidence of the relationship between CNT structure and carrier mobility. We demonstrated that DWCNTs have higher carrier mobility than SWCNTs. The carrier mobility of SWCNTs is limited by the defect concentration in the film. This method can potentially be applied to a wide range of p-type conducting materials and opens new opportunities toward understanding the carrier mobility and other material properties, including thermoelectric properties. Measurements of n-type materials using photoacid generators are currently in progress.

Supporting Information.

Conductance as a function of UV-irradiation time for CNT–PBG films (PDF)

Corresponding Author

*E-mail for Q.W.: [email protected]., M.M.: [email protected] or Y.Z.: [email protected]

Acknowledgments This research was partly supported by JST, PRESTO, JPMJPR17R1.

Scheme 1. Photoinduced dedoping reaction of p-type materials using a photobase generator.

Figure 1. (a) Schematic of the device fabrication process for the mobility measurement used for the photoinduced dedoping reaction. Photographs of the (b) measurement setup and (c) electrode geometry. (d) Optical spectra of the UV lamp used in this study. The full width at half maximum (FWHM) is approximately 3 nm.

Figure 2. FTIR spectra of a PBG film dried at 80°C for 120 min.

Figure 3. Cross-sectional (a) bright-field and (b) dark-field STEM images of PBG–PEDOT/PSS films. (c) A high-magnification STEM image and (d) sulfur maps of PBG–PEDOT/PSS films.

Mobility (cm2/Vs)

103 102 101 100 10-1 10-2

0

20

40

60

80

100

120

Drying Time at 80 °C (min) Figure 4. Carrier mobility extracted from the photoinduced dedoping reaction plotted as a function of the drying time of PBG films at 80°C.

0.0025 0.0020 0.00193

0.0015

Conductance G (S)

Conductance G (S)

0.0030

0.0010

0.00192 0.00191 0.00190 0.00189

0.0005

0.00188

0.0000

0

10

20

30

Time t (s)

0

10

20 Time t (s)

30

40

Figure 5. Conductance as a function of UV-irradiation time for a graphene–PBG film (closed circles) and a PEDOT/PSS–PBG film (open circles). The inset shows the same result for the PEDOT/PSS–PBG film at a different Y-scale.

Table 1. Carrier mobility of different films Sample

Carrier mobility (cm2/V s)

PEDOT/PSS (PH1000, without additive)

5.9×10-2 ± 3.7×10-2

PEDOT/PSS (PH1000, with EG)

13 ± 6

PEDOT/PSS (PH500, without additive)

1.7×10-2 ± 0.3×10-2

PEDOT/PSS (PH500, with EG)

10.1 ± 4.2

Graphene (AirMembrane)

742 ± 157

Figure 6. SEM images of films of (a) eDIPS SWCNTs, (b) Tuball SWCNTs and (c) eDPIS DWCNTs. (d) and (e) Raman spectra of different samples.

Table 2. Carrier mobility of different CNT filmsa Sample

Carrier mobility (cm2/V s)

Tuball

11.7 ± 9.2

eDIPS SWCNT

55.8 ± 36.5

eDIPS DWCNT

116 ± 55.8

a. The average mobility value and standard deviations were calculated from 13–18 devices for each condition.

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Graphical Abstract

Highlights

1. A method to extract carrier mobility in conducting polymers and nanocarbon materials was developed.

2. The reliability of this method is verified by using Graphene as a standard sample. 3. This method is extended to study the relationship between CNT structure and carrier mobility.