Extracting Carrier Mobility in Conducting Polymers Using a

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C: Energy Conversion and Storage; Energy and Charge Transport

Extracting Carrier Mobility in Conducting Polymers Using a Photoinduced Charge Transfer Reaction Qingshuo Wei, Masakazu Mukaida, and Takao Ishida J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b04885 • Publication Date (Web): 03 Jul 2018 Downloaded from http://pubs.acs.org on July 9, 2018

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The Journal of Physical Chemistry

Extracting Carrier Mobility in Conducting Polymers Using a Photoinduced Charge Transfer Reaction Qingshuo Wei*,†,‡,§, Masakazu Mukaida*,†,‡, and Takao Ishida†,‡ †

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 ‡

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 §

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

Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan Corresponding Author * [email protected] * [email protected]

ACS Paragon Plus Environment

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ABSTRACT.

Recently, the electrical conductivity of conducting polymers has been significantly improved. Unfortunately, there are nearly no reports on the carrier mobility and carrier density measurements at this stage because traditional measurement approaches are difficult to apply in highly doped disordered systems. In this study, we developed a method to extract the carrier mobility in a highly doped disordered system using a solid state photoinduced charge transfer reaction. Combining the number of the charges transferred from photobase generators to the conducting polymers, the carrier mobility can be extracted from the conductivity change. This approach was successfully applied to conducting polymers with different conductivities. Potentially, this method could also be used to measure the carrier mobility of other disordered systems such as p-type and n-type carbon nanotube films.


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1. INTRODUCTION The charge carrier mobility is a key performance criteria for organic semiconductors.1-2 For undoped or low doped organic semiconductors, the charge carrier mobility can be measured using thin film transistors (TFTs)3, the time-of-flight (TOF) technique4, space charge limited current (SCLC) methods5, and time-resolved microwave conductivity measurements (TRMCs)6. This is because the charge number introduced from the electrode or irradiation by these approaches is much higher than the initial charge number inside the semiconductors. Conversely, for highly doped organic semiconductors, extracting the carrier mobility is challenging using the above characterizations due to the high initial carrier concentration. One approach is to use a Hall effect measurement, which is similar to doped inorganic semiconductors. This works for organic semiconductors with ordered structure and high carrier mobility.7-11 In an ordered structure, the charge carrier can be viewed as a free electron and treated within the effective mass approximation. Conversely, in disordered systems, the charge carriers hold very different mobility values depending on the morphology.1 Poorly conducting films have carrier mobilities that are too low to measure and at high conductivity the interpretation of the Hall coefficient is difficult in disordered systems.12-13 Nevertheless, chemists have significantly improved the material quality of conducting polymers very recently.14-15 Very high electrical conductivities of solution processable conducting polymers (>4000 S/cm)16-18 and vapor phase polymerized films (>8000 S/cm) have been reported.19-21 The material properties are getting close to their inorganic counterparts, and these materials could potentially be used in flexible electronic devices. To further improve the


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electrical conductivity of these materials, to understand the transport properties of these disordered systems, and to explore further applications, measurements of the carrier mobility and doping density are critically important. Unfortunately, there are nearly no reports on the carrier mobility and carrier density measurements at this stage. Recently, we developed a solid state photoinduced dedoping reaction based on a photobase generator 2-(9-Oxoxanthen-2-yl)propionic acid 1,5,7-triazabicyclo[4.4.0]dec-5-ene salt (PBG) combined with conducting polymers (Scheme 1).22 By varying the UV-irradiation time, the electrical conductivity in these films is precisely controlled over more than three orders of magnitude. This is due to the formation of a strong base 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD)23 from PBG during UV irradiation, which transfers the electrons to the 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

Scheme 1. Photoinduced dedoping reaction of conducting polymers using a photobase generator.


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is similar to the extraction of carrier mobility from a thin film transistor in the depletion mode. 2. EXPERIMENTAL SECTION Chemicals. PEDOT/PSS (Clevios PH1000) was purchased from H.C. Starck. Ethylene glycol and 2-(9-Oxoxanthen-2-yl)propionic acid 1,5,7-triazabicyclo[4.4.0]dec-5-ene salt (PBG) was purchased from TCI Chemicals. Methanol (super dehydrated) was purchased from Wako Chemicals. Film Preparation. A 1-mm-thick cross-linked polydimethylsiloxane (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 PBG methanol solution (25 mg/mL) was drop casted on the PDMS substrate, and the substrate was heated on a hot plate at 80°C for 20 min. The PBG film on the PDMS was used for X-ray diffraction measurements. Other solvents like ethanol could also be used to fabricate PBG film, but the film quality is lower than methanol. The PEDOT/PSS film was spin coated onto glass substrates at 2000–6000 rpm, which gave a typical film thickness of 30–80 nm. After annealing at 150°C for 10 min in air, the prepared PBG/PDMS film was laminated on a PEDOT/PSS surface by hand. After removing the PDMS from the PEDOT/PSS, the PBG film remained on the PEDOT/PSS surface. Characterization. The film thickness was measured using a surface profilometer (Surfcoder ET 200, Kosaka Laboratory Ltd.). The set-up for the conductance measurement was constructed in-house. The probes were made of gold wire with a diameter of 0.5 mm. The channel length between the probes was fixed to 1 cm. The conductance change with time was recorded using a Dual Channel Digital Multimeter (DMM 7352A, ADCMT). The room temperature conductivity of the different samples was cross checked with a four-probe conductivity test meter (MCP-T600,


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Mitsubishi Chemical Corporation). The UV–Vis–NIR spectra were acquired using a UV–Vis– NIR spectrometer (UV2500PC, Shimadzu). X-ray scattering and diffraction were performed at room temperature using an X-ray diffractometer (D8 Discover, Bruker). An Xe lamp with a monochromator (Bunko Keiki, SM-25) was used to control the light irradiation. The light intensity was adjusted using a Reflective Neutral Density Filter (SIGMAKOKI) and measured using a standard silicon solar cell (Bunkou Keiki) in the wavelength range of 300–400 nm.

3. RESULTS AND DISCUSSION

Figure 1. Schematic of the device fabrication process for the mobility measurement used for the photo-induced dedoping reaction. To accurately estimate the number of charges transferred from the photobase generator to the polymer, we constructed the photobase generator/PEDOT bilayer structure using a polydimethylsiloxane (PDMS) stamp. There are several advantages to making the bilayer structure using the PDMS stamping method. The first and most important one is that the interface is clear. The photoinduced charge transfer reaction from photobase generator to the conducting polymers should start first at the interface. In the bilayer structure, the molecular


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density at the first layer can be estimated. The second advantage is that the polymer film and the photobase generator film can be prepared separately, and therefore the solvent effect on the film morphology in both layers during film formation can be minimized. Figure 1 shows the device fabrication process. A 1-mm-thick cross-linked PDMS film with a size of 1 cm × 2.5 cm was prepared. The PBG methanol solution was drop casted on the PDMS substrate after treating the substrate with a UV-ozone cleaner for 10 min. After drying the

Figure 2. Photographs of the PEDOT/PSS−PBG films before (a) and after (b) UV irradiation. (c) Absorption spectra of films of as-cast PEDOT/PSS, PEDOT/PSS−PBG, and UV-irradiated PEDOT/PSS-PBG films. ACS Paragon Plus Environment

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PDMS/PBG film on a hot plate at 80°C for 20 min, the film was laminated on a target conducting polymer film surface by hand. After removing the PDMS, the photobase generator film remained on the conducting polymers. The UV light was irradiated from the conducting polymer side, and the conductance was recorded with the irradiation time. Figures 2a and 2b show photographs of the PEDOT/PSS-PBG bilayer films before and after 340-nm UV irradiation for 30 min. The size of PEDOT/PSS film is around 2.5 cm × 2.5 cm and the size of the photobase generator film is around 1 cm × 1 cm. The change in the film color at the PBG part is clearly observed. The blue color is attributed to the absorption of neutral PEDOT. Figure 2c shows the absorption spectra of an as-cast PEDOT/PSS film, a PEDOT/PSS-PBG film, and the PEDOT/PSS-PBG film after UV irradiation. The PEDOT/PSS film is nearly transparent in the visible region and has increasing absorption in the NIR region. After transferring the PBG film onto the PEDOT/PSS film, the base line shifted due to the refection from the PEDOT/PSSPBG interface. The absorption lower than 400 nm from the PBG significantly increased. After UV irradiation (2 mW/cm2, 340 nm) for ~30 min the film showed a strong absorption from 600 to 700 nm, which is attributed to the π→π* transition in the neutral PEDOT.24 This result provides evidence of the photoinduced dedoping reaction at the bilayer film. With the decreasing carrier concentration in PEDOT/PSS, the conductivity of the film should decrease. The relationship between the carrier mobility, carrier density, and electrical conductivity is written as σ = neµ, where σ is the electrical conductivity, n is the carrier density,

µ is the carrier mobility, and e is the elementary charge. Assuming the carrier mobility is not changed, the conductivity change with irradiation time is directly related to the carrier concentration charge, and therefore

= .

(1)


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In the actual devices, we measured the conductance G of the film. Because the film geometry is the same, σ varies in direct proportion to the conductance G.

=

=

(2)

The carrier concentration charge is due to the charge transfer reactions from 1,5,7triazabicyclo[4.4.0]dec-5-ene, and 1,5,7-triazabicyclo[4.4.0]dec-5-ene formation is directly related to the absorbed photon number:

=

=

× ×

=

×( )× ×

,

(3)

where I is the light energy absorbed by the first layer of the photobase generator film, 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 the light, I is the light intensity, and ABS is the absorbance of the first layer at

λ. Therefore,

=

×

× (1 − 10

!" )

× Φ$ .

(4)

The carrier mobility can be calculated using the following equation: =(

&

× ℎ × $)/( × ( × (1 − 10

!" )

× Φ$ ).

(5)


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The quantum yield of the TBD formation is reported to be 0.64.23 We assumed this value in our devices for simplicity. This may cause some errors in the mobility calculation; however, it should be acceptable at this stage because the electrode contact and the estimation of the absorbance at the first layer can have a larger effect using this method (see discussion later). Therefore,

Figure 3. (a) 2D scattering patterns of the PBG films on PDMS using transmission geometry and (b) 2D diffraction patterns of the PBG films on PDMS. =

×

+ ()/* )

./

× , - × $ (*) × 1.56.

(6)

To estimate the molecular density at the interface, we performed morphology studies of the photobase generator film on PDMS using 2D X-ray diffraction and scattering. As shown in Figure 3, both transmission and grazing-incidence angle geometries show no clear diffraction suggesting that the film is amorphous. This is unexpected. As an organic salt, PBG shows


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crystallinity. The amorphous nature of the PBG film prepared from a methanol solution may be related to the hydrogen bonding between PBG and methanol, which affects the crystallization of the PBG. If the film is put in the air for more than 1 week, clear diffraction patterns can be observed in the X-ray scattering measurement. Due to the amorphous nature of the PBG film before transfer printing, we can estimate the molecular density of the PBG on the surface by assuming a totally random structure. The film d is measured to be ca. 1.4 g/cm3, and the molecular weight Mw of PBG is 407.47 g/mol. The molecular density at the first layer is calculated to be 3=(

45

× 6 )./7 = 1.62 × 10/ 9:. = 2.69 × 10 :?@ = 3 × A

(8)

Therefore the carrier mobility is =

×

()/*

+)

×

B+.CD × E

E ×F

./

G

× $ (*) × 1.56.

(9)

In most of our measurements, we used a wavelength of 340 nm, which is much closer to the peak absorption of PBG. The absorbance at 340 nm shows a good linear relationship even at a very high concentration ranges (Figure 4b). The molar absorption coefficient at 340 nm is 5884 M−1 cm−1. Under 340 nm irradiation, (:. /HI) =

×

()/*

+)

× 1563.

(10)

The unit cm2/vs of the carrier mobility in an organic semiconductor is more commonly used; therefore, (9:. /HI) =

×

(*)/&*

+)

× 1.563 × 10K .

(11)


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Figure 4. (a) The molar absorption coefficient versus the wavelength graph of PBG in methanol. (b) Absorbance at 340 nm versus the solution concentration of PBG in methanol. To confirm that the charge transfer reaction is directly related to the formation of 1,5,7triazabicyclo[4.4.0]dec-5-ene at the interface, we monitored the conductance change of the PEDOT/PSS-PBG film experiment under irradiation of different wavelengths. As shown in Figure 5a, the changes in the conductance are very small when the wavelength is smaller than 320 nm and significantly increases from 330 nm to 360 nm. When the wavelength is longer than 380 nm, the conductance of the film remains constant. If we plot the slope of the conductance change dG/dt divided by the light intensity at different wavelengths, we can find a clear maximum value at 340 nm. This is consistent with the absorption spectrum of the PBG, suggesting this dedoping reaction is sensitive to the formation of TBD. Using the molar absorption coefficient at different wavelengths, as shown in Figure 4a, we can extract the carrier mobility using equation (9). As shown in Figure 5b, the calculated mobility value for the PEDOT/PSS film is identical from 310 nm to 360 nm and ranges from 10 cm2/vs to 20 cm2/vs.


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Figure 5. (a) Stepwise change in conductance for a PEDOT/PSS-PBG film switching the UV illumination on and off at different wavelengths. (b) Plot of dG/dt/I0 versus the irradiation wavelength for a PEDOT/PSS-PBG film (closed circles). Plot of the calculated mobility using dG/dt/I0 at different wavelengths (open circles). This value is close to the mobility for trap-free organic single crystals.25 This is reasonable because a fraction of the mobile charges fills the disorder induced traps, therefore significantly increasing the mobility of the remaining charge carriers.26-27 Another important cause is that PEDOT nanocrystals in the films form ordered layered structures, which enhance the mobility by increasing the π-orbital overlap.28 A wavelength longer than 370 nm will give a much higher mobility value. This is because the absorption coefficient is very low in this range. In principle, the conductance of the film should not change significantly. However, due to the distribution of the monochromatic light (the bandwidth of our set-up is approximately 20 nm), the conductance still decreases. This could result in an overestimation of the mobility values when using equation (9). To minimize the effect of the irradiation bandwidth, we fixed the irradiation wavelength at 340 nm in the following experiments.


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Figure 6a shows the conductance change of the PEDOT/PSS-PBG film under the 340-nm

Figure 6. (a) Stepwise change in the conductance for a PEDOT/PSS-PBG film switching a 340-nm UV illumination on and off at different light intensities. (b) Plot of (dG/dt)/(dG/dt)0 versus I/I0 for a PEDOT/PSS-PBG film (closed circles). Plot of the calculated mobility using dG/dt/I0 at different light intensities (open circles). irradiation with different light intensities. Cutting the light using a neutral-density filter reduced the slope of the conductance change. Interestingly, it shows a nearly linear relationship with the light intensity and the calculated carrier mobility is constant. This suggests that the photoinduced dedoping reaction is not very sensitive to the light intensity in this energy range from 0.5 mW/cm2 to ca. 3 mW/cm2. It is known that the addition of a high boiling point solvent, such as ethylene glycol (EG) or dimethyl sulfoxide (DMSO), can dramatically increase the electrical conductivity.29-30 A morphology change has been confirmed using X-ray diffraction and Raman spectroscopy.28, 31 However, changes in the carrier mobility and carrier density due to the addition of solvents are still not clear. It is generally accepted that the carrier mobility increases and the carrier density is not changed. This is because the doping of p-type semiconductors is an oxidation reaction and


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Figure 7. (a) Conductance as a function of the UV-irradiation time for PEDOT/PSS−PBG films with different amounts of ethylene glycol. (b) Carrier mobility and carrier density in the PEDOT/PSS film plotted as a function of the amount of ethylene glycol. oxidizing agents are necessary for doping. A co-solvent such as EG or DMSO cannot oxidize PEDOT. The mobility measurement approach proposed in this study could be an ideal method to apply here to systematically study the effect of solvent addition on the carrier mobility and carrier density changes. Because both the PEDOT/PSS layer and the PBG layer can be prepared separately, the effect of solvents on the film morphology can be ignored. As shown in Figure 7a, at the addition of the co-solvent EG, the conductance of the film changes more than three orders suggesting that the conductivity of the film increased by more than three orders. After the transfer of PBG to the surface of the PEDOT/PSS film, 340-nm UV light is irradiated from the PEDOT/PSS side and the conductance starts to decrease. We extracted the carrier mobility and carrier concentration using equation (11) for all samples. As a summary in Figure 7b, with the addition of EG, the carrier mobility significantly increased by approximately two orders. This is because the co-solvents improved the crystallinity of PEDOT and the ordering of the PEDOT


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nanocrystals in the solid films. Interestingly, the calculated carrier density also increased from ca. 4 × 1019 cm−3 to ca. 6 × 1020 cm−3. As we mentioned before, the doping of p-type semiconductors is an oxidation reaction and a co-solvent such as EG or DMSO cannot oxidize PEDOT. One possible explanation is, with the addition of co-solvents, the previously inactive PEDOT, which is isolated by PSS, rearranged the electrically percolating network of the PEDOT crystal and therefore the carrier concentration increased. The reaction between PEDOT and oxygen in the air during annealing may also increase the carrier density, which cannot be denied at this stage. In particular, it has been reported that the addition of co-solvents could significantly decrease the Seebeck coefficient of the PEDOT/PSS films.32 The Seebeck coefficient is less sensitive to the film morphology, and a decreasing Seebeck coefficient may suggest a change in the oxidation level of PEDOT.33

4. CONCLUSIONS In conclusion, we proposed a novel approach to extract the carrier mobility in highly doped disordered systems using a photoinduced charge transfer reaction. Combining the number of charges transferred from the photobase generators to the conducting polymers, the carrier mobility can be extracted from the conductivity change. This approach was successfully applied to a benchmark conducting polymer PEDOT/PSS with different conductivities. We demonstrated that the addition of co-solvents increased both the carrier mobility and the carrier concentration in the polymer films. Note that this approach is not limited to p-type conducting polymers. By choosing photoactive materials such as photo-acid generators, extracting the carrier mobility in an n-doped material is also possible. Measuring the carrier mobility in both n-type and p-type carbon nanotube films is currently a much researched topic.


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AUTHOR INFORMATION Corresponding Author *E-mail for Q.W.: [email protected], and M.M.: [email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This research was partly supported by JST, PRESTO, JPMJPR17R1. REFERENCES 1. Coropceanu, V.; Cornil, J.; da Silva Filho, D. A.; Olivier, Y.; Silbey, R.; Brédas, J.-L., Charge Transport in Organic Semiconductors. Chem. Rev. 2007, 107, 926-952. 2. Tiwari, S.; Greenham, N. C., Charge Mobility Measurement Techniques in Organic Semiconductors. Opt. Quantum Electron. 2009, 41, 69-89. 3. Horowitz, G., Organic Field-Effect Transistors. Adv. Mater. 1998, 10, 365-377. 4. Haber, K. S.; Albrecht, A. C., Time-of-Flight Technique for Mobility Measurements in the Condensed Phase. J. Phys. Chem. 1984, 88, 6025-6030. 5. Blom, P. W. M.; deJong, M. J. M.; Vleggaar, J. J. M., Electron and Hole Transport in Poly(P-Phenylene Vinylene) Devices. Appl. Phys. Lett. 1996, 68, 3308-3310. 6. Saeki, A.; Koizumi, Y.; Aida, T.; Seki, S., Comprehensive Approach to Intrinsic Charge Carrier Mobility in Conjugated Organic Molecules, Macromolecules, and Supramolecular Architectures. Acc. Chem. Res. 2012, 45, 1193-1202. 7. Kang, K.; Watanabe, S.; Broch, K.; Sepe, A.; Brown, A.; Nasrallah, I.; Nikolka, M.; Fei, Z.; Heeney, M.; Matsumoto, D., et al., 2d Coherent Charge Transport in Highly Ordered conducting Polymers Doped by Solid State diffusion. Nat. Mater. 2016, 15, 896. 8. Wang, S.; Ha, M.; Manno, M.; Daniel Frisbie, C.; Leighton, C., Hopping Transport and the Hall Effect near the Insulator–Metal Transition in Electrochemically Gated Poly(3Hexylthiophene) Transistors. Nat. Commun. 2012, 3, 1210. 9. Podzorov, V.; Menard, E.; Rogers, J. A.; Gershenson, M. E., Hall Effect in the Accumulation Layers on the Surface of Organic Semiconductors. Phys. Rev. Lett. 2005, 95, 226601. 10. Jun, T.; Kazuhito, T.; Yoshinobu, A.; Taishi, T.; Yoshihiro, I., Hall Effect of Quasi-Hole Gas in Organic Single-Crystal Transistors. Jpn. J. Appl. Phys. 2005, 44, L1393.


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Extracting Carrier Mobility in Conducting Polymers Using a

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