Photoinduced Dedoping of Conducting Polymers: An Approach to

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Photoinduced Dedoping of Conducting Polymers: An Approach to Precise Control of the Carrier Concentration and Understanding Transport Properties Qingshuo Wei,*,† Masakazu Mukaida,*,† Kazuhiro Kirihara,† Yasuhisa Naitoh,‡ and Takao Ishida*,† †

Nanomaterials Research Institute, Department of Materials and Chemistry, National Institute of Advanced Industrial Science and Technology, 1-2-1 Namiki, Tsukuba, Ibaraki 305-8564, Japan ‡ Nanoelectronics Research Institute, Department of Electronics and Manufacturing, National Institute of Advanced Industrial Science and Technology, Higashi 1-1-1, Tsukuba, Ibaraki 305-8562, Japan S Supporting Information *

ABSTRACT: Exploring the various applications of conjugated polymers requires systematic studies of their physical properties as a function of the doping density, which, consequently, calls for precise control of their doping density. In this study, we report a novel solid-state photoinduced charge-transfer reaction that dedopes highly conductive polyelectrolyte complexes such as poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate). Varying the UV-irradiation time of this material allows the carrier density inside the film to be precisely controlled over more than 3 orders of magnitude. We extract the carrier density, carrier mobility, and Seebeck coefficient at different doping levels to obtain a clear image of carrier-transport mechanisms. This approach not only leads to a better understanding of the physical properties of the conducting polymer but also is useful for developing applications requiring patterned, large-area conducting polymers. KEYWORDS: semiconducting materials, photobase generator, polymer, carrier concentration, carrier mobility

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INTRODUCTION Improvement of the performance of organic electronic devices requires improvement of the electrical properties of conjugated polymers. To attain this goal, a better understanding of the physical properties of conjugated polymers as a function of the doping level is of fundamental importance.1,2 Pioneering works have concentrated on neutral conjugated polymers whose carrier density is controlled by a thin-film transistor (TFT) configuration.3−7 Because inorganic dielectrics benefit from sophisticated fabrication processes and well-understood characterization methods, TFTs comprising inorganic and polymer dielectrics are widely used to study how the carrier density affects the transport properties of conjugated polymers. With the TFT configuration, doping and dedoping is reversibly controlled, which is advantageous for many studies. Unfortunately, the carrier density inside the conjugated-polymer film is not persistent but decays in the absence of a nonzero gate voltage. This approach is also limited by the low carrier density obtained in traditional TFTs that result from the low dielectric constant of the dielectric layers. Another drawback is that the carrier distribution orthogonal to the dielectric layer depends on the total carrier density (i.e., gate voltage in TFTs).8,9 A much greater carrier density could be obtained by using electrolyte-gated or electrochemical transistors, but the doping process for these techniques is based on ion diffusion in the conjugated polymer, which can affect the film morphology and thus the density of disorder-induced traps in the film.10 © XXXX American Chemical Society

Another approach to control the carrier density inside such films is to start from predoped polymers. Films fabricated from predoped polyelectrolyte complexes such as poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS) or from in situ synthesized conducting polymers such as PEDOT/ tosylate offer high carrier density and stable electrical conductivity.11 On the basis of this approach, depletion-mode electrochemical transistors can be fabricated, and the carrier density inside these films can be controlled over a large range.12−15 To improve the control and uniformity of doping, we propose herein the use of a solid-state photoinduced charge-transfer reaction to dedope the highly conductive polymer PEDOT/PSS. This approach provides an excellent platform to study the physical properties of conjugated polymers from high carrier density to low carrier density. By variation of the illumination time, the carrier density inside the film can be continuously and easily controlled over more than 3 orders of magnitude. Furthermore, we obtain the carrier mobility and Seebeck coefficient as a function of the carrier density, which provides information on the charge-transfer mechanisms in the films. Moreover, this reaction may potentially be used for large-area photolithographic patterning of conducting polymers. Received: October 31, 2015 Accepted: January 6, 2016

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DOI: 10.1021/acsami.5b10453 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces Scheme 1. Photoinduced Dedoping Reaction of PEDOT/PSS Using the Photobase Generator

Figure 1. Schematic of the patterning process of the PEDOT/PSS film used for the photoinduced dedoping study.

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RESULTS AND DISCUSSION

Photoinduced Dedoping Reactions. Photoinduced doping of conjugated polymers involves exposing them to optical illumination, which increases their carrier density through photoexcitation or photooxidation.16 Using photoacid generators, photoinduced doping of nondoped semiconducting polymers was also reported.17−19 For photoinduced dedoping, a photobase generator is mixed with a conducing polymer so that dedoping occurs upon optical illumination because of the photodecarboxylation reaction in the photobase generator.20 Scheme 1 presents the photoinduced dedoping reaction exploited in this study. 2-(9-Oxoxanthen-2-yl)propionic acid 1,5,7-triazabicyclo[4.4.0]dec-5-ene salt (PBG; see Scheme 1) is a commercially available water-soluble photobase generator used in low-temperature UV-curing systems.21 When irradiated at 365 nm, the quantum yield of PBG is high for formation of 1,5,7triazabicyclo[4.4.0]dec-5-ene (TBD). TBD is a strong base with a pKa value as high as 26.22 Thus, doped PEDOT+ should accept electrons from TBD to form neutral PEDOT0.23 The first step to realize photoinduced dedoping is to fabricate PEDOT/PSS−PBG hybrid films. PEDOT/PSS films stably disperse water with low pH. Adding a salt to a PEDOT/PSS dispersion should affect the stability of the solution; in fact, adding 1 wt % PBG to a PEDOT/PSS dispersion results in a gellike mixture, which prevents its use for spin coating. Thus, in this study, we fabricate PEDOT/PSS films by first spin coating and then immersing the as-prepared PEDOT/PSS films into a 50 mg mL−1 PBG solution to afford PEDOT/PSS−PBG films (see Figure 1). The surface roughness is not significantly changed before and after immersion into a PBG solution (Figure S1). To confirm photoinduced dedoping in the hybrid films, UV− vis−near-IR (NIR) spectroscopy, which exploits the fact that neutral PEDOT and doped PEDOT have different absorption spectra in the visible and NIR, was conducted. Figure 2a shows

Figure 2. (a) Absorption spectra of films of as-cast PEDOT/PSS, PEDOT/PSS−PBG, and UV-irradiated PEDOT/PSS. Inset: Photographs of PEDOT/PSS−PBG films with (right) and without (left) UV irradiation. PEDOT/PSS patterns fabricated using (b) metal masks and (c) inkjet-printed PET masks.

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 almost transparent in the visible region and has growing absorption in the NIR region. After the PEDOT/PSS film is immersed into a PBG solution, the bipolaron absorption above 1200 nm slightly decreases and the polaron absorption near 900 nm slightly increases. This result suggests that immersing PEDOT/PSS into the PBG solution slightly modifies the number of bipolarons and polarons in the film. The blue spectrum in Figure 2a is for the PEDOT/PSS−PBG film after UV irradiation (0.5 mW cm−2, 365 nm) for ∼20 min. The film becomes almost transparent in the NIR and has a strong absorption from 600 to 700 nm. This absorption is due to the π → π* transition in neutral PEDOT. The change in the film color due to UV irradiation is clearly observed, as shown in the inset of Figure 2a. This result provides B


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ACS Applied Materials & Interfaces

the PEDOT/PSS film but exposed to various durations of UV irradiation. For all devices, the conductivity of the film increases with the temperature from 100 K to room temperature, suggesting a hopping-like charge transport in the films.25 For the film with an initial electrical conductivity greater than 400 S cm−1 (red squares), the electrical conductivity shows almost the same temperature dependence from 100 K to room temperature. The activation energy calculated using Arrhenius’ equation is ∼2.4 meV, which is very low for organic semiconductors. With a longer irradiation time and lower electrical conductivity, the slope of σ versus T increases. The activation energy increases to 10 (26) meV for a film with an electrical conductivity of 8 (0.14) S cm−1. This result suggests that charge carriers must overcome a large activation energy for transport in a film with low carrier density. This can be explained by the significant decrease in the mobility at low electrical conductivity due to deep Coulomb traps (see the discussion later). The decrease in the electrical conductivity upon UV irradiation may be attributed to a decrease in the carrier density resulting from charge transfer from TBD to PEDOT+. However, another contributing factor may be the carrier mobility, which may also change because it may depend on the carrier density. To determine the carrier density and mobility as a function of the irradiation time, we fabricated TFTs on Si/SiO2 substrates. By spin coating, we deposited ∼15 nm PEDOT/PSS films on Si/ SiO2 substrates. After drying at 150 °C for 10 min, part of each film was removed by cotton sticks to make ∼3-mm-wide lines free of PEDOT/PSS. After evaporation of gold electrodes on each film perpendicularly across the 3-mm-wide lines, each film was immersed into a PBG solution and then dried under vacuum. The channel length L (i.e., distance between gold electrodes) is 100 μm, and the width W is 3 mm. By using the four-probe method, we measured the electrical conductivity of the transistors in vacuo. As shown in Figure 4a,b, the output curves of a film with an electrical conductivity of 628 S cm−1 show no clear field effect due to the gate voltage. The source−drain current in the transfer curves shown in Figure 4a are relatively independent of the gate voltage but undergo a large hysteresis for constant source−drain voltage when the gate voltage is scanned forward and backward (Figure 4b). Thus, changes in the carrier density due to gate bias are not clearly detected, and, hence, the observed source−drain IV characteristics may result from high conductance in the channel. A detectable field effect appears at a conductivity of ∼300 S cm−1 (Figure 4d). The transfer curves show only a weak hysteresis, and the source−drain current clearly increases with the gate voltage. The carrier mobility was calculated using

evidence of the photoinduced dedoping reaction proposed in this study. Because the electrical properties of neutral conjugated polymers are close to those of insulating plastics, this approach may be used to make large-area patterns of conducting lines. We demonstrate this scheme by irradiating these films through a variety of shadow masks. Figure 2b shows a pattern in the PEDOT/PSS film made using a metal mask with 100-μm lines. The well-resolved interface between doped and dedoped PEDOT is clear. Because photoinduced dedoping occurs upon irradiation at 365 nm, we can use shadow masks fabricated on poly(ethylene terephthalate) (PET) substrates with an inkjet printer. Figure 2c shows the institute logo and Chinese characters patterned on the PEDOT/PSS film on glass using an inkjetprinted PET mask. Electrical Conductivity, Carrier Density, and Carrier Mobility. To further confirm the photoinduced dedoping reactions and study the transport properties of these films, we measured the electrical conductivity of the film using an in situ four-probe technique. The samples were prepared on glass substrates, and the gold electrodes in the van der Pauw geometry were vacuum-evaporated on the film. The as-prepared PEDOT/ PSS films have an electrical conductivity of 900 S cm−1, which slightly decreased to ∼800 S cm−1 after immersion in the PBG solution. This result may be attributed to a decrease in the carrier density, as indicated by the UV−vis−NIR spectra, because a smaller absorption at the NIR range suggested a lower carrier concentration.24 As shown in Figure 3a, for the PEDOT/PSS

Figure 3. (a) Electric conductivity as a function of the UV-irradiation time for a PEDOT/PSS film (open circles) and a PEDOT/PSS−PBG film (closed circles). The inset shows the device structure for the electrical conductivity measurement. (b) Temperature dependence of the electrical conductivity for the PEDOT/PSS−PBG film with various UV-irradiation times.

films without PBG, the electrical conductivity remains almost constant despite being exposed to the same 0.5 mW cm−2 UV irradiation, suggesting that PEDOT/PSS is stable under this irradiation. For PEDOT/PSS−PBG films, the electrical conductivity starts to decrease immediately (on the measurement time scale) once films are exposed to UV irradiation. Specifically, the electrical conductivity decreases more than 5 orders of magnitude after 15 min of UV irradiation. This result further supports the conclusion that photoinduced dedoping occurs because of UV irradiation. This photoinduced dedoping reaction ends immediately once UV irradiation ceases, offering an excellent way to control the doping density (Figure S2). This approach therefore provides a possible route for fine control of the doping density of a single device by simple control of the UVirradiation time. Figure 3b shows, as a function of the temperature, the normalized electrical conductivity of identical devices made from

μ = gm

∂I L 1 = DS W CoxVDS ∂VG

where gm =

∂IDS ∂VG

VDS

L 1 W CoxVDS

is the transconductance, L is the channel VDS

length, W is the channel width, and Cox is the capacitance of the dielectric, which is 10.7 nF cm−2 for 300-nm-thick SiO2. For the film with an electrical conductivity of 284 S cm−1, the carrier mobility calculated based on the transfer curve in Figure 4d is 8.2 cm2 V−1 s−1. The carrier density is 2.2 × 1020 cm−3 using n = σ/μe. With an increasin the UV-irradiation time, the electrical conductivity decreases. For a film with an electrical conductivity of 19 S cm−1 (Figure 4e,f), the carrier mobility decreases slightly to 5.1 cm2 V−1 s−1. The calculated carrier density is 2.4 × 1019 C


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Figure 4. (a, c, e, g, and i) Drain−source current as a function of the drain−source voltage for PEDOT/PSS−PBG transistors with different UVirradiation times. The gate voltages are given in each plot. (b, d, f, h, and i) Drain−source current as a function of the gate voltage for the same films. Each pair of panels in a given row corresponds to a single sample.

cm−3, which suggests that a decrease in the electrical conductivity at the early stages of UV irradiation is mainly due to a decrease in the carrier density. At the later stages of UV irradiation, the carrier mobility significantly decreases. For a film with an electrical conductivity of 1.9 S cm−1 (Figure 4g,h), the carrier mobility is 1.1 cm2 V−1 s−1 and the carrier density is 1.1 × 1019 cm−3. Finally, for a film with an electrical conductivity of 1.2 ×

10−3 S cm−1 (Figure 4i,j), the carrier mobility decreases by over 4 orders of magnitude to 2.7 × 10−3 cm2 V−1 s−1 and the carrier density decreases to 2.9 × 1018 cm−3. Figure 5 shows the carrier mobility as a function of the carrier density extracted from the transistor results (i.e., Figure 4). Note that the data in Figures 4 and 5 are extracted from a single device exposed to different durations of UV irradiation. The results of D


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electrical conductivity. For an as-cast film, the electrical conductivity is ∼900 S cm−1, and the calculated carrier density reaches 7 × 1020 cm−3. If we assumed a PEDOT/PSS weight ratio of 1:2.531 and a film density of 1.5 g cm−3,32 the calculated charge density is ∼0.4 holes per thiophene ring. The carrier density calculated here is greater than other published values, which are around one positive charge per three to four ethylenedioxythiophene (EDOT) units, as obtained from electrochemical measurements and by monitoring the consumption of EDOT and the catalyst peroxodisulfate during polymerization.31,33−35 It is suggested that, besides charge-transfer reactions, other reactions may also contribute to the increase in the concentration of mobile charges. One possible reaction is that a proton interacts with the polymer and charges delocalized along the polymer chains, thereby providing electronic conductivity. This phenomenon is well studied in polyaniline and polypyrrole. It is important to note that when the molecular weight of PEDOT is not high, the α position of the end thiophene can be protonated, which may provide an explanation for the high carrier density in highly conductive PEDOT/PSS. In support of this conclusion are recent studies that show that the electrical conductivity of PEDOT/PSS is improved by treatment with various acids.11,36,37 Seebeck Coefficient and Thermoelectric Power. Recent studies show that conducting polymers are promising materials for thermoelectric power conversion,38−40 and photoinduced dedoping may be a good way to understand the optimized carrier density in organic thermoelectric materials. Such experiments could also be conducted using a single device. Previous studies have shown that the results for the Seebeck coefficient S for PEDOT/PSS may be affected by humidity because PEDOT/ PSS readily absorbs atmospheric water vapor.32,41 To avoid this effect and simplify our discussion, we measured the Seebeck coefficient in a N2-filled glovebox, where water and oxygen are at concentrations below 100 ppm. The samples were prepared on glass substrates, and gold electrodes in the van der Pauw geometry were vacuum-evaporated on the film. The temperature difference ΔT and the electromotive force (EMF) ΔV were measured simultaneously by probing with a pair of gold electrodes connected to a digital multimeter. The slope of ΔV versus ΔT gives S. In addition, the electrical conductivity was measured using the four-probe technique. As shown in Figure 6a, the Seebeck coefficient of an asprepared PEDOT/PSS film is ∼18 μV K−1 and increases slightly to 20 μV K−1 after immersion into a PBG solution. This Seebeck coefficient is almost identical with that obtained in previous studies.42,43 For a PEDOT/PSS−PBG film, the electrical conductivity decreases with increasing irradiation time and the slope of the EMF versus temperature difference increases, which suggests an increasing Seebeck coefficient. This is a reasonable result because a greater Seebeck coefficient generally corresponds to films with lower carrier density. We calculated the carrier density in the film based on the results shown in Figure 5 and for a film with an electrical conductivity greater than 300 S cm−1. For these calculations, we used a constant mobility of 8.2 cm2 V−1 s−1. Figure 6b shows, as a function of the carrier density, the Seebeck coefficient, and Figure 6c shows the power factor PF = σS2 for the PEDOT/PSS film. Also plotted in the same figure are the corresponding results for a H2SO4-treated PEDOT/PSS film with a four-probe electrical conductivity of 1800 S cm−1 and a Seebeck coefficient of 8 μV K−1. A maximum power factor of ∼42 μW mK−2 occurs at a carrier concentration of ∼5 × 1020 cm−3 (with an electrical conductivity of ∼590 S cm−1), and the

Figure 5. Carrier mobility as a function of the carrier density for transistors based on PEDOT/PSS−PBG thin films.

this experiment may be attributed to fine control of the photoinduced dedoping reactions, as proposed above. More than 10 individual devices are fabricated using the same approach; all devices show an initial mobility value on the order of 1 cm2 V−1 s−1 and decreased over 3−4 orders after UV irradiation. The results of Figure 5 raise several questions regarding PEDOT/PSS. The first question regards the maximum carrier mobility in PEDOT/PSS. A mobility of 8.2 cm2 V−1 s−1 is very high compared with that of most organic semiconductors and is close to the mobility for trap-free organic single crystals.26 This result is partly attributed to the high carrier concentrations in the films because a fraction of the mobile charges fill the disorderinduced traps, thereby significantly increasing the mobility of the remaining charge carriers.27,28 Another important cause behind the maximum carrier mobility in PEDOT/PSS is that PEDOT nanocrystals in the films form highly ordered layered structures, which enhance the mobility by increasing the π-orbital overlap.29 The second question involves the significant decrease in the mobility for carrier densities below 1019 cm−3. A decrease of 4 orders of magnitude in the carrier mobility with only 1 order of magnitude decrease in the carrier density is too large to explain by invoking only the effect of disorder-induced traps.28,30 A larger effect contributing to the decrease in the mobility is expected from deep Coulomb traps.4−6 Note that the PSS− counterions remain in the film during photoinduced dedoping, and these PPS− ions exert strong Columbic forces on the moving carriers. At high carrier density, this Columbic interaction may be screened by a fraction of the carriers, leaving the remaining carriers with high mobility. Photoinduced dedoping decreases the number of high-mobility carriers, thereby increasing the activation energy of the Columbic traps. This is the reason why the increase in the conductivity with the temperature is more pronounced at the low electrical conductivity range, as shown in Figure 3b. This effect is similar to that which occurs in electrochemical transistors made from organic semiconductors.4,5 Because photoinduced dedoping occurs in the solid state and no ion diffusion occurs during dedoping, this reaction could serve as a good platform for building an analytical model to understand the relationship between Columbic traps and carrier density. The third question to discuss involves the maximum carrier density in PEDOT/PSS films. For a film with an electrical conductivity greater than 300 S cm−1, the carrier mobility cannot be directly extracted. Because the carrier mobility saturates at a carrier density of 5 × 1019 cm−3, the maximum carrier density may be extracted using the saturated mobility and the maximum E


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the UV-irradiation time, we find a maximum power factor of ∼42 μW mK−2 at a carrier density of ∼5 × 1020 cm−3. Thus, the use of photoinduced dedoping not only provides a platform for understanding the physical properties of conducting polymers but also is useful for developing applications requiring patterned, large-area conducting polymers.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b10453. Details of the Experimental Section (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Q.W.). *E-mail: [email protected] (M.M.). *E-mail: [email protected] (T.I.). Author Contributions

Q.W. conceived the project, performed the experiments, and wrote the paper. K.K., Y.N., and T.I. supported the electrical conductivity measurements. M.M. and T.I. supported the Seebeck coefficient measurements. All authors discussed data and commented on the manuscript. M.M. and T.I. supervised this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by NEDO (TherMAT) and Grant-inAid for Young Scientists (B) 15K17927 from MEXT, Japan.

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REFERENCES

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Figure 6. (a) EMF as a function of the temperature difference for obtaining the Seebeck coefficient of a PEDOT/PSS−PBG film with a given UV-irradiation time. The inset shows the device structure for the Seebeck coefficient and electrical conductivity measurements. (b) Resulting Seebeck coefficient. (c) Power factor as a function of the carrier density.

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CONCLUSION In summary, we present a solid-state photoinduced dedoping reaction based on photobase generators combined with PEDOT/PSS. By variation of the UV-irradiation time, the carrier density in these films is precisely controlled over more than 3 orders of magnitude. The carrier mobility in the film varies from 10−3 to 8.2 cm2 V−1 s−1 depending on the carrier density. We attribute this result to variation in the screening of Coulomb traps from PSS− at different carrier concentrations.4,5 In addition, by measuring the Seebeck coefficient of the film as a function of F


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