PSS: Effect on Terahertz Optoelectronic

Article pubs.acs.org/JPCC

Solvent Doping of PEDOT/PSS: Effect on Terahertz Optoelectronic Properties and Utilization in Terahertz Devices Fei Yan, Edward P. J. Parrott, Benjamin S.-Y. Ung, and Emma Pickwell-MacPherson* Department of Electronic Engineering, The Chinese University of Hong Kong, Shatin, Hong Kong, China ABSTRACT: Poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) (PEDOT/PSS) is a conducting polymer and is a promising material for use in optoelectronic devices. Adding dopants to PEDOT/PSS significantly affects its optoelectronic properties: in this article we use terahertz time domain spectroscopy (THz-TDS) to probe the effects of dopants dimethyl sulfoxide (DMSO) and ethylene glycol. The carrier density, mobility, and conductivity are calculated from the THz measurements by fitting the dielectric permittivity to the Drude−Smith model. This gives us an insight into the conductivity enhancement mechanisms, and we find evidence to suggest that both carrier delocalization and charge screening play a role, although the relative importance of these two mechanisms depends upon both dopant polarity and concentration. To demonstrate an application of this finding, we design and fabricate broadband terahertz neutral density filters based upon 6% DMSO doped PEDOT/PSS thin films of varying thickness and demonstrate optical densities between 0.14 and 0.53 from 0.5 to 2.2 THz with a comparable frequency variation to commercially available optical frequency ND filters. ize the material properties of nanocrystals,12,13 carbon nanotubes,14,15 and graphene.16,17 Currently, THz-TDS lacks many affordable optical devices and materials, such as polarizers,18,19 waveplates,20 and filters,21 resulting in research groups putting effort into their development. Among them, a tunable dual band terahertz band-pass filter, and a band stop filter have been fabricated using VO222 and Si23 metamaterials, respectively. However, the insertion loss is quite high, and the bandwidth needs to be increased. In this work, THz-TDS is combined with Drude−Smith modeling24 to study the effects on the refractive index, carrier density, mobility, and conductivity of PEDOT/ PSS thin films resulting from the addition of either DMSO or EG in various quantities. By tailoring the optoelectronic properties of the PEDOT/PSS film we develop and demonstrate low-cost broadband variable step THz neutral density (ND) filters using 6% DMSO doped PEDOT/PSS films of various thicknesses, with comparable performance to commercially available visible frequency devices.

1. INTRODUCTION The conductive polymer poly(3,4-ethylenedioxythiophene)/ poly(4-styrenesulfonate) (PEDOT/PSS) has emerged as a promising material for electrodes in optoelectronic (EO) devices. It has many advantages over other conducting polymers, such as a high transparency, excellent thermal stability, and it can be processed in aqueous solution.1,2 Consequently, it has been used as a transparent conductive layer in organic thin film transistors (OTFTs) and solar cells.3,4 Although the optical transparency of pristine PEDOT/PSS is close to that of indium−tin−oxide (ITO), its relatively low conductivity has remained an obstacle in these optoelectronic applications. Studies have shown that chemical processing of PEDOT/PSS plays a major role in changing the morphology, structure, and conductivity.5 By mixing PEDOT/PSS with various solvents such as dimethyl sulfoxide (DMSO),2,6 ethylene glycol (EG),7 and dimethylformamide (DMF),8 the conductivity varies by more than 1 order of magnitude. One theory suggests that the high-boiling-point solvent acts as a plasticizer, thereby aiding in the reorientation of the PEDOT/ PSS chains at high temperatures to form better connections between the conducting PEDOT chains and possibly even the formation of percolating networks of high conductivity as the percolation threshold decreases 9,10 Other studies have proposed that polar solvents with a high dielectric constant induce strong screening effects between the positively charged PEDOT and the negatively charged PSS dopant, thereby reducing the Coulomb interactions between the counterions and the charge carriers.11 However, the relative validity of these two theories is not known; consequently the carrier transport mechanism is not yet fully understood. In recent years noncontact terahertz time-domain spectroscopy (THz-TDS) measurements have been used to character© XXXX American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. Single-side-polished z-cut quartz and high-resistive silicon substrates are ultrasonically cleaned for 10 min successively in acetone, isopropyl alcohol, and deionized (DI) water and then treated by O2 plasma for 5 min to improve the hydrophilicity. The aqueous PEDOT/PSS solutions (PH 1000, Heraeus Ltd., Leverkusen, Germany) with a PEDOT/PSS concentration of 1.3% and a weight to weight ratio of PSS to PEDOT of 2.5:1 are filtered through a 0.45 μm Received: January 16, 2015 Revised: February 15, 2015

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DOI: 10.1021/acs.jpcc.5b00465 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C syringe filter and mixed with either DMSO or EG. The resulting mixtures are stirred for 12 h to improve the uniformity of the PEDOT/PSS thin films. To study the effects of DMSO and EG doping on the optoelectronic properties of PEDOT/ PSS, PEDOT/PSS samples on z-cut quartz are doped with DMSO and EG of different concentrations. As for the applications for THz ND filters, the 6% DMSO doped PEDOT/PSS thin films with different thicknesses are spin coated on high resistive silicon substrate by controlling the spin coating speed. The details are summarized in Table 1. The Table 1. Details of the PEDOT/PSS Samples Used in the Optoelectronic Properties Study and the ND Filters Investigation application

doping solvent

spin coating speed (rpm)

OE properties OE properties OE properties OE properties OE properties ND filter ND filter ND filter ND filter

none (pristine) 2% DMSO 6% DMSO 2% EG 6% ED 6% DMSO 6% DMSO 6% DMSO 6% DMSO

2000 2000 2000 2000 2000 8000 6000 4000 2000

spin layers

layer thickness (nm)

5 5 5 5 5 1 1 1 1

500 500 500 500 500 31 43 63 129

Figure 1. Schematic of the sample holder used in the THz-TDS measurements. A manual linear stage is used to switch between reference (Er) and sample (Es) measurements.

where the FP(ω) represents the Fabry−Perot term given by 1 FP(ω) = ⎛ np − na ⎞⎛ np − ns ⎞ ωd 1 − ⎜ n + n ⎟⎜ n + n ⎟ exp⎡⎣ −2in p c ⎤⎦ ⎝ p a ⎠⎝ p s ⎠ (2) Here, c is the vacuum light speed, d is the PEDOT/PSS thickness, np, ns, and na are the complex refractive indexes of PEDOT/PSS, z-cut quartz substrate and air, respectively. The complex transmission coefficient can also be expressed as Ts(ω) = A exp(−iΔφ). Both amplitude transmission A and phase shift Δφ can be measured experimentally by THz-TDS by Fourier transforming the recorded time-domain terahertz electric field of the sample and reference. Then the frequencydependent complex refractive index of PEDOT/PSS sample (np = n + iκ) can be self-consistently determined by numerically solving the complex eq 1 using the experimentally obtained amplitude transmission A and phase shift Δφ. Once n and κ are known, the real and imaginary components of the frequency-dependent dielectric permittivity (ε = εr + iεi) are calculated:

thicknesses of PEDOT/PSS thin films are measured by a stepprofiler (Alpha-Step 500, Tencor). To improve the crystallinity, the prepared thin films are then annealed at 100 °C for 20 min in a nitrogen atmosphere. 2.2. THz Measurements. THz-TDS measurements are performed using our free-space THz-TDS system. Briefly, this consists of 10 mW, sub-100 fs, and 800 nm center wavelength pulses generated by a Ti:sapphire femtosecond laser (Vitesse, Coherent Inc., USA) focused in a free space modality onto LTgrown GaAs photoconductive devices acting as an emitter and detector (Menlo Systems TERA8−1, Martinsried, Germany). By varying the optical delay between the pump pulse (emitter gating) and probe pulse (detector gating) using a rapid scanning stage (ScanDelay 50, APE Berlin, Germany) operating at 4 Hz, the terahertz pulse is measured over a 45 ps window and recovered using a software lock-in amplifier. One thousand repeats are performed for each measurement and the average taken, resulting in a peak signal-to-noise ratio (SNR) greater than 70 dB and a 40 dB region between 0.3 and 3 THz. Terahertz time-domain waveforms are recorded for both the dopant samples on z-cut quartz substrates (Es(t)) and a bare zcut quartz surface as a reference (Er(t)) by moving the sample hold as shown in Figure 1. The fast Fourier transform (FFT) allows us to obtain the frequency spectra information (amplitude and phase) of the sample Es(ω) and reference Er(ω). The complex transmission coefficient of the sample Ts(ω) can be obtained by dividing the signal with the sample by the signal without the sample by taking the multiple reflections into consideration as detailed in ref 25: Ts(ω) =

εr = n2 − κ 2 εi = 2nκ

If the high frequency limit of the dielectric permittivity (ε∞) is determined then the frequency-dependent complex conductivity σ̃ = σr + iσi can be solved using the following equations: σr(ω) = ε0ωεi σi(ω) = ε0ω(ε∞ − εr)

(4)

2.3. Drude−Smith Modeling. The Drude−Smith model is fitted to the dielectric response calculated from the measured data using eq 3. From this the model parameters, ε∞, ωp, γ, and c1 in eq 5 are extracted so as to gain insight into the carrier transport mechanisms for the doped PEDOT/PSS materials. Carrier localization and/or trapping commonly occur in conductive polymers with nanostructures, such as the PEDOT/PSS materials investigated herein. Smith considered the backscattering of carriers and proposed the Drude−Smith model.24 In this investigation, only a single backscatter event is considered in eq 5.

2n p(na + ns) Es(ω) = Er(ω) (na + n p)(n p + ns)

⎡ ωd ⎤ exp⎢ −i(n p − na) ⎥FP(ω) ⎣ c ⎦

(3)

ε(ω) = ε∞ −

(1) B

⎡ ⎤ γ ⎢1 + c1 ⎥ (γ − iω) ⎦ (ω + iωγ ) ⎣ ωp2

2

(5)

DOI: 10.1021/acs.jpcc.5b00465 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C The parameter ε∞ is the permittivity at infinite frequency (which is subsequently used to derive the complex conductivity (eq 4)), ωp is the plasma frequency, γ is the damping rate, and c1 represents the fraction of the carrier’s initial velocity that was retained after experiencing a collision. The parameter c1 can vary between 0 and −1, corresponding to the Drude permittivity for c1 = 0 and complete carrier backscattering/ localization for c1 = −1. 2.4. Neutral Density Filter Calculations. Figure 2a,b schematically illustrates the transmission functions for both the

Figure 3. (a) Refractive index and (b) extinction coefficient from 0.3 to 2.5 THz for the PEDOT/PSS samples as a function of frequency. Figure 2. (a) Transmission at the air−substrate interface. (b) Transmission at the air−PEDOT/PSS−substrate interface. (c) Equivalent transmission line circuit of that in panel b.

parts of the dielectric permittivity are extracted, as detailed in eq 3, and displayed in Figure 4a,b, respectively.

substrate reference and the thin-film-on-substrate THz ND filter sample. The theory can be understood by an equivalent transmission line circuit with characteristic impedances (Figure 2c).26,27 For a thin conducting PEDOT/PSS film with conductivity σ and thickness d, where d is much smaller than the skin depth, the normalized transmission (t) can be extracted:28 ts E (ω) ns + na = st = t0 E0t(ω) ns + na + Z0σ (̃ ω)d ns + 1 = ns + 1 + Z0σ (̃ ω)d

t=

(6)

Here, Est(ω) is the complex field amplitude that transmitted through the PEDOT/PSS on high-resistive silicon substrate, E0t(ω) is the complex field amplitude transmitted through the high-resistive silicon substrate, Z0 is the free space impedance, σ̃(ω) is the conductivity of PEDOT/PSS, d is the thickness of PEDOT/PSS, ns is the refractive index of the high-resistive silicon substrate, na is the refractive index of air, and na = 1. Therefore, on the basis of eq 6, the THz transmission can be tuned by controlling both the conductivity and thickness of PEDOT/PSS thin film. Furthermore, if the conductivity is predominantly real and constant across a frequency range, the THz transmission modulation will be essentially frequency independent.

Figure 4. (a) Real and (b) imaginary parts of the dielectric permittivity as a function of frequency between 0.3 and 2.5 THz. The black lines are the best Drude−Smith model fittings derived from eq 5.

Figure 4 shows that the dielectric response of the 6% doped PEDOT/PSS samples differs significantly from the pristine and 2% samples. The real permittivity (Figure 4a), in particular, shows a marked difference, as the value is almost frequency independent for both 6% DMSO doped and 6% EG doped samples between 0.5 and 2.5 THz. The curvature of the imaginary permittivity (Figure 4b) is also more pronounced for the 6% doped samples, particularly when compared to the pristine PEDOT/PSS sample. The best fit parameters of the

3. RESULTS AND DISCUSSION The real (n) and imaginary (κ) parts of the refractive index of the PEDOT/PSS thin films as a function of frequency are given in Figure 3a,b, respectively. The values of both n and κ for all samples lie between a good conductor such as gold (on the order of 102−3)29 and a semiconducting material such as silicon (n = 3.42). From the n and κ values, the real and imaginary C

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The Journal of Physical Chemistry C Table 2. Drude−Smith Parameters, Carrier Density, Mobility, and DC Conductivity Fitted to the Experimental Data samples

ωp/2π (THz)

γ/2π (THz)

c1

ε∞

N (1019/cm3)

μ (cm2/V·s)

σdc (S/cm)

pristine 2% DMSO 6% DMSO 2% EG 6% EG

52.5 67.4 98.6 80.4 96.5

2.15 2.27 6.98 3.18 4.31

−0.75 −0.64 −0.27 −0.67 −0.53

468 48.2 92.6 202 280

2.81 4.64 9.91 6.59 9.50

39.6 53.8 35.7 35.6 37.2

178 399 566 375 566

Drude−Smith model (eq 5) are determined by fitting the dielectric permittivity data; these are shown in Figure 4 as the solid and dashed lines. Adequate fits were achieved for all samples, with particularly good results obtained for the doped samples. The simpler Drude model, which is often used for metal, graphene, and other highly conductive materials, was unable to produce satisfactory results because it does not account for carrier localization and/or trapping, which commonly occurs in conductive polymers. From these fits, the charge carrier density N, carrier mobility μ, and dc conductivity σdc are determined following the methods described in ref 30. The values for the Drude−Smith parameters, as well as those detailed by eq 7, are given in Table 2. Here, ε0 is the vacuum permittivity; m* is the effective mass of the charge carrier and is 0.82m07 (m0 is the free electron mass) for the calculations in this article. N= μ=

reducing conductivity. In the present cases, the dopant is screening the negatively charged PSS. Comparing the changes in conductivity with dopant, a bigger increase is seen in the conductivity for 2% DMSO compared to 2% EG, but similar conductivities for the 6% samples. Furthermore, the increase in the backscatter parameter is much larger for 6% DMSO than for 6% EG, despite the similar backscatter parameter increases at 2% dopant concentration. This difference may be due to the relative polarizability of the two dopants: DMSO has a larger dipole moment (3.96 D) compared to EG (2.36 D).32 Therefore, the charge screening effect is higher at low dopant concentrations for DMSO compared to EG, resulting in a larger conductivity; however, at 6% concentration the larger dipole moment of DMSO is “overscreening” the negative charge from the PSS, thus reducing the overall conductivity to less than that of EG, despite the large increase in carrier delocalization. At even higher dopant concentrations the conductivities of the films would decrease as has been observed previously.4 Consequently, both carrier delocalization and charge screening effects of dopants appear to alter the conductivity of PEDOT/ PSS films, but the extent to which each effect alters the conductivity is dependent on both dopant concentration and dopant polarity. To observe the changes in the frequency-dependent real (σr) and imaginary (σi) parts of conductivity of PEDOT/PSS, the values are calculated using eq 4 and plotted in Figure 5.

ε0ωp2m* e2 (1 + c1)e γm*

σdc = (1 + c1)

ε0ωp2 γ

(7)

For the PEDOT/PSS samples doped with DMSO, the dc conductivity increases from 178 S/cm for pristine PEDOT/ PSS, to 399 and 566 S/cm with 2% and 6% doping concentration, respectively. For PEDOT/PSS doped with EG, the dc conductivity increases to 375 S/cm and 566 S/cm with 2% and 6% doping concentration, respectively. These conductivities for EG doped PEDOT/PSS agree well with the dc conductivity measured in ref 7. From the fitting parameters of both DMSO and EG doping, the carrier density (which is associated with the plasma frequency) increases as the doping concentration increases from 0% to 6%. The carrier mobility increases with 2% doping mainly due to the larger back scattering parameter, while it decreased slightly with 6% doping as a result of the faster damping rate compared to pristine PEDOT/PSS. References 10 and 11 suggest that the enhanced carrier transport observed for doped PEDOT/PSS samples is due to either increased carrier delocalization (through less backscatter) or the screening effects of the dopant. First, the increased carrier delocalization results in the less negative values of the c1 parameter as the dopant concentration increases, which is observed for all doped samples investigated herein. Consideration of the screening effects of the dopant is more complex. Research into the screening of impurities in graphene suggests that dipolar molecules improve the graphene conductivity by screening the charges from the impurities.31 Above certain concentrations, once the dipolar molecules have fully screened the charge impurities they become scattering centers themselves, therefore

Figure 5. (a) Real and (b) imaginary parts of the conductivity of the PEDOT/PSS doped films between 0.3 and 2.5 THz. The black lines are the conductivities as derived from the Drude−Smith parameters in Table 2 D

DOI: 10.1021/acs.jpcc.5b00465 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

4. CONCLUSIONS PEDOT/PSS films doped with DMSO and EG represent a useful material for the development of terahertz optical devices. In this article, we develop a thorough understanding of the changes in permittivity with doping; this is required to make informed decisions on the optimum doping materials and concentrations when designing optical devices. The terahertz spectroscopic properties of PEDOT/PSS doped with 2% and 6% concentrations of either EG or DMSO showed that both carrier delocalization and increased charge screening are partly responsible for the higher conductivity observed; however, the relative importance of each effect is dopant dependent. The higher polarity of DMSO relative to EG results in increased charge screening at low concentrations for DMSO, but reduced conductivity at higher concentrations as the DMSO becomes a charge scattering center. This affects the frequency-dependent conductivity of the PEDOT/PSS thin films and makes it possible to tune the conductivity for an almost frequencyindependent response: this is the ideal behavior for a terahertz ND filter. Six percent DMSO doped PEDOT/PSS thin films were found to be good materials for a ND filter due to their near frequency-independent conductivity in the terahertz frequency range. Our fabricated ND filters with thicknesses between 31 and 129 nm achieved ODs between 0.14 and 0.53, which is close to those of commercially available optical frequency ND filters. Fine tuning of the doping concentration should further improve the frequency independence. Additionally these thin films should also lend themselves to other broadband optical devices, including antireflection coatings.

As seen in Figure 5a, the real conductivity of 6% DMSO and 6% EG doped PEDOT/PSS samples appears less frequencydependent than for the other samples; for the 6% DMSO doped one, it is almost frequency independent in the range 0.3−2.5 THz with a value of approximately 560 S/cm, and the imaginary conductivity is less than 27% of the real part across the frequency range shown. As discussed in Section 2.4, the frequency independent value of the real part of the conductivity (in particular) of 6% DMSO doped PEDOT/PSS should make it a good material for broadband THz filters, as it ensures that the normalized transmission (eq 6) will be frequency independent. To test this hypothesis, 6% DMSO doped thin films of varying thicknesses (see Table 1) were spin coated onto silicon substrates, and their normalized transmission (T) was calculated using t from eq 6:

T = |t |2 = 10−OD

(8)

OD in eq 8 refers to the optical density. The subsequent normalized transmissions for the four thin films measured are plotted as a function of frequency in Figure 6.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +852 3943 8260. Notes

The authors declare no competing financial interest.

Figure 6. Normalized THz transmission of 6% DMSO doped PEDOT/PSS thin films of varying thickness. The gray bars represent a 7% transmission interval for the four samples between 0.5 and 2.2 THz.

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ACKNOWLEDGMENTS

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REFERENCES

This work was partially supported by the Research Grants Council of Hong Kong (415313), the CUHK Direct Grant (4055001), and The Hong Kong Innovation and Technology Scheme (ITS/198/12).

As the thickness of 6% DMSO doped PEDOT/PSS thin film is increased from 31 to 129 nm, the normalized transmission remains relatively flat. Between 0.5 and 2.2 THz this value varies by less than 7%, which is comparable to commercially available ND filters for optical frequencies (approximately 5% variation between 450 and 600 nm). Median normalized transmissions of 72%, 68%, 51%, and 30% are recorded for the 31, 43, 63, and 129 nm films, respectively, corresponding to ODs between 0.14 and 0.53. The small frequency variation that remains is a result of the remaining frequency dependence of the imaginary conductivity (see Figure 5b). The effect is small because of the relative size of the real and imaginary parts, and so further optimization of the doping levels of the PEDOT/PSS thin film may improve the frequency independence further. Currently there have been few studies of THz ND filters, particularly for broadband applications. In addition to the band stop filters mentioned previously, a terahertz attenuator has been demonstrated using a series of closely spaced silicon wafers angled at the Brewster angle to minimize multiple reflections.33 The silicon wafer solution remains large, relatively complex, and more expensive compared to the doped thin film solution presented herein.

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DOI: 10.1021/acs.jpcc.5b00465 J. Phys. Chem. C XXXX, XXX, XXX−XXX