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The conductivity of poly(3,4-ethylene dioxythiophene)/poly(4-styrenesulfonate) (PEDOT/PSS) is significantly enhanced on adding some organic solvent such as ethylene glycol (EG). In this paper, the optoelectronic properties of EG doped PEDOT/PSS on transmission and anti-reflection effects are investigated in detail by terahertz time domain spectroscopy (THz-TDS). The transmission line circuit theory gives us an insight into the THz transmission mechanisms of the main and second pulses. In particular, we show that the conductivities of 10% EG doped PEDOT/PSS are nearly frequency independent from 0.3 to 1.5 THz. To demonstrate applications of this property, we design and fabricate broadband terahertz neutral density filters and anti-reflection coatings based on 10% EG doped PEDOT/PSS thin films with varying thickness. Our measurements highlight the capability of THz-TDS to characterize the conductivity of EG doped PEDOT/PSS, which is essential for broadband optoelectronic devices in THz region.
© 2017 Optical Society of America
1. Introduction
Poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) (PEDOT/PSS) is a promising material which has thermal stability [1–3], high transparency [4,5] and aqueous solution properties [6,7]. Recently, its use as a conducting layer in optoelectronic devices, e.g., organic thin film transistors (OTFTs) [8,9], organic light-emitting diodes (OLEDs) [10,11] and solar cells [12,13], has attracted increasing interest. The optical transparency of PEDOT/PSS is comparable to that of the traditional transparent conductive oxide indium-tin-oxide (ITO), yet its relatively low conductivity is still an obstacle in optoelectronic applications [14,15]. Research has shown that chemical doping changes the morphology, structure, and conductivity [6,15,16]. By adding various solvents such as dimethylsulfoxide (DMSO), dimethylformamide (DMF), and ethylene glycol (EG) in PEDOT/PSS, the conductivity can be improved by more than 2 orders of magnitude [17].
In recent years, terahertz time-domain spectroscopy (THz-TDS) technique has been advancing rapidly to characterize the properties of thin-films material [18–22]. Some researchers investigated the chemical doping effects on the optoelectronic properties of graphene and PEDOT/PSS thin films for optoelectronic devices in THz region [23–25]. However, it is a challenge to produce the low-cost and broadband optical devices such as THz polarizers, filters, and waveplates, which has attracted interest from many research groups [26–28]. There are numerous reports concerning on THz devices, e.g., the active THz filters fabricated by VO2 and Si metamaterials [29], the high performance THz antireflection coatings demonstrated by H2SO4 doped graphene stacks [30], and the DMSO doped PEDOT/PSS [31], yet the fabrication complexity and cost still need to be reduced. Furthermore, the toxicity of the selected dopant also has to be considered to reduce the risk for the fabrication. EG is a kind of low-cost materials less toxic than DMSO [32]. In this work, THz-TDS is combined with transmission line circuit theory to study the effects of EG doping concentration on the conductivity of PEDOT/PSS thin films. By fixing the optimized EG doping amount which the conductivity of PEDOT/PSS is nearly frequency independent from 0.3 to 1.5 THz, we further develop and demonstrate low-cost broadband variable step THz neutral density (ND) filters and antireflection coatings based on EG doped PEDOT/PSS films with various thicknesses.
2. Experimental method
Double-side-polished 0.5 mm thick quartz substrates are ultrasonically cleaned for 10 min successively in acetone, isopropyl alcohol, and deionized (DI) water and then surface 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 0.4 μm syringe filter and mixed with EG. The mixed solutions are stirred for 24 h to improve the uniformity of the PEDOT/PSS thin films. To study the effects of EG doping on the optoelectronic properties of PEDOT/PSS, PEDOT/PSS samples on quartz are doped with EG of different concentrations. As for the applications for THz ND filters and antireflection coatings, the 10% EG doped PEDOT/PSS thin films with different thicknesses are spin coated on quartz substrate by controlling the spin coating speed. The thicknesses of PEDOT/PSS thin films are measured by a step profiler. To improve the crystallinity, the PEDOT/PSS thin films are then annealed at 100 °C for 20 min in a nitrogen atmosphere.
THz-TDS measurements are performed using free-space THz-TDS system. The system consists of 300 mW in mode-lock operation, 800 nm center wavelength and 84 MHz repetition rate pulse generated by a Ti:sapphire oscillator which is pumped by a 2.2 W 532 nm Nd:YV04 laser (SproutTM, Lighthouse Photonics). A GaAs semiconductor antenna is used for the THz pulse generation and a ZnTe crystal is employed for electro-optical detection. THz spectra were recorded from 0 to 3.3 mm (equal to a time window ranging from 0 to 22 ps), with a scan speed of 5 μm per step and an interval time of 300 ms, resulting a nominal resolution of 45 GHz. The THz-TDS system has a highest signal to noise ratio 60dB at 0.6 THz. The measurements were conducted in the typical transmission geometry at room temperature (∼298 K). All samples were fabricated on a sample holder with a circular area by ~3 mm in diameter, the optics were purged using nitrogen gas to remove the water vapor from the air to decrease the humidity down to below 5%. The usable frequency range of the system is from 0.1 to 2.5 THz, the optimized and valid range has been presented from 0.3 to 1.5 THz in this work due to signal attenuation by the sample. Terahertz time-domain waveforms are recorded for pristine and doped samples on quartz substrate Es(t) and a bare quartz surface as a reference Er(t) by moving the sample holder. The fast Fourier transform (FFT) allows us to obtain the frequency spectra 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 following [33].
3. Results and discussion
The THz spectroscopic properties of the PEDOT/PSS thin films show that the effect of the conductivity enhancement is partly responsible for the dopant polarity and dopant concentration [31]. In order to study the effects of EG doping concentration on the THz optoelectronic properties, 90 nm thick pristine, 2%, 4%, 6%, 8% and 10% EG doped PEDOT/PSS thin films are fabricated on the 0.5 mm thick quartz substrates. Figure 1 shows the THz time-domain transmission waveforms of the quartz reference and PEDOT/PSS thin films with different doping concentration. The amplitudes of the main pulse and the second pulse for each waveform change with the doping concentration. Considering the frequency dependent transmission of the main pulse, the effects of doping concentration is clearly shown in Fig. 2. For the pristine PEDOT/PSS, the transmission decreases from 82% to 69% between 0.3 to 1.5 THz. The transmission trend as function of frequency is totally different when there is a 2% EG doping in PEDOT/PSS. It increases from 56% to 67% in the same frequency range. By increasing the doping concentration to 4%, the transmission becomes flatter though there is an increase at the low frequency region from 0.3 to 0.6 THz. The transmission trend of 6% EG doped PEDOT/PSS is similar to that of 4% EG doped sample, but the value is lower. As for the doping concentration is further increased to 8%, the transmission increases comparing with 6% EG doped sample but decreases from 67% at 0.3 THz to 57% at 1.5 THz. As can be find, the transmission of 10% doped PEDOT/PSS is much more flat than that of 4% and 6% doped samples. It is nearly frequency independent (~65%) with a fluctuation of less than 5%. Furthermore, there is a polarity change of the second pulse for all the EG doped PEDOT/PSS thin films, which indicates a π phase change. Figures 3(a) and 3(b) schematically illustrate the transmission and reflection for both the bare substrate reference and the thin film on substrate samples. The transmission line circuit theory [34,35] as depicted in Fig. 3(c) can be used to explain the THz transmission properties of the main and second pulses.
Fig. 2 The frequency dependent transmissions of the main pulse for the PEDOT/PSS thin films on quartz normalized to the bare substrate.
Fig. 3 (a) Transmission and reflection at substrate-air interface; (b) Transmission and reflection at substrate-PEDOT/PSS-air interface; (c) The equivalent transmission line circuit of that in (b).
For a PEDOT/PSS thin film with the thickness much less than the skin depth of THz wave, the transmission and reflection coefficients in Figs. 3(a) and 3(b) are given as follows:
where Z0 = 377 Ω is the impedance of free space, ns is the refractive index of substrate, na is the refractive of air, σ is the conductivity of PEDOT/PSS, and d is the thickness of PEDOT/PSS thin film.The transmission of the main pulse (tp1) and the second pulse (tp2) can be calculated by using Eqs. (1) and (2), respectively:
By assuming the imaginary conductivity of PEDOT/PSS is 0, the refractive index of quartz substrates and air is 2.0 and 1.0, respectively. The real conductivities of PEDOT/PSS with different doping levels are calculated and shown in Fig. 4 by using Eq. (3) based on Fig. 2. We find that the almost frequency independent THz transmission of the main pulse of 10% EG doped PEDOT/PSS in Fig. 2 is due to its nearly real and constant conductivity (~480 S/cm). To understand the polarity change of the second pulses, we analyze the transmission parts in Eqs. (2) and (3). As the PEDOT/PSS thin films have the same thickness, only the conductivity was considered in this situation. If the conductivity is low, i.e., ns−na−Z0σ∙d>0, then the second pulse has the same polarity as the main pulse. On the contrary, if the conductivity is higher than a critical value, i.e., ns−na−Z0σ∙d<0, then there is a π phase change of the second pulse. Because the conductivity for all the PEDOT/PSS thin films are higher than that of the critical conductivity about 290 S/cm for 90 nm thick PEDOT/PSS thin films, the corresponding polarity changes of the second pulse for all the PEDOT/PSS samples are observed in Fig. 1.
By tuning the conductivity and thickness of PEDOT/PSS, we are able to obtain a defined transmission level for THz neutral density filters and eliminate the second pulse for THz antireflection coatings. Frequency independent conductivity is an essential property for the broadband THz neutral density filters and antireflection coatings [31]. Based on the analysis above, 10% EG doped PEDOT/PSS emerges as a good candidate thanks to its extremely frequency independent, low cost and green chemical properties. To demonstrate the applications of 10% EG doped PEDOT/PSS in broadband THz devices, we fabricate 10% EG doped PEDOT/PSS thin films of different thicknesses by controlling the spin coating speed. The thickness of PEDOT/PSS layer as function of spin coating speed is measured by a step profiler and depicted in Fig. 5. As the spin coating speed increases from 2000 to 4000, 6000, and 8000 rpm, the thickness of PEDOT/PSS thin film decreases from 102 to 82, 55, and 34 nm, respectively. The transmission time-domain waveforms of bare quartz and PEDOT/PSS thin films coated quartz substrates are recorded and shown in Fig. 6(a), and the corresponding transmission frequency domain spectra of the samples normalized to quartz reference are shown in Fig. 6(b), in which the theoretical calculation results are denoted by dash lines. We can see from Fig. 6(b) that the transmission of the main pulse is close to constant between 0.3 and 1.5 THz for each PEDOT/PSS sample, and the transmission matches very well to the theoretical calculation based on Eq. (3). It demonstrates that 10% EG doped PEDOT/PSS is a good candidate for broadband THz neutral density filter, which enables the attainability of any transmission level by controlling the thickness of 10% EG doped PEDOT/PSS thin film.
Fig. 6 (a) THz time domain waveforms of quartz reference and 10% EG doped PEDOT/PSS thin films of different thicknesses on quartz; (b) Normalized THz transmission of the main pulse of 10% EG doped PEDOT/PSS samples.
The antireflection effects of the reflected pulses are the key factors for antireflection coatings [31]. It is also interesting to analyze the change of the second pulse caused by the reflection at the substrate-air or substrate-PEDOT/PSS-air interface. The time domain signal of the second pulse is highlighted in Fig. 7(a). When the thin film thickness is 34 nm, the second pulse remains the same polarity as the quartz reference. The reason is that the thickness of 34 nm is less than the critical thickness which can be calculated from Eq. (2) as 55.3 nm, resulting in rs>0. Additional, the amplitude reduces when the thickness of sample is 34 nm which obviously indicates antireflection effects. If we look at the results of 55 nm thickness sample, the THz signal of the second pulse is almost eliminated. By further increasing the thickness to 82 nm and 102 nm, we can observe that the polarity changes sa rs<0 and the amplitude increases.
Fig. 7 (a) THz time domain waveforms of the second pulse of quartz reference and 10% EG doped PEDOT/PSS thin films of different thicknesses on quartz; (b) Normalized transmission of the second pulse of PEDOT/PSS on quartz substrate (red squares denote the theoretical calculation and black triangles signify the experiment results).
To quantitatively evaluate the antireflection effects, the normalized transmission of the second pulse is analyzed. It is defined by the ratio between the peak-to-peak signal of the second pulse of PEDOT/PSS on the quartz and that of the quartz reference. The results shown in Fig. 7(b) demonstrate that as the thickness of PEDOT/PSS on quartz increases from 34 to 55, 82, and 102 nm, the normalized transmission of the second pulse decreases from 30% to 5%, −30%, and −38%, respectively, which is signified with black triangles. We find that the experiment results match quite well with theoretical calculations of Eq. (3) that are denoted by red squares, indicating a real and constant conductivity of 10% EG doped PEDOT/PSS. Therefore, 10% EG doped PEDOT/PSS can be applied in THz antireflection coatings. By tuning the thin film thickness to ~55 nm the reflection at the substrate-PEDOT/PSS-air interface can be nearly eliminated for quartz. This type of material can also be applied to other substrates such as silicon and sapphire.
4. Conclusion
PEDOT/PSS thin films doped with EG represent a type of promising material for THz optical devices. In this article, we study the effects of EG doping concentration on the conductivity by transmission THz-TDS and find that the conductivity of 10% EG doped PEDOT/PSS is nearly frequency independent at 0.3-1.5 THz region. This property makes it a good candidate for broadband THz neutral density filters and antireflection coatings. We fabricate neutral density filters with thickness between 34 and 102 nm and achieve transmission between 62% and 83%. Furthermore, the second pulse is almost suppressed by 55 nm thickness PEDOT/PSS thin film.
Funding
National Natural Science Foundation of China (61575125 and 61471246); the Guangdong Foundation of Outstanding Young Teachers in Higher Education Institutions (YQ2015141 and YQ2013141); Guangdong Special Support Program of Top-notch Young Professionals (2015TQ01R453 and 2014TQ01X273); and Shenzhen Scientific Research and Development Funding Program (JCYJ20150324141711587).
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