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Microwave characterization of conducting polymer PEDOT: PSS film using a microstrip line for humidity sensor application
Accepted Manuscript Microwave characterization of conducting polymer PEDOT: PSS film using a microstrip line for humidity sensor application Tae-Gyu Kang, Jin-Kwan Park, Byung-Hyun Kim, Jung Joon Lee, Hyang Hee Choi, Hee-Jo Lee, Jong-Gwan Yook PII: DOI: Reference:
S0263-2241(19)30041-7 https://doi.org/10.1016/j.measurement.2019.01.032 MEASUR 6276
To appear in:
Measurement
Received Date: Revised Date: Accepted Date:
12 July 2017 8 November 2017 14 January 2019
Please cite this article as: T-G. Kang, J-K. Park, B-H. Kim, J.J. Lee, H.H. Choi, H-J. Lee, J-G. Yook, Microwave characterization of conducting polymer PEDOT: PSS film using a microstrip line for humidity sensor application, Measurement (2019), doi: https://doi.org/10.1016/j.measurement.2019.01.032
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.
Microwave characterization of conducting polymer PEDOT: PSS film using a microstrip line for humidity sensor application Tae-Gyu Kang, 1 Jin-Kwan Park, 1 Byung-Hyun Kim, 1 Jung Joon Lee, 2 Hyang Hee Choi, 2 Hee-Jo Lee, 3 and Jong-Gwan Yook1 1
School of Electrical and Electronic Engineering, Yonsei University, Seoul, 120-749, South Korea
2
Institute of Engineering Research, Yonsei University, Seoul, 120-479, South Korea
3
Department of Physics Education, Daegu University, Gyeongsan, Gyeongbuk, 38453, South Korea
In this paper, we characterize the thin conducting polymer film, PEDOT: PSS, using a microstrip line in a humid environment at microwave frequencies. To characterize the thin film alone over the microstrip line, a de-embedding technique of thru-reflect-line (TRL) calibration is utilized. After the de-embedding procedure, the thin film is modeled with a transmission line, and the film’s R, L, G, and C parameters are extracted from measured S-parameters. In addition, the characteristics of the thin film has been tested in terms of electrical conductivity with increasing relative humidity (RH). In the future, it is expected that this work can contribute to the improvement of stability as well as performance of conducting polymer-based electronic sensing devices.
1
1. Introduction Over the past few decades, microwave engineers have proposed numerous approaches and methodologies for measurement techniques to characterize thin materials such as dielectric and magnetic films in both the narrow and broad frequency regions [1-3]. In particular, these techniques based on planar strip lines have been widely used for electrical characterization of material properties in the microwave regime [4-8]. Among these approaches, the technique using the coplanar waveguide (CPW) and the microstrip line have been most widely used when extracting the electrical characteristics of the substrate or thin film sample [9]. The material properties based on planar strip lines can be characterized by a transmission line (over a broadband) and with a resonator method (over a narrow band). Both approaches accurately extract the microwave/millimeterwave characteristics when the material-under-test (MUT) is placed on a bare sample [10, 11]. Although these methods have shown accurate results, the de-embedding process for removing parasitic components must be included. Thus, many researchers have employed the method of MUT on a resonators, a bulky type like dielectric resonator (DR), due to simple and direct measurement capability [12-14]. For this reason, a user-friendly calibration kit for materials characterization in the broad band should be carefully designed to easily extract the characteristics of unknown materials. In this study, to characterize a desired material via the designed calibration kit, we have used the PEDOT: PSS (poly (3, 4-ethylenedioxythiophene) polystyrene sulfonate) film, which is one of the well-known conducting polymer [15-21]. PEDOT: PSS film is critical to predicting changes in electrical properties depending on environmental conditions (such as humidity or temperature) for the stability of electrical devices [22]. Several research groups have studied the influence of relative humidity (RH) on the electrical properties of PEDOT: PSS [23-26]. In particular, the high-frequency property of 2D thin film of PEDOT: PSS on printed circuit boards (PCBs) has attracted significant attention because of advances in highspeed integrated-circuit (IC) technology [27, 28]. Thus, high frequency characteristics of PEDOT: PSS thin film deposited on the microstrip line depending on humidity change are investigated in this work. In this study, we present elaborately approaching process to characterize the thin conducting polymer film between the microstrip lines depending on relative humidity at microwave frequencies. Section 2 presents the sample preparation of the thin film on PCB and the integrated thru-reflect-line (TRL) de-embedding technique for analyzing the propagation properties of the thin film. Finally, in section 3, we extract the microwave characteristics (such as impedance, propagation constant, and lumped circuit models) to predict the electrical properties of the film depending on humidity using the measured scattering parameters(S-parameters).
2
2. Experimental setup 2.1 Sample preparation
Figure 1. Chemical structure of the PEDOT: PSS film.
Figure 2. Schematic of microstrip line device for characterization of PEDOT: PSS thin film.
The chemical structure of the poly (3, 4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT: PSS) is as shown in Fig.1 [29]. PEDOT was originally developed as an insoluble polymer, however its high conductivity soon made the material the subject of strong scientific interest. The insolubility problem of PEDOT has been overcome by the incorporation of poly (styrene sulfonate) PSS material and water. As a result, PEDOT: PSS came to have high conductivity and can be easily transformed to arbitrary shape as a water-soluble polyelectrolyte material [30]. For the experimental study, we used the microstrip line with SMA connectors to achieve a two-port measurement system, as illustrated in Fig.2. The sample preparation was as follows: a 50 ohm microstrip line was fabricated on an FR-4 substrate, and
3
a gap with a dimensions 1.15 mm × 2.3 mm was etched at the middle of the line. The gap in the PCB substrate was treated with an oxygen plasma for 1800 sec to intensify the adhesion between the PCB surface and the organic polymer. Next, using the bar-coating method, polymer thin-films was placed onto the PCB substrate by heating the film at 80
for 10 min on a
hotplate in ambient lab conditions. With this process, the thin film replaces a small part of the copper signal line. It is clear that a uniform layer of the film could be deposited in the gap area on the microstrip line. The thickness of the thin film is roughly 10 um, and it was measured by an AS 500 alpha-step surface profiler (KLA-Tencor Co., USA). The AFM image of the film reveals a uniform surface with an average roughness of 2.192nm, as shown in Fig. 3. In this work, all organic materials were used without further purification. PEDOT which is the solid content is 1.1 wt% and the weight ratio of PEDOT to PSS is 1:2.5. To optimize the electrical property of the PEDOT: PSS film, dimethyl sulfoxide (DMSO, 99%) was added to the aqueous PEDOT: PSS solution (5.0 v/v%) and stirred at room temperature. The solution was then filtered by a syringe filter. We used PCB substrates FR-4 with a permittivity of 4.4, a loss tangent of 0.02, and a thickness of 1.2mm. Also, the thickness of the copper layer is 0.035mm.
Figure 3. AFM image of thin conducting polymer film
2.2 TRL calibration To obtain the microwave characteristics of the thin conducting polymer PEDOT: PSS film, it is necessary to use the deembedding technique [31]. The sample illustrated in Fig. 2 contains signal lines and SMA connectors on both sides of the thin film, so, both components have a strong effect on measuring the microwave characteristics of the thin film itself. Using the thru-reflect-line (TRL) calibration, we have conducted the TRL de-embedding procedure up to the reference plane and extracted the propagation properties of the film. Fig. 4(a) shows the custom-designed TRL calibration set used in this experiment. These known three components allow the characterization of the additional structures and it is possible to predict the performance of the PEDOT: PSS thin film itself. The thru component is made by directly connecting port 1 to port 2 at the desired reference planes. In this work, a non-zero thru component with a length of 2 L1 is used, where L1 denotes the length from the end of the SMA port to the calibration
4
reference plane. The reflect component with a half-length of the thru component has known load value, such as open. The line component has a length of 2 L1+L2, where L2 is a certain line length used in the calibration process. Also, these custom-designed TRL calibration sets are made with the same SMA connector, substrate and line dimensions as the DUT sample. After the TRL calibration procedure, part of L1 can be matched with 50 ohm and no longer has loss or phase delay even though the part does have physical length. Note that the dashed line shown in Fig. 2 defines the reference plane used in the TRL extraction algorithm, thus allowing characterization of the PEDOT: PSS layer itself. The details of the mathematical procedure related to TRL calibration are described in reference [32].
Figure 4. Fabricated TRL calibration kit. (a) component of thru-reflect-line, (b) validation of calibration using a thru component.
Before measurement of the DUT sample, the performance of a TRL calibration kit is necessary. Fig. 4(b) shows the frequency response from connecting the thru component after TRL calibration. The signal transmissions (
and
)
between the two ports should be close to 0 dB level. This means that the signals propagate with very small reflection, because the non-zero thru has no electrical length and loss within the calibration process despite having physical length. From the results, the TRL calibration kit for characterizing the thin conducting polymer film is successfully performed so that the impedance of each port set to 50 ohm and reference plane is shifted to the dashed line in Fig.2.
2.3 Extraction of electrical parameters After performing the TRL calibration, the sample can be modeled, as shown in Fig. 5(a). In this stage, an interconnected microstrip line consisting of the thin conducting polymer film with characteristic impedance physical length
, propagation constant
is placed between the two-port networks with 50 ohm reference impedance. The element
parameter matrix, is determined by driving port with an incident power
and measuring the output power
of the Sfrom port .
Because these microstrip lines are physically symmetrical, the S-parameters is symmetrical as well as reciprocal, and
.
5
and
Figure 5. (a) Transmission line model of the DUT, (b) lumped circuit model of the conducting polymer film after TRL calibration.
The relationship between the measured S-parameters and the transmission line parameters, ,
and
, can be expressed as
follow [33,36]: (1a) , (1b) where
. The S-parameter matrix can be converted to ABCD parameters, which
more explicitly represent the propagation constant and the impedance
. The equivalent ABCD matrix is [33]:
(2) Also, the relationship between the S-parameters and the ABCD matrix can be represented as follows: (3a) (3b) (3c) (3d) where From equations (1)-(3),
. and
can be obtained as follows:
(4) (5a) (5b)
6
Equations (4) and (5) represent a complete solution to a microwave network that includes impedance discontinuity. In this work, impedance discontinuity occurs by placing the thin film of impedance the characteristic impedance
between the reference transmission lines with
. The equations (4) and (5) can often transcend solutions that often have ambiguous physical
meanings, such as negative attenuation constants even if the wave attenuates. With this in mind, a solution that matches the modeling situation should be chosen. Fig. 5(b) shows the equivalent lumped circuit model of the thin film after de-embedding the reference transmission line. A transmission line with infinitesimal length can be modeled as a lumped circuit, as shown in Fig. 5(b), where R, L, G and C have per-unit-length quantities. Once
and
are obtained, the distributed circuit parameters and the corresponding
electrical characteristics can be determined from the standard transmission line relationships below: (6) (7) and, (8a) (8b) (8c) (8d) Thus, the lumped-element transmission line model parameters are determined from the above equations (8a)-(8d).
3. Results and discussion. The sample shown in Fig. 2 with characteristic impedance of
and a propagation constant of γ is deposited on a grounded
FR-4 substrate, replacing the signal line. Fig. 6 shows the frequency response of the thin film on PCB in a variety of relative humidity (RH) levels (10%, 30%, and 50%) in a frequency range from 1 to 10 GHz. The magnitude of small, less than -15 dB, which indicates that the impedance mismatch is not significant. However,
(or
(or
) are very
) is intuitive to
understand because it indicates how well the signal is transmitted through the transmission line over a two-port network. The measured magnitude of
(or
) was about -1 dB over the entire frequency region, which indicates that the thin film
material sufficiently has high conductivity to transmit the signal with little losses. We can also see that the value of increases with increased humidity, which means that the signal transmission is improved by enhancement the conductivity of the thin film.
7
Figure 6. S-parameters of the thin conducting polymer film. (a) Magnitude of , and (d) phase of
, (b) phase of
, (c) magnitude of
This high frequency performance change can be explained by the change in molecular structure of PEDOT: PSS as a function of relative humidity. Fig. 1 shows the chemical structures of PEDOT: PSS, where the top structure is PSS and the lower structure is PEDOT. The lower structure of the PEDOT is made with the benzoid and quinoid forms. The benzoid structure has a -electron localized, conjugated structure that remains strongly unaffected by external factors. In contrast, the quinoid structure of PEDOT has a delocalized state of
-electrons, which can be affected by other polar groups [34]. The
benzoid structure may also be the best structure for coil conformation, while the quinoid structure may be the best structure for linear structure. As humidity increases, the water vapor induces additional doping of the conjugated polymer PEDOT: PSS. This phenomenon could cause more of the thin film to change from a benzoid structure to a quinoid structure. Also, while PSS is separated from PEDOT: PSS due to the water vapor’s penetration into the thin film, the bond energy between the PEDOT chains may increase. Therefore, the conductivity of the film is enhanced [35]. Fig.7 shows the attenuation constant and phase constant of the thin conducting polymer film on PCB using the S-parameters obtained from the experiments. The attenuation constant ( ) shown in Fig 7(a) represents the attenuation of electromagnetic wave propagating through the thin film per unit length. The attenuation is originated from the conductor as well as dielectric losses. It is clear that the attenuation constant of the thin film is increased by decreasing humidity. The phase constant ( ) shown in Fig 7(b) expresses the amount of phase shift per unit length that occurs when the wave travels the thin film. Note that there are no noticeable difference in the phase constant with relative humidity level. However, the attenuation constant shows clean distinction between different levels of humidity. Thus, this difference can be utilized for humidity sensor applications using transmission differences at high frequencies..
8
Figure 7. Propagation constant of the thin conducting polymer film. (a) The attenuation constant and (b) the phase constant extracted from measured S-parameters of the thin film.
The measured S-parameters are also used to extract lumped-element circuit parameters R, L, C and G. On the transmission line, the signal is transmitted in the form of an electric and magnetic fields. This complex phenomenon can be understood by an equivalent lumped-element circuit model. Fig 8(a) shows the series resistance of the thin film on the PCB. This series resistance represents the resistance due to the finite conductivity of the conducting polymer film used in this experiment. This series resistance significantly decreases with increasing humidity at all frequencies. This experimental result can be explained why
increases and the attenuation
constant decreases with humidity. The shunt conductance, G, or dielectric loss on the FR-4 substrate is shown in Fig 8(b). The change of dielectric loss is negligible at all frequencies. Figs 8(c) and (d) show the inductance and capacitance of the conducting polymer film, respectively. Note that the variation of the inductance and capacitance due to the humidity can be negligible compared to that of the resistance due to the humidity.
9
Figure 8. Transmission line parameters. (a) Resistance, (b) conductance, (c) inductance, and (d) capacitance of the microstrip-structure conducting polymer film.
4. Conclusion This study has investigated the microwave transmission properties of the thin conducting polymer, PEDOT: PSS film, on PCB with humidity variation based on microstrip line structure. We have confirmed that the changes in the high-frequency characteristics of the thin film as a function of relative humidity are closely related to the thin film’s equivalent resistance change. We found that, as the relative humidity increased from 10% to 50%, the series resistance has been decreased by an average of 6.43% in the observed frequency regime (1-10 GHz). This result can be utilized for the development of highfrequency fast-response humidity sensor systems.
Acknowledgments This research was supported by the MSIP (Ministry of Science, ICT and Future Planning), Korea, under the ITRC (Information Technology Research Center) support program (IITP-2017-2013-0-00680) supervised by the IITP (Institute for Information & communications Technology Promotion) This research was also supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2015R1C1A1A02037804) and by the Mid-career
10
Researcher Program through the National Research Foundation of South Korea (NRF) funded by the Science and Engineering, (NRF-2017R1A2B4012051).
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S0263-2241(19)30041-7 https://doi.org/10.1016/j.measurement.2019.01.032 MEASUR 6276
To appear in:
Measurement
Received Date: Revised Date: Accepted Date:
12 July 2017 8 November 2017 14 January 2019
Please cite this article as: T-G. Kang, J-K. Park, B-H. Kim, J.J. Lee, H.H. Choi, H-J. Lee, J-G. Yook, Microwave characterization of conducting polymer PEDOT: PSS film using a microstrip line for humidity sensor application, Measurement (2019), doi: https://doi.org/10.1016/j.measurement.2019.01.032
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.
Microwave characterization of conducting polymer PEDOT: PSS film using a microstrip line for humidity sensor application Tae-Gyu Kang, 1 Jin-Kwan Park, 1 Byung-Hyun Kim, 1 Jung Joon Lee, 2 Hyang Hee Choi, 2 Hee-Jo Lee, 3 and Jong-Gwan Yook1 1
School of Electrical and Electronic Engineering, Yonsei University, Seoul, 120-749, South Korea
2
Institute of Engineering Research, Yonsei University, Seoul, 120-479, South Korea
3
Department of Physics Education, Daegu University, Gyeongsan, Gyeongbuk, 38453, South Korea
In this paper, we characterize the thin conducting polymer film, PEDOT: PSS, using a microstrip line in a humid environment at microwave frequencies. To characterize the thin film alone over the microstrip line, a de-embedding technique of thru-reflect-line (TRL) calibration is utilized. After the de-embedding procedure, the thin film is modeled with a transmission line, and the film’s R, L, G, and C parameters are extracted from measured S-parameters. In addition, the characteristics of the thin film has been tested in terms of electrical conductivity with increasing relative humidity (RH). In the future, it is expected that this work can contribute to the improvement of stability as well as performance of conducting polymer-based electronic sensing devices.
1
1. Introduction Over the past few decades, microwave engineers have proposed numerous approaches and methodologies for measurement techniques to characterize thin materials such as dielectric and magnetic films in both the narrow and broad frequency regions [1-3]. In particular, these techniques based on planar strip lines have been widely used for electrical characterization of material properties in the microwave regime [4-8]. Among these approaches, the technique using the coplanar waveguide (CPW) and the microstrip line have been most widely used when extracting the electrical characteristics of the substrate or thin film sample [9]. The material properties based on planar strip lines can be characterized by a transmission line (over a broadband) and with a resonator method (over a narrow band). Both approaches accurately extract the microwave/millimeterwave characteristics when the material-under-test (MUT) is placed on a bare sample [10, 11]. Although these methods have shown accurate results, the de-embedding process for removing parasitic components must be included. Thus, many researchers have employed the method of MUT on a resonators, a bulky type like dielectric resonator (DR), due to simple and direct measurement capability [12-14]. For this reason, a user-friendly calibration kit for materials characterization in the broad band should be carefully designed to easily extract the characteristics of unknown materials. In this study, to characterize a desired material via the designed calibration kit, we have used the PEDOT: PSS (poly (3, 4-ethylenedioxythiophene) polystyrene sulfonate) film, which is one of the well-known conducting polymer [15-21]. PEDOT: PSS film is critical to predicting changes in electrical properties depending on environmental conditions (such as humidity or temperature) for the stability of electrical devices [22]. Several research groups have studied the influence of relative humidity (RH) on the electrical properties of PEDOT: PSS [23-26]. In particular, the high-frequency property of 2D thin film of PEDOT: PSS on printed circuit boards (PCBs) has attracted significant attention because of advances in highspeed integrated-circuit (IC) technology [27, 28]. Thus, high frequency characteristics of PEDOT: PSS thin film deposited on the microstrip line depending on humidity change are investigated in this work. In this study, we present elaborately approaching process to characterize the thin conducting polymer film between the microstrip lines depending on relative humidity at microwave frequencies. Section 2 presents the sample preparation of the thin film on PCB and the integrated thru-reflect-line (TRL) de-embedding technique for analyzing the propagation properties of the thin film. Finally, in section 3, we extract the microwave characteristics (such as impedance, propagation constant, and lumped circuit models) to predict the electrical properties of the film depending on humidity using the measured scattering parameters(S-parameters).
2
2. Experimental setup 2.1 Sample preparation
Figure 1. Chemical structure of the PEDOT: PSS film.
Figure 2. Schematic of microstrip line device for characterization of PEDOT: PSS thin film.
The chemical structure of the poly (3, 4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT: PSS) is as shown in Fig.1 [29]. PEDOT was originally developed as an insoluble polymer, however its high conductivity soon made the material the subject of strong scientific interest. The insolubility problem of PEDOT has been overcome by the incorporation of poly (styrene sulfonate) PSS material and water. As a result, PEDOT: PSS came to have high conductivity and can be easily transformed to arbitrary shape as a water-soluble polyelectrolyte material [30]. For the experimental study, we used the microstrip line with SMA connectors to achieve a two-port measurement system, as illustrated in Fig.2. The sample preparation was as follows: a 50 ohm microstrip line was fabricated on an FR-4 substrate, and
3
a gap with a dimensions 1.15 mm × 2.3 mm was etched at the middle of the line. The gap in the PCB substrate was treated with an oxygen plasma for 1800 sec to intensify the adhesion between the PCB surface and the organic polymer. Next, using the bar-coating method, polymer thin-films was placed onto the PCB substrate by heating the film at 80
for 10 min on a
hotplate in ambient lab conditions. With this process, the thin film replaces a small part of the copper signal line. It is clear that a uniform layer of the film could be deposited in the gap area on the microstrip line. The thickness of the thin film is roughly 10 um, and it was measured by an AS 500 alpha-step surface profiler (KLA-Tencor Co., USA). The AFM image of the film reveals a uniform surface with an average roughness of 2.192nm, as shown in Fig. 3. In this work, all organic materials were used without further purification. PEDOT which is the solid content is 1.1 wt% and the weight ratio of PEDOT to PSS is 1:2.5. To optimize the electrical property of the PEDOT: PSS film, dimethyl sulfoxide (DMSO, 99%) was added to the aqueous PEDOT: PSS solution (5.0 v/v%) and stirred at room temperature. The solution was then filtered by a syringe filter. We used PCB substrates FR-4 with a permittivity of 4.4, a loss tangent of 0.02, and a thickness of 1.2mm. Also, the thickness of the copper layer is 0.035mm.
Figure 3. AFM image of thin conducting polymer film
2.2 TRL calibration To obtain the microwave characteristics of the thin conducting polymer PEDOT: PSS film, it is necessary to use the deembedding technique [31]. The sample illustrated in Fig. 2 contains signal lines and SMA connectors on both sides of the thin film, so, both components have a strong effect on measuring the microwave characteristics of the thin film itself. Using the thru-reflect-line (TRL) calibration, we have conducted the TRL de-embedding procedure up to the reference plane and extracted the propagation properties of the film. Fig. 4(a) shows the custom-designed TRL calibration set used in this experiment. These known three components allow the characterization of the additional structures and it is possible to predict the performance of the PEDOT: PSS thin film itself. The thru component is made by directly connecting port 1 to port 2 at the desired reference planes. In this work, a non-zero thru component with a length of 2 L1 is used, where L1 denotes the length from the end of the SMA port to the calibration
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reference plane. The reflect component with a half-length of the thru component has known load value, such as open. The line component has a length of 2 L1+L2, where L2 is a certain line length used in the calibration process. Also, these custom-designed TRL calibration sets are made with the same SMA connector, substrate and line dimensions as the DUT sample. After the TRL calibration procedure, part of L1 can be matched with 50 ohm and no longer has loss or phase delay even though the part does have physical length. Note that the dashed line shown in Fig. 2 defines the reference plane used in the TRL extraction algorithm, thus allowing characterization of the PEDOT: PSS layer itself. The details of the mathematical procedure related to TRL calibration are described in reference [32].
Figure 4. Fabricated TRL calibration kit. (a) component of thru-reflect-line, (b) validation of calibration using a thru component.
Before measurement of the DUT sample, the performance of a TRL calibration kit is necessary. Fig. 4(b) shows the frequency response from connecting the thru component after TRL calibration. The signal transmissions (
and
)
between the two ports should be close to 0 dB level. This means that the signals propagate with very small reflection, because the non-zero thru has no electrical length and loss within the calibration process despite having physical length. From the results, the TRL calibration kit for characterizing the thin conducting polymer film is successfully performed so that the impedance of each port set to 50 ohm and reference plane is shifted to the dashed line in Fig.2.
2.3 Extraction of electrical parameters After performing the TRL calibration, the sample can be modeled, as shown in Fig. 5(a). In this stage, an interconnected microstrip line consisting of the thin conducting polymer film with characteristic impedance physical length
, propagation constant
is placed between the two-port networks with 50 ohm reference impedance. The element
parameter matrix, is determined by driving port with an incident power
and measuring the output power
of the Sfrom port .
Because these microstrip lines are physically symmetrical, the S-parameters is symmetrical as well as reciprocal, and
.
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and
Figure 5. (a) Transmission line model of the DUT, (b) lumped circuit model of the conducting polymer film after TRL calibration.
The relationship between the measured S-parameters and the transmission line parameters, ,
and
, can be expressed as
follow [33,36]: (1a) , (1b) where
. The S-parameter matrix can be converted to ABCD parameters, which
more explicitly represent the propagation constant and the impedance
. The equivalent ABCD matrix is [33]:
(2) Also, the relationship between the S-parameters and the ABCD matrix can be represented as follows: (3a) (3b) (3c) (3d) where From equations (1)-(3),
. and
can be obtained as follows:
(4) (5a) (5b)
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Equations (4) and (5) represent a complete solution to a microwave network that includes impedance discontinuity. In this work, impedance discontinuity occurs by placing the thin film of impedance the characteristic impedance
between the reference transmission lines with
. The equations (4) and (5) can often transcend solutions that often have ambiguous physical
meanings, such as negative attenuation constants even if the wave attenuates. With this in mind, a solution that matches the modeling situation should be chosen. Fig. 5(b) shows the equivalent lumped circuit model of the thin film after de-embedding the reference transmission line. A transmission line with infinitesimal length can be modeled as a lumped circuit, as shown in Fig. 5(b), where R, L, G and C have per-unit-length quantities. Once
and
are obtained, the distributed circuit parameters and the corresponding
electrical characteristics can be determined from the standard transmission line relationships below: (6) (7) and, (8a) (8b) (8c) (8d) Thus, the lumped-element transmission line model parameters are determined from the above equations (8a)-(8d).
3. Results and discussion. The sample shown in Fig. 2 with characteristic impedance of
and a propagation constant of γ is deposited on a grounded
FR-4 substrate, replacing the signal line. Fig. 6 shows the frequency response of the thin film on PCB in a variety of relative humidity (RH) levels (10%, 30%, and 50%) in a frequency range from 1 to 10 GHz. The magnitude of small, less than -15 dB, which indicates that the impedance mismatch is not significant. However,
(or
(or
) are very
) is intuitive to
understand because it indicates how well the signal is transmitted through the transmission line over a two-port network. The measured magnitude of
(or
) was about -1 dB over the entire frequency region, which indicates that the thin film
material sufficiently has high conductivity to transmit the signal with little losses. We can also see that the value of increases with increased humidity, which means that the signal transmission is improved by enhancement the conductivity of the thin film.
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Figure 6. S-parameters of the thin conducting polymer film. (a) Magnitude of , and (d) phase of
, (b) phase of
, (c) magnitude of
This high frequency performance change can be explained by the change in molecular structure of PEDOT: PSS as a function of relative humidity. Fig. 1 shows the chemical structures of PEDOT: PSS, where the top structure is PSS and the lower structure is PEDOT. The lower structure of the PEDOT is made with the benzoid and quinoid forms. The benzoid structure has a -electron localized, conjugated structure that remains strongly unaffected by external factors. In contrast, the quinoid structure of PEDOT has a delocalized state of
-electrons, which can be affected by other polar groups [34]. The
benzoid structure may also be the best structure for coil conformation, while the quinoid structure may be the best structure for linear structure. As humidity increases, the water vapor induces additional doping of the conjugated polymer PEDOT: PSS. This phenomenon could cause more of the thin film to change from a benzoid structure to a quinoid structure. Also, while PSS is separated from PEDOT: PSS due to the water vapor’s penetration into the thin film, the bond energy between the PEDOT chains may increase. Therefore, the conductivity of the film is enhanced [35]. Fig.7 shows the attenuation constant and phase constant of the thin conducting polymer film on PCB using the S-parameters obtained from the experiments. The attenuation constant ( ) shown in Fig 7(a) represents the attenuation of electromagnetic wave propagating through the thin film per unit length. The attenuation is originated from the conductor as well as dielectric losses. It is clear that the attenuation constant of the thin film is increased by decreasing humidity. The phase constant ( ) shown in Fig 7(b) expresses the amount of phase shift per unit length that occurs when the wave travels the thin film. Note that there are no noticeable difference in the phase constant with relative humidity level. However, the attenuation constant shows clean distinction between different levels of humidity. Thus, this difference can be utilized for humidity sensor applications using transmission differences at high frequencies..
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Figure 7. Propagation constant of the thin conducting polymer film. (a) The attenuation constant and (b) the phase constant extracted from measured S-parameters of the thin film.
The measured S-parameters are also used to extract lumped-element circuit parameters R, L, C and G. On the transmission line, the signal is transmitted in the form of an electric and magnetic fields. This complex phenomenon can be understood by an equivalent lumped-element circuit model. Fig 8(a) shows the series resistance of the thin film on the PCB. This series resistance represents the resistance due to the finite conductivity of the conducting polymer film used in this experiment. This series resistance significantly decreases with increasing humidity at all frequencies. This experimental result can be explained why
increases and the attenuation
constant decreases with humidity. The shunt conductance, G, or dielectric loss on the FR-4 substrate is shown in Fig 8(b). The change of dielectric loss is negligible at all frequencies. Figs 8(c) and (d) show the inductance and capacitance of the conducting polymer film, respectively. Note that the variation of the inductance and capacitance due to the humidity can be negligible compared to that of the resistance due to the humidity.
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Figure 8. Transmission line parameters. (a) Resistance, (b) conductance, (c) inductance, and (d) capacitance of the microstrip-structure conducting polymer film.
4. Conclusion This study has investigated the microwave transmission properties of the thin conducting polymer, PEDOT: PSS film, on PCB with humidity variation based on microstrip line structure. We have confirmed that the changes in the high-frequency characteristics of the thin film as a function of relative humidity are closely related to the thin film’s equivalent resistance change. We found that, as the relative humidity increased from 10% to 50%, the series resistance has been decreased by an average of 6.43% in the observed frequency regime (1-10 GHz). This result can be utilized for the development of highfrequency fast-response humidity sensor systems.
Acknowledgments This research was supported by the MSIP (Ministry of Science, ICT and Future Planning), Korea, under the ITRC (Information Technology Research Center) support program (IITP-2017-2013-0-00680) supervised by the IITP (Institute for Information & communications Technology Promotion) This research was also supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2015R1C1A1A02037804) and by the Mid-career
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Researcher Program through the National Research Foundation of South Korea (NRF) funded by the Science and Engineering, (NRF-2017R1A2B4012051).
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