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Abstract
We study a conductive polymer PEDOT: PSS for infrared thermal detection. We propose to use PEDOT: PSS simultaneously as the thermal sensitive material, infrared absorption layer, thermal insulation layer, as well as the electrical connection material. We demonstrate theoretically and confirm experimentally that PEDOT: PSS has a high temperature coefficient of resistance (TCR) of ∼0.6%/K at room temperature, which is comparable to those inorganic materials commonly used in commercial bolometers. We then experimentally characterize the infrared absorption of a thin PEDOT: PSS film, and we find that it has considerable absorption (over 0.2) in a broad range even with a film as thin as 100nm. This is beneficial to reduce the thermal capacitance and thereby increase the response speed. Finally, we characterize the infrared response of PEDOT: PSS film, and the detectivity is found to be 2×108 cmHz1/2/W. This work is especially inspirational in the design and fabrication of versatile, and highly integrated sensors and detectors, as well as other polymer-based devices and systems.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Infrared thermal detectors have found a wide range of commercial and military applications, as they offer small footprint, high degree of integration, instant and room temperature operations, as well as cost-effective mass production. Depending on the operation principle, several types of thermal detectors: thermopiles, Golay cells, pyroelectric sensors, and bolometers [1–4] have been developed and reported. The main advantage of thermal detectors is broad spectral range of operation, which is typically not obtainable with photon detectors. Furthermore, unlike cooled photon detectors, thermal detectors can operate at room temperatures, and therefore, suitable for compact and remote industrial sensing or imaging applications. The main drawback of thermal detectors compared to photon detectors is that it has relatively inferior sensitivity and detectivity. Nevertheless, by using modulated sources or choppers, one can significantly enhance overall detectivity of thermal detectors.
Among those thermal detectors, bolometers are currently considered as the prime candidates for real time imaging and sensing applications, due to their relatively high sensitivity and established fabrication technology. The bolometer element is operated by measuring the changes in the resistance upon infrared radiation. The main constituents of a bolometer element (or pixel) is summarized in Fig. 1. The infrared radiation is incident onto an absorbing layer and the thermal sensitive element (heat capacity ${\textrm{C}_\textrm{A}}$ and ${\textrm{C}_\textrm{S}}$) connected via a thermal link (conductance ${\textrm{G}_\textrm{L}}$) to a heat sink (reservoir at temperature ${\textrm{T}_\textrm{S}}$). The time constant (time constant ${\tau } = ({\textrm{C}_\textrm{A}} + {\textrm{C}_\textrm{S}})/{\textrm{G}_\textrm{L}}$) and detectivity are the two main factors commonly used to evaluate the performance of bolometers. The detectivity is mainly determined by the thermal-sensitive material, which converts temperature-dependent resistance changes into electrical changes, as well as the thermal conductance between the element and its surroundings. Traditional thermal sensitive material routinely based on inorganic materials (typically amorphous silicon or vanadium oxide), because of their high thermal coefficient of resistance (TCR) [5,6]. Nevertheless, due to the high reflecting nature of inorganic materials in infrared region, an infrared absorbing layer (i.e. carbon black, metamaterials) or resonant structures needs to be employed in order to enhance the infrared absorbance [7,8]. This inevitably compromises on the response speed due to the increase of the pixel capacitance. Furthermore, in order to prevent the heat leaking from the thermal sensitive layer to the substrate before the temperature changes caused by infrared absorption disappear, suspended micro-bridges with low thermal conductivity are routinely fabricated to connect the sensing pixel to the readout circuit. The micro-bridges with long legs and a small cross-sectional area are commonly adopted, which would achieve a small thermal conduction between the sensing element and its surroundings. Currently, inorganic material, such as Si3N4 with good micro-machining compatibility, are commonly used for the micro-bridges fabrication [9,10]. However, such a material has a relatively high thermal conductivity [11,12] (∼90W m−1 K−1), which could result in the leakage of the heat. Ultra-thin and narrow bridges are required to reduce thermal conductance, which, on the other hand, comprises on the mechanical stability and overall yield. Additionally, in order to ensure electrical connection between the sensing element and the read-out electronic, a metal layer has to be deposited on the bridge. Nevertheless, this step not only makes the fabrication process more complicated, but also increase the element capacitance and thereby the time constant.
The advantage of conductive polymer provides a better alternative for developing bolometer sensing element. First, polymers generally exhibit much lower thermal conductivity than inorganic materials [13–16], and typically show a value below 1W m−1 K−1. This makes it a good candidate for the fabrication of the micro-bridges for thermal insulation. Second, some semiconductors or conductive polymers are temperature sensitive and have relatively high TCR. Furthermore, some polymers show a wider and higher absorption in the infrared region, which is potential avoid the use of the absorbing layer and further reduce the pixel capacitance, and thereby, the time constant. Moreover, since some conductive polymers have high conductivity, they can be directly used for electrical connection. Finally, due to the solubility of most organic materials, one can conveniently choose solution-based processing method, which is extremely suitable for cost-effective mass production.
Due to those appealing advantages, conductive polymers including PEDOT: PSS have been studied for infrared thermal detection [17–19]. Particularly, in [17], the authors investigated PEDOT: PSS as the active material, and tested thermal response of PEDOT: PSS film. Later in 2009 [18], the authors characterized the TCR of PEDOT: PSS films. By using an appropriate thermal treatment, one can further improve the thermal sensing capability of PEDOT: PSS films. More recently, Hankansson demonstrated a freestanding layer of PEDOT: PSS for infrared detection [19].
In this paper, we propose to use PEDOT: PSS as the sensitive material, thermal insulation bridge, infrared absorption layer, as well as the electrical connection layer, simultaneously. We study the TCR of PEDOT: PSS thin film in the range of 100K-480K and study the thermal sensing mechanism. Then, we study the infrared absorption of PEDOT: PSS film in the infrared region and explore the underlying principle. Furthermore, we characterize the infrared response of a thin PEDOT: PSS film. We believe that the work in this paper is inspirational and relevant in the design of versatile, and highly integrated thermal infrared sensors and detectors, among other polymer devices and systems.
2. Conductive polymer PEDOT: PSS
Poly (3, 4-ethylenedioxythiophene) or simply PEDOT is a conducting polymer, which has been considered as one of the most successful polymers, from fundamental and practical perspectives. Unfortunately, PEDOT itself is insoluble and presents low conductivity, nevertheless, this drawback can be circumvented by doped with a water-dispersible polymer (polystyrene sulfonic acid (PSS)). The introduction of PSS into PEDOT has two main functions: on one side, it facilitates the charge transportation, on the other side, it allows the dispersion of the PEDOT in water, generating a complex where the oligomeric PEDOT segments are attached to the long chains of the PSS.
The charge transport in PEDOT: PSS follows a Mott variable-range hopping (VRH) model [20,21]. In this model, charge carriers hop from a localized state to a nearby localized state of different energy. The optimal hopping distance decreases with increasing temperature, hence the conductivity increases. The temperature-dependent conductivity of PEDOT: PSS thin films can be described by the follow equation [22]:
3. Thermal sensitivity characterizations of the PEDOT: PSS film
In order to characterize the sensitivity of PEDOT: PSS material for infrared thermal detection, we measure the TCR value of a PEDOT: PSS film. An aqueous dispersion of PEDOT: PSS (commercially known as Clevious P) was purchased for this work. A thin film with a thickness of ∼800nm is spin-coated onto a glass substrate with patterned electrodes. Prior to the spin-coating process, the substrates were thoroughly cleaned, and the coated thin film is subsequently thermal-treated at 100°C for 10 minutes to evaporate the water. The current-voltage (I-V) characteristics of the thin film are measured by using a comprehensive analysis system (model: ASEC-03), which has high current sensitivity (1pA) and wide temperature range (100-480K), as shown in Fig. 2(a). The test is carried out at room temperature. From the I-V curves, the corresponding resistance of the thin film at certain temperatures can be easily obtained. As observed from Fig. 2(b), the resistance of PEDOT: PSS film decreases exponentially with the increasing temperature, which agrees well with Eq. 1.
Fig. 2. (a) Measured current-voltage curves of PEDOT: PSS film at temperatures ranging from 90 K to 410 K. (b) The derived resistance of the PEDOT: PSS film at different temperatures ranging from 90 K to 480 K and their exponential fitting. Squares and circles represent two sequential measurements, which shows good repeatability. (c) The derived TCR values of PEDOT: PSS film at different temperatures.
The thermal sensing capability of the material can be evaluated by its temperature coefficient of resistance (TCR), which is written as:
From Fig. 2(b), we derive the TCR values of PEDOT: PSS at different temperatures, as shown in Fig. 2(c). We note that it varies from ∼10−3 to ∼10−2%/K at different spectral range. This is also comparable with commercially available thermal sensitive materials such as silicon, and VO2 [23,24].4. Infrared absorption test of PEDOT: PSS film
Infrared absorption is another critical factor which influences the responsibility and detectivity of bolometers. In this work, we also study the absorption of thin PEDOT: PSS film. To this end, we first choose and characterize the transmission spectra of highly transparent substrates in the infrared range. Then, we prepare a thin film of PEDOT: PSS (∼100nm) on the same substrates by using a spin coating method, and measure the transmission spectra. The reference and sample characterization are performed in sequence in the same setup, in order to guarantee same experimental and optical coupling conditions. The difference in the transmission spectra of the sample and reference measurement can be considered as the absorption of the PEDOT: PSS film. For the spectral range from 200nm to 2500nm, we choose highly transparent quartz glass as the reference substrate, and for the far infrared range between 2500-25000nm, we choose a Germanium (Ge) substrate which has high transmission in this range. PEDOT: PSS film with a thickness of ∼100nm is prepared on the substrate, which is then ready for subsequent characterizations. In both cases, we measure the transmission spectra of the substrates before and after applying the PEDOT: PSS film. The transmission spectra are illustrated in Fig. 3. As is obvious from Fig. 3, PEDOT: PSS film is highly transparent in the visible range, but shows a considerable absorption in the mid-infrared and infrared regions of the spectrum with an estimated absorption over 0.2. This optical behavior is related to presence of charge carriers, which has been explained with a so-called localization modified Drude model [25]. The carriers in the PEDOT: PSS material can be considered as a free electron gas, and the mean-free path is restricted by the defects and disorders along the polymer chains. The absorption band of PEDOT: PSS is mainly due to the intra-band transitions of the free carriers. Nevertheless, the inter-band and intra-chain transitions may also make certain contribution to the infrared absorption behavior of PEDOT: PSS film [26]. In fact, it is reported that one can even tune the absorption band by controlling the doping level, as reported in [25]. This is of great significance when designing infrared sensors and detectors, to target specific spectral ranges.
Fig. 3. Transmission spectra of the substrates with and without PEDOT: PSS thin film. (a) Glass substrate is chosen for the range from 200 nm to 2500 nm. (b) Ge substrate is chosen for the range from 2500 nm to 25000 nm.
5. Thermal response test of PEDOT: PSS film
In order to evaluate the thermal response of the PEDOT: PSS material, we fabricate a thin PEDOT: PSS film on a glass substrate with pre-defined electrodes. In order to guarantee good adhesion on the substrate and electrical contact with the conductive polymer, thin layers of Cr/Au with thicknesses of 50nm, 100nm, are deposited sequentially on the substrate via a sputtering deposition method. In order to evaluate the infrared response, we apply a step voltage (∼1V) on the element, and measure the current response upon the infrared radiation. We choose a black body with a temperature of 600K (corresponding peak radiation wavelength ∼5µm) to mimic the infrared radiation source. In Fig. 4, the electrical output of such a polymer infrared device upon a modulated infrared radiation is shown. It is evident that the thin PEDOT: PSS film shows a current change under infrared radiation. The calibrated power density of this infrared source is 30µW/cm2, the sensing area of the element is 1.6×10−3cm2. The detectivity D* amounts to almost 2×108cmHz1/2/W, which is comparable to commercially available bolometers [23,27–28]. The absence of any measurable delay in the response of the device is consistent with the view that the charges on the polymer chain are responsible for the absorption of infrared radiation [28]. The energy delivered by the absorbed photon is dissipated very quickly, leading to a fast rise in the effective temperature for the charge carriers. The recovery of the device after illumination seems to be governed by heat transport from the PEDOT: PSS to its surroundings.
Fig. 4. Response of the PEDOT: PSS film with patterned electrodes. Voltage over the device 1 V. Current: ∼1.27×10−6A. Integration time of measurement: 80 ms. Illumination source: a black body at 600 K. Active area: Beam diameter 5 mm. The responsivity is calculated to be 5×10−3A/W. The noise level of the element is 3×10−10A.
6. Discussions
When designing a bolometer, some desirable features are a low thermal conductance between the sensing element and its surrounding, a high absorption of the infrared radiation, a temperature sensing material with a high TCR, and fair electrical connection, as well as a sufficiently low bolometer thermal time constant. Traditional designs rely on several different materials to address each of those requirements. In this paper, we study a conductive polymer PEDOT: PSS for infrared thermal detection. Right from the outset of this project, the motivation is to find one single versatile material, which has high temperature coefficient of resistance, considerable infrared absorption, low thermal conductivity, as well as fair electrical conductivity. The main advantage of this approach is simplicity in the bolometer design and manufacturing, while retaining comparable or superior performance offered by the bolometers based on traditionally inorganic materials.
Polymers typically have much lower thermal conductivity, so we start with a conductive polymer named PEDOT: PSS, and characterize the TCR values at a wide range of temperatures, measure the infrared absorption spectrum of PEDOT: PSS film, and test its response upon infrared radiation. Our experimental results and analysis indicate that PEDOT: PSS has a very good thermal sensitivity with high TCR in a wide temperature range, and considerable infrared absorption. Furthermore, this film itself is conductive, which can also be simultaneously used for the electrical connection. In fact, in this work, for preliminary demonstrations, we simply choose one cost-effective PEDOT: PSS dispersion with moderate conductivity (1S/cm). By introducing secondary dopants into PEDOT: PSS dispersions, which will result in highly conductive films (over 1000S/cm), we can further optimize the performance. Additionally, the conductivity of PEDOT: PSS film can be enhanced by more than three orders or even higher in magnitude by adding secondary dopants including polyalcohols or high dielectric solvents, such as methyl sulfoxide (DMSO) and N, Ndimethylformamide into the PEDOT: PSS solution [29,30].
One critical factor when evaluating the performance of a bolometer is the lifetime and repeatability of the thermal sensitive film. In 2009, Vitoratos et al. have studied the thermal degradation of PEDOT: PSS film. It is reported that the exponent ${\alpha }$ in Eq. 1 is decreased after multiple circles of thermal treatment in several days, which indicates that the conduction mechanism is changed during the thermal treatment [35]. That is to say, although organic materials exhibit good thermal and electrical performance, which can compete with inorganic materials, improvements need to be made in order to increase their lifetime.
Additionally, in practical implementation, it is possible to use a single-pixel detector for thermal sensing and imaging by raster scanning the single pixel, however, this would significantly compromise on the detection speed [36–38]. One way to increase the acquisition speed is to reduce the number of measurements based on compressive sensing realm. In one implementation, one could raster scan the single pixel at certain selected positions in certain trajectory. In another, one could also fix the position of the single pixel, and apply controllable and reconfigurable mask/modulator in the optical path. In both cases, the data acquisition is limited by the time constant of the pixel. In our future work, we will optimize the fabrication process of the suspended pixels, and investigate the response time and the data acquisition speed.
Now, we would like to comment on the design of the bolometer based on PEDOT: PSS for future practical implementations. Heat transfer between the sensing element and the heat sink is always the limiting factor of the bolometer performance. Heat transfer takes place in three forms: convection, radiation and conduction. Since the pixels are always packaged in a low pressure environment, convection is negligible compared to radiation and conduction. Radiant heat transfer is usually also very small. Therefore, thermal conductance is always the limiting mechanism of heat flow from the element to the readout circuit. Traditional bolometers are based on inorganic materials, which has relatively high thermal conductivity. In order to reduce conductance, one has to use micro-bridge with very thin and narrow dimensions on the price of comprising on the structure robustness and manufacturing complexity. In this work, since PEDOT: PSS has much lower thermal conductivity [31,32] (∼0.3 W m−1 K−1), which is two orders lower than that of Si3N4, it, therefore, permits huge freedom to design a micro-bridge with relative big dimensions to enhance the mechanical performance, as well as to simplify the manufacturing process. In this work, our preliminary design is shown in Fig. 5.
The practical demonstration of bolometers based on PEDOT: PSS requires fabrication of suspended PEDOT: PSS bridges, as shown in Fig. 5. Several traditional approaches including transfer and printing technique, sacrificial layer micromachining can be used to address this problem. Additionally, by introducing a cross-linking agent into PEDOT: PSS mixtures, one can conveniently achieve suspended films [33,34], which is also another approach to realize thus-designed sensing structures. Our group are currently working on optimizing the recipes and fabricating PEDOT: PSS bridges using the above-mentioned approaches, and we will report the results in the future publications.
7. Summary
In summary, we study a conductive polymer named PEDOT: PSS for infrared thermal detection. We use PEDOT: PSS simultaneously as the infrared absorption layer, thermal sensitive film, thermal insulation layer, as well as the electrical connection layer. By using experimental characterizations and analysis, we find PEDOT: PSS film has a relatively high TCR values (∼0.6%/K at room temperature), and a considerable absorption (∼0.2) in the infrared region. Then, we performed the infrared response test of the PEDOT: PSS film, which projects a detectivity of 2×108cmHz1/2/W. This is comparable to commercially available bolometers. We believe this work is inspirational for developing polymer-based devices and systems.
Funding
National Natural Science Foundation of China (11904135).
Acknowledgments
Experiments were performed at National Key Laboratory of Science and Technology on Surface Engineering, Lanzhou Institute of Physics, with support of Shengzhu Cao and Lanxi Wang.
Disclosures
The authors declare no conflicts of interest. References and links
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