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Polymer dielectric layer functionality in organic field-effect transistor based ammonia gas sensor
Organic Electronics 14 (2013) 3453–3459
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Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Polymer dielectric layer functionality in organic field-effect transistor based ammonia gas sensor Wei Huang, Junsheng Yu ⇑, Xinge Yu, Wei Shi State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Optoelectronic Information, University of Electronic Science and Technology of China (UESTC), Chengdu 610054, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 30 July 2013 Received in revised form 3 September 2013 Accepted 11 September 2013 Available online 25 September 2013 Keywords: Organic field-effect transistors (OFETs), Ammonia sensor Polymer dielectric Recovery property Low detect limitation
a b s t r a c t Ammonia (NH3) gas sensors based on pentacene organic field-effect transistors (OFETs) are fabricated using polymers as the dielectric. Compared with those incorporating poly(vinyl alcohol), poly(4-vinylphenol) or poly(methyl methacrylate) dielectric, a low detect limitation of 1 ppm and enhanced recovery property are obtained for OFETs with polystyrene (PS) as gate dielectric. By analyzing the morphologies of pentacene and electrical characteristics of the OFETs under various concentrations of NH3, the variations of the sensing properties of different dielectrics based OFET-sensors are proved to be mainly caused by the diversities of dielectric/pentacene interfacial properties. Furthermore, low surface trap density and the absence of polar groups in PS dielectric are ascribed to be responsible for the high performance of NH3 sensors. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Environmental monitoring, especially the air quality monitoring has become a global issue due to terrible air pollution around the world [1,2]. Gas sensors as an effective way to monitor the gas quality have been developed for more than 30 years [3,4]. Among various gas sensors, organic field-effect transistor (OFET) based sensors have many advantages over semiconductor resistor sensors and optical gas sensors [5–7]. Apart from the low cost and light weight features of OFET [8,9], the sensing property of such devices can be modulated by changing the working conditions and integrating them in oscillators or amplifier circuits. As is well known, organic semiconductor layer is a key factor in achieving high performance OFET-sensor. To date, many attempts in improving the sensing properties of organic semiconductor layers have been made [10,11]. For example, H. Katz et al. developed a sensor array with ⇑ Corresponding author. Tel.: +86 28 83207157. E-mail address: [email protected] (J. Yu). 1566-1199/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.orgel.2013.09.018
distinct response pattern and unambiguous recognition ability for individual analytes based on naphthalenetetracarboxylic diimide derivatives and copper phthalocyanine [12]. D. Yang et al. reported organic nitrogen dioxide sensors based on zinc phthalocyanine (ZnPc) nanofiber network with much better recovery characteristics and shorter response/recovery time compared with polycrystalline ZnPc film devices [13,14]. In addition, the dielectrics of OFETs also play an important role in sensing [15]. It is well known that proper dielectric materials or dielectrics surface modification can significantly enhance the performance of OFETs due to an improvement of the organic semiconductor/dielectric interface property or a better crystallization of the upper organic semiconductor [16–19]. In recent years, electrolyte-gate OFET sensors show great advantages in the detection of biomaterials in aqueous solution [20–22]. Q. Tang et al. designed sulfur dioxide sensors based on gas dielectric CuPc nanowire FET with detect limitation down to sub ppm levels (0.5 ppm) [23]. A. Klug et al. demonstrated a top-gate bottom contact OFET-sensor using pH-sensitive, xanthene-dye functionalized, active-sensing dielectrics
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with ammonia (NH3) detect limitation of 100 ppm [24]. The above researches imply that dielectrics of OFETs, especially the dielectric surface properties have great influence on the sensitive characteristics of OFET-sensors. However, as polymer dielectrics hold the potential of being utilized in flexible and low-cost electronic devices, the influence of such materials on performance in OFET gas sensors is still lack of research. Herein we report NH3 sensors based on OFETs with four kinds of polymer dielectrics, including polystyrene (PS), poly(vinyl alcohol) (PVA), poly(methyl methacrylate) (PMMA) and poly(4-vinylphenol) (PVP). Through analyzing the electrical characteristics of the devices under different NH3 concentrations and the surface morphologies of dielectric and pentacene, we identify the variations of sensing properties are mainly generated by the dielectrics/pentacene interface diversities. The low trap density and the absence of polar groups in PS dielectric contribute to the high performance of sensors based on pentacene OFETs, which show low detect limitation down to 1 ppm, high sensitivity and reliable recovery property.
2. Experimental 2.1. Device preparation The OFETs were fabricated on indium tin oxide glass substrates. The architecture of the OFETs is shown in Fig. 1, along with the chemical structures of the polymer dielectrics. Before the dielectric layers were spin-coated, the substrates were successively ultrasonic cleaned in acetone, deionized water and isopropyl alcohol. PS (average Mw 280,000), PVA (Mw 146,000–186,000, 99+% hydrolyzed), PMMA (average Mw 120,000) and PVP (average Mw 25,000) were dissolved in xylene, deionized water, anisole and butyl acetate, respectively. All the polymers are purchased from Sigma–Aldrich. The polymer dielectrics were spin-coated on ITO substrate at room temperature and dried in an oven under 100 °C for 1 h. The thickness of dielectrics was measured by a Dektak 150 stylus profiler. The finger source/drain electrodes (30 nm) were successively deposited under 3 103 Pa through a shadow mask. Subsequently, the devices were moved to another
Fig. 1. (a, b, c and d) Molecular structures of PS, PVA, PMMA and PVP in this study, respectively. (e) Schematic of OFET based gas sensors.
chamber for the deposition of 30 nm pentacene (Sigma–Aldrich) under 2 104 Pa at the rate of 0.2–0.3 Å/s. The devices with different dielectrics are referred as device A with PS dielectric, device B with PVA dielectric, device C with PMMA dielectric, device D with PVP dielectric. The electrical characteristics of the OFETs were carried out with a Keithley 4200 sourcemeter in nitrogen at room temperature. Charge carrier mobility (l) and threshold voltage (VTH) were extracted in the saturation regime from the highest slope of |IDS|1/2 vs. VGS plots using the following equation:
IDS ¼ ðW=2LÞlC i ðV GS V TH Þ2
ð1Þ
where L (100 lm) is the channel length, W (1 cm) is the channel width, Ci is the capacitance (per unit area) of the dielectric, VGS is the gate voltage, and IDS is the drain-source current. 2.2. Film characterization and sensor test The morphologies of the dielectrics and pentacene films were characterized by atom force microscopy (AFM) (MFP3D-BIO, Asylum Research) in tapping mode. The OFET-sensor was stored in an airtight test chamber (approximately 16 mL). Dry air and 500 ppm standard NH3 gas were purchased from Sichuan Tianyi Science & Technology Co., and a mixture with the appropriate concentrations was introduced into the test chamber by mass flow controllers (S48 300/HMT, Beijing BORIBA METRON Instruments Co.). The flow rate in the test was fixed at 100 sccm (standard cm3 per min). The testing environment with specific relative humidity was realized by utilizing a standard humidity generator (BSF-6X-2, Beijing Naisida New Technology Development Co.). 3. Results and discussion Six cycles of the real-time IDS responding to the dynamic switches in different NH3 concentration (10–100 ppm) at room temperature for these four devices are shown in Fig. 2. The testing process of the time dependence of the drain-source current is not started until the drain-source current reached a stable state (the drain-source current
Fig. 2. Response curves of the four kinds of devices to NH3 pulses.
W. Huang et al. / Organic Electronics 14 (2013) 3453–3459
barely decrease with time), and both VGS and the drain voltage VDS are 40 V. Y axis represents percentage changes of IDS, which is defined as DI = |(INH3 IAIR)/IAIR| 100%. It is obvious that the sensor based on device A holds the best sensing properties. The sensitivities of devices A and B are similar (about 6% decrease in IDS at 10 ppm concentration of NH3 and 20% decrease at 100 ppm concentration), while device A exhibits fairly better recovery property than that of device B. Meanwhile, the sensitivities of devices C and D, which are about 8% and 6% reduction of IDS at 100 ppm concentration, are lower than those of devices A and B, also poor recovery properties are observed in these two devices. In order to study the essential reason for the deviation in sensing performance, AFM is utilized to observe the morphologies of the dielectrics and the pentacene films grown on them. As shown in Fig. A.1 (see Appendix A), all the surface of the four kinds of dielectrics are fairly flat, with root mean square roughness of 194.5 pm on PS, 289.6 pm on PVA, 235.3 pm on PMMA and 201.5 pm on PVP. Similar morphologies of the dielectrics indicate that the molecular structures of dielectrics play a much more important role in affecting the sensing property rather than the morphologies. Fig. 3 shows the morphologies of pentacene films on four different dielectrics. Similar wellordered island shape grains are formed. Pentacene on PVA and PS both share about 0.8 lm length and 0.5 lm
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width. Meanwhile, pentacene on PMMA and PVP exhibits bigger average grain size of about 1.8 lm length and 1 lm width, which is approximately 4–5 times bigger in area than that on PVA and PS. Smaller grains induce more grain boundaries, which could reduce the electric characteristics, especially the mobility of the OFETs. On the other hand, at the aspect of sensing, more grain boundaries will increase the surface area-to-volume ratio, thus more NH3 molecules can diffuse into the pentacene and the interface of dielectric/pentacene in devices A and B [25]. Consequently, more obvious variations of IDS in devices A and B are observed compared with those of devices C and D, as shown in Fig. 2. However, the diversity of morphologies of pentacene can still not explain good recovery property of device A. In order to systematically investigate the sensing mechanism of these devices, we have tested the transfer curves of the four kinds of devices under different concentration of ammonia. All the devices were exposed to a specific concentration of NH3 for 2 min before measuring. The gate voltage VGS is 20 to 40 V and the drain voltage VDS is 40 V. Fig. 4a shows the transfer curves of PS based OFET in different NH3 concentrations (transfer curves of the other three kinds of OFETs in different NH3 concentrations are shown in Fig. A.2). There are several parameters to evaluate the performance of OFETs, such as on-state current (ION), mobility, threshold voltage and subthreshold
Fig. 3. AFM topography images of the pentacene films on dielectrics of PS (a), PVA (b), PMMA (c), and PVP (d), respectively.
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Fig. 4. (a) Transfer curves of PS-based OFET-sensor under a specific concentration of NH3. (b, c) Percentage variations of ION, l and VTH of all the devices at different NH3 concentrations. (d) Trap density and percentage variations of SS of all the devices at different NH3 concentrations.
slope (SS). These multiple parameters of the OFETs can be extracted from Fig. 4a and Fig. A.2, and their percentage changes (defined by DY = (YNH3 YAIR)/YAIR 100%) are presented in Fig. 4b–d as a function of NH3 concentration. As shown in Fig. 4b, the percentage variations of ION of all the devices decrease along with the increasing concentration of NH3. When the NH3 molecules diffuse into the pentacene film through the grain boundaries, the polar molecules may disturb charge transport in organic materials by increasing the amount and magnitude of energetic disorder through charge-dipole interactions [26,27]. Thus, NH3 molecules are usually regarded as scattering center or energy barrier for charge transport in pentacene OFETs. Due to smaller grain size in devices A and B, ION of such devices decrease about 25% and 30%, respectively, while only about 20% decrease in devices C and D are observed. The difference in grain size of pentacene can also explain the variations of charge carrier mobilities in devices A, C and D. However, the charge mobility of device B presents a slight increase before decrease, and the decreasing rate is relatively slow compared with those of devices A, C and D. It is well known that PVA based OFETs suffer from obvious hysteresis due to charge trapping and mobile ions in dielectric, which will dramatically increase Ci after applying a negative gate voltage [28,29]. According to Eq. (1), although IDS decreases, under the consideration of negative shifted VTH (as shown in Fig. 4c) and increased Ci which is set constant when calculation, l may stay unchanged or increase.
On the other hand, VTH of device A barely changes while the VTH of the other three devices shifts to more negative positions under the exposing to NH3, as shown in Fig. 4c. For VTH is usually referred to charge trapping at the interface of dielectric/pentacene, more NH3 molecules trap at the interface, more immobilized positive charges are located, then a stronger negative gate voltage is needed to turn the transistor on. Thus, more trap sites which will trap NH3 molecules are assumed to exist in the surface of dielectrics which consist of PVA, PMMA or PVP. Furthermore, SS is proportional to the trap density at the interface of dielectric and organic semiconductor, and the trap density (N) can be extracted by the following equation [9,23]:
SS ¼ ðkT=qÞ ln 10ð1 þ qN=CÞ
ð2Þ
where q is the electronic charge, k is Boltzmann’s constant, T is absolute temperature, and C is the areal capacitance of the dielectric structure. As shown in Fig. 4d, the SS decreases about 20% in device B, 10% in device C and 13% in device D at 100 ppm NH3 concentration, respectively. Nevertheless, the SS of device A keeps almost unchanged under the concentration of 10–100 ppm NH3. Through calculation using Eq. (2), the variations of N along with the NH3 concentration are also plotted in Fig. 4d. It is obvious that, device A hold the smallest N of about 3.1 1012 cm2 eV1, and stay the same density under NH3 concentration from 10 to 100 ppm. Meanwhile, as the NH3 concentration increases, trap densities of devices B, C and D which reveal larger amounts compared with
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that of device A will decrease. As PVA and PVP possess hydroxyl groups (–OH) which will facilitate the trapping of hole charge carriers [30], and the function groups of –COOCH3 in PMMA also possess the ability to act as hole trap sites [31], the trap densities reveal relative large amounts of 1.92 1013 cm2 eV1 in device B, 9.81 1012 cm2 eV1 in device C and 8.57 1012 cm2 eV1 in device D, respectively. While with a much stable structure, the function groups of PS (benzene rings) is proved to have the smallest hole trap density compared to that of PVA, PMMA and PVP [30]. Furthermore, hydroxyl groups in polymers have been reported to have the ability to interact with ammonia to form structure like ammonia water, thus the trapped ammonia at the dielectric surface may not be removed easily under the environment of dry air [32]. For PMMA dielectric, the dipole moment of the –COOCH3 has the ability to absorb NH3 molecules [31], while such interactions will not happen between ammonia and benzene rings. In addition, the polarities of PVA, PMMA and PVP are all higher than that of PS, which indicates dipole–dipole interactions can occur much more frequently between the above three dielectrics and polar ammonia. As a result, the trapped NH3 at the interface of dielectric/ pentacene can effectively reduce the density of negative interface states [32], which is consistent with the reduction of SS and interface trap density as shown in Fig. 4d, while with rare NH3 absorbed at the interface of PS/pentacene, almost constant SS and N are realized. From the above discussion, it can be seen that the sensing mechanism of NH3 with pentacene OFETs based on polymer dielectric lies mainly in two parts: (1) NH3 which diffuses into the grain boundaries of pentacene acts as scattering center or energy barrier for charge transport; (2) NH3 that is trapped by the dielectric acts as immobilized positive charge which can reduce SS and negatively
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shift VTH. When sensors are exposed to NH3, as shown in Fig. 5a and c, NH3 molecules will diffuse into pentacene and the interface. Under the condition of PVA, PVP or PMMA dielectric (Fig. 5a), both parts play important roles in the sensing mechanism. For OFETs based on PS dielectric (Fig. 5c), the first part mainly affects the sensing property. For the surface trap density of PS is low and benzene rings of PS can hardly react with NH3, the amount of NH3 that is trapped by dielectric is rare. When NH3 is removed from the testing environment, NH3 molecules that are trapped by the dielectrics will not be easily removed for the interaction force between NH3 and dielectric is relatively stronger (Fig. 5b). As a result, poor recovery properties in devices B, C and D are obtained. However, most of the NH3 molecules can be removed from PS based OFETs (Fig. 5d), thereby enhanced recovery property is achieved compared with those of OFETs based on PVA, PMMA and PVP. Furthermore, in our experiment, it is noteworthy that OFET based on PS shows NH3 detection down to 1 ppm, as shown in Fig. 6. Obvious response and recovery cycles along with the variation of NH3 concentration are observed. With a preferable interface of dielectric/organic semiconductor, sensing parameters, including sensitivity, detect limitation and response/recovery property, are tremendously improved. Moreover, according to a large number of previous reports, H2O vapor has been proven to be the key factor that would affect pentacene OFETs. Therefore, the humidity effect on the electrical characteristics of OFET based on PS is also studied. Relative humidity (RH) from 0% to 70% are introduced to the testing chamber, and the transfer curves of the PS based OFET under different RH are shown in Fig. A.3. It is found that the humidity would strongly change the off state current. However, the negligible changes of ION, l and VTH can be extracted as shown in
Fig. 5. (a, b) Represent the absorption and separation process of NH3 in devices B, C and D. (c, d) Represent the absorption and separation process of NH3 in device A.
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Acknowledgements This research was funded by the National Science Foundation of China (NSFC) (Grant No. 61177032), the Foundation for Innovation Groups of National Science Foundation of China via No. 61021061, the Fundamental Research Funds for the Central Universities (Grant No. ZYGX2010Z004), SRF for ROCS, SEM (Grant No. GGRYJJ0805). Appendix A. Supplementary material
Fig. 6. Response curve of OFET-sensor based on PS dielectric to NH3 concentration ranging from 1 to 10 ppm.
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.orgel.2013.09.018. References
Fig. 7. Percentage variations of ION, l, VTH and SS of PS based OFET at different relative humidity.
Fig. 7. These results are consistent with the previous reports [33,34]. Compared with Fig. 4, PS based OFET NH3 sensors were realized by obvious negative shift of saturation current and hole mobility under different NH3 concentrations. Therefore, PS based OFET sensors can distinguish NH3 from water when working in environment. 4. Conclusion In summary, we fabricated OFET based NH3 sensors with various polymer dielectric materials including PS, PVA, PVP, and PMMA. Among all these OFETs, the sensor based on PS dielectric exhibits a reliable recovery property under various concentrations of NH3. The enhancement of the sensing properties is attributed to the relatively smaller grain size of pentacene on PS dielectric, low surface trap density and chemically stable molecular structure consisting of benzene rings. Thus, NH3 can diffuse into the transistor easily and remove from it with less residual, leading to a remarkable detect limitation of 1 ppm. To our best knowledge, this is the lowest detect limitation of OFET based NH3 sensor fabricated with polymer dielectric on glass substrate. Also, such results demonstrate that the combination of p-type pentacene and proper polymer dielectrics in OFET can realize high performance NH3 sensors, which is helpful to developed low cost and high sensitive gas sensors.
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Contents lists available at ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Polymer dielectric layer functionality in organic field-effect transistor based ammonia gas sensor Wei Huang, Junsheng Yu ⇑, Xinge Yu, Wei Shi State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Optoelectronic Information, University of Electronic Science and Technology of China (UESTC), Chengdu 610054, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 30 July 2013 Received in revised form 3 September 2013 Accepted 11 September 2013 Available online 25 September 2013 Keywords: Organic field-effect transistors (OFETs), Ammonia sensor Polymer dielectric Recovery property Low detect limitation
a b s t r a c t Ammonia (NH3) gas sensors based on pentacene organic field-effect transistors (OFETs) are fabricated using polymers as the dielectric. Compared with those incorporating poly(vinyl alcohol), poly(4-vinylphenol) or poly(methyl methacrylate) dielectric, a low detect limitation of 1 ppm and enhanced recovery property are obtained for OFETs with polystyrene (PS) as gate dielectric. By analyzing the morphologies of pentacene and electrical characteristics of the OFETs under various concentrations of NH3, the variations of the sensing properties of different dielectrics based OFET-sensors are proved to be mainly caused by the diversities of dielectric/pentacene interfacial properties. Furthermore, low surface trap density and the absence of polar groups in PS dielectric are ascribed to be responsible for the high performance of NH3 sensors. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Environmental monitoring, especially the air quality monitoring has become a global issue due to terrible air pollution around the world [1,2]. Gas sensors as an effective way to monitor the gas quality have been developed for more than 30 years [3,4]. Among various gas sensors, organic field-effect transistor (OFET) based sensors have many advantages over semiconductor resistor sensors and optical gas sensors [5–7]. Apart from the low cost and light weight features of OFET [8,9], the sensing property of such devices can be modulated by changing the working conditions and integrating them in oscillators or amplifier circuits. As is well known, organic semiconductor layer is a key factor in achieving high performance OFET-sensor. To date, many attempts in improving the sensing properties of organic semiconductor layers have been made [10,11]. For example, H. Katz et al. developed a sensor array with ⇑ Corresponding author. Tel.: +86 28 83207157. E-mail address: [email protected] (J. Yu). 1566-1199/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.orgel.2013.09.018
distinct response pattern and unambiguous recognition ability for individual analytes based on naphthalenetetracarboxylic diimide derivatives and copper phthalocyanine [12]. D. Yang et al. reported organic nitrogen dioxide sensors based on zinc phthalocyanine (ZnPc) nanofiber network with much better recovery characteristics and shorter response/recovery time compared with polycrystalline ZnPc film devices [13,14]. In addition, the dielectrics of OFETs also play an important role in sensing [15]. It is well known that proper dielectric materials or dielectrics surface modification can significantly enhance the performance of OFETs due to an improvement of the organic semiconductor/dielectric interface property or a better crystallization of the upper organic semiconductor [16–19]. In recent years, electrolyte-gate OFET sensors show great advantages in the detection of biomaterials in aqueous solution [20–22]. Q. Tang et al. designed sulfur dioxide sensors based on gas dielectric CuPc nanowire FET with detect limitation down to sub ppm levels (0.5 ppm) [23]. A. Klug et al. demonstrated a top-gate bottom contact OFET-sensor using pH-sensitive, xanthene-dye functionalized, active-sensing dielectrics
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with ammonia (NH3) detect limitation of 100 ppm [24]. The above researches imply that dielectrics of OFETs, especially the dielectric surface properties have great influence on the sensitive characteristics of OFET-sensors. However, as polymer dielectrics hold the potential of being utilized in flexible and low-cost electronic devices, the influence of such materials on performance in OFET gas sensors is still lack of research. Herein we report NH3 sensors based on OFETs with four kinds of polymer dielectrics, including polystyrene (PS), poly(vinyl alcohol) (PVA), poly(methyl methacrylate) (PMMA) and poly(4-vinylphenol) (PVP). Through analyzing the electrical characteristics of the devices under different NH3 concentrations and the surface morphologies of dielectric and pentacene, we identify the variations of sensing properties are mainly generated by the dielectrics/pentacene interface diversities. The low trap density and the absence of polar groups in PS dielectric contribute to the high performance of sensors based on pentacene OFETs, which show low detect limitation down to 1 ppm, high sensitivity and reliable recovery property.
2. Experimental 2.1. Device preparation The OFETs were fabricated on indium tin oxide glass substrates. The architecture of the OFETs is shown in Fig. 1, along with the chemical structures of the polymer dielectrics. Before the dielectric layers were spin-coated, the substrates were successively ultrasonic cleaned in acetone, deionized water and isopropyl alcohol. PS (average Mw 280,000), PVA (Mw 146,000–186,000, 99+% hydrolyzed), PMMA (average Mw 120,000) and PVP (average Mw 25,000) were dissolved in xylene, deionized water, anisole and butyl acetate, respectively. All the polymers are purchased from Sigma–Aldrich. The polymer dielectrics were spin-coated on ITO substrate at room temperature and dried in an oven under 100 °C for 1 h. The thickness of dielectrics was measured by a Dektak 150 stylus profiler. The finger source/drain electrodes (30 nm) were successively deposited under 3 103 Pa through a shadow mask. Subsequently, the devices were moved to another
Fig. 1. (a, b, c and d) Molecular structures of PS, PVA, PMMA and PVP in this study, respectively. (e) Schematic of OFET based gas sensors.
chamber for the deposition of 30 nm pentacene (Sigma–Aldrich) under 2 104 Pa at the rate of 0.2–0.3 Å/s. The devices with different dielectrics are referred as device A with PS dielectric, device B with PVA dielectric, device C with PMMA dielectric, device D with PVP dielectric. The electrical characteristics of the OFETs were carried out with a Keithley 4200 sourcemeter in nitrogen at room temperature. Charge carrier mobility (l) and threshold voltage (VTH) were extracted in the saturation regime from the highest slope of |IDS|1/2 vs. VGS plots using the following equation:
IDS ¼ ðW=2LÞlC i ðV GS V TH Þ2
ð1Þ
where L (100 lm) is the channel length, W (1 cm) is the channel width, Ci is the capacitance (per unit area) of the dielectric, VGS is the gate voltage, and IDS is the drain-source current. 2.2. Film characterization and sensor test The morphologies of the dielectrics and pentacene films were characterized by atom force microscopy (AFM) (MFP3D-BIO, Asylum Research) in tapping mode. The OFET-sensor was stored in an airtight test chamber (approximately 16 mL). Dry air and 500 ppm standard NH3 gas were purchased from Sichuan Tianyi Science & Technology Co., and a mixture with the appropriate concentrations was introduced into the test chamber by mass flow controllers (S48 300/HMT, Beijing BORIBA METRON Instruments Co.). The flow rate in the test was fixed at 100 sccm (standard cm3 per min). The testing environment with specific relative humidity was realized by utilizing a standard humidity generator (BSF-6X-2, Beijing Naisida New Technology Development Co.). 3. Results and discussion Six cycles of the real-time IDS responding to the dynamic switches in different NH3 concentration (10–100 ppm) at room temperature for these four devices are shown in Fig. 2. The testing process of the time dependence of the drain-source current is not started until the drain-source current reached a stable state (the drain-source current
Fig. 2. Response curves of the four kinds of devices to NH3 pulses.
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barely decrease with time), and both VGS and the drain voltage VDS are 40 V. Y axis represents percentage changes of IDS, which is defined as DI = |(INH3 IAIR)/IAIR| 100%. It is obvious that the sensor based on device A holds the best sensing properties. The sensitivities of devices A and B are similar (about 6% decrease in IDS at 10 ppm concentration of NH3 and 20% decrease at 100 ppm concentration), while device A exhibits fairly better recovery property than that of device B. Meanwhile, the sensitivities of devices C and D, which are about 8% and 6% reduction of IDS at 100 ppm concentration, are lower than those of devices A and B, also poor recovery properties are observed in these two devices. In order to study the essential reason for the deviation in sensing performance, AFM is utilized to observe the morphologies of the dielectrics and the pentacene films grown on them. As shown in Fig. A.1 (see Appendix A), all the surface of the four kinds of dielectrics are fairly flat, with root mean square roughness of 194.5 pm on PS, 289.6 pm on PVA, 235.3 pm on PMMA and 201.5 pm on PVP. Similar morphologies of the dielectrics indicate that the molecular structures of dielectrics play a much more important role in affecting the sensing property rather than the morphologies. Fig. 3 shows the morphologies of pentacene films on four different dielectrics. Similar wellordered island shape grains are formed. Pentacene on PVA and PS both share about 0.8 lm length and 0.5 lm
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width. Meanwhile, pentacene on PMMA and PVP exhibits bigger average grain size of about 1.8 lm length and 1 lm width, which is approximately 4–5 times bigger in area than that on PVA and PS. Smaller grains induce more grain boundaries, which could reduce the electric characteristics, especially the mobility of the OFETs. On the other hand, at the aspect of sensing, more grain boundaries will increase the surface area-to-volume ratio, thus more NH3 molecules can diffuse into the pentacene and the interface of dielectric/pentacene in devices A and B [25]. Consequently, more obvious variations of IDS in devices A and B are observed compared with those of devices C and D, as shown in Fig. 2. However, the diversity of morphologies of pentacene can still not explain good recovery property of device A. In order to systematically investigate the sensing mechanism of these devices, we have tested the transfer curves of the four kinds of devices under different concentration of ammonia. All the devices were exposed to a specific concentration of NH3 for 2 min before measuring. The gate voltage VGS is 20 to 40 V and the drain voltage VDS is 40 V. Fig. 4a shows the transfer curves of PS based OFET in different NH3 concentrations (transfer curves of the other three kinds of OFETs in different NH3 concentrations are shown in Fig. A.2). There are several parameters to evaluate the performance of OFETs, such as on-state current (ION), mobility, threshold voltage and subthreshold
Fig. 3. AFM topography images of the pentacene films on dielectrics of PS (a), PVA (b), PMMA (c), and PVP (d), respectively.
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Fig. 4. (a) Transfer curves of PS-based OFET-sensor under a specific concentration of NH3. (b, c) Percentage variations of ION, l and VTH of all the devices at different NH3 concentrations. (d) Trap density and percentage variations of SS of all the devices at different NH3 concentrations.
slope (SS). These multiple parameters of the OFETs can be extracted from Fig. 4a and Fig. A.2, and their percentage changes (defined by DY = (YNH3 YAIR)/YAIR 100%) are presented in Fig. 4b–d as a function of NH3 concentration. As shown in Fig. 4b, the percentage variations of ION of all the devices decrease along with the increasing concentration of NH3. When the NH3 molecules diffuse into the pentacene film through the grain boundaries, the polar molecules may disturb charge transport in organic materials by increasing the amount and magnitude of energetic disorder through charge-dipole interactions [26,27]. Thus, NH3 molecules are usually regarded as scattering center or energy barrier for charge transport in pentacene OFETs. Due to smaller grain size in devices A and B, ION of such devices decrease about 25% and 30%, respectively, while only about 20% decrease in devices C and D are observed. The difference in grain size of pentacene can also explain the variations of charge carrier mobilities in devices A, C and D. However, the charge mobility of device B presents a slight increase before decrease, and the decreasing rate is relatively slow compared with those of devices A, C and D. It is well known that PVA based OFETs suffer from obvious hysteresis due to charge trapping and mobile ions in dielectric, which will dramatically increase Ci after applying a negative gate voltage [28,29]. According to Eq. (1), although IDS decreases, under the consideration of negative shifted VTH (as shown in Fig. 4c) and increased Ci which is set constant when calculation, l may stay unchanged or increase.
On the other hand, VTH of device A barely changes while the VTH of the other three devices shifts to more negative positions under the exposing to NH3, as shown in Fig. 4c. For VTH is usually referred to charge trapping at the interface of dielectric/pentacene, more NH3 molecules trap at the interface, more immobilized positive charges are located, then a stronger negative gate voltage is needed to turn the transistor on. Thus, more trap sites which will trap NH3 molecules are assumed to exist in the surface of dielectrics which consist of PVA, PMMA or PVP. Furthermore, SS is proportional to the trap density at the interface of dielectric and organic semiconductor, and the trap density (N) can be extracted by the following equation [9,23]:
SS ¼ ðkT=qÞ ln 10ð1 þ qN=CÞ
ð2Þ
where q is the electronic charge, k is Boltzmann’s constant, T is absolute temperature, and C is the areal capacitance of the dielectric structure. As shown in Fig. 4d, the SS decreases about 20% in device B, 10% in device C and 13% in device D at 100 ppm NH3 concentration, respectively. Nevertheless, the SS of device A keeps almost unchanged under the concentration of 10–100 ppm NH3. Through calculation using Eq. (2), the variations of N along with the NH3 concentration are also plotted in Fig. 4d. It is obvious that, device A hold the smallest N of about 3.1 1012 cm2 eV1, and stay the same density under NH3 concentration from 10 to 100 ppm. Meanwhile, as the NH3 concentration increases, trap densities of devices B, C and D which reveal larger amounts compared with
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that of device A will decrease. As PVA and PVP possess hydroxyl groups (–OH) which will facilitate the trapping of hole charge carriers [30], and the function groups of –COOCH3 in PMMA also possess the ability to act as hole trap sites [31], the trap densities reveal relative large amounts of 1.92 1013 cm2 eV1 in device B, 9.81 1012 cm2 eV1 in device C and 8.57 1012 cm2 eV1 in device D, respectively. While with a much stable structure, the function groups of PS (benzene rings) is proved to have the smallest hole trap density compared to that of PVA, PMMA and PVP [30]. Furthermore, hydroxyl groups in polymers have been reported to have the ability to interact with ammonia to form structure like ammonia water, thus the trapped ammonia at the dielectric surface may not be removed easily under the environment of dry air [32]. For PMMA dielectric, the dipole moment of the –COOCH3 has the ability to absorb NH3 molecules [31], while such interactions will not happen between ammonia and benzene rings. In addition, the polarities of PVA, PMMA and PVP are all higher than that of PS, which indicates dipole–dipole interactions can occur much more frequently between the above three dielectrics and polar ammonia. As a result, the trapped NH3 at the interface of dielectric/ pentacene can effectively reduce the density of negative interface states [32], which is consistent with the reduction of SS and interface trap density as shown in Fig. 4d, while with rare NH3 absorbed at the interface of PS/pentacene, almost constant SS and N are realized. From the above discussion, it can be seen that the sensing mechanism of NH3 with pentacene OFETs based on polymer dielectric lies mainly in two parts: (1) NH3 which diffuses into the grain boundaries of pentacene acts as scattering center or energy barrier for charge transport; (2) NH3 that is trapped by the dielectric acts as immobilized positive charge which can reduce SS and negatively
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shift VTH. When sensors are exposed to NH3, as shown in Fig. 5a and c, NH3 molecules will diffuse into pentacene and the interface. Under the condition of PVA, PVP or PMMA dielectric (Fig. 5a), both parts play important roles in the sensing mechanism. For OFETs based on PS dielectric (Fig. 5c), the first part mainly affects the sensing property. For the surface trap density of PS is low and benzene rings of PS can hardly react with NH3, the amount of NH3 that is trapped by dielectric is rare. When NH3 is removed from the testing environment, NH3 molecules that are trapped by the dielectrics will not be easily removed for the interaction force between NH3 and dielectric is relatively stronger (Fig. 5b). As a result, poor recovery properties in devices B, C and D are obtained. However, most of the NH3 molecules can be removed from PS based OFETs (Fig. 5d), thereby enhanced recovery property is achieved compared with those of OFETs based on PVA, PMMA and PVP. Furthermore, in our experiment, it is noteworthy that OFET based on PS shows NH3 detection down to 1 ppm, as shown in Fig. 6. Obvious response and recovery cycles along with the variation of NH3 concentration are observed. With a preferable interface of dielectric/organic semiconductor, sensing parameters, including sensitivity, detect limitation and response/recovery property, are tremendously improved. Moreover, according to a large number of previous reports, H2O vapor has been proven to be the key factor that would affect pentacene OFETs. Therefore, the humidity effect on the electrical characteristics of OFET based on PS is also studied. Relative humidity (RH) from 0% to 70% are introduced to the testing chamber, and the transfer curves of the PS based OFET under different RH are shown in Fig. A.3. It is found that the humidity would strongly change the off state current. However, the negligible changes of ION, l and VTH can be extracted as shown in
Fig. 5. (a, b) Represent the absorption and separation process of NH3 in devices B, C and D. (c, d) Represent the absorption and separation process of NH3 in device A.
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Acknowledgements This research was funded by the National Science Foundation of China (NSFC) (Grant No. 61177032), the Foundation for Innovation Groups of National Science Foundation of China via No. 61021061, the Fundamental Research Funds for the Central Universities (Grant No. ZYGX2010Z004), SRF for ROCS, SEM (Grant No. GGRYJJ0805). Appendix A. Supplementary material
Fig. 6. Response curve of OFET-sensor based on PS dielectric to NH3 concentration ranging from 1 to 10 ppm.
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.orgel.2013.09.018. References
Fig. 7. Percentage variations of ION, l, VTH and SS of PS based OFET at different relative humidity.
Fig. 7. These results are consistent with the previous reports [33,34]. Compared with Fig. 4, PS based OFET NH3 sensors were realized by obvious negative shift of saturation current and hole mobility under different NH3 concentrations. Therefore, PS based OFET sensors can distinguish NH3 from water when working in environment. 4. Conclusion In summary, we fabricated OFET based NH3 sensors with various polymer dielectric materials including PS, PVA, PVP, and PMMA. Among all these OFETs, the sensor based on PS dielectric exhibits a reliable recovery property under various concentrations of NH3. The enhancement of the sensing properties is attributed to the relatively smaller grain size of pentacene on PS dielectric, low surface trap density and chemically stable molecular structure consisting of benzene rings. Thus, NH3 can diffuse into the transistor easily and remove from it with less residual, leading to a remarkable detect limitation of 1 ppm. To our best knowledge, this is the lowest detect limitation of OFET based NH3 sensor fabricated with polymer dielectric on glass substrate. Also, such results demonstrate that the combination of p-type pentacene and proper polymer dielectrics in OFET can realize high performance NH3 sensors, which is helpful to developed low cost and high sensitive gas sensors.
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