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Organic Field-Effect Transistors with Macroporous Semiconductor Films as High-Performance Humidity Sensors Shaohua Wu,† Guiheng Wang,† Zhan Xue,† Feng Ge,† Guobing Zhang,†,‡ Hongbo Lu,†,‡ and Longzhen Qiu†,‡,* †

Key Lab of Special Display Technology, Ministry of Education, National Engineering Lab of Special Display Technology, State Key Lab of Advanced Display Technology, Academy of Opto-Electronic Technology, Hefei University of Technology, Hefei 230009, China ‡ Key Laboratory of Advanced Functional Materials and Devices, Anhui Province School of Chemistry and Chemical Engineering, Hefei University of Technology, Hefei 230009, China S Supporting Information *

ABSTRACT: In this study, we designed and fabricated a high-performance humidity sensor based on a donor−acceptor polymer transistor. To improve its sensing performance, a polymeric semiconductor film with macroporous structure was prepared using a facilitated phase-separation method. The relationship between the sensing performance and the pore size was systematically investigated by testing the humiditysensing performance. The results suggested that the sensitivity of the sensor was improved with increasing pore size within a certain range. The sensor based on the macroporous film with an average pore size of 154 nm exhibited a sensitivity of 415 and a response time of 0.68 s, as the low relative humidity (RH) changed from 32% RH (9146 ppm) to 69% RH (20 036 ppm). These sensitivity values are better than those obtained by other reported humidity sensors based on organic field-effect transistors. KEYWORDS: organic field-effect transistors, humidity sensors, phase separation, macroporous film, controllable pore size ultrathin film,26,27 air-dielectric device,28 and porous film.29,30 Compared to those of ultrathin film and air-dielectric devices, porous films have simpler and more stable structures. The presence of pores in the film provides an efficient and direct pathway for the diffusion of gas molecules throughout the conductive channels to yield organic thin-film transistors (OTFTs) with improved performances. For example, Zan et al.31 and Kang et al.32 prepared porous pentacene films using polystyrene sphere template and m-bis(triphenylsilyl)benzene (TSB3) surface inducer, respectively. The sensitivity of the sensor based on porous semiconductor film increased by two to three orders of magnitude compared to the sensor without pores. Besides, we have recently reported a high-performance chemical sensor using donor−acceptor (D−A) polymer semiconductor layer with macroporous structure, which displayed a 100-fold increased sensitivity than a flat film.33 Nevertheless, these high-performance biological and chemical sensors only showed the effect of the macroporous structure on the sensing performance. The quantitative relationship between pore size and performance is still unclear. The determination of

1. INTRODUCTION Recently, organic field-effect transistors (OFETs) have gained significant interests in research and development because of their promising use in flexible displays,1−4 smart-card badges,5 electronic skin,6−9 and a wide variety of sensors.10−13 Especially, biological and chemical sensors based on OFETs, which play an important role in daily life, have attracted much attention, as OFET sensors have many advantages, such as high sensitivity,14−17 low cost,18−20 simple fabrication,21−23 and multiparameter operation.13,24 They have been exposed to different volatile gases and biomolecules and found promising in the detection of low concentrations through interactions between the analytes and the active layer. In OFETs, the surface potential at the interface between the organic layer and the gate dielectric creates a layer with thickness of less than 5 nm for charge transport.25 In conventional OFET-based sensors, target analytes are difficult to diffuse into the chargetransport layer to interact with the active layer. This results in relatively low sensitivities. Therefore, to prepare high-performance biological and chemical sensors, OFETs require specific structures that allow the analytes to quickly and adequately penetrate the active channels. Three main effective approaches were used to facilitate the exposure of the charge-transport layer to the analyte molecules: © XXXX American Chemical Society

Received: February 8, 2017 Accepted: April 13, 2017 Published: April 13, 2017 A

DOI: 10.1021/acsami.7b01865 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. (a) Schematic illustration of the synthesis of macroporous PBIBDF-BT films. (b) AFM images of the as-prepared PBIBDF-BT/PBA blend films with different PBIBDF-BT contents: 100, 90, 70, and 50% from left to right. (c) AFM images of the PBIBDF-BT/PBA blend films washed by acetone corresponding to the top panel. min each and then dried in a clean room under a nitrogen stream. After 15 min treatment with O3, CYTOP was spin-coated onto the cleaned SiO2/Si substrates at 3000 rpm for 60 s, followed by heat treatment at 180 °C in a heated platen for 15 min. CF solutions containing different ratios of polymers/PBA were then spin-coated onto the CYTOP-treated SiO2/Si substrates at 4000 rpm and room temperature for 60 s. The obtained PBIBDF-BT films were annealed at 150 °C for about 15 min in a nitrogen-filled glovebox, followed by washing with acetone to remove any remaining PBA. Finally, a 30 nm thick gold layer was deposited on the spin-coated PBIBDF-BT films as source and drain electrodes using the shadow mask method. 2.2. Instrument and Measurement. The sensing properties of the humidity sensors were characterized in a customized transparent box with controllable humidity atmosphere. The box was provided with inlet and outlet valves for the flow of dry and wet nitrogen gas. To increase humidity in the box, the nitrogen flow was passed through water and then the water vapor-saturated gas was allowed to flow into the box. The humidity was controlled by adjusting the flow ratio of the dry and wet nitrogen gas. Meanwhile, a mixed gas of varying RH was passed into the humidity chamber at a flow rate of 1.5 L/min. To test the selectivity of the OFET sensors toward methanol, ethanol, isopropanol, diethyl ether, anisole, methylbenzene, acetone, CF, ethyl acetate, anisole, and hexane, vapors from these interferences were obtained by flowing pure N2 in a vessel containing the corresponding solvent liquid. The concentration of the resulting vapor could be controlled by introducing pure N2. The humidity chamber was fabricated and equipped with feedthroughs for the electrical source, drain and gate contacts. The electrical measurements were performed

this relationship would lead to the fabrication of highperformance OTFT sensors. In addition, humidity levels have widely been studied and applied in numerous sensors because relative humidity (RH) is very important in scientific research, industrial and agricultural production, and food safety.34−37 In this study, a highly sensitive and selective OFET-based humidity sensor comprising porous semiconductor films was fabricated and tested. The D− A-conjugated polymer molecule (PBIBDF-BT) based on bis(2oxoindolin-3-ylidene)-benzodifuran-dione (BIBDF) and poly(1,4-butylene adipate) (PBA) was selected as the active layer and an additive to prepare the macroporous semiconductor film through washing off the PBA. By measuring the sensing performance of devices with different pore sizes as a function of humidity levels, a quantitative relationship between the pore size and the sensing performance was obtained.

2. EXPERIMENTAL SECTION 2.1. Materials Preparation and Device Fabrication. Polymer semiconductors (PBIBDF-BT; Mw = 58 852 g/mol) were synthesized as previously reported.38 Chloroform (CF) and PBA (Mw = 2000 g/ mol) were purchased from Sigma-Aldrich Chemical Co., and CYTOP from Asahi Glass Co., Ltd. Heavily doped silicon wafers as gate electrodes with thermally grown 300 nm thick SiO2 as dielectric layers were used as substrates. The SiO2/Si substrates were first ultrasonically washed with pure acetone, pure alcohol, and deionized water for 15 B


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Figure 2. Pore size distributions of macroporous films prepared with different PBIBDF-BT contents: (a) 90 wt %, (b) 70 wt %, and (c) 50 wt %.

Figure 3. (a) Schematic diagram of the device structure with the macroporous PBIBDF-BT film. The PBA concentration dependence of mobility of the PBIBDF-BT films in (b) P-channel and (c) N-channel. using a Keithley 4200-SCS. A commercial humidity meter was used for the measurement of the resulting RH in the chamber. The sensitivity (S) of the sensors is defined by eq 1.

S1 =

S2 =

3. RESULTS AND DISCUSSION 3.1. Macroporous Film Fabrication and Characterization. As illustrated in Figure 1a, the macroporous films were obtained by mixing the solutions of the polymer semiconductor (PBIBDF-BT) and the low Mw PBA polymer. The figure also shows the chemical structures of PBA and PBIBDF-BT conjugated polymers. During the spin-coating process of PBIBDF-BT/PBA blends, a phase separation occurred between the PBIBDF-BT and PBA components because of the immiscibility of PBA and the crystallinity of PBIBDF-BT at room temperature. The macroporous film was then formed by washing off the PBA component from the blend film using acetone. The morphological characteristics of the organic thin films were investigated by atomic force microscopy (AFM). Figure 1b shows the AFM images of the original PBIBDF-BT thin films deposited on the devices. These diagrams show the as-prepared blend membranes with different ratios of polymer semiconductor and insulating oligomers. The blend membrane had PBIBDF-BT contents of 100, 90, 70, and 50 wt % (from left to right in the figure). It will be noted that the pure PBIBDF-BT film displayed a flat and smooth surface with a

Igas ‐ off Igas ‐ on

(1)

Igas ‐ off − Igas ‐ on Igas ‐ off

(2)

where Igas‑off and Igas‑on are the sensor’s current in the timing mode when the gas was off and on, respectively. When the change in current was more than one order of magnitude, the sensitivity of the sensor was calculated by eq 1. However, when the current variation was less than one order of magnitude, eq 2 was employed instead of eq 1. For gas sensors, response time is an important characteristic that evaluates the response speed of the sensor toward the gas. The response time (Tres) and the recovery time (Trec) are defined as the times taken by the device to reach 90% response when the gas is on and off, respectively. C


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a certain extent on the transport of hole and electron carriers, which has been reported previously.31 The enhanced carrier mobility of the film may thus be attributed to the orderly aggregation of the PBIBDF-BT nanowires in the channel. However, when the mass fraction of PBA exceeded 10%, the electrical properties of the devices deteriorated. The latter could be explained as follows: (i) decreased PBIBDF-BT contents may lead to reductions in the effective channel length of the semiconductor layer, which may reduce the current between the source and drain, and (ii) the pore size of the semiconductor layer gradually increased with increasing PBA content. The contact area between the semiconductor and air also increased, and moisture in the air caused more traps inside the semiconductor that captured a portion of the carriers. Overall, higher-performance OFET sensors could be prepared by controlling the proportion of the blended components for a semiconductor layer of appropriate pore size. 3.3. Selectivity and Sensitivity of OFET-Type Humidity Sensors. In OFET-based sensors, chemical or physical adsorption of target analytes induces changes in the channel current. The latter depends on several factors, including the analyte composition, concentration, and operating conditions. To test the sensing performance of the devices, a closed box was used as a testing chamber to control the concentration of the analytes (Figure 4a). The PBIBDF-BT sensor devices were first exposed to different chemical vapors, and their impacts on the performance of devices were monitored by varying different parameters. In Table 1, these molecules are divided into two

root-mean-square roughness of 1.35 nm. As PBA increased, the surface roughness of the blended films gradually increased from 1.35 to 10.20 nm. After washing off the PBA, the surface roughness increased from 1.35 to 13.50 nm, and macropores appeared in the morphology (Figure 1c). The results of water contact angle tests of these membranes are shown in Figure S1. The contact angle of water gradually decreased from 104.1 to 94.6°, and the contact angle of pure PBA film was the smallest recorded value (62.1°), as depicted in Figure S1a−d. The latter exhibited an increase in PBA content on the membrane surface. After immersion in acetone for 15 min at room temperature, the contact angle of water showed the opposite trend with changes from 104.2 to 110.1°. The surface roughness with micro/nanostructure was expected to increase the water contact angle of the surface.39,40 To clarify the relationship between PBA content and pore size, the pore sizes were analyzed using a Nanoscope analysis software to estimate the average pore diameter of each membrane using data taken from the frequency distribution histograms. Figure S2 shows the analysis process, where the crowd blue sections represent the superficial area of the pores. In this process, incomplete holes were not counted at the edges of the AFM diagrams. The resulting specific data are shown in Figure 2 in the form of a frequency distribution histogram. It can be observed that as the PBIBDF-BT component reduced from 100 to 90, 70, and 50 wt %, the average pore diameter gradually increased. The calculated results showed that the average pore diameter increased from 82 to 109 and 154 nm. The pattern showing the influence of different PBA ratios demonstrated the specific relationship between the morphology of the blending films and the PBA ratio. This illustrated that the size of pores in the semiconductor films increased as the PBA content gradually raised. 3.2. Electrical Properties of OFET. OFET-based sensors with macroporous organic semiconductor (OSC) layers were prepared using the bottom-gate top-contact configuration shown in Figure 3a. The PBA concentration dependence of mobility of the PBIBDF+BT films using macroporous films as OSC layers is depicted in Figure 3b,c. In general, the field-effect mobility is calculated using eq 3 at the saturation stage. IDS =

WC i μ(VGS − VTH)2 2L

Table 1. Chemical Nature of Gas Molecules Used To Study the Response of the Devicesa solvent water (H+) methanol (H+) ethanol (H+) isopropanol (H+) diethyl ether toluene acetone CF ethyl acetate hexane anisole

(3)

where IDS is the drain-source current, VGS is the gate-source voltage, μ is the mobility, VTH is the threshold voltage, Ci is the capacitance of the gate dielectric, L is the channel length, and W is the channel width. Figure S3a−d illustrates the N-type properties of different pore size membranes. These devices were tested using a drainsource voltage (VDS) of 80 V and a gate-source voltage (VGS) range of −10 to 100 V. The calculated mobility is shown in Figure 3b, and Figure S3a−d also represents the transfer curves of devices made of macroporous membranes with sizes of 0, 82, 109, and 154 nm, corresponding to mobilities of 0.258, 0.626, 0.140, and 0.036 cm2 V−1 s−1. Figure S3e−h indicates the Ptype properties of different pore size membranes with mobilities of 0.078, 0.201, 0.084, and 0.046 cm2 V−1 s−1, respectively. It will be noted that the obtained trend was similar to that of the N-type device (Figure 3c). Figure 3b,c shows the change in the electrical properties of the devices as a function of PBIBDF-BT content. The addition of small amounts of PBA to the devices improved their electrical properties. This indicates that the macropores in the PBIBDF-BT layer have an impact to

dipole moment (D)

dielectric constant (k)

polarity

sensitivity

1.86 1.71 1.69 1.70

78.5 32.6 32.6 24.3

10.2 6.6 4.3 4.3

269.8 6.4 15.9 46.6

1.30 0.40 2.88 1.10 1.90 0 1.29

4.3 2.4 20.7 4.7 6.03 1.88 4.3

2.9 2.4 5.4 4.4 4.3 0.06 2.8

3.85 94.7 11.1 27.3 15.7 3.64 27.6

a

Three types of parameter were used to measure the polarity of the gas molecules: dipole moment (D), dielectric constant (k), and polarity.

categories: (i) protic molecules (names followed by H+) and (ii) aprotic molecules (names without H+). The table also contains dipole moments, dielectric constants, and polarity empirical constants, characterizing the strength of the materials. A macroporous film with a pore size of about 109 nm was used to test the response of the devices to various chemical vapors. A total of 10 different gas molecules with different dipole moments were used to investigate the effect of polarization of adsorbed molecules. The response characteristic curves were obtained at different concentrations of vapors using a bias drain voltage of 80 V, and gate pulse trains of 0.1 Hz were applied at the gate electrode between duty cycles of 1 and 100%. The “on” gate voltage was fixed at −80 V, whereas the “off” gate voltage was fixed at 0 V, according to the work of D


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Figure 4. (a) Testing equipment used for the electrical characterization of humidity sensors with macroporous OSC layers. (b) Response of devices to different chemical vapors. The concentration of water changes from 32% RH (9146 ppm) to 69% RH (20 036 ppm), whereas the concentration of other organic solvents ranged from 0 to 50 000 ppm.

Figure 5. Effect of RH on the device performance. (a−d) Changes in the transfer characteristic curve of the macroporous film devices as a function of RH: (a) 100 wt %, (b) 90 wt %, (c) 70 wt %, (d) 50 wt %. (e−h) Relationships between the mobility of the macroporous film device and the RH in (a)−(d), respectively.

Yang et al.41 The IDS changes in the response characteristic curve after passing elevated concentrations of vapors are shown in Figure S4. The sensitivity was considered as the current ratio between the on and off states. The concentrations of all organic vapors were controlled by the ratio of saturated organic vapor to high-purity nitrogen. The test concentration was varied between 0 to 50 000 ppm, except for water vapor. To test the usefulness of the sensors in daily life, a small RH detection range was selected for comparison with other organic vapors. In daily life, humidity in air changes within a small range and RH changes from 32 to 69% corresponding to 9147 to 20 036 ppm, respectively. It can be observed that the devices showed a significant response to polar protic molecules compared with both polar aprotic and nonpolar molecules. Polar molecules located on a semiconductor surface and grain boundaries will be more likely

to interact with semiconductors to induce charge−dipole interactions.42 Figure 4b shows the resulting sensitivities to 10 s exposure of different analyte gases. The polar protic solvent (water) exhibited the largest IDS change in the transfer characteristic curves, whereas other chemical vapors showed relatively smaller variations. The protic molecules (water, methanol, ethanol, isopropanol) have relatively larger polarities, which play the role of traps in the electronic transmission. The good selectivity for humidity is mainly due to the high polarity and high dielectric constant of the water molecules, and the smaller molecular volume is a secondary factor, which more easily diffuses into the grain boundary.37 Overall, the device showed a lower response to aprotic or nonpolar molecules, such as diethyl ether, acetone, CF, ethyl acetate, and hexane. However, toluene exhibited a higher sensitivity to aprotic molecules. The reason for this could be related to the πE


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3.4. Sensing Performance of Humidity Sensor Based on OFETs with Macroporous Semiconductor Film. The variation in drain current at RH was monitored using a bias drain voltage of 80 V, and gate pulse trains of 0.1 Hz were applied at the gate electrode between duty cycles of 1 and 100%. The on gate voltage was fixed at 80 V, whereas the off gate voltage was fixed at 0 V. Figure 7 shows the stability and reproducibility of the measured drain current of the sensors as RH varied from 32% RH to 69% RH for several cycles. It will be noted that the sensors had stably repeated responses. The response or recovery times of the sensors were measured at 90% of the total current response, and the results are shown in Table 2 (calculated from Figure 7). The structure of the device and the humidity of the sensitive material determine the performance of the sensor. Here, materials highly sensitive to humidity and active layers with macroporous films were used. Table 2 reveals that the response times are among the fastest values reported to date for OFET humidity sensors. It should be noted that the evaluation of response time is important for humidity sensors, where only a handful of reports evaluated the response time in OFET humidity sensors. Mori et al.43 reported that pentacene-based humidity sensor exhibited good response and recovery times of 3 and 8 s, respectively. Zhu et al.44 measured the response and recovery times of humidity sensor based on pentacene as 20 and 15 min, respectively. According to a recent report, the insulating layer of a device could also affect its performance. A device with an insulating layer comprising three-layer structures was reported by Subbarao et al.37 They found that the sensors exhibited good response and recovery times of 0.73 and 0.52 s, respectively. Park et al.35 also reported that the pentacene-based humidity sensor exhibited response and recovery times of 10 and 40 s, respectively. The response parameters of all of the humidity sensors based on OSCs are listed in Table S1. A few other reports are available on resistive- or capacitive-type humidity sensors based on nanowire, carbon nanotubes, or graphene, which showed an ultrafast response to humidity.45−53 Here, the response times shown in Table 2 showed a significant decrease with increasing pore size, and the shortest response time was recorded as 0.44 s. However, compared to those of other humidity sensors based on OFETs, the devices reported in this study showed the fastest responses with remarkable stability. Figure 8a showed that the device with a PBIBDF-BT content of 70 wt % could be tested for several cycles under different humidity environments without significant deterioration. The drain current showed some very regular changes when the RH changed from 25 to 35, 46, 57, and 68% RH. The surface of the OSC layer condensed water droplets when the RH was high, leading to device instability. Figure 8 shows the relationship between the PBIBDF-BT content, pore size of macroporous film, and sensitivity of the humidity sensor. It will be noted that lower PBIPDF-BT contents induced greater pore size in the OSC layer, which, in turn, led to higher sensitivities and better sensor performance.

conjugated structure in the benzene ring, which forms a charge trap that reduces charge density. To further evaluate the sensitivity of the devices to humidity, the current transfer curves were evaluated upon exposure to humidity at various concentrations from high-purity nitrogen to 84% RH. Figure 5a−d indicates the response of the transfer curves at different RHs of high-purity nitrogen: 26, 41, 63, 71, and 84%. The current (IDS) reached the maximum value when the device was under high-purity nitrogen environment. As the RH gradually increased, more water molecules diffusing into the pores increased the trap density and led to a decrease in drain current. The mobility and threshold voltage were calculated from the transfer characteristic curve of the device at different RHs, and the results are shown in Figures 5e−h and S5, respectively. The mobility gradually decreased with increasing RH. As the PBIBDF-BT contents in the blend films decreased, the devices exhibited a higher sensitivity to humidity. The humidity-sensing mechanism is mainly attributed to the reduction in carrier mobility. When the device is exposed to an environment rich in water molecules, diffusion of water molecules into grain boundaries induces charge−dipole interactions with semiconductors. These interactions increased the energy barrier for charge-carrier transport, thus reducing the mobility of the devices. It is important to note that changes in threshold voltage were irregular because of the adherence of the condensed water to the device’s surface. This, in turn, affected the stability of the device. Otherwise, the trend of the threshold voltage was similar to that of mobility (Figure S5c). The response of P-type channel devices is shown in Figure S6. The mobility obtained with these devices obviously decreased as the RH increased. The changes in the transfer curves of all devices with changes of RH from N2 to ∼58% RH are shown in Figure S7. It can be seen that all devices with different PBIBDF-BT contents showed good uniformity. In addition, Figure 6 comprehensively depicts the quantitative

Figure 6. Relationship between sensitivity and RH in devices with different PBIBDF-BT contents.

relationship between sensitivity and the PBIBDF-BT contents. The device with different pore diameters clearly showed the same trend with the increase of water, meaning that IDS decreased more rapidly as pore size increased. This also shows that the pores played a very important role in the sensing performance of the devices. Through the pores in the semiconductor film, the water molecules are more easily diffused into the charge-carrier transport layer. As a result, the contact area between the water molecules and the channelcarrier transport layer is increased, resulting in a high sensitivity of the sensor.

4. CONCLUSIONS A simple and controllable phase-separation approach was proposed for the fabrication of high-performance chemical sensors based on macroporous PBIBDF-BT semiconductor thin films. These sensors exhibited high selectivity for water vapor detection. The humidity-sensing measurements then indicated that larger sensor layer apertures induced greater F


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Figure 7. Changes in the measured drain current of OFET devices at VGS = VDS = 80 V as humidity varied between 32 and 69% RH in cycle test with different PBIBDF-BT contents: (a) 100 wt %, (b) 90 wt %, (c) 70 wt %, and (d) 50 wt %. (e−h) Enlarged sections of the graphs corresponding to (a)−(d). The red area represents the response periods, and the green area represents the recovery periods.

monitoring, industrial and agricultural production, health care,

Table 2. Sensing Parameters of OFET-Based Macroporous PBIBDF-BT films with Different PBIBDF-BT Contents response time (s) recovery time (s) sensitivity (32−69% RH)

100 wt %

90 wt %

70 wt %

50 wt %

10.02 67.10 29

2.40 45.57 126

0.44 46.23 269

0.68 45.21 415

and food safety.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the sensitivities and shorter response times. This demonstrated the quantitative relationship between the pore size and sensing performance parameters. Overall, highly sensitive (Ioff/Ion = 415) and fast-response (Tres = 0.68 s) humidity OFET-type sensors with macroporous OSC layers were fabricated by the solution-processed method. This study may provide a new convenient pathway to regulate and control macroporous semiconducting films and to obtain macroporous highperformance humidity sensors suitable for use in environmental

ACS Publications website at DOI: 10.1021/acsami.7b01865. Water contact angle; pore size estimation using AFM images; P-type and N-type transfer characteristic curves; sensing characteristic curves of different gases; the relationship between RH and threshold voltage; the changes of P-type transfer characteristic curves; the distribution of the sensing properties of five devices (PDF)

Figure 8. (a) Variation in drain current at different RH values: 25 to 35, 46, 57, and 68% RH. (b) Relationship between PBIBDF-BT content, pore size of macroporous films, and sensitivity. G


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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Longzhen Qiu: 0000-0001-9454-3206 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by National Natural Science Foundation of China (NSFC, Grant no. 51573036) and the Program for New Century Excellent Talents in University (Grant no. NCET-12-0839).

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