Achieving a Good Life Time in a Vertical-Organic-Diode Gas Sensor

In this study, we investigate the keys to obtain a sensitive ammonia sensor with high air stability by using a low-cost polythiophene diode with a vertical channel and a porous top electrode. Poly(3-hexylthiophene) (P3HT) and air-stable poly(5,5′-bis(3-dodecyl-2-thienyl)-2,2′-bithiophene) (PQT-12) are both evaluated as the active sensing layer. Two-dimensional current simulation reveals that the proposed device exhibits numerous connected vertical nanometer junctions (VNJ). Due to the de-doping reaction between ammonia molecules and the bulk current flowing through the vertical channel, both PQT-12 and P3HT VNJ-diodes exhibit detection limits of 50-ppb ammonia. The P3HT VNJ-diode, however, becomes unstable after being stored in air for two days. On the contrary, the PQT-12 VNJ-diode keeps an almost unchanged response to 50-ppb ammonia after being stored in air for 25 days. The improved storage lifetime of an organic-semiconductor-based gas sensor in air is successfully demonstrated.


Introduction
Solid-state gas sensors have drawn considerable attention for their applications in environmental pollution monitoring [1], toxic or explosive gas detection [2], food condition tracking [3] and non-invasive diagnostics through breath analysis [4][5][6]. Among these applications, sensors based on organic semiconductor (OSC) materials are particularly promising because of their low-cost process, room-temperature operation and wide selection of material properties [4,7]. However, when OSC materials are stored in air, oxidation due to oxygen and moisture is known to destroy the electric property in OSC materials within a few days [8,9]. Recently, researchers proposed various air-stable OSC materials and successfully demonstrated air-stable organic thin-film transistors (OTFTs) [10][11][12]. A 120-day storage time was reported for OTFT with an air-stable OSC layer. In an OSC-based gas sensor, however, the air stability after long-term storage has been not reported yet.
In this work, we studied the air stability of a sensitive ammonia gas sensor based on a vertical organic diode. Two kinds of polythiophene materials are used in the proposed sensor. One is poly(3-hexylthiophene) (P3HT); the other is air-stable poly(5,5'-bis(3-dodecyl-2-thienyl)-2,2'-bithiophene) (PQT-12). In recent years, abundant research focused on developing nanostructure gas sensors [13][14][15][16]. Sensors with a nanostructure can increase the surface-to-volume ratio and, thus, lead to higher performance. Chen shows that under ultraviolet light illumination during gas sensing, single-walled carbon nanotubes can detect ammonia, nitric oxide and nitrogen dioxide in parts per trillion level [17]. Besides, Hassan reports that vertical zinc oxide nanorod arrays exhibit high sensitivity to hydrogen [18]. Here, instead of changing the sensing layer into a nanostructure, we develop an electrode with nanopores to form vertical nano-channels in the sensing layer. In our previous work, we had used a P3HT-based vertical diode with a porous top electrode to detect the breath ammonia of rats [19]. The vertical diode exhibits numerous vertical nanometer junctions and is named the vertical nanometer junction (VNJ)-diode. In this work, we demonstrate that the P3HT VNJ-diode exhibits air stability only for 1-2 days. Using PQT-12 to replace P3HT, we successfully realize a sensitive ammonia sensor with a detection limit of 50 ppb after being stored in air for 25 days. In a previous report, a resistor-type sensor using PQT-12 film was found to have almost no response to 1000 ppm ammonia [20]. In that work, the addition of carbon nanotubes (CNTs) into PQT-12 can greatly improve the sensing response. In this work, we demonstrate that using the VNJ-diode structure, the bulk current flowing through PQT-12 responds to 50-ppb ammonia. Together with the simple structure and low-cost process, the improved air stability demonstrated in this work facilitates the future commercialization of OSC-based gas sensors.

Experimental Section
The structure of the proposed VNJ-diode is shown in Figure 1a. Figure 1b shows the simulated current distribution of the P3HT VNJ-diode. A Silvaco TCAD simulator (Silvaco Inc., Santa Clara, CA, USA) was used to perform the simulation with the parameters provided in [21]. The molecular structures of two polythiophene-based organic semiconductor materials, P3HT and PQT-12, are shown in Figure 1c,d, respectively.  The ITO electrode is treated by 100 W oxygen plasma for 15 min. For the PQT-12 VNJ-diode, PQT-12 (American Dye Source, Inc., Quebec, Canada, molecular weight 15,000-50,000) material dissolved in chloroform (purchased from Aldrich, St. Louis, MO, USA) (0.8 wt%) was spun coated on a substrate to form a PQT-12 layer. After the PQT-12 film was annealed at 140 °C for 30 min, the thin-film PQT-12 was spin-rinsed with p-xylene (purchased from Aldrich). The resulting PQT-12 layer exhibits a thickness of 60-100 nm. The substrate was then submerged into a dilute ethanol solution of negatively charged polystyrene (PS) spheres (Fluka). PS spheres with a diameter of 200 nm were adsorbed on the substrate as the shadow mask. Optimized sphere densities (about 5 #/µm 2 ) were obtained by using the concentration of the PS sphere as 0.24 wt% with 40 s soaking time. The wet substrate was dipped into boiling isopropyl alcohol (IPA) for 10 s. The substrate was blow-dried immediately after dipping into IPA. Aluminum (Al) of 40 nm was thermal evaporated as the top electrode with an active area of 1 mm 2 . Adhesive tape (Scotch, 3 M) was used to remove the PS spheres. An aluminum top electrode with high-density nanopores was formed. For the P3HT VNJ-diode, P3HT (Rieke Metals, 2.5 wt% in chlorobenzene, molecular weight 50,000-70,000) was spun coated on ITO substrate. The P3HT film was annealed at 200 °C for 10 min. After the P3HT layer was spin-rinsed with p-xylene, a film thickness of 40 nm was formed. The optimized PS sphere density (about 5 #/µm 2 ) can be also obtained by using ethanol with the 0.24 wt % PS sphere and 40-s soaking time. The PQT-12 VNJ-diode or the P3HT VNJ-diode was placed in a micro-fluid sensing chamber containing a high purity (99.9999%) nitrogen gas. We used an electrical syringe pump to inject the 100 ppm ammonia (NH3) into a tube to mix with high-purity nitrogen gas. The nitrogen gas flow was controlled by a mass-flow controller, and the ammonia concentration was obtained by adjusting the injection speed of the syringe pump. The gas mixture then entered the micro-fluid system.

Results and Discussion
The simulated two-dimensional current distribution of the VNJ-diode is shown in Figure 1b [21]. Because of the low conductivity of polythiophene materials, such as P3HT or PQT-12, the vertical current flows are limited within the regions with overlapping top and bottom electrodes. Hence, the VNJ-diode operates like vertical nanometer junctions connected in parallel. Figure 3a shows the current densities as a function of the applied bias (J-V) of the PQT-12 VNJ-diode before ammonia sensing (solid line) and after 200-s, 500-ppb ammonia sensing (dashed line). Those of the P3HT VNJ-diode are shown in Figure 3b. Because the highest-occupied molecular-orbital (HOMO) level of PQT-12 is 5.3 eV, hole injection from oxygen-plasma-treated ITO (the work function is about 5 eV) to PQT-12 exhibits an energy barrier. Hence, in Figure 3a, the current density at low biasing voltage is small. For the P3HT VNJ-diode, the hole injection barrier is small, since the HOMO level of P3HT (5 eV) is similar to the work function of oxygen-plasma-treated ITO. Regardless of the different hole injection conditions, both the P3HT and PQT-12 VNJ-diodes exhibit obvious current drops after ammonia exposure, because ammonia molecules act as de-doping agents to reduce the carrier concentration in polythiophene-based OSCs [22,23]. The real-time current variations of fresh PQT-12 VNJ-diode when detecting 500-ppb ammonia are shown in Figure 4a. Four devices fabricated in different runs were measured. When 500-ppb ammonia is injected into the sensing chamber (marked by the triangle symbols), a significant current drop is observed. After removing the ammonia (marked by the star symbols), the current drop can be recovered after about 800-1000 s. The sensing and recovery responses can be repeatedly obtained when 500-ppb ammonia is injected into and removed from the sensing chamber repeatedly. It is noted that devices exhibit device-to-device variation, because of the slightly difference in P3HT film thickness and nanopore density on the top electrode. Such a device-to-device variation, however, can be greatly suppressed when using a current variation ratio to represent the sensing response, as shown in Figure 4b. The current variation ratio, (I−I 0 )/I 0 , is defined as the current minus the initial current (I 0 ) divided by I 0 . The responses of four devices are all about 42% under exposure of ammonia for 200 s. With a long enough recovering time (e.g., 800 s), an almost full recovery can be obtained (as shown in Figure 4a). Finally, with a fixed reading time of 200 s, the sensing response of the fresh PQT-12 VNJ-diode as a function of ammonia concentration is shown in Figure 4c. Data with error bars were obtained from four independent devices. A detection limit of 50 ppb ammonia is obtained in the proposed device. Data obtained from the fresh P3HT VNJ-diode are also represented by the dashed line in Figure 4c. The relationship between the response of VNJ-diode and ammonia concentration is not a linear relationship, but closer to a power-law relationship [24,25].
So far, we show that the proposed PQT-12 and P3HT VNJ-diodes can detect ammonia molecules in the ppb-regime. The lifetime of P3HT in air, however, is known to be limited. When the P3HT VNJ-diode is exposed to air for two days, the sensor current exhibits a five-times increase at a 2-V bias, and the response to 500-ppb ammonia becomes unstable (shown later in Figure 5d). For real applications, the sensor stability in air needs to be improved. Here, we demonstrate a good air-stability in the OSC-based gas sensor by using PQT-12 to serve as the sensing layer. PQT-12 exhibits the same conjugated backbone as P3HT, but has a better immunity to the humidity-related oxidation in air [26,27]. The J-V curves of the fresh and aged PQT-12 VNJ-diode are shown in Figure 5a. The black J-V curve is measured right after fabricating the device. Red, green and blue J-V curves are measured after putting the PQT-12 VNJ-diode in air for 2, 10 and 25 days, respectively. In Figure 5a, the J-V curves of the PQT-12 VNJ-diode change a bit after being stored in air for several days. However, such a change in the J-V curve does not significantly influence the sensor response to ammonia. The responses of the fresh and aged sensor as a function of sensing time to the 500-ppb and 50-ppb ammonia concentrations are shown in Figure 5b,c, respectively. The dashed line represents the recovery behavior after removing ammonia from the micro-fluid system.  The sensor lifetimes of the P3HT VNJ-diode and the PQT-12 VNJ-diode are compared in Figure 5d. The sensors response readings at 200 s are plotted as a function of aging time. The P3HT VNJ-diode fails to deliver a stable response to 500-ppb ammonia after being stored in air for two days. For the PQT-12 VNJ-diode, quantitatively speaking, the responses to 500-ppb ammonia are −0.41 and −0.35 for the fresh sensor and the 25-day aged sensor. The responses to 50-ppb ammonia are −0.13 and −0.11 for the fresh sensor and 25-day aged sensor. About a 15%-16% degradation is observed after 25 days. However, a clear difference between the responses to the 50-ppb and 500-ppb ammonia concentrations can still be obtained. In particular, for low-concentration applications, such as breath ammonia detection [6], the PQT-12 VNJ-diode is able to detect 50-ppb ammonia with an almost unchanged response after being stored 25 days in air.

Conclusions
In conclusion, this study presents a sensitive ammonia sensor based on an air-stable organic vertical diode. It is known that many organic semiconductor materials suffer from a short lifetime in air due to the oxidation effect between oxygen, moisture and organic molecules. In this work, we investigated the lifetime of an organic-based ammonia sensor in air. The proposed sensitive ammonia sensor exhibited numerous vertical nanometer junctions (VNJ). Two kinds of polythiophene materials, P3HT and air-stable PQT-12, were used as the organic sensing layer. Using low-cost colloidal lithography (i.e., using self-assembled PS nanospheres as a hard mask), a porous top electrode was fabricated to allow ammonia molecules to diffuse easily into the bulk of the polythiophene sensing layer. The bulk current flowing through vertical nano-junctions (VNJ) was decreased due to the de-doping reaction between ammonia and the polythiophene material. When the P3HT VNJ-diode exhibits a detection limit to 50-ppb ammonia, after two days of being stored in air, the P3HT VNJ-diode becomes unstable due to the oxidation effect. On the other hand, the PQT-12 VNJ-diode exhibits a detection limit of 50-ppb ammonia after being stored in air for 25 days. It emerged that, during the 25-day storage time, the current of the PQT-12 VNJ-diode (biased at 2 V) changes significantly (i.e., from 0.13 to 0.04 mA/cm 2 ). The response to 50-ppb ammonia, represented by the current variation ratio, remained almost unchanged during the 25 storage days (i.e., from 0.13 to 0.11). Along with the simple structure and low-cost process, the improved air stability demonstrated in this work facilitates future commercialization of OSC-based gas sensors. In our previous work, we already demonstrated that the proposed polythiophene sensor exhibits a significant response to ammonia and has almost no response to carbon dioxide, acetone and ethanol [19]. In future work, it is expected that the proposed sensor may also be able to detect other kinds of amine-based gas molecules for applications in environmental air pollution detection. Furthermore, by using different kinds of sensing materials in the proposed sensor, we may form a sensing array to further improve the sensing selectivity, as well as the sensing sensitivity.