A robust organic hydrogen sensor for distributed monitoring applications

Main

The unique properties of diatomic hydrogen (H₂), such as its rapid diffusion rate and high energy density, make it suitable for various applications in different industries, including energy, transportation, petroleum refining, defence, space, food and pharmaceuticals^1,2,3,4. However, its combustible nature, low flammability point (approximately 4 vol% H₂ in the air) and the inability of people to detect it using their senses create safety concerns for hydrogen production, storage and usage⁵. There is, thus, a growing demand for sensitive and selective hydrogen sensors that can provide an early warning of potential leaks. To meet the large volume demands of the emerging hydrogen economies, such sensors must be cost-effective to manufacture, consume little power, operate at ambient conditions and have a minimal environmental impact. Among the different types of hydrogen sensors available, which include resistive, capacitive, optical, acoustic and magnetic systems^{6,7,8,9,10,11,12,13,14}, resistive-based technologies are arguably the most promising because they are simpler and more scalable to manufacture and can be integrated with incumbent electronics^{15,16,17,18,19,20}.

Various materials have been explored as the active element of such resistive hydrogen sensors to address the trade-offs between sensor performance (including sensitivity, rise time and fall time), shape, size, circuit complexity, power consumption and cost. These materials include noble metals such as palladium (Pd)²¹ and oxide materials such as zinc oxide (ZnO), molybdenum trioxide (MoO₃), titanium dioxide (TiO₂), niobium pentoxide (Nb₂O₅), tin(IV) oxide (SnO₂), tungsten oxide (W₂O₃) and indium(III) oxide (In₂O₃)^{6,22,23,24,25,26,27}. However, the typically low responsivity, high-temperature operation (requiring a power-hungry heating element) and long response time have limited their practical applications.

Various metal composites^{28,29,30,31,32,33} and organic materials, such as polypyrrole nanofibres and polyaniline^34,35,36, have also been investigated for H₂ detection. Polypyrrole nanofibre and polyaniline-based hydrogen sensors can exhibit a decrease in resistance with increasing hydrogen concentration due to the interaction with charged amine nitrogen sites on the polymers³⁶. Pentacene-based organic field-effect transistors have also been employed to detect hydrogen by a different mechanism in which the contact resistance of the device at room temperature increases with the expansion of the palladium electrodes caused by the presence of hydrogen^37,38. However, the suboptimal performance of these organic sensors in terms of sensing parameters, including low responsivity (≤0.9) and slow response time (up to 6 s), has impeded their deployment in practical applications.

In this article, we describe a hydrogen sensor that uses an organic semiconductor as the active layer. Our approach is based on the p-doping of organic semiconductors by oxygen species^39,40,41. The current changes as a result of the interaction between hydrogen and the physisorbed oxygen species. The sensor offers high responsivity, fast response, low power consumption, low limit of detection (LoD) and good stability in harsh testing conditions. We also show that it can outperform a state-of-the-art commercial device in different practical sensing demonstrations.

Sensor fabrication and performance

Our sensor has interdigitated platinum electrodes patterned lithographically (Fig. 1a, right and Extended Data Fig. 1) and a thin layer of the organic polymeric semiconductor poly[3,6-bis(5-thiophen-2-yl)-2,5-bis(2-octyldodecyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione-2,2′-diyl-alt-thieno[3,2-b]thiophen-2,5-diyl] (DPP-DTT) deposited on top by spin-coating or printing (Fig. 1a, left). The polymer forms a conformal and smooth layer with a root mean square surface roughness of ~0.6 nm (Extended Data Fig. 1). Before the polymer layer was deposited by photoelectron spectroscopy, the work function of the platinum electrode was measured to be 5.3 eV (Extended Data Fig. 1), which matches well with the highest occupied molecular orbital energy level of DPP-DTT of −5.2 eV, making it a good hole-injecting contact. Completed devices were stored in dry air inside a dry box for 10 h before testing. The device was then transferred into a test chamber and exposed to a stream of synthetic air mixed with varying concentrations of H₂ following the experimental procedures described in Methods (Extended Data Fig. 2).

**Fig. 1: Device schematic and characterization of H₂ sensing.**

Figure 1b shows the current versus voltage characteristics of the optimized device (Supplementary Fig. 1) recorded before and after exposure to hydrogen for a few minutes at room temperature. Under 100% synthetic air (0 ppm H₂), a linear current versus voltage (I–V) characteristic was recorded, indicating the presence of Ohmic conduction under equilibrium conditions. Upon inserting various amounts of H₂ from 100 to 1,000 ppm, the sensor current decreased logarithmically by 10,000-fold from microampere to sub-nanoampere levels (Fig. 1c) at room temperature and under 1.0 V bias, demonstrating its high sensitivity to H₂. Such a drastic current decrease can be attributed only to carrier depletion induced upon exposure to H₂. The current reduction in the low-voltage regime (0–0.1 V) was followed by a second conduction regime (>0.1 V), manifested as a sublinear I–V dependence. Qualitatively similar I–V characteristics have previously been attributed to Schottky barriers at both contacts in two-terminal semiconductor diodes⁴². We hypothesize that ambient oxygen p-dopes the DPP-DTT and shifts the Fermi energy so that the Pt/DPP-DTT becomes more Ohmic while the device conductivity increases (Extended Data Fig. 3). Exposure to H₂ gas de-dopes the organic semiconductor and reduces its work function, giving rise to a higher Schottky barrier at each Pt–organic semiconductor interface to form a back-to-back diode equivalent circuit. Modulating the Schottky barriers by p-doping and subsequent de-doping leads to the strong current modulation observed in the I–V characteristics (Fig. 1b and Extended Data Fig. 4a) due to its exponential dependence on the barrier height.

The H₂ concentration-dependent responsivity, defined as ΔR/R₀, where R is the electrical resistance at the measured H₂ concentration, and R₀ is the resistance before H₂ exposure, was calculated from the I–V characteristics for an applied bias between 0.1 and 1.0 V (Fig. 1d, Extended Data Fig. 4b and Supplementary Fig. 2). We found that the responsivity at higher hydrogen concentrations (1,000 ppm) saturated at around 1.0 V. Accordingly, 1.0 V was chosen for further temporal hydrogen response measurements at higher hydrogen concentrations. A bias-dependent ΔR/R₀ of more than six orders of magnitude was observed upon increasing the H₂ concentration from 100 to 1,000 ppm. The signal evolution at constant H₂ concentrations recorded at 0.5 V, shown in Extended Data Fig. 4c, confirms the robust sensing behaviour of the device. The extracted LoD of the DPP-DTT sensor was approximately 192 ppb (Extended Data Fig. 4b).

The dynamic response of the sensor was studied next by subjecting the DPP-DTT device to pulses of synthetic air containing different H₂ concentrations (Extended Data Fig. 4d) inside the gas-sensing chamber, which was maintained at 20 °C with 15% relative humidity (RH). Exposure of the sensor to 1% H₂ (10,000 ppm) yielded a notably fast signal response with a rise time of ~0.84 s (Fig. 1e). The rapid response to hydrogen was also demonstrated by leaking hydrogen gas (99.99%) close to the organic sensor (Supplementary Video 1). The longer-term stability of the sensing process was confirmed by recording the response of the sensor over 1,000 sensing cycles at 0.1% H₂ (1,000 ppm) (Extended Data Fig. 4e). A statistical analysis of the rise and fall times yields 0.83 (±0.08) s and 6.52 (±0.21) s, respectively (Extended Data Fig. 4f). These results demonstrate the excellent sensing capabilities of the organic device, which surpass most hydrogen sensor performance metrics as defined by the US Department of Energy and those published in the literature (Supplementary Table 1)^43,44,45.

Mechanism of hydrogen detection

The proposed macroscopic and atomic-level mechanism for hydrogen sensing in our device is presented in Fig. 2a (left and right, respectively). In step (i), the organic sensor is introduced into an oxygen-containing environment (for example, ambient air) and is p-doped by oxygen molecules. This leads to an increase in the device current, and the biasing leads to the catalytic formation of oxygen adatoms on the surface of the Pt electrodes. In step (ii), hydrogen is introduced and immediately permeates through the organic semiconductor. In step (iii), the hydrogen molecules are split at the electrodes into adatoms and subsequently react with oxygen adatoms to form water. This reduces the oxygen-induced doping and is accompanied by a fast reduction in the device current, so that hydrogen is detected. The water molecules formed leave the device through evaporation, and the current remains low as long as hydrogen is present. In step (iv), the device remains exposed to ambient air although hydrogen exposure has ceased. Oxygen again p-dopes the organic semiconductor, leading to a rapid increase in the current (signal recovery) and the cessation of hydrogen sensing.

**Fig. 2: Working principle of the device and theoretical data.**

To verify the proposed sensing mechanism, we undertook a series of density functional theory (DFT) calculations (Extended Data Fig. 5). The calculations targeted several processes and corresponding molecular structures involving oxygen- and hydrogen-related species in the bulk of the organic semiconductors of interest, as well as at the surface of the Pt electrode. We first examined the stability of oxygen molecules next to polymer chains of DPP-DTT and two other organic semiconductors, namely poly(indacenodithiophene-co-benzothiadiazole) (C₁₆-IDTBT) and poly(3-hexyl)thiophene (P3HT) (Extended Data Fig. 5a,b). In all cases, we found that it is energetically favourable for an oxygen molecule to dissociate and form sulfonyl groups on the polymers (other O-chemisorbed structures are also stable, but their energies are higher than that of the sulfonyl configuration). However, as shown in a previous DFT study⁴¹, the activation energy for the process of a physisorbed O₂ molecule next to a P3HT polymer chain reacting together to create a sulfonyl group on the thiophene is very high (approximately 2.0 eV). Hence, this process is largely suppressed in our device, and any oxygen molecules that have diffused into the bulk of P3HT (or DPP-DTT or C₁₆-IDTBT, which have similar S-based heterocyclic groups) will probably remain as O₂ in physisorbed configurations (Fig. 2b). Interestingly, density of states (DOS) calculations of the corresponding structures revealed that physisorbed O₂ molecules create unfilled energy states inside the bandgaps and close to the respective valence bands of DPP-DTT, P3HT and C₁₆-IDTBT. Figure 2c shows the DOS results for DPP-DTT. The results for P3HT and C₁₆-IDTBT are presented in Extended Data Fig. 5a,b. The existence of these acceptor states is consistent with the effect of p-doping of oxygen on the polymers of interest (as well as other typical organic semiconductors)^46,47.

Having identified the physisorbed oxygen molecules as the source of p-doping, we next aimed to understand the processes that enable the de-doping of the organic semiconductors in the presence of H₂. The first mechanism considered was that hydrogen molecules react directly with physisorbed O₂ molecules. We probed such a scenario with DFT-nudged elastic band (DFT-NEB) calculations and found that the transformation of physisorbed H₂ and O₂ into an H₂O molecule and an S-O group on a P3HT polymer chain is an exothermic process with a prohibitively high energetic barrier of 2.50 eV. Another possibility was that hydrogen molecules reacted with O species chemisorbed on the polymer. However, as we mentioned earlier, the formation of such species is also suppressed due to their very high activation energies.

As the barriers to reactions involving O₂ in these organic polymers are high, an alternative scenario focuses on the role of the Pt electrodes. It is known that Pt is efficient in dissociating hydrogen molecules and creating atomic H adatoms on its surface⁴⁸. Accordingly, we performed a series of DFT-NEB calculations to investigate reactions of such H adatoms with O species on a Pt surface. The corresponding results are presented in Extended Data Fig. 5c–f. From these results, we deduce that, first, oxygen molecules are chemisorbed on the Pt surface, and then, in a multistep process, H adatoms react with the O adatoms to create H₂O molecules. The latter have relatively small binding energies to the surface (0.2 eV), so some of these molecules will desorb. This process of eliminating p-type dopant oxygen species from the organic semiconductor in the presence of hydrogen leads to a reduction in current, as observed in our devices. To verify the role of the Pt electrode, a DPP-DTT device was prepared with gold (Au) electrodes. The experimental results (Supplementary Fig. 3) indicate that the DPP-DTT device did not show any significant response in the presence of hydrogen with Au electrodes, suggesting that the catalytic effects of Pt are crucial to the working mechanism of the device.

Finally, to exclude the possibility of electrochemical effects, we used a mixed ionic–electronic conductor, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate, which exhibits high ionic conductivity like that of organic electrolytic materials, as the sensing layer (Supplementary Fig. 4). The high ionic conductivity overwhelmed the effect of oxygen p-doping, so that the device did not respond to hydrogen, which supports our conclusion regarding the resistive nature of our organic sensors.

Stress testing the organic hydrogen sensor

To investigate whether the p-doping and de-doping processes by O₂ and H₂, respectively, could be observed in other organic semiconductors, we studied several other hole-transporting organic semiconductors: C₁₆-IDTBT, P3HT, 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene), poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) and one electron-transporting material, namely 2,2′-((2Z,2’Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno-[2″,3″:4′,5′]thieno[2′,3′:4,5]pyrrolo[3,2-g]thieno-[2′,3′:4,5]thieno[3,2-b]indole-2,10-diyl)bis(methanylylidene))-bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (Y6). The detailed analysis of the response to hydrogen exposure for all sensors developed is presented in Fig. 3a and Extended Data Fig. 6a–d. The DPP-DTT sensor exhibited the highest responsivity of over three orders of magnitude, while the P3HT device showed the lowest. The Y6-based sensor had a low current due to the poor electron injection caused by the large mismatch of the Pt work function with the lowest unoccupied molecular orbital and highest occupied molecular orbital levels of Y6. The sensor showed no evidence of oxygen-induced p-doping and no response to H₂ (Extended Data Fig. 6e). These findings also provide further support to the proposed sensing mechanism that relies on O₂-induced p-doping and the subsequent de-doping upon exposure to hydrogen. Due to its superior performance among the range of active-layer materials tested, the DPP-DTT sensor was chosen for further analysis.

**Fig. 3: Performance characteristics of different organic-based hydrogen sensors under various conditions.**

The performance of the DPP-DTT sensors was evaluated under different environmental conditions, such as elevated temperatures, different interfering volatile gases and varying RH (Fig. 3b–f). A small increase was observed in the responsivity up to 40 °C for H₂ concentrations up to 800 ppm (Fig. 3b). However, the responsivity of the sensor decreased on increasing the temperature from 60 to 120 °C. We attribute this to the lower adsorption rate for oxygen⁴⁹ and H₂ at higher temperatures⁵⁰. Figure 3c shows the real-time response of the sensor to H₂ concentrations from 0 to 1,000 ppm at 120 °C, which demonstrates its robust sensing capabilities even under such harsh conditions (full sets of the sensing characteristics of the DPP-DTT sensor measured at various temperatures from 40 to 120 °C are shown in Extended Data Fig. 7a–e). We also studied the long-term thermal stability of the DPP-DTT devices (Supplementary Fig. 5) and found that their response to hydrogen, in terms of current change, remained largely independent of thermal stress. Thus, the organic sensor does not require extended stabilization periods before it can be used for quantitative H₂ concentration measurements.

The selectivity of the DPP-DTT sensor to H₂ was also examined in the presence of various volatile organics (interfering species), including ethanol, methanol, acetone and toluene, which exhibit flammability and reducing properties like those of hydrogen and are commonly found in household environments⁵¹ (Fig. 3d and Extended Data Fig. 8a). The change in absolute responsivity for H₂ was approximately four orders of magnitude higher than that for the other compounds, even in a lower responsivity region (0.5 V), indicating the possibility of a further reduction in the power consumption of the organic hydrogen sensor. In further experiments, the DPP-DTT organic device did not exhibit any response to non-volatile reducing methane gas (0.1% in air) when measured at 20 °C and 15% RH (Supplementary Fig. 6). The current response of the DPP-DTT sensor was also examined in the presence of the oxidizing-gas nitrogen dioxide. The oxidation of the DPP-DTT layer produced holes that increased the device current, in contrast to the effect observed with H₂ (Supplementary Fig. 7). However, the device did not return to its initial state due to persistent conductivity and exhibited a very slow recovery, probably due to the irreversible oxidation of DPP-DTT. Importantly, the sensor was able to detect the presence of H₂ in a complex gas mixture containing 1.0% H₂, 1.0% O₂, 1.0% N₂ and 1.0% CO₂ in Ar (Extended Data Fig. 9a). Before H₂ exposure, the sensor showed a slight increase in the current when exposed to a stream of Ar gas containing 10% CO₂ and O₂ (Extended Data Fig. 9b,c); however, these small changes occurred at different timescales and did not interfere with the ability of the sensor to detect the presence of H₂ rapidly in real time (Extended Data Fig. 9d).

The organic sensor showed no response when balloons filled with Ar, N₂ and dry air were popped under the same experimental conditions to simulate sudden changes in the stoichiometry of the ambient atmosphere (Extended Data Fig. 10d), highlighting the specificity of the sensor to H₂. The performance of the hydrogen sensor was also examined under atmospheric (21%) and high (99.9%) oxygen conditions for comparison. The corresponding I–V characteristics were measured at hydrogen concentrations of 0 and 1,000 ppm (Supplementary Fig. 8). We found that under 99.9% O₂ conditions, the device current increased for both 0 and 1,000 ppm hydrogen concentrations. However, the device current remained largely unchanged despite the fourfold increase in O₂ concentration, indicating that the responsivity of the device in various O₂ environments remained unaffected. The only condition where the sensor failed to recover was when the atmosphere lacked O₂. Extended Data Fig. 9e,f shows that the sensor response to H₂ when inert gases were used as the carrier (Ar or N₂) was initially strong, but its signal failed to recover due to the absence of O₂, which is responsible for the subsequent p-doping.

To determine the impact of humidity on the H₂-sensing capabilities of the device, we conducted measurements across a range of RH from 15% to 80% for various H₂ concentrations ranging from 0 to 1,000 ppm (Fig. 3e and Extended Data Fig. 8b–d). The hydronium ions in water doped the organic semiconductor, such that the device current increased with humidity, as reflected in the I–V characteristics. In the presence of hydrogen, the organic semiconductor was de-doped, causing a decrease in the device current. However, hydrogen adatoms also react with hydroxyl ions, so that the device current increased and the hydrogen response reduced as the humidity was increased. The device exhibited robust dynamic operation with more than two orders of magnitude of current change upon repeated exposure to 0 and 1,000 ppm of H₂ at 80% RH for various bias conditions ranging from 0.2 to 1.0 V in 0.2 V increments (Fig. 3f). In long-term stability testing of the device under high humidity conditions (80% RH), the device was found to be stable (Supplementary Fig. 9). Finally, the long-term stability of the DPP-DTT sensors was studied by storing them in ambient air and humidity (20–24 °C and 50–60% RH) while sporadically testing their response to 1,000 ppm of H₂ under controlled conditions of 20 °C and 15% RH at 1 V. These data are plotted in Extended Data Fig. 10e. The sensor remained functional for nearly 2 years (646 days), with its response plateauing after an initial ‘burn-in’ period of ~60 days.

Our experiments demonstrate that the organic hydrogen sensor meets most of the target specifications for hydrogen safety applications. These include signal responsivity (0.25 relative resistance at 1% H₂), LoD (<0.1%), response speed (t₉₀ < 1 s at 1% H₂), recovery speed (t₁₀ < 60 s) and cost (potentially well under US$40 per unit), as defined by the US Department of Energy and in the literature (Supplementary Table 1)^43,44,45. t₉₀ is defined as the time required for the device current or resistance to change from 10% to 90% in the presence of hydrogen, while t₁₀ is the time taken for the device current or resistance to change from 90% to 10% in the absence of hydrogen. This led us to investigate the practical utility of this technology in realistic sensing applications, which we will discuss next.

Quantitative hydrogen sensing in practical scenarios

Before deploying the sensor in the quantitative detection of H₂ in practical scenarios, the device was calibrated with hydrogen concentrations ranging from background levels (500 ppb) to 1,000 ppm in ambient conditions (~20 °C and ~50% RH) using the same measuring electronics used in the demonstrations (Supplementary Fig. 10). We then studied the capability of the organic sensor to detect H₂ under three sensing scenarios.

In the first scenario, the sensor acted as a dynamic leak detector (Fig. 4a) tasked to monitor the H₂ concentration leaking from a pipe connected to a cylinder containing 5% H₂ in synthetic air under ambient conditions. The leak was controlled by a mass-flow controller. Throughout the experiment, we maintained a flow rate of 500 standard cubic centimetres per minute. The sensing unit was slowly moved by hand. A rapid reduction of the current was observed as it was moved through the leak area, corresponding to a H₂ concentration of 610 ppm (Fig. 4d).

**Fig. 4: Evaluation of the response of the organic hydrogen sensor in different sensing scenarios and comparisons with a commercial sensor.**

In the second scenario, we used the sensor as a warning system for H₂ leaks inside a closed room (volume ~18 m³, at room temperature and 55% RH) and compared its response with a commercial hydrogen sensor (details in Methods). H₂-filled balloons were placed 100 cm below the sensor unit and popped with a needle, as shown in Fig. 4b. The response of the organic sensor was recorded wirelessly in real time. Figure 4e shows the response signal from the organic sensor from the three sequentially burst balloons. The hydrogen concentrations measured by the organic sensor were 300 ppm (balloon 1), 280 ppm (balloon 2) and 275 ppm (balloon 3). The organic sensor exhibited a fast response of ~3 s, which was maintained even after several consecutive balloon bursts (inset in Fig. 4e). Extended Data Fig. 10c shows the real-time signals from both the commercial and organic sensors at distances of 100, 150 and 200 cm from the bursting balloon. In each case, the organic sensor exhibited a faster and larger response than the commercial sensor. Moreover, the responsivity of the organic sensor was almost double that of the commercial sensor (Fig. 4f). The results indicate that, as a hydrogen-sensing unit (electronics together with sensor), the wireless organic sensor demonstrates better device performance in terms of responsivity compared to the wired commercial apparatus.

In the third scenario, two sensing nodes (nodes 1 and 2) were deployed at different positions inside a closed room (volume ~16 m³) with a ventilation system (Fig. 4c). The sensors were positioned on opposite walls, approximately 2.5 m apart. Supplementary Fig. 11 is a photograph showing the dimensions of the room. Approximately 15 l of 100% hydrogen were released from several popped balloons placed on the floor of the room. The distances from the hydrogen source to nodes 1 and 2 were approximately 3.0 and 2.2 m, respectively. The released H₂ gas diffused throughout the room, and its concentration was monitored wirelessly in real time by the two nodes (Fig. 4g). The hydrogen concentrations measured at nodes 1 and 2 were approximately 605 and 740 ppm, respectively. This simple yet powerful demonstration shows the potential for these organic detectors to be used in wireless sensor networks.

The organic sensor was even used in a quantitative analysis of H₂ concentration under dynamic conditions. Specifically, the wireless sensor was mounted on a drone and quantitatively detected the hydrogen concentration while flying through an area where an H₂ leak had just occurred (Supplementary Fig. 12). The measured hydrogen concentration was approximately 320 ppm. Similarly, its high-fidelity dynamic sensing capability was also observed in a water-splitting experiment. Several cycles of H₂ generation were monitored with an average rise time of 14 (±4) s (Extended Data Fig. 10a,b and Supplementary Video 2). The extracted hydrogen concentration in the exhaust gas (Extended Data Fig. 10b) was ~335 ppm, as determined from the calibration curve in Supplementary Fig. 10 using a measured RH of ~52% at 20 °C (Supplementary Fig. 13). The dynamic response of the organic sensor was also compared against that of the commercial sensor by monitoring the evolution of H₂ concentration during a chemical reaction between Mg and acetic acid inside a round-bottom flask (Supplementary Fig. 15). The organic sensor showed faster response and recovery times than the commercial device.

Finally, to investigate the potential for cost-effective fabrication of these devices, we used a screen-printing process to deposit DPP-DTT onto interdigitated Pt electrodes and measured the performance of the devices (Supplementary Fig. 14a,b). The responsivity of the screen-printed devices was comparable to devices prepared by spin-coating, thus demonstrating that this technology is amenable to cheap, high-throughput manufacturing. Compared to other sensor technologies (Fig. 4h,i and Supplementary Table 2), our organic hydrogen sensor has the potential to be simpler to manufacture with superior performance (higher responsivity, lower power consumption and so on) and thus offers a promising technical solution for the safe and widespread use of hydrogen technologies.

Conclusions

We have described a reversible hydrogen-induced de-doping mechanism in organic semiconductors, which we used to develop a robust hydrogen sensor. The organic sensor exhibits a high responsivity (1.19 × 10⁴), fast response time (<1 s), low power consumption (≤2 μW; with no requirement for an external heating element), low LoD (~192 ppb), stability in harsh testing conditions and high selectivity for hydrogen even in complex gas environments. We also showed that our organic hydrogen sensor can outperform a commercial device in different practical sensing demonstrations. With its potential for scalable and cost-effective fabrication due to the attractive properties of organic semiconductors, our hydrogen sensor could be of widespread use in the emerging hydrogen economy.

Methods

Interdigitated electrodes

Titanium (50 nm)/platinum (150 nm) interdigitated electrodes with 5-μm gaps (IDE2) patterned on a glass substrate were purchased from Micrux and used as electrodes. All IDE2 samples were cleaned by ultrasonication in water, isopropanol and acetone for 5 min each before depositing the organic semiconductor.

Active materials

This work used the following organic semiconducting materials: DPP-DTT, C₁₆-IDTBT, P3HT, TIPS-pentacene and Y6. DPP-DTT, P3HT and TIPS-pentacene were purchased from Ossila, and PTAA and Y6 were purchased from 1-Material. All commercially supplied materials were used as received without further purification. C₁₆-IDTBT (Mn 65 kg mol⁻¹, dispersity of a polymer (Ð) 2.4) was synthesized in our laboratory.

Material preparation

DPP-DTT (5 mg ml⁻¹, chlorobenzene), C₁₆-IDTBT (6 mg ml⁻¹, chlorobenzene), P3HT (8 mg ml⁻¹, chlorobenzene), PTAA (8 mg ml⁻¹, chlorobenzene), TIPS-pentacene (5 mg ml⁻¹, toluene) and Y6 (10 mg ml⁻¹, chlorobenzene) were dissolved overnight on a hotplate at 70 °C while being stirred at 1,000 rpm. The DPP-DTT solution was spin-coated at 4,000 rpm for 60 s on the interdigitated Pt electrodes, followed by annealing at 100 °C for 90 min. C₁₆-IDTBT, P3HT, PTAA and Y6 were spin-coated at 2,000 rpm for 60 s on the interdigitated Pt electrodes, followed by annealing at 100 °C for 90 min. The TIPS-pentacene solution (10 μl) was drop cast onto the interdigitated Pt electrodes and annealed at 50 °C for 5 min. The thicknesses of the different organic semiconducting layers, measured with a stylus surface profilometer (Bruker), were 21 (±5), 32 (±6), 42 (±9), 49 (±8), 32 (±5) and 463 (±42) nm for DPP-DTT, C₁₆-IDTBT, P3HT, PTAA, Y6 and TIPS-pentacene, respectively.

Characterization

All hydrogen sensors were electrically characterized inside a Linkam HFS600E-PB4 chamber. The equilibrium of the gas flow was achieved with two mass-flow controllers (ALICAT). The temperature of the sensing device inside the chamber was varied to study the temperature-dependent H₂ sensing. All the electrical characterization measurements were obtained with Agilent B1500A instruments. Custom MATLAB programs were used to control the mass-flow controllers when mixing and diluting the sensing gas according to experimental requirements. This program was also used to provide hydrogen in pulsed form on top of the sensing surface, and this process was employed to evaluate the response of the device. The response from the other volatile gases (not H₂) was measured using the DPP-DTT device inside a custom-designed chamber. For the response measurements, a particular amount of ethanol, methanol, acetone or toluene was soaked in tissue paper and placed in the chamber to achieve a 1% environment of each gas. A DHT11 humidity sensor, which has a measuring accuracy of ±5%, was used to calibrate the humidity inside the gas-sensing chamber (Linkam) as well as the ambient humidity level for all the humidity-dependent H₂-sensing measurements. A commercially available Pt Clark-type electrode-based H₂-detecting system (Unisense, approximately US$1,200 per electrode) was also used to detect H₂. We compared its response with that of our organic hydrogen-sensing device. The surface morphology of the DPP-DTT-coated interdigitated Pt electrode was studied using an atomic force microscope (NT-MDT) in tapping mode. The work function of the Pt electrodes was measured by photoelectron spectroscopy in air using RIKEN KEIKI equipment. Gas cylinders were purchased from either Abdullah Hashim Industrial Gases & Equipment or Air Liquide. The responsivity of the sensor to hydrogen gas was calculated as (R − R₀)/R₀, where R₀ is the device current (or resistance) before hydrogen exposure and R is the device current after exposure to air containing 1,000 ppm H₂. The LoD of the DPP-DTT device was extracted from a linear fit of the H₂ concentration-dependent responsivity curve using 3.3 × (standard deviation of the intercept/slope of the linear curve), where the standard deviation of the intercept = (number of data points used in the measurement times the standard error of the intercept). The power consumption of the sensor was calculated as P = V × I, where V is the bias applied to the device and I is the measured sensing current.

DFT methodology

DFT studies were carried out with the Vienna Ab Initio Simulation Package⁵² using a plane-wave basis (with an energy cutoff of 400 eV), projector augmented waves⁵³, the generalized gradient approximation for Perdew–Burke–Ernzerhof⁵⁴ exchange-correlation functional and the DFT-D3 method for the van der Waals interactions⁵⁵.

To simulate the effect of oxygen and hydrogen reactions with the polymers of interest, we modelled them with a single periodic chain. The sampling of the reciprocal space in calculations of the electronic DOS used the tetrahedron method with 18 k-points in the direction of the periodic chain⁵⁶. The structures were rendered with the software VESTA⁵⁷. The activation energies of pertinent processes were obtained with the nudged elastic method⁵⁸, which models the so-called minimum energy pathway as a ‘chain’ of subsequent configurations (termed images) along the reaction coordinate.

Proof-of-concept demonstrations

We have demonstrated hydrogen sensing in three example use cases: hydrogen-generating chemical reactions, water electrolysis and the bursting of a H₂-filled balloon. We used prototype electronics⁵⁹ with the organic sensor to send sensing data wirelessly to an Android mobile phone, which analysed the data in real time. Photographs of the circuit layout and the printed circuit board used for the measuring electronics are shown in Supplementary Fig. 16. In these demonstrations, a commercial hydrogen sensor from UNISENSE was used for comparison. Its low power consumption (~2 μW) makes the organic sensor suitable for safe hydrogen detection applications as this reduces the risk of sparking. The organic sensor can detect hydrogen concentrations well below the flammable range (>40,000 ppm), enabling pre-emptive security alerts before hydrogen reaches hazardous levels.

To demonstrate the detection of hydrogen from chemical reactions (Supplementary Fig. 15), a three-necked 500-ml round-bottom flask equipped with a condenser and two septa was charged with a mixture of magnesium shavings (20 g) and water (50 ml). The top of the condenser was fitted with the DPP-DTT organic sensor and the commercial hydrogen sensor, and a continuous flow of air was introduced through one of the septa. Once the responses of the sensors had become stable, 1 M aq. acetic acid (0.1 ml) was added through the other septum. Data from both sensors were simultaneously recorded as the response to the generated hydrogen. After hydrogen evolution ceased, another portion of the aqueous acetic acid was added, and this process was repeated several times.

For the water hydrolysis experiments, we designed a small reaction chamber made of glass. We used steel springs as electrodes inside the chamber. Further, 2.5 M aq NaOH was used as the electrolyte. The spring electrodes were partially covered with small cylindrical plastic pipes, which collected H₂ and O₂ from the cathode and anode electrodes, respectively. The reaction chamber was connected to a 9.0 V battery (Duracell). Data from the organic sensor were recorded remotely by the bespoke electronics.

The balloon-bursting experiments were performed at room temperature in ambient air. The 100% hydrogen-filled balloons were placed at varying distances (100–200 cm) from the organic and commercial hydrogen sensors at stationary positions. The balloons were then burst using a sharp needle, and the response from the two sensors was recorded simultaneously in real time.