High-Performance Humidity Sensing in π-Conjugated Molecular Assemblies through the Engineering of Electron/Proton Transport and Device Interfaces

Abstract

The development of systems capable of responding to environmental changes, such as humidity, requires the design and assembly of highly sensitive and efficiently transducing elements. Such a challenge can be mastered only by disentangling the role played by each component of the responsive system, thus ultimately achieving high performance by optimizing the synergistic contribution of all functional elements. Here, we designed and synthesized a novel [1]benzothieno [3,2-b][1]benzothiophene derivative equipped with hydrophilic oligoethylene glycol lateral chains (OEG-BTBT) that can electrically transduce subtle changes in ambient humidity with high current ratios (>10 4 ) at low voltages (2 V), reaching state-of-the-art performance. Multiscale structural, spectroscopical, and electrical characterizations were employed to elucidate the role of each device constituent, viz., the active material's BTBT core and OEG side chains, and the device interfaces. While the BTBT molecular core promotes the self-assembly of (semi)conducting crystalline films, its OEG side chains are prone to adsorb ambient moisture. These chains act as hotspots for hydrogen bonding with atmospheric water molecules that locally dissociate when a bias voltage is applied, resulting in a mixed electronic/protonic long-range conduction throughout the film. Due to the OEG-BTBT molecules' orientation with respect to the surface and structural defects within the film, water molecules can access the humidity-sensitive sites of the SiO 2 substrate surface, whose hydrophilicity can be tuned for an improved device response. The synergistic chemical engineering of materials and interfaces is thus key for designing highly sensitive humidity-responsive electrical devices whose mechanism relies on the interplay of electron and proton transport.

Diffusion-Induced Ingress of Angiotensin-Converting Enzyme 2 into the Charge Conducting Path of a Pentacene Channel for Efficient Detection of SARS-CoV-2 in Saliva Samples

“…The sharp increase in current when water is dropped on the sensor is due to the additional proton generation from the dissociation of water molecules on the semiconducting channel’s surface. 43 However, the current saturates within a few seconds. The saliva sample is dropped once the current saturates.…”

“…A small volume (1 μL) of the saliva sample is further dropped on the water droplet. The sharp increase in current when water is dropped on the sensor is due to the additional proton generation from the dissociation of water molecules on the semiconducting channel’s surface . However, the current saturates within a few seconds.…”

Humidity Sensors Based on Metal–Organic Frameworks

“…When the sensing materials adsorb water molecules, their physical properties will change, and the changed physical properties are then converted into measurable signals, including fluorescence intensity, frequency, impedance, resistance, and capacitance. The hydrophilicity of the substrate, the number of electrodes, and the electrode material show various effects on the signals [ 60 ]. As an example, electrodes of electrically transduced humidity sensors are usually prepared on the substrates with the interdigital structure ( Figure 2 a).…”

Nanofloating gate modulated synaptic organic light-emitting transistors for reconfigurable displays

“…To broaden the control over the device operation, thereby enabling functional diversification, electric spiked memory transistors could offer a versatile solution when coupled with sensors by collecting external physical or chemical stimuli and converting them into electrical inputs for data storage. The sensory data for intelligent bionic system could be thus encoded by using different types of stimuli, whose precision and reliability for technological applications have been already demonstrated through the fabrication of tactile sensors (7)(8)(9)(10), light sensors (11)(12)(13), humidity sensors (14,15), and chemical sensors (16)(17)(18). As an alternative to conventional three-terminal memory architectures, nanofloating gate (NFG) transistors (NFGTs) are particularly suitable for the realization of both volatile and nonvolatile memory devices, because they combine a well-established working mechanism, reliable memory operations, and a gigantic memory capacity (19)(20)(21)(22).…”