High-Performance Flexible Gas Sensors Based on Layer-by-Layer Assembled Polythiophene Thin Films

Abstract

Gas sensors using organic semiconductors (OSCs) as active materials possess many advantages, including tunable selectivity and intrinsic flexibility, but still suffer from slow response and limited sensitivity. Ultrathin OSC films with high surface-to-volume ratios can improve the sensing performance; however, fabrication of continuous and uniform ultrathin OSC films using conventional methods remains challenging. In this work, we introduce a layer-by-layer assembly method, which is derived from the concept of atomic/molecular layer deposition technology, to prepare polythiophene OSC films under precise control. The resulting polythiophene film exhibits a linear growth as a function of the deposition cycle, and the nanoscale ultrathin films present a good thickness uniformity and smooth surface, in contrast to the uneven porous structure of spin-coated films. The assembly mechanism is revealed based on the detailed characterization of the polythiophene films. Further, the fabricated gas sensor using the layer-by-layer assembled polythiophene film as an active layer exhibits a 17%/ppm sensitivity to ammonia and a response time of less than 2 s, which is much superior to its counterpart with the spin-coated polythiophene film and is the best performance compared to previous reports. Flexible gas sensors are further demonstrated by depositing the film on a polymer substrate using the abovementioned method. Therefore, this work provides a widely applicable method to precisely deposit ultrathin and uniform OSC films for advanced applications.

Mechanistic Insights into Oxidative Molecular Layer Deposition of Conjugated Polymers

“…Conversely, the oxidation potentials of EDOT, PDA, and Py of 1.21, 0.74, and 0.87 V vs SHE fall below the oxidation power of Mo­(V)­Cl 5 → Mo­(IV)­Cl 4 and Mo­(IV)­Cl 4 →Mo­(III)­Cl 3 , allowing for continued polymer growth of pEDOT, pPDA, and pPy. We also note that oMLD using the 3HT monomer is viable due to the electron donating hexyl group which lowers the onset potential for oxidation, similar to the ethylene dioxyl group on EDOT.…”

“…As shown in the inset of Figure a, the Thi monomer represents the core functional structure of EDOT, but without the ethylene-dioxyl substituent. Considering the successful oMLD growth of EDOT:MoCl 5 , as well as recent reports of successful oMLD of Py:MoCl 5 and 3HT:MoCl 5 , one would expect that the Thi monomer would undergo oMLD growth, but it does not. This highlights a gap in mechanistic understanding of the oMLD growth mechanism.…”

“…oMLD is distinct from gas phase polymerization studies in which an oxidant and monomer are codosed in the gas phase to produce polymer films, − because it separates the gas phase chemical precursors into sequential exposure steps. To date, oMLD processes have been demonstrated using 3,4-ethylenedioxythiophene (EDOT, Figure b), ,, pyrrole (Py, Figure c), p-phenylenediamine (PDA, Figure d), and 3-hexylthiophene (3HT, Figure e) monomers and MoCl 5 , ReCl 5 , and SbCl 5 chemical oxidants. These species undergo self-limiting surface reactions to grow thin-film polymers monomer-by-monomer.…”

“…Oxidative molecular layer deposition (oMLD) is a relatively new deposition technique used to form electrically (semi)­conductive and redox-active conjugated polymers. oMLD is of growing interest because it provides molecular-level control of conjugated polymer film thickness, yielding uniform thin film polymers that are useful for electrochemical energy conversion and storage, − sensors, , and textiles. − The oMLD technique is a subcategory of molecular layer deposition (MLD), in which complementary bifunctional organic molecules are dosed in alternating exposures to form polymer films , and is akin to common atomic layer deposition (ALD) processes used to form inorganic films . The oMLD process uses the same monomers and oxidants that are used in conventional solution phase polymerization, but doses these species in the gas phase in sequential chemical exposures, as depicted in Figure a.…”

Atomic/Molecular Layer Deposition for Designer's Functional Metal–Organic Materials

“…New organic components have also been developed for the purely organic MLD processes; the MLD material library already includes, besides the initially introduced polyimides [ 15,17–22,137–143 ] and polyamides, [ 15,23–28,144–149 ] many other polymers: polyurea, [ 29,30,37,38,51,150–164 ] polythiourea, [ 52 ] polyurethane, [ 165,166 ] polyazomethine, [ 167–172 ] poly(3,4‐ethylenedioxy‐thiophene), [ 173–177 ] polyimide–polyamide, [ 141 ] poly(ethylene terephthalate) (PET), [ 50,178–180 ] and others. [ 31,32,39–44,176,181–200 ] In recent years, the organic precursor library has been rapidly expanding. We have collected in Table 1 the organic precursors so far used in ALD/MLD processes, together with the heating temperatures employed for their evaporation in the corresponding process conditions; [ …”