Investigation of ionic liquid induced doping in all-solid-state organic electrochemical transistors

The momentous discovery of conducting polymers in 1977 completely shattered the outlook of the polymer science as a branch of insulators and set forth monumental developments leading to the evolution of a comprehensive library of π conjugated polymers. Notwithstanding their menial charge carrier mobility, they have unique advantages over the conventional semiconductors such as the potential for altering their properties for specific applications through molecular engineering, their compatibility with various fabrication technologies such as solution processing and vacuum processing, and their physical attributes such as softness, flexibility, and low weight. Organic electrochemical transistors (OECTs) are a new generation of transistors in the subfield of bioelectronics which are structured like field effect transistors, except the drain current is controlled by the volumetric injection of ions into the channel from an electrolyte. This distinction is one of the defining characteristics of OECTs which is responsible for their unique ion-to-electron transduction properties. However, the use of liquid electrolyte limits the application of OECTs as the doping process is complicated by the presence of water and changes in the hydration radius of the ions present during doping. Moreover, they are susceptible to electrolyte leakage, and thus not suitable for wearable sensors. Solid electrolytes, however, are not susceptible to these drawbacks, and opens other applications for OECTs beyond bioelectronics such as pressure sensors, photodetectors etc. In the present study, a solid gel electrolyte made from the polymer poly(vinylidene fluoride-co-hexafluoropropene) (PVDF-co-HFP), and ionic liquids (ILs) 1-Ethyl-3-methylimidazolium tetrafluoroborate (EMIM BF4), and 1-Ethyl-3-methylimidazolium trifluoromethanesulfonate (EMIM OTF) are utilized to study the performance of solid state OECTs (SSOECTs). The effect of IL concentration on the static and dynamic response of devices is studied in detail. It is found that the electrochemical doping ability of the electrolyte is affected by the ionicity of the ionic liquids used in the electrolyte. Further, the performance gap between the ILs is bridged by improving the electrochemical accessibility through additive engineering the channel layer Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS). Towards that end, the effects of an ionic additive 1-Ethyl-3-methylimidazolium chloride (EMIM Cl), and a non-ionic additive Polyethylene glycol (PEO) are studied using various material characterisations such as insitu Raman, spectroelectrochemistry, X-ray photoelectron spectroscopy (XPS), GIWAX, cyclic voltammetry, electrochemical impedance spectroscopy, and AFM.Doctor of Philosoph

Contact modulated ionic transfer doping in all-solid-state organic electrochemical transistor for ultra-high sensitive tactile perception at low operating voltage

Ionic-electronic coupling across the entire volume of conjugated polymer films endows organic electrochemical transistors (OECTs) with high transconductance and low operating voltage. However, OECTs utilize liquid electrolytes, which limit their long-term operation, reproducibility, and integration while solid electrolytes typically result in inefficient ion transport. Here we show that a solid polymer electrolyte, can facilitate good electrochemical response in conjugated polymers and yield high OECT performance. This allows for the OECT based pressure sensors, modulated through a pressure sensitive ionic doping process. The pressure sensor exhibits the highest sensitivity ever measured (~10000 kPa-1) and excellent stability. Flexible sensor arrays realize a static capture of spatial pressure distribution and enable monitoring of dynamic pressure stimuli. Our findings demonstrate that all-solid-state OECTs are good candidates for providing rich tactile information, enabling applications for soft robotics, health monitoring and human-machine interfaces.Agency for Science, Technology and Research (A*STAR)Ministry of Education (MOE)Nanyang Technological UniversitySubmitted/Accepted versionW.L.L. would like to acknowledge funding support from her NTU start-up grant (M4081866), Ministry of Education (MOE) under ACRF Tier 2 grant (2018-T2-1-075), and A*STAR AME Young Individual Research Grant (Project Number A1784c019)

Self-healable organic electrochemical transistor with high transconductance, fast response, and long-term stability

The major challenges in developing self-healable conjugated polymers for organic electrochemical transistors (OECTs) lie in maintaining good mixed electronic/ionic transport and the need for fast restoration to the original electronic and structural properties after the self-healing process. Herein, we provide the first report of an all-solid-state OECT that is self-healable and possesses good electrical performance, by utilizing a matrix of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and a nonionic surfactant, Triton X-100, as a channel and an ion-conducting poly(vinyl alcohol) hydrogel as a quasi-solid-state polymer electrolyte. The fabricated OECT exhibits high transconductance (maximum 54 mS), an on/off current ratio of ∼1.5 × 103, a fast response time of 6.8 ms, and good operational stability after 68 days of storage. Simultaneously, the OECT showed remarkable self-healing and ion-sensing behaviors and recovered ∼95% of its ion sensitivity after healing. These findings will contribute to the development of high-performance and robust OECTs for wearable bioelectronic devices.Ministry of Education (MOE)W.L.L. would like to acknowledge funding support from her NTU start-up grant (M4081866), Ministry of Education (MOE), under the AcRF Tier 2 grant (2018-T2-1-075) and the A*STAR AME Young Individual Research Grant (Project no. A1784c019). We would like to acknowledge the Facility for Analysis, Characterization, Testing and Simulation (FACTS), Nanyang Technological University, Singapore, for use of their GIWAXS facility (Xenocs Nano-inXider)

Enhancing the electrochemical doping efficiency in diketopyrrolopyrrole-based polymer for organic electrochemical transistors

The increasing interest in organic electrochemical transistors (OECTs) for next-generation bioelectronic applications motivates the design of novel conjugated polymers with good electronic and ionic transport. Many conjugated polymers developed for organic field-effect transistors (OFETs) exhibit high charge carrier mobilities but they are not suitable for OECTs due to poor ion-uptake arising from the non polar alkyl chain substituted on the conjugated backbone. They are also sensitive to moisture, resulting in poor performance in aqueous electrolytes. Herein, the widely used conjugated building block diketopyrrolopyrrole (DPP) is used and functionalized it with polar triethylene glycol side chains (PTDPP-DT) to promote ion penetration. The electrical performance of PTDPP-DT based OECT in two types of aqueous electrolytes is studied and the electrochemical doping response is investivated. It is found that the tetrafluoroborate (BF4−) anion with large crystallographic radius allows high-efficiency electrochemical doping of the PTDPP-DT polymer, and thus gives rise to the high transconductance of 21.4 ± 4.8 mS with good device stability, where it maintained over 91 % of its doped-state drain current after over 500 cycles of pulse measurement.</p

All inorganic mixed halide perovskite nanocrystal-graphene hybrid photodetector : from ultrahigh gain to photostability

Hybrid graphene-perovskite photodetectors embrace the excellent photoabsorption properties of perovskites and high carrier mobility of graphene in a single device. Here, we demonstrate the integration of halide-ion-exchanged CsPbBr x I3-x nanocrystals (NCs) as a photoabsorber and graphene as a transport layer. The NCs conform to a cubic lattice structure and exhibit an optical band gap of 1.93 eV. The hybrid device attained a maximum responsivity of 1.13 × 104 A/W and specific detectivity of 1.17 × 1011 Jones in low light intensity (∼80 μW/cm2). Specifically, an ultrahigh photoconductive gain of 9.32 × 1010 is attained because of fast hole transit time in the graphene transistor and long recombination lifetime in the perovskite NCs simultaneously. The phototransistor also shows good stability and can maintain ∼95% of the photocurrent under continuous illumination over 5 h and ∼82% under periodic illumination over 37 h. Our results also revealed that the common issue of ion separation and segregated halide domains in mixed halide perovskite NCs do not occur under low light intensities. The intensive degradation of CsPbBr x I3-x NCs is only observed under stronger light excitation (≥55 mW/cm2), reflecting as emission shifts. Our work establishes the use of fully inorganic perovskite NCs as highly stable photodetectors with high responsivity and low power light detection.NRF (Natl Research Foundation, S’pore)ASTAR (Agency for Sci., Tech. and Research, S’pore)MOE (Min. of Education, S’pore)Accepted versio

Recent technological advances in fabrication and application of organic electrochemical transistors

Organic electrochemical transistors (OECTs), where conjugated polymers undergo doping/ de-doping from an electrolyte, have been widely studied for applications ranging from switching elements, artificial synapses to transducers for biological sensing. The concurrent transport of electronic and ionic charges within the channel provide OECTs with excellent amplification capability and efficient ion-to-electron transduction at low operating voltages (<1V). Herein, the latest research advances in the fabrication techniques and applications of OECTs are summarized. Particular focus is given on the emerging applications including sensors, neuromorphic devices and electrophysiological monitoring. Finally, the future challenges and opportunities of conventional OECTs and novel all-solid-state OECTs are discussed.Agency for Science, Technology and Research (A*STAR)Ministry of Education (MOE)Nanyang Technological UniversitySubmitted/Accepted versionW.L.L. would like to acknowledge funding support from her NTU start-up grant (M4081866), Ministry of Education (MOE) under AcRF Tier 2 Grant (No. 2018-T2-1-075), ASTAR AME IAF-ICP Grant (No. I1801E0030), and A*STAR AME Young Individual Research Grant (Project Number A1784c019)

Self-powered organic electrochemical transistors with stable, light-intensity independent operation enabled by carbon-based perovskite solar cells

Wearable sensors and electronics for health and environment monitoring are mostly powered by batteries or external power supply, which requires frequent charging or bulky connecting wires. Self-powered wearable electronic devices realized by integrating with solar cells are becoming increasingly popular due to their ability to supply continuous and long-term energy to power wearable devices. However, most solar cells are vulnerable to significant power losses with decreasing light intensity in the indoor environment, leading to an errant device operation. Therefore, stable autonomous energy in a reliable and repeatable way without affecting their operation regime is critical to attaining accurate detection behaviors of electronic devices. Herein, we demonstrate, for the first time, a self-powered ion-sensing organic electrochemical transistor (OECT) using carbon electrode-based perovskite solar cells (CPSCs), which exhibits a highly stable device operation and independent of the incident light intensity. The OECTs powered by CPSCs maintained a constant transconductance (gm) of ~60.50±1.44 μS at light intensities ranging from 100 mW cm-2 to 0.13 mW cm-2. Moreover, this self-powered integrated system showed good sodium ion sensitivity of -69.77 mV decade-1, thereby highlighting its potential for use in portable, wearable, and self-powered sensing devices.Agency for Science, Technology and Research (A*STAR)Ministry of Education (MOE)W.L.L. would like to acknowledge funding support from Ministry of Education (MOE) under AcRF Tier 2 grant Nos. (2018-T2-1-075 and 2019- T2-2-106), A*STAR AME IAF-ICP Grant (No. I1801E0030), and National Robotics Programme (W1925d0106)

Ionic-liquid doping enables high transconductance, fast response time, and high ion sensitivity in organic electrochemical transistors

Organic electrochemical transistors (OECTs) are highly attractive for applications ranging from circuit elements and neuromorphic devices to transducers for biological sensing, and the archetypal channel material is poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate), PEDOT:PSS. The operation of OECTs involves the doping and dedoping of a conjugated polymer due to ion intercalation under the application of a gate voltage. However, the challenge is the trade-off in morphology for mixed conduction since good electronic charge transport requires a high degree of ordering among PEDOT chains, while efficient ion uptake and volumetric doping necessitates open and loose packing of the polymer chains. Ionic-liquid-doped PEDOT:PSS that overcomes this limitation is demonstrated. Ionic-liquid-doped OECTs show high transconductance, fast transient response, and high device stability over 3600 switching cycles. The OECTs are further capable of having good ion sensitivity and robust toward physical deformation. These findings pave the way for higher performance bioelectronics and flexible/wearable electronics.ASTAR (Agency for Sci., Tech. and Research, S’pore)Accepted versio

Large area, high efficiency and stable perovskite solar cells enabled by fine control of intermediate phase

Organic-inorganic lead halide perovskites have shown great potential in efficient photovoltaic devices. However, there are issues related to device stability and reliability and the high power conversion efficiencies (PCE) are typically demonstrated on cell areas much less than 0.2 cm2. The main challenges which limit high efficiencies in larger area devices lie on the low temperature solution processing methods which typically produce lower quality perovskites with defects (pinholes and traps) and the undesired increase in series resistance with cell area. Herein, the control of the dimethyl sulfoxide (DMSO) adduct intermediate phase for the formation of the defect-free perovskite layer and their suitability for larger area solar cells are investigated. We have also selected different conducting substrates, namely indium tin oxide (ITO) with sheet resistance of 10 Ω/□ and fluorine doped tin oxide (FTO) substrates with sheet resistances of 7 and 15 Ω/□ to characterize the effect of substrate sheet resistance and transparency on the photovoltaic performance in large area devices. We demonstrate high PCEs of 18.2% for small area devices (0.16 cm2) and 15.1% for large area device (2 cm2) using the DMSO-enriched recipe. In addition, enhanced device stability was observed, where the devices sustained 94% of their initial efficiency after 105 days without encapsulation. These results confirm that the fine control of adduct intermediate phase for reduced-defect perovskite film provides a simple and universal solution for larger area, efficient and stable perovskite solar cells.Agency for Science, Technology and Research (A*STAR)Ministry of Education (MOE)Nanyang Technological UniversityNational Research Foundation (NRF)Submitted/Accepted versionT.Y. thank the support from “the Fundamental Research Funds for the Central Universities” (No. 2018RC022). W.L.L. would like to acknowledge funding support from her NTU start-up grant (M4081866), Ministry of Education (MOE) under AcRF Tier 2 grant (2018-T2-1-075) and A*STAR AME Young Individual Research Grant (Project Number A1784c019). The authors would also like to acknowledge the funding from Office of Naval Research Global (ONRG-NICOP-N62909-17-1- 2155) and Intra-CREATE Collaborative Grant (NRF2018-ITC001-001)

Universal spray-deposition process for scalable, high-performance, and stable organic electrochemical transistors

Organic electrochemical transistors (OECTs) with high transconductance and good operating stability in an aqueous environment are receiving substantial attention as promising ion-to-electron transducers for bioelectronics. However, to date, in most of the reported OECTs, the fabrication procedures have been devoted to spin-coating processes that may nullify the advantages of large-area and scalable manufacturing. In addition, conventional microfabrication and photolithography techniques are complicated or incompatible with various nonplanar flexible and curved substrates. Herein, we demonstrate a facile patterning method via spray deposition to fabricate ionic-liquid-doped poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)-based OECTs, with a high peak transconductance of 12.9 mS and high device stability over 4000 switching cycles. More importantly, this facile technique makes it possible to fabricate high-performance OECTs on versatile substrates with different textures and form factors such as thin permeable membranes, flexible plastic sheets, hydrophobic elastomers, and rough textiles. Overall, the results highlight the spray-deposition technique as a convenient route to prepare high-performing OECTs and will contribute to the translation of OECTs into real-world applications.ASTAR (Agency for Sci., Tech. and Research, S’pore)MOE (Min. of Education, S’pore)Accepted versio