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Organic thin film transistors (OTFTs) have been extensively investigated for biosensing and bioelectronic applications. The conducting polymer poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulfonate) (PEDOT:PSS) has been widely used in these devices, typically in the form of solid thin films. These films, while flexible, exhibit mechanical properties that differ significantly from soft biological materials, such as brain tissue. Hydrogels, by contrast, are intrinsically better suited to interfacing with soft, delicate biological materials. In this work, we investigate PEDOT:PSS-based hydrogels, for use as the top gate electrode in low-voltage hygroscopic insulator field effect transistors (HIFETs). We describe our method for fabricating patterned PEDOT:PSS hydrogel electrodes and report their electrical and rheological properties. We also show that HIFETs using PEDOT:PSS hydrogel gate electrodes exhibit good transistor characteristics and operate at low voltages (−1 V ≤ Vds ≤ 0 V). Further, we demonstrate that hydrogel-gated HIFETs exhibit excellent sensitivity to aqueous potassium chloride (KCl) and sodium chloride (NaCl), as well as hydrogen peroxide (H2O2) over a range of concentrations. Lastly, we show that HIFET sensors can be translated to selective sensing of ions by demonstrating selective sensing of K+ ions. Our results represent an important step forward in the development of HIFETs as soft and conformable sensors.

Thermally activated delayed fluorescent (TADF) materials are extensively investigated as organic light-emitting diodes (OLEDs) with TADF emitting layers demonstrating high efficiency without the use of heavy metal complexes. Therefore, solution-processable and printable TADF emitters are highly desirable, moving away from expensive vacuum deposition techniques. In addition, using emissive materials not requiring an external host simplifies the fabrication process significantly. Herein, OLEDs using a solution-processable TADF polymer that do not need an external host are introduced. The non-conjugated TADF polymer features a TADF emitter (4-(9H-carbazol-9-yl)-2-(3′-hydroxy-[1,1′-biphenyl]-3-yl)-isoindoline-1,3-dione) as a side chain, as well as a hole-transporting side chain and an electron-transporting side chain on an inactive polymer backbone. All organic layers of the OLEDs are fabricated using solution processing methods. The OLEDs with inkjet-printed emissive layers have comparable maximum current and external quantum efficiency as their spin-coated counterparts, exceeding luminance of 2000 cd m−2. The herein-explored strategy is a viable route toward self-hosted printable TADF OLEDs.

Flexible pressure sensors are an attractive area of research due to their potential applications in biomedical sensing and wearable devices. Among flexible and wearable pressure sensors, capacitive pressure sensors show significant advantages, owing to their potential low cost, ultralow power consumption, tolerance to temperature variations, high sensitivity, and low hysteresis. In this work, we develop capacitive flexible pressure sensors using graphene based conductive foams. In these soft and porous conductive foams, graphene is present either as a coating of the pores in the foam, inside the structure of the foam, or as a combination of both. We demonstrate that they are durable and sensitive at low pressure ranges (<10 kPa). Systematic analysis of the different pressure sensors revealed that the porous foams with graphene coated pores are the most sensitive (∼0.137 kPa–1) in the pressure range 0–6 kPa, with a limit of detection of 50 Pa. Further, we demonstrated the potential applications of our pressure sensors by showing detection of weak physiological signals of the body. Our work is highly relevant for research in flexible pressure sensors based on conductive foams as it shows the impact of different ways of incorporating conductive material on performance of pressure sensors.

Hygroscopic insulator field effect transistors (HIFETs) are a class of solid-state, low-voltage organic thin film transistors with promising sensitivity to hydrogen peroxide (H2O2). While work has progressed on the development and understanding of HIFET-based sensors, important questions regarding the fundamental H2O2 sensing mechanisms need to be fully investigated. In this work, a detailed study of the effects of H2O2 on the transient drain currents and transfer characteristics of HIFETs, while varying analyte concentrations and device voltages, is presented. The sensing mechanism is found to be consistent with the direct oxidation of the organic semiconductor channel, poly(3-hexylthiophene-2,5-diyl), by H2O2 diffused into the device through the permeable top gate electrode and hygroscopic insulator. Further, the importance of having a transistor structure in sensing H2O2, as compared to a two-terminal device, is demonstrated. These results represent important progress in the understanding of HIFET-based sensors, revealing paths for future applications and optimization.

Given the many and varied roles of ions in living organisms, biocompatible organic ion sensors are a matter of considerable interest. In this work, simple, low-voltage, solid-state hygroscopic insulator field effect transistors (HIFETs) have been tested to characterize their ion-sensitive properties. Two biologically relevant salt solutions, sodium chloride (NaCl) and potassium chloride (KCl), were tested. To assess pH sensitivity, solutions of hydrochloric acid (HCl) and sodium hydroxide (NaOH) were also tested. The salts and acidic solutions caused similar, concentration-dependent changes in HIFET performance from 10 mM to 1 M, consistent with an increase in ion concentration in the hygroscopic insulator, increasing device capacitance. By contrast, basic solutions caused an overall decrease in device performance, consistent with a net removal of ions due to acid-base reactions between the insulator and the analyte. These results show that HIFETs exhibit promising sensitivity to a range of ions and can therefore serve as platforms for future ion-selective devices.

Confocal fluorescence microscopy provides a means to map charge carrier density within the semiconductor layer in an active organic thin film transistor (OTFT). This method exploits the inverse relationship between charge carrier density and photoluminescence (PL) intensity in OTFTs, originating from exciton quenching following exciton-charge energy transfer. This work demonstrates that confocal microscopy can be a simple yet effective approach to gain insight into doping and de-doping processes in OTFT sensors. Specifically, the mechanisms of hydrogen peroxide sensitivity are studied in low-voltage hygroscopic insulator field effect transistors (HIFETs). While the sensitivity of HIFETs to hydrogen peroxide is well known, the underlying mechanisms remain poorly understood. Using confocal microscopy, new light is shed on these mechanisms. Two distinct doping processes are discerned: one that occurs throughout the semiconductor film, independent of applied voltages; and a stronger doping effect occurring near the source electrode, when acting as an anode with respect to a negatively polarized drain electrode. These insights offer important guidance to future studies and the optimization of HIFET-based sensors. More importantly, the methods reported here are broadly applicable to the study of a range of OTFT-based sensors. This work demonstrates that confocal microscopy can be an effective research tool in this field.

Organic Light Emitting Diodes (OLEDs) are a critical part of current consumer electronics, from mobile device displays to solid-state lighting. Thus demand for the development of industrially compatible OLED materials continues to increase. Herein, we introduce a series of solution-processible polymers incorporating a Thermally Activated Delayed Fluorescence (TADF) emitter and a host species in the side chain to finely adjust the charge transport properties. For balanced charge transport, a bipolar host polymer based on carbazole and alpha-carboline was investigated in addition to the commonly employed unipolar host mCP and an electron transporting version thereof. We demonstrate that the combination of unipolar co-hosts on one polymer chain can generate, with less synthetic effort, the same optoelectronic properties as a polymer carrying the corresponding bipolar host molecule as a pendant.

Solution-processing of multilayered Organic Light Emitting Diodes (OLEDs) remains a challenge that is often addressed by cross-linking polymer precursors into insoluble networks. Herein, we blend an emissive polymer carrying a Thermally Activated Delayed Fluorescence (TADF) emitter and a host species with a photo-cross-linkable polymer containing ortho-methylbenzaldehyde and maleimide groups as reactive cross-linkers to form a Semi-Interpenetrating Polymer Network (SIPN) upon irradiation at 365 nm. The progress of the cross-linking via Diels–Alder [4 + 2]-cycloaddition is monitored by FT-IR-spectroscopy and is correlated with the solvent resistance of the SIPN. Furthermore, the influence of the molecular weight and the cross-linker content on the efficiency of the cross-linking are investigated. The resulting polymer films show a high solvent resistance evidenced by photoluminescence and AFM measurements and are thus suitable for a successive solution-processed layer. Furthermore, a comonomer carrying the commercial host molecule 1,3-bis(N-carbazolyl)benzene (mCP) was synthesized in high yields, copolymerized and integrated in the emissive SIPN with good resistance against organic solvents. Lastly, the polymer blends were processed with an ink-jet printer and turned into an insoluble SIPN.

Organic thin film transistors, employing diverse device architectures, materials and form factors, have been demonstrated as effective sensors of a variety of analytes, including ions. In many such devices, it is essential to use materials with both electronic conductivity and ion permeability, that are also stable under aqueous conditions. Hygroscopic insulator field effect transistors (HIFETs) are simple, solution processable, and low-voltage transistors with promising sensing properties when the analyte is introduced into the device through a permeable top gate electrode. It is important that the gate electrode is water stable, to prevent loss or damage to the electrode in aqueous conditions and ensure consistent performance. Here, we report two different methods to fabricate stable gate electrodes for HIFETs using crosslinked PEDOT:PSS, a conductive and ion-permeable polymer. We compare the two common crosslinkers, divinyl sulfone (DVS) and (3-glycidyloxypropyl)trimethoxysilane (GOPS), and present methods to overcome challenges associated with using either for crosslinking a top gate electrode. We show that DVS crosslinking can take place at room temperature, but freestanding films need to be formed separately then attached to the transistor to avoid damage to underlying layers by the crosslinker. GOPS crosslinking requires high temperatures but can be done in situ and bonds well to underlying layers. Using these optimised methods, we show that the stable PEDOT:PSS films perform well as gate electrodes for HIFET sensors. Our report is a significant step forward in the development of stable HIFET-based sensors, and our methods may have broader applicability to similar stable organic electronic devices. This journal is

Multifunctional operation of tetracene/polycyclopentadithiophene-benzothiadiazole (PCPDTBT), and tetracene sensitized PCPDTBT:fullerene bulk heterojunctions (BHJs) that work as an organic photovoltaic (OPV), organic light emitting diode (OLED), and organic photodetector (OPD) is unveiled. The multifunctional operation is underlined by a synergetic interplay between singlet and triplet energy transfer and the judicious alignment of energy levels of donor–acceptor cascade. Introduction of tetracene is shown to enhance electroluminescence (EL) at 800 nm despite the observation of a near-complete photoluminescence quenching in BHJ systems. Signatures of triple energy transfer mediated via charge transfer states of BHJ to tetracene become apparent as the proportion of fullerene in the BHJ is increased. An optimal balance between exciton population is achieved with BHJ 1:2 that produces EL peaks at 530 and 800 nm while maintaining a respectable OPV and OPD performance. The proposed ternary system unlocks the potential of having multifunctionality within one single diode structure, and it also offers new design rules for multispectral OLEDs.

Amongst emissive materials for organic light emitting diodes (OLEDs), thermally activated delayed fluorescence (TADF) materials have shown substantial promise in the last few years. For OLEDs, solution processable and printable emissive materials are highly desirable as printing allows for precise patterning without masks, enables deposition of nanometer scale thicknesses and results in minimal wastage of material. Herein, we introduce a solution processable TADF emitting polymer as an emissive material for OLEDs. The bespoke polymer structure features the TADF emitter 4-(9H-carbazol9-yl)-2-(3′-hydroxy-[1,1′-biphenyl]-3-yl)isoindoline-1,3-dione as a pendant group on a poly(methyl methacrylate) based polymer chain. The resulting OLEDs have a peak emission wavelength of 520 nm with a maximum luminance of around 4700 cd m−2. The peak emission wavelength can be blue-shifted by exciplex management to achieve a peak wavelength of 494 nm. Critically, we employed the TADF-containing polymer system to ink-jet print OLEDs, demonstrating that such polymers are viable for printable OLEDs.

Flexible and low-power consuming integrated circuits are some of the basic requirements for smart wearable devices. High mobility solution-processed organic field-effect transistors (OFETs) have the potential to make a big impact in printed electronic circuits, but their overall performance is currently limited by unusually high threshold voltages (Vth). Herein, systematic optimization of donor–acceptor conjugated polymer, based on dithienothiophene (DTT) and thiophene-flanked diketopyrrolopyrrole (DPP), namely, PDPPT–DTT, OFETs by application of self-assembled monolayers (SAMs) at the semiconductor–dielectric, and semiconductor–metal interfaces is reported. The results clearly exhibit that simultaneous application of octyltrichlorosilane (OTS) as semiconductor–dielectric interface modifying layer and pentafluorobenzene thiol (PFBT) as semiconductor–metal interface modifying layer results in significantly lower Vth and subthreshold slope values from −14.07 V and 13.26 (V Dec−1) to +1.06 V and 7.11 (V Dec−1), respectively. This tailored approach is also beneficial in enhancing hole mobility values by an order of magnitude from 0.01 to 0.5 cm2 V−1 s−1 along with the possibility of switching from hole accumulation mode (Vth = −3.75 V) to depletion mode (Vth = +1.06 V) through device engineering. Simultaneous interface engineering reveals OFET electronic properties can be fine-tuned for robust circuits and low power electronic applications.

Metal halide perovskite nanocrystals (PeNCs) are emerging as one of the most promising materials for optoelectrical devices such as light-emitting diode (LED) owing to their tunable band gaps, high color purity, and tolerance to defects. Nevertheless, the materials have not yet demonstrated their full potential in practical applications due to the poor stability of PeNCs and their unsatisfactory performance in devices. In this research, LED devices based on CsPbBr3 NCs are successfully demonstrated as an active light-emitting layer with enhanced efficiency via potassium doping. The study of the effect of potassium dopant on physicochemical properties of inorganic CsPbBr3 PeNCs shows that potassium cations (K+) remain in the crystal structure of perovskite compound as interstitial defects, which results in better coverage of surface ligands on the PeNCs and improved energy band alignment with adjacent electron injection layer (2,2′,2′-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole)) and hole injection layer (poly(3,4-ethylenedioxythiophene) polystyrene sulfonate) in an LED. As a consequence, the green LED with the K-doped CsPbBr3 PeNCs shows luminance up to 5759 cd m-2 and external quantum efficiency (EQE) of 5.6%, which is superior to the pristine device without K-doping (luminance: 4579 cd m-2, EQE: 4.8%).

Hygroscopic insulator field-effect transistors (HIFETs) are a class of low-voltage-operation organic transistors that have been successfully demonstrated for biosensing applications through modification of the gate electrode. However, modification of the gate electrode often leads to nonideal transistor characteristics due to changes in its intrinsic electrical properties. This work investigates the effect of gate conductance in HIFETs using poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) as a model gate electrode. It is revealed that a reduction in gate conductance results in a reduction in the effective gate voltage and plays an important role in defining HIFET characteristics. Key figures of merit, including ON/OFF ratio, threshold voltage, transconductance, and saturation mobility increase with increasing gate conductance and reach a plateau once sufficient gate conductance is attained. This effect is attributed to a decrease in the effective gate voltage along the gate electrode arising from its resistivity when a gate leakage current is present. These results are widely applicable and serve as design rules for HIFET device optimization.

Low‐cost flexible organic light‐emitting diodes (OLEDs) with nanoemitter material from waste open up new opportunities for sustainable technology. The common emitter materials generated from waste are carbon dots (CDs). However, these have poor luminescent properties. Further solid‐state emission quenching makes application in display devices challenging. Here, flexible and rigid OLED devices are demonstrated using self‐assembled 2D arrays of CDs derived from waste material, viz., human hair. High‐performance CDs with a quantum yield (QY) of 87%, self‐assembled into 2D arrays, are achieved by improving the crystallinity and decreasing the CDs' size distribution. The CD island array exhibits ultrahigh hole mobility (≈10−1 cm2 V−1 s−1) and significant reduction in solid‐state emission quenching compared to pristine CDs; hence, it is used here as an emitting layer in both indium tin oxide (ITO)‐coated glass and ITO‐coated flexible poly(ethylene terephthalate) (PET) substrate OLED devices, without any hole‐injection layer. The flexible OLED device exhibits a stable, voltage‐independent blue/cyan emission with a record maximum luminescence of 350 cd m−2, whereas the OLED device based on the rigid glass substrate shows a maximum luminescence of 700 cd m−2. This work sets up a platform to develop next‐generation OLED displays using CD emitters derived from the biowaste material.