Organic electrochemical transistors (OECTs) leverage ion injection from an electrolyte into an organic semiconductor film to yield compelling advances in biological interfacing, printed logic circuitry and neuromorphic devices. Their defining characteristic is the coupling between electronic and ionic charges within the volume of an organic film. In this review we discuss the mechanism of operation and the 2 materials that are being used, overview the various form factors, fabrication technologies and proposed applications, and take a critical look at the future of OECT research and development.

We report a design strategy that allows the preparation of solution processable n-type materials from low boiling point solvents for organic electrochemical transistors (OECTs). The polymer backbone is based on NDI-T2 copolymers where a branched alkyl side chain is gradually exchanged for a linear ethylene glycol-based side chain. A series of random copolymers was prepared with glycol side chain percentages of 0, 10, 25, 50, 75, 90, and 100 with respect to the alkyl side chains. These were characterized to study the influence of the polar side chains on interaction with aqueous electrolytes, their electrochemical redox reactions, and performance in OECTs when operated in aqueous electrolytes. We observed that glycol side chain percentages of >50% are required to achieve volumetric charging, while lower glycol chain percentages show a mixed operation with high required voltages to allow for bulk charging of the organic semiconductor. A strong dependence of the electron mobility on the fraction of glycol chains was found for copolymers based on NDI-T2, with a significant drop as alkyl side chains are replaced by glycol side chains.

We have measured the electrical characteristics and the efficiencies of single-layer organic light-emitting diodes based on poly͓2-methoxy-5-͑2-ethylhexoxy͒-1,4-phenylene vinylene͔ ͑MEH-PPV͒, with Au anodes and Ca, Al, and Au cathodes. We show that proper accounting of the built-in potential leads to a consistent description of the current-voltage data. For the case of Au and Al cathodes, the current under forward bias is dominated by holes injected from the anode and is space-charge limited with a field-dependent hole mobility. The Ca cathode is capable of injecting a space-charge-limited electron current. ͓S0163-1829͑98͒52844-5͔Organic light-emitting diodes ͑OLED's͒ have emerged over the past ten years as viable candidates for application in display technologies. 1 In their simplest configuration, a fluorescent semiconducting polymer is sandwiched between two metal electrodes, an anode with a high and a cathode with a low work function. Under the application of an electric field, holes and electrons are injected into the valence and the conduction band of the polymer, respectively. A fraction of these charges combine to form excitons that decay radiatively, giving rise to light emission. While the technology of OLED's is advancing rapidly, fundamental studies of the device operation are lagging behind. Even in PPV derivatives, which were the first polymers to show electroluminescence 2,3 and are by far the best studied, the relative importance of charge injection as opposed to charge transport as the factor limiting the efficiency of OLED's is still under debate. [4][5][6] For the case of large barriers at the cathode ͑anode͒, inefficient electron ͑hole͒ injection is the limiting process. 4,7 However, since the trap-free drift mobilities are not known for both carriers, it is not clear whether the efficiency of the best devices is limited by injection or by bulk transport. One experimental degree of freedom is the electrode work function which one can change to alter the barrier for electron or hole injection into the polymer, thus changing the magnitude of the electron ͑hole͒ current. Parker 4 has performed a systematic study of ͑mostly unipolar͒ devices with different electrode combinations.In the case of bipolar devices, where there is a significant difference between the work functions of the anode and the cathode, a built-in potential (V bi ) is established in the polymer layer at zero bias ͑see Fig. 1͒. 8 This built-in potential fundamentally affects the operating characteristics of the diode: For applied bias (V appl ) less than V bi the electric field inside the polymer opposes charge injection and forward drift current. ͑Current may flow by diffusion.͒ In the simplest picture, where the bands of the polymer remain rigid, V bi is equal to the work-function difference ͑⌬͒ between the anode and the cathode. The above picture is surely rather simplistic: Instead of extended bands, the electronic levels of conjugated polymers are best described as a ͑Gaussian͒ distribution of localized states. Charge transp...

Multimodal and multidisciplinary approaches lead to next-generation technologies for reading and modulating neural function.

Transition metal complexes have emerged as promising candidates for applications in solid-state electroluminescent devices. These materials serve as multifunctional chromophores, into which electrons and holes can be injected, migrate and recombine to produce light emission. Their device characteristics are dominated by the presence of mobile ions that redistribute under an applied field and assist charge injection. As a result, an efficiency of 10 lm/W--among the highest efficiencies reported in a single layer electroluminescent device--was recently demonstrated. In this article we review the history of electroluminescence in transition metal complexes and discuss the issues that need to be addressed for these materials to succeed in display and lighting applications.