Alexander A. Zakhidov

Doped organic transistors operating in the inversion and depletion regime.

The inversion field-effect transistor is the basic device of modern microelectronics and is nowadays used more than a billion times on every state-of-the-art computer chip. In the future, this rigid technology will be complemented by flexible electronics produced at extremely low cost. Organic field-effect transistors have the potential to be the basic device for flexible electronics, but still need much improvement. In particular, despite more than 20 years of research, organic inversion mode transistors have not been reported so far. Here we discuss the first realization of organic inversion transistors and the optimization of organic depletion transistors by our organic doping technology. We show that the transistor parameters—in particular, the threshold voltage and the ON/OFF ratio—can be controlled by the doping concentration and the thickness of the transistor channel. Injection of minority carriers into the doped transistor channel is achieved by doped contacts, which allows forming an inversion layer.

Detection of Transmitter Release from Single Living Cells Using Conducting Polymer Microelectrodes

The advent of organic electronics has made available a host of materials and devices with unique properties for interfacing with biology.1–2 One example is the use of conducting polymer coatings on metal electrodes that are implanted in the central nervous system and interface electrically with neurons, providing stimulation and recording the neurons electrical activity.3–5 Coating a metal electrode with a conducting polymer has been shown to lower the electrical impedance and decrease the mechanical properties mismatch at the interface with tissue, with beneficial effects on the lifetime of the implant.3, 6 Conducting polymers can also be functionalized with biomolecules that stimulate neural growth and minimize the immune response to the implant.3–5, 7 Other examples are organic electronic ion pumps,8 and ion transistors,9 which are recently invented devices capable of precise delivery of neurotransmitters to neurons. These devices were recently implanted in the ear of a guinea pig and were shown to control its hearing.10 Conducting polymers such as poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulfonate) (PEDOT:PSS), a material that has been shown to be biocompatible with a variety of different cells,1 have been used for these applications. These examples highlight the main advantages that organic electronic materials bring to the interface with biology, including their “soft” nature, which offers better mechanical compatibility with tissue than traditional electronic materials, and natural compatibility with mechanically flexible substrates, which paves the way for the development of implants that better conform to the non-planar shape of organs. Finally, the ability of organics to transport ionic in addition to electronic charge creates the opportunity to interface with electrically active cells in novel ways, as the work on ion pumps indicates.

Acid-sensitive semiperfluoroalkyl resorcinarene: an imaging material for organic electronics.

An acid-sensitive semiperfluoroalkyl resorcinarene was synthesized, and its lithographic properties were evaluated. Its solubility in segregated hydrofluoroether solvents enables the patterning of delicate organic electronic materials.

Orthogonal processing: A new strategy for organic electronics

The concept of chemical orthogonality has long been practiced in the field of inorganic semiconductor fabrication, where it is necessary to deposit and remove a layer of photoresist without damaging the underlying layers. However, these processes involving light sensitive polymers often damage organic materials, preventing the use of photolithography to pattern organic electronic devices. In this article we show that new photoresist materials that are orthogonal to organics allow the fabrication of complex devices, such as hybrid organic/inorganic circuitry and full-colour organic displays. The examples demonstrate that properly designed photoresists enable the fabrication of organic electronic devices using existing infrastructure.

Dry photolithographic patterning process for organic electronic devices using supercritical carbon dioxide as a solvent

The particular challenge of micropatterning organic materials has stimulated numerous approaches for making effective and repeatable patterned structures with fine features. Among all the micropatterning techniques photolithography, being the preferred method for the inorganic semiconductor industry, did not create much impact due to its incompatibility with the majority of organic electronic materials. Here we introduce a novel, chemically benign approach to dry photolithographic patterning of organic materials using super-critical carbon dioxide (scCO2) as a green developing solvent. We illustrate the possible applications of the new technique by patterning conducting polymers and light emitting polymers for organic light emitting diodes.

Nanoimprinted Perovskite Nanograting Photodetector with Improved Efficiency

Recently, organolead halide-based perovskites have emerged as promising materials for optoelectronic applications, particularly for photovoltaics, photodetectors, and lasing, with low cost and high performance. Meanwhile, nanoscale photodetectors have attracted tremendous attention toward realizing miniaturized optoelectronic systems, as they offer high sensitivity, ultrafast response, and the capability to detect beyond the diffraction limit. Here we report high-performance nanoscale-patterned perovskite photodetectors implemented by nanoimprint lithography (NIL). The spin-coated lead methylammonium triiodide perovskite shows improved crystallinity and optical properties after NIL. The nanoimprinted metal-semiconductor-metal photodetectors demonstrate significantly improved performance compared to the nonimprinted conventional thin-film devices. The effects of NIL pattern geometries on the optoelectronic characteristics were studied, and the nanograting pattern based photodetectors demonstrated the best performance, showing approximately 35 times improvement on responsivity and 7 times improvement on on/off ratio compared with the nonimprinted devices. The high performance of NIL-nanograting photodetectors likely results from high crystallinity and favored nanostructure morphology, which contribute to higher mobility, longer diffusion length, and better photon absorption. Our results have demonstrated that the NIL is a cost-effective method to fabricate high-performance perovskite nanoscale optoelectronic devices, which may be suitable for manufacturing of high-density perovskite nanophotodetector arrays and to provide integration with state-of-the-art electronic circuits.

Orthogonal Processing and Patterning Enabled by Highly Fluorinated Light‐Emitting Polymers

a m i p p f o d f a p m o s t Solution processing of organic electronic materials is a highly attractive processing option for many applications, particularly organic light emitting diodes (OLEDs) for display and solidstate lighting. It is a low cost approach with no limitations with regard to substrate size. While highly effi cient full color displays are rather straightforward to fabricate via vacuum-assisted shadow mask deposition of organic small molecules, it is challenging to achieve solution-processed full color displays due to the limitations imposed by compatibility issues among active light-emitting components and other chemicals and solvents used in the device patterning process. Much work has been done on the patterning of organic electronic materials, which has been summarized in the review by Menard et al. [ 1 ] However, many of the novel patterning techniques described in the review, such as inkjet printing and screen printing, suffer from the disadvantages of low resolution and low throughput. As such, photolithography is still the ideal technique for patterning of organic light-emitting materials, since it has good resolution, high-throughput, easy scalability to large substrates, good registration between multiple layers and is very well established in the semiconductor industry. However, the standard organic and polar solvents used in the processing of photosensitive materials can damage the organic light-emitting materials used as active layers. Many approaches have been proposed to overcome this problem. Several groups [ 2–6 ] have demonstrated light-emitting polymers with side-groups that can be cross-linked under light activation to produce insoluble polymer networks in desired areas, hence they can be directly patterned in a way similar to standard photoresist materials. Huang et al. [ 7 ] showed that inserting a photocurable interlayer

Coherent mode coupling in highly efficient top-emitting OLEDs on periodically corrugated substrates

Bragg scattering at one-dimensional corrugated substrates allows to improve the light outcoupling from top-emitting organic light-emitting diodes (OLEDs). The OLEDs rely on a highly efficient phosphorescent pin stack and contain metal electrodes that introduce pronounced microcavity effects. A corrugated photoresist layer underneath the bottom electrode introduces light scattering. Compared to optically optimized reference OLEDs without the corrugated substrate, the corrugation increases light outcoupling efficiency but does not adversely affect the electrical properties of the devices. The external quantum efficiency (EQE) is increased from 15 % for an optimized planar layer structure to 17.5 % for a corrugated OLED with a grating period of 1.0 μm and a modulation depth of about 70 nm. Detailed analysis and optical modeling of the angular resolved emission spectra of the OLEDs provide evidence for Bragg scattering of waveguided and surface plasmon modes that are normally confined within the OLED stack into the air-cone. We observe constructive and destructive interference between these scattered modes and the radiative cavity mode. This interference is quantitatively described by a complex summation of Lorentz-like resonances.

Molecular doping for control of gate bias stress in organic thin film transistors

The key active devices of future organic electronic circuits are organic thin film transistors (OTFTs). Reliability of OTFTs remains one of the most challenging obstacles to be overcome for broad commercial applications. In particular, bias stress was identified as the key instability under operation for numerous OTFT devices and interfaces. Despite a multitude of experimental observations, a comprehensive mechanism describing this behavior is still missing. Furthermore, controlled methods to overcome these instabilities are so far lacking. Here, we present the approach to control and significantly alleviate the bias stress effect by using molecular doping at low concentrations. For pentacene and silicon oxide as gate oxide, we are able to reduce the time constant of degradation by three orders of magnitude. The effect of molecular doping on the bias stress behavior is explained in terms of the shift of Fermi Level and, thus, exponentially reduced proton generation at the pentacene/oxide interface.

Fabrication of polymer-based electronic circuits using photolithography

We exploited the concept of solvent orthogonality to enable photolithography for high-resolution, high-throughput fabrication of electronic circuits based on a polymeric semiconductor. An array of ring oscillators utilizing top contact polymer thin film transistors with 1 μm channel length has been fabricated on a 100 mm wafer scale. We used high performance, air stable poly(2,5-bis(thiophene-2-yl)-(3,7-ditri-decanyltetrathienoacene) as our active semiconducting material. Owing to the small channel length and small overlap length, these devices have a signal propagation delay as low as 7 μs/stage.