George G. Malliaras

Charge injection and recombination at the metal–organic interface

Abstract We consider the mechanism of charge injection from metals into amorphous organic semiconductors. By first treating charge recombination at the interface as a hopping process in the image potential, we obtain an expression for the surface recombination rate. The principle of detailed balance is then used to determine the injection current. This simple approach yields the effective Richardson constant for injection from metal to organic, and provides a means to derive the electric field dependence of thermionic injection. The result for the net current, injected minus recombination, is in agreement with a more exact treatment of the drift–diffusion equation.

In vivo recordings of brain activity using organic transistors

In vivo electrophysiological recordings of neuronal circuits are necessary for diagnostic purposes and for brain-machine interfaces. Organic electronic devices constitute a promising candidate because of their mechanical flexibility and biocompatibility. Here we demonstrate the engineering of an organic electrochemical transistor embedded in an ultrathin organic film designed to record electrophysiological signals on the surface of the brain. The device, tested in vivo on epileptiform discharges, displayed superior signal-to-noise ratio due to local amplification compared with surface electrodes. The organic transistor was able to record on the surface low-amplitude brain activities, which were poorly resolved with surface electrodes. This study introduces a new class of biocompatible, highly flexible devices for recording brain activity with superior signal-to-noise ratio that hold great promise for medical applications.

TEMPERATURE- AND FIELD-DEPENDENT ELECTRON AND HOLE MOBILITIES IN POLYMER LIGHT-EMITTING DIODES

We have studied the transport properties of electron- and hole-dominated MEH-PPV, poly(2-methoxy,5-(2′-ethyl-hexoxy)-p-phenylene vinylene), devices in the trap-free limit and have derived the temperature-dependent electron and hole mobilities (μ=μ0eγ√E) from the space-charge-limited behavior at high electric fields. Both the zero-field mobility μ0 and electric-field coefficient γ are temperature dependent with an activation energy of the hole and electron mobility of 0.38±0.02 and 0.34±0.02 eV, respectively. At 300 K, we find a zero-field mobility μ0 on the order of 1±0.5×10−7 cm2/V s and an electric-field coefficient γ of 4.8±0.3×10−4 (m/V)1/2 for holes. For electrons, we find a μ0 an order of magnitude below that for holes but a larger γ of 7.8±0.5×10−4 (m/V)1/2. Due to the stronger field dependence of the electron mobility, the electron and hole mobilities are comparable at working voltages in the trap-free limit, applicable to thin films of MEH-PPV.

Humidity sensors based on pentacene thin-film transistors

When a pentacene thin-film transistor is exposed to humidity, its saturation current decreases. This decrease was found to be reversible and can therefore be used to measure the amount of relative humidity in atmosphere. The sensitivity was found to depend on the thickness of the pentacene layer. The microscopic origin of the sensing mechanism is discussed.

An organic electronics primer

Weak intermolecular interactions, low dielectric constants, and the availability of a nearly unlimited number of different molecules determine the scope of organic semiconductors as systems for exploring and exploiting solid-state phenomena.

Photovoltaics from soluble small molecules

Solution-processable small molecules have attractive features for application in photovoltaic cells. They offer the facile processing associated with polymers, yet are easier to synthesize and purify, are monodisperse, and typically show higher charge carrier mobilities. Recent progress in solution-processable small molecule blends has yielded photovoltaic cells with efficiencies exceeding 1%. This article reviews progress in this nascent field and discusses the requirements imposed by the need for charge separation within an interpenetrating network, energy level tuning for light absorption and voltage output, and processing techniques to achieve phase separation on excitonic length scales. Design criteria for next-generation materials are provided.

Solid-state electroluminescent devices based on transition metal complexes

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.

The roles of injection and mobility in organic light emitting diodes

Numerical methods have been used to solve the bipolar current problem for a single emitting layer between electrodes with explicit injection characteristics. We consider ohmic and tunneling contacts at the anode and/or cathode for various ratios of hole to electron mobility. Diffusion is included and found to have minimal effect on the recombination efficiency. The recombination profile is dictated mainly by the ratio of mobilities, less so by contacts. Maximum efficiency is obtained for two ohmic contacts. When the injection is imbalanced, higher efficiencies are achieved when the majority carrier has the lower mobility. At sufficiently high voltages, the current tends towards balance, allowing for maximum efficiency.

NeuroGrid: recording action potentials from the surface of the brain

Recording from neural networks at the resolution of action potentials is critical for understanding how information is processed in the brain. Here, we address this challenge by developing an organic material–based, ultraconformable, biocompatible and scalable neural interface array (the ‘NeuroGrid’) that can record both local field potentials(LFPs) and action potentials from superficial cortical neurons without penetrating the brain surface. Spikes with features of interneurons and pyramidal cells were simultaneously acquired by multiple neighboring electrodes of the NeuroGrid, allowing for the isolation of putative single neurons in rats. Spiking activity demonstrated consistent phase modulation by ongoing brain oscillations and was stable in recordings exceeding 1 weeks duration. We also recorded LFP-modulated spiking activity intraoperatively in patients undergoing epilepsy surgery. The NeuroGrid constitutes an effective method for large-scale, stable recording of neuronal spikes in concert with local population synaptic activity, enhancing comprehension of neural processes across spatiotemporal scales and potentially facilitating diagnosis and therapy for brain disorders.

Photolithographic patterning of organic electronic materials

Hydrofluoroethers are shown to be benign solvents to a wide variety of organic electronic materials, even at extreme conditions such as boiling temperature. Coupled with fluorous functional photoresist-acidsensitive semi-perfluoroalkyl resorcinarene, they open new frontiers for photolithographical patterning for organic electronic systems. Summary of Research: Organic electronics is emerging as a promising technology to enable mechanically flexible devices through solution processing of organic materials [1]. As with traditional electronics, organic devices require active functional materials to be tailored into micropatterned and multi-layered device components. While the former relies on photolithographic patterning techniques, the latter is restricted from adopting such robust, high-resolution and high-throughput techniques because of the chemical compatibility issue between organic materials and patterning agents [3]. Namely, deterioration of materials’ performance occurs during the photoresist deposition and removal stages due to aggressive organic solvents, as well as in the pattern development steps by aqueous base solutions. In our search for universal, materials-friendly solvents, we have identified environmentally benign fluorous solvents combined with specifically tailored patterning materials as a possible solution to this complex problem. Fluorous solvents are poor solvents for non-fluorinated organic materials [2]. Among the variety of fluorous solvents, segregated hydrofluoroethers (HFEs) attracted our attention because of their nonflammability, zero ozone-depletion potential and low toxicity for humans [3]. We tested the impact of HFEs solvents on wellcharacterized and commercially available organic electronic materials. We demonstrated that HFE solvents do not damage or alter electronic and optoelectronic properties of wide class of organic electrnic materials, including: organic semiconductors (pentacene and poly-3-hexylthiophene (P3HT)), conducting polymer Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) and organic light emitting polymers and small molecule compounds (polyfluorenes, and [Ru(bpy)3] (PF6 –)2 complex). To further demonstrate the orthogonality of HFEs to active organic material as well as to organic/metal interface we used aforementioned organic materials to make organic light emitting diodes (OLEDs) and thin film transistors (TFTs) which we characterized before and after exposing to HFE [4]. We found HFE did not significantly change the characteristic of tested devices even at elevated temperatures. For example, Figure 1 shows [Ru(bpy)3] (PF66 –)2 based electroluminescent device [5] in boiling HFE 7100 (61°C). We operated the device in the boiling HFE for one hour and did not observe any substantial change in its performance. This new dimension in solvent orthogonality which is enabled by the use of HFEs offers unique opportunities for the chemical processing of organic electronic materials. One example is in the area of photolithographic processing: One can use a photoresist that is properly fluorinated to be processable in HFEs [6]. We have successfully demonstrated this approach