Nano and Micro Patterned Organic Devices : From Neural Interfaces to Optoelectronic Devices

Abstract: This thesis describes fabrication methods for optoelectronic devices (light emitting diodes (LEDs) and photodiodes) and structures for neural interfacing (neural electrodes and nerve guidance structures) using semiconducting and conducting organic materials as the active elements. Special importance has been attached to the patterning and processing of these devices. Soft lithographic patterning methods constitute the key element for the fabrication of the optoelectronic devices while photolithography is the base for the fabrication of neural electrodes.When fabricating organic optoelectronic devices material properties of the semiconducting polymers put demands on device geometry. Low mobility of the materials sets restriction on the active layer thicknesses, usually less than 100 nm. For a photodiode a thicker layer increases light absorption but decreases the possibility to extract the generated charge. An intriguing approach to solve this conflict is to impose a light collecting structure on the active layer. Active layer thickness can be kept small while light absorption is increased. These structures may be submicrometer sized and cover large areas ( ~cm2). A soft lithographic patterning method, soft imprint, for fabricating submicrometer features over large areas was developed and used to fabricate LEDs and photodiodes.A tentative alternative route to increase light collection in thin layers is to include three-dimensional optical elements in the device. Using selfassembly of water to form microdomes, I devised a method of fabricating micrometer sized optical lenses in a polymer substrate. These structures were successfully used as substrates for building organic photodiodes with an "inverted" geometry.Efficient function of "standard" organic optoelectric devices relies, among else, on a thin homogenous layer of an organic film sandwiched between two metal electrodes. The last processing step of the "standard"device is the deposition of the top metal electrode, by vacuum evaporation, on the organic layer. The heat from the evaporation and the momentum of metal atoms may be destructive to the thin organic layer, creating short circuits in the device. Breaking the standard planar geometry of a device with an imposed topography, up to two orders the magnitude of the layer thickness, increases the risk of defects. An "inverted" device geometry, where the last evaporation step was substituted with a method of spin coating a conducting polymer as the top electrode, was thus explored. This approach to apply the top electrode in an "inverted" structure was proven in successful fabrication of organicphotodiodes.A polymer approach to a neural interface was devised by the use of a polymer hydrogel electrode. Metal electrodes used for neural excitation depend on electrochemical reactions at the metal surface to generate a stimulating, faradaic, current. As unwanted products from the electrochemical reactions may have deleterious consequences for the surrounding tissue, these currents should be minimized. The polymer hydrogel electrode acts as a reservoir of charge that can be expelled by the application of a potential to the electrode. By applying a small stimulating potential, faradaic current is kept at a minimum and substituted by a capacitive current, which avoids electrochemical reactions at the interface. Simulations of a polymer hydrogel electrode show that the electrode has the electrical requirements for exciting myelinated nerve fibers. To selectively electrically address a part of a nerve fiber population, it is of interest to use biology and sort nerve fibers into different compartments with a low signal crosstalk. Using chemical cues it was possible to sort regenerating motor and sensory axons into different branches of a silicone Y-tube.