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Simple techniques to make plastic semi conductors more efficient

Simple techniques to make plastic semi conductors more efficient

14-May-10
Silicon is used dominantly in modern electronic devices. However, silicon requires crystallization along with careful and delicate etching processes that increase the cost of devices like solar panels and light-emitting diodes (LEDs). One alternative is to use carbon-based semiconductors, including certain polymers that can be dissolved in solution and then spread or sprayed onto a surface to leave a thin film. This fabrication method is potentially much economical and faster than conventional semiconductor preparation. Most electronic devices require several different layers of semiconductor materials to be sandwiched together to give functional structures - a difficult process. Addition of subsequent layers can often re-dissolve the earlier layer, thus destroying the conducting property of conducting polymers.
A team of collaborating researchers from the National University of Singapore, the University of Cambridge and Cambridge Display Technology, UK, has developed a simple alternative. The team added bis-fluorophenyl azide (developed in-house) to the polymer mix before applying the polymers to the substrate. Exposing the aside to deep ultraviolet light triggered a chemical reaction that induced the photo cross-linking of side chains, forming bonds among the polymers chains. The cross-linked polymer thus becomes insoluble and does not reduce conductivity. This relatively simple technique was used to make a variety of devices, including the light-harvesting part of a photovoltaic cell, a field-effect transistor (the basis of modern integrated circuits), and light-emitting diodes (LEDs). In LED, the addition of a 10 nm-thick layer of a polymer called TFB made the LED ten times more efficient by confining charge carriers (electrons and �holes�) to the light-emitting film. The team also used this technique to make adjacent donor-acceptor heterostructures in which the electron and hole conduction paths have built-in continuity, thereby increasing the photon-to-electron conversion efficiency of the device. The scientists say that their photo cross-linking technique can be applied to many different polymers, and should make it much simpler and quicker to build prototypes of a wide range of new electronic devices.
In another research, a team at the University of Chicago and Lawrence Berkeley National Laboratory has developed �electronic glue" that could accelerate advances in semiconductor-based technologies, including solar cells and thermoelectric devices that convert sun light and waste heat, respectively, into useful electrical energy. Commercial solar cells, computer chips and other semiconductor technologies typically use large semiconductor crystals. But that is expensive and can make large-scale applications such as rooftop solar-energy collectors prohibitive. For those uses, engineers see great potential in semiconductor nanocrystals, sometimes just a few hundred atoms each. Nanocrystals can be readily mass-produced and used for device manufacturing via inkjet printing and other solution-based processes. But a problem remains: The crystals are unable to efficiently transfer their electric charges to one another due to surface ligands�bulky, insulating organic molecules that cap nanocrystals. The "electronic glue" solves the legend problem. Substituting the insulating organic molecules with novel inorganic molecules dramatically increases the electronic coupling between nanocrystals.
 
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