Organic semiconductors key to cheap solar cells

A novel organic semiconductor device from Cornell University researchers uses ionic junctions that electroluminesce and act as a photovoltaic cell. The idea could lead to displays on cloth or paper, and cheap solar cells. Organic semiconductors can be made in thin, flexible sheets. And, "Flexible means low-cost fabrication,"says George Malliaras, Cornell-associate professor of materials-science and engineering. "Large quantities could be made cheaply by feeding films from rolls." Semiconductors — organic or otherwise — are materials that contain either an excess of free electrons (N-type) or "holes" ( Ptype). Holes are spaces where an atom ought to have an electron but doesn't, representing a positive charge. N and P-type materials can be joined to form diodes and transistors. The Cornell group went a step further by making a diode out of organic semiconductors that also contain free ions, molecules with an electrical charge. They laminated two organic layers, one containing free positive ions, and the other, negative ions. They then added thin conducting films on top and bottom; the top conductor is transparent to let light in and out. Where the two films meet, negative ions migrate across the junction to the positive side and vice versa, until reaching equilibrium. This is analogous to what takes place in a silicon diode, where electrons and holes migrate across the junction, say researchers. A voltage applied across the top and bottom electrodes makes current flow through the junction. The migration of ionic charge across the junction causes a higher voltage potential than normal and affects the way electrons combine with holes. This raises the energy of the molecules, which quickly releases as intense light. Conversely, bright light shone on the device is absorbed by the molecules, causing them to kick out electrons. The ionic charges create a preferential direction for the electrons to move,