Organic semiconductors have been studied since the early 1950s, and the large amount of work devoted to them has allowed a better understanding of their charge-transport properties. However owing to their very poor semiconducting characteristics, they were merely considered as exotic materials with little potential interest for applications until the late 1980s, when two significant steps simultaneously appeared in the literature. Richard Friend's group showed that light-emitting diodes could be made from a conjugated semiconducting polymer, and our laboratory showed that efficient field-effect transistors (FETs) could be realized from short-conjugated oligomers. These two results launched intensive research on these two types of organic-based devices, and the extensive work accomplished since has largely confirmed the technological pertinence of organic semiconductors, showing the promise for applications in flexible and large-area electronics. Two categories of organic semiconductors are actually under development: (1) conjugated polymer-based ones whose amorphous state is favorable to strong luminescence and (2) conjugated oligomer-based ones, in which charge-transport efficiency is directly related to the long-range packing of molecules in the semiconducting film. In fact conjugated oligomers can be said to be forming molecular polycrystals whose electrical properties are essentially controlled by molecular order. Thus performance of sexithiophene-based FETs has been improved by a factor of nearly 50 by controlling the molecular ordering in the evaporated film, from a disordered three-dimensional structure to a well-ordered two-dimensional organization where all the molecules stack along a packing axis nearly parallel to the substrate surface.
Elimination of Lateral Resistance and Current Crowding in Large-Area LEDs by Composition Grading and Diffusion-Driven Charge Transport
