METAL-INSULATOR-SEMICONDUCTOR SCHOTTKY-TYPE DIODES OF DOPED THIOPHENE OLIGOMERS

Schottky barrier diodes were made from films of the thiophene oligomer alpha,alpha'-coupled dodecathiophene substituted with four n-dodecyl side chains, T12d4(2,5,8,11), which were doped in solution with various amounts of mainly 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). Eutectic Ga-In alloy or evaporated In were applied as Schottky contacts. The diodes exhibited strong rectification up to 10(6) at +1 V/-1 V, diode quality factors between 1.2 and 2, and forward current densities up to a few A cm-2 at 1 V forward bias. Analysis of complex impedance spectra and capacitance-voltage measurements showed that the impedance of the diodes is due to the actual Schottky junction, which is similar for all doped diodes, and to a 'non-conducting' interface layer which is formed by chemical reaction upon application of the Schottky metal. Current transport at high forward bias is not bulk limited, but due to charge injection across this thin interfacial film and, hence, manifested as space charge limited current. It will be argued that the diodes can tentatively be described as metal-insulator-semiconductor (MIS) Schottky diodes with a graded dopant profile. This interpretation is also supported by a correlation found between forward and saturation current densities.