METAL-INSULATOR-SEMICONDUCTOR SCHOTTKY-TYPE DIODES OF DOPED THIOPHENEOLIGOMERS

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 variou s amounts of mainly 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). E utectic 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 impedanc e of the diodes is due to the actual Schottky junction, which is simil ar for all doped diodes, and to a 'non-conducting' interface layer whi ch is formed by chemical reaction upon application of the Schottky met al. Current transport at high forward bias is not bulk limited, but du e to charge injection across this thin interfacial film and, hence, ma nifested as space charge limited current. It will be argued that the d iodes can tentatively be described as metal-insulator-semiconductor (M IS) Schottky diodes with a graded dopant profile. This interpretation is also supported by a correlation found between forward and saturatio n current densities.