Observations on the memory effect in diodes fabricated from carbon-bridged dithiophenes

A number of laboratories including our own have reported hysteresis, i.e. a memory effect, in the current-voltage characteristics of semiconducting polymers. Here we report further results on the hysteresis observed in electropolymerised films of a carbon-bridged dithiophene. In particular, we present spectroscopic evidence that shows the effect requires a high degree of conjugation and moderate doping of the polymer. The presence of mobile ions per se in the film is insufficient to cause the effect. Successive film samples produced from the same monomer solution show progressive changes in both the UV and FT-IR spectra consistent with reduced conjugation in the polymer coupled with reduced doping by the BF4− counterions. These changes correlate with a reduction in hysteresis and a general lowering of film conductivity. From the spectroscopic evidence we develop a possible band energy scheme which suggests that both aluminium and ITO should make rectifying contacts to the polymer thus confirming our earlier findings.