Memory effect in the current–voltage characteristic of a low-band gap conjugated polymer

Diodes formed by electrodeposition of the low-band gap polymer poly(4-dicyano methylene-4H-cyclopenta[2,1-b:3,4-b^′]dithiophene), onto glass slides coated with indium tin oxide (ITO) and furnished with evaporated aluminum counterelectrodes exhibit a reversible bistability in their current–voltage (I–V) characteristics. Applying +5 V to the ITO electrode induces a “high” conductance state while applying −5 V induces a “low” conductance state. The effect is identical in most respects to recent observations in diodes formed from thin films of chromium-doped ${\mathrm{SrZrO}}_3$ sandwiched between ${\mathrm{SrRuO}}_3$ and gold electrodes. A number of mechanisms are discussed but the evidence points to the controlling influence of an interfacial depletion layer at the ITO–polymer interface. It is also shown that the high capacitances associated with such layers can lead to higher than expected displacement currents being generated during the automated collection of I–V data. The presence of such currents distorts the I–V characteristics in the low-bias low-current regime.