Effects of space charge at the conjugated polymer/electrode interface

Films of the polymer poly(4-dicyanomethylene-4H-cyclopenta[2,1-b:3,4-b′]dithiophene (PCDM) were electrodeposited onto indium tin oxide (ITO) coated glass substrates. The formation of a polymer was confirmed by the presence of an absorption band at ∼900 nm (∼1.4 eV) previously attributed to a π–π* transition. Diodes formed by evaporating aluminum counterelectrodes onto the films displayed nonlinear current–voltage characteristics that were independent of voltage polarity. Interestingly, at low applied voltages, the diodes exhibited two, stable, reversible conductance states differing by about an order of magnitude. The effect is similar to that reported recently in diodes formed from chromium-doped SrZrO3. ac admittance measurements carried out over a range of frequency coupled with the voltage dependence of the device capacitance at low frequency suggests that the two states arise from the presence of a field-dependent space charge at the ITO/PCDM interface.