Electrochemical properties of luminescent polymers and polymer light-emitting electrochemical cells

The electrochemical p-doping potentials (φp) and n-doping potentials (φn) of 10 soluble poly(1,4-phenylene vinylene) (PPV) derivatives and of the conjugated polymer blend in a light-emitting electrochemical cell (LEC) were measured by cyclic voltammetry. The energy levels corresponding to the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the polymers were determined from the onset potentials for n-doping (φn′) and p-doping (φp′), respectively. The electrochemical energy gap, Eg′=Δφ=φp′−φn′, agrees well with the optical energy gap (Eg). For copolymers of poly(2-methoxy, 5-(2′-ethyl-hexyloxy) paraphenylene vinylene) (MEH-PPV) and poly(2-butyl, 5-(2′-ethyl-hexyl) paraphenylene vinylene) (BuEH-PPV), the n-doping potentials are nearly independent of the ratio of MEH-PPV to BuEH-PPV. However, p-doping potentials depend strongly on the ratio, the higher the MEH-PPV fraction, the lower the p-doping potential of the copolymer. For cyano-PPV (CN-PPV), both p-doping and n-doping potentials were shifted by ca. 0.6 V (more electronegative) as a result of the electron withdrawing effect of cyano side group. The electrochemical p-doping and n-doping processes of the MEH-PPV polymer blend in the LEC device were confirmed from linear sweep voltammetry of the LEC; the doping onset potentials were the same as for an MEH-PPV film measured in a liquid electrolyte.