Electroabsorption spectroscopy of self-assembled (PPV/SPS) films: Size dependent blue shift

We have studied the electroabsorption (EA) of self-assembled organic heterostructures (PPV/SPS)_n consisting of alternate layers of poly(p-phenylenevinylene) (PPV) and "spacer" layers of poly(styrene-4-sulfonate), SPS. The heterostructures were prepared utilizing a layer-by-layer self-assembly technique using the precursor pre-polymer, followed by heat treatment to convert the pre-polymer into the conjugated polymer. For comparison, we have also studied the electroabsorption of a device made of spin-coated PPV. The thickness of the spacer layer, as determined by X-ray reflectivity, is varied from ≈ 5Å to 45Å where the average thickness of the active PPV layers are maintained coarsely constant, 16 ± 3Å for the ten layers of each device. In all our measurements here the applied electric field is perpendicular to the plane of the films. The EA spectra of the self-assembled films is significantly blue-shifted with increasing thickness of the spacer in the bilayers. The spectral width of the main peak in the EA spectrum for all the heterostructures is significantly broadened with respect to that of the spin-coated PPV, but it is almost independent of the spacer thickness. It exhibits a quadratic dependence on the applied electric field but no spectral shift with increasing field strength. The blue shift of the EA spectra is consistent with the blue shift in the absorption spectra as demonstrated in this work and in previous studies. The EA results, and particularly the blue-shifted spectra in the self-assembled films, are tentatively attributed to confinement effects, reduction of the PPV centrosymmetry and to an increase of the in-plane disorder of the active PPV layers.