Characterization of Fluorene - Thiophene Copolymer Layers by the Surface Photovoltage Method

Summary : The surface photovoltage method was used for the study of thin polymer layers made of poly{(9,9-dihexylfluorene-2,7-diyl)-3-(3-methylbutyl)thiophene-2,5-diyl]} prepared by spin-coating on ITO coated glass substrates. The surface photovoltage (SPV) in a thin polymer layer is a result of dissociation of photogenerated free excitons in electric field of the space charge region (SCR) which forms spontaneously at the surface. The photovoltage signal was formed between a transparent electrode capacitively coupled to the surface of the sample and the back ITO electrode. Photovoltaic spectra were measured under illumination both the free surface (SPV_fs) and the substrate (SPV_ITO) side and for various layer thicknesses. The shape of the SPV_ITO spectrum is very similar to that of the absorption spectrum, whereas the SPV_fs spectrum maximum is shifted to longer wavelengths. Fitting theory to the experimental data, the thickness of the space charge region and the diffusion lengths were evaluated. The results are discussed also in relation to photoluminescence study of thin films.