X-ray photoemission spectroscopy study of vertical phase separation in F8BT:PDI/ITO films for photovoltaic applications

We present a depth-resolved X-ray photoemission spectroscopy study of the Poly(9,9’-dioctylfluorene-cobenzothiadiazole): Perylene tetracarboxylic diimide blend (briefly, F8BT:PDI), employed for the realization of the light harvesting layer in organic photovoltaic devices. We address the problem of the vertical distribution of PDI molecules in the blend, relevant for the optimization of the photo-generated charge collection in such devices. The depth resolution is obtained by sputtering the organic layer with Ar⁺ ions. A thorough investigation of the effects of different sputtering treatments on the F8BT:PDI film surface is presented. Changes in the stoichiometry of the organic layer, as well as the cleavage of molecular bonds are detected, even after mild sputtering. In particular, we report about the formation of a carbon-rich surface layer. Finally, a method is proposed for the calculation of the PDI concentration, which relies on the detection of specific chemical markers and is robust against sputter-induced artifacts. As a case study, we evaluated the PDI concentration in a 10 nm thick F8BT:PDI layer spin coated on indium tin oxide.