In organic optoelectronics, the organic film morphologies deeply affect the characteristics of the devices. In result, the understanding and the control of film morphologies have been the focus of extensive research, particularly in the field of polymer-based devices, as these represent the lowest cost solutions for organic electronics [1,2]. In particular, the control of film morphology is remarkably important to Organic Photovoltaic cells (OPVs) combining electron donor and electron acceptor materials. The most efficient OPVs are based on interpenetrating networks of the two components, segregated into domains, whose dimensions, down to the nanoscale, favour charge generation and transport [3].
Here, we report on a versatile, simple, and low cost method to tailor polymer film morphologies and thicknesses, through controlling phase separation occurring during the deposition, by spin coating, of polymer blends. The studied blends are composed of a cross-linkable semiconducting electroluminescent (EL) polymer, namely a polyfluorene derivative, and an insulating polymer (polystyrene). Upon cross-linking, the EL polymer originates an insoluble network, allowing for the removal of the insulating polymer by dissolution in a solvent. Upon varying the blend characteristics and spinning velocity, columnar-grain films and porous films, with variable features dimensions and thicknesses were obtained (see examples in Figure 1). The use of cross-linkable polymers is particularly advantageous, because the cross-linked films resist further deposition of solutions, hence allowing for the fabrication of organic multilayer structures, all fabricated from solution. On the basis of a qualitative description for phase-separation occurring during the spin coating process, we present strategies to tune film morphologies to match optimal features for several applications, such as OPVs.