The morphology of bulk heterojunction organic photovoltaic cells controls many of the performance characteristics of devices. However, measuring this morphology is challenging because of the small length-scales and low contrast between organic materials. Here we use nanoscale photocurrent mapping, ultrafast fluorescence and exciton diffusion to observe the detailed morphology of a high-performance blend of PTB7:PC₇₁BM. We show that optimized blends consist of elongated fullerene-rich and polymer-rich fibre-like domains, which are 10–50 nm wide and 200–400 nm long. These elongated domains provide a concentration gradient for directional charge diffusion that helps in the extraction of charge pairs with 80% efficiency. In contrast, blends with agglomerated fullerene domains show a much lower efficiency of charge extraction of ~45%, which is attributed to poor electron and hole transport. Our results show that the formation of narrow and elongated domains is desirable for efficient bulk heterojunction solar cells.

The latex technology concept is applied for the preparation of graphene/polystyrene nanocomposites. Aqueous dispersions of graphene are obtained viaoxidation and exfoliation of graphite and subsequent reduction in the presence of surfactant. The quality of the prepared nanofillers is characterized by atomic force microscopy (AFM). Different amounts of aqueous graphene dispersions are then mixed with polystyrene (PS) latex and composites are prepared by freeze-drying and subsequent compression molding. The final bulk and local conductivities of the composites are respectively measured by a four-point method and by means of conductive AFM (C-AFM) analysis. The morphology of the conductive nanocomposites is studied with charge contrast scanning electron microscopy imaging (SEM). The percolation threshold for conduction is below 1 wt% of graphene in the composites, and a maximum conductivity of about 15 S m⁻¹ can be achieved for 1.6–2 wt% nanofiller.