Determining the optimum morphology in high-performance polymer-fullerene organic photovoltaic cells

• Published in: Nature communications Vol. 4; no. 1; p. 2867
• Main Authors: Hedley, Gordon J, Ward, Alexander J, Alekseev, Alexander, Howells, Calvyn T, Martins, Emiliano R, Serrano, Luis A, Cooke, Graeme, Ruseckas, Arvydas, Samuel, Ifor D W
• Format: Journal Article
• Language: English
• Published: England Nature Publishing Group 17.12.2013; Nature Pub. Group
• Subjects: Morphology; Photovoltaic cells; Physics; Polymers; United Kingdom > UK
• ISSN: 2041-1723; 2041-1723
• DOI: 10.1038/ncomms3867

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.