Oxidised carbon nanotubes as solution processable, high work function hole-extraction layers for organic solar cells

Abstract

For efficient hole-extraction in solution processed organic solar cells the transparent indium-tin oxide (ITO) electrode is invariably pre-coated with a thin layer of the high work function conducting polymer poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonate). Herein we show that thin films of partially oxidised multi-wall and single-wall carbon nanotubes are equally effective at facilitating hole-extraction in efficient (∼2.7%) bulk-heterojunction organic solar cells based on poly(3-hexylthiophene) (P3HT): [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) blends. Crucially, in contrast to PEDOT:PSS, deposition is from aqueous solutions of low acidity (pH 6-7) ensuring compatibility with ITO and other emerging conducting oxides. Furthermore, thin oxidised carbon nanotube films offer greater transparency in the near-infrared as compared to PEDOT:PSS films of comparable thickness. The functionality of these nano-structured films is demonstrated in relatively large area devices (∼0.35 cm2) and the performance rationalised based on measurements of the electronic structure and morphology. © 2009 Elsevier B.V. All rights reserved.