Laterally Ordered Bulk Heterojunction of Conjugated Polymers: Nanoskiving a Jelly Roll

Authors


  • This work was supported by the DOE under DE-FG02-00ER45852 and the NSF under CHE-0518055. The authors used shared facilities supported by the NSF through the MRSEC program under award DMR-0213805 and through the NSEC program under award PHY-0117795. The authors thank Dr. Emily A. Weiss and Dr. Michael D. Dickey for helpful discussions and Dr. Richard Schalek for training on the ultramicrotome. W. F. R. acknowledges a training grant from NIH award number T32 GM007598. This work was performed in part using the facilities of the Center for Nanoscale Systems (CNS), a member of the National Nanotechnology Infrastructure Network (NNIN), which is supported by the National Science Foundation under NSF award no. ECS-0335765. CNS is part of the Faculty of Arts and Sciences at Harvard University.

Abstract

This paper describes the fabrication of a nanostructured heterojunction of two conjugated polymers by a three-step process: i) spin-coating a multilayered film of the two polymers, ii) rolling the film into a cylinder (a “jelly roll”) and iii) sectioning the film perpendicular to the axis of the roll with an ultramicrotome (nanoskiving). The conjugated polymers are poly(benzimidazobenzophenanthroline ladder) (BBL, n-type) and poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV, p-type). The procedure produces sections with an interdigitated junction of the two polymers. The spacing between the phases is determined by spin-coating (∼15 nm to 100 nm) and the thickness of each section is determined by the ultramicrotome (100 to 1000 nm). The minimum width of the MEH-PPV layers accessible with this technique (∼15 nm) is close to reported exciton diffusion lengths for the polymer. When placed in a junction between two electrodes with asymmetric work functions (tin-doped indium oxide (ITO) coated with poly(3,4-ethylenedioxythiophene:poly(styrenesulfonate) (PEDOT:PSS), and eutectic gallium-indium, EGaIn) the heterostructures exhibit a photovoltaic response under white light, although the efficiency of conversion of optical to electrical energy is low. Selective excitation of BBL with red light confirms that the photovoltaic effect is the result of photoinduced charge transfer between BBL and MEH-PPV.

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