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6 February 2004 Enhanced photovoltaic efficiency in polymer-fullerene composites by thermally controlled interdiffusion
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The interface between polymer and fullerene in organic photovoltaic devices is improved by thermally induced interdiffusion. Starting from a bilayer of 2-methoxy-5-(2’-ethylhexyloxy)-1,4-phenylenevinylene copolymer (MEH-PPV) and the Buckminsterfullerene (C60) devices are heated in the vicinity of the glass transition temperature creating a gradient bulk-heterojunction. Interdiffused devices show photoluminescence quenching with concomitant improvements in photocurrents. Variation of the polymer layer thickness shows an increase in photocurrents with decreasing layer thickness within the examined thickness regime as transport of the separated charges out of the device is improved. The interdiffusion was observed in situ by monitoring the photocurrents during the heating step. Cross-sectional transmission electron microscopy reveals C60 clusters of up to 30 nm in diameter in the interdiffused devices. The clustering of the fullerene molecules puts a significant constraint on the interdiffusion process.
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Martin Drees, James Randolph Heflin, Richey Davis, Daniela Topasna, and Paige Stevenson "Enhanced photovoltaic efficiency in polymer-fullerene composites by thermally controlled interdiffusion", Proc. SPIE 5215, Organic Photovoltaics IV, (6 February 2004);

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