Effect of molecular aggregation by thermal treatment on photovoltaic properties of MEH-PPV: Fullerene-based solar cells

The effect of a thermal annealing treatment at various temperatures on the performance of bulk heterojunction photovoltaic cells based on poly[2-methoxy-5-(2′-ethyl-hexyloxy)-p-phenylene vinylene] (MEH-PPV) and buckminsterfullerene (C₆₀) composites was investigated. With the increase in temperature, three stages were observed: from 60 to 120 °C, the photovoltaic performance of the composite devices became better with the rising temperature; from 120 to 180 °C, the photovoltaic performance decreased to a minimum; but, at about 200 °C, a sharp rise occurred, followed by a gradual decline. The obvious red-shift of the photoluminescence (PL) spectra and the new PL peak and shoulder at about 620 and 660 nm, respectively, indicated the formation of molecular aggregation and the lengthened and the ordered conjugated segments. It was suggested that the crystallization of C₆₀ after thermal annealing at 200 °C, which matched the nano-sized ordered domains of MEH-PPV led to a sharp enhancement in photovoltaic performance.