Investigation of Photoinduced Charge Transfer in Composites of a Novel Precursor PPV Polymer and Fullerenes
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ABSTRACT Homogenous blends of a processable methanofullerene, [6,6]-Phenyl C61 - butyric acid methyl ester (PCBM), with poly (p-phenylene vinylene) (PPV) made from a novel nonionic precursor route were investigated by optical spectroscopy and by photocurrent measurements. The conversion process of the precursor polymer to the PPV in the blends with PCBM was monitored by IR absorption. In composites of PPV/PCBM strong quenching of the PPV luminescence was observed. LESR (light induced electron spin resonance) and PIA (photoinduced absorption) studies confirmed the occurrence of photoinduced electron transfer from the PPV to PCBM. Photovoltaic devices made from PPV/PCBM blends showed monochromatic power conversion efficiencies of app. 0.3%. The spectral photocurrent was observed to follow the absorption profile of the PPV.
INTRODUCTION Efficiencies of the first polymeric solar cells, based on hole conducting conjugated polymers (mainly polyacetylene) were rather discouraging [1]. Encouraging breakthrough to higher efficiencies was achieved by switching to different classes of electron donor type conjugated polymers (polythiophenes (PT), polyphenylenevinylenes (PPV) and their derivatives) and by mixing them with suitable electron acceptors [2].The strongest hindrance to use organic polymeric semiconductors as the active component in photovoltaic devices during the last two decades was the rather inefficient charge
generation process as well as the unbalanced transport properties of the hole conducting polymers. Prototypes of photovoltaic devices based on a polymeric donor / acceptor network showed solar energy conversion efficiencies of around 1% [3]. Due to the application potential, the photophysics of conjugated polymer/fullerene solid composites have been well investigated in the last years [4]. From the conjugated polymers of the first generation, PPV was the most successful candidate for single layer polymer photovoltaic devices [5].Unsubstituted PPV is generally produced from a soluble precursor polymer with subsequent heat conversion. Polymer electroluminescence (polymer light emitting diodes (LEDs)), which is close to market application now, was investigated for the first time in PPV and then optimized for soluble conjugated polymers [6, 7, 8, 9 ]. There are many important aspects with respect to the basic processes that control the life time of polymer devices, including substrate smoothness, barrier properties, presence of oxygen and moisture as well as the cristallinity and the morphology of two component blends [10, 11, 12]. Compared to the new generation of soluble, substituted PPVs, the PPV from the precursor polymer is therefore still interesting due to its environmental as well as mechanical stability, serving therefore as a model system for polymer devices. With the observation of photoinduced charge transfer between non degenerate ground state polymers and fullerenes by Santa Barbara group and Osaka group [2, 4, 13, 14], new efforts were put again on the investigation of polymer based photovoltai
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