Photovoltaic action of conjugated polymer/fullerene bulk heterojunction solar cells using novel PPE-PPV copolymers

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Abstract
The design of novel conjugated polymers suitable for use in plastic solar cells is one of today's challenges aiming towards improved key properties like the increase of photocurrent and open circuit voltage of such devices. In this work we present first results on arylene-ethynylene/arylene-vinylene hybrid polymers 3 (poly(-2,5-dioctyloxy-1,4-phenylene-diethynylene-2,5-dioctyloxy-1,4-phenylene-vinylene-2,5-di(2′-ethyl)hexyloxy-1,4-phenylene-vinylene)) and 5 (poly(2,5-dioctyloxy-1,4-phenylene-ethynylene-9,10-anthracenylene-ethynylene-2,5-dioctyloxy-1,4-phenylene-vinylene-2,5-di(2′-ethyl)hexyloxy-1,4-phenylene-vinylene)), demonstrating photovoltaic action in combination with the soluble C60 derivative 1-(3-methoxycarbonyl) propyl-1-phenyl [6,6]C61 (PCBM). Devices with an active layer thickness of about 100 nm yielded power conversion efficiencies of up to 2% under 100 mW cm−2 AM 1.5 white light illumination. The coarse grained morphology of the active layers was identified as the main limitation for the photocurrent, revealed by AFM measurements. The photovoltaic devices were characterized by current–voltage and spectral photocurrent measurements. The results show that the open circuit voltage is weakly dependent on the HOMO (highest occupied molecular orbital) level of the conjugated polymer used as donor.