Application of self-assembling photosynthetic dye for organic photovoltaics
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Susumu Yoshikawab) Institute of Advanced Energy, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan; and Core Research of Evolutional Science & Technology (CREST), Japan Science Technology Agency (JST), Gokasho, Uji, Kyoto 611-0011, Japan (Received 2 May 2010; accepted 27 July 2010)
We have developed novel photovoltaic systems composed of the fullerene derivative (6,6)-phenyl C61 butyric acid methyl ester as electron acceptor with a second functional organic molecule, in this case bacteriochlorophyll c, as the light-harvesting and photosensitizing part. It was found that heat treatment of a thin film of bacteriochlorophyll c altered the morphological states of the aggregates and conductivity of the thin film could be regulated through the annealing process. Blended fullerene derivative and bacteriochlorophyll c thin films were fabricated on the surface of an indium-tin oxide/ poly(ethylene dioxythiophene) doped with polystyrene sulfonic acid substrate layer and their photovoltaic properties were characterized and evaluated. Formation of fullerene-coordinated bacteriochlorophyll c complex was confirmed by changes in the visible absorption spectra and by FTIR. Such complexation promoted generation of photocurrent in the region of the Qy band and the current density of the thin film increased. A maximum incident photon-to-current conversion efficiency of 5.1% was attained at 745 nm.
I. INTRODUCTION
Photosynthesis is a highly studied natural phenomena since the solar energy conversion efficiency is quite high in terms of an almost 100% quantum efficiency of charge separation. For example, bacteriochlorophylls (BChls) c, d, and e are the main light-harvesting pigments of green photosynthetic bacteria that self-assemble into nanostructures within the chlorosomes, forming efficient antennas of photosynthetic organisms.1 Recently, solar cells have attracted much attention relevant to global environmental issues. Particularly, organic semiconductors have become attractive as lightweight, flexible, and color-tunable materials that are expected to lead to simple fabrication processes with the potential for ubiquitous utilization.2 In this regard, we have developed photovoltaic systems composed of organic semiconductors of poly(3-hexylthiophene)s (P3HT) and fullerene derivative (6,6)-phenyl C61 butyric acid methyl ester (PCBM).3 Currently, a combination of a novel polymer of poly[4,8bis-2-ethylhexyloxy-benzo [1,2-b:4,5-b9]dithiophene-2,6-diyl-alt-4-heptylcarbonylthieno[3,4-b]thiophene-2,6-diyl] with PC70BM resulted in a remarkable enhancement of the power conversion efficiency (PCE) of 7.7%.4 Matsuo et al. reported a columnar
structured bulk-heterojunction solar cell based on synthetic tetrabenzoporphyrin and bis(dimethylphenylsilylmethyl)[60]fullerene showing a relatively higher PCE of 5.2%.5 However, such new materials are prepared by a complicated synthetic route in fairly small amounts. In this context, application of environmentally benign photosynthetic dyes for a photovoltaic system is promising in terms of the biomimetic a
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