Application of self-assembling photosynthetic dye for organic photovoltaics

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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-b']dithiophene-2,6-diyl-alt-4-heptylcarbonyl-thieno[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 approach, which allows us to prepare large amounts of various self-assembling chlorophyllous pigments that exist in nature with tailored properties of highly efficient charge separation and light-harvesting through simple cultivation of organisms. In this context, self-assembling analogues of the natural chlorophylls,⁶ bacteriochlorophylls,^7-9 and chlorins^7,9 have already been applied for organic thin-film photovoltaics, but due to spontaneous aggregation and very fast charge recombination, the device performance was rather modest. In order to regulate and adjust the self-aggregation of the photosynthetic dyes, we report here the novel application of photosynthetic dye of bacteriochlorophyll c (BChl c) as light-harvesting and/or photosensitizing part for the bulk heterojunction organic thin-film solar cell.

II.

EXPERIMENTAL

Chlorobium tepidum was grown and its chlorosomes were isolated by using methods similar to those previously reported¹⁰ with a slight modification. Cultivation was carried out at 40 °C under continuous illumination of 20 W fluorescent...