Chemically Functionalized Nanocarbons for Artificial Photosynthesis and Solar Energy Conversion

Recently, we have conducted the regioisomer separation of hexyloxycarbonyl-substituted dihydronaphthyl-based [60]fullerene bis-adducts (C6BA) and applied the separated bis-adducts to PSCs for the first time.^1,2 To reduce the plausible number of the fullerene bis-adduct isomers, a symmetrical dihydronaphthyl group was chosen as the substituent. We expected that symmetrical introduction of two long alkoxycarbonyl chains into the dihydronaphthyl groups would facilitate the isomer separation and purification. Indeed, repeated separations by HPLC equipped with a Buckyprep column afforded trans-1, trans-2, trans-3, trans-4, e, and a mixture of cis-2 and cis-3 (denoted as cis-2+cis-3) isomers. The LUMO energy levels of C6BA isomers estimated by cyclic voltammetry are 0.1 – 0.2 eV higher than those of the corresponding mono-adduct (C6MA) and [60]PCBM. However, the best PCE (1.44%) achieved by the trans-4 isomer of C6BA (denoted as C6BA–trans-4) in combination with regioregular poly(3-hexylthiophene) (P3HT) was lower than those of the corresponding mono-adduct C6MA and [60]PCBM-based PSCs (1.71 and 2.10%, respectively). The inferior efficiencies may be caused by the long aliphatic side chains, which disrupt the optimal packing and efficient electron transport of the fullerene molecules in the active layer. Hence, we decided to shorten the n-hexyl chain of C6BA to n-butyl and ethyl (C4BA and C2BA). We expected that we could systematically address the effects of alkyl chain length as well as substituent pattern of fullerene bis-adducts on the photovoltaic properties of PSCs.³ In this talk I will present an overview of our chemically functionalized nanocarbons for artificial photosynthesis and solar energy conversion.

[1] S. Kitaura, K. Kurotobi, M. Sato, Y. Takano, T. Umeyama, and H. Imahori, Chem. Commun., 48, 8550-8552 (2012).

[2] T. Umeyama and H. Imahori, J. Mater. Chem. A (Feature Article), 2, 11545-11560 (2014).

[3] R. Tao, T. Umeyama, K. Kurotobi, and H. Imahori, ACS Appl. Mater. Interface, in press.