Development of New Fullerene-based Electron Acceptors for Efficient Organic Photovoltaic Cells
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Development of New Fullerene-based Electron Acceptors for Efficient Organic Photovoltaic Cells Yutaka Matsuo1 1 Department of Chemistry, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 1130033, JAPAN ABSTRACT This article describes design of fullerene-based electron-accepting materials to obtain high performance in organic thin-film photovoltaic devices. A 1,4bis(dimethylphenylsilylmethyl)[60]fullerene gives higher open circuit voltage than 1,2-diadduct because of smaller π-conjugated systems, and enables columnar fullerene-core array for high electron mobility and thermal crystallization for ideal phase separation with electron-donor materials. A 56π-electron fullerene derivative possessing the dihydromethano group as the smallest carbon addend does not disrupt fullerene-fullerene contact in solid state, giving high open-circuit voltage without decreasing of short-circuit current density and fill factor. INTRODUCTION Because [60]fullerene is widely employed in the development of organic photovoltaic (OPV) devices, the control of its electronic state and morphology by addition of organic addends to the fullerene core has become a very important issue. For instance, an increasing number of addends can reduce the π-conjugation length, raise the LUMO level, and hence raise the opencircuit voltage (VOC) of the OPV device, which is beneficial for the device performance. However, the addends inevitably change the crystal packing, typically reduce the fullerene– fullerene contact, and may also reduce the carrier mobility. We report new fullerene-based electron acceptors, 1,4-bis(silylmethyl)[60]fullerenes (SIMEFs) [1,2] and 56πdihydromethano[60]fullerene derivative [3] that raise VOC without decreasing of short-circuit current density (JSC). EXPERIMENTAL DETAILS Synthesis of the mono-adduct: C60(CH2SiMe2Ph)H. To a solution of C60 (2.00 g, 2.78 mmol) in 1,2-dichlorobenzene (500 mL) containing N,N-dimethylformamide (6.45 mL, 83.3 mmol) was added a THF solution of PhMe2SiCH2MgCl (9.80 mL, 0.850 M, 8.33 mmol) at 25 °C. Color of the solution immediately changed from purple to dark green. After stirring for 10 min, saturated aqueous NH4Cl solution (1.0 mL) was added to terminate the reaction. The resulting dark red solution was subjected to vacuum distillation to remove 1,2-dichlorobenzene. The residue containing a small amount of 1,2-dichlorobenzene (ca. 0.5~3 mL) was dissolved in toluene (200 mL), and passed through a pad of silica gel to remove magnesium salts. The toluene solution was concentrated to obtain a solution (ca. 5~10 mL) containing the product, which was reprecipitated by addition of methanol (ca. 100~200 mL) to obtain brown powder. Purification of the product was carried out with preparative HPLC separation (Buckyprep column, eluent: toluene/2-propanol = 7/3) to obtain the title compound (1.99 g, 2.28 mmol, 82% isolated yield,
analytically pure). 1H NMR (500 MHz, CDCl3): δ 0.89 (s, 6H, SiCH3), 3.16 (s, 2H, CH2), 6.39 (s, 1H, C60H), 7.44–7.46 (m, 3H, Ph), 7.88–7.90 (m, 2H, Ph); 13C NMR (125 MHz, CDCl3): δ
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