Functionalized Methanofullerenes Used as n-Type Materials in Bulk-Heterojunction Polymer Solar Cells and in Field-Effect Transistors

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Abstract
The synthesis of two well-solubilized [60]methanofullerene derivatives (p-EHO-PCBM and p-EHO-PCBA) is presented for usage in organic solar cells and in field-effect transistors. The para position of the PCBM’s phenyl ring was substituted with a branched alkoxy side chain, which contributes to higher solubility, facilitating synthesis, purification, and processing. We find a small change of the open-circuit voltage (Voc) as a slight improvement in performance upon application in P3HT/[60]methanofullerene bulk-heterojunction-photovoltaic cells, when compared to PCBM, because of the electron donation of the alkoxy group. In the case of the devices with a TiOx layer, the best power conversion efficiencies (PCE, ηe) is observed in a layered structure of P3HT/p-EHO-PCBA/TiOx (ηe = 2.6%), which slightly exceeds that of P3HT/PCBM/TiOx (ηe = 2.3%) under conditions reported here. This can be attributed, in part, to the carboxylic acid group in p-EHO-PCBA that leads to an effective interface interaction between the active layer and TiOx phase. In addition, n-channel organic field-effect transistor (OFET) devices were fabricated with thin films of p-EHO-PCBM and p-EHO-PCBA, respectively cast from solution on SiO2/Si substrates. The values of field-effect mobility (μ) for p-EHO-PCBM and p-EHO-PCBA are 1 × 10−2 and 1.6 × 10−3 cm2/V·s, respectively. The results in this paper demonstrate the effects of a carboxylic acid group and an electron-donating substituent in [60]methanofullerenes as n-type materials with respect to organic solar cells and OFET applications.