Buckminsterfullerene derivatives bearing a fluoroalkyl group for use in organic photovoltaic cells
Graphical abstract
Novel [60]fullerene derivatives bearing a fluoroalkyl group were synthesized and their photophysical, electrochemical, and photovoltaic properties were investigated. [60]Fullerene derivatives containing a C4-fluoroalkyl group showed the best photovoltaic performances among the fluoroalkyl C60 derivatives synthesized. This is so far the first report of the successful use of fluoroalkyl fullerene derivatives as active materials in organic photovoltaic cell.
Highlights
► We synthesized six novel fluoroalkyl fullerene derivatives. ► Photovoltaic cells using the fullerene derivatives were characterized. ► The cell performances were notably affected by the substituents on the fullerenes. ► This is the first example of the use of fluoroalkyl fullerenes for the solar cells.
Introduction
Organic photovoltaic (OPV) cells based on conjugated polymers and soluble fullerene derivatives have attracted a great deal of attention because of their solution processability and mechanical flexibility properties and their potential low-cost large-module fabrication [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]. The soluble buckminsterfullerene derivative, [6,6]-phenyl C61 butyric acid methyl ester (PC61BM), is currently the most promising acceptor material for OPV cells. The power conversion efficiency (PCE) of poly(3-hexylthiophene) (P3HT)/PC61BM bulk-heterojunction (BHJ) solar cells has been improved up to 6% under specially optimized conditions [1], [2], [3], [4], [5].
To make further progress in OPV materials and physics, developing a deeper understanding of the compatibility, miscibility, and dispersibility in OPV materials is necessary to allow for the fabrication of an ideal p–n juncture between a donor and an acceptor material in a blend film. We cannot, however, select an alternative acceptor material to PC61BM for OPV fabrication at present. This lack of alternative acceptor materials has led to an overall lack of fundamental understanding and has severely limited the development of any further improvement in OPV performance. In light of these limitations, several new fullerene derivatives have been developed in recent years [15], [16], [17], [18], [19], [20], [21]. Cao et al. [15] reported the synthesis of methanofullerenes containing several long-alkyl ester groups from PC61BM. They subsequently concluded that the synthesis of an acceptor that has good compatibility with donor polymer materials is important for improving device performance. Blom et al. [16] reported that fullerene bis-adducts with higher LUMO levels effectively enhanced the open-circuit voltage and PCE values to a greater extent than that of the corresponding fullerene mono-adducts. In 2008, Wudl et al. [17] developed functionalized methanofullerenes based on the PC61BM with a branched alkoxy side chain on a phenyl group. These compounds provided PCE values of 1.73% from a standard photovoltaic cell. The efficiency was further improved up to 2.64% when a TiOx layer was introduced as a spacer between the active layer and the Al cathode. In a separate development in 2009, Wudl et al. [18] reported an isomeric iminofullerene and its positive effect in enhancing short-circuit currents. In a more detailed study, two other groups [19], [20], [21] investigated the relationship between material structure and photovoltaic performance, and the studies revealed that the solubility of the fullerene derivatives and their intermolecular interactions had an impact on the OPV cell parameters. These reports have provided us with some important information about the relationship between the chemical structures of the materials and the device characteristics. The suitably modified chemical structures of the materials can induce not only the function of high carrier mobility but also the compatibility in the blend film, which are important characteristics of the materials.
Chikamatsu et al. developed long-chain perfluoroalkyl-substituted C60 derivatives and investigated their field-effect transistor (FET) properties. They demonstrated a field-effect mobility of the order of 10−1 cm2/V s as well as the device-operation air stability of the compounds [22], [23]. We believed that high performance materials for n-channel FETs such as perfluoroalkyl substituted C60 derivatives could be potential candidates for acceptor materials in the next generation of OPV cells. The characteristic nature of the perfluoroalkyl group, however, must be considered. For instance, Hashimoto et al. reported the attempted use of a fullerene derivative with a fluoroalkyl chain (F-PC61BM) in an OPV cell. The F-PC61BM did not work as the active material of the BHJ type device, but was used as an additive to form a self-organized buffer layer, improving the fill factor of the device as a consequence of its strong phase separation ability [24]. As stated above, there are no reported examples of the use of fullerene derivatives containing fluoroalkyl groups as the active acceptor materials in OPV cell.
In the current paper, we report the synthesis of several new fullerene derivatives containing fluoroalkyl groups of different lengths, together with their photophysical, electrochemical, and photovoltaic properties. Our study of the BHJ OPV cell using P3HT:fullerene-derivative blend film as the active layer provided a PCE value up to 0.53% under simulated AM 1.5 G solar irradiation at 100 mW/cm2. This represents so far the first demonstration of the successful use of a fluoroalkyl fullerene derivative as the active material in organic photovoltaic cells.
Section snippets
Materials
Reagents were purchased from Wako Pure Chemical Industries, Tokyo Kasei Chemical Industries, Merck Ltd., and Aldrich, and used without further purification. [60]Fullerene was purchased from Honjo Chemical Corporation. Compounds 1, 8, 9 (Scheme 1), amino acid derivatives [2-(methylamino)decanoic acid, N-dodecylglycine, and 2-(dodecylamine)hexadecanoic acid], and perfluroalkylbenzaldehyde derivatives (4-heptadecafluorooctylbenzaldehyde, 4-tridecafluorohexylbenzaldehyde, and
Synthesis of fluoroalkyl [60]fullerene derivatives
The motivation for the design and synthesis of these fullerene derivatives was to look for novel fullerene derivatives for application as the acceptors in next generation OPV materials. We were also keen to investigate the effect that incorporating fluoroalkyl chains in fullerene derivatives have on the photovoltaic properties. Our strategy for developing novel fullerene derivatives with high carrier mobility involved the introduction of a fluoroalkyl group onto the fullerene framework through
Conclusion
Novel [60]fullerene derivatives bearing a fluoroalkyl group were synthesized, and their absorbance, electrochemical, and photovoltaic properties were investigated. All of the fullerene derivatives exhibited absorption ranges and reduction potential values similar to those of PC61BM. The substituents on the pyrrolidine ring did not affect the frontier-orbital energy levels of the corresponding fullerene derivatives. The BHJ OPV cells were fabricated with an ITO/PEDOT-PSS/P3HT:fullerene
Acknowledgements
This work was supported through a cooperative research program with Daikin Co., Ltd. We would like to extend our thanks to the Comprehensive Analysis Center at the Institute for Scientific and Industrial Research of Osaka University for their assistance with the elemental analyses and NMR spectroscopic measurements.
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