RESEARCHORGANIC PV
Based on this strategy, the researchers tested a prototypical photovoltaic device using a molecular compound semiconductor which consists of electron-donor and electron-acceptor molecules, and molecular conductor materials as the highly efficient electron- and hole-ejecting electrodes. This study was supported by the New Energy and Industrial Technology Development Organization through its Innovative Solar Cell program.
Details of Research
The researchers used a molecular compound, DBTTF-TCNQ, as the molecular compound semiconductor, that is composed of alternated stacks of electron donor, DBTTF, and electron acceptor, TCNQ, molecules in the crystal. The material exhibits strong photo-absorption at near- infrared range where the electronic photo- excitation is possible by smaller photon energy of about 1/2 ~ 1/3 as compared to that of conventional organic semiconductors (Fig. 2).
They manufactured a prototypical photovoltaic device using the DBTTF-TCNQ single crystal as the semiconducting layer, and highly electron-ejecting and hole-ejecting molecular conductor materials as an anode and a cathode, respectively, that were deposited on top of the single crystal surfaces (metal-insulator-metal-type diode). The device shows notable rectification and photovoltaic characteristics by the photo-irradiation at near- infrared range.
Furthermore, they carried out Laser-Beam-Induced Current measurement using laser light focused to the diffraction limit, so as to observe diffusion and charge-separation processes of photo-generated excitons. As a result they observed extremely long diffusion length, reaching as large as 20 µm, which is 1,000 times longer than that of fullerene as is used for conventional OPCs (Fig. 3).
The wavelength dependence of the diffusion length indicates that the generated excitons are separated, and electrons and holes are generated just after the photo-excitation, which should be the origin of the long diffusion length as long as 20 µm (Fig. 4). This feature should be advantageous to improve the efficiency of OPCs, since the lifetime before the electron-hole recombination becomes extremely long. The group is now trying to develop a multi-layered photovoltaic cell using charge- transfer optical absorption in order to realize highly efficient OPCs.
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Figure 3 : Diffusion and charge-separation characteristics of photo- generated excitons (Intermolecular charge-transfer absorption and intramolecular absorption)
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Figure 4 : Diffusion lengths at different wavelengths of excitation light
www.solar-pv-management.com Issue I 2011
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