Tatsuo Hasegawa (Leader), Jun'ya Tsutsumi (Researcher) and others of the Correlated Materials Photoelectronics Group, the Photonics Research Institute (Director: Masanobu Watanabe), the National Institute of Advanced Industrial Science and Technology (AIST; President: Tamotsu Nomaguchi), have substantiated an organic photovoltaic cell (OPC) based on a novel concept where optical absorption due to the charge-transfer between different organic molecules is utilized.

Research and development of OPC technologies have currently been conducted worldwide, because the technologies are expected to realize light-weight, flexible photovoltaic sheets. In this study, the researchers designed and fabricated a prototypical OPC using a molecular compound composed of two different kinds of organic molecules. It was found that the cell presents a photovoltaic effect due to the irradiation of near-infrared light whose wavelength is longer than 1 µm, although such a photoelectric conversion of near-infrared light has been very difficult in conventional OPCs. Furthermore, the lifetime and the diffusion length of excitons and charge carriers in the device were found to be three orders of magnitude longer than those of the conventional OPCs. Based on the concept, we can expect to realize the more efficient conversion from light energy to electric energy.

As the findings enable the utilization of near-infrared light that has been a crucial subject in the R&D of OPCs, the new concept is expect to improve the efficiency of OPCs. The details of this study will be published in Physical Review Letters (November 26, 2010), a journal of the American Physical Society, and published online on November 24, 2010 (EST).

Figure 1 : Photoelectric conversion of near-infrared light using charge-transfer photo-absorption

OPCs are now intensively studied in the world because of their advantages over silicon-based solar cells in the capability of producing light-weight and flexible solar cell sheets. In addition, as OPCs can be manufactured in the process neither under vacuum nor with high temperature, it is feasible to realize the large-area and low-cost cells. The feature is expected as a key technology to realize the green innovation for the low-carbon society.

The efficiency of OPCs has reached about 7~8% in the past few years, although the further improvement in the conversion efficiency should be necessary for the practical use. The factors hindering the improvement in the efficiency include (1) a limited wavelength range of optical absorption, i.e. visible light (wavelength < 800 nm), which results in the inactivity of OPCs to near-infrared light that holds about 40% of the total solar radiation energy and (2) an extremely short lifetime of photo-excited states, which results in the large energy loss before the photoelectric conversion. These problems originate from the inherent nature of organic semiconductors, i.e. the excitonic states are strongly confined to the individual molecules. Consequently, it is considered to be quite difficult to give fundamental solutions to these problems.

At AIST, researchers have been developing a new type of OPC that is based on the optical absorption due to the charge-transfer arisen between different molecules. Since the wavelength region of the charge-transfer optical absorption varies depending on the combination of the molecules, it becomes possible to use near-infrared light for photovoltaic conversion that is impossible for conventional organic materials. Based on this strategy, the researchers fabricated and 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.

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).

Figure 2 : Optical absorption spectra of a single-component molecular semiconductor and a molecular compound semiconductor

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. Resultantly, 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.

Figure 3 : Diffusion and charge-separation characteristics of photo-generated excitons (Intermolecular charge-transfer absorption and intramolecular absorption)

Figure 4 : Diffusion lengths at different wavelengths of excitation light

We are now trying to develop a multi-layered photovoltaic cell using charge-transfer optical absorption in order to realize highly efficient OPCs.