Extended Exciton Diffusion in Rubrene Single-Crystalline Organic Solar Cells
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Extended Exciton Diffusion in Rubrene Single-Crystalline Organic Solar Cells Tetsuhiko Miyadera1, 2 a), Noboru Ohashi2, Tetsuya Taima2, 1, Toshihiro Yamanari2, Yuji Yoshida2 1
Japan Science and Technology Agency, PRESTO, 4-1-8 Honcho Kawaguchi, Saitama 332-0012, Japan 2 National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1, Higashi, Tsukuba, Ibaraki 305-8565, Japan
ABSTRACT Single-crystalline organic solar cells were investigated. Rubrene single crystals made by train sublimation method were used for the active layer of the solar cells. Typical solar cell characteristics and external quantum efficiency (EQE) were observed with the film thickness of several micrometers. In spite of their large film thickness, the EQE spectra showed no screening effect, which means that absorbed photons efficiently converted to electric charges. This can be attributed to the extended exciton diffusion due to uniform and trap free characteristic of rubrene single crystal.
INTRODUCTION Organic solar cells (OSCs) currently attract a great deal of attention as the next generation electric power source due to their flexibility, colorfulness, and cost reduction. Fundamental studies for organic semiconductors are still necessary for the improvement of OSCs because establishment of device physics and design principles for OSCs are underway. Exciton diffusion in organic films is an important factor for the enhancement of the current density of OSCs. The exciton diffusion length limits the optimum thickness of the active layer of OSCs. Exciton diffusion length in vacuum-deposited organic thin films is typically 10-50 nm 1. The short exciton diffusion length disturbs the efficient photon to current conversion in organic semiconducting layers. In order to overcome the shortcoming of the exciton diffusion length, bulk hetero junction system has been developed 2. There are, however, few reports which intend to extend the exciton diffusion length itself. In order to enhance the exciton diffusion length, we focused on organic single crystals because of its structurally uniform and trap-free feature. An advantage of the
use of organic single crystals was studied in the field of organic thin film transistors (OTFTs). The field effect mobility was enhanced by 4 orders of magnitude by the use of rubrene single crystals 3. The enhancement of exciton diffusion length is also expected by the use of organic single crystals. Pioneering study of single-crystal OSCs was reported by R. J. Tseng et. al. 4. They reported the film-thickness-dependent current density of tetracene single-crystal solar cells. They also found that the photocurrent depended on the polarization of incident photons. In this study, rubrene single-crystal solar cells were fabricated and based on the solar-cell characteristics analysis, exciton diffusion in organic single crystal was discussed.
EXPERIMENTAL Rubrene (Aldrich) single crystals were fabricated by means of train sublimation, where the sublimation of organic materials and re-crystallization take plac
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