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Materials Molecular Science Hiramoto Group

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Location: Yamate, No.4 Bldg. Room602
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Organic Semiconductors, Organic Solar Cells, ppm-Doping

Organic Solar Cells

Organic solar cells have been intensively studied due to many advantages like flexible, printable, light, low-cost, fashionable, etc. In 1991, Hiramoto invented “blended junction” (Fig. 1)1) and “tandem junction”2) of organic solar cells, which are fundamental concepts in the present organic solar cells. We have been focused on the establishment of “bandgap science for organic solar cells.”3) We believe that the following features are indispensable. (i) A ppm-level doping strategy should be applied to sub-ppm purified organic semiconductors for the complete removal of uncontrollable doping by oxygen from the air. (ii) Complete pn-control, i.e., the observation that every single and blended organic semiconductor shows both n- and p-type characteristics by impurity doping alone, should be achieved. (iii) Doping in the bulk of organic single crystals without grain boundaries is necessary for precise clarification of the nature of the doping effects.

Recently, we have succeeded the fabrication of ppm-level doped organic single crystals and measurements of their Hall effects using our original ultra-slow deposition technique at a rate of 10-9 nm/s (Fig. 2)4). This is a foundation for the first organic single crystal solar cells.

We fervently hope that young researchers will take up the challenge of working in the interdisciplinary field of organic solar cells to help develop the next generation of cells for renewable energy.


Fig. 1 Concept of blended junction.

2018_hiramoto2.pngFig. 2 ppm-doped organic single crystal (AFM) formed by ultra-slow deposition of 10- 9 nm/s having a rotating shutter.

Selected Publications

  1. M. Hiramoto , H. Fujiwara , M. Yokoyama, “Three-layered organic solar cell with a photoactive interlayer of codeposited pigments”, Appl. Phys. Lett., 58, 1062-1064 (1991).
  2. M. Hiramoto, M. Suezaki, M. Yokoyama, “Effect of thin gold interstitial-layer on the photovoltaic properties of tandem organic solar cell”, Chem. Lett., 1990, 327-330 (1990).
  3. M. Hiramoto, M. Kikuchi, S. Izawa, “Parts-per-Million-Level Doping Effects in Organic Semiconductor Films and Organic Single Crystals”, Adv. Mater.,30, 1801236 (15 pages) (2018) (Invited Progress Report).
  4. C. Ohashi, S. Izawa, M. Hiramoto et al., “Hall effect in bulk-doped organic single crystal”, Adv. Mater., 29, 1605619 (2017).
  5. N. Shintaku, M. Hiramoto, S. Izawa, “Effect of trap-assisted recombination on open-circuit voltage loss in phthalocyanine/fullerene solar cells”, Org. Electron., 55, 69-74 (2018).