The main issues confronting people of the 21st century are said to be energy, the environment and resources. Consequently, solar cell research is an attractive research field from both an academic and industrial viewpoint. Organic solar cells made from organic materials such as conjugated polymers and fullerenes convert photon energy to electric current through a principle that differs from silicon-based solar cells. Organic solar cells also have many outstanding features, and as such are currently being actively researched. In addition to understanding the primary processes such as exciton generation and charge separation and recombination on a molecule’s temporal scale, it is also absolutely imperative to design and build a precise device structure on a molecule’s spatial scale in order to control these primary processes and achieve high conversion efficiency. As such, at our research laboratory we use transient absorption spectroscopy based on ultrashort pulsed lasers to observe transient species from exciton generation through to charge collection to each electrode over a wide time scale, and elucidate on a molecule’s temporal scale the mechanisms that lead to the photon-to-current energy conversion. Furthermore, using technology for the fabrication of ultrathin polymer films we are attempting to design the layered structure on a molecule’s spatial scale and control these primary processes. In this way, we can understand the structure and mechanism of organic solar cells, and carry out research to develop new organic solar cells with improved performance.

[Dynamics of Excitons and Charged Carriers in Polymer Solar Cells]

Currently, there are several types of polymer-based solar cells: polymer/fullerene blends, polymer/polymer blends, and polymer/inorganic semiconductor hybrids. Using transient absorption spectroscopy, we detect transient species of excitons and charged carriers formed in such blend films, and observe a series of fundamental processes such as exciton generation, exciton diffusion, charge separation, charge dissociation, charge recombination, and charge transport over a wide temporal range from the femtosecond through to the millisecond scale. On the basis of the spectroscopic study, we are attempting to shed light on the mechanism and dynamics of photon-to-current energy conversion in polymer solar cells.

Recent Publications
1. S. Yamamoto, J. Guo, H. Ohkita, S. Ito, Adv. Funct. Mater., 18, 2555-2562 (2008).
2. T. A. Ford, H. Ohkita, S. Cook, J. R. Durrant, N. C. Greenham, Chem. Phys. Lett., 454, 237-241 (2008).
3. H. Ohkita, S. Cook, Y. Astuti, W. Duffy, S. Tierney, W. Zhang, M. Heeney, I. McCulloch, J. Nelson, D. D. C. Bradley, J. R. Durrant, J. Am. Chem. Soc., 130, 3030-3042 (2008).
4. S. Cook, H. Ohkita, Y. Kim, J. J. Benson-Smith, D. D. C. Bradley, J. R. Durrant, Chem. Phys. Lett., 445, 276-280 (2007).
5. H. Ohkita, S. Cook, Y. Astuti, W. Duffy, M. Heeney, S. Tierney, I. McCulloch, D. D. C. Bradley, J. R. Durrant, Chem. Commun., 3939-3941 (2006).
6. S. Cook, H. Ohkita, J. R. Durrant, Y. Kim, J. J. Benson-Smith, J. Nelson, D. D. C. Bradley, Appl. Phys. Lett., 89, 101128/1-101128/3 (2006).
7. H. Ohkita, S. Cook, T. A. Ford, N. C. Greenham, J. R. Durrant, J. Photochem. Photobiol. A, 182, 225-230 (2006).

[Nanometric Design of Highly Efficient Polymer Solar Cells with Ultrathin Films]

In order to perform photon-to-current energy conversion efficiently, it is necessary to control the architecture of the device on a molecule’s spatial scale and to optimize the diffusion and transport of excitons and charged carriers. To date we have employed the layer-by-layer deposition technique to fabricate multilayered ultrathin polymer films and recently developed triple-layered organic solar cells comprised of a hole-transporting layer, a light-harvesting layer, and an electron-transporting layer by a combination of spincoating and the layer-by-layer deposition technique. On the basis of the precise design and the layered nanostructures, we found the important device physics and design rules to achieve highly efficient charge separation and charge transport.

Recent Publications
1. M. Ogawa, M. Tamanoi, H. Ohkita, H. Benten, S. Ito, Sol. Energy Mater. Sol. Cells, DOI:10.1016/j.solmat.2008.11.050.
2. K. Masuda, M. Ogawa, H. Ohkita, H. Benten, S. Ito, Sol. Energy Mater. Sol. Cells, DOI:10.1016/j.solmat.2008.09.028.
3. H. Benten, N. Kudo, H. Ohkita, S. Ito, Thin Solid Films, in press.
4. H. Benten, M. Ogawa, H. Ohkita, Shinzaburo Ito, Adv. Funct. Mater., 18, 1563-1572 (2008).
5. M. Ogawa, N. Kudo, H. Ohkita, S. Ito, H. Benten, Appl Phys Lett, 90, 223107/1-223107/3 (2007).

[Development of Organic-Inorganic Hybrid Solar Cells]

We are conducting research on organic-inorganic hybrid solar cells comprised of conjugated polymers and metal oxide semiconductors, and focusing our attention on the chemical stability and superior electron mobility of metal oxide semiconductors such as titanium dioxide. The charge separation efficiency of organic/inorganic heterojunction is improved through chemical modification of the semiconductor surface with fullerene or dye molecules. In this way, we are examining electron transfer and charge separation mechanisms of organic/inorganic heterojunction and designing new optoelectronic devices.

Recent Publications
1. N. Kudo, S. Honda, Y. Shimazaki, H. Ohkita, S. Ito, H. Benten, Appl. Phys. Lett., 90, 183513/1-183513/3 (2007).
2. N. Kudo, Y. Shimazaki, H. Ohkita, M. Ohoka, S. Ito, Sol. Energy Mater. Sol. Cells, 91, 1243-1247 (2007).
3. H. Ohkita, Y. Shimazaki, M. Ohoka, S. Ito, Chem. Lett., 33, 1598 (2004).



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