Browsing by Author "Segawa, Hiroshi"

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Effects of end- on oriented polymer chains at the donor/ acceptor interface in organic solar cells
(2018-12-07)
Wang, Fanji
;
Nakano, Kyohei
;
Yoshida, Hiroyuki
;
Hashimoto, Kazuhito
;
Segawa, Hiroshi
Molecular alignment at the interface between the electron donor and acceptor (D/A) materials in thin films may have a large effect on the charge generation process in organic solar cells (OSCs). In this study, the effects of the polymer chain orientations at the D/A interface were investigated in OSCs with planar heterojunction (PHJ) structures. Poly(3-alkylthiophene) films with edge-on (main chain parallel to the interface) and end-on (main chain vertical to the interface) orientations at the surface were used to construct PHJs with a fullerene derivative. Ultraviolet photoelectron spectroscopy and low-energy inverse photoelectron spectroscopy revealed that the difference in polarization energy through the charge-permanent quadrupole interaction causes a rigid energy shift of 0.5eV between the two orientations. The PHJ device with the end-on oriented polymer chains at the D/A interface showed a smaller photovoltage loss expected from the energy structure and a more efficient charge separation than the device with the edge-on oriented polymer chains. These improvements were attributed to charge delocalization along the end-on oriented polymer chains normal to the D/A interface, through which the effective binding energy of the charge pairs in the interfacial charge-transfer state was reduced. These results provide important information about molecular arrangements at the D/A interface that may enable further improvements in OSC performance.
Theoretical studies on the absorption spectra of cis-[Ru(4,4 \'-COO-2,2 \'-bpy)(2)(X)(2)](4-), (X = NCS, Cl) and panchromatic trans-terpyridyl Ru complexes including strong spin-orbit coupling
(2015-01-01)
Mishima, Kenji
;
Kinoshita, Takumi
;
Hayashi, Michitoshi
;
Jono, Ryota
;
Segawa, Hiroshi
In this paper, we theoretically and experimentally investigate the photophysical and chemical characteristics and absorption spectra of various ruthenium complexes in solution used as efficient dye-sensitized solar cells. The target molecules are two well-known complexes, cis-[Ru(4,4\'-COO-2,2\'-bpy)(2)(X)(2)](4-), (X = NCS, Cl) and trans-terpyridyl Ru. The experimental absorption spectra of these molecules, which show strong spin-orbit (SO) coupling, are simulated using first-order perturbation theory based on time-dependent density functional theory and quantum chemistry calculations. It turns out that the theory can simulate the experimental data very well, which indicates that SO coupling is very important and the mixing between singlet and triplet states is strong in these molecules because of the large SO coupling constant of the Ru atom. The exact absorption spectra can only be reproduced by including the perturbation by SO coupling. The physical and chemical differences between cis-[Ru(4,4\'-COO-2,2\'-bpy)(2)(X)(2)](4-), (X = NCS, Cl) and trans-terpyridyl Ru complexes are elucidated by natural bond orbital and natural transition orbital analyses. From these analyses, we have found that the two kinds of Ru complexes are quite different in terms of photoexcitation response and chemical bonding between the central Ru atom and the surrounding ligands.