Co-sensitizer effect of black (N749) dye by DFT molecular dynamics investigations of TiO2(101)/black dye/acetonitrile interfaces

Koharu Aikawa^a, Yusuke Ootani^a, Liyuan Han^a^,^b, Yoshitaka Tateyama^a^,^b^,^c, Keitaro Sodeyama^a^,^c

^aNational Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki, 305, Japan

^bJapan Science and Technology Agency (JST), Saitama 332-0012

^cKyoto University, Japan, Goryo-Ohara, Nishikyo-ku, Kyoto 615-8245, Japan

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)

Roma, Italy, 2015 May 11th - 13th

Organizer: Filippo De Angelis

Poster, Yoshitaka Tateyama, 387
Publication date: 5th February 2015

Although perovskite solar cells attract significant attention now, fundamental mechanisms of conventional dye-sensitized solar cells (DSC) have not been fully understood yet, and still deserve being investigated. Here we addressed a question why Ru 749 dye (black dye:BD) gives better efficiency than the others, which is useful for further improvement of DSCs. We examined stable structures and photoexcitation character of BD adsorption to TiO₂ anatase (101) interface immersed in bulk acetonitrile (AN) solution, a most representative electrode interface in DSCs. Density functional theory based molecular dynamics (DFT-MD) with explicit solvent molecules was used to take into account the fluctuations of solvation shells and adsorbed molecules. We demonstrated that BD adsorption via deprotonated carboxylate two anchors (d2) is the most stable at the interface, while the one protonated carboxyl anchor (p1) has the average energy only slightly higher than the d2. This indicates that the p1 state can still coexist with the d2 under equilibrium. It is in contrast to the calculated large stability of the p1 in vacuo. Inhomogeneous charge distribution and anchor fluctuation enhanced by AN solution causes this d2 stabilization. The calculated projected densities of states and the photoabsorption spectra clearly show that the d2 state has larger driving force of the electron injection into the TiO2, whereas the photoabsorption in the wavelength region over 800 nm, a characteristic of BD sensitizer, is found mainly attributed to the p1 even in the AN solution. Consequently, the better performance of BD DSC can be understood in terms of the cosensitizer framework of the d2 and p1 states.
(left) Major adsorption mode with deprotonated two anchors on TiO2, and (right) minor adsorption mode with protonated one anchor.
Tateyama, Y.; Sumita, M.; Ootani, Y.; Aikawa, K.; Jono, R.; Han, L; Sodeyama, K. Acetonitrile Solution Effect on Ru N749 Dye Adsorption and Excitation at TiO2 Anatase Interface. J. Phys. Chem. C 2014, 118, 16863-16871.

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