Nonadiabatic Molecular Dynamics Study of Electron Transfer from Alizarin to the Hydrated Ti 4+ Ion Walter R. Duncan and Oleg V. Prezhdo* Department of Chemistry, UniVersity of Washington, Seattle, Washington 98195-1700 ReceiVed: May 16, 2005; In Final Form: July 14, 2005 Ab initio real-time nonadiabatic (NA) molecular dynamics (MD) simulations are performed in order to investigate the photoinduced electron transfer (ET) from alizarin to the hydrated Ti 4+ ion and compare it with the ET into bulk TiO 2 that forms the basis of the Gra ¨tzel type solar cell. The experimental data and electronic structure calculations indicate that the photoexcitation spectra of alizarin attached to either bulk TiO 2 or the Ti 4+ ion in solution are very similar. In contrast, the NAMD simulations at ambient temperature predict marked differences between the ET dynamics that follow the photoexcitation in the two systems. The simulation of ET between alizarin and the TiO 2 surface shows predominantly adiabatic transfer that occurs within 8 fs (Duncan et al. J. Am. Chem. Soc. 2005, 127, 7941), in agreement with the time-resolved experimental data. The simulation of alizarin attached to the hydrated Ti 4+ ion reported presently predicts that the ET does occur, but on a slower 30 fs time scale, with a substantially reduced amplitude and by a predominantly NA mechanism. The differences are attributed to the disparity in the acceptor states of bulk TiO 2 and the Ti 4+ ion in solution. It is shown that the predicted alizarin-Ti 4+ ET dynamics can be verified experimentally. I. Introduction The dye-sensitized nanocrystalline solar cell, also known as the Gra ¨tzel cell, is a promising alternative to the more costly traditional solar cell. 1-5 It employs organic or transition-metal- based chromophores that are tuned to visible light and adsorbed to highly porous nanocrystalline titanium dioxide. This inex- pensive semiconductor, although it absorbs in the ultraviolet region and is thus not suitable for solar light harvesting on its own, does facilitate the electron-hole separation and conducts free electrons. The cell’s electron-hole separation begins with the interfacial electron transfer (ET) from the chromophore to the semiconductor that occurs after the light absorption. The relative yields and rates of electron injection, recombination, and the decay of the chromophore excited state influence the efficiency of the solar cell. 3,6 The competition of charge recombination with the photoinduced ET lowers the photocur- rent. The positioning of the photoexcited state energy relative to the bottom edge of the semiconductor conduction band determines the injection efficiency and the magnitude of the voltage loss. While the injection from photoexcited states high above the conduction band edge is most effective, the relaxation of the injected electron to the bottom of the conduction band leads to an experimentally observed photovoltage that is below the theoretical maximum. Improving the photon-to-electron yield and the voltage of solar devices requires a thorough understand- ing of the competing reaction mechanisms. Considerable research efforts have been focused on the photoinduced ET dynamics across the chromophore-semicon- ductor interface. 7-41 The experimental data clearly point to a complex dependence of the ET rates and mechanisms on the chemical and electronic structure of the organic dyes, inorganic semiconductor, and dye-semiconductor binding. Ultrafast laser techniques have shown that electron injection can occur on a femtosecond time scale, 7-22 which is faster than the redistribu- tion of the vibrational energy, and is therefore poorly described by the traditional ET models. 13,42 Two competing mechanisms that require different conditions for optimum performance have been proposed to explain the observed ultrafast ET. 16,17 In the adiabatic mechanism, the coupling between the dye and the semiconductor is large, and ET occurs through a transition state along the reaction coordinate that involves a concerted motion of the nuclei. The electron remains in the same Born- Oppenheimer (adiabatic) state, which changes localization from the dye to the semiconductor along the reaction coordinate. In the nonadiabatic (NA) mechanism, the coupling between the dye and the semiconductor is small, and the ultrafast ET is achieved through multiple direct transitions from the dye state into a manifold of the conduction band acceptor states. The adiabatic transfer can be described by Marcus transition state theory, while the NA ET is typically treated by a perturbation theory, such as the Fermi Golden rule. 42 Intermediate couplings can entail a combination of both ET mechanisms. The ab initio NA molecular dynamics (MD) simulations of the interfacial ET carried out in our group 35-40 were the first theoretical studies that established the electron injection mech- anisms in real time and at the atomistic level of detail. Our studies investigated ET from isonicotinic acid 35-38 and alizarin 38-41 into TiO 2 . The alizarin system constitutes a particularly interesting case, since the chromophore photoex- cited-state energy is near the bottom of the TiO 2 conduction band. The dynamics of ET from alizarin into TiO 2 are strongly modulated by the fluctuations of the donor and acceptor state energies that are induced by nuclear vibrations. The NAMD simulation reproduced the ultrafast experimental ET time scale and demonstrated that the adiabatic mechanism dominates. 38-40 The adiabatic injection is made possible by a strong alizarin- TiO 2 coupling that leads to efficient electron injection at the edge of the conduction band. The injection scenario that exists in the alizarin-TiO 2 system has the potential to aid in the design * Corresponding author. E-mail: prezhdo@u.washington.edu. 17998 J. Phys. Chem. B 2005, 109, 17998-18002 10.1021/jp052570x CCC: $30.25 © 2005 American Chemical Society Published on Web 09/01/2005