Temperature Independence of the Photoinduced Electron Injection in Dye-Sensitized TiO2 Rationalized by Ab Initio Time-Domain Density Functional Theory

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• 作者：Walter R. Duncan ; Oleg V. Prezhdo
• 刊名：Journal of the American Chemical Society
• 出版年：2008
• 出版时间：July 30, 2008
• 年：2008
• 卷：130
• 期：30
• 页码：9756-9762
• 全文大小：256K
• 年卷期：v.130,no.30(July 30, 2008)
• ISSN：1520-5126

文摘

Time-domain density functional theory simulations resolve the apparent conflict between the central role that thermal fluctuations play in the photoinduced chromophore−TiO₂ electron transfer (ET) in dye-sensitized semiconductor solar cells [J. Am. Chem. Soc. 2005, 127, 18234; Isr. J. Chem. 2003, 42, 213] and the temperature independence of the ET rate [e.g., Annu. Rev. Phys. Chem. 2005, 56, 119]. The study, performed on the alizarin−TiO₂ interface at a range of temperatures, demonstrates that the ET dynamics, both adiabatic and nonadiabatic (NA), are dependent on the temperature, but only slightly. The adiabatic rate increases with temperature because a fluctuation toward a transition state (TS) becomes more likely. A classical TS theory analysis of the adiabatic ET gives a Gibbs energy of activation that is equal to k_BT at approximately 50 K, and a prefactor that corresponds to multiple ET pathways. The NA rate increases as a result of changes in the distribution of photoexcited-state energies and, hence, in the density of accessible TiO₂ levels, as expressed in the Fermi Golden Rule. In the system under investigation, the photoexcited state lies close to the bottom of the TiO₂ conduction band (CB), and the chromophore−semiconductor coupling is strong, resulting in primarily adiabatic ET. By extrapolating the simulation results to chromophores with excited states deeper inside the CB and weaker donor−acceptor coupling, we conclude that the interfacial ET is essentially independent of temperature, even though thermal ionic motions create a widespread of initial conditions, determine the distribution of injected electron energy, and drive both adiabatic and NA ET.