Time-Domain ab Initio Study of Charge Relaxation and Recombination in Dye-Sensitized TiO₂

Abstract

In order to investigate the electron dynamics at the alizarin/I2-/TiO2 interface this study uses a novel state-of-the-art quantum-classical approach that combines time-dependent density functional theory with surface hopping in the Kohn-Sham basis. Representing the dye-sensitized semiconductor Grätzel cell with the I-/I3- mediator, the system addresses the problems of an organic/inorganic, molecule/bulk interface that are commonly encountered in molecular electronics, photovoltaics, and photoelectrochemistry. The processes studied include the relaxation of the injected electron inside the TiO2 conduction band (CB), the back electron transfer (ET) from TiO2 to alizarin, the ET from the surface to the electrolyte, and the regeneration of the neutral chromophore by ET from the electrolyte to alizarin. Developing a theoretical understanding of these processes is crucial for improving solar cell design and optimizing photovoltaic current and voltage. The simulations carried out for the entire system that contains many electronic states reproduce the experimental time scales and provide detailed insights into the ET dynamics. In particular, they demonstrate the differences between the optimized geometric and electronic structure of the system at 0 K and the experimentally relevant structure at ambient temperature. The relaxation of the injected electron inside the TiO2 CB, which affects the solar cell voltage, is shown to occur on a 100 fs time scale and occurs simultaneously with the electron delocalization into the semiconductor bulk. The transfer of the electron trapped at the surface to the ground state of alizarin proceeds on a 1 ps time scale and is facilitated by vibrational modes localized on alizarin. If the electrolyte mediator is capable of approaching the semiconductor surface, it can form a stable complex and short-circuit the cell by accepting the photoexcited electron on a subpicosecond time scale. The ET from TiO2 to both alizarin and the electrolyte diminishes the solar cell current. Finally, the simulations show that the electrolyte can efficiently regenerate the neutral chromophore. This is true even though the two species do not form a chemical bond and, therefore, the electronic coupling between them is weaker than in the TiO2-chromophore and TiO2-electrolyte donor-acceptor pairs. The chromophore-electrolyte coupling can occur both directly through space and indirectly through bonding to the semiconductor surface. The ET events involving the electrolyte are promoted primarily by the electrolyte vibrational modes.

Calculation of non-adiabatic coupling vectors in a local-orbital basis set

“…The surface hopping method has been implemented within DFT 20,33,34 for calculations of systems on the order of 100 atoms and with durations of up to a few picoseconds. In particular, surface hopping simulations using a time-dependent KS (TDKS) approach 20 have been used to analyze non-adiabatic dynamics in a number of relatively large systems; [42][43][44] this TDKS approach 20 has been compared with the more accurate linear-response time-dependent DFT (LR-TDDFT) method 34 by performing non-adiabatic simulations for different systems. 45 The results show that the simpler and computationally more efficient TDKS approach yields a very reasonable description of the systems analyzed, the agreement with the more accurate method being better for larger systems.…”

Transforming Benzophenoxazine Laser Dyes into Chromophores for Dye‐Sensitized Solar Cells: A Molecular Engineering Approach

“…Ab initio molecular dynamics, which marries concepts of quantum mechanical forces and excitations, presents a more sophisticated approach to study dye···TiO 2 buried interfaces; [39][40][41] these enable both dynamic structural and energetic information. Such studies are generally being reported on a single system at this stage, owing to their signifi cant computational outlay.…”

Donor/Acceptor Adsorbates on the Surface of Metal Oxide Nanoporous Films: A Spectroscopic Probe for Different Electron Transfer Pathways

“…On the other hand, it has been shown both theoretically [17,45,54] and experimentally [24,46] that the alizarin excited state is positioned near the TiO 2 conduction band edge. The simulation for the alizarin/TiO 2 coupled system showed that the electron is initially injected into localized surface states and subsequently delocalized on a !…”

“…Moreover, the alizarin-sensitized TiO 2 system, where the molecular photoexcited state is at the edge of the conduction band, [17,24,45] represents an interesting case of a dye/semiconductor coupled system. In spite of this, an extremely fast injection for alizarin-sensitized TiO 2 nanoparticles in solutions in the sub-10 femtosecond time scale has been observed in the experiment [13] and was confirmed by theory.…”