Electron and Hole Dynamics in Dye-Sensitized Solar Cells: Influencing Factors and Systematic Trends

We investigate electron and hole dynamics upon photon excitation in dye-sensitized solar cells, using a recently developed method based on real-time evolution of electronic states through time-dependent density functional theory. The systems we considered consist of organic sensitizers and nanocrystalline TiO2 semiconductors. We examine the influence of various factors on the dynamics of electrons and holes, including point defects (vacancies) on the TiO2 surface, variations in the dye molecular size and binding geometry, and thermal fluctuations which result in different alignments of the electronic energy levels. Two clear trends emerge: (a) dissociated adsorption of the dye molecules leads to faster electron injection dynamics by reducing interfacial dipole moments; (b) oxygen vacancy defects stabilize dye adsorption and facilitate charge injection, at the cost of lower open circuit voltage and higher electron−hole recombination rate. Understanding of these effects at the atomic level suggests tunable parameters through which the electronic characteristics of dye-sensitized solar cell devices can be improved and their efficiency can be maximized

Natural Dyes Adsorbed on TiO₂ Nanowire for Photovoltaic Applications: Enhanced Light Absorption and Ultrafast Electron Injection

We investigate the electronic coupling between a TiO2 nanowire and a natural dye sensitizer, using state-of-the-art time-dependent first-principles calculations. The model dye molecule, cyanidin, is deprotonated into the quinonoidal form upon adsorption on the wire surface. This results in its highest occupied molecular orbital (HOMO) being located in the middle of the TiO2 bandgap and its lowest unoccupied molecular orbital (LUMO) being close to the TiO2 conduction band minimum (CBM), leading to greatly enhanced visible light absorption with two prominent peaks at 480 and 650 nm. We find that excited electrons are injected into the TiO2 conduction band within a time scale of 50 fs with negligible electron−hole recombination and energy dissipation, even though the dye LUMO is located 0.1−0.3 eV lower than the CBM of the TiO2 nanowire

Quantum Mode Selectivity of Plasmon-Induced Water Splitting on Gold Nanoparticles

Plasmon induced water splitting is a promising research area with the potential for efficient conversion of solar to chemical energy, yet its atomic mechanism is not well understood. Here, ultrafast electron–nuclear dynamics of water splitting on gold nanoparticles upon exposure to femtosecond laser pulses was directly simulated using real time time-dependent density functional theory (TDDFT). Strong correlation between laser intensity, hot electron transfer, and reaction rates has been identified. The rate of water splitting is dependent not only on respective optical absorption strength, but also on the quantum oscillation mode of plasmonic excitation. Odd modes are more efficient than even modes, owing to faster decaying into hot electrons whose energy matches well the antibonding orbital of water. This finding suggests photocatalytic activity can be manipulated by adjusting the energy level of plasmon-induced hot carriers, through altering the cluster size and laser parameter, to better overlap adsorbate unoccupied level in plasmon-assisted photochemistry

Molecular Orbital Insights into Plasmon-Induced Methane Photolysis

As a promising way to reduce the temperature for conventional thermolysis, plasmon-induced photocatalysis has been utilized for the dehydrogenation of methane. Here we probe the microscopic dynamic mechanism for plasmon-induced methane dissociation over a tetrahedral Ag20 nanoparticle with molecular orbital insights using time-dependent density functional theory. We ingeniously built the relationship between the chemical bonds and molecular orbitals via Hellmann–Feynman forces. The time- and energy-resolved photocarrier analysis shows that the indirect hot hole transfer from the Ag nanoparticle to methane dominates the photoreaction at low laser intensity, due to the strong hybridization of the Ag nanoparticle and CH4 orbitals, while indirect and direct charge transfer coexist to facilitate methane dissociation in intense laser fields. Our findings can be used to design novel methane photocatalysts and highlight the broad prospects of the molecular orbital approach for adsorbate–substrate systems

Collective Behavior of Single-Atom Catalysts: A Synergistic Effect between Strain and Site Configuration

Fe single-atom catalysts on N-doped graphene (Fe–NC) exhibit good and variable catalytic activity linked to the active site density and configuration. Here, we comprehensively investigate the Fe–NC catalysts under various strained states and site densities to address the interplay between the active site density, local strain, site geometry, and oxygen evolution reaction (OER) activity. It is found that the active site density is closely associated with in-plane strain, which can be tuned by popping up Fe single atoms from the graphene film and, thereby, modulating the OER catalytic activity. Further analysis indicates that there exist three orientations of the FeN4 active site, each introducing specific anisotropic strain. As a result, the in-plane strain correlates with both the orientation and density of the active site, ultimately influencing catalytic activity. Our findings demonstrate the synergistic effects of multiple factors in single-atom catalysts, providing new insights into the rational design and fine tuning single-atom catalysts via collective interactions

Predicting Energy Conversion Efficiency of Dye Solar Cells from First Principles

In this work we target on accurately predicting energy conversion efficiency of dye-sensitized solar cells (DSC) using parameter-free first principles simulations. We present a set of algorithms, mostly based on solo first principles calculations within the framework of density functional theory, to accurately calculate key properties in energy conversion including sunlight absorption, electron injection, electron–hole recombination, open circuit voltages, and so on. We choose two series of donor-π-acceptor dyes with detailed experimental photovoltaic data as prototype examples to show how these algorithms work. Key parameters experimentally measured for DSC devices can be nicely reproduced by first-principles with as less empirical inputs as possible. For instance, short circuit current of model dyes can be well reproduced by precisely calculating their absorption spectra and charge separation/recombination rates. Open circuit voltages are evaluated through interface band offsets, namely, the difference between the Fermi level of electrons in TiO₂ and the redox potential of the electrolyte, after modification with empirical formulas. In these procedures the critical photoelectron injection and recombination dynamics are calculated by real-time excited state electronic dynamics simulations. Estimated solar cell efficiency reproduces corresponding experimental values, with errors usually below 1–2%. Device characteristics such as light harvesting efficiency, incident photon-to-electron conversion efficiency, and the current–voltage characteristics can also be well reproduced and compared with experiment. Thus, we develop a systematic ab initio approach to predict solar cell efficiency and photovoltaic performance of DSC, which enables large-scale efficient dye screening and optimization through high-throughput first principles calculations with only a few parameters taken from experimental settings for electrode and electrolyte toward a renewable energy based society

Ultrafast carrier relaxation and its Pauli drag in photo-enhanced melting of solids

Ultrafast light-matter interaction is a powerful tool for the study of solids. Upon laser excitation, carrier multiplication and lattice acceleration beyond thermal velocity can occur, as a result of far-from-equilibrium carrier relaxation. The roles of electron-electron and electron-phonon scatterings are identified by first-principles dynamic simulations, from which a unified phase diagram emerges. It not only explains the experimentally-observed "inertial" melting but also predicts abnormal damping by Pauli Exclusion Principle with a new perspective on ultrahigh-intensity laser applications

Presentation1_Plasmon-Induced Water Splitting on Ag-Alloyed Pt Single-Atom Catalysts.pdf

A promising route to realize solar-to-chemical energy conversion resorts to water splitting using plasmon photocatalysis. However, the ultrafast carrier dynamics and underlying mechanism in such processes has seldom been investigated, especially when the single-atom catalyst is introduced. Here, from the perspective of quantum dynamics at the atomic length scale and femtosecond time scale, we probe the carrier and structural dynamics of plasmon-assisted water splitting on an Ag-alloyed Pt single-atom catalyst, represented by the Ag19Pt nanocluster. The substitution of an Ag atom by the Pt atom at the tip of the tetrahedron Ag20 enhances the interaction between water and the nanoparticle. The excitation of localized surface plasmons in the Ag19Pt cluster strengthens the charge separation and electron transfer upon illumination. These facts cooperatively turn on more than one charge transfer channels and give rise to enhanced charge transfer from the metal nanoparticle to the water molecule, resulting in rapid plasmon-induced water splitting. These results provide atomistic insights and guidelines for the design of efficient single-atom photocatalysts for plasmon-assisted water splitting.</p

Momentum-resolved TDDFT algorithm in atomic basis for real time tracking of electronic excitation

Ultrafast electronic dynamics in solids lies at the core of modern condensed matter and materials physics. To build up a practical ab initio method for studying solids under photoexcitation, we develop a momentum-resolved real-time time dependent density functional theory (rt-TDDFT)algorithm using numerical atomic basis, together with the implementation of both the length and vector gauge of the electromagnetic field. When applied to simulate elementary excitations in two-dimensional materials such as graphene, different excitation modes, only distinguishable in momentum space, are observed. The momentum-resolved rt-TDDFT is important and computationally efficient for the study of ultrafast dynamics in extended systems