Simulating Dye-Sensitized TiO2 Heterointerfaces in Explicit Solvent: Absorption Spectra, Energy Levels, and Dye Desorption

Dye-sensitized solar cells (DSCs) represent a valuable, efficient, and low-cost alternative to conventional semiconductor photovoltaic devices. A deeper understanding of the dye/semiconductor heterointerface and of the dye-sensitized semiconductor/ electrolyte interactions are fundamental for further progress in DSC technology. Here we report an ab initio molecular dynamics simulation of a dye-sensitized TiO2 heterointerface “immersed” in an explicit water environment for an efficient organic dye, followed by TDDFT excited state calculations of the coupled dye/semiconductor/ solvent system. This new computational protocol and the extended model system allows us to gain unprecedented insight into the excited state changes occurring for the solvated dye-sensitized heterointerface at room temperature, and to provide an atomistic picture of water-mediated dye desorption

Theoretical Investigations of the Effects of J-Aggregation on the Linear and Nonlinear Optical Properties of E-4-(4-Dimethylaminostyryl)-1-methylpyridinium [DAMS + ]

J-type aggregation of organic chromophores into inorganic host matrixes provides a useful route toward materials showing strong second-order nonlinear optical (NLO) response. The increased NLO response of J-aggregates is related to the peculiar arrangement of the NLO-phores into the host matrix, which produces the appearance of a narrow and intense band in the material electronic absorption spectrum, red-shifted with respect to the main absorption band of the isolated NLO-phore. A theoretical investigation, based on DFT, TDDFT, and ZINDO calculations on the relationship between the structural features of various or oligomeric aggregates and their linear and nonlinear optical properties shows that the appearance of a new red-shifted absorption band, typical of J-aggregation, is associated with interchromophoric transitions of charge-transfer character, due to the splitting of HOMO and LUMO levels. The intensity of this latter band increases by increasing the number of NLO-phores in a model of oligomeric arrangement of J-aggregates. The calculated quadratic hyperpolarizabilities for the mostly responsive J-type trimeric aggregates of [DAMS + ] are found to largely exceed that of three isolated NLO-phores, confirming a cooperative NLO strong contribution due to J-aggregation. Finally, our DFT and TDDFT calculations on eclipsed or with opposite dipole dimeric H-aggregates of [DAMS + ] show a splitting of HOMO and LUMO levels, which gives place to interchromophoric transitions of chargetransfer character but blue-shifted, as observed experimentally

White-light phosphorescence emission from a single molecule: application to OLED

A simple mononuclear cyclometallated iridium(III) complex exhibits white photo- and electro- luminescence in the wavelength range from 440 to 800 nm, which originates from a single emitting excited state of mixed character.Bolink Henk, [email protected] ; Coronado Miralles, Eugenio, [email protected]

Impact of Spin–Orbit Coupling on Photocurrent Generation in Ruthenium Dye-Sensitized Solar Cells

Relativistic TDDFT calculations have been performed employing a novel computational approach to evaluate the impact of spin–orbit coupling (SOC) in the optical and photovoltaic properties of panchromatic RuII dyes for dye-sensitized solar cells (DSCs). The employed computational setup accurately reproduces the optical properties of the investigated dyes, allowing an assessment of the factors responsible for the varying SOC with the dye metal–ligand environment. While for the prototypical panchromatic black dye sensitizer a negligible SOC effect is found, the SOC-induced spectral broadening calculated for the recently reported DX1 dye partly enhances the light-harvesting efficiency and consequently the photocurrent generation in DSCs based on this dye

TDDFT Modeling of Spin-Orbit Coupling in Ruthenium and Osmium Solar Cell Sensitizers

We report on the relevance of spin-orbit coupling on the optical properties of Ru(II) and Os(II)- polypyridyl dyes effectively employed in Dye-sensitized Solar Cells (DSCs). We include relativistic effects on TDDFT calculations of selected complexes, by using different levels of calculations, i.e. the scalar zero-order regular approximation (ZORA) and the fully relativistic ZORA including spin-orbit coupling, in such a way to disentangle and evaluate the spin-orbit effect. The widely investigated [M(bpy)3]2+ (M=Ru(II) and Os(II)) have been selected as benchmark complexes in our calculations, followed by investigation on “realistic” dyes used in DSCs, such as the prototypical N3 dye, its Os-based analogue and a panchromatic Os(II) dye. We find that in Ru(II) complexes spin-orbit coupling lead to a slight correction of the spectral shape, while only including the spin-orbit coupling we are able to reproduce the low energy absorption bands characteristic of the Os(II) complexes. This study allows us to find a quantitative correlation between the strength of spin-orbit coupling and the metal center, highlighting the secondary effect of the different ligands experienced by the metal center

Everything you always wanted to Know about Black Dye (but Were Afraid to Ask): A DFT/TDDFT Investigation

We report an exhaustive theoretical and computational investigation of the electronic, optical, redox and acid-base properties, along with the adsorption mode on TiO2, of Black Dye (BD), the prototypical panchromatic dye for solar cell applications. We investigated in detail the variation of the relevant dye properties as a function of the solution pH, corresponding to the stepwise deprotonation of the carboxylic groups. Our results reproduced the expected blue-shift of the optical absorption spectrum and the experimental trend of oxidation potentials by increasing pH, which turned both out to be in excellent agreement with experimental values. Also, our calculated excited state oxidation potential is in good agreement with available experimental data. We then looked at the pKa of the various deprotonation steps, finding lowest pKa values for the stepwise dissociation of three protons of 2.71, 3.69 and 5.20, in excellent agreement with experimental pKa values of 3 and 5, for two and one protons dissociation, respectively. We finally investigated the adsorption of BD on TiO2, finding the most stable adsorption to occur via two dissociated monodentate carboxylic groups. Inspection of the electronic structure and alignment of energy levels for N719 and BD revealed a reduced driving force for electron injection in the latter dye, which could possibly lead to energetically unfavorable electron transfer from the excited dye to the TiO2 conduction band. Our results may constitute a reference study for future investigations and optimization of BD-based dye-sensitized solar cells

A computational approach to the electronic, optical and acid–base properties of Ru(II) dyes for photoelectrochemical solar cells applications

We provide a unified review of recent work carried out on computational investigations of a large series of Ru(II)–polypyridyl complexes effectively employed as solar cells sensitizers in dye-sensitized solar cells (DSCs). The use of methods rooted into Density Functional Theory (DFT) and its Time-Dependent extension (TDDFT) are demonstrated to be powerful tools to describe the electronic and optical properties of metallorganic ruthenium solar cells sensitizers, allowing us to unravel the interplay between their UV–Vis spectral changes and the complexes acid–base properties. This theoretical approach can be used to reproduce and understand the experimental data and also to design and predict the electronic and optical properties of new ruthenium-based sensitizer dyes

Ab Initio Determination of Ground and Excited State Oxidation Potentials of Organic Chromophores for Dye-Sensitized Solar Cells

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