601 research outputs found
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
Solvent Effects on the Adsorption Geometry and Electronic Structure of Dye-Sensitized TiO2: A First-Principles Investigation
The performance of dye-sensitized solar cells (DSSCs)
depends significantly on the adsorption geometry of the dye on the
semiconductor surface. In turn, the stability and geometry of the adsorbed
molecules is influenced by the chemical environment at the electrolyte/
dye/TiO2 interface. To gain insight into the effect of the solvent on the
adsorption geometries and electronic properties of dye-sensitized TiO2
interfaces, we carried out first-principles calculations on organic dyes and
solvent (water or acetonitrile) molecules coadsorbed on the (101) surface
of anatase TiO2. Solvent molecules introduce important modifications on
the dye adsorption geometry with respect to the geometry calculated in
vacuo. In particular, the bonding distance of the dye from the Ti anchoring
atoms increases, the adsorption energy decreases, and the two C−O bonds
in the carboxylic moieties become more symmetric than in vacuo. Moreover, the adsorbed solvent induces the deprotonation of
the dye due to the changing the acid/base properties of the system. Analysis of the electronic structure for the dye-sensitized
TiO2 structures in the presence of coadsorbed solvent molecules shows an upward shift in the TiO2 conduction band of 0.2 to
0.5 eV (0.5 to 0.8 eV) in water (acetonitrile). A similar shift is calculated for a solvent monolayer on unsensitized TiO2. The
overall picture extracted from our calculations is consistent with an upshift of the conduction band in acetonitrile (2.04 eV vs
SCE) relative to water (0.82 eV vs SCE, pH 7), as reported in previous studies on TiO2 flatband potential (Redmond, G.;
Fitzmaurice, D. J. Phys. Chem. 1993, 97, 1426−1430) and suggests a relevant role of the solvent in determining the dye−
semiconductor interaction and electronic coupling
Relativistic Solar Cells
Hybrid AMX3 perovskites (A=Cs, CH3NH3; M=Sn, Pb; X=halide) have
revolutionized the scenario of emerging photovoltaic technologies. Introduced
in 2009 by Kojima et al., a rapid evolution very recently led to 15% efficient
solar cells. CH3NH3PbI3 has so far dominated the field, while the similar
CH3NH3SnI3 has not been explored for photovoltaic applications, despite the
reduced band-gap. Replacement of Pb by the more environment-friendly Sn would
facilitate the large uptake of perovskite-based photovoltaics. Despite the
extremely fast progress, the materials electronic properties which are key to
the photovoltaic performance are relatively little understood. Here we develop
an effective GW method incorporating spin-orbit coupling which allows us to
accurately model the electronic, optical and transport properties of CH3NH3SnI3
and CH3NH3PbI3, opening the way to new materials design. The different
CH3NH3SnI3 and CH3NH3PbI3 properties are discussed in light of their
exploitation for solar cells, and found to be entirely due to relativistic
effects.Comment: 16 pages, 4 figure
The Ferroelectric-Ferroelastic Debate about Metal Halide Perovskites
Metal halide perovskites (MHPs) are solution-processed materials with exceptional photoconversion efficiencies that have brought a paradigm shift in photovoltaics. The nature of the peculiar optoelectronic properties underlying such astounding performance is still controversial. The existence of ferroelectricity in MHPs and its alleged impact on photovoltaic activity have fueled an intense debate, in which unanimous consensus is still far from being reached. Here we critically review recent experimental and theoretical results with a two-fold objective: we argue that the occurrence of ferroelectric domains is incompatible with the A-site cation dynamics in MHPs and propose an alternative interpretation of the experiments based on the concept of ferroelasticity. We further underline that ferroic behavior in MHPs would not be relevant at room temperature or higher for the physics of photogenerated charge carriers, since it would be overshadowed by competing effects like polaron formation and ion migration
Inherent electronic trap states in TiO2 nanocrystals: effect of saturation and sintering
We report a quantum mechanical investigation on the nature of electronic trap states in realistic models of
individual and sintered anatase TiO2
nanocrystals (NCs) of ca. 3 nm diameter. We find unoccupied
electronic states of lowest energy to be localized within the central part of the NCs, and to originate
from under-coordinated surface Ti atoms lying mainly at the edges between the (100) and (101) facets.
These localized states are found at about 0.3–0.4 eV below the fully delocalized conduction band states,
in good agreement with both electrochemical and spectro-electrochemical results. The overall DensityOf-States (DOS) below the conduction band (CB) can be accurately fitted to an exponential distribution
of states, in agreement with capacitance data. Water molecules adsorbed on the NC surface raise the
energy and reduce the number of localized states, thus modifying the DOS. As a possible origin of
additional trap states, we further investigated the oriented attachment of two TiO2
NCs at various
possible interfaces. For the considered models, we found only minor differences between the DOS of
two interacting NCs and those of the individual constituent NCs. Our results point at the presence of
inherent trap states even in perfectly stoichiometric and crystalline TiO2
NCs due to the unavoidable
presence of under-coordinated surface Ti(IV) ions at the (100) facets
Quaterpyridine Ligands for Panchromatic Ru(II) Dye Sensitizers
A new general synthetic access to carboxylated quaterpyridines (qpy), of interest as ligands for panchromatic dyesensitized solar cell organometallic sensitizers, is presented. The strategic step is a Suzuki−Miyaura cross-coupling reaction,
which has allowed the preparation of a number of representative unsubstituted and alkyl and (hetero)aromatic substituted qpys.
To bypass the poor inherent stability of 2-pyridylboronic acid derivatives, we successfully applied N-methyliminodiacetic acid
(MIDA) boronates as key reagents, obtaining the qpy ligands in good yields up to (quasi)gram quantities. The structural,
spectroscopic (NMR and UV−vis), electrochemical, and electronic characteristics of the qpy have been experimentally and
computationally (DFT) investigated. The easy access to the bis-thiocyanato Ru(II) complex of the parent species of the qpy
series, through an efficient route which bypasses the use of Sephadex column chromatography, is shown. The bis-thiocyanato
Ru(II) complex has been spectroscopically (NMR and UV−vis), electrochemically, and computationally investigated, relating its
properties to those of previously reported Ru(II)−qpy complexes.“This document is the Accepted Manuscript version of a Published Work that appeared in final form in [The Journal of Organic Chemistry], copyright © American Chemical Society after peer review and technical editing by the publisher
Stem cell plasticity and dormancy in the development of cancer therapy resistance
Cancer treatment with either standard chemotherapy or targeted agents often results in the emergence of drug-refractory cell populations, ultimately leading to therapy failure. The biological features of drug resistant cells are largely overlapping with those of cancer stem cells and include heterogeneity, plasticity, self-renewal ability, and tumor-initiating capacity. Moreover, drug resistance is usually characterized by a suppression of proliferation that can manifest as quiescence, dormancy, senescence, or proliferative slowdown. Alterations in key cellular pathways such as autophagy, unfolded protein response or redox signaling, as well as metabolic adaptations also contribute to the establishment of drug resistance, thus representing attractive therapeutic targets. Moreover, a complex interplay of drug resistant cells with the micro/macroenvironment and with the immune system plays a key role in dictating and maintaining the resistant phenotype. Recent studies have challenged traditional views of cancer drug resistance providing innovative perspectives, establishing new connections between drug resistant cells and their environment and indicating unexpected therapeutic strategies. In this review we discuss recent advancements in understanding the mechanisms underlying drug resistance and we report novel targeting agents able to overcome the drug resistant status, with particular focus on strategies directed against dormant cells. Research on drug resistant cancer cells will take us one step forward toward the development of novel treatment approaches and the improvement of relapse-free survival in solid and hematological cancer patients
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