Yu-Ai Duan

Theoretical characterization and design of small molecule donor material containing naphthodithiophene central unit for efficient organic solar cells

To seek for high‐performance small molecule donor materials used in heterojunction solar cell, six acceptor–donor–acceptor small molecules based on naphtho[2,3‐b:6,7‐b′]dithiophene (NDT) units with different acceptor units were designed and characterized using density functional theory and time‐dependent density functional theory. Their geometries, electronic structures, photophysical, and charge transport properties have been scrutinized comparing with the reported donor material NDT(TDPP)2 (TDPP = thiophene‐capped diketopyrrolopyrrole). The open circuit voltage (Voc), energetic driving force(ΔEL‐L), and exciton binding energy (Eb) were also provided to give an elementary understanding on their cell performance. The results reveal that the frontier molecular orbitals of 3–7 match well with the acceptor material PC61BM, and compounds 3–5 were found to exhibit the comparable performances to 1 and show promising potential in organic solar cells. In particular, comparing with 1, system 7 with naphthobisthiadiazole acceptor unit displays broader absorption spectrum, higher Voc, lower Eb, and similar carrier mobility. An in‐depth insight into the nature of the involved excited states based on transition density matrix and charge density difference indicates that all S1 states are mainly intramolecular charge transfer states with the charge transfer from central NDT unit to bilateral acceptor units, and also imply that the exciton of 7 can be dissociated easily due to its large extent of the charge transfer. In a word, 7 maybe superior to 1 and may act as a promising donor candidate for organic solar cell.

Theoretical Insight into the Origin of Large Stokes Shift and Photophysical Properties of Anilido-Pyridine Boron Difluoride Dyes

The geometric and electronic structures and photophysical properties of anilido-pyridine boron difluoride dyes 1-4, a series of scarce 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) derivatives with large Stokes shift, are investigated by employing density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations to shed light on the origin of their large Stokes shifts. To this end, a suitable functional is first determined based on functional tests and a recently proposed index-the charge-transfer distance. It is found that PBE0 provides satisfactory overall results. An in-depth insight into Huang-Rhys (HR) factors, Wiberg bond indices, and transition density matrices is provided to scrutinize the geometric distortions and the character of excited states pertaining to absorption and emission. The results show that the pronounced geometric distortion due to the rotation of unlocked phenyl groups and intramolecular charge transfer are responsible for the large Stokes shift of 1 and 2, while 3 shows a relatively blue-shifted emission wavelength due to its mild geometric distortion upon photoemission, although it has a comparable energy gap to 1. Finally, compound 4, which is designed to realize the rare red emission in BODIPY derivatives, shows desirable and expected properties, such as high Stokes shift (4847 cm(-1)), red emission at 660 nm, and reasonable fluorescence efficiency. These properties give it great potential as an ideal emitter in organic light-emitting diodes. The theoretical results could complement and assist in the development of BODIPY-based dyes with both large Stokes shift and high quantum efficiency.

Cyano or o-nitrophenyl? Which is the optimal electron-withdrawing group for the acrylic acid acceptor of D-π-A sensitizers in DSSCs? A density functional evaluation.

AbstractWe report a DFT, TDDFT and DFTB investigation of the performance of two donor-π-acceptor (D-π-A)-type organic dyes bearing different electron-withdrawing groups (EWG) for dye-sensitized solar cells (DSSCs) to evaluate which EWG is better for an acrylic acid acceptor, i.e., Cyano (–CN) or o-nitrophenyl (o-NO2–Ph). A series of theoretical criteria applied successfully in our previous work to explain the different performance of organic dyes related to open-circuit photovoltage (Voc) and short-circuit current density (Jsc) were used to evaluate the performance of the dyes with just different EWG. Our calculated results reveal that dye 2 with o-NO2–Ph has a larger vertical dipole moment, more electrons transferred from the dye to the semiconductor and a lower degree of charge recombination, which could lead to larger Voc; while the larger driving force and comparable light harvesting efficiency could lead to higher Jsc , highlighting the potential of o-NO2–Ph as an EWG in an acrylic acid acceptor. FigureCN or o-NO2-Ph? Which is better for acrylic acid acceptor of donor-π-acceptor (D-π-A) dyes used in dye-sensitized solar cells (DSSCs) has been evaluated by DFT/TDDFT calculations.

The influence of molecular solid packings on the photoluminescence and carrier transport properties for two bow-shaped thiophene compounds: a theoretical study

Abstract The quantum mechanical and quantum mechanical/molecular mechanical methods were employed to investigate the photoluminescence and carrier transport properties of two bow-shaped thiophene compounds, whose optimized geometries were proved to have large deviation between the single-molecule and the solid-phase models. The results show that the molecular packings have large influence on the geometrical structures for both systems, but barely affect their energy levels of the frontier molecular orbitals and the reorganization energies, thus indicating little effect on the charge transport properties. While the obvious blue shifts of absorption and emission spectra in solid phase compared with single molecule are considered to relate with the intermolecular interactions, the difference in center thiophene units between two bow-shaped compounds induces the decided differences in optoelectronic properties, especially in carrier transport abilities.

Density functional studies on photophysical properties and chemical reactivities of the triarylboranes: effect of the constraint of planarity.

AbstractThe geometric and electronic structures, absorption spectra, transporting properties, chemical reactivity indices and electrostatic potentials of the planar three-coordinate organoboron compounds 1-2 and twisted reference compound Mes3B, have been investigated by employing density functional theory (DFT) and conceptual DFT methods to shed light on the planarity effects on the photophysical properties and the chemical reactivity. The results show that the planar compounds 1-2 exhibit significantly lower HOMO level than Mes3B, owing to the stronger electronic induction effect of boron centers. This feature conspicuously induces a blue shifted absorption for 1, although 1 seemingly possesses more extended conjugation framework than Mes3B. Importantly, the reactivity strength of the boron atoms in 1-2 is much lower than that in Mes3B, despite the fact that the tri-coordinate boron centers of 1-2 are completely naked. The interesting and abnormal phenomenon is caused by the strong p-π electronic interactions, that is, the empty p-orbital of boron center is partly filled by π-electron of the neighbor carbon atoms in 1-2, which are confirmed by the analysis of Laplacian of the electron density and natural bond orbitals. Furthermore, the negative electrostatic potentials of the boron centers in 1-2 also interpret that they are not the most preferred sites for incoming nucleophiles. Moreover, it is also found that the planar compounds 1-2 can act as promising electron transporting materials since the internal reorganization energies for electron are really small. FigureThe planar effects significantly affect the frontier molecular orbital levels, absorption wavelengths, transporting properties, and chemical reactivities of compounds 1-2. The underlying origin has been revealed by density functional theory and conceptual density functional theory calculations