Yun Geng

Density functional theory characterization and design of high-performance diarylamine-fluorene dyes with different π spacers for dye-sensitized solar cells

To rationalize the marked difference in the energy conversion efficiency of dye sensitized solar cells (DSSCs) based on organic dyes 1 and 2 different only in their π spacer, density functional theory (DFT) and time-dependent DFT calculations of the geometries, electronic structures and absorption spectra of the organic dyes before and after binding to titanium oxide were carried out. These enable us to determine factors such as dipole moments associated with the open-circuit photovoltage (Voc), and to quantify parameters such as the light harvesting efficiency, the electron injection efficiency associated with the short-circuit photocurrent density (Jsc). The results reveal that compared to 2 with a thiazole spacer, 1 with a thiophene spacer could cause a red shift of the absorption spectrum, increase the oscillator strength and improve the driving force for electron injection, thus leading to the larger Jsc, in good agreement with experimental data. As for Voc, our results stress that apart from the generally emphasized vertical dipole moment of the dyes pointing outward from the semiconductor surface, the number of photoinjected electrons from the dye to the semiconductor is also crucial to obtain high performance dyes with high Voc. After justifying the reliability of the quantum-chemical methods, we designed another four dyes with different π spacers to screen more efficient organic dyes. Fortunately, taking 1 as reference, we find that dye 4 with a thienothiophene spacer displays an enhanced Jsc and Voc, indicating that it will be a more efficient diarylamine-fluorene-based organic dye used in DSSCs, which will play a theoretical guiding role in the design and synthesis of new organic dyes.

Theoretical discussions on electron transport properties of perylene bisimide derivatives with different molecular packings and intermolecular interactions

Seven perylene bisimide derivatives with different molecular packings and intermolecular interactions were investigated in detail within Marcus-Levich-Jortner formalism at the level of density functional theory (DFT). In theory, we further proved the report that different halogen substitutions in the core position of perylene bisimide lead to different molecular packings in their single crystals and thus obviously different electron transport properties. Here, insight into the geometries, the character of the frontier molecular orbitals, the decompositions of reorganization energies and transfer integrals in different directions was provided to shed light on the relationship between structures and properties. The molecular dynamics (MD) simulations and band structures calculations were also employed to give a multiscale understanding of their transport properties. The results show that there are small discrepancies of the intramolecular electron reorganization energies among these compounds and the transfer integrals determine their electron transport properties. Compounds 1a, 3a and 3b, with typical “brick” packing, π-stacked face-to-face packing and “herringbone” packing, respectively, have larger electron mobilities among these systems and possess different transport dimensionalities. Moreover, we also find there is close relationship between the intermolecular interaction energy and the transfer integral.

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.

A theoretical discussion on the relationships among molecular packings, intermolecular interactions, and electron transport properties for naphthalene tetracarboxylic diimide derivatives

Three naphthalene tetracarboxylic diimide derivatives 1–3 with high electron mobilities and long-term ambient stabilities were investigated employing Marcus–Levich–Jortner formalism at the density functional theory (DFT) level. The complicated relationships among molecular packings, intermolecular interactions, and transport properties for these compounds were focused on and analyzed through investigating the sensitivities of transfer integrals to intermolecular relative orientations, the optimizations of the major transport pathways and the calculations of intermolecular interaction energies by using dispersion-corrected DFT. The results show that the transfer integrals are sensitive to the subtle changes of relative orientations of molecules, especially for core-chlorinated compounds, and there is an interplay between intermolecular interaction and molecular packing. It is found that the transfer integrals associated with the molecular packing motifs of these systems determine their electron mobilities. Interestingly, further discussions on band structures, the anisotropies and temperature dependences of mobilities, and the comparisons of mobilities before and after optimization indicate that the intermolecular packing motifs in the film state may be different from those in the crystalline state for 2. Finally, we hope that our conjecture would facilitate the future design and preparation of high-performance charge-transport materials.

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.

Forward molecular design for highly efficient OLED emitters: A theoretical analysis of photophysical properties of platinum(II) complexes with N-heterocyclic carbene ligands

The electronic structures and photophysical properties of eight Pt-complexes with different N-heterocyclic carbene ligands and potential to serve as light emitting diode materials were investigated by density functional theory and time-dependent density functional theory, employing the BP86 functional for geometry optimisations, SAOP potential for excited state calculations and all-electron TZ2P basis set throughout. Non-radiative and radiative decay rate constants were determined for each system through analyses of the geometric relaxations, d-orbital splitting and spin-orbit couplings at the optimised S(0) and T(1) geometries. Three Pt-systems bound to two N-heterocyclic carbenes were shown to be nonemissive, while a fourth was shown to be emissive from the T(1) excited state. Similar T(1)-initated emission was observed for three other Pt-systems investigated, each bound to four N-heterocyclic carbenes, while a fourth similarly tetra-ligated system showed T(2)-initation of emission. The results highlight the coupling of ligand-identity to photophysical properties and more importantly, the potential for rational optimisation and tuning of emission wavelengths and phosphorescent efficiencies. Encouragingly, two of the tetra-N-heterocyclic carbene ligated systems show strong potential to serve as highly-efficient blue and green light emitting materials, respectively.

Theoretical study of the bridging effect on the charge carrier transport properties of cyclooctatetrathiophene and its derivatives

The bridging effect on the charge transport properties of cyclooctatetrathiophene and its derivatives (systems 1–4) was investigated at the level of density functional theory (DFT). Insights into their geometries, frontier molecular orbitals, reorganization energies, transfer integrals and band structures were provided in detail. Increasing charge mobilities for both holes and electrons were predicted in cyclooctatetrathiophene derivatives as the number of bridging sulfur atoms increased. The improved charge transport from system 1 to 4 can be interpreted from two contributions: (i) decreased reorganization energy with improved molecular planarity under the consideration of intermolecular interactions; and (ii) enhanced transfer integral derived from the π-stacking arrangement for 2 and 3, and multidimensional S⋯S interactions are found to contribute to charge transport in system 4 besides π⋯π interactions. The charge transport properties were also analyzed with a band-like model and the results were in agreement with those gained from the hopping model in the fact that the paths with large transfer integrals are all along the directions with large dispersions in the valence band or conduction band.

The interplay of intermolecular interactions, packing motifs and electron transport properties in perylene diimide related materials: a theoretical perspective

Perylene diimide (PDI) and its derivatives hold great promise, since they are undeniably considered as an important family of organic n-type semiconductors with both high carrier mobilities and air stabilities comparable to p-type ones, although they traditionally stand out as a class of high-performance dyes and pigments. In this feature article, we summarize the influences of substituents on different positions (imide, ortho, bay) of PDI on their electronic and morphological (packing) properties, which are in close connection with the ability for carrier transport. Then representative molecular packing motifs for PDIs are also classified, with an emphasis on the intricate interplay of intermolecular interactions, packing motifs and electron transport properties of perylene imide related carrier transport materials from a theoretical point of view, towards paving the way for boosting and improving the electron transport mobilities and air stabilities of PDIs-based materials.

The influence of the diphenylphosphoryl moiety on the phosphorescent properties of heteroleptic iridium(III) complexes and the OLED performance: a theoretical study

A series of heteroleptic iridium(III) complexes were investigated by using the density functional theory/time-dependent density functional theory (DFT/TD-DFT) approach to determine the influence of the diphenylphosphoryl (Ph2PO) moiety on the electronic structures, phosphorescent properties and the organic light-emitting diode (OLED) performance. The results reveal that the introduction of the Ph2PO group could not only dramatically change the electron density distributions of the LUMO and cause red shifts of the emission wavelengths, but also increase the oscillator strengths and the metal character, thus leading to larger radiative decay rates. Additionally, compared with FIrpic (a widely used kind of blue guest material in OLED devices), those complexes with Ph2PO substituents could improve the electron injection/balance ability, increase the Forster energy transfer rate and confine the triplet excitons to the guest phosphors, hence resulting in better OLED performance. Interestingly, further analysis indicates that, compared to IrpicPO with the Ph2PO group sited at the phenyl ring of the phenylpyridine (ppy) ligands, IrpicPOpy with the Ph2PO group sited at the pyridine ring of the ppy ligands performs better in the hole-trapping and hole-injection ability. Finally, we hope our investigations will facilitate the future design of high efficient phosphorescent materials.

A promising anchor group for efficient organic dye sensitized solar cells with iodine-free redox shuttles: a theoretical evaluation

The advantages and disadvantages of a new anchor group 2-(1,1-dicyanomethylene)rhodamine (DCRD) in D–π–A dyes for dye sensitized solar cells, compared with the commonly used anchor group cyanoacrylic acid (CA), were firstly investigated through DFT/TDDFT calculations on the dye/(TiO2)48/electrolyte interfacial electronic dynamics. It was found that the dissociative bidentate bridging mode is the most stable adsorption configuration on the TiO2 anatase (101) surface for DCRD. The calculated results indicate that in contrast to dyes with a CA group, the DCRD anchor group-based dye has a red-shifted absorption spectrum, which is beneficial for the enhancement of Jsc. However, the S atom in the DCRD group near the semiconductor surface has strong interactions with I2, which could lead to a higher I2 concentration in the vicinity of the dye-coated TiO2 surface, accelerating interfacial charge recombination. To avoid such shortcomings, a further optimization strategy for DCRD-based dyes was also proposed from a theoretical point of view. More importantly, we anticipate that dyes with the DCRD anchor group could show better performance in iodine-free redox shuttles-based DSSCs, in which the dye–I2 interaction could be avoided.