Xian-Kai Chen

An efficient strategy for designing n-type organic semiconductor materials—introducing a six-membered imide ring into aromatic diimides

The aromatic diimides are among the most promising and versatile candidates for organic optoelectronic materials due to their commercial availability, low cost, excellent optical and electric performance, such as naphthalene, anthracene and perylene diimides. But, so far, the problem is not clarified—is a five- or six-membered imide ring more helpful for n-type organic semiconductor materials? The work investigated in detail and compared various properties for molecules with a five-/six-membered imide ring from the following aspects: (1) molecular stability, reaction activity, geometries, frontier molecular orbitals as well as oxidation and reduction abilities at the single-molecule level; (2) the variation of transfer integrals at the various molecular stacking motifs; (3) the estimate of carrier mobility and its anisotropy for the actual molecule crystals. The results indicate that molecules with a six-membered imide ring should be more suitable for n-type organic semiconductor materials.

Theoretical Study on Charge Transport Properties of Intra- and Extra-Ring Substituted Pentacene Derivatives

A series of pentacene derivatives, halogen-substituted and thiophene- and pyridine-substituted, have been studied with a focus on the electronic properties and charge transport properties using density functional theory and classical Marcus charge-transfer theory. The transport properties of holes and electrons have been studied to get insight into the effect of halogenation and heteroatom substitution on transport and injection of charge carriers. The calculation results revealed that fluorination and chlorination can effectively lower the lowest unoccupied molecular orbital (LUMO) level, modulate the hole and electron reorganization energy, improve the stacking mode of the crystal structure, and enhance the ambipolar characteristic. Chlorination gives a better ambipolar characteristic. On the basis of halogen substitution, the substitution of terminal benzene ring of triisopropyl-silylethynyl-pentacene (TIPS-PEN) by a thiophene or pyridine will greatly lower the LUMO level and improve the stacking mode, leading to more suitable ambipolar materials. Hence, both intra- and extra-ring substitution are favorable to enhance the ambipolar transport property of TIPS-PEN.

Theoretical study on optoelectronic properties of fluorene derivatives with pyrene-functional groups: effect of the heteroatoms and pyrene’ substituting position

The geometries and photoelectric properties of pyrene-functional fluorene derivatives are investigated by detailed quantum-chemical methods, including density functional theory (DFT), time-dependent density functional theory (TD-DFT) and polarizable continuum model (PCM). The effects of the following factors on the properties were systemically analyzed: (1) the substituting position of the pyrene groups on the central fluorene; (2) the substitution of the central C by the heteroatoms; (3) the introduction of phenyl at the central C position. The rigid pyrene groups play a dominant role in determining their frontier molecular orbitals and reducing the reorganization energies. In these studied molecules, due to its large ⊿EST (the splitting between the lowest singlet and triplet excitation energies) and excellent and balanceable charge transport properties, DPF-2 should be a promising emission layer material. The host–guest systems composed of these emitters and rubrene are suitable for emission layer materials of white organic light-emitting devices.

Understanding the effects of the number of pyrazines and their positions on charge-transport properties in silylethynylated N-heteropentacenes.

AbstractThe charge-transport properties of a series of silylethynylated N-heteropentacenes (TIPS-PEN-xN; x = 2, 4) were systematically investigated using Marcus electron-transfer theory coupled with kinetic Monte Carlo simulations. Electronic structure calculations showed that introducing more pyrazine rings decreases the energy levels of the lowest unoccupied molecular orbitals (LUMOs) and should aid electron transfer. The number and the positions of the pyrazine rings greatly influence the molecular packing in crystals and hence the intermolecular electronic coupling. Furthermore, the introduction of internal (rather than external) pyrazine rings leads to a better charge-transport network. Transport parameters evaluated from the hopping and band-like models both demonstrate that, among the TIPS-PEN-xN molecules, B-TIPS-PEN-4N—which has two internal pyrazine rings—is the most promising n-type material. Graphical AbstractThe effects of the number of pyrazines and their positions on charge-transport properties in silylethynylated N-heteropentacenes are investigated with Marcus transport mechanism as well as coherent band-like transport model