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The discovery and utilization of metal-free organic emitters with thermally activated delayed fluorescence (TADF) is a huge breakthrough toward high-performance and low-cost organic light-emitting diodes. Time-dependent second-order perturbation theory including spin−orbit and nonadiabatic couplings, combined with time-dependent density functional theory, is employed to reveal the nature of highly efficient TADF in pure organic emitters. Our results demonstrate that except energy gaps between the lowest singlet (S 1 ) and triplet (T 1 ) excited states the nonadiabatic effect between low-lying excited states should play a key role in the T 1 → S 1 upconversion for TADF emitters, especially donor−acceptor−donor (D−A−D) molecules. We not only clarify the reason why D−A−D molecules with large S 1 −T 1 energy gaps show efficient TADF but also explain the experimental observation that D−A−D-type compounds with S 1 −T 1 gaps close to those of their D−A-shape counterparts display more efficient T 1 → S 1 upconversion.

We have carried out nonadiabatic molecular dynamics simulations combined with time-dependent density functional theory calculations to compare the properties of the two-dimensional (2D) (BA) 2 (MA)Pb 2 I 7 and three-dimensional (3D) MAPbI 3 (where MA = methylammonium and BA = butylammonium) materials. We evaluate the different impacts that the 2D-confined spacer layer of butylammonium cations and the 3D-confined methylammonium cations have on the charge carrier dynamics in the two systems. Our results indicate that, while both the MA + and BA + cations play important roles in determining the carrier dynamics, the BA + cations exhibit stronger nonadiabatic couplings with the 2D perovskite framework. The consequence is a faster hotcarrier decay rate in 2D (BA) 2 (MA)Pb 2 I 7 than in 3D MAPbI 3 . Thus, tuning of the functional groups of the organic spacer cations in order to reduce the vibronic couplings between the cations and the Pb−I framework can offer the opportunity to slow down the hot-carrier relaxations and increase the carrier lifetimes in 2D lead-halide perovskites.

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.

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The 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.