The interface characteristic is a crucial factor determining the power conversion efficiency of organic solar cells (OSCs). In this work, our aim is to conduct a comparative study on the interface characteristics between the very famous non-fullerene acceptor, ITIC, and a fullerene acceptor, PC71BM by combining molecular dynamics simulations with density functional theory. Based on some typical interface models of the acceptor ITIC or PC71BM and the donor PBDB-T selected from MD simulation, besides the evaluation of charge separation/recombination rates, the relative positions of Frenkel exciton (FE) states and the charge transfer states along with their oscillator strengths are also employed to estimate the charge separation abilities. The results show that, when compared with those for the PBDB-T/PC71BM interface, the CT states are more easily formed for the PBDB-T/ITIC interface by either the electron transfer from the FE state or direct excitation, indicating the better charge separation ability of the former. Moreover, the estimation of the charge separation efficiency manifests that although these two types of interfaces have similar charge recombination rates, the PBDB-T/ITIC interface possesses the larger charge separation rates than those of the PBDB-T/PC71BM interface. Therefore, the better match between PBDB-T and ITIC together with a larger charge separation efficiency at the interface are considered to be the reasons for the prominent performance of ITIC in OSCs.

In contrast to the traditional view that the small organic molecules emit fluorescence, more and more experiments manifest their special luminescence types, such as the thermally activated delayed fluorescence (TADF) and roomtemperature phosphorescence. Why the similar organic molecules exhibit different luminescence types is focused on in this work on the basis of density functional theory/timedependent density functional theory calculations on a series of small organic molecules with phenoxazine or carbazole as a donor and diphenyl-triazine as an acceptor. The deep analysis of the geometrical and electronic structures shows how the structure, especially for the donor−acceptor dihedral angle, determines the singlet−triplet energy gap and the property of excited states. The explorations on the electron−hole pairs of natural transition orbitals and the contribution of the key heteroatom (N) to different molecular orbitals reveal the distinct electron transition processes of excitation to singlet and triplet states, explain the reason for different energy-level distributions of excited states, and identify which pairs have more favorable intersystem crossing for these molecules, while the calculations of spin−orbit coupling and reorganization energy display the efficiency of the different luminescence types. Meanwhile, considering the potential application of TADF materials in organic light-emitting diodes, we also separately modified the phosphorescent molecule and the prompt fluorescent molecule through the introduction of methyls to increase the steric hindrance and realize the perpendicular orientation of donor and acceptor unit, and finally to screen the excellent TADF molecules theoretically.

The case that aggregation has a large influence on the structure and fluorescent properties of 5-(4-(1,2,2-triphenylvinyl)phenyl)thiophene-2-carbaldehyde (P TA) is investigated in detail herein by employing quantum mechanics and molecular mechanics. Besides the isolated molecule, the aggregated molecule in water and in the crystalline state was studied by focusing on the comparison of photoelectronic properties, including the geometrical and electronic structures at ground and excited states, emission and internal conversation properties. For the aggregation state, the intermolecular interaction was used to explain the difference in structure, emission color and intensity of different polymorphs. The noticeable contribution from low-frequency region, corresponding to the four phenyl rings twisting vibration, to the Huang-Rhys factor and reorganization energy, as well as the possible potential energy surface crossing between S and S states for isolated molecules was considered as the reason of its aggregation-induced emission (AIE) performance. Importantly, the aggregation process in water simulated at the same time helps us to have a deeper understanding of the AIE behavior of P TA, which also provides another perspective to explore the AIE phenomenon in theory.