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Organic solar cells (OSCs) have made rapid advances in power conversion efficiency during the past decades, which is boosted partly by the various designs of new materials, especially in donor mate...

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 room-temperature phosphorescence. Why the similar organic molecules exhibit different luminescence types is focused on in this work on the basis of density functional theory/time-dependent 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 trip...