The thermally activated delayed fluorescence (TADF) mechanism has recently attracted significant interest in the field of organic light-emitting diodes (OLEDs). TADF relies on the presence of a very small energy gap between the lowest singlet and triplet excited states. Here, we demonstrate that time-dependent density functional theory in the Tamm-Dancoff approximation can be very successful in calculations of the lowest singlet and triplet excitation energies and the corresponding singlet-triplet gap when using nonempirically tuned range-separated functionals. Such functionals provide very good estimates in a series of 17 molecules used in TADF-based OLED devices with mean absolute deviations of 0.15 eV for the vertical singlet excitation energies and 0.09 eV [0.07 eV] for the adiabatic [vertical] singlet-triplet energy gaps as well as low relative errors and high correlation coefficients compared to the corresponding experimental values. They significantly outperform conventional functionals, a feature which is rationalized on the basis of the amount of exact-exchange included and the delocalization error. The present work provides a reliable theoretical tool for the prediction and development of novel TADF-based materials with low singlet-triplet energetic splittings.

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flexibility, and tunable optical transparency, which are advantageous for applications in building-integrated photovoltaic (PV) and wearable electronics compared with conventional inorganic cells. [1][2][3][4][5] To achieve high efficiency, it is essential to develop suitable donor (D) and acceptor (A) materials that can be easily processed into a finely phase-separated morphology in active layer with low energy loss (E loss ) in OSCs. [6][7][8][9] Recently, the vibrant progress made on nonfullerene acceptors (NFAs), [10,11] especially the Y-series NFAs, [12] with facilely tailored chemical structures and precisely tuned bandgap, energy levels, and crystallization properties, has enabled OSCs to achieve very high power conversion efficiencies (PCEs) ≈19%. [13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32] The bulk-heterojunction (BHJ) architecture-based devices prepared from mixing D and A in solution is the most widely adopted method for depositing photoactive layer and have successfully increased the PCE of single-junction OSCs to 19% recently. [31] Another emerging strategy is to deposit neat D and A materials A record power conversion efficiency (PCE) of over 19% is realized in planarmixed heterojunction (PMHJ) organic solar cells (OSCs) by adopting the asymmetric selenium substitution strategy in making a pseudosymmetric electron acceptor, BS3TSe-4F. The combined molecular asymmetry with more polarizable selenium substitution increases the dielectric constant of the D18/ BS3TSe-4F blend, helping lower the exciton binding energy. On the other hand, dimer packing in BS3TSe-4F is facilitated to enable free charge generation, helping more efficient exciton dissociation and lowering the radiative recombination loss (ΔE 2 ) of OSCs. As a result, PMHJ OSCs based on D18/BS3TSe-4F achieve a PCE of 18.48%. By incorporating another mid-bandgap acceptor Y6-O into D18/BS3TSe-4F to form a ternary PMHJ, a higher open-circuit voltage (V OC ) can be achieved to realize an impressive PCE of 19.03%. The findings of using pseudosymmetric electron acceptors in enhancing device efficiency provides an effective way to develop highly efficient acceptor materials for OSCs.

Figure 1. a) The synthetic routes of IC-Br(in) and IC-Br(out). b) Chemical structures of PY-IT, PY-OT and PY-IOT.

The combination of rigid acridine donor and 1,8-naphthalimide acceptor has afforded two orange-red emitters of NAI-DMAC and NAI-DPAC with high rigidity in molecular structure and strongly pretwisted charge transfer state. Endowed with high photoluminescence quantum yields (Φ ), distinct thermally activated delayed fluorescence (TADF) characteristics, and preferentially horizontal emitting dipole orientations, these emitters afford record-high orange-red TADF organic light-emitting diodes (OLEDs) with external quantum efficiencies of up to 21-29.2%, significantly surpassing all previously reported orange-to-red TADF OLEDs. Notably, the influence of microcavity effect is verified to support the record-high efficiency. This finding relaxes the usually stringent material requirements for effective TADF emitters by comprising smaller radiative transition rates and less than ideal Φ s.