Efficient organic emitters in the deep‐red region are rare due to the “energy gap law”. Herein, multiple boron (B)‐ and nitrogen (N)‐atoms embedded polycyclic heteroaromatics featuring hybridized π‐bonding/ non‐bonding molecular orbitals are constructed, providing a way to overcome the above luminescent boundary. The introduction of B‐phenyl‐B and N‐phenyl‐N structures enhances the electronic coupling of those para‐positioned atoms, forming restricted π‐bonds on the phenyl‐core for delocalized excited states and thus a narrow energy gap. The mutually ortho‐positioned B‐ and N‐atoms also induce a multi‐resonance effect on the peripheral skeleton for the non‐bonding orbitals, creating shallow potential energy surfaces to eliminate the high‐frequency vibrational quenching. The corresponding deep‐red emitters with peaks at 662 and 692 nm exhibit narrow full‐width at half‐maximums of 38 nm, high radiative decay rates of ca. 108 s−1, ≈100 % photo‐luminescence quantum yields and record‐high maximum external quantum efficiencies of ca. 28 % in a normal planar organic light‐emitting diode structure, simultaneously.

The rapid development of low-bandgap (LBG) nonfullerene acceptors and wide-bandgap (WBG) copolymer donors in recent years has boosted the power conversion efficiency (PCE) of organic solar cells (OSCs) to the 18% level [1−21] . The commercialization of OSCs is highly expected. However, critical issues like the cost and the stability also determine whether OSCs can enter the market or not [22] . Active materials, i.e. donors and acceptors, are the key materials determining the performance and cost of OSCs [23] . Nowadays, the state-of-the-art donors and acceptors like D18 [4] , PM6 [24] , Y6 [3] , IT-4F [25] and CO i 8DFIC [11] generally contain fluorine atoms, presenting high synthesis cost. Replacing fluorine with chlorine to make chlorinated donors or acceptors is an effective strategy to lower the cost while maintain the high efficiency for organic solar cells [26] . In the past few years, remarkable progress has been made in Cl-containing donors. In 2014, Wang et al. reported a chlorinated phenazine copolymer PCTClP with a low bandgap and a deep HOMO level [27] . Solar cells based on PCTClP and a fullerene acceptor PC 71 BM gave a PCE of 4.06%. In 2015, Pei et al. designed a chlorinated isoindigo copolymer Cl-IIDT [28] . Thanks to the chlorination, Cl-IIDT shows reduced crystallinity and a preferred faceon orientation, delivering a PCE of 4.60% in fullerene-based solar cells. In 2017, He et al. used monochlorinated benzothiadiazole unit as the building block to construct an asymmetric copolymer donor PBDTHD-ClBTDD [29] . The PBDTHD-ClBT-DD:PC 71 BM cells afforded decent PCEs up to 9.11%. In the same year, Peng et al. developed an efficient small molecular

The establishment of simple molecular design strategy to realize red-shifted emission while maintaining good color purity for multi-resonance induced thermally activated delayed fluorescent (MR-TADF) materials remains an appealing yet challenging...

High-triplet-energy hosts with favorable carrier injection/transporting abilities are realized, endowing efficient blue TADF devices with record-low voltages.

Orange-emitting phosphorescent organic light-emitting diodes (PHOLEDs) are drawing more and more attention; however, high-performance hosts designed for orange PHOLEDs are rare. Here, four indolocarbazole/1, 3, 5-triazine hybrids are synthesized to optimize the singlet and triplet energies, as well as transporting properties, for ideal orange PHOLEDs. By introducing moieties with different electronegativity, a graded reduction of the singlet and triplet energies is achieved, resulting in minimum injection barrier and minimum energy loss. Besides, the charge transporting abilities are also tuned to be balanced on the basis of the bipolar features of those materials. The optimized orange PHOLED shows a maximum external quantum effi ciency (EQE) of 24.5% and a power effi ciency of 64 lm W -1 , both of which are among the best values for orange PHOLEDs. What is more, the effi ciency roll-off is extremely small, with an EQE of 24.4% at 1000 cd m -2 and 23.8% at 10 000 cd m -2 , respectively, which is the lowest effi ciency roll-off for orange PHOLEDs to date, resulting in the highest EQE for orange PHOLEDs when the luminance is above 1000 cd m -2 . Besides the balanced charges, the small roll-off is also attributed to the wide recombination zone resulting from the bipolar features of the hosts.