Molecular orientation is one of the most crucial factors to boost the efficiency of organic light-emitting devices. However, active control of molecular orientation of the emitter molecule by the host molecule is rarely realized so far, and the underlying mechanism is under discussion. Here, we systematically investigated the molecular orientations of thermally activated delayed fluorescence (TADF) emitters in a series of carbazole-based host materials. Enhanced horizontal orientation of the TADF emitters was achieved. The degree of enhancement observed was dependent on the host material used. Consequently, our results indicate that π-π stacking, CH/n (n = O, N) weak hydrogen bonds, and multiple CH/π contacts greatly induce horizontal orientation of the TADF emitters in addition to the molecular shape anisotropy. Finally, we fabricated TADF-based organic light-emitting devices with an external quantum efficiency (η ext) of 26% using an emission layer with horizontal orientation ratio () of 79%, which is higher than that of an almost randomly oriented emission layer with of 62% (η ext = 22%).

Pendant sulfone/phenoxazine-based homo- and copolymers were synthesized via the radical (co)­polymerization of 4-bromophenyl vinyl sulfide (BPVS) and styrene (St) followed by oxidation of the thus-produced sulfides, P­(BPVS) and P­(BPVS-r-St)­s, to the corresponding sulfones and Pd-catalyzed coupling with phenoxazine-substituted phenyl boronate ester (PXZPhBpin) to afford P­(PVSO2-PhPXZ) and P­(PVSO2-PhPXZ-r-St)­s, respectively. The prepared homo- and copolymers, featuring a π-conjugated phenylene unit between the electron-accepting sulfone (SO2) unit and the electron-donating phenyl phenoxazine (PhPXZ) unit in their side chains, exhibited thermally activated delayed fluorescence (TADF) and were used to fabricate organic light-emitting diodes (OLEDs). The device with spin-coated P­(PVSO2-PhPXZ) as an emitting layer showed a maximum external quantum efficiency (EQEmax) of 1.0%, whereas that with an emitting layer of spin-coated 75 mol %-St P­(PVSO2-PhPXZ-r-St) exhibited efficient bluish-green emission with EQEmax = 4.1%. The presented design strategy allows one to manipulate the polarity and electrochemical properties of pendant units as well as design molecular structures with suitable spacers between donor and acceptor units in the TADF side chain, thus helping to achieve the desired (opto)­electronic properties. Thus, this work significantly extends the range of nonconjugated TADF polymers and paves the way for the fabrication of next-generation OLEDs.

We applied lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) salt for electron injection layers (EILs) in solution-processed organic light-emitting devices (OLEDs). The application result indicates that the alone Li-TFSI EIL was insufficient. Meanwhile, zinc oxide (ZnO) nanoparticle (NP)/polyethyleneimine ethoxylated (PEIE)/Li-TFSI trilayer was a highly efficient EIL compared with conventional lithium (Li) 8-quinolate and ZnO NPs/PEIE EILs and we realized a low-driving voltage OLED. Possibly, Li cations were reduced to Li atoms by the thermally evaporated aluminum (Al) atoms with adjacent Al cathode, and Li atoms reduced the work function of the cathode, resulting in the driving voltage reduction of the OLED. In the triple EIL structures, Li-TFSI was changed to magnesium and silver salts. Among the three types of TFSI salts, Li-TFSI exhibited the most efficient electron injection property.