Theoretical study of the influence of fluorination/chlorination on the electronic properties of donor:acceptor complexes.

In this study, the electronic properties of J50:N2200 (benzodithiophene-alt-benzotriazole: NDI-bithiophene) interface before and after fluorination/chlorination were investigated based on the first-principles density functional theory (DFT). The results reveal that the donor (D) and acceptor (A) molecules exhibit direct band gap whether to be fluorinated/chlorinated or not, and the six D:A pairs constructed all display indirect band gap. Next, for the fluorinated/chlorinated D molecule J50, the slope of total density of states (TDOS) curve edge at the highest occupied molecular orbital (HOMO) energy level enlarges, indicating high electron locality; the fluorination/chlorination of the A molecule N2200 reduces the slope of the TDOS at the HOMO level, and the electron delocalization strengthens. Then, the difference ΔE1 of the lowest unoccupied molecular orbital (LUMO) levels between D and A, the difference ΔE2 of HOMO levels between D and A, and the difference ΔE3 between the HOMO level of the D and the LUMO level of the A were calculated about the D:A complexes. The consequences present that by using fluorine/chlorine (F/Cl) substitution at J50, ΔE1 and ΔE2 both decrease, and ΔE3 increases; for N2200, both ΔE1 and ΔE2 increase, and ΔE3 decreases. Since the higher open circuit voltage (VOC) is directly proportional to ΔE3, again ΔE1 and ΔE2 afford the driving force for charge transport, these expose that the fluorination/chlorination of J50 is beneficial to obtain the higherVOC, meanwhile, the F/Cl replacement in N2200 facilitates the separation of excitons. In addition, by the Bader charge analysis, the F/Cl substitution at D in D:A blends will promote the intramolecular charge transfer and enhance the molecular polarity; moreover, the substitution at A will improve the intermolecular charge transfer and the dipole electric field may be enhanced. Finally, the details also depend on the type of element and the position of substitution.

Highly efficient non-doped blue fluorescent OLEDs with low efficiency roll-off based on hybridized local and charge transfer excited state emitters.

Designing a donor-acceptor (D-A) molecule with a hybridized local and charge transfer (HLCT) excited state is a very effective strategy for producing an organic light-emitting diode (OLED) with a high exciton utilization efficiency and external quantum efficiency. Herein, a novel twisting D-π-A fluorescent molecule (triphenylamine-anthracene-phenanthroimidazole; TPAAnPI) is designed and synthesized. The excited state properties of the TPAAnPI investigated through photophysical experiments and density functional theory (DFT) analysis reveal that its fluorescence is due to the HLCT excited state. The optimized non-doped blue OLED using TPAAnPI as a light-emitting layer exhibits a novel blue emission with an electroluminescence (EL) peak at 470 nm, corresponding to the Commission International de L'Eclairage (CIE) coordinates of (0.15, 0.22). A fabricated device termed Device II exhibits a maximum current efficiency of 18.09 cd A-1, power efficiency of 12.35 lm W-1, luminescence of ≈29 900 cd cm-2, and external quantum efficiency (EQE) of 11.47%, corresponding to a high exciton utilization efficiency of 91%. Its EQE remains as high as 9.70% at a luminescence of 1000 cd m-2 with a low efficiency roll-off of 15%. These results are among the best for HLCT blue-emitting materials involved in non-doped blue fluorescent OLEDs. The performance of Device II highlights a great industrial application potential for the TPAAnPI molecule.