Versatile hole injection of VO2: Energy level alignment at N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine/VO2/fluorine-doped tin oxide

Energy level alignments at the interface of N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB)/VO₂/fluorine-doped tin oxide (FTO) were studied by photoemission spectroscopy. The overall hole injection barrier between FTO and NPB was reduced from 1.38 to 0.59 eV with the insertion of a VO₂ hole injection layer. This could allow direct hole injection from FTO to NPB through a shallow valence band of VO₂. Surprisingly, VO₂ can also act as a charge generation layer due to its small band gap of 0.80 eV. That is, its conduction band is quite close to the Fermi level, and thus electrons can be extracted from the highest occupied molecular orbital (HOMO) of NPB, which is equivalent to hole injection into the NPB HOMO.     

Chiral Thermally Activated Delayed Fluorescence Materials Based on R/S-N2,N2′-Diphenyl-[1,1′-binaphthalene]-2,2′-diamine Donor with Narrow Emission Spectra for Highly Efficient Circularly Polarized Electroluminescence

In this study, two pairs of chiral thermally activated delayed fluorescence (TADF) materials enabling circularly polarized luminescence (CPL) named R/S-p- BAMCN (rod-shape) and R/S-o- BAMCN (helix-shape) are prepared based on a new pair of chiral donor (D*), R/S-N²,N²′-diphenyl-[1,1′-binaphthalene]-2,2′-diamine (R/S- BAM ), and two cyano (CN) acceptors. Due to the rigid molecular structure and special intramolecular arrangement, the chiral TADF materials show high photoluminescence quantum efficiency (up to 0.86) with narrow full-width at half-maximum (38 nm in cyclohexane, 51 nm in toluene) and photoluminescence dissymmetry factor (|g_PL|) up to 5.3 × 10⁻³. Particularly, the circularly polarized OLEDs (CP-OLEDs) with rod-shaped R/S-p- BAMCN as the emitter show high device performances with the maximum external quantum efficiency nearing 28%. Meanwhile, the semi-transparent CP-OLEDs based on helix-shaped R/S-o- BAMCN exhibit obvious circularly polarized electroluminescence (CPEL) properties with the electroluminescence |g_EL| factors around 4.6 × 10⁻³. The strategy of rigid D*-A-D* structure with special arrangement of chiral donor provides a direct way to obtain efficient CP-TADF materials with narrow emission spectra and promising CPL properties for better CPEL performance.