Hsiao-Fan Chen

Solid-state white light-emitting electrochemical cells (LECs) based on ionic transition metal complexes are capable of generating efficient white electroluminescence (EL). Recently, blue-emitting LECs combined with embedded red color conversion layers (CCLs), which were inserted between indium tin oxide (ITO) and a glass substrate, have been reported to offer an external quantum efficiency (EQE) >12%. However, the output spectrum of EL from such white LECs changed with time due to the altered microcavity effect when the recombination zone was moving. Furthermore, the device efficiency should be further improved to realize practical applications. In this work, TiO₂ nanoparticles (NPs) of two sizes are incorporated into red CCLs to improve device performance. Large NPs (250 nm) can scatter and redirect the light passing through red CCLs and further enhance the extracted light. Small NPs (25 nm) increase the refractive index of red CCLs and raise the amount of light intensity in the evanescent tail of the waveguide mode near the ITO layer. Therefore, more trapped light in the evanescent tail of the waveguide mode can be recycled to the external mode by scattering. The peak EQE and power efficiency of white LECs employing scattering red CCLs reach 20.0% and 39.5 lm W⁻¹, respectively. Furthermore, with scattering NPs to average the EL in different directions, white EL is almost invariant with time. These results show that blue-emitting LECs combined with scattering red CCLs would be effective to provide efficient and stable white EL.

The combination of both very high brightness and deep blue emission from phosphorescent organic light-emitting diodes (PHOLED) is required for both display and lighting applications, yet so far has not been reported. A source of this difficulty is the absence of electron/exciton blocking layers (EBL) that are compatible with the high triplet energy of the deep blue dopant and the high frontier orbital energies of hosts needed to transport charge. Here, we show that N-heterocyclic carbene (NHC) Ir(III) complexes can serve as both deep blue emitters and efficient hole-conducting EBLs. The NHC EBLs enable very high brightness (>7,800 cd m⁻²) operation, while achieving deep blue emission with colour coordinates of [0.16, 0.09], suitable for most demanding display applications. We find that both the facial and the meridional isomers of the dopant have high efficiencies that arise from the unusual properties of the NHC ligand—that is, the complexes possess a strong metal–ligand bond that destabilizes the non-radiative metal-centred ligand-field states. Our results represent an advance in blue-emitting PHOLED architectures and materials combinations that meet the requirements of many critical illumination applications.

Transient electron paramagnetic resonance (TREPR) spectroscopy has been used to study the spin–spin interactions in a novel U-shaped electron donor–chromophore–acceptor–radical (D–C–A–R^•) system in which a xanthene bridge holds a tert-butylphenyl nitroxide (BPNO^•) radical in close proximity to a naphthalene-1,8:4,5-bis(dicarboximide) (NDI) acceptor. Photoexcitation of the 4-aminonaphthalene-1,8-dicarboximide (ANI) chromophore results in rapid, two-step electron transfer to generate the triradical (D^+•–C–A^–•–R^•). The large through-bond distance between A^–• and R^• makes their spin–spin exchange interaction (2J_AR) negligibly small, whereas their short through-space distance results in a strong dipolar interaction (D_AR), which is observed as a set of broad lines in the TREPR spectra of D^+•–C–A^–•–R^• in solid toluene solution at 85 K. Transient nutation experiments show that these transitions belong to a species with spin S = 1, whereas experiments on D^+•–C–A^–•–R^• in the oriented nematic liquid crystal 4-cyano-4′-n-pentylbiphenyl at 85 K demonstrate the anisotropy of D_AR.

Solid-state white light-emitting electrochemical cells (LECs) have attracted research attention owing to their advantages of simple device structure, low operation voltage and compatibility with solution processes. In this work, we demonstrate a simple approach to obtain white electroluminescence (EL) from non-doped LECs based on a blue-emitting complex. With a relatively thicker emissive layer, red emission can be additionally enhanced by the microcavity effect when the recombination zone moves to appropriate positions. Hence, white EL can be harvested by combining blue emission from the complex and red emission from the microcavity effect. These non-doped white LECs show external quantum efficiencies and power efficiencies up to 5% and 12 lm W⁻¹, respectively. These results show that efficient white EL can be obtained in simple non-doped LECs.

Ordered multi-spin assemblies are required for developing solid-state molecule-based spintronics. A linear donor–chromophore–acceptor (D–C–A) molecule was covalently attached inside the 150 nm diam. nanopores of an anodic aluminum oxide (AAO) membrane. Photoexcitation of D–C–A in a 343 mT magnetic field results in sub-nanosecond, two-step electron transfer to yield the spin-correlated radical ion pair (SCRP) ¹(D⁺˙–C–A⁻˙), which then undergoes radical pair intersystem crossing (RP-ISC) to yield ³(D⁺˙–C–A⁻˙). RP-ISC results in S–T₀ mixing to selectively populate the coherent superposition states |S′〉 and |T′〉. Microwave-induced transitions between these states and the unpopulated |T₊₁〉 and |T₋₁〉 states result in spin-polarized time-resolved EPR (TREPR) spectra. The dependence of the electron spin polarization (ESP) phase of the TREPR spectra on the orientation of the AAO membrane pores relative to the externally applied magnetic field is used to determine the overall orientation of the SCRPs within the pores at room temperature.

Highly efficient exciplex emission in solid-state light-emitting electrochemical cells (LECs) was achieved by mixing two ionic fluorescent emitters with opposite bipolarity. A triarylamine derivative as a hole-transport material (HTM) and a 1,3,5-triazine derivative as an electron-transport material (ETM) were selected due to their superior carrier mobilities. The ionic character of HTM and ETM was achieved by chemically attaching three pendant methylimidazolium groups as peripheries, rendering them amenable to LEC applications. Strong exciplex green emission can be observed either from photoluminescence in solid states or electroluminescence in LEC devices when HTM and ETM are mixed. By optimizing the mixing ratio, the external quantum efficiency and power efficiency can be achieved up to 3.04% and 10.29 lm W⁻¹, respectively. These results are among the highest reported for fluorescent LECs.