Yi-Hsiang Huang

A diarylborane-substituted carbazole as a universal bipolar host material for highly efficient electrophosphorescence devices

Recently bipolar phosphorescent host materials have attracted wide attention since they can achieve better charge balance and hence better device performance. In this work, we report the synthesis and physical properties of a novel bipolar host material containing the dimesityl borane/carbazole hybrid, CMesB. With a high triplet energy, CMesB is considered a promising universal host material and has been applied to phosphorescent OLEDs of various colors. Red/green/blue/white (RGBW) OLEDs based on CMesB all show high external quantum efficiencies (20.7% for red, 20.0% for green, 16.5% for blue, and 15.7% for white) at practical brightnesses. The results indicate that the bipolar host CMesB with high triplet energy has high potential in manufacturing RGBW OLEDs for display or lighting applications.

Incorporation of a CN group into mCP: a new bipolar host material for highly efficient blue and white electrophosphorescent devices

A novel bipolar host material mCPCN has been designed and synthesized by incorporating the electron-accepting CN group into the well-known benchmark host material mCP. Compared to mCP, the incorporation of the simple and small CN group significantly improves thermal/morphological stabilities (Tg = 97 °C and Td = 313 °C) and increases the electron affinity, while keeping electronic transition energies unaltered and maintaining a high triplet energy (ET = 3.03 eV). Characteristics of single-carrier devices containing mCPCN indicate its rather balanced hole/electron injection and transport properties. Highly efficient blue phosphorescent organic light emitting devices (PhOLEDs) with maximum external quantum, current and power efficiencies of 26.4%, 58.6 cd A−1, and 57.6 lm W−1, respectively, were achieved using mCPCN as the bipolar host material and bis[(4′,6′-difluorophenyl)pyridinato-N,C2′]iridium(III) picolinate (FIrpic; ET = 2.65 eV) as the triplet emitter. Furthermore, blue PhOLEDs adopting mCPCN exhibit impressively low efficiency roll-offs, retaining high quantum efficiencies of ∼25% at 1000 cd m−2 and ∼20% even at 8000 cd m−2. mCPCN has also been successfully used in implementing highly efficient white PhOLEDs having external quantum efficiencies in excess of 23%.

Evaluation of propylene-, meta-, and para-linked triazine and tert-butyltriphenylamine as bipolar hosts for phosphorescent organic light-emitting diodes

Three representative bipolar hybrids – tBu-TPA-p-TRZ, tBu-TPA-m-TRZ, and tBu-TPA--TRZ with triplet energies, ET = 2.5, 2.7 and 3.0 eV, respectively – were synthesized and characterized for a comprehensive evaluation of their potential as a host for phosphorescent organic light-emitting diodes (PhOLEDs) using red-emitting Ir(piq)3 as the dopant with an ET value of 2.1 eV. Formation of charge transfer complexes, CTCs, was diagnosed by fluorescence bathochromism in increasingly polar solvents. Both intra- and inter-molecular charge transfer processes are invoked to explain CTC formation in all three hybrids. The ppp-hybrid is by far the most susceptible to CTC formation both in solution and neat solid film, resulting in PhOLEDs with reduced external quantum efficiency, EQE, despite the best balance between charge fluxes across the emitting layer, EML, as revealed by the electron- and hole-only devices in addition to PhOLEDs containing a sensing layer. The highest EQE is achieved with the mm-hybrid thanks to the compromise between balanced charge fluxes and CTC formation. The -hybrid is the least prone to CTC formation while suffering charge flux imbalance to yield an EQE intermediate between those of the mm- and ppp-hybrids. The least CTC formation involving the -hybrid is advantageous in accommodating the most singlets and triplets readily transferrable to both red and blue phosphors on account of its relatively high ET value. Furthermore, the -hybrid offers the best morphological stability of the desired glassy EML, thus holding promise for the fabrication of superior PhOLEDs overall.

Organic light-emitting devices integrated with internal scattering layers for enhancing optical out-coupling

Abstract— In this work, studies on organic light-emitting devices (OLEDs) incorporating nanoparticle-based nanocomposite scattering layers as the internal extraction structures (i.e., between the substrate and the OLED structure) were concluded. By adjusting the nanoparticle sizes and concentrations, the optical properties (optical scattering and integrated transmittance) of the nanocomposite scattering layers can be widely tuned. With appropriate compositions, nanoparticle-based nanocomposite scattering layers with strong optical scattering, high integrated transmittance, and yet enough flatness for device integration can be obtained. The use of such nanocomposite internal scattering layers provides a convenient and effective approach for simultaneously achieving large efficiency enhancement (1.96× for quantum efficiency and 2.04× for cd/A efficiency) and improving viewing characteristics (more stable colors and emission patterns over angles). The fabrication of nanoparticle-based nanocomposite scattering films is based on solution-processing, which is relatively simple and convenient. These features may make it highly attractive for various OLED applications and mass production.

Enhancing light out-coupling of organic light-emitting devices using indium tin oxide-free low-index transparent electrodes

With its increasing and sufficient conductivity, the conducting polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) has been capable of replacing the widely used but less cost-effective indium tin oxides (ITOs) as alternative transparent electrodes for organic light-emitting devices (OLEDs). Intriguingly, PEDOT:PSS also possesses an optical refractive index significantly lower than those of ITO and typical organic layers in OLEDs and well matching those of typical OLED substrates. Optical simulation reveals that by replacing ITO with such a low-index transparent electrode, the guided modes trapped within the organic/ITO layers in conventional OLEDs can be substantially suppressed, leading to more light coupled into the substrate than the conventional ITO device. By applying light out-coupling structures onto outer surfaces of substrates to effectively extract radiation into substrates, OLEDs using such low-index transparent electrodes achieve enhanced optical out-coupling and external quantum efficiencies in comparison with conventional OLEDs using ITO.

Unlocking the Full Potential of Conducting Polymers for High‐Efficiency Organic Light‐Emitting Devices

By carefully tuning the thicknesses of low-optical index PEDOT:PSS and high-index ITO layers in organic light-emitting devices (OLEDs), very high optical coupling efficiencies can be obtained through the generation of appropriate microcavity effects. These experiments result in an external quantum efficiency (EQE) of 33.7% for green phosphorescent OLEDs and even higher EQEs of 54.3% can be obtained by adopting an external out-coupling lens.

Highly efficient tandem organic light-emitting devices utilizing the connecting structure based on n-doped electron-transport layer/HATCN/hole-transport layer

In this work, we conducted studies of tandem organic light-emitting devices (OLEDs) based on the connecting structure consisting of n-doped electron-transport layer (n-ETL)/1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HATCN)/hole-transport layer. We investigated effects of different n-ETL materials and different HATCN thicknesses on characteristics of tandem OLEDs. Results show that the tandem OLEDs with n-BPhen and a 20 nm layer of HATCN in the connecting structure exhibited the best performance. With these, highly efficient and bright green phosphorescent two-emitting-unit tandem OLEDs, with drive voltages significantly lower than twice that of the single-unit benchmark device and current efficiencies higher than twice that of the single-unit benchmark device, were demonstrated.

White Organic Light-Emitting Diode With Linearly Polarized Emission

An organic light emitting diode (OLED) with a linearly polarized white light emission is demonstrated using a nanograting structure. The aluminum based grating structure is fabricated by laser interference lithography and formed on the back side of the glass substrate of the OLED. The nanograting structure functions as a polarizer to select the transverse magnetic wave in the wavelength range of 400-700 nm. The polarization characteristics are studied experimentally and theoretically in detail. The experimental results agree well with the simulation by a rigorous coupled wave analysis. The polarization ratio of the polarizer can reach as high as 93.4%.

Plasmonic ITO-free polymer solar cell

The aluminum and sliver multilayered nano-grating structure is fabricated by laser interference lithography and the intervals between nanoslits is filled with modified PEDOT:PSS. The grating structured transparent electrode functions as the anti-reflection layer which not only decreases the reflected light but also increases the absorption of the active layer. The performances of P3HT:PC₆₁BM solar cells are studied experimentally and theoretically in detail. The field intensities of the transverse magnetic (TM) and transverse electrical (TE) waves distributed in the active layer are simulated by rigorous coupled wave analysis (RCWA). The power conversion efficiency of the plasmonic ITO-free polymer solar cell can reach 3.64% which is higher than ITO based polymer solar cell with efficiency of 3.45%.

Design and fabrication of birefringent nano-grating structure for circularly polarized light emission

Three different nano-grating structures are designed as phase retarders that can transform linearly polarized light to circularly polarized emission for the wavelengths of 488 nm, 532 nm and 632.8 nm, respectively. Gold based nano-grating structures with various periods are fabricated by utilizing laser interference lithography. The ellipticity of all circularly polarized emission can reach around 90% such that the structure has great potential in the applications of three-dimensional (3D) display. The effects of the slit width and metal thickness modulations are simulated by rigorous coupled wave analysis (RCWA) method. Besides, the field intensity and phase of the transmitted TM and TE waves are also simulated to understand their polarization characteristics.