Ting-Yi Cho

Microcavity two-unit tandem organic light-emitting devices having a high efficiency

In pursuit of further enhancement in luminance and efficiency of organic light-emitting devices (OLEDs), it is worth exploring what benefits could be obtained by combining two luminance-enhancement techniques, i.e., microcavity and tandem OLEDs. In this letter, we have investigated theoretically and experimentally the characteristics of noncavity and microcavity tandem OLEDs. Results show that with well designed microcavity and device structures (i.e., consistent with resonant and antinode conditions), a fivefold enhancement in luminance can be achieved with cavity tandem devices having only two emitting units. A very high efficiency of 200cd∕A has been demonstrated with a phosphorescent cavity two-unit device.

Enhancing light outcoupling of organic light-emitting devices by locating emitters around the second antinode of the reflective metal electrode

Due to generally low conductivity and low carrier mobilities of organic materials, organic light-emitting devices (OLEDs) are typically optimized for light outcoupling by locating emitters around the first antinode of the metal electrode. In this letter, by utilizing device structures containing conductive doping, we investigate theoretically and experimentally the influences of the location of emitters relative to the metal electrode on OLED emission, and show that substantial enhancement in light outcoupling (1.2 times) or forward luminance (1.6 times) could be obtained by placing emitters around the second antinode instead of the first antinode. Depending on the detailed condition, the second-antinode device may also give more directed emission as often observed in strong-microcavity devices yet without suffering a color shift with viewing angles.

Highly bright blue organic light-emitting devices using spirobifluorene-cored conjugated compounds

An efficient and morphologically stable pyrimidine-containing spirobifluorene-cored oligoaryl, 2,7-bis[2-(4-tert-butylphenyl)pyrimidine-5-yl]-9,9′-spirobifluorene (TBPSF), as an emitter or a host for blue organic light-emitting devices (OLEDs), is reported. The steric hindrance inherent with the molecular structure renders the material a record-high neat-film photoluminescence (PL) quantum yield of 80% as a pure blue emitter (PL peak at 430 nm) of low molecular weight, and a very high glass-transition temperature (Tg) of 195 °C. Blue OLEDs employing this compound as the emitter or the emitting host exhibit unusual endurance for high currents over 5000 mA/cm2. When TBPSF is used as a host for perylene in a blue OLED, maximal brightness of ∼80 000 cd/m2 had been achieved, representing the highest values reported for blue OLEDs under dc driving.

Microcavity top-emitting organic light-emitting devices integrated with microlens arrays: Simultaneous enhancement of quantum efficiency, cd/A efficiency, color performances, and image resolution

A long bothering issue in microcavity organic light-emitting devices (OLEDs) is the difficulty to simultaneously achieve enhanced cd/A efficiency, enhanced external quantum efficiency, enhanced color saturation, and stable colors with viewing angles in the same device design. In this work, we show that microcavity top-emitting OLEDs integrated with microlenses may provide a universal approach for simultaneously achieving all these desired nice characteristics. Furthermore, the pixel blurring often occurring in employment of microlenses to conventional bottom-emitting OLEDs is significantly suppressed by combination of top-emitting microcavity OLEDs and microlenses.

Organic light-emitting devices integrated with solar cells: High contrast and energy recycling

In this letter, the authors report that by integrating organic light-emitting devices (OLEDs) with solar cells, luminous ambient-light reflection as low as 1.4% (even superior to that achieved with polarizers) can be achieved without compromising the electroluminescence efficiency for high-contrast display applications. Furthermore, in such a configuration, the photon energies of the incident ambient light and the portion of OLED emission not getting outside of the device can be recycled into useful electrical power via the photovoltaic action, instead of being totally wasted as in other reported contrast-enhancement techniques. These features, the authors believe, shall make this technique attractive for high-contrast display applications and portable/mobile electronics that are highly power aware.

Fuzzy-junction organic light-emitting devices

A “fuzzy-junction” organic light-emitting device (OLED) containing a graded organic–organic interface is reported. Such graded junction is effectively produced utilizing interdiffusion through an ultrathin interfacial fusing layer sandwiched between two functional layers. With a glass transition temperature (Tg) lower than remaining layers, this fusing layer permits smooth interdiffusion and mixing of neighboring layers by annealing above its Tg. With appropriate material combinations, fuzzy-junction OLEDs thus prepared exhibit both reduced voltage and enhanced emission efficiency in comparison with conventional abrupt-junction devices. As an instance, a green fluorescent OLED with such fuzzy junction shows a high peak power efficiency of ∼20 lm/W, substantially higher than ∼14 lm/W of a corresponding abrupt-junction device.

Energy-recycling pixel for active-matrix organic light-emitting diode display

The authors report a pixel structure for active-matrix organic light-emitting diode (OLED) displays that has a hydrogenated amorphous silicon solar cell inserted between the driving polycrystalline Si thin-film transistor and the pixel OLED. Such an active-matrix OLED pixel structure not only exhibits a reduced reflection (and thus improved contrast) compared to conventional OLEDs but also is capable of recycling both incident photon energies and internally generated OLED radiation. Such a feature of energy recycling may be of use for portable/mobile electronics, which are particularly power aware.

Three-color reconfigurable organic light-emitting devices

This report reveals utilization of phase transitions and corresponding changes in physical properties of organic semiconductors for implementing reconfigurable organic optoelectronic devices, i.e., a device whose configurations and characteristics can be programed after fabrication. Specifically, glass transitions of amorphous molecular materials have been exploited to demonstrate reconfigurable organic light-emitting devices (OLEDs) capable of generating any of the three primary colors. The capability to pattern such devices into fine color pixels with thermal imaging renders it attractive for applications in high-resolution full-color OLED displays and as active optical memory devices.

Energy‐recycling high‐contrast organic light‐emitting devices

— It is reported that by integrating OLEDs with solar cells, ambient-light reflection as low as 1.4% (even superior to that achieved with polarizers) can be achieved without compromising the EL efficiency for high-contrast display applications. Furthermore, in such a configuration, the photon energies of both the incident ambient light and the portion of OLED emission not getting outside of the device can be recycled into useful electrical power via the photovoltaic action, instead of being wasted as in other reported contrast-enhancement techniques. These features, we believe, shall make this present technique attractive for high-contrast display applications and portable/mobile electronics that are highly power-aware.

Optical characteristics of tandem and microcavity tandem organic light-emitting devices

In pursuit of the further enhancement of the luminance and efficiency of organic light- emitting devices (OLEDs), it is worthy of exploring what benefits could be obtained by combining two luminance-enhancement techniques, i.e., microcavity and tandem OLEDs. Furthermore, a deeper understanding of the optics in tandem OLEDs will be useful for the design and optimization of tandem OLEDs. In this paper, the optical characteristics of noncavity and microcavity tandem OLEDs are theo- retically and experimentally investigated. By the use of rigorous electromagnetic modeling of OLEDs, the radiation characteristics of tandem OLEDs as a function of device structures are analyzed and correspondingly, the guidelines for optimizing the performance of tandem devices are suggested. By making use of the analytical results, it is shown that with well-designed microcavity conditions and device structures, a five-fold enhancement in luminance in the normal direction can be achieved with cavity-tandem devices having only two emitting units. A very high efficiency of 200 cd/A for a rather broad brightness range of 100-4000 nits is demonstrated with a phosphorescent cavity two-unit device.