Meng-Ting Lee

Suppression of efficiency roll off in blue phosphorescent organic light-emitting devices using double emission layers with additional carrier-transporting material

By employing double emission layers (DELs) into blue phosphorescent organic light-emitting device (PHOLED), the device shows improved current efficiency by a factor of 1.8 as compared with that of device using single emission layer. Furthermore, by doping additional carrier-transporting material into DELs, the device shows only a slight efficiency roll off of 24% from low (1u2002mA/cm2 and 400u2002cd/m2) to high current density (40u2002mA/cm2 and 10u2009000u2002cd/m2). The dramatic improvement in device performances can be attributed to the formation of a broader carrier recombination zone and the elimination of carrier accumulation at the interface. A blue PHOLED with a current efficiency of 29.5 cd/A, a power efficiency of 21 lm/W, and a low driving voltage of 4.4 V with a Commission Internationale deL’Eclairage (CIEx,y) of (0.16, 0.35) at a practical brightness of 1000u2002cd/m2 can be achieved.

Abnormal hole mobility of biaxial strained Si

The strain effect on the hole mobility is investigated by bulk Si field-effect transistor, substrate-strained Si devices, and these devices under biaxial tensile mechanical strain. The hole mobility along ⟨110⟩ direction on (001) Si substrate degrades at small biaxial tensile strain (<∼0.3%) but enhances at the biaxial tensile strain larger than ∼0.3%. This abnormal behavior can be understood in terms of the effective hole conductive mass which is the population average of heavy-hole and light-hole masses. The effective mass is more heavy-hole-like at small strain, since the heavy-hole band has a larger density of state than light-hole band. As the biaxial tensile strain increases, the hole population in the light-hole band increases due to the upshift and crossover of the light-hole band above the heavy-hole band. Therefore, the effective mass with larger biaxial tensile strain decreases significantly due to the small mass of light hole. The effective hole mass, which increases at small strain, then decr...

Improvement in carrier transport and recombination of white phosphorescent organic light-emitting devices using a composite blue emitter

We demonstrate high-efficiency white phosphorescent organic light-emitting devices (PHOLEDs) based on a yellow and composite blue emitters. The composite blue emitter is constructed from a wide-band-gap host, organometallic iridium dopant, and a carrier-transporting material. Under the same driving current density and emissive color, the current efficiency of the white PHOLEDs can be enhanced by a factor of 1.4 comparing to that of using typical blue emitter composed of host and dopant only. Attaching an outcoupling enhancement film onto glass substrate, the white PHOLEDs with a current efficiency of 47 cd/A, a power efficiency of 32 lm/W, and a CIEx,y (CIE: Commission Internationale dEclairage) of (0.40,0.44) at a practical brightness of 1000u2002cd/m2 can be achieved.

Low-voltage, high-efficiency blue phosphorescent organic light-emitting devices

Low-voltage, high-efficiency blue phosphorescent organic light-emitting devices based on a composite emitter, including a wide-band-gap host, a carrier-transporting material, and an organometallic iridium dopant, have been demonstrated. The devices exhibit an external quantum efficiency of 12%, a power efficiency of 17lm∕W and a low voltage of 4.8V at a practical brightness of 1000cd∕m2 with a CIEx,y of (0.16, 0.35), which was twofold higher than that of using the typical emitter composed of host and dopant only. The dramatic enhancement in performance can be attributed to the transport of carriers into the wide-band-gap host, which can be promoted through doping a carrier-transporting material in emitter for increasing carrier recombination.

Promising a-Si:H TFTs with High Mechanical Reliability for Flexible Display

The high mechanical reliability of a-Si:H TFTs have been fabricated on plastic substrate for flexible display applications. The promising TFT backplane has been successfully applied for AMLCD on colorless polyimide (PI) substrate. The tri-layer of Ti/Al/Ti with 10 mum width are used as scan lines and data lines to replace Cr for stress compensation, and can sustain mechanical bending cycles. The TFTs at 200degC on PI substrate have superior stability with external strain and bending cycles. The redistribution of trap states is analyzed by modeling simulation. The promising a-Si:H TFTs on PI substrate after bending cycles still have superior operation and electrical stress stability and make it possible for AMOLED applications. The Si-based TFTs with high mechanical reliability are highly potential candidate for flexible active-matrix display beyond FPD (flat panel display) generation

Evidence of Si/SiGe heterojunction roughness scattering

The separation distance between the electron channel at oxide∕Si interface and the strained-Si∕relaxed-SiGe heterojunction can significantly affect the effective electron mobility of metal–oxide–silicon field-effect transistors due to the roughness scattering of the underneath Si∕SiGe heterojunction. The mobility degradation due to the Si∕SiGe heterojunction with the roughness of 7 nm becomes insignificant when the strained-Si thickness is larger than ∼20nm. A clear hole confinement shoulder is observed in the accumulation region of the capacitance–voltage curves, indicating that the abrupt transition from the SiGe buffer to strained Si is maintained at the rough heterojunction. The heterojunction roughness scattering not only degrades the electron mobility, but also degrades the device characteristics such as the transconductance and cut-off frequency.

P-221L: Late-News Poster: New Blue Phosphorescent Host for High-efficiency White OLED

A new blue phosphorescent host with an electron transporting dibenzothiophene molecular skeleton connected to hole transporting carbazolyl moiety has been developed. Using this material, an all-phosphorescent white OLED with a high power efficiency of 60 lm/W, a low driving voltage of 3.6V and a color-rendering index of 75 at practical brightness of 1000 cd/m2 can be achieved.

Growth of high-quality relaxed SiGe films with an intermediate Si layer for strained Si n-MOSFETs

An intermediate Si layer in Si1?xGex film, replacing the conventionally compositional graded buffer layer, was used to fabricate a relaxed SiGe substrate of high quality. The intermediate Si layer changes the relaxation mechanism of the SiGe thin film via the generation of {3?1?1} dislocation loops. The {3?1?1} dislocation loops are formed in the intermediate Si layer to prompt a state of relaxation in the SiGe overlayer, provide the defects for trapping of threading dislocations (TDs) and leave a SiGe top layer with low dislocation density. For the SiGe/Si/SiGe samples, the residual strain and TDs on the top SiGe layer are independent of the SiGe underlayer thickness. With a 700 nm thick Si0.8Ge0.2 overlayer, such a Si0.8Ge0.2/Si/Si0.8Ge0.2 heterostructure with a smooth surface has a TD density of 8.9 ? 105 cm?2 and 3% residual strain. Owing to the different main relaxation mechanisms in SiGe films, the surface root mean square roughness of this relaxed buffer with a low density of surface pits was measured to be about 3 nm, which is lower than that of the sample without any intermediate Si layer (13 nm). Relaxation of the SiGe overlayer depends on the thickness of the intermediate Si layer. Optimization on relaxation in the SiGe/Si/SiGe structure with an intermediate Si layer of 50 nm is done. Strained Si n-channel metal-oxide-semiconductor field effect transistors with various buffer layers were fabricated and examined. The effective electron mobility for the strained Si device with this novel substrate technology was found to be 80% higher than that of the Si control device. The SiGe thin films with the intermediate Si layer serve as good candidates for high-speed strained Si devices. The global strain in the Si channel with a SiGe/Si/SiGe buffer is still beneficial for short channel devices.

Thermal stability study of Si cap/ultrathin Ge/Si and strained Si/Si1-xGex/Si nMOSFETs with HfO2 gate dielectric

The thermal stabilities of MOSFETs with high-K gate dielectric on both Si/ultrathin Ge/Si (SGS) and strained Si on relaxed Si1?xGex (SS) substrates are studied. Though an initial drivability enhancement of 29% is shown for the SGS nMOSFET, annealing at 750 ?C has resulted in drastic degradation in its drivability, lowering its Id beyond that of the Si nMOSFETs by 52%. Despite lowering in the junction leakage current, Ge diffusion to the near surface region, indicated by Vth and surface roughness change, degrades the SGS device performance significantly. For the SS nMOSFET, drivability varies with Ge content, whereby a maximum of 86% improvement over that of the Si nMOSFET is observed for 30% Ge. In contrast to the SGS nMOSFET, the SS nMOSFET is able to retain its Id improvement, even after annealing at 950 ?C, as the in-plane tensile strain is preserved. Ge diffusion to the surface does not affect the device significantly, as the strained Si thickness is about 10 nm compared to a Si cap thickness of only 1.5 nm for the SGS substrate.

P‐154: High Efficiency White‐Phosphorescent OLED with Host‐Free Yellow Emitter

A white PHOLED with high power efficiency of 55 lm/W has been demonstrated by using a host-free yellow phosphorescent emitter sandwiched by double blue phosphorescent emitters. The white PHOLED with host-free yellow phosphorescent emitter achieved a comparable device performance to that of using complicated host-guest doped system. Based on this device concept as well as the molecular engineering of blue host material, highly efficient white PHOLED can be achieved.