Mao-Feng Hsu

High-efficiency blue organic light-emitting diodes using a 3,5-di(9H-carbazol-9-yl)tetraphenylsilane host via a solution-process

We present a solution-processed blue organic light-emitting diode (OLED) with markedly high current efficiency of 41.2 cd A−1 at 100 cd m−2 and 31.1 cd A−1 at 1000 cd m−2. The high efficiency was partly attributed to the use of a molecular host, 3,5-di(9H-carbazol-9-yl)tetraphenylsilane, which possesses a wide triplet band gap, high carrier mobility, ambipolar transport property and high glass transition temperature. Besides the intrinsically good physical properties, the solution-process also played an important role in fabricating the high-efficiency device, since it could make the molecular distribution of host and guest homogeneous in the emissive layer. Moreover, the device efficiency at higher brightness could be markedly enhanced by using an electron-blocking layer. As the microlens was introduced on the glass substrate to enhance the light outcoupling, the resultant device efficiency of the blue OLED further increased to 50.1 cd A−1 at 100 cd m−2 and 37.3 cd A−1 at 1000 cd m−2.

Depth Profiling of Organic Films with X-ray Photoelectron Spectroscopy Using C60+ and Ar+ Co-Sputtering

By sputtering organic films with 10 kV, 10 nA C60+ and 0.2 kV, 300 nA Ar+ ion beams concurrently and analyzing the newly exposed surface with X-ray photoelectron spectroscopy, organic thin-film devices including an organic light-emitting diode and a polymer solar cell with an inverted structure are profiled. The chemical composition and the structure of each layer are preserved and clearly observable. Although C60+ sputtering is proven to be useful for analyzing organic thin-films, thick organic-devices cannot be profiled without the low-energy Ar+ beam co-sputtering due to the nonconstant sputtering rate of the C60+ beam. Various combinations of ion-beam doses are studied in this research. It is found that a high dosage of the Ar+ beam interferes with the C60+ ion beam, and the sputtering rate decreases with increasing the total ion current. The results suggest that the low-energy single-atom projectile can disrupt the atom deposition from the cluster ion beams and greatly extend the application of the cluster ion-sputtering. By achievement of a steady sputtering rate while minimizing the damage accumulation, this research paves the way to profiling soft matter and organic electronics.

Extraordinarily high efficiency improvement for OLEDs with high surface-charge polymeric nanodots.

The efficiency of highly efficient blue, green, red, and white organic light-emitting diodes (OLEDs) has been substantially advanced through the use of high surface-charge nanodots embedded in a nonemissive layer. For example, the blue OLEDs markedly high initial power efficiency of 18.0 lm W(-1) at 100 cd m(-2) was doubled to 35.8 lm W(-1) when an amino-functionalized polymeric nanodot was employed. At high luminance, such as 1000 cd m(-2) used for illumination applications, the efficiency was improved from 12.4 to 21.2 lm W(-1), showing a significant enhancement of 71%. The incorporated highly charged nanodots are capable of effectively modulating the transportation of holes via a blocking or trapping mechanism, preventing excessive holes from entering the emissive layer and the resulting carrier-injection imbalance. Furthermore, in the presence of a high-repelling or dragging field arising from the highly charged nanodots, only those holes with sufficient energy are able to overcome the included barriers, causing them to penetrate deeper into the emissive layer. This penetration leads to carrier recombination over a wider region and results in a brighter emission and, therefore, higher efficiency.

A New Door for Molecular-Based Organic Light-Emitting Diodes

Long life-time molecular-based organic electronics, such as organic light-emitting diodes (OLEDs), organic solar cells, or organic transistors etc, inevitably demand their constituent molecules to be highly thermal-stable. Coupling with special needs in molecular design, the resultant increasing molecular weight (MW) will eventually make the molecules difficult to deposit if via dry-process, while using wet-process would frequently result in undesired relatively poorer efficiency. Surprisingly, two high-molecule composing OLEDs with relatively high-efficiency were obtained by using solution-process. A blue OLED with a blue dye doped in a novel high-MW, wide band-gap host, 3,5-di(9H-carbazol-9-yl) tetraphenylsilane (SimCP2), yielded 24 lm/W (38 cd/A) at 100 nits, and a green OLED using a novel high-MW green dye, bis[5-methyl-7-trifluoromethyl-5H-benzo (c)(1,5) naphthyridin-6-one] iridium (picolinate) (CF3BNO), yielded 70 lm/W (89 cd/A), while their dry-processed blue and green counterparts yield 1.7 and 21 lm/W, respectively. Importantly, although the comparatively high MW has made the resulting molecules extremely difficult to vacuum-evaporate and has resulted in poor device performance, the wet-process has been proven effective in fabricating two high molecule-containing OLEDs with relatively high efficiency. The successful demonstration suggests that the same approach may as well be extended to other organic devices that compose or should compose high molecules.

High-Efficiency Phosphorescent and Fluorescent Pure-White Organic Light-Emitting Diodes by Incorporating Small Nano-Dot in Non-emissive Layer

High-efficiency pure-white organic light-emitting diodes (OLEDs) were fabricated using small polysilicic acid nanodot embedded polymeric hole-transporting layer. By incorporating the nanodot, the efficiency of a solution-processed phosphorescent white OLED was increased from 6.8 to 23.7 Im/W, an improvement of 250%. 17.1 Im/W was obtained while the same concept was applied on a mixed-host composed fluorescent white OLED.