Shihao Liu

Top-emitting quantum dots light-emitting devices employing microcontact printing with electricfield-independent emission

Recent breakthroughs in quantum dot light-emitting devices (QD-LEDs) show their promise in the development of next-generation displays. However, the QD-LED with conventional ITO-based bottom emission structure is difficult to realize the high aperture ratio, electricfield-independent emission and flexible full-color displays. Hence, we demonstrate top-emitting QD-LEDs with dry microcontact printing quantum dot films. The top-emitting structure is proved to be able to accelerate the excitons radiative transition rate, then contributing to stable electroluminescent efficiency with a very low roll-off, and preventing spectra from shifting and broadening with the electric field increases. The results suggest potential routes towards creating high aperture ratio, wide color gamut, color-stable and flexible QD-LED displays.

Silver/germanium/silver: an effective transparent electrode for flexible organic light-emitting devices

Mechanical resilience is important for future organic light-emitting devices (OLEDs) in lighting and displays. A key constraint is the electrode used in OLEDs. Here we demonstrate ultra-thin silver/germanium/silver (AGA)-based flexible organic light-emitting devices, with comparable efficiency to their ITO-based counterparts. The fabrication process of AGA electrodes is compatible with that of the OLEDs, and they show excellent optical, electrical and mechanical properties. They can be used as transparent cathodes and anodes for top-emitting, bottom-emitting and transparent flexible OLEDs.

An efficient flexible white organic light-emitting device with a screen-printed conducting polymer anode

A screen-printed poly(3,4-ethylenedioxythiophene) : poly(styrenesulfonate) (PEDOT : PSS) layer was used as the anode for a flexible white organic light-emitting device (FWOLED). After 700 bending cycles, the sheet resistance of the anode was increased to 1.8 times the initial value (296 Ω/). The average transmission of the PEDOT : PSS layer on the PET substrate was 74% in the visible light region. At a brightness of 1000 cd m−2, the current and power efficiency of the FWOLED were 26.4 cd A−1 and 10.8 lm W−1, respectively. And after 100 outer bending cycles, the device also had a power efficiency of 5.0 lm W−1 at 1000 cd m−2.

Efficient inverted organic light-emitting devices with self or intentionally Ag-doped interlayer modified cathode

Green phosphorescent inverted organic light-emitting devices (IOLEDs) with self or intentionally Ag-doped interlayer modified cathode were demonstrated. The IOLEDs show low driving voltage and high efficiency. For example, the efficiency of inverted bottom-emitting OLED with ITO cathode is comparable with the conventional bottom-emitting OLED with ITO anode. The top-emitting IOLED with Ag cathode shows high current efficiency of 76.4 cd/A which is 2.38 times of that of the conventional bottom-emitting OLED with ITO anode. The results indicate that the electron injection from cathode was observably improved by the Ag-doped interlayer and such interlayer is cathode independent relatively.

Angle-stable RGBW top-emitting organic light-emitting devices with Ag/Ge/Ag cathode

Ag/Ge/Ag (AGA) is investigated as a transparent cathode for top-emitting organic light-emitting devices (TEOLEDs). TEOLEDs of different colors with excellent performances can be gained by simply adopting a corresponding emitting layer, without changing the thickness of the device and cathode. Especially, the blue and white TEOLEDs exhibit high efficiency as well as the bottom OLEDs and show an excellently angle-stable characteristic. The white TEOLED exhibits a maximum current efficiency of 12.4 cd/A, and the CIE coordinates at 6 V only shift by (0.048, 0.046) from 0° to 60°. It can be attributed to the less angle-dependent cavity emission of the TEOLED with AGA cathode.

Ultrasonic spray coating polymer and small molecular organic film for organic light-emitting devices

Ultrasonic spray coating process (USCP) with high material -utilization, low manufacture costs and compatibility to streamline production has been attractive in researches on photoelectric devices. However, surface tension exists in the solvent is still a huge obstacle to realize smooth organic film for organic light emitting devices (OLEDs) by USCP. Here, high quality polymer anode buffer layer and small molecular emitting layer are successfully realized through USCP by introducing extra-low surface tension diluent and surface tension control method. The introduction of low surface tension methyl alcohol is beneficial to the formation of poly (3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) films and brings obvious phase separation and improved conductivity to PEDOT:PSS film. Besides, a surface tension control method, in which new stable tension equilibrium is built at the border of wetting layer, is proposed to eliminate the effect of surface tension during the solvent evaporation stage of ultrasonic spray coating the film consists of 9,9-Spirobifluoren-2-yl-diphenyl-phosphine oxide doped with 10 wt% tris [2-(p -tolyl) pyridine] iridium (III). A smooth and homogenous small molecular emitting layer without wrinkles is successfully realized. The effectiveness of the ultrasonic spray coating polymer anode buffer layer and small molecular emitting layer are also proved by introducing them in OLEDs.

Efficient Trilayer Phosphorescent Organic Light-Emitting Devices Without Electrode Modification Layer and Its Working Mechanism

At present, numerous functional layers are introduced to improve the carrier injection and balance the carrier transport in organic light-emitting devices (OLEDs). Although it may be a good way to enhance the efficiency of devices, the introduction of functional layers would also result in extra process and long manufacture period. Actually, with the enrichment of material system, many appropriate materials could be chosen to share two or even more functions in OLEDs. Here, via impedance spectroscopy and transient electroluminescence analysis, di-[4-(N,N-ditolyl-amino)-phenyl] cyclohexane (TAPC) and 4,7-diphenyl-1,10-phenanthroline (Bphen) are demonstrated to serve as carrier injection and transport layers simultaneously. As a result, efficient trilayer OLEDs are achieved with comparable performances to conventional multilayer devices. Further studies have also been carried out to analyze the recombination and quenching mechanisms in devices. TAPC can block electrons effectively, while Bphen avoids the accumulation of holes. It makes carriers in emitting layer become more balanced, resulting in the reduction of efficiency roll-off.

Efficiently alternating current driven tandem organic light-emitting devices with (Ag/4,7-diphenyl-1,10-phenanthroline)n interconnecting layers

Alternating current (AC) driven tandem organic light-emitting devices (OLEDs) in which an invert unit is connected to a normal unit with [Ag/4,7-diphenyl-1,10-phenanthroline (Bphen)]n interconnecting layers (ILs) are demonstrated. The current-voltage and capacitance-voltage characteristics of different devices with or without ILs demonstrated that (Ag/Bphen)n ILs possess excellent charge generation and electron injection abilities. Thus, (Ag/Bphen)n ILs could offer sufficient electrons for two units to realize radiation-recombination under AC bias. Efficient AC-driven OLEDs in which the color and brightness can be tuned independently by changing the AC bias are achieved.