Yuehua Chen

A Solution-Processed Resonance Host for Highly Efficient Electrophosphorescent Devices with Extremely Low Efficiency Roll-off.

Solution-processible N-P=O resonance-host molecules are applied successfully in spin-coated phosphorescent light-emitting diodes (PHOLEDs) to selectively and self-adaptively tune the electrical properties for balanced charge transportation. The resonance-molecule-hosted PHOLEDs exhibit a high maximum quantum efficiency of 16.5% and a low efficiency roll-off down to 0.7%, highlighting the significant progress of these solution-processed PHOLEDs with high efficiency and flat efficiency roll-off achieved simultaneously.

Stable pure-blue polymer light-emitting devices based on β-phase poly(9,9-dioctylfluorene) induced by 1,2-dichloroethane

Thin films of β-phase poly(9,9-dioctylfluorene) (PFO) were realized by dipping amorphous-phase PFO thin films in 1,2-dichloroethane for several seconds. Absorption, photoluminescence, and electroluminescence spectra indicate that the β-phase is generated in the thin films. A stable pure-blue polymer light-emitting device based on the β-phase PFO film was fabricated. The efficiency of the device with the β-phase is increased to 2.53 cd/A, which is 3.3 times higher than that of the device with the amorphous phase. Moreover, it is found that the device with β-phase emission shows high color purity as well as stable spectra.

Miniature spectrometer based on diffraction in a dispersive hole array.

We present an ultra-compact spectrometer that uses a 10×10 hole array as the dispersive component. Our analysis shows that the two-dimensional intensity distribution can be modeled by a system of simultaneous linear equations when the size of each hole in the dispersive component has been pre-designed appropriately. One can readily recover the spectral contents of the input radiation by solving the linear equation system with regularized procedure. Experimental results show that the reconstruction range is at least within the entire visible band, which can be further extended if a near-infrared CCD is used. One therefore envisions strong potential for many wavelength analysis applications.

Efficient phosphorescent polymer light-emitting devices using a conjugated starburst macromolecule as a cathode interlayer

We present the results of a systematic study of a conjugated starburst macromolecule TrOH cathode interlayer produced by solution-processing for the fabrication of highly efficient multilayered phosphorescent polymer light-emitting devices (PhPLEDs). It was found that the performance of the PhPLEDs was strongly dependent on the solvent composition of the TrOH coatings. The devices with the interlayer deposited from the mixed-solvent of water and ethanol showed much better device performance than that of the device with ethanol as solvent. From the impedance spectra of the devices and UV-vis absorption spectra of the emission layers (EMLs) treated by ethanol or mixed-solvent, the variation in device performance is mainly attributed to washing out the electron transport material in the EML due to the rinse effect. The erosion of the EML could be greatly suppressed by adding an appropriate amount of water into ethanol. The optimized green device with a maximum luminous efficiency of 23.4 cd A−1 and an external quantum efficiency of 6.7% was obtained when the ethanol : water ratio approached 7 : 3 (v/v). The peak efficiency of 14.6 cd A−1 was also achieved in a single emissive layer white PhPLED with a TrOH interlayer. In order to further understand the effect of the TrOH interlayer deposited from different solutions on the device performance, atomic force microscopy and contact angles were used to investigate the surface properties of the TrOH interlayer. The results indicate that the interfacial morphology is mainly controlled by the wetting characteristics of the EML and TrOH solution. The inferior device performance can be ascribed to the discontinuous TrOH film when the amount of water in TrOH solution is above 40%, which leads to the subdued electron injection from cathode into the EML.

Arylfluorene based universal hosts for solution-processed RGB and white phosphorescent organic light-emitting devices

A series of solution-processible arylfluorenes (AFs)/carbazole hybrid compounds (SmeFCz, DMeFCz, SMeOFCz, DMeOFCz, SFFCz and DFFCz) have been designed and synthesized via a simple one-step reaction. These compounds exhibit good thermal stability, excellent solubility and high triplet energy levels, which can be used as universal hosts for solution-processed phosphorescent organic light-emitting devices (PhOLEDs). Employing the developed materials, highly efficient blue (DMeFCz : CEmax: 20.0 cd A−1, EQEmax: 10.0%), green (DMeFCz : CEmax: 26.0 cd A−1, EQEmax: 7.8%) and red (SMeFCz : CEmax: 13.7 cd A−1, EQEmax: 8.2%) PhOLEDs fabricated by spin-coating in different device configurations have been demonstrated. Furthermore, complementary-color, single-emitting layer white phosphorescent device were also obtained. The SMeOFCz-based white device exhibited highly efficient white-light emission with a maximum efficiency of 27.1 cd A−1 (EQE of 12.0%). The state-of-art performance indicates that AFs/carbazole hybrid hosts have a favorable prospect for applications in solution-processed full-color displays and white PhOLEDs.

Highly efficient solution-processed phosphorescent organic light-emitting devices with double-stacked hole injection layers

In this paper, solution-processed nickel oxide (NiOx) is used as hole-injection layers (HILs) in solution-processed phosphorescent organic light-emitting diodes (PhOLEDs). Serious exciton quenching is verified at the NiOx/emitting layer (EML) interface, resulting in worse device performance. The device performance is significantly improved by inserting a layer of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS) between the EML and NiOx. The solution-processed blue PhOLED with the double-stacked NiOx/PEDOT:PSS HILs shows a maximum current efficiency of 30.5 cd/A, which is 75% and 30% higher than those of the devices with a single NiOx HIL and a PEDOT:PSS HIL, respectively. Improvement of device efficiency can be attributed to reducing exciton quenching of the PEDOT:PSS layer as well as the electron blocking effect of the NiOx layer.

Inverted polymer light-emitting devices using a conjugated starburst macromolecule as an interlayer

We present an investigation of inverted polymer light-emitting diodes (PLEDs) with an alcohol-soluble conjugated starburst macromolecule TrOH as an interlayer. The introduction of a TrOH interlayer between the light-emitting polymer layer and ZnO has been shown to significantly enhance the device efficiency due to efficient electron injection as well as efficient blocking of exciton quenching. It was found that the thickness of the TrOH interlayer is an important factor impacting the device performance. As a result, an efficient inverted PLED with a maximum luminous efficiency of 1.43 cd A−1 was achieved using a blue polymer poly(9,9-dioctylfluorene) (PFO) as the emitting layer.