Zhiming Zhong

Highly efficient, solution processed electrofluorescent small molecule white organic light-emitting diodes with a hybrid electron injection layer.

Highly efficient, solution-processed, and all fluorescent white organic light-emitting diodes (WOLEDs) based on fluorescent small molecules have been achieved by incorporating a low-conductivity hole injection layer and an inorganic-organic hybrid electron injection layer. The light-emission layer is created by doping a fluorescent π-conjugated blue dendrimer host (the zeroth generation dendrimer, G0) with a yellow-emitting fluorescent dopant oligo(paraphenylenevinylene) derivative CN-DPASDB with a doping ratio of 100:0.15 (G0:CN-DPASDB) by weight. To suppress excessive holes, the high-conductivity hole injection layer (PEDOT:PSS AI 4083) is replaced by the low-conductivity PEDOT:PSS CH 8000. To facilitate the electron injection, a hybrid electron injection layer is introduced by doping a methanol/water-soluble conjugated polymer poly[(9,9-bis(30-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFNR2) with solution-processed cesium fluoride (CsF). The device achieves a maximum luminous efficiency of 17.0 cd A(-1) and a peak power efficiency of 15.6 lm W(-1) at (0.32, 0.37) with a color rendering index of 64.

An Alkane-Soluble Dendrimer as Electron-Transport Layer in Polymer Light-Emitting Diodes

Polymer light-emitting diodes (PLEDs) have attracted broad interest due to their solution-processable properties. It is well-known that to achieve better performance, organic light-emitting diodes require multilayer device structures. However, it is difficult to realize multilayer device structures by solution processing for PLEDs. Because most semiconducting polymers have similar solubility in common organic solvents, such as toluene, xylene, chloroform, and chlorobenzene, the deposition of multilayers can cause layers to mix together and damage each layer. Herein, a novel semiorthogonal solubility relationship was developed and demonstrated. For the first time, an alkane-soluble dendrimer is utilized as the electron-transport layer (ETL) in PLEDs via a solution-based process. With the dendrimer ETL, the external quantum efficiency increases more than threefold. This improvement in the device performance is attributed to better exciton confinement, improved exciton energy transfer, and better charge carrier balance. The semiorthogonal solubility provided by alkane offers another process dimension in PLEDs. By combining them with water/alcohol-soluble polyelectrolytes, more exquisite multilayer devices can be fabricated to achieve high device performance, and new device structures can be designed and realized.

Effects of a random copolymer's component distribution on its opto-electronic properties

The effects of the component distribution of a red light-emitting random copolymer poly(9,9-dioctylfluorene-co-(4,7-dithienyl-benzothiadiazole)) (DOF : DBT = 9 : 1 mol/mol, PFO-DBT10) on its optoelectronic properties are systematically investigated. The PFO-DBT10 copolymers with various molecular weights (MWs) have identical absorption spectra and energy levels. However, as the molecular weight increases, so does the photoluminescence intensity ratio of DBT to DOF segments. For polymer light-emitting diodes, PFO-DBT10 with a large MW shows an enhanced charge trapping effect, leading to increased operation voltage under any given current density, worse luminous efficiency, and more saturated red emission, compared to the one with a small MW. The experiments are performed within the framework of the components molar ratio dispersion of random copolymer chains induced by the MW using the binomial distribution formula. According to the formula, the high MW random copolymer is analogous to the ideal PFO-DBT10 in terms of the uniformly distributed units. The low MW random copolymer is more likely to be a mixture of pure PFO and PFO-DBTx, in which the molar ratio x varies, due to its dispersive component distribution. This work offers a new approach to study the opto-electronic properties of random copolymers in the light of component distribution.

In situ patterning of microgrooves via inkjet etching for a solution-processed OLED display

Inkjet-printing a solvent onto an insulating polymer layer is employed to in situ build microgrooves as bank structures in the application of a solution-processed OLED display. The inkjet-etching process not only eliminates photolithographys shadow mask and photo exposure, but is also capable of constructing bank structures on any functional layer. The orthogonal solubility between the CYtop polymer and the organic layer avoids any solvent erosion. A pixelated display is successfully fabricated by inkjet-printing a blue-emitting polymer onto inkjet-etched CYtop microgrooves with a pixel resolution of 140 lines per inch. Forming a bank structure in situ on any layer as needed offers more choices to design a new panel structure, device architecture, and deposition methods.

Achieving Highly Efficient Blue Light-Emitting Polymers by Incorporating a Styrylarylene Amine Unit

A series of novel blue light-emitting polymers were designed and synthesized by incorporating a blue styrylarylene amine (DV) chromophore into the backbone of poly(9,9-dioctylfluorene). All the resultant polymers exhibit blue emission peaking at around 465 nm. The fabricated single layer polymer light-emitting devices based on PF-TD2DV2 consisting of 2 mol% of the DV moiety present an impressively high luminous efficiency of 5.47 cd A−1 with Commission International de L’Eclairage (CIE) coordinates of (0.15, 0.14). The luminous efficiency can be further improved to over 8 cd A−1 upon the incorporation of a hole transport layer of polyvinylcarbazole and the blending of a hole transport material N-([1,1′-biphenyl]-4-yl)-N-(9,9-dimethyl-9H-fluoren-2-yl)-9,9′-spirobifluoren-2-amine (BSFN) into the emissive layer. Of particular importance is that the luminous efficiency remains 7.40 cd A−1 at a brightness of 1000 cd m−2, and the devices exhibit excellent stability of electroluminescence with the variation of driving voltage from 3 to 10 V. These results demonstrate the great potential of this class of polymers for application in solution processed blue light-emitting diodes.