LEDs Wonderland

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Additive effect improves polymer yellow LEDs

A group of Dongge Ma from Chinese Academy of Sciences has reported an efficient way to increase the thickness of solution processed small molecule based electron-transmiting layer in yellow PLEDs. Solution-phase processing would be a highly desirable method of multilayer diode manufacturing as it's much easier and cheaper to perform. They have noted that commonly chosen electron-transmitting materials ( TmPyPB, TAZ and TPBL) can only produce a maximum of 15 nm thick electron-transmitting layer when processed in solution. That makes corresponding fabricated diodes reach their efficiency lim it quickly. Interestingly, when mixture of various compositions of these small molecules were tested, ideal combination was discovered. The TPBI:TnPyPB:TAZ mixture (1.3:0.7:1 w/w) allowed to obtain 40 nm thick layer and reach external quantum efficiency of 41.7 cd/A and 12.7%. The maximum brightness was 23926 cd/m2.

The original article and supporting information were published by ACS Applied Materials & Interfaces:
Efficient Phosphorescent polymer Yellow-Light-Emitting Diodes Based on Solution-Processed Small Molecule Electron Transporting Layer
Tengling Ye, Shiyang Shao, Jiangshan Chen, Lixiang Wang, Dongge Ma
DOI: dx.doi.org/10.1021/am1010018

Ion motion controls LEDs time response

Research group of Thuc-Quen Nguen at UC Santa Barbara has published a study aimed to design anionic conjugated polyelectrolyte (CPE) that could enable improved temporal response through facilitation of ion motion. Such an improvement would be greatly beneficial for the development of better PLEDs based displays. They had designed their polyelectrolyte to contain ion-conducting polyethylene oxide pendant (PFPEOCO2Na) as electron injection layers (EILs). Pristine PLEDs containing PFPEOCO2Na exhibit luminance response times on the order of tenths of seconds. This delay is attributed to the formation of ordered structures within the CPE film that slow ion migration and therefore result in a longer temporal response time. The researchers have found out that presence of the ethylene oxide units on PFPEOCO2Na did not improve the luminance response times of as cast PLEDs (46s). Yet it is possible to accelerate the response by a combination of thermal and voltage treatments that “lock” ions within the interfaces adjacent to PFPEOCO2Na resulting in the response time of 200 micro seconds (105 fold enhancement). This study may lead to the development of efficient multilayer solution-processable PLEDs with stable high work function cathodes and fast luminance response time.

Original article is published by the Journal of American Chemical Society DOI 10.1021/ja106268w