Abstract
In this study, we demonstrate a series of high-efficiency blue oganic light emitting diodes with a solution-processed emissive layer. Their result and discussion will be shown in three parts. In the first part, we present a high-efficiency blue organic light-emitting diode (OLED) with a solution-processed emissive layer composing a melucular-based host of 3,5-di(9H-carbazol-9-yl) tetraphenylsilane. The device exhibits a current efficiency of 41.2 cd/A with an external quantum efficiency (EQE) of 21.0% and power efficiency of 24.9 lm/W at 100 cd/m2 or 31.1 cd/A (15.8%, 15.4 lm/W) at 1,000 cd/m2. The high efficiency is partly attributed to the use of a novel molecular host, which possesses wide triplet band gap, high carrier mobility, ambipolar transport property and high glass transition temperature. Besides the intrinsically good physical properties, solution-process also plays an important role to fabricate the high-efficiency device, since it could make the molecular distribution of host and guest homogeneous in the emissive layer to facilitate host-to-guest energy transfer. Moreover, the device efficiency at higher brightness could be markedly enhanced by using an electron-blocking layer. In the sencond part, we also demonstrate a high-efficiency blue OLED with a solution-processed emissive layer composing an oligomeric host of 3-(carbazol-9-ylmethyl)-3-methyloxetane that possesses high triplet-energy and especially high electron-mobility. The device exhibits a current efficiency of 40.4 cd/A with an external quantum efficiency (EQE) of 21.6% and power efficiency of 28.2 lm/W at 231 cd/m2 or 24.7 cd/A (10.3%, 15.5 lm/W) at 1,000 cd/m2. The high efficiency may be attributed to the host that possesses a wide triplet band-gap, effectively facilitating energy-transfer from the host to guest. Moreover, the high electron-mobility favors the transport of electron, resulting to a more balanced carrier-injection in the emissive layer. The device efficiency has been further enhanced to 42.6 cd/A (22.9%, 29.7 lm/W) at 124 cd/m2 or 28.8 cd/A (15.4%, 17.8 lm/W) at 1,000 cd/m2 by pre-heating the emissive solution at elevated temperature before spin-coating. In the last part, the efficiency of highly efficient blue OLEDs has been substantially advanced through the use of high surface-charge nanodots embedded in a non-emissive layer. Amonst, the blue OLED’s markedly high initial power efficiency of 18.0 lm/W at 100 cd/m2 was doubled to 35.8 lm/W when an amino-functionalized polymeric nanodot was employed. At high luminance, such as 1,000 cd/m2, used for illumination applications, the efficiency was improved from 12.4 to 21.2 lm/W 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.