We report efficient tandem white organic light-emitting diodes (WOLEDs) by using bathocuproine:Cs2CO3∕MoO3 as an effective interconnecting layer. We utilized two primary colors of sky blue and orange fluorescent emitters to obtain efficient white electroluminescence. Although single WOLEDs using two adjacent emitting layers showed a maximum current efficiency of 7.96cd∕A with Commission Internationale d’Eclairage (CIE) coordinates of (0.28, 0.34), the tandem WOLED device made by stacking two single color OLEDs in series demonstrated doubled maximum current efficiency of 17.14cd∕A with CIE coordinates of (0.28, 0.41). The stacking of different single color OLEDs in series instead of double stacking of WOLEDs can be useful to achieve highly efficient WOLEDs because it can reduce the number of layers of the devices.

Although significant progress has been made in the development of vacuum‐deposited small‐molecule organic light‐emitting diodes (OLEDs), one of the most desired research goals is still to produce flexible displays by low‐cost solution processing. The development of solution‐processed OLEDs based on small molecules could potentially be a good approach but no intensive studies on this topic have been conducted so far. To fabricate high‐performance devices based on solution‐processed small molecules, the underlying nature of the produced films and devices must be elucidated. Here, the distinctive characteristics of solution‐processed small‐molecule films and devices compared to their vacuum‐deposited counterparts are reported. Solution‐processed blue OLEDs show a very high luminous efficiency (of about 8.9 cd A–1) despite their simplified structure. A better hole‐blocking and electron‐transporting layer is essential for achieving high‐efficiency solution‐processed devices because the solution‐processed emitting layer gives the devices a better hole‐transporting capability and more electron traps than the vacuum‐deposited layer. It is found that the lower density of the solution‐processed films (compared to the vacuum‐deposited films) can be a major cause for the short lifetimes observed for the corresponding devices.

Cesium azide (CsN3) is employed as a novel n‐dopant because of its air stability and low deposition temperature. CsN3 is easily co‐deposited with the electron transporting materials in an organic molecular beam deposition chamber so that it works well as an n‐dopant in the electron transport layer because its evaporation temperature is similar to that of common organic materials. The driving voltage of the p‐i‐n device with the CsN3‐doped n‐type layer and a MoO3‐doped p‐type layer is greatly reduced, and this device exhibits a very high power efficiency (57 lm W−1). Additionally, an n‐doping mechanism study reveals that CsN3 was decomposed into Cs and N2 during the evaporation. The charge injection mechanism was investigated using transient electroluminescence and capacitance–voltage measurements. A very highly efficient tandem organic light‐emitting diodes (OLED; 84 cd A−1) is also created using an n–p junction that is composed of the CsN3‐doped n‐type organic layer/MoO3 p‐type inorganic layer as the interconnecting unit. This work demonstrates that an air‐stable and low‐temperature‐evaporable inorganic n‐dopant can very effectively enhance the device performance in p‐i‐n and tandem OLEDs, as well as simplify the material handling for the vacuum deposition process.

The authors report the effect of thermal treatment of hole-transporting interlayers between a polymeric hole injection layer and an emitting layer (EML) on the luminous efficiency and the lifetime performance in blue polymer light-emitting diodes. As the thermal annealing temperature of the interlayer increased, the hole mobility of the interlayer tended to decrease, which results in reducing the hole current injected into the EML in the devices. Hence, the device luminous efficiency decreased due to lower electron-hole balance. Nevertheless, the device lifetime increased, which can be attributed to the formation of the thicker interlayer and the better defined interlayer/EML interface.

A new blue light-emitting fluorene derivative 2,7-di [8-(7,10-diphenylfluoranthenyl)]-9,9-dimethylfluorene (DFDF) with good thermal stability at 420 °C has been synthesized and characterized. An organic lightemitting device (OLED) with the structure of ITO/NPB (70 nm)/DFDF (30 nm)/TPBI (20 nm)/LiF (0.5 nm)/ Al (100 nm) has been investigated, where DFDF serves as a nondoped host emitter. Such a device possesses high current and power efficiencies of 3.8 cd/A and 2.6 lm/W, respectively, and stable bright blue-light emission at λ ) 474 nm with Commission Internationale de L'Eclairage coordinates of (0.16, 0.23) over a wide range of operating voltages. The present results verify that DFDF is a promising candidate for a fluorescent bluelight-emitting OLED.