**1.3. Electron injection in inverted OLEDs**

The active matrix displays based on OLEDs have been successfully applied to portable electronic devices, e.g., mobile phones and music players. In order to facilitate the large-scale commercialization for active matrix OLED displays, it is of great necessity to reduce their fabrication cost. Hence, the n-channel amorphous silicon thin-film transistor technology may preferably be utilized to drive the light emitting elements, but requires the OLED structure with inverted layer sequence [8, 9]. Compared to the regular OLEDs, higher driving voltages are always obtained in the inverted ones due to the poor electron injection as a consequence of the inefficient metal penetration into organics [10,11]. Thus, the n-doped electron transport layers (n-ETLs) are adopted to enhance the electron current in inverted OLEDs [4], e.g., lithium doped bathcuproine (BCP). However, the electron injection from the cathode into n-ETL in the inverted OLED is found always less efficient than that in the regular OLED. Thomschke et al. [12] improved the electron injection in a n-i-p OLED via inserting an interlayer of Bphen double-doped with cesium and silver between the silver cathode and cesium-doped Bphen, followed by the thermal annealing of the whole device. Though this method led to almost the same electrical properties in the inverted OLEDs as those in the regular OLEDs, it is relatively complicated and therefore unsuitable for use in the mass production of the active matrix OLED displays. Here, we introduce the increased electron injection in inverted bottomemission OLEDs (IBOLEDs) via using the combination of two n-ETLs [13-16].
