Highly efficient organic electroluminescent diodes realized by efficient charge balance with optimized electron and hole transport layers

Highly efficient organic electroluminescent devices (OLEDs) were developed based on 4,7-diphenyl-1, 10-phenanthroline (BPhen) as the electron transport layer (ETL), tris-(8-hydroxyquinoline) aluminum (Alq3) as the emission layer (EML) and N,Ń-bis-[1-naphthy(-N,Ńdiphenyl-1,1’-biphenyl-4,4’-diamine)] (NPB) as the hole transport layer (HTL). The typical device structure was glass substrate/ ITO/ NPB/ Alq3/ BPhen/ LiF/ Al. Since BPhen possesses a considerable high electron mobility of 5×10−4 cm2 V −1 s−1, devices with BPhen as ETL can realize an extremely high luminous efficiency. By optimizing the thickness of both HTL and ETL, we obtained a highly efficient OLED with a current efficiency of 6.80 cd/A and luminance of 1361 cd/m2 at a current density of 20 mA/cm2. This dramatic improvement in the current efficiency has been explained on the principle of charge balance.