Efficient tandem organic light-emitting diodes with non-doped structures

Highly efficient tandem organic light-emitting diodes (TOLEDs) were achieved based on a non-doped charge generation unit (CGU) consisting of LiF/Al/C60/4,4’,4”tris(N-3-methylphenyl-N-phenyl-amino) triphenylamine (m-MTDATA) and ultrathin emitting layers. The current–voltage characteristics of the CGU devices and electron-only devices and the capacitance–voltage characteristics of the CGU-based capacitance devices were characterized to explore the charge generation and injection mechanisms. The charge generation process occurs at the interface of C60/m–MTDATA through electron transferring from the highest occupied molecular orbital of m-MTDATA to the lowest unoccupied molecular orbital of C60. It is found that the thinner C60 layer contributes to efficient electron injection. Under the optimal structure, the blue TOLEDs exhibit a maximum current efficiency (CE_max) of 43.3 cd/A. The CE_max and maximum external quantum efficiency (EQE_max) of the white TOLEDs reach 84.6 cd/A and 26.7%, respectively.