Charge Separation Process in an Ultrathin Electron-Injecting Bilayer-Assisted Charge Generation Unit for Tandem Organic Light-Emitting Diodes

We have systematically investigated charge separation process in an ultrathin electron-injecting bilayer-assisted charge generation unit (CGU) consisting of n-type/p-type junction for tandem organic light-emitting diode (OLED). Charge generation occurs in the p-type side, which is formed by doping a strong electron acceptor in an electron-rich organic semiconductor (OSC). The n-type side is an ultrathin bilayer that acts as electron-injecting layer (EIL) for assisting electron separation from the p-type doped OSC. The energy barrier for electron tunneling is significantly reduced by means of increasing the evaporation rate of Al in the bilayer and acceptor concentration in OSC. Moreover, insertion of an interlayer with a deep-lying lowest unoccupied molecular orbital (LUMO) level between the n-type and p-type materials induces a tunneling path with low barrier, which is further proved by current density–voltage (J–V) and capacitance–voltage (C–V) characteristics of external carrier-excluding devices. Tandem OLED with CGU including the interlayer exhibits better performances such as lower driving voltage and higher power efficiency, demonstrating that the interlayer with deep-lying LUMO plays an important role in promotion of charge separation process.