Effect of Interlayer on Doped Organic p–n Heterojunction Charge Generation Layers Using Impedance Spectroscopy

Tandem organic light-emitting diodes (OLEDs) are two or more emitting units that are connected in series with charge generation layer(s) (CGLs). Although these devices can achieve higher efficiencies and longer operating lifetimes, the CGL is a key element that determines the lifetime and efficiency of these devices. This study investigates the charge generation and operation mechanisms in pristine and aged organic p–n heterojunction CGLs with and without an interlayer (IL) using impedance spectroscopy (IS) and equivalent circuit simulations. Current density and voltage (J–V) analyses show a nearly three times higher current density of the CGL devices with an IL, requiring lower operating voltage and an unchanged onset voltage after aging, demonstrating device stability. The IS and equivalent circuit simulation results reveal that the charge generation efficiency of CGL devices with an IL can be attributed to the lower energy barrier imposed by the IL at the p–n heterojunction and the stability of its molecules after electrical aging. Further investigations providing a clear understanding of the reason behind the stability and efficient operating mechanism in these devices intuitively demonstrate that IS and equivalent circuit simulations can be effectively employed for electrical stability research on multilayered organic devices.