The metal interlayer in the charge generation layer of tandem organic light-emitting diodes

This work studies the interface in the charge generation layer (CGL), consisting of aluminum (Al) doped in poly(ethylene glycol) dimethyl ether as an n-type layer and 2, 3, 5, 6-tetrafluoro-7, 7, 8, 8-tetracyanoquinodimethane (F₄-TCNQ) doped in N,N′-bis-(1-naphthyl)-N,N′-diphenyl-1, 1′-biphenyl-4-4′-diamine as an p-type layer, in tandem organic light-emitting diodes (OLEDs). Introducing a thin high work function metal interlayer (e.g., Ag or Au) effectively improves the transport and inhibits the accumulation of charges in the CGL, which markedly reduces the operating voltage and enhances the efficiency of tandem OLEDs. We attribute that the high density of surface states on metal clusters (interlayer) reduce the junction barrier to facilitate the transport of carriers through CGL. Experimental results show enhancements of tandem OLEDs by an additional metal interlayer as follows: luminous efficiency increases from 37.2 to 51.4 cd A^-1, the light turn-on voltage decreases from 9.2 to 6.6 V, and luminescence at 10 mA cm ^-2 increases from 3712 to 5211 cd m^-2.