Improved Performance of Organic Light Emitting Diodes by Balanced Charge Injection

Abstract

Balanced charge injection in organic light emitting diodes (OLEDs) has been investigated towards improving their efficiency and stability. Balanced charge injection was achieved through selective insertion and elimination of some charge injection and charge transport layers. Poly(ethylene-dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) was used as hole injecting layer (HIL) but because of its hygroscopic nature, it caused rapid degradation in device performance. Elimination of PEDOT:PSS and insertion of thin films of hole transporting N,N′-Di (1-naphthyl)-N,N′-diphenyl- (1,1′-biphenyl) 4,4′-diamine (NPD), and hole blocking bathocuproine (BCP) along with electron injecting 8-hydroxyquinolinolato lithium (Liq) layer led to balanced charge injection into light emitting tri(8-hydroxyquinolinato) aluminium (Alq3) layer that resulted in improved current efficiency (CE) of OLEDs. The reference device with PEDOT:PSS showed maximum CE of 0.4 cd/A, whereas the device with balanced charge injection exhibited much enhanced CE of 1.75 cd/A. Furthermore, after ~15 h of continuous operation, the reference device retained only ~15% of its initial CE whereas the device with
balanced charge injection retained over 80% of its initial CE, demonstrating substantially improved operational stability by balanced charge injection.