On the problem of microcavity effects on the top emitting OLED with semitransparent metal cathode

Abstract

Recent interest in the top emission organic light emitting devices (TEOLEDs) has been stimulated by their potential application for the development of full color flat panel displays. We have fabricated the bottom emitting, ITO/α-NPD(50 nm)/Alq3(35 nm)/BCP(10 nm)/Ca(10 nm)/Ag(120 nm) device with a microcavity length of L = 95 nm and three top emitting structures, namely Ni/α-NPD(50 nm)/Alq3(35 nm)/ BCP(10 nm)/Ca(10 nm)/Ag(10 nm) (L = 95 nm), Ni/α-NPD(50 nm)/Alq3(35 nm)/BCP(5 nm)/Ca(10 nm)/ Ag(10 nm) (L = 90 nm), and Ni/α-NPD(35 nm)/Alq3(50 nm)/BCP(5 nm)/Ca(10 nm)/Ag(10 nm) (L = 90 nm). These devices were characterized by electroluminescence (EL) and current–voltage (I–V) measurements. Results on the top and the bottom emitting devices have been compared. In Ni/α-NPD(50 nm)/ Alq3(35 nm)/BCP(10 nm)/Ca(10 nm)/Ag(10 nm) structure, the EL yield is lowest and Fabry-Perot interference fringes are observed, due to microcavity effects because of the presence of the residual reflection at the semitransparent Ca/Ag cathode. The turn-on voltage is significantly high (7 V), about two times compared to the bottom emitting structure. The device performance could be improved by properly optimizing the microcavity length. No interference fringes are noticed in TEOLEDs with the microcavity length of L = 90 nm. Enhanced EL and very low threshold voltage (2.75V) are reported in the Ni/α-NPD(50 nm)/Alq3(35 nm)/BCP(5 nm)/Ca(10 nm)/Ag(10 nm) TEOLED. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)