Investigation of high-efficiency organic light-emitting devices using stacked structure

Abstract

Stacked organic light-emitting device (OLED) with at least two electroluminescent (EL) units connected electrically in series was first proposed in 1996, which exhibits same brightness at a lower current density and hence increases the luminous efficiency compared with that of traditional OLEDs with a single EL unit. In this regard, the operation stability of a stacked OLED can be enhanced in principle. In my work, several organic based connecting units which combine two EL units together are investigated. These connecting units include: (i) Tris(8-hydroxyquinoline)aluminum(III) (Alq3) doped with Mg/4,4’,4”-tris{N,-(3-methylphenyl)-N-phenylamino}-triphenylamine (m-MTDATA) doped with tetrafluorotetracyanoquinodimethane (F4-TCNQ); (ii) Alq3/ m-MTDATA doped with F4-TCNQ and (iii) Alq3 doped with Mg/ m-MTDATA. It is found that the insertion of some of the representative structures enables efficient injection of electrons and holes to the bottom and to the top emissive cell respectively, leading to a stacked OLED with luminous efficiency and operational half-life doubled than that of an OLED with single emissive unit. Nevertheless, few of them do not work for the improvement of luminous efficiency and operational stability in stacked light-emitting device. Since the connecting unit in stacked OLED is of paramount importance, understanding its working mechanism is certainly beneficial for the design and optimization of stacked OLED structures. Therefore, electrical properties and electronic structures of various interfaces in the stacked OLED were studied by transport measurements and several photoemission spectroscopic techniques. The mechanisms were discussed based on energy level diagrams derived from the ultraviolet photoelectron spectroscopy. Stacked structure not only drastically improves the luminous efficiency of OLED that emits in single-color, but also becomes a novel device architecture for white-emitting device if either blue and yellow or red, green and blue emitting materials are stacked in series. Last but not least, demonstration of a high-efficiency white-emitting OLED using stacked structure is one of the goals in my present work.

Date of Award	3 Oct 2006
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Shuit Tong LEE (Supervisor)