WL090401CurrentOpinioninSolidStateMaterialsScience(139)

Mechanisms
of organic EL
Burrows et a/. [7**] have shown that small molecule-based OLEDs may be modeled by assuming that both current conduction and EL yield is ultimately controlled by space charge effects in the presence of an intrinsic trap distribution. The depth and distribution of the traps is
236
Prospects and applications
Paul E Burrows*‘,
Efficient electroluminescence
for organic light-emitting
devices
Stephen
R Forrest* and M&-k
progress towards meeting these goals from January 1995
from an organic thin film at in 1987. Since then, of flat panel in In early 1997, of organic Advances to compete flat panels. thin films create of full color and
Introduction
the field of organic light-emitting devices steadily increasing research interest [ 1,2,3**,4,5**,6*]. The enormous extent of commercial
(OLEDs) has generated
leaves them poised emissive properties of organic
as monochrome, for realizing
on the unique displays.
a new generation
Addresses *Center for Photonic and Optoelectronic Materials (POEM), Department of Electrical Engineering, C436 ECIuad, Princeton University, Princeton, NJ 08544, USA te-mail: burrows@ *Department of Chemistry, University of Southern California, Los Angeles, CA 90089, USA Current Opinion 2~236-243 in Solid State & Materials Science 1997,
Inቤተ መጻሕፍቲ ባይዱrecent
years,
and academic investment [6*] in OLEDs, illustrated in Table 1, is largely stimulated by their potential application to low cost, lightweight, low voltage and full color flat panel displays. Successful development of emissive, OLED-based displays has the potential to revolutionize a $30 billion a year market [l]. Commercial success, however, requires that such displays compete with existing flat panel technologies such as back-lit liquid crystal displays, necessitating high brightness, high efficiency (i.e. low operating voltage and current), good color saturation, and long lifetime. Satisfactory performance for indoor and portable display applications requires a brightness of ~100 cd/m2 at an operating voltage of between 5 and lSV, an efficiency of =5 lm/W and a continuous operational lifetime of at least 10000 hours. In this paper, we will review
molecules used for red (R), green (G) and blue (B) light emission are shown in Figure 2, along with three examples of charge-transporting molecules. In many cases (Alq3, for example) the functions of electron transporter and light emitter may be combined in a single layer, resulting in ‘single heterostructure’ devices.
low drive voltage was first reported
the possibility of realizing a new generation both industrial and academic the improvement light-emitting commercially based flexible the potential devices laboratories.
displays has driven an increasing volume of research in the efficiency and lifetime
to the present. We also highlight the unique properties of organic materials which allow for display applications unobtainable with conventional materials. The basic structure of an OLED is shown in Figure 1 and typically consists of three organic semiconductor layers (an organic ‘double heterostructure’) sandwiched between two electrodes. The top, electron-injecting electrode consists of a low work function metal alloy, typically Mg-Ag or Li-Al, deposited by vacuum evaporation. The bottom, hole-injecting, electrode is typically a thin-film of the transparent semiconductor indium tin oxide (ITO), deposited onto the substrate by sputtering or electron beam evaporation. Light is emitted through this electrode when the device is operated in ‘forward bias’, that is with the IT0 biased positive with respect to the top electrode. The organic semiconductor layers are either spin-coated polymeric materials [3,6*] or vacuum-deposited ‘small molecules’ [l,Z], each layer being 100-1000 A thick in both cases. Vacuum deposition offers the advantages of better control over layer thickness and uniformity (critical parameters for displays), and access to higher purity starting materials. In contrast, spin coating offers potentially lower cost and ease of use over relatively small areas. Deposited on the anode is a hole transporting layer (HTL) of a material such as TPD (N,N’-diphenyl-N,N-bis[3methylphenyll-l,l’biphenyl_4,4’diamine), shown with other widely used molecules in Figure 2. This is followed by the light-emitting (EL) layer consisting of, for example, Alq’zOPh (bis-[&hydroxy] quinalidine aluminum phenoxide) for blue light, or Alq3 (tris-[&hydroxyquinoline] aluminum) for green light. An electron transporting layer (ETL), consisting of a material such as TAZ (l-phenylZ[4-ten-butylphenyll-5biphenyl-1,3,4-triazole) completes the double heterostructure. Doping a small amount (-1 mol%) of a fluorescent dye into the EL emission layer can significantly improve the device efficiency, is now routinely obtained [Z]. Representative and >l Im/W examples of