Organic light-emitting devices (LEDs) have attracted much attention due to their potential application for flat-panel displays. People apply themselves to develop the light-emitting materials with high performance and light-emitting devices with high efficiency and stability. In this paper

we discuss the design of the molecular structure and the method to improve the emitting characteristics of the LEDs. The materials used for the LEDs include light-emitting materials

carrier-transporting materials

and electrode materials. The performance of these materials can be optimized by the design of the molecular structure. The materials with good performance must have high photoluminescent quantum efficiency

good carrier transporting ability and thermal stability. To date

efficiencies for organic LEDs have been limited by the use of fluorescent emissive materials. This limit arises since fluorescence only involves singlet relaxation

thus eliminating the participation of the triplet exciton population. By employing a phospherescent dye where both singlet and triplet excited states participate

the LEDs internal efficiency can

in principle

be increased by as high as 100%. Another method to improve the emitting efficiency is that employing the Frster excitation energy transfer process. Blending a donor with an emitting acceptor for the energy transfer highly reduces the concentration quenching of the generated excitons. Therefore

the energy transfer in the blend systems works effectively to improve the external quantum efficiency of the light-emitting diodes. But it is difficult for the doping system to produce red LEDs with high luminance efficiency. Because the energy transfer from host to guest is not very complete due to the weak overlap between the emission of host and the absorption of red dyes

and in addition

the concentration quenching at high doping concentration. Recently

there are some reports to use two-step energy transfer to increase the emission characteristics of organic LEDs. The emitting layer of the LEDs are co-doped with three different fluorescent materials (A

B and C)

and the energy transfer process is from A to B and then from B to C

which is a promising method for obtaining red LEDs. The structure of the LEDs is important for the improvement of the brightness and emitting efficiency of the LEDs. When design the structure of the LEDs

the match of the energy band and layer thickness and refractive index

and the balance of the injected carriers between the carrier-transporting and emitting layers must be taken into account.