**Author details**

Dashan Qin *Institute of Polymer Science and Engineering, School of Chemical Engineering, Hebei University of Technology, Tianjin, People's Republic of China* 

Jidong Zhang

172 Organic Light Emitting Devices

**4. Conclusions** 

**Author details** 

Dashan Qin

injection into the bottom unit, relative to IS2.

It should be pointed out that, due to the poor ability of Li2CO3:PTCDA to inject electrons into the traditional electron transport materials (e.g., Alq3, BCP), an n-doped layer with high-lying LUMO level, n-doped BCP must be involved to facilitate the electron injection from n-doped PTCDA into the traditional electron transport materials [13]. Thus, IS1 cannot only reduce the ohmic loss of current conduction, but also can offer comparable electron

*3.2.5. The design concept of uniting two n-doped layers and its significance in OLEDs* 

The n-doped organic electron acceptors, e.g., n-NTCDA, n-PTCDA and n-C60, possess markedly higher conductivities but lower capabilities of injecting electrons into electron transport layer (such as BCP, Alq3, etc.), as compared to the frequently used n-doped materials (such as n-BCP, n-Alq3, etc.) in organic light emitting diodes (OLEDs). In this section, we study the combination of the above two classes of n-doped materals, called the structure of uniting double n-doped layers (n-ETL1/ n-ETL2/ ETL). The characteristics for the structure of n-ETL1/ n-ETL2/ ETL are as follows: n-ETL1 features the LUMO level greater than 4.0 eV (i.e., the LUMO level lies below 4.0 eV) and possesses higher conductivity of transporting electrons; n-ETL2 features the LUMO level less than 3.4 eV (i.e., the LUMO level lies above 3.4 eV) and possesses better capability of injecting into ETL; at the n-ETL1/ n-ETL2 interface, due to the quasi Fermi level alignment. In this caes, the energy barrier for the electron injection from n-ETL1 to n-ETL2 is usually less than 0.2 eV. Such a constructed combination cannot only significantly reduce ohmic loss in electron conduction, but also possess strong capability of injecting electrons into ETL. It has been demonstrated that the structure of uniting double n-doped layers enables remarkable enhancement in both electron current and luminous performance for inverted and tandem OLEDs. Therefore, it can be considered as an advanced electron injection technology, which can significantly push forward the commercializations of organic flat-panel displays and solid-state lighting.

Currently, the commerical application of organic lighting is being held back by the fact that the power efficiency of OLEDs drops drastically with the increase in active area.This means that the luminous flux of OLEDs does not increase automatically if the emissive area is increased. Thus, it becomes the top priority to develop the charge injection techniques which can increase the power and emittance of OLEDs. The charge injection techniques discussed in this chapter are still in their very early stages, and their further development depends

mostly by inventing new materials which can meet all the requirements listed above.

*Institute of Polymer Science and Engineering, School of Chemical Engineering,* 

*Hebei University of Technology, Tianjin, People's Republic of China* 

*State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin Province, People's Republic of China* 
