*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.
