*3.1.3. The significance and future development of the donor/ acceptor interface in OLEDs*

Very recently, the planar and mixed donor/acceptor interfaces have also been used as very efficient interconnecting structures for tandem OLEDs [27], showing the versatility of the donor/acceptor pairs. However, it should be noted that all the donors and acceptors currently used to form the hole-generating interfaces in OLEDs exhibit significant absorption in the visible-light range, thus reducing the light out-coupling from the devices. Therefore, it is of great importance to develop wide-bandgap donors and acceptors. In addition, in order to increase the amplitude of hole current at the donor/acceptor interfaces, it is of much necessity to narrow down the offset between the HOMO of donor and LUMO of acceptor.
