Mixed excitation was observed based on solid-state cathodoluminescence (SSCL) structures under alternating-current (AC) applied voltages, and it combines charge carrier injection recombination and hot electron impact excitation. Under AC voltages of 500 Hz, the electroluminescence from an ITO/SiOx/MEH-PPV/SiOx/Al device contained two emission bands: a blue emission band peak at 3.02 eV (410nm) and an orange emission band peak at 2.10eV (590nm). The source of these two emission bands was investigated. Our experimental results suggest that the orange emission comes from the recombination of MEH-PPV excitons, and the blue emission is from oxygen defects and interface states of SiOx.

An improved performance of organic light-emitting diodes has been obtained by using 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4Hpyran (DCJTB) as an ultrathin emitting layer. When 0.1nm DCJTBwas inserted between the hole-transporting layer and electron-transporting layer, for an unoptimized device indiumtin oxide (ITO)/naphtylphenyliphenyl diamine (NPB)/DCJTB (0.1nm)/8-hydroxyquinoline aluminum (Alq3)/Al, the maximum brightness was 1531 cdm−2 at 15V. Compared with doped devices ITO/NPB/Alq3:DCJTB (1%)/Alq3/LiF/Al, a higher efficiency has been achieved. Compared with the conventional device ITO/NPB/Alq3/Al, the inserted device has a slightly higher current efficiency and lower turn-on voltage. We suggest the ultrathin DCJTB layer acts as trap for carriers, and the accumulated holes at the hole-transport layer/electron-transport layer interface have enhanced the electric field in the electron-transport layer and improved the electron injection at the cathode.

The photoluminescence (PL) and electroluminescence (EL) properties of OLEDs based on poly(N-vinylcarbazole)(PVK) and 2, 9-dimethyl-4, 7-diphenyl-1, 10-phenanthroline (BCP) have been studied. A new emission at 595 nm has been observed in EL spectra but not in PL spectra of the devices.We suggest the new emission originates from the electroplex occuring at PVK-BCP interface. White emission via electroplex has been obtained in all the OLEDs. CIE coordinates of device ITO/PEDOT:PSS/PVK/BCP/Alq3/LiF/Al is (0.31, 0.33) at 18 V. Alq3 layer plays an important role in tuning the recombination zone and enhancing the electroplex emission.

The PL and EL spectra of poly(N-vinylcarbazole) (PVK): 2, 9-dimethyl-4, 7-diphenyl-1, 10-phenanthroline (BCP) (1:1w/w) film were found completely different. The PL spectrum is a single peak at 415 nm that originates from excitons emission from PVK, and the tail of the spectrum is suggested to be excimer emission from BCP molecules. However, a new emission at 595 nm was found in the EL spectra of devices ITO/PEDOT:PSS(50nm)/PVK:BCP (1:1)(100nm)/Al. After aggregate, exciplex and product of electrochemical reaction were ruled out, the new emission was proposed to be electroplex emission that occurred between PVK and BCP molecules. Under high voltage, only electroplex emission can be observed in the EL spectra.

Electroplex emission based on poly(N-vinylcarbazole) (PVK) and 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) has been improved dramatically by using a multi-layer device structure indium-tin oxide (ITO)/poly(3,4-ethylenedioxythiophene): poly(styrenesulphonic acid) (PEDOT:PSS)/PVK/BCP/PVK/BCP/LiF/Al. Electroplex emission at 595nm has been improved about 10 times under low voltage and four times under high voltage compared to the double layer device ITO/PVK/ BCP/Al. The maximum brightness of the device also has been improved about eight times. Bright white emission via electroplex formation can be obtained with Commission International d’Eclairage (CIE) coordinates (0.336, 0.320) at 26V with a brightness of 123cd/m2. Based on the analysis of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the materials, we suggest the enhancement is mainly ascribed to the confinement effect of the quantum-well-like multi-layer device structure. Every hole and electron has more possibilities to cross recombination at the PVK/BCP interface.