We systematically studied the excitation and the fluorescence steady-state spectroscopy of poly[2-methoxy-5-(2-ethylhexoxy)-p-phenylene vinylene] (MEH-PPV) in two solvents and several concentrations. Fluorescence spectra were recorded for solutions in several concentrations (10-5mg/ml to 10-3mg/ml), showing that tetrahydrofuran (THF) and toluene solvate the polymer chain differently. Dilute solution (10-5mg/ml) in THF exhibit broader fluorescence spectra due to greater conformation disorder. The degree of the aggregation depends on both the solvent and the polymer concentration. Aggregation is promoted in toluene solution and hindered in THF solvent.

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.

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.

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.

The capacitance effect on ITO/poly[2-methoxy-5-[(2´-ethylhexyl)oxy]-p-phenylenevinylene] (MEH-PPV)/Al is studied by impedance spectroscopy technology, and the electroluminescence (EL) mechanism of this device driven by a sinusoidal alternating-current (AC) bias is suggested. By calculating the RC time constant of the device, we find that it is in good agreement with the lag-time between the EL and applied AC bias. Also, the influence of operating frequency on the EL intensity of the device is presented and it is concluded that a low operating frequency is good for a high device performance.

The definition and role of "host" in polymer LED materials are studied in the present work. "Primary host" and "associate host" have been proposed and the rules of how to select an associate host are reported. Based on our experiments and the analysis of the energy scheme of the devices, we suggest that the values of the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) are critical determinant in selecting a suitable associate host. On one hand, the associate host should be a hole-blocking material. This can confine the excitons in the active layer. On the other hand, the associate host should have a suitable LUMO that is convenient for electrons to transport.

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.

We report the effect of inorganic nanolayers on electron injection in the polymer light-emitting diodes (PLEDs) in which a hole is the major charge carrier. The inorganic nanolayers with different dielectric constants were placed between the emitting layer and the aluminum cathode, and their influence on the device performance was investigated. The device with a nanolayer of lower dielectric constant demonstrated higher efficiency, which is significantly higher than that of one without an insulating layer. The enhancement may result from the lowering of the effective barrier height for electron injection while increasing electron-tunneling probability, which improve the balanced injection of hole and electrons

The steady current-voltage characteristics of single layer organic devices based on MEH-PPV and N, N0-diphenyl-N, N0-bis(40-[N, N-bis(naphth-1-yl)-amino]-biphenyl-4-yl)-benzidine (TPTE) blend with different TPTE concentrations was investigated. The thickness dependence of the current-voltage relationship clearly demonstrates that the current at low voltage and at high voltage are all space charge limited. The current density-electric field characteristic proves the blend polymer LEDs to operate in the tunneling-controlled model. The effective hole mobility is directly determined by space charge limited current at high voltage and increases with increasing TPTE content in the blend. The EL efficiency shows concentration dependence, which is attributed to the change of the transport of holes in the blend film.