Atsuo Sadakata

Probing interfacial charge accumulation in ITO/α-NPD/Alq3/Al diodes under two electroluminescence operational modes by electric-field induced optical second-harmonic generation

By using electric field induced optical second harmonic generation measurement, charge accumulation at the double-layer interface of ITO/α-NPD/Alq3/Al diodes was verified under two electroluminescence (EL) operational modes, which were activated in the low and high frequency regions, respectively, with application of large ac square voltage. Results supported our proposed idea [A. Sadakata et al., J. Appl. Phys. 110, 103707 (2011)] that accumulated holes suppress hole injection in the low frequency region and lead to the decrease of the EL intensity activated by the recombination of holes and electrons injected from opposite electrodes. On the one hand, the accumulated holes assist electron injection in the high frequency region and result in the increase of EL intensity activated by the recombination of the interfacial accumulated holes and injected electrons from Al electrode.

Analyzing two electroluminescence modes of indium tin oxide/α-NPD/Alq3/Al diodes by using large alternating current square voltages

By applying large A.C. square voltages to double-layer ITO/α-NPD/Alq3/Al diodes, we studied the generation of electroluminescence (EL) in terms of the carrier injection and transport. The EL intensity gradually decayed with the increase of the frequency of the applied A.C. square voltage, and reached a minimum at the frequency corresponding to the carrier transit time. Interestingly, the EL intensity was again increased by the application of further higher frequency A.C. square voltages, relying on the D.C. component of the A.C. square voltages. The results suggest the presence of two EL modes. Using the Maxwell-Wagner effect model analysis, we have proposed a model that accounts for the two EL modes.

Electroluminescence Generated from ITO/α-NPD/Alq3/Al Diodes by Applying A.C. Square Voltage

We measured the electroluminescence (EL) generated from double-layer ITO/α-NPD/Alq3/Al diodes by applying large A.C. square voltages for understanding carrier behaviors. Two EL modes were identified in high and low frequency regions, relying on the D.C. component of the applied A.C. square voltages. This indicates that two different carrier behaviors are responsible for the EL emission from the double-layer EL diodes. Modeling the two EL modes, we showed the contribution of the D.C. component of applied A.C. square voltages to the two EL modes in terms of the interfacial Maxwell-Wagner charging.

Electroluminescence Enhanced from Electrode Interface in ITO/Tetracene/Al Diodes

We investigated emission spectra of ITO/tetracene/Al diodes for fully understanding carrier behavior in the device. By applying an A.C. square voltage, we showed that two distinct peaks were enhanced at around 545 nm and 615 nm. The peak at 545 nm is ascribed to intrinsic carrier process leading to bulk-originated EL, whereas the peak at 615 nm is due to interfacial carrier process via carrier traps nearby the electrodes, resulting in OLED degradation. To discriminate interface- and bulk-originated EL, A.C. frequency dependence of the EL spectrum was analyzed with taking into account carrier transit and carrier injection.

Influence of Pentacene Interface Layer in ITO/α-NPD/Alq3/Al Organic Light Emitting Diodes by Time-Resolved Electric-Field-Induced Optical Second-Harmonic Generation Measurement.

By using I-V, EL-V, displacement current measurement (DCM) and time-resolved electric-field-induced optical second-harmonic generation (TR-EFISHG) measurement, we studied the influence of interface pentacene layer inserted between ITO and a-NPD layers in ITO/α-NPD/Alq3/Al OLEDs. All experiments were carried out for the OLEDs with and without a pentacene interface layer. The I-V and EL-V measurements showed the decrease of operating voltage of EL, the DCM showed the lowering of inception voltage of carrier injection by inserting a pentacene interface layer. The TR-EFISHG measurement showed the faster accumulation of holes at the interface between the a-NPD and Alq3 layers, which resulted in the relaxation of electric field of a-NPD layer accomplished by the increase of the conductivity and the increase of the electric field in the Alq3 layer. We conclude that TR-EFISHG measurement is helpful for understanding I-V and EL-V characteristics, and can be combined with other methods to give significant information which are impacted by the interface layer.