Efficient Trilayer Phosphorescent Organic Light-Emitting Devices Without Electrode Modification Layer and Its Working Mechanism

Abstract

At present, numerous functional layers are introduced to improve the carrier injection and balance the carrier transport in organic light-emitting devices (OLEDs). Although it may be a good way to enhance the efficiency of devices, the introduction of functional layers would also result in extra process and long manufacture period. Actually, with the enrichment of material system, many appropriate materials could be chosen to share two or even more functions in OLEDs. Here, via impedance spectroscopy and transient electroluminescence analysis, di-[4-(N,N-ditolyl-amino)-phenyl] cyclohexane (TAPC) and 4,7-diphenyl-1,10-phenanthroline (Bphen) are demonstrated to serve as carrier injection and transport layers simultaneously. As a result, efficient trilayer OLEDs are achieved with comparable performances to conventional multilayer devices. Further studies have also been carried out to analyze the recombination and quenching mechanisms in devices. TAPC can block electrons effectively, while Bphen avoids the accumulation of holes. It makes carriers in emitting layer become more balanced, resulting in the reduction of efficiency roll-off.

Organic Light-Emitting Diodes with Electrospun Electrodes for Double-Side Emissions

“…To underline the change point between leakage or diffusion-limited current caused by ohmic contact (low voltage region) and volume-controlled current with an exponential distribution of traps, known as the opening voltage of diodes, the capacitance-voltage measurements have been performed ( Figure 5 a,b). Recently, capacitance-voltage spectroscopy was used to investigate the physical mechanisms of OLEDs, where the first inflection point corresponds to the turn-on voltage [ 27 ]. In the case of the devices with electrospun cathodes, the inflection point was measured at 2.5 V while in the case of the neat film electrodes at around 2 V.…”

“…Capacitance-voltage measurements confirm the turn-on voltage point between the ohmic zone and the zone dominated by the exponential distribution of the traps. This point marks a constant injection in the devices when the OLED starts to be driven by the applied voltage, increasing the capacitance [ 27 ].…”

Operational Stability Analysis of Blue Thermally Activated Delayed Fluorescence Organic Light-Emitting Diodes Using the Capacitance-Voltage Method

“…These methods also sometimes prevent the measured OLED pixels from being used again. On the other hand, it is possible to quantitatively analyze the degradation characteristics of the aged device considering the charge accumulation feature at the organic interfaces using C-V characteristics and the Cole-Cole plot [18,19]. In this study, an impedance analyzer, which is non-destructive measuring equipment, was used to investigate the degradation features of blue TADF OLEDs by measuring the capacitance-voltage (C-V) characteristics.…”