Effect of the electric field during annealing of organic light emitting diodes for improving its on/off ratio

Abstract

If an organic light emitting diode is to be used as part of a matrix addressed array, it should exhibit low reverse leakage current. In this paper we present a method to improve the on/off ratio of such a diode by simultaneous application of heat and electric field post device fabrication. A green OLED with excellent current efficiency was seen to be suffering from a poor on/off ratio of 10(2). After examining several combinations of annealing along with the application of a reverse bias voltage, the on/off ratio of the same device could be increased by three orders of magnitude, specifically when the device was annealed at 80 °C under reverse bias (-15 V) followed by slow cooling also under the same bias. Simultaneously, the forward characteristics of the device were relatively unaffected. The reverse leakage in the OLED is mainly due to the injection of minority carriers in the hole transport layer (HTL) and the electron transport layer (ETL), in this case, of holes in tris-(8-hydroxyquinoline)aluminum(Alq3) and electrons in 4,4',4''-tris(N-3-methylphenyl-N-phenylamino)triphenylamine (m-MTDATA). Hence, to investigate these layers adjacent to the electrodes, we fabricated their single layer devices. The possibility of bulk traps present adjacent to electrodes providing states for injection was ruled out after estimating the trap density both before and after the reverse biased annealing. The temperature independent current in reverse bias ruled out the possibility of thermionic injection. The origin of the reverse bias current is attributed to the availability of interfacial hole levels in Alq3 at the cathode work function level in the as-fabricated device; the suppression of the same being attributed to the fact that these levels in Alq3 are partly removed after annealing under an electric field.

Sulfonyl-Substituted Heteroleptic Cyclometalated Iridium(III) Complexes as Blue Emitters for Solution-Processable Phosphorescent Organic Light-Emitting Diodes

“…1 mA/cm 2 (Figure 5d), is typical of some OLEDs at the device turn-on due to a slight change in material morphological defects caused by the initial current passing through the device. 52…”

Mediation of exciton concentration on magnetic field effects in NPB : Alq3-based heterojunction OLEDs

“…According to the spin-statistics principle, the ratio of PP S to PP T based on spin pairing within organic electroluminescent devices is 1 : 3. 30,31 In fluorescent devices, S 1, Alq3 generates fluorescence, while T 1 does not contribute directly to the luminescence due to the forbidden transition. It was found that under certain conditions, S 1, Alq3 and T 1, Alq3 can be interconverted, and two T 1, Alq3 can produce an S 1, Alq3 and a ground-state molecule, a process known as TTA, 32–35 creating delayed fluorescence.…”

High‐Resolution 3D Printing of Freeform, Transparent Displays in Ambient Air

“…[45,46] For example, the maximum current efficiency of the OLED using SPW-111 appears at low current density, but the maximum current efficiency of the other OLED using TBADN:DPAVBi appears at high current density. [52,53] As shown in Figure S4 in the Supporting Information, the properties (turnon voltage, luminance at 11 V, and maximum current efficiency) of the e-jet-printed OLEDs with or without the thermal drying process (130 °C for 10 min) were not significantly different. As shown in Table S4 in the Supporting Information, the properties of the e-jet-printed OLED were comparable to the properties in previous results.…”

“…Also, the processing steps of this hybrid 3D printing system require no additional thermal drying/annealing steps immediately after the materials were printed because our e‐jet system uses fine nozzles to facilitate the ejection of small droplets, which are dried very quickly during their flight . However, additional thermal treatments of the fully fabricated OLEDs can improve the device performances further by enhancing the diffusivity of carriers . As shown in Figure S4 in the Supporting Information, the properties (turn‐on voltage, luminance at 11 V, and maximum current efficiency) of the e‐jet‐printed OLEDs with or without the thermal drying process (130 °C for 10 min) were not significantly different.…”