Effects of aquaregia treatment of indium–tin–oxide substrates on the behavior of double layered organic light-emitting diodes

Abstract

The effects of a controlled aquaregia treatment of indium–tin–oxide (ITO) substrates on the behavior of highly efficient vacuum evaporated double layered 8-tris-hydroxyquinoline Al (Alq3)-based light-emitting diodes are described. It is found that in suitably treated devices, both current injection and the electroluminescence (EL) are significantly enhanced. The enhancement is believed to result from the greater ITO/hole transporting layer contact areas and the contact conditions. The observed dependence of I(V), the EL output, and the EL efficiency on the ITO surface morphology indicates that space-charge-limited currents dominate the behavior of the devices.

Correlation between the Indium Tin Oxide morphology and the performances of polymer light-emitting diodes

“…We obtained similar values among the samples around 1.9 ± 0.1 Ω µm which indicates that the composition of the ITO layer is not affected by the acidic treatment. Moreover, Li et al [3] did not observe, by XPS, any chemical difference between aqua regia-treated and untreated ITO.…”

“…But, without surface modification of the ITO, PLED devices usually exhibit poor performance. Many surface treatments of the ITO have been employed to improve the device performance, including aqua regia treatment [3], oxygen plasma treatment [4], [5], [6] and [7], atmospheric plasma treatment [8], ultra-violet ozone treatment [9], carbon tetrafluoride/oxygen treatment [10] and various coating treatments such as for example self-assembled monolayers [11] and [12]. Alternatively, by inserting an additional thin layer, such as copper-phtalocyanine [13], polyaniline [14], poly(3,4-ethylene dioxythiophene) doped with polystyrene sulfonate (PEDOT-PSS) [15] and [16], inorganic or organic insulators [17] and [18], emission intensity and device stability can be improved by lowering the driving voltage.…”

Parametric Studies of Pulsed Nd:YAG Laser Deposition of ITO for OLED Application

“…higher rate of charge injection, a wider operating voltage range (+1.7 V) and hence an enhanced maximum brightness (+74 %). Wantz et al 25 and Li et al 26 reported that increasing the ITO surface roughness, by diluted aquaregia surface treatment, could greatly enhance their bi-layer OLED brightness although the lifetime was not investigated. They attributed the surface roughness effect to an increase of the anodic contact surface area and unevenness in enhancing the hole injection from the ITO layer into the hole transport layer.…”

Device Optimization and Transient Electroluminescence Studies of Organic light Emitting Devices

“…The most common is UV-ozone treatment, where the ITO film is exposed to ozone produced by a UV lamp. 86 Other procedures involve partial etching of the ITO in aqua regia 87 or plasma etching. 88 However, since the excess oxygen typically evolves out of the treated ITO within a few hours, the organic layers must be deposited on the ITO immediately after the treatment.…”

“…Before the deposition of the organic layers, it was also partially etched by aqua regia (25% HNO3, 75% HC1) to improve the devices' performance. 87 This treatment was used in place of the more standard UV-ozone treatment. The desired sheet resistance after the aqua regia treatment was ~ 40-50 £2/sq.…”