Surface Treatment of Indium Tin Oxide Using Radio Frequency Atmospheric and Low Pressure Plasma for OLEDs

Abstract

We investigated the effect of atmospheric-pressure plasma ͑APP͒ and low-pressure plasma ͑LPP͒ treatments on the performance of organic light emitting diodes ͑OLEDs͒ with an indium tin oxide ͑ITO͒ layer. The Owens-Wendt and Lifshitz-van der Waals acid-base methods revealed that the increase of surface energy was mainly attributed to polar component ͑␥ s p ͒ and Lewis base ͑␥ s − ͒ interactions, respectively, independent of either APP or LPP treatments. Unlike APP treatment, LPP treatment more plentifully produced reactive oxygen species in the plasma. Therefore, the LPP-treated ITO surfaces slowly proceeded with reorientation compared to APP-treated ITO. The carbon content in untreated ITO was approximately 0.045%, while those of Ar APP-, Ar/O 2 APP-, and O 2 LPP-treated ITO were 0.014, 0.011, and 0.010%, respectively, mostly containing incorporated reactive oxygen. The O 2 LPP-treated surface showed more uniform roughness than Ar or Ar/O 2 APP-treated surfaces. The highest work-function ͑⌽͒ value ͑4.58 eV͒ was obtained from O 2 LPP-treated ITO, intermediate values ͑4.47-4.48 eV͒ from Ar and Ar/O 2 APP-treated ITOs, and the smallest value from untreated ITO ͑4.46 eV͒. Thus, OLED fabricated on the surface of O 2 LPP-treated ITO substrate exhibited superior performance among all plasma-treated samples.Indium tin oxide ͑ITO, ϳ9 to 10 mol % tin oxide in indium oxide͒ is a highly degenerate n-type semiconductor with a wide bandgap ͑3.5-4.5 eV͒. ITO films have excellent electrical and optical properties due to their easy availability ͑fabrication of liquidcrystal industry͒, good optical transparency ͑Ͼ90% at 550 nm͒, good conductivity due to oxygen vacancies in In 2 O 3 and doped impurities, i.e., Sn, low electrical resistivity, and the ease with which it can be patterned. Therefore, ITO has been widely used in organic light emitting diodes ͑OLEDs͒. [1][2][3][4][5][6] Recently, OLEDs have received much attention through their potential use in full-color display because of many merits like low driving voltage ͑3-15 V͒, high brightness ͑Ͼ1000 cd/m 2 ͒, high response time, wide view angle, slim and light, and simple process. OLED devices are usually made up of ITO anode/organic layer/ metal cathode contact. Because the organic film is in direct contact with the ITO, there exists an interfacial barrier formed between the organic layer and ITO. The barrier influences the injection of holes from ITO to the organic layer and results in lower emission efficiency due to abnormal behaviors such as shorting, unstable current-voltage ͑I-V͒ characteristics, and damage on the surface of the top cathode contact after continuous operation of the device in the OLED built on bare, cleaned ITO substrates. Because the increase in work function of ITO decreases the barrier height 7 through modifying the surface properties of ITO, many researchers have attempted various methods, including chemical processes ͑aquage-ria, degreasing, and RCA protocol͒ 8 and physical treatments using oxygen, 9-14 argon plasma, 15,16 and UV ozone treatment, 17,1...

Improving the Morphological and Optical Properties of Sputtered Indium Tin Oxide Thin Films by Adopting Ultralow-Pressure Sputtering

“…On the basis of the above results, we can confirm that we have obtained ITO films that have lower surface-roughness values ͑R rms : 0.6 nm͒ than those previously reported ͑R rms : Ͻ3.2 nm͒. 4,10,16 No additional processing steps, such as the addition of H 2 O, chemical mechanical polishing, or ion-beam treatment, were used in our methods. According to the electrical data ͑resistiv-ity and carrier density͒ in Table I, decreasing the SP is also found to affect the electrical properties of the ITO thin films.…”

Spray-Coating Thin Films on Three-Dimensional Surfaces for a Semitransparent Capacitive-Touch Device

“…The subsequent deposition of the material from these droplets into morphologically uniform (i.e., a continuous layer of material) films has been considered a challenge , primarily because of the large number of parameters involved in the drying process. However, uniform thin film morphology is essential in many devices (e.g., solar cells, OLEDs, field-effect transistors, saturable absorbers, and TCFs , ) to reduce defects, improve efficiency or field-effect mobility, and achieve a percolating film. , The ink properties such as boiling point ( B p ), η, and γ lv can affect morphological uniformity of these films. Increasing η can hinder the movement of particulates and could likely improve the morphological uniformity of a sprayed film; however, increasing the η of an ink often requires the addition of a rheology modifier, which significantly reduces the electrical, mechanical, and optical performance of a TCF.…”

“…36 The subsequent deposition of the material from these droplets into morphologically uniform (i.e., a continuous layer of material) films has been considered a challenge 37,38 primarily because of the large number of parameters involved in the drying process. However, uniform thin film morphology is essential in many devices (e.g., solar cells, 38 OLEDs, 39 field-effect transistors, 26 saturable absorbers, 28 and TCFs 20,21 ) to reduce defects, improve efficiency or field-effect mobility, 26 and achieve a percolating film. 18,40 The ink properties such as boiling point (B p ), η, and γ lv 29 can affect morphological uniformity of these films.…”

Indium Tin Oxide (ITO) based Ohmic Contacts on Bulk n-GaN Substrate

“…A summary of ohmic contact resistances to GaN was reported by Greco et al 4 The band diagram for ITO/n-GaN heterostructure is shown in ). 40,41 The high doping in semiconductor facilitates to narrow down the barrier, allowing electrons to directly tunnel through the interface due to significant band bending of the conduction band. The experimental results from the C-V measurements shows an order magnitude increment (see Table I) in the carrier concentration for the plasma treated bulk n-GaN.…”