Physical and chemical plasma pretreatment of indium tin oxide and influence on organic light emitting diodes
Presentation held at Plastic Electronics, PE 2010, October 19-21, 2010, Dresden
A commercial, vertical inline deposition tool for mass production of OLEDs (provided by Applied Materials) with a maximum substrate size of 370x470 mm2, was equipped with two different types of plasma sources for pre-treatment of indium tin oxide (ITO). We have studied the difference in ITO surface properties after chemical and physical plasma treatment. The physical bombardment of ITO with high energetic Ar+ and/or O+ ions showed an increase in surface energy, sheet resistance and transmittance in the near infrared range in respect to the kinetic ion energy and the amount of oxygen in the process gas. We believe that changes of electrical and optical properties of ITO were caused by penetration of oxygen deeper into the bulk material. On the other hand, a chemical treatment of ITO with oxygen radicals increased surface energy as well, but did not change optical and electrical properties. Due to the deposition of hole-only devices on plasma treated ITO, we could measure an improvement in injection current of more than one order of magnitude, compared with hole-only devices on untreated or ion bombarded ITO. The raising in injection current could be explained by an oxidized surface which forms a stronger surface dipole. That should increase the ITO work function and lower the barrier height for holes at the ITO/organic interface. Organic light emitting diodes with molecular doped transport layers and orange phosphorescence emitter were deposited on those ion bombarded or oxidized ITO. By measuring the J-V characteristic, we observed a ~0.4 V low voltage at a luminescence of 1,000 cd/m2 for OLEDs deposited on oxidized ITO. Thereby, we achieved the maximum power efficiency of 28.3 lm/W for such single unit OLED stack. However, the current efficiencies with 26.1 cd/A for OLEDs built on ion bombarded ITO were only slightly higher than 24.6 cd/A for OLEDs onto ITO, pre-treated with oxygen radicals. Surprisingly, such subtle distinction in current efficiencies yielded with 8,000 hours in five times longer life time, as OLEDs on oxidized ITO.