Tunneling effect on enhanced OLED performance using Al<inf>2</inf>O<inf>3</inf> buffer layer

Abstract

It has been found that introduction of buffer layers between organic holes transport layer and anode layer plays an important role in improving device stability and hole injection efficiency of organic light-emitting diodes (OLED). As for the mechanism of the improvement due to the buffer layer, it is still controversial. To understand the mechanism behind the enhanced performance of OLED by the buffer layer, a new model is therefore proposed which combines classical model and quantum tunneling model to explain the OLED performance improvement. A thin Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> insulating buffer layer deposited on indium tin oxide (ITO) anode by atomic layer deposition has been investigated for OLED. The observed power efficiency and current efficiency improvement at the optimal thickness of 1.4 nm are well explained by the model. A series of Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> films of different thicknesses were deposited on ITO anode and characterized. Their roughness, sheet resistance, surface potential, and resulted OLED current density were investigated. It is believed that the blocking of hole inject by the Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> buffer layer makes more balanced carrier density in emission layer, thus enhances the current efficiency. Though less number of holes are injected in OLED due to the insertion of Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> layer, quantum tunneling through the ultra-thin buffer layer play an important role to contribute to the hole injection, which avoids crossing the interface barrier, resulting in less energy consumed and power efficiency enhanced.