Built-in Voltage in Alq3 Based Organic Light-Emitting Diodes Incorporating PEDOT:PSS and LiF Layer

• Published in: Molecular crystals and liquid crystals (Philadelphia, Pa. : 2003) Pa. : 2003), 2008, Vol.491 (1), p.145-151
• Main Authors: Lee, Eun Hye, Yoon, Hee Myoung, Han, Wone Keun, Kim, Tae Wan, Ahn, Joon Ho, Jang, Kyung Uk, Hong, Jin Woong
• Format: Article
• Language: English
• Subjects: built-in voltage; modulated photocurrent; organic light-emitting diodes
• ISSN: 1542-1406; 1563-5287; 1543-5318
• DOI: 10.1080/15421400802329574

Built-in voltage was measured in Alq 3 based organic light-emitting diodes incorporating a PEDOT:PSS and LiF layer by using a modulated photocurrent technique at ambient conditions. A device was made with a structure of anode/Alq 3 /cathode. Either an ITO or an ITO/PEDOT:PSS was used as an anode, and a LiF/Al was used as a cathode. The built-in voltage of the device is determined from a bias voltage-dependent photocurrent. The applied bias voltage when the magnitude of modulated photocurrent is zero corresponds to a built-in voltage. It was found that an incorporation of PEDOT:PSS layer between the ITO and Alq 3 increases a built-in voltage by about 0.4 V. This is consistent to a difference of a highest occupied energy states of ITO and PEDOT:PSS. This implies that a use of PEDOT:PSS layer in anode lowers a hole barrier height. With a use of bilayer cathode system LiF/Al, it was found that a built-in voltage increases as the LiF layer thickness increases in the thickness range of 0 ∼ 1 nm. It indicates that a very thin alkaline metal compound LiF lowers an electron barrier height. These results could be related to an improvement of device efficiency brought about by the insertion of PEDOT:PSS and LiF layer.