Melting Effect of Hole-Injecting Layer on the Performance of Passive Matrix Organic Light-Emitting Displays

Abstract

Here we report improved operation stability of passive matrix organic light-emitting displays (PM-OLED) by melting a hole-injecting layer (HIL) that is the first organic layer contacting anode. The PM-OLED displays fabricated in this work are consisted of 128� 128 pixels in which each pixel has a dimension of 200μm� 200μm. The exact thermal tran- sition behaviour of hole-injecting material was first examined using a differential scanning calorimeter in order to decide the melting temperature for the HIL melting process (300 o C/3min). Results show that the display with the untreated (as- coated) HIL exhibited large leakage current which eventually resulted in damages (black cross-talk lines) to the display during operation. However, no cross-talk defect was observed for the PM-OLED display with the thermally treated (melted) HIL, which was supported by the absence of leakage current at reverse bias. Since the breakthrough works on organic light-emitting devices (OLED) based on either low-molecular-weight mate- rials (i.e., small molecules) (1) or polymers (2), OLED dis- plays are now in market even though their applications are limited to small size display for MP3 players, mobile phones, car front panel devices, shavers, etc (3, 4). This suc- cessful debut of OLED display into market can be mainly attributed to remarkable advances in organic semiconductor materials and process technology (3-5). However, these OLED displays do still suffer from their short lifetime though the thermal stability of organic materi- als themselves has been significantly improved when it comes to the test device measurement (4, 6, 7). In case of OLED displays made using small molecules, it has been re- ported that a progressive electrical short (PES) phenomenon is responsible for the degradation of display pixels during long time operation (8). This report claimed that the PES phenomenon is closely related to the formation of unstable defects in organic layers which eventually leads to cata- strophic degradation of whole layers in pixels. In this report the PES phenomenon could be healed by thermal treatment at temperatures below 100 o