Examination of Spontaneous Orientation Polarization in Wet‐Processed Tris(8‐hydroxyquinolinato)aluminum Film Measured by Rotary Kelvin Probe Method

Abstract

Recently, a spontaneous orientation polarization (SOP) has attracted much attention as an important factor improving the performance of organic light‐emitting devices. However, so far, SOP is reported only for films fabricated by vacuum vapor deposition, and no direct attempt is made to examine the existence of SOP for wet‐processed film. To examine this phenomenon, it is necessary to completely exclude light illumination in the sample system, as photocarriers can reduce or eliminate the surface potential caused by the SOP formation. Herein, the SOP of wet‐processed tris(8‐hydroxyquinolinato)aluminum (Alq3) film on an indium‐tin‐oxide substrate is investigated using a novel rotary Kelvin probe method. Film formation and surface potential measurement are conducted under completely dark condition. A surface potential shift of −0.33 V is observed for a 1 μm thick film of Alq3, indicating that the SOP of wet‐processed Alq3 film is negligible, in contrast to vacuum‐evaporated film.