Displacement current measurement (DCM) is a simple but powerful tool for exploring charge carrier dynamics in organic semiconductor devices. In the first section, we review the basic concept of DCM and how it detects the charge injection, extraction, accumulation, and trapping behaviors in organic semiconductor devices within a quasistatic regime. Subsequently, we present applications of this technique to investigate the device properties of tris-(8-hydroxyquinolate) aluminum ()-based organic light-emitting diodes. We observed that light irradiation during device fabrication induces additional negative space charges and charge traps in the layer. In addition, the device containing the illuminated film exhibits a lower luminous efficiency and shorter lifetime compared to the device fabricated in dark conditions, possibly because of the additional hole accumulation in the illuminated film. DCM detects the formation of charge traps in the aged devices, decay of the negative space charge, and increase in hole injection voltage with device aging. The origins of these behaviors can be attributed to orientation polarization and charge traps in film.