Spectral features of the Stark signals that are observed in photoinduced optical spectra for dye-sensitized solar cell systems are examined using continuous-wave-photoinduced absorption techniques combined with bias application. The Stark signals are found to almost disappear with increasing the bias voltage. The disappearance of Stark signal indicates that there exists a unidirectional built-in electric field acting on neutral states of dye molecules. Time-dependent measurements demonstrate that the Stark signal slowly rises, whereas the photocurrent exhibits a rapid rise and then decays slowly. This finding indicates that, although the screening effect occurs at the beginning of the cell operation, it is gradually lost during the operation. We conclude that the occurrence of the Stark effect is attributed to the electrolyte ions that gradually become deactivated or transfer away from the TiO2 layer. (C) 2013 Elsevier B.V. All rights reserved.