We measured the time profiles of terahertz (THz) radiation emitted from a GaAs-based solar cell under weak excitation with ultrashort optical pulses. The time-domain THz waveform directly reflects the characteristic saturation and overshooting of the drift current under high fields in the thin depletion layer of the solar cell, from which we can evaluate the internal electric field quantitatively. We also measured the time profiles of THz radiation for simultaneous excitation with ultrashort pulses and continuous light and observed the reduction of the internal field in the depletion layer of the solar cell during operation. Since nonlinear responses of the drift current in the depletion layer can be observed, we conclude that THz measurements provide a key technology for characterizing the solar cell performance that can be expected under practical operating conditions. (C) 2018 Author(s).