We demonstrate the sensitive measurement of photocarriers in an active layer of a GaAs-based photovoltaic device using time-resolved terahertz reflection spectroscopy. We found that the reflection dip caused by Fabry-Perot interference is strongly affected by the carrier profile in the active layer of the p-i-n structure. The experimental results show that this method is suitable for quantitative evaluation of carrier dynamics in active layers of solar cells under operating conditions. Published by AIP Publishing.