In this paper, CdS/CdTe solar cells with conversion efficiency values of η = 5.1, 3.6, 3.5, and 2.9% are analyzed and the effects of the device parameters, such as the diode ideality factor (n), the saturation current-density (Jo), and the series resistance (Rs,) on the fill factor, both in the dark and under illumination are investigated. The solar cells with efficiency η = 5.1, 3.6, and 2.9% were grown from 3N source material. A solar cell with 3.5% efficiency was made of a 6N CdTe source. The temperature current density-voltage (J-U-T) and capacitance-voltage characteristics (C-U-T) of CdS/CdTe solar cells were measured in a temperature range of 303-383 K. For all studied cells, the diode ideality factor in the dark is much higher than 2 and the saturation current density increases under illumination, while both the series and shunt resistances decrease under illumination and temperature.