If all the solar cells in a solar panel have the same electrical characteristics, and they all experience the same insolation and temperature, then all the cells will operate at the exact same current and voltage. In this case, the PV module's IV curve has the same shape as a single cell, but with increased voltage and current.
For a group of identically connected solar cells, the total current is simply the current of a single cell multiplied by the number of cells connected in parallel. Thus. The total voltage is the voltage of a single cell multiplied by the number of cells in series.

If the solar cells are identical, the fill factor does not change when the cells are connected in parallel or in series. However, there is usually a mismatch in the cells, so when the cells are combined, the fill factor decreases. Cell mismatches can come from manufacturing, or from differences in light across the cells, i.e. one cell has more light than the other.