The variable that represents potential difference is V, which stands for voltage.

Potential Difference is measured in Volts

There is no such thing as a 'voltage difference'! 'Voltage' means 'potential difference', so what you appear to be asking is "How do you get the largest potential difference difference?'!

Potential difference is caused by the separation of charges between two points. The greater the amount of charge separation, the greater the potential difference.

electric potential is potential difference between two points in closed circuit.

but electromotive force is potential difference in any open circuit.

'Force' isn't measured in volts. Potential and potential difference are measured in volts.