The contact potential of a pn-junction diode is signified by the turn-on or barrier voltage, which is the voltage beyond which non-negligible current con be measured flowing in the forward-bias direction. To put it simply, one can run a variable potential difference across a diode in it forward-bias direction until one measures a current. That is the contact potential of the diode. Theoretically, the contact potential is a function of the temperature and doping concentration, and intrinsic hole-electron pair concentrations. However, in the real world, there maybe other factors that will affect the contact potential of a diode.

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.