Theory of open-circuit voltage and the driving force of charge separation in pn-junction solar cells

We have derived the formula to calculate the open-circuit voltage in a pn-junction solar cell from carrier densities by considering the driving force of charge separation without using the equation for current. The excess amount of chemical potential of charge carriers is proposed as the origin of the driving force of charge separation, and the voltage formula is derived from the gradient of excess chemical potential. The calculated voltage is shown to agree with the result of a rigorous device simulation for symmetrical pn-homojunction devices with band gaps of 0.6–1.8 eV and majority-carrier densities of 1015–1019 cm-3. The developed formula is, therefore, valid for the pn-homojunction devices, indicating that the driving force of charge separation stems from the excess chemical potential.