The open circuit voltage is derived as an explicit function of the carrier concentrations at the junction edges through the concept of the quasi-Fermi-levels. Then, the carrier concentrations, and hence the open circuit voltage is studied as a function of the solar concentration ratio. The limit of the cells with and without back confinement (back surface field), the importance of effective cooling, and the effects of band narrowing and the cell dimensions are examined. Back confinement cells have higher photovoltages and can operate at higher concentration ratios efficiently for a given cooling scheme. A relatively light 10**1**5cm** minus **3 doping is chosen for the sample cells.