Built-in Potential and Open Circuit Voltage of Heterojunction CuIn 1-x GaxSe 2 Solar Cells

The reasons why the open circuit voltage (V oc ) of high-x CuIn 1-x Ga x Se 2 (CIGS)/ZnO solar cells remain low are discussed. Here it is shown that the V oc ceiling can be interpreted simply on the basis of a model that the valence-band energy (E v ) of CIGS is almost immovable irrespective of x . When the conduction-band energy (E c ) of ZnO is lower than that of high-x CIGS (DEc bi ) that arises from the separation between the Fermi energies of the two materials. If the Ev (and therefore the Fermi energy) of p-type CIGS is constant with increasing x, the V bi and V oc that follows the V bi remain unchanged since the Fermi energy of ZnO is constant. This unchangeable V oc reduces the conversion efficiency of high-x CIGS cells in cooperation with reduced photocurrents due to a larger bandgap. A positive offset, ΔE c >o gives rise to a photoelectrons barrier in the conduction-band that partially cancels V oc , thus the V oc of a low-x CIGS cell is governed by the E c of CIGS. Based upon this concept, a material selection guideline is given for the windows and transparent electrodes appropriate for high-x CIGS absorbers-based solar cells.