We investigate Cr-doped ZnS (ZnS: Cr) as a potential deep-level intermediate band material for high efficiency solar cells. We study n-ZnO:Al/ZnS:Cr/p-GaP heterojunction cell for the first time, and this paper presents an interpretation of the performance of the solar cell in the framework of intermediate band solar cells. We conclude that the ZnS:Cr used in this work has two characteristic energy levels at 0.88 eV and 2.68 eV below the conduction band. This material also has a quasi-continuum of energy levels between the former level and the valence band maximum. This quasi-continuum results in thermal carrier escape that limits the open-circuit voltage to the lowest energy gap in ZnS: Cr, similar or equal to 0.8 V.