Energy offsets between the Er 3+ 4 f -states and the GaAs energy levels important for photon ratchet intermediate-band solar cells

The lifetime of the carriers present in the intermediate bands is short, and thus an additional band at slightly lower energies is introduced in photon ratchet intermediate-band solar cells to reduce the impact of the intermediate-band lifetime. This so-called ratchet band should have a long lifetime and allow a fast energy transfer from the intermediate band. Here, we investigate the optical properties of Er-doped GaAs, where the Er atoms provide the ratchet states and the intermediate states are formed by Er-related oxygen defects, to elucidate the actual energy offsets in this system. The temperature dependence of our photoluminescence data shows two activation energies, 214 meV and 10 meV, and we attribute the former to thermal activation from the Er3+ excited state labelled 4I11/2 to the GaAs conduction band and the latter to thermal activation from the ratchet state (4I13/2) to localized states related to the oxygen defects. By using photoluminescence excitation spectroscopy, we identified the 4I9/2 state within the GaAs conduction band, which suggests that the Er3+ ground-state manifold (4I15/2) is energetically aligned with the GaAs valence-band maximum. These findings are useful in developing strategies that lead to photon-ratchet intermediate-band solar cells with higher efficiencies.