Superlattice intermediate band solar cell with resonant upper-conduction-band assisted photo-absorption and carrier extraction

In this work we propose and theoretically evaluate a superlattice intermediate band solar cell design, wherein a superlattice comprising lattice matched layers of electronically mismatched alloys and thin barriers are inserted within the intrinsic region of a wide bandgap p-i-n diode. The shallow valence band offsets in the design favor the minority hole extraction, and the intermediate levels is build through superlattice minibands formed by coupling lower band gap wells (lower E-conduction branch of mismatched alloys like GaAsN) and higher bandgap Kane-like semiconductors. In the proposed design the upper conduction band E+ of the mismatched alloys is maintained in resonance with the barrier and bandgap of host material to promote an efficient extraction of electrons and preserve the 3D nature of the upper band, thus favoring a strong intermediate to band second photon absorption. In this design carriers can be promoted either directly to the conduction band or via the intermediate band, permitting the absorption of low energy photons whilst maintaining a high cell voltage. Also the characteristic lengths of the wells are substantially smaller than typical diffusion lengths of the electronically mismatched alloy which should overcome the minority carrier losses observed in bulk like devices fabricated with these alloys. To attain the necessary combination of high and low bandgaps and low dislocation density, we use materials that are lightly strained or lattice-matched to an GaAs (or Ge) substrate. GaAsN (Sb) quantum well layers are incorporated into direct bandgap low Al content (x<;30%)) AlGaAs host material, to attain high bandgaps of 1.7-1.9 eV, and low-energy bandgaps of 1.1-1.3 eV. A preliminary detailed balance evaluation of the proposed device that incorporates calculation of the absorption properties of the SL region and the host AlGaAs crystal suggest potential for exceeding 1sun and 1000 sun efficiencies of 39% and 55% respectively.