Short-circuit current enhancement in p-i-n GaAs/Ge solar cells with tuned superlattices, embedded in the mid-region

High efficiency in crystalline solar cells is currently of great importance and can be achieved by a combination of high and low band gaps in heterostructure designs. Such devices absorb in both short and long wavelengths, thus covering wider absorption areas from the solar spectrum. On the other hand, p-i-n cells provide space for excess absorption at specific wavelengths through layers grown in the mid-region of the cell. In this communication, the host device is a GaAs-GaAs-Ge solar cell with the mid-layer (GaAs) hosting a short-period GaAs/Ge superlattice. Under illumination, the device generates two current density components (a) bulk current density of 18mA/cm² and (b) excess current density of 12mA/cm², due to thermionic escape mechanisms in quantum wells, with a total current in the neighborhood of 30mA/cm². We calculate from first principles thermionic currents from quantum traps under one-sun illumination, with quantum wells tuned to 1eV incident solar photons. We predict 12mA/cm² from a structure with the following geometry: twenty-period superlattice with 20nm-width of Ge-layers (0.66eV as low gap material) and 200nm barriers (1.42eV, GaAs as wide-gap medium, with negligible tunneling). The repeat distance of the superlattice is 220nm which for 20 layers translates to 4.4 μm of total superlattice thickness. For an 80μm, bulk GaAs-GaAs-Ge solar cell, this means that the excess carrier contribution would come from a superlattice layer which is under 6% of total device thickness. We predict that the total short circuit current density is in excess of 30mA/cm², which along with open-circuit voltage increase, makes the device desirable for high current cells and subsequently for high efficiency cells as well. By appropriate n-doping of the germanium layers we show that it is possible to increase total thermal currents right off the quantum wells in GaAs-Ge layers. The figure below depicts- a short superlattice in the middle of the p-i-n PV device: green stands for Ge layer and the rest stands for GaAs. As seen from the second figure (next page), collection efficiency is depicted in excess of 28% with open-circuit voltage ~1.00V, FF factor 90%, short circuit current 30 ma?cm² under nominal 100mW/cm² solar radiation. Ideal applications of such designs would apply for tandem devices, where open circuit voltage increase due to the series connection would increase overall OC voltage (and hence efficiency) by a factor of 2.