Development of high-efficiency Al0.2Ga0.8As solar cells and interconnect schemes for Al0.2Ga0.8As/Si mechanically-stacked cascade cells

Al_0.2Ga_0.8As/Si mechanically-stacked cascade cells offer potential AMO power conversion efficiencies >25 percent. The demonstration of a high-performance stack requires three critical components, a high efficiency (≃18 percent) Al_0.2Ga_0.8As top cell, a high-conductance interconnect, and a high-efficiency Si bottom cell. In this work, the authors present the development of a 17 percent (active-area) efficient p⁺-n Al_0.2Ga_0.8As solar cell. The cell has a V_oc of 1.22 V, a J_sc of 21.8 mA/cm² , and a fill factor of 0.865. The Al_0.2Ga_0.8As cells were grown by low-pressure organometallic vapor phase epitaxy. The introduction of an undoped spacer layer between the n-base and the p⁺-emitter is found to improve the fill factor and the red response of the cells. Second, they report an interconnect scheme for the mechanical stacking of the AlGaAs cells onto a Si substrate. The technique involves an eutectic-metal-bond (EMB) using a Au/Sn/Au metallization between GaAs (contact layer to the base of the AlGaAs cell) and Si. Preliminary evaluation of this EMB interconnect reveals a resistivity of ≃4.2×10^-2 ohm-cm² which is adequate for AMO operation. The advantages of this interconnect for the p⁺-n cell configuration, including suitability in manufacturing environment for the production of low-cost AlGaAs/Si mechanically-stacked cells, are presented