Performance Evaluation and Comparison of Monolithic and Mechanically Stacked Dual Tandem InGaP/GaAs Heterojunction on Ge Cell: A TCAD Study

The photovoltaic characteristics of a mechanically and monolithic stacked tandem solar cell of the heterojunction InGaP/GaAs and Ge sub cells, were numerically simulated under 1-sun air mass 1.5 global spectrum (AM1.5G) at ambient temperature (300 K) using the two-dimensional device simulator Silvaco–Atlas. Our tandem structure consists of a thin upper cell with heterojunction of indium and gallium phosphide on gallium arsenide (In0.49Ga0.51P/GaAs), on a relatively thick germanium (Ge) substrate which acts as a lower cell in order to obtain good performances of such a structure. We studied both cells, stacked mechanically (four terminal:4T) and monolithic (two terminal:2T) using Silvaco ATLAS Virtual Wafer FabricationTool. First, we have simulated the single InGaP/GaAs and Ge solar cells with fixed thicknesses at 1.4 µm and 210 µm respectively. They presented a conversion efficiencies (ƞ) of 30.32% and 10.96% respectively. The efficiency of mechanically stacked tandem solar is 30.96% and short current density of 26.16 mA/cm2 which is limited by the lower short current density of both sub-cells. Using the method of current matching, by varying the base thicknesses of the InGaP/GaAs top and Ge bottom sub-cells, the numerical simulation results presented a matched maximum current Jsc value of 29.12 mA/cm2 obtained at base thicknesses of 0.605 and 209.9 μm for the InGaP/GaAs top and Ge bottom sub-cells respectively, leading to a high power conversion efficiency (ƞ) of the mechanically stacked sub cells of 34.77%, the open-circuit voltage and the fill factor are 1.329 V and 88.96%, respectively. Next, the sub-cells were interconnected with tunnel junctions (TJs), p-GaAs/n-GaAs to allow carrier transport, the results of the monolithic stacked sub-cells are converged with results of the mechanically stacked sub-cells, and are represented in the following results of the tandem cell: power conversion efficiency (ƞ) of 32.96%, the open-circuit voltage of 1.343 V, the short current of 29.19 mA/cm2 and the fill factor of 84.05%.