No abstract

Electrical current mismatching is a well-known limitation of triple junction solar cells that lowers the final conversion efficiency. Several solutions have been proposed to face this issue, including the insertion of a multiple quantum well structure as the intermediate junction’s active material. With a better matching in the current among the junctions, the total current increases, thus modifying the working conditions of the overall device. In this way, the InGaP top junction needs to be optimized to such new condition. In this work, numerical simulations were carried out aiming the enlargement of the electrical current density of an InGaP pn junction to achieve the proper current matching in triple junction solar cell for spatial applications. The optimized structure has been grown in a GaAs substrate and characterized as a single junction solar cell. Although the measured short circuit current density and conversion efficiency are still well below the theoretically predicted values, processing improvement should lead to adequate cell performance.

The solar cells with highest efficiencies are the tandem cells with three or four pn junctions in series, connected by tunnel diodes. The current produced by the stacked pn junctions is limited by the smallest one. Therefore, to optimize the full solar cell efficiency it is crucial to match the current produced by the different junctions. This issue has been extensively investigated over the years. In the most usual case of three pn junctions, the current is limited by the middle junction. Recently, the use of multiple quantum wells to further improve the current generated at the intermediate pn junction was proposed. When such a middle junction is used, new optimization of the top cell is required to reach a better current matching. In this work we have used a commercial software, Comsol, to optimize the top solar cell for a triple junction structure to be used in space applications, meaning subjected to the AM0 spectrum. The newly designed solar cell is based on an InGaP pn junction with AlGaInP n-doped window. The layers are lattice matched to GaAs and to Ge, which is the material used for the bottom pn junction. These materials were chosen based on the fact that they are more resistant to radiation, which is of paramount importance for use in satellites. The first step to fabricate the designed solar cell is the optimization of each individual layer. The different InGaP and AlGaInP semiconductor layers of the designed solar cell have been grown by metalorganic vapor phase epitaxy at 675 ºC. The alloys' composition was calibrated. High doping levels of InGaP were achieved. However, difficulties in reaching a doping level of the AlGaInP window layer of around 1x10 18 cm −3 , as required, were faced and should be discussed. Additionally, luminescence and x-ray diffraction data of the grown samples will be presented.