Design of AlxGa1−xAs/GaAs/InyGa1−yAs triple junction solar cells with anti-reflective coating

Multi-junction solar cells (SC) made from III–V compound semiconductors are still in the development phase. Here, we perform calculations for multi-junction cells: AlxGa1−xAs top junction, GaAs middle junction and InyGa1−yAs bottom junction (all of these materials with band-gaps between 2.1 and 0.8 eV) in order to obtain the optimal band gap and thickness for each junction under the AM1.5 solar radiation spectrum. The ideal photo-current density is around 15.5 mA/cm2. In order to reduce the natural reflectivity, an anti-reflective coating (ARC) was chosen, based on a MgF2/ZnS double layer, allowing for a significant increase of the current density with respect to a cell without it. Calculations of external quantum efficiency (QE) were also performed for the three cases mentioned above: ideal one, taking into account the total reflection and with the ARC double layer. Finally, when more realistic calculations are done, taking into account the carrier recombination at each sub-cell, and the light reflection for a tandem cell with the designed ARC on top, the expected conversion efficiency (η), under the AM 1.5 spectrum (without concentration), was determined to be around 38.5%, making this an attractive III–V compound tandem cell to be investigated in the near future.