Theoretical efficiency limit for a two-terminal multi-junction step-cell using detailed balance method

Here we present detailed balance efficiency limit for a novel two-terminal dual and triple junction step-cell under AM 1.5G and AM 0 incident spectrums. The step-cell is a multi-junction (MJ) solar cell in which part of the top cell is removed, exposing some of the bottom cell area to unfiltered incident light, thus increasing bottom cell's photogenerated current. Optical generation of the bottom cell is modeled in two parts: step part, limited by the bottom cell bandgap, and conventional part, additionally limited by the top cell absorption. Our results show that conventionally designed MJ cell with optimized bandgap combination of 1.64 eV/0.96 eV for dual junction and 1.91 eV/1.37 eV/0.93 eV for triple junction has the highest theoretical efficiency limit. However, the step-cell design provides significant efficiency improvement for cells with non-optimum bandgap values. For example, for 1.41 eV (similar to GaAs)/Si dual junction under AM 1.5G, efficiency limit increases from similar to 21% in a conventional design to 38.7% for optimized step-cell. Similar benefits are observed for three-junction step-cell and for AM 0 spectrum studied here. Step-cell relaxes bandgap requirements for efficient MJ solar cells, providing an opportunity for a wider selection of materials and cost reduction. (C) 2016 AIP Publishing LLC.