Detailed balance analysis of area de-coupled double tandem photovoltaic modules

This paper describes how layers of area de-coupled top and bottom cells in photovoltaic tandem modules can increase the efficiency of two-terminal tandem devices. The point of the area de-coupling is to allow the number of top cells to differ from the number of bottom cells. Within each of the layers, the cells can be horizontally series-connected and the layers can then be current- or voltage-matched with each other in a tandem module. Using detailed balance modeling, it is shown that two-terminal tandem modules of this type can achieve the same theoretical efficiency as stacks of independently operated cells, often referred to as four-terminal cells. Optimal ratios of the number of bottom cells to the number of top cells are calculated. Finally, it is shown that modules with a bottom layer consisting of 60 cells with a band gap of 1.11 eV, resembling standard silicon modules, offer sufficient resolution to optimize the number of top cells and achieve high efficiency over a large range of top cell band gaps. This result extends the list of materials that can be used as top cells in two-terminal tandem modules with silicon bottom cells.