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The photovoltaics industry is adopting bifacial systems which offer improved energy harvesting compared to monofacial ones. This stems from the collection of light reflected by the surroundings on the rear side of the modules, leading to system-level gains typically ranging from 5 to 25%rel. The question arises, however, whether this bifacial gain also applies to two-terminal (2T) tandem solar cells, since the series-connected sub-cells must generate equivalent currents to achieve an optimal performance. Using comprehensive simulations based on realistic device characteristics and typical meteorological data at different locations, we demonstrate that 2T tandem solar cells can indeed benefit from bifaciality provided that their design is tailored to this configuration. The top cell needs to absorb significantly more light than when designed for monofacial operation. In the geographical locations and system configurations simulated here, a broad performance optimum is found at a mismatch value of 6 mA cm -2 under front illumination, corresponding to a top cell generating ~23 mA cm -2 compared to ~17 mA cm -2 in the bottom one. With such specific design, bifacial tandems can yield up to 1.2 times the energy output of bifacial single-junction devices across a wide range of locations.

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