Origin of the Voltage Gap and Recombination Losses in All‐Perovskite Tandem Solar Cells

ABSTRACT All‐perovskite tandem solar cells with narrow and wide bandgap perovskite absorbers are promising candidates for low‐cost and high‐efficiency photovoltaic applications. However, the open circuit voltage of typical tandem structures is generally smaller than the sum of the individual voltages in the single‐junction form; a quantity we call the voltage gap. Subcell optimization can only begin once nonradiative losses associated with each absorber layer can be properly identified. To address this, we used absolute electroluminescence hyperspectral imaging to construct external radiative efficiency maps of each subcell within the tandem stack and compare these measurements with single junction devices. These measurements were then combined with additional electro‐optical characterization and modeling to construct subcell current vs voltage curves. We find that the narrow band gap subcell contributes the most toward the voltage gap, and therefore fabrication and processing efforts should focus on better growth and passivation strategies for the narrow band gap absorber.