Identifying Recombination Mechanisms in Bifacial Perovskite Solar Cells: Consequences for High Efficiency Tandems

Perovskite solar cells (PSCs) are promising for high‐efficiency tandem devices exceeding the Shockley‐Queisser limit, whose performance is largely determined by individual subcells. A key, often overlooked factor is subcell orientation. While single‐junction cells are typically optimized for bottom illumination in pin configuration, tandem applications require illumination through the top transparent electrode. Depending on the illumination direction, performance losses are dominated by nonradiative recombination either at one of the interfaces between the perovskite and transport layers or within the bulk perovskite. Identifying which of these dominates the losses remains challenging. Here, we introduce an experimental method to identify the limiting nonradiative recombination pathway and its position in bifacial PSCs. By illuminating devices from either side with red, blue, and white light, the wavelength‐dependent fill factor response is used to probe the different recombination pathways. Extensive drift‐diffusion simulations, varying 35 parameters and modeling a wide variety of cells, reveal characteristic fill factor traces associated with four recombination scenarios and show 95% accuracy in identifying the dominant loss mechanism using this method. Finally, the method is applied to a vapor‐deposited, bifacial PSC for tandem applications, showing that the electron transport layer‐perovskite interface limits the performance of this particular device.