Characterizing Fill Factor Limitations in Perovskite‐Silicon Tandem Solar Cells

Monolithic perovskite‐silicon tandem solar cells have achieved power conversion efficiencies approaching 35%. However, a significant gap remains to their thermodynamic limit, primarily due to fill factor ( FF ) losses. This work presents a methodology to identify the underlying mechanisms of the losses. Electroluminescence characterization reveals the potential FF without series resistance losses but cannot fully explain the observed FF limitations. An additional loss mechanism is the photoshunt, a phenomenon which the parallel shunt resistance apparently reduces under illumination in perovskite solar cells. Using an analytical model, we show that the photoshunt originates from competition between charge extraction, governed by transport‐layer properties and electric field, and recombination in the absorber and at interfaces. An equivalent circuit model quantifies its impact on subcell and tandem FF . Additionally, the two‐diode property of the Si subcell can also influence the tandem FF losses. Measurements on tandem devices under spectrally independent LED illumination show that photoshunt losses can be hidden under bottom‐cell‐limited conditions, explaining the preference for slightly bottom‐cell‐limited tandem designs. Reducing the photoshunt through improved charge transport and reduced recombination could enable operation closer to current matching while minimizing FF and efficiency losses.