Hole Transporting Bilayers for Efficient Micrometer-Thick Perovskite Solar Cells

Achieving high efficiencies in halide perovskite solar cells with thicknesses >1 µm is necessary for developing perovskite-Si tandem cells based on small pyramidal structures. To achieve this goal, not only is the perovskite layer quality to be optimized but also the properties of the charge-transport layers must be tuned to reduce charge-collection losses. The transport layers provide a non-ohmic resistance that modulates the Fermi-level splitting inside the perovskite absorber. The finite conductivity of the transport layers can lead to losses in the fill factor (FF) and short-circuit current, even at infinite charge-carrier mobility in the absorber layer. These losses notably scale with the absorber layer thickness, which implies that higher-conductivity transport layers are required for thicker perovskite absorbers. One strategy to improve charge collection and thereby FFs in thick inverted perovskite solar cells is to use bilayers of hole-transport layers. In this study, the combination of poly[bis(4-phenyl) (2,4,6-trimethylphenyl)amine] with self-assembled monolayers provides the best photovoltaic performance in single-junction devices.