Hole Transporting Bilayers for Efficient Micrometer‐Thick Perovskite Solar Cells

Abstract 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 FF s 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.