2D/3D Hybrid Cs2AgBiBr6 Double Perovskite Solar Cells: Improved Energy Level Alignment for Higher Contact‐Selectivity and Large Open Circuit Voltage

Since their introduction in 2017, the efficiency of lead‐free halide perovskite solar cells based on Cs2AgBiBr6 has not exceeded 3%. The limiting bottlenecks are attributed to a low electron diffusion length, self‐trapping events and poor selectivity of the contacts, leading to large non‐radiative VOC losses. Here, 2D/3D hybrid double perovskites are introduced for the first time, using phenethyl ammonium as the constituting cation. The resulting solar cells show an increased efficiency of up to 2.5% for the champion cells and 2.03% on average, marking an improvement by 10% compared to the 3D reference on mesoporous TiO2. The effect is mainly due to a VOC improvement by up to 70 mV on average, yielding a maximum VOC of 1.18 V using different concentrations of phenethylammonium bromide. While these are among the highest reported VOC values for Cs2AgBiBr6 solar cells, the effect is attributed to a change in recombination behavior within the full device and a better selectivity at the interface toward the hole transporting material (HTM). This explanation is supported by voltage‐dependent external quantum efficiency, as well as photoelectron spectroscopy, revealing a better energy level alignment and thus a better hole‐extraction and improved electron blocking at the HTM interface. Solar cells comprised of a lead‐free Cs2AgBiBr6‐based 2D/3D hybrid double perovskite with PEA+ as constituting cation show an increase of the power conversion efficiency by almost 10% due to a large increase in the VOC. This is attributed to a better contact selectivity caused by an improved energy level alignment at the interface between the perovskite layer and the hole‐transporting material.