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

Abstract

to solve several issues arising from their lead-based cousins featuring excellent optoelectronic properties. [1] Changing the simple perovskite ABX 3 structure to a 2 × 2 × 2 supercell, halide double perovskites have the general formula A 2 B I B III X 6 where two bivalent cations B 2+ are exchanged by a combination of one monovalent cation B + (e.g., Cu + , Ag + , Au + , In + ) and one trivalent cation B 3+ (e.g., Bi 3+ , Sb 3+ ). [2] Several theoretical calculations have been performed on this structural motif, providing a large variety of proposed thermodynamically stable compounds of which Cs 2 AgBiBr 6 proved to be the most promising material so far. [3][4][5][6][7] Cs 2 AgBiBr 6 was characterized first by McClure et al. [8] and, being long-term stable at ambient conditions and providing alternative element combinations that exclude toxic elements, [2] this material was moved into the focus of research, especially due to promising optoelectronic properties of single crystals (for instance a charge carrier lifetime of >500 ns) [9,10] and the possibility to solution-process the material for thin films synthesis. [11] After the first report on solar cells using Cs 2 AgBiBr 6 as an active layer in 2017, [11] several groups reported solar cells Since their introduction in 2017, the efficiency of lead-free halide perovskite solar cells based on Cs 2 AgBiBr 6 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 V OC 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 TiO 2 . The effect is mainly due to a V OC improvement by up to 70 mV on average, yielding a maximum V OC of 1.18 V using different concentrations of phenethylammonium bromide. While these are among the highest reported V OC values for Cs 2 AgBiBr 6 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.