One of the major challenges in perovskite research today is the fabrication of stable perovskite materials for optoelectronic applications without performance loss.
To improve the stability of metal halide perovskite films and devices, layered (2D) perovskites are frequently used as a passivating layer. This improved long-term stability, however, often comes at the expense of device performance. More fundamental understanding of the perovskite material is needed to understand this effect. In a recent study, we therefore used pulsed, transient photoconductivity, to estimate the long-range mobility over a wide range of charge carrier densities. By comparing different fabrication routes, we could show that long-range mobility can be used as a measure for the quality of a perovskite absorber layer.^[1] We then improved our mobility estimation by including effects like exciton species and early-time recombination. This allowed us to use the same method to investigate two-dimensional perovskite materials. To our surprise, we discovered that PEA₂PbI₄, a well-studied 2D material, has an 8 times higher long-range mobility than FA_0.9Cs_0.1PbI₃, a typical three-dimensional perovskite.^[2] To gain a better understanding, we used optical probe terahertz spectroscopy to measure short-range mobility and found an identical value for the mobility of the 2D material, indicating superior material quality. We also hypothesise that the main reasons for the underperformance of perovskite device stacks with 2D passivations are the high exciton fraction and the anisotropy of charge carrier transport.

Using our newly acquired knowledge, we attempted to find an improved 2D passovation layer in this study. We begin by screening a variety of candidates using similar methods to those used in previous studies in order to find one with better optoelectronic properties than PEA₂PbI₄. Most importantly, we are attempting to modify the exciton binding energy and structural properties of the materials. Our best candidate is then used to passivate FAPbI₃ thin films, resulting in an increase in open-circuit voltage while preserving other device parameters. We use structural and optoelectronic characterization to separate the factors that contribute to improved device performance. Our findings show that multiple parameters influence the formation of 2D passivation, which sheds light on their long-term stability under various conditions.