Charge‐Carrier Dynamics, Mobilities, and Diffusion Lengths of 2D–3D Hybrid Butylammonium–Cesium–Formamidinium Lead Halide Perovskites

Abstract

Perovskite solar cells (PSCs) have improved dramatically over the past decade, increasing in efficiency and gradually overcoming hurdles of temperature-and humidity-induced instability. Materials that combine high charge-carrier lifetimes and mobilities, strong absorption, and good crystallinity of 3D perovskites with the hydrophobic properties of 2D perovskites have become particularly promising candidates for use in solar cells. In order to fully understand the optoelectronic properties of these 2D-3D hybrid systems, the hybrid perovskite BA x (FA 0.83 Cs 0.17 ) 1-x Pb(I 0.6 Br 0.4 ) 3 is investigated across the composition range 0 ≤ x ≤ 0.8. Small amounts of butylammonium (BA) are found that help to improve crystallinity and appear to passivate grain boundaries, thus reducing trap-mediated chargecarrier recombination and enhancing charge-carrier mobilities. Excessive amounts of BA lead to poor crystallinity and inhomogeneous film formation, greatly reducing effective charge-carrier mobility. For low amounts of BA, the benevolent effects of reduced recombination and enhanced mobilities lead to charge-carrier diffusion lengths up to 7.7 µm for x = 0.167. These measurements pave the way for highly efficient, highly stable PSCs and other optoelectronic devices based on 2D-3D hybrid materials.

Resolving the Ultrafast Charge Carrier Dynamics of 2D and 3D Domains within a Mixed 2D/3D Lead‐Tin Perovskite

“…Notably, both ϕk 2 and ϕ 2 k 3 in the 2D film are approximately double that of the 3D film. These differences are not on the scale of those previously reported between 3D and n = 1 RP perovskites [2,27,60,62] . For example, Kober-Czerny et al measured bimolecular recombination rates with multiple techniques for the n = 1 RP perovskite PEA 2 PbI 4 and a 3D mixed cation lead iodide perovskite.…”

“…Notably, both ϕ k 2 and ϕ 2 k 3 in the 2D film are approximately double that of the 3D film. These differences are not on the scale of those previously reported between 3D and n = 1 RP perovskites [ 2,27,60,62 ] . For example, Kober–Czerny et al.…”

Enhanced Charge Transport via Mixed-Dimensional Heterostructures in 2D–3D Perovskites and Their Relevance to Solar Cells

“…As shown in Table S1, the lattice constant a of the 3D perovskite decreased from 8.840 to 8.786 Å when the 2D ratio increased from x = 0.00 to x = 0.10. The reduced lattice constant can be attributed to the grain-to-grain strain and lattice distortion induced by the different ionic radii of BA (2.7 Å) and MA (2.1 Å). , As mentioned later in the TEM measurements, some amorphous phases may also modify the lattice constant in the 2D–3D mixed perovskites.…”

“…The reduced lattice constant can be attributed to the grain-to-grain strain and lattice distortion induced by the different ionic radii of BA (2.7 Å) and MA (2.1 Å). 23,29 As mentioned later in the TEM measurements, some amorphous phases may also modify the lattice constant in the 2D−3D mixed perovskites.…”

“…SEM images (Figure a–c) show that an x = 0.00 film has dense grains of 400 nm size, and a grain size decrease of about 30 nm is exhibited with an increasing x ratio. When small amounts of 2D perovskite were added ( x = 0.02), the long chain of BAI impeded grain growth during nucleation and consequently reduced the grain size. ,, The grain size was also found to vary with the addition of the 2D perovskite, as observed by atomic force microscopy (AFM) measurements (see Figure S2). For x = 0.04, rod-shaped grains appeared together with dense grains.…”