Growth-Controlled Broad Emission in Phase-Pure Two-Dimensional Hybrid Perovskite Films

Abstract

Two-dimensional hybrid metal halide perovskites (2D perovskites) are attractive for light-emitting devices and other applications because their emission is tunable across the visible spectrum. The emission profile of 2D perovskites can be broadened via a variety of mechanisms and is further complicated by the presence of impurities. Here, the challenge of making phase-pure films in Ruddlesden–Popper phases [(A′)2(A) n−1B n X3n+1 structure] is overcome by using a single A/A′-site cation, ethylammonium (EA), whose optimal size also prohibits the formation of off-target phases. In the (EA)2(EA) n−1Pb n Br3n+1 family, the low-energy, broad emission observed in bulk crystals is reduced in spin-cast, polycrystalline films. This decrease in broad emission, attributed to phonon-mediated processes, is correlated with the strain in polycrystalline films that is observed by X-ray scattering. Photothermal deflection spectroscopy shows that strain also increases the electronic disorder near the free exciton absorbance. Broad emission in films can be recovered by slowing growth kinetics, which removes the strain acquired from spin-casting and increases the domain size. These results help extend the utility of 2D perovskites by suggesting design rules for the growth of thin films with the targeted phase and emission.

Deterministic fabrication of 3D/2D perovskite bilayer stacks for durable and efficient solar cells

“…This result is well correlated with the calculated x-ray penetration depth curve for the RP-2D HaP, showing a critical angle (α c ) of 0.181° (Fig. 3E) ( 28 , 29 ). The integrated area under the (020) peak (2D signature feature) and the (111)/(001) peak (3D signature feature) from the GIWAXS spectra for various incident angles (0° to 0.5°) (Fig.…”

Enhancing and Extinguishing the Different Emission Features of 2D (EA1−xFAx)4Pb3Br10 Perovskite Films

“…We also showed previously that strain from rapid casting suppresses broad emission in (EA) 4 Pb 3 Br 10 , and that the intensity of broad emission is recovered by slowing the casting kinetics. [ 52 ] Such broad emission is thought to originate from STEs that might couple to a low‐energy defect. [ 52 ] STE emission has been correlated with distortions to the inorganic structure: in an “ideal” lattice, such as cubic MAPbBr 3 , the Pb‐Br bond lengths are all equal and the bond angles are all 180°.…”

“…Bulk crystals of (EA) 4 Pb 3 Br 10 ( x FA = 0.00) oriented on the substrate with the (0 k 0) peaks out‐of‐plane (i.e., 2D sheets parallel to the substrate), consistent with prior work. [ 52 ] As an example, the (020) plane is shown in Figure 1a. All (0 k 0) peaks are observed for x FA = 0.09 and 0.17 indicating that an n = 3 structure is retained (Figure 1e).…”

“…(EA) 4 Pb 3 Br 10 ( x FA = 0.00) bulk crystals have three emission features: a free exciton (FE) near 2.9 eV, and two features at lower energy, that arise from coupling of the free exciton to phonons ( Figure ). [ 52 ] We call these two features “Ph‐1” and “Ph‐2,” for phonon‐coupled 1 and 2. Ph‐1 is narrow, and located near 2.7 eV, which is only 0.2 eV below the free exciton (see prior work for peak fits [ 52 ] ).…”

“…[ 65 ] Recent work shows that film strain can be used to remove the lowest‐energy feature of (EA) 4 Pb 3 Br 10 . [ 52 ] In order to fully tune the emission color of 2D films, we must further explore how to change the optoelectronic properties of 2D phases through composition and processing.…”