Abstract
We investigate the origin of the broadband visible emission in layered hybrid lead-halide perovskites and its connection with structural and photophysical properties. We study ⟨001⟩ oriented thin films of hexylammonium (HA) lead iodide, (C6H16N)2PbI4, and dodecylammonium (DA) lead iodide, (C12H28N)2PbI4, by combining first-principles simulations with time-resolved photoluminescence, steady-state absorption and X-ray diffraction measurements on cooling from 300 to 4 K. Ultrafast transient absorption and photoluminescence measurements are used to track the formation and recombination of emissive states. In addition to the excitonic photoluminescence near the absorption edge, we find a red-shifted, broadband (full-width at half-maximum of about 0.4 eV), emission band below 200 K, similar to emission from ⟨110⟩ oriented bromide 2D perovskites at room temperature. The lifetime of this sub-band-gap emission exceeds that of the excitonic transition by orders of magnitude. We use X-ray diffraction measurements to study the changes in crystal lattice with temperature. We report changes in the octahedral tilt and lattice spacing in both materials, together with a phase change around 200 K in DA2PbI4. DFT simulations of the HA2PbI4 crystal structure indicate that the low-energy emission is due to interstitial iodide and related Frenkel defects. Our results demonstrate that white-light emission is not limited to ⟨110⟩ oriented bromide 2D perovskites but a general property of this class of system, and highlight the importance of defect control for the formation of low-energy emissive sites, which can provide a pathway to design tailored white-light emitters.
Highlights
Lead halide perovskite solar cell and light-emitting diode (LED) efficiencies have increased substantially in recent years,[1−3] but their instability in air[4] and ultraviolet light[5] challenges commercial application
Examples of 2D perovskite LEDs8 and 2D/3D mixed perovskite solar cells have been reported, but the solar cell efficiencies still fall behind their pure 3D analogues, even 2D/3D mixtures, which allow for separation of initial excitonic states.[9,10]
The structure consists of sheets of cornersharing PbI42− octahedral (Figure 1a), which are separated by Article is at 525 nm for HA2PbI4, and 500 nm for DA2PbI4, both show a slight broadening in the emission below the main narrow peak
Summary
Lead halide perovskite solar cell and light-emitting diode (LED) efficiencies have increased substantially in recent years,[1−3] but their instability in air[4] and ultraviolet light[5] challenges commercial application. Whole or partial substitution of the standard cation amines in the 3D bulk perovskites[6,7] with longer aliphatic chains was suggested to alleviate the instability Use of such larger-cation amines changes the crystal structure into a 2D perovskite[8] with semiconducting lead-halide perovskite sheets separated by a layer of insulating aliphatic chain (organic spacer). White-lightemitting hybrid perovskites contain widely differing organic spacers and show large differences in crystal structure. This has so far limited controlled studies into the white-light emission in hybrid perovskites, in particular with regard to compositional and structural effects
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