Wide‐Bandgap Double Perovskites with Multiple Longitudinal‐Optical Phonon Scattering

Abstract

Alloyed lead-free double perovskites display intense photoluminescence, are environmentally friendly, and their devices show long-term operation. Thanks to these properties, which make them excellent warm white-emitting materials, they have recently received great attention in lighting applications. An important factor to tune the optical properties of alloyed lead-free double perovskites is the presence of self-trapped excitons. Here, it is demonstrated that in leadfree double perovskites, the strong electron-phonon coupling plays a crucial role in the generation of self-trapped excitons. The strong electron-phonon coupling is confirmed by a large Huang-Rhys factor and by the presence of multiphonon transitions. In particular, sharp emission lines superimposed on the broad photoluminescence emission band of one of these samples (Cs 2 Ag 0.6 Na 0.4 InCl 6 0.5%Bi) are observed; these are due to the strong coupling of longitudinal-optical phonons with excited electronic states caused by the tetragonally distorted AgCl 6 octahedrons. Such a strong coupling of longitudinal-optical phonons to electrons can effectively modulate the photophysical properties of alloyed double perovskites, and its understanding is, thus, of paramount importance for the design of future optoelectronic devices.

Impact of bismuth-doping on enhanced radiative recombination in lead-free double-perovskite nanocrystals

“…These values were similar to the bulk phonon energy (∼50 meV). 79 Smaller optical phonon energy values of the doped samples imply that more phonons are produced, which would serve as scattering centers, 77 possibly due to the strained crystalline lattice by doped ions. The inhomogeneous broadening of PL FWHM might be contributed by the surface defect-induced trap state.…”

The making and breaking of perovskite photochromism through doping

“…It has been reported that Na + alloying could effectively break the lattice symmetry, and therefore relax the parity-forbidden transitions, leading to an improvement of luminescence intensity. [28][29][30] In an attempt to increase the photoluminescence, sodium ions were used as a co-dopant. Upon increasing the Na + content, the color of the crystals turned from dark purple to pink, and eventually to colorless (Fig.…”

White light emission with unity efficiency from Cs2Na1−xAgxIn1−yBiyCl6double perovskites: the role of bismuth and silver

“…1b as Na amount is increased. 39 Similarly, with increasing Bi content in Cs 2 NaIn 1Ày Bi y Cl 6 , the (220) reflection (Fig. 1e) is sequentially shifted to lower 2y values, indicating the formation of alloyed In/Bi compounds.…”

“…2. Following literature, 25,28,[41][42][43] the observed peaks were assigned to breathing (T 2g ) and stretching (E g , A 1g ) modes of [MCl 6 ] 5À and [M 0 Cl 6 ] 3À octahedra. In the Cs 2 Na 1Àx Ag x InCl 6 series (Fig.…”

“…2a), both [AgCl 6 ] 5À and [InCl 6 ] 3À octahedra are supposed to contribute to the three modes, as Ag and In have similar elemental properties, while the T 2g /A 1g intensity ratio increases with increasing Na content, allegedly due to progressive improvement of the crystal lattice quality. 42 Incremental substitution of In with Bi was clearly detected by (i) the decrease in intensity, together with the shift to lower wavenumbers, of the T 2g [InCl 6 ] 3À breathing mode, initially located at 138 cm À1 , (ii) the complementary rise of the corresponding T 2g [BiCl 6 ] 3À mode at 108 cm À1 , and (iii) the shift of the A 1g stretching mode from 291 to 275 cm À1 (Fig. 2b).…”