Extrinsic nature of the broad photoluminescence in lead iodide-based Ruddlesden–Popper perovskites

Abstract

Two-dimensional metal halide perovskites of Ruddlesden-Popper type have recently moved into the centre of attention of perovskite research due to their potential for light generation and for stabilisation of their 3D counterparts. It has become widespread in the field to attribute broad luminescence with a large Stokes shift to self-trapped excitons, forming due to strong carrier-phonon interactions in these compounds. Contrarily, by investigating the behaviour of two types of lead-iodide based single crystals, we here highlight the extrinsic origin of their broad band emission. As shown by below-gap excitation, in-gap states in the crystal bulk are responsible for the broad emission. With this insight, we further the understanding of the emission properties of low-dimensional perovskites and question the generality of the attribution of broad band emission in metal halide perovskite and related compounds to self-trapped excitons.

Effect of Structural Defects and Impurities on the Excited State Dynamics of 2D BA2PbI4 Perovskite

“…[2] All depositions were started from fully synthesized BA 2 PbI 4 crystals in order to minimize the effect of the precursor ratios on the thin film preparation. [26][27][28] Subsequently, solutions of BA 2 PbI 4 were prepared by dissolving the synthesized single crystals or powders in dimethylformamide (DMF) at 343 K under magnetic stirring for 2 h. The fresh solutions were spin-coated on hot quartz substrates at 433 K, with a ramp of for 30 s and post-annealed for 15 min. We optimized the spin-coating conditions in order to obtain the best quality thin films.…”

“…These low wavelength emission peaks have a longer lifetime (~3 -5 ns) than the common exciton emission (~490 nm and 516 nm) in 2D perovskites (~1 -2 ns) (see Figure S1 in Supporting Information) and are attributed in the literature to broadband emission from self-trapped excitons due to strong phononcoupling with the distorted soft crystal structure and/ or in extrinsic defect states. [26][27][28][29][30]33] The intensity of the broad-band emission features (and hence the concentration of self-trapped excitons in defects) is higher for the films prepared from BA 2 PbI 4 amorphous powders and non-optimized films than for the ones prepared from BA 2 PbI 4 single crystals (see Figure 3,b, 3,d and 3,f). These defects, and subsequent changes in photoluminescence spectra and dynamics, are associated to slight differences in stoichiometry and dynamic behavior of ions, particularly the diffusion of halides ions due to vacancies or interstitial defects.…”

“…[21,22] Some studies have reported complex absorption and emission spectra (many transitions energies) at low temperatures of which the origin has been related to several effects such as phase transitions, strong many-body interactions between the excitons in the inorganic layer and/ or formation of higher order biexcitons and selftrapped excitons. [12,[23][24][25][26][27][28][29][30][31] A comparison of all these studies reveals that there are large differences in the relative intensities of the different features in the spectra, especially in the broad band emission intensities at~560-800 nm that are associated with selftrapped excitons. [26][27][28][29][30] Studies to design broadly emitting perovskite compounds associate efficient self-trapped exciton emission from lattice distortions of the inorganic cages.…”

“…[12,[23][24][25][26][27][28][29][30][31] A comparison of all these studies reveals that there are large differences in the relative intensities of the different features in the spectra, especially in the broad band emission intensities at~560-800 nm that are associated with selftrapped excitons. [26][27][28][29][30] Studies to design broadly emitting perovskite compounds associate efficient self-trapped exciton emission from lattice distortions of the inorganic cages. [26 -30] However, the large differences in the relative intensities point to self-trapped excitons in defects introduced during the preparation methods.…”

“…[26 -30] However, the large differences in the relative intensities point to self-trapped excitons in defects introduced during the preparation methods. [26] This suggests that the quality and preparation methods of 2D perovskites are critical to understand the complexities in the observed excited state dynamics.…”