Broadband Extrinsic Self‐Trapped Exciton Emission in Sn‐Doped 2D Lead‐Halide Perovskites

Abstract

Metal-halide perovskites represent a class of promising light absorbers for efficient solar cells. [1][2][3][4][5] The propensity of perovskite films for low-cost solution processing also encourages scientists to explore potential applications beyond solar cells. [6][7][8][9] In particular, as emitters, perovskites exhibit intriguing luminescent properties such as narrowband emission, spectral tunability, and high quantum efficiency, which enables applications in the microlasers and light-emitting diodes (LEDs). [10][11][12][13] The luminescence efficiency of perovskites generally relies on nanostructures that can spatially confine excitons, and consequently reduce the possibility of nonradiative recombination during the carrier/ exciton migration. However, nanocrystals, due to boundary scattering of carriers, generally face the problem of poor charge transport, which is undesirable for LED performance. 2D perovskites, where bulky organic layers and inorganic layers are alternately and periodically arranged, feature natural quantum-well structures. This quantum-well structure is regarded as promising LED emitters for decades. [14][15][16] However, low photoluminescence quantum yields (PLQYs, typically < 1%) of 2D perovskites at room temperature is a bottleneck to achieving high-performance LEDs. [17] The low PLQYs may be attributed to insufficient confinement of Wannier type excitons within the inorganic layers [18] as suggested by the long charge-carrier/exciton diffusion length (60 nm). [19] Engineering crystal structures of low-dimensional (0D to 2D) perovskites by employing suitable organic ammonium cations is the predominant methods for the tuning of luminescence, both in spectral coverage and efficiency. [20,21] In these cases, severe structural distortion of metal halide octahedra is a common feature because of the size mismatch between organic and inorganic components, which results in potential fluctuations. [22,23] Such fluctuations of potential within an inorganic layer of perovskite sometimes, but not always, [20,21] slow the diffusion of carriers or excitons, and consequently induce self-trapped excitons (STEs), which represents a type of bound states for efficient radiative recombination. However, the occurrence of exciton self-trapping in semiconductors is the exception rather than the rule. [24] In parallel, compositional engineering As emerging efficient emitters, metal-halide perovskites offer the intriguing potential to the low-cost light emitting devices. However, semiconductors generally suffer from severe luminescence quenching due to insufficient confinement of excitons (bound electron-hole pairs). Here, Sn-triggered extrinsic self-trapping of excitons in bulk 2D perovskite crystal, PEA 2 PbI 4 (PEA = phenylethylammonium), is reported, where exciton self-trapping never occurs in its pure state. By creating local potential wells, isoelectronic Sn dopants initiate the localization of excitons, which would further induce the large lattice deformation around the impurities to accommodate the se...

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