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.

The effective mass of charge carriers is a crucial parameter for the design of any optoelectronic device. The estimated values of the effective mass of 2D halide perovskites currently span a broad range, providing an unwelcome source of confusion in this promising material system. Here we highlight how the distortion imposed by the organic spacers, and orbital hybridization effects by the metal cation, govern the effective mass. As a result, the effective mass in 2D halide perovskites can be easily tailored over a wide range. To demonstrate this, we have directly measured the reduced effective mass of charge carriers in phenethylamine (PEA)-based 2D halide perovskites. Combining the experimental results with electronic bandstructure calculations, we propose a scaling diagram for the effective mass value versus the distortion of the octahedra imposed by the organic cations.

Extreme magnetic field reveals the secret of dark states and bright emission of 2D layered perovskites.

2D perovskite materials have recently reattracted intense research interest for applications in photovoltaics and optoelectronics. As a consequence of the dielectric and quantum confinement effect, they show strongly bound and stable excitons at room temperature. Here, the band-edge exciton fine structure and in particular its exciton and biexciton dynamics in high quality crystals of (PEA) 2 PbI 4 are investigated. A comparison of bulk and surface exciton lifetimes yields a room temperature surface recombination velocity of 2 × 10 3 cm s −1 and an intrinsic lifetime of 185 ns. Biexciton emission is evidenced at room temperature, with a binding energy of ≈45 meV and a lifetime of 80 ps. At low temperature, exciton state splitting is observed, which is caused by the electron-hole exchange interaction. Transient photoluminescence resolves the low-lying dark exciton state, with a bright/dark splitting energy estimated to be 10 meV. This work contributes to the understanding of the complex scenario of the elementary photoexcitations in 2D perovskites. materials such as transition metal dichalcogenides, phosphorene, and graphene. They can be easily grown by both solution methods and vapor transport methods at low temperature, [14][15][16][17] with a tunable bulk direct bandgap. [18] These advantages make them appealing for future optoelectronic and photonic applications.Unlike their 3D counterparts, the dielectric and quantum confinement of carriers in the 2D perovskite layers gives rise to unusually strong excitonic effects. [19,20] It has been experimentally observed that excitons are tightly confined in the inorganic layers with binding energy as high as a few hundred millielectronvolts (significantly higher than that of 3D perovskites). [21] This greatly enhanced exciton binding energy makes them particularly interesting for light-emitting applications. [8,22] Moreover, 2D perovskites can exhibit a variety of multiexciton species, including biexcitons and trions. [20,23,24] The presence of these quasiparticles is exciting due to their unique role, leading to a better understanding of many body effects and their great promise for photonic applications. In addition, recent experiments reveal an important role of electron-phonon couplings on the exciton dynamics in 2D lead-iodide perovskite, suggesting a complex scenario for carrier relaxation and exciton formation. [25,26] It is therefore crucial to understand elementary photoexcitations in these layered materials. However, exciton fine structures and their properties are usually masked by local energy fluctuations resulting from disorder in thin films or broad emission due to the formation of self-trapped excitons. [27] Whereas their steady-state optical properties have

No abstract