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

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In this article it is investigated how the hole extraction layer (HEL) influence the charge recombination and performance in half tin and half lead (FASn Pb I ) based solar cells (HPSCs). FASn Pb I film grown on PEDOT:PSS displays a large number of pin-holes and open grain boundaries, resulting in a high defect density and shunts in the perovskite film causing significant bulk and interfacial charge recombination in the HPSCs. By contrast, FASn Pb I films grown on PCP-Na, an anionic conjugated polymer, show compact and pin-hole free morphology over a large area, which effectively eliminates the shunts and trap states. Moreover, PCP-Na is characterized by a higher work function, which determines a favorable energy alignment at the anode interface, enhancing the charge extraction. Consequently, both the interfacial and bulk charge recombination in devices using PCP-Na HEL are considerably reduced giving rise to an overall improvement of all the device parameters. The HPSCs fabricated with this HEL display power conversion efficiency up to 16.27%, which is 40% higher than the efficiency of the control devices using PEDOT:PSS HEL (11.60%). Furthermore, PCP-Na as HEL offers superior performance in larger area devices compared to PEDOT:PSS.

2D perovskites offers a rich playing field to explore exciton physics and they possess a great potential for a variety of opto-electronic applications. Whilst their photophysics shows intricate interactions of excitons with the lattice, most reports have so far relied on single compound studies. With the exception of variations of the organic spacer cations, the effect of constituent substitution on the photophysics and the nature of emitting species, in particular, have remained largely under-explored. Here PEA 2 PbBr 4 , PEA 2 PbI 4 , and PEA 2 SnI 4 (where PEA stands for phenylethylammonoium) are studied through a variety of optical spectroscopy techniques to reveal a complex set of excitonic transitions at low temperature. Weak high-energy features are attributed to vibronic transitions breaking Kasha's, for which the responsible phonons cannot be accessed through simple Raman spectroscopy. Bright peaks at lower energy are due to two distinct electronic states, of which the upper is a convolution of the free exciton and a localized dark state and the lower is attributed to recombination involving shallow defects. This study offers deeper insights into the photophysics of 2D perovskites through compositional substitution and highlights critical limits to the communities' current understanding of processes in these compounds.

Ruddlesden−Popper perovskite films deposited with different methods show very diverse phase segregation and composition. When DMSO is used as solvent, the conventional method based on spin-coating and annealing produces very poor devices, whereas the vacuum-assisted method proposed here allows obtaining devices with efficiency up to 14.14%. The conventional method gives rise to a three-dimensional (3D)-like phase on the top of the film but dominant n = 2 phase with large domains (∼40 μm) at the bottom of the film. These n = 2 domains are oriented with their inorganic slabs parallel to the substrate and inhibit the charge transport in the vertical direction. Consequently, severe monomolecular and bimolecular charge recombination occurs in the solar cells. Conversely, the vacuum-assisted method yields films with a 3Dlike phase dominant throughout their entire thickness and only a small amount of n ≤ 2 domains of limited dimensions (∼3 μm) at the bottom, which facilitate charge transport and reduce charge recombination.