Knowledge of the mechanism of formation, orientation, and location of phases inside thin perovskite films is essential to optimize their optoelectronic properties. Among the most promising, low toxicity, lead-free perovskites, the tin-based ones are receiving much attention. Here, an extensive in situ and ex situ structural study is performed on the mechanism of crystallization from solution of 3D formamidinium tin iodide (FASnI 3 ), 2D phenylethylammonium tin iodide (PEA 2 SnI 4 ), and hybrid PEA 2 FA n−1 Sn n I 3n+1 Ruddlesden-Popper perovskites. Addition of small amounts of low-dimensional component promotes oriented 3D-like crystallite growth in the top part of the film, together with an aligned quasi-2D bottom-rich phase. The sporadic bulk nucleation occurring in the pure 3D system is negligible in the pure 2D and in the hybrid systems with sufficiently high PEA content, where only surface crystallization occurs. Moreover, tin-based perovskites form through a direct conversion of a disordered precursor phase without forming ordered solvated intermediates and thus without the need of thermal annealing steps. The findings are used to explain the device performances over a wide range of composition and shed light onto the mechanism of the formation of one of the most promising Sn-based perovskites, providing opportunities to further improve the performances of these interesting Pb-free materials.

To date, there are no reports of 3D tin perovskite being used as a semiconducting channel in field‐effect transistors (FETs). This is probably due to the large amount of trap states and high p‐doping typical of this material. Here, the first top‐gate bottom‐contact FET using formamidinium tin triiodide perovskite films is reported as a semiconducting channel. These FET devices show a hole mobility of up to 0.21 cm2 V−1 s−1, an ION/OFF ratio of 104, and a relatively small threshold voltage (VTH) of 2.8 V. Besides the device geometry, the key factor explaining this performance is the reduced doping level of the active layer. In fact, by adding a small amount of the 2D material in the 3D tin perovskite, the crystallinity of FASnI3 is enhanced, and the trap density and hole carrier density are reduced by one order of magnitude. Importantly, these transistors show enhanced parameters after 20 months of storage in a N2 atmosphere.

metal halide octahedra.This opens a rich playing field for tuning the photophysical properties of these 2D compounds targeted to a variety of opto-electronic applications. [1] Many studies have considered 2D compounds to increase the stability of 3D perovskites in solar cells and to facilitate charge extraction or to act as efficient emitters in light emitting devices. [2,3] The choice of the spacer cation can affect factors, such as moisture resistivity, [4] or the degree of distortion in the inorganic slabs, which partially governs the band gap energy [5] or the carrier effective masses. [6] Recently, also the imposition of optical chirality [7] or the mixing of states between spacers and inorganic moieties have become important topics of research. [8] Traditionally, most of the organic spacers have been based on primary amines, such as the prototypical butylammonium (BA + ) or phenylethylammonium (PEA + ) giving rise to A 2 MX 4 chemical formulas in so-called Ruddlesden-Popper (RP) compounds, where A is the spacer cation, M the metal cation, and X the halide anion. [1] More recently, diammonium cations, forming AMX 4 Dion-Jacobson (DJ) structures, have moved into the spotlight of research. [9,10] Instead of two spacer cations per unit cell, these compounds possess only one organic molecule with two functional groups to stabilize the layered structure. Although employed in solar cells and characterized for this purpose, [11][12][13] fundamental studies on their photophysics are so far scarce.The photophysics of 2D perovskites exhibits several intensively studied aspects. In particular, low-temperature studies reveal a rich substructure of the excitonic luminescence and absorption. High-energy features are commonly explained through exciton-phonon interactions. However, there is no consensus on the origin of the vibrational modes responsible for the observed spectral features and the actual mechanisms at play. [14][15][16] Slightly Stokes-shifted band edge emission and split sub-peaks have been analyzed in the framework of dark and bright excitons, [17,18] bright doublets, [19] localized defects, [16,20] and have been shown to depend on the angle of detection. [20,21] At even lower energy, broad emission bands are sometimes attributed to self-trapped excitons, but have recently been proposed to be defect-related. [22] Here, we report on the photophysics of the prototypical compounds PDMAPbI 4 and PDMASnI 4 , which are based on the The photophysics of 2D perovskites incorporating 1,4-phenylenedimethanammonium (PDMA) as spacer cations is studied. PDMAPbI 4 and PDMASnI 4 exhibit absorption and luminescence spectra dominated by excitonic transitions and an emission due to two different states. Low-temperature studies reveal a time-dependent red shift of 12 meV that is correlated with grain-specific luminescence spectra observed in optical micrographs. For the Pb-variant, grains of red-shifted and lower intensity band edge emission simultaneously exhibit a more pronounced luminescence from a broad defect-related band...

In article number 2001294, Maria A. Loi, Giuseppe Portale, and co-workers unveil the mechanism of crystallization of Ruddlesden-Popper Sn-based perovskites using in situ X-ray measurements. The addition of a small fraction of the 2D component to the 3D perovskite causes suppression of the bulk crystallization and promotes oriented surface crystallization. Knowledge of the mechanism of crystal formation will help scientists to further optimize the efficiency of lead-free perovskite solar cells.

Scarce information is available on the thin film morphology of Dion-Jacobson halide perovskites. However, the microstructure can have a profound impact on a material’s photophysics and its potential for optoelectronic...