Extremely low surface trap densities and ultrahigh sensitivity to oxygen and water molecules are demonstrated in perovskite single crystals.

International audienceHybrid organometal halide perovskites have been demonstrated to have outstanding performance as semiconductors for solar energy conversion. Further improvement of the efficiency and stability of these devices requires a deeper understanding of their intrinsic photophysical properties. Here, the structural and optical properties of high-quality single crystals of CH3NH3PbI3 from room temperature to 5 K are investigated. X-ray diffraction reveals an extremely sharp transition at 163 K from a twinned tetragonal I4/mcm phase to a low-temperature phase characterized by complex twinning and possible frozen disorder. Above the transition temperature, the photoluminescence is in agreement with a band-edge transition, explaining the outstanding performances of the solar cells. Whereas below the transition temperature, three different excitonic features arise, one of which is attributed to a free-exciton and the other two to bound excitons (BEs). The BEs are characterized by a decay dynamics of about 5 μs and by a saturation phenomenon at high power excitation. The long lifetime and the saturation effect make us attribute these low temperature features to bound triplet excitons. This results in a description of the room temperature recombination as being due to spontaneous band-to-band radiative transitions, whereas a diffusion-limited behavior is expected for the low-temperature range

A long-lived hot carrier population is critical in order to develop working hot carrier photovoltaic devices with efficiencies exceeding the Shockley–Queisser limit. Here, we report photoluminescence from hot-carriers with unexpectedly long lifetime (a few ns) in formamidinium tin triiodide. An unusual large blue shift of the time-integrated photoluminescence with increasing excitation power (150 meV at 24 K and 75 meV at 293 K) is displayed. On the basis of the analysis of energy-resolved and time-resolved photoluminescence, we posit that these phenomena are associated with slow hot carrier relaxation and state-filling of band edge states. These observations are both important for our understanding of lead-free hybrid perovskites and for an eventual future development of efficient lead-free perovskite photovoltaics.

Formamidinium lead iodide (FAPbI3) is a newly developed hybrid perovskite that potentially can be used in high-efficiency solution-processed solar cells. Here, the temperature-dependent dynamic optical properties of three types of FAPbI3 perovskite films (fabricated using three different precursor systems) are comparatively studied. The time-resolved photoluminescence (PL) spectra reveal that FAPbI3 films made from the new precursor (a mixture of formamidinium iodide and hydrogen lead triiodide) exhibit the longest lifetime of 439 ns at room temperature, suggesting a lower number of defects and lower non-radiative recombination losses compared with FAPbI3 obtained from the other two precursors. From the temperature-dependent PL spectra, a phase transition in the films is clearly observed. Meanwhile, exciton-binding energies of 8.1 and 18 meV for the high- and low-temperature phases are extracted, respectively. Importantly, the PL spectra for all of the samples show a single peak at room temperature, whereas at liquid-helium temperature the emission features two peaks: one in higher energy displaying a fast decay (0.5 ns) and a second red-shifted peak with a decay of up to several microseconds. These two emissions, separated by ~18 meV, are attributed to free excitons and bound excitons with singlet and triplet characters, respectively.

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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.

Layered halide perovskites have recently shown extraordinary potential for low‐cost solution‐processable optoelectronic applications because of their superior moisture stability over their 3D counterparts. However, few studies have investigated the effect of light on layered hybrid perovskites. Here, the mechanically exfoliated nanoflakes of the 2D perovskite (PEA)2PbI4 (PEA, 2‐phenylethylammonium) are used as a model to investigate their intrinsic photostability. The light‐induced degradation of the flakes is investigated by using in situ techniques including confocal laser scanning microscopy, wide‐field fluorescence microscopy, and atomic force microscopy. Under resonant photoexcitation, (PEA)2PbI4 degrades to PbI2. It is clearly shown that this process is initiated at the crystal edges and from the surface. As a consequence, the photoluminescence of (PEA)2PbI4 is progressively quenched by surface traps. Importantly, the light‐induced degradation can be suppressed by encapsulation using hexagonal boron nitride (hBN) flakes and/or polycarbonates. This report sheds light on a specific mechanism of light‐induced degradation in layered perovskites and proposes a new encapsulation method to improve their photostability.

The higher dielectric constant and electron donating properties of PTEG-1 electron extraction layer synergistically helps to eliminate the light soaking effect and enhance device performance.