RESUMEN
The optical response of 2D layered perovskites is composed of multiple equally-spaced spectral features, often interpreted as phonon replicas, separated by an energy Δ ≃ 12 - 40 meV, depending upon the compound. Here the authors show that the characteristic energy spacing, seen in both absorption and emission, is correlated with a substantial scattering response above ≃ 200 cm-1 (≃ 25 meV) observed in resonant Raman. This peculiar high-frequency signal, which dominates both Stokes and anti-Stokes regions of the scattering spectra, possesses the characteristic spectral fingerprints of polarons. Notably, its spectral position is shifted away from the Rayleigh line, with a tail on the high energy side. The internal structure of the polaron consists of a series of equidistant signals separated by 25-32 cm-1 (3-4 meV), depending upon the compound, forming a polaron vibronic progression. The observed progression is characterized by a large Huang-Rhys factor (S > 6) for all of the 2D layered perovskites investigated here, indicative of a strong charge carrier - lattice coupling. The polaron binding energy spans a range ≃ 20-35 meV, which is corroborated by the temperature-dependent Raman scattering data. The investigation provides a complete understanding of the optical response of 2D layered perovskites via the direct observation of polaron vibronic progression. The understanding of polaronic effects in perovskites is essential, as it directly influences the suitability of these materials for future opto-electronic applications.
RESUMEN
Recent works demonstrate that polyelectrolytes as a hole transport layer (HTL) offers superior performance in Ruddlesden-Popper perovskite solar cells (RPPSCs) compared to poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS). The factors contributing to such improvement need to be systematically investigated. To achieve this, we have systematically investigated how the two HTLs affect the morphology, crystallinity, and orientation of the Ruddlesden-Popper perovskite (RPP) films as well as the charge extraction of the RPPSCs. PEDOT:PSS as a HTL leads to RPP films of low crystallinity and with a number of large pinholes. These factors lead to poor charge carrier extraction and significant charge recombination in the RPPSCs. Conversely, a PCP-Na HTL gives rise to highly crystalline and pinhole-free RPPSC films. Moreover, a PCP-Na HTL provides a better energy alignment at the perovskite/HTL interface because of its higher work function compared to PEDOT:PSS. Consequently, devices using PCP-Na as HTLs are more efficient in extracting charge carriers.
RESUMEN
Formamidinium lead iodide (FAPbI3) is one of the most extensively studied perovskite materials due to its narrow band gap and high absorption coefficient, which makes it highly suitable for optoelectronic applications. Deposition of a solution containing lead iodide (PbI2) and formamidinium iodide (FAI) or sequential deposition of PbI2 and FAI usually leads to the formation of films with a poor morphology and an unstable crystal structure that readily crystallize into two different polymorphs: the photoinactive yellow phase and the photoactive black phase. In this work, 2D 2-phenylethylammonium lead iodide (PEA2PbI4) thin films are deposited by a scalable doctor-blade coating technique and used as a growth template for the high-quality 3D FAPbI3 perovskite thin films which are obtained by organic cation exchange. We report the structural, morphological and optical properties of these converted 3D FAPbI3 perovskite films which we compare to the directly deposited 3D FAPbI3 films. The converted FAPbI3 thin films are compact, smooth, and highly oriented and exhibit better structural stability in comparison with the directly deposited 3D films. These results not only underscore the importance of the employed deposition techniques in fabricating highly crystalline and stable perovskite thin films but also provide a strategy to easily obtain very compact perovskite layers using doctor-blade coating.
RESUMEN
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.
RESUMEN
The optical properties of the newly developed near-infrared emitting formamidinium lead triiodide (FAPbI3 ) nanocrystals (NCs) and their polycrystalline thin film counterpart are comparatively investigated by means of steady-state and time-resolved photoluminescence. The excitonic emission is dominant in NC ensemble because of the localization of electron-hole pairs. A promisingly high quantum yield above 70%, and a large absorption cross-section (5.2 × 10-13 cm-2 ) are measured. At high pump fluence, biexcitonic recombination is observed, featuring a slow recombination lifetime of 0.4 ns. In polycrystalline thin films, the quantum efficiency is limited by nonradiative trap-assisted recombination that turns to bimolecular at high pump fluences. From the temperature-dependent photoluminescence (PL) spectra, a phase transition is clearly observed in both NC ensemble and polycrystalline thin film. It is interesting to note that NC ensemble shows PL temperature antiquenching, in contrast to the strong PL quenching displayed by polycrystalline thin films. This difference is explained in terms of thermal activation of trapped carriers at the nanocrystal's surface, as opposed to the exciton thermal dissociation and trap-mediated recombination, which occur in thin films at higher temperatures.
RESUMEN
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.
RESUMEN
One of the limiting factors to high device performance in photovoltaics is the presence of surface traps. Hence, the understanding and control of carrier recombination at the surface of organic-inorganic hybrid perovskite is critical for the design and optimization of devices with this material as the active layer. We demonstrate that the surface recombination rate (or surface trap state density) in methylammonium lead tribromide (MAPbBr3) single crystals can be fully and reversibly controlled by the physisorption of oxygen and water molecules, leading to a modulation of the photoluminescence intensity by over two orders of magnitude. We report an unusually low surface recombination velocity of 4 cm/s (corresponding to a surface trap state density of 108 cm-2) in this material, which is the lowest value ever reported for hybrid perovskites. In addition, a consistent modulation of the transport properties in single crystal devices is evidenced. Our findings highlight the importance of environmental conditions on the investigation and fabrication of high-quality, perovskite-based devices and offer a new potential application of these materials to detect oxygen and water vapor.
RESUMEN
We use first-principles density functional theory based calculations to determine the stability and properties of silicene, a graphene-like structure made from silicon, and explore the possibilities of modifying its structure and properties through incorporation of transition metal ions (M: Ti, Nb, Ta, Cr, Mo and W) in its lattice, forming MSi(2). While pure silicene is stable in a distorted honeycomb lattice structure obtained by opposite out-of-plane displacements of the two Si sub-lattices, its electronic structure still exhibits linear dispersion with the Dirac conical feature similar to graphene. We show that incorporation of transition metal ions in its lattice results in a rich set of properties with a clear dependence on the structural changes, and that CrSi(2) forms a two-dimensional magnet exhibiting a strong piezomagnetic coupling.
