Hybrid organic-inorganic semiconductors feature complex lattice dynamics due to the ionic character of the crystal and the softness arising from non-covalent bonds between molecular moieties and the ...inorganic network. Here we establish that such dynamic structural complexity in a prototypical two-dimensional lead iodide perovskite gives rise to the coexistence of diverse excitonic resonances, each with a distinct degree of polaronic character. By means of high-resolution resonant impulsive stimulated Raman spectroscopy, we identify vibrational wavepacket dynamics that evolve along different configurational coordinates for distinct excitons and photocarriers. Employing density functional theory calculations, we assign the observed coherent vibrational modes to various low-frequency (≲50 cm
) optical phonons involving motion in the lead iodide layers. We thus conclude that different excitons induce specific lattice reorganizations, which are signatures of polaronic binding. This insight into the energetic/configurational landscape involving globally neutral primary photoexcitations may be relevant to a broader class of emerging hybrid semiconductor materials.
Despite the widespread use of solution-processable hybrid organic-inorganic perovskites in photovoltaic and light-emitting applications, determination of their intrinsic charge transport parameters ...has been elusive due to the variability of film preparation and history-dependent device performance. Here we show that screening effects associated to ionic transport can be effectively eliminated by lowering the operating temperature of methylammonium lead iodide perovskite (CH3NH3PbI3) field-effect transistors. Field-effect carrier mobility is found to increase by almost two orders of magnitude below 200 K, consistent with phonon scattering-limited transport. Under balanced ambipolar carrier injection, gate-dependent electroluminescence is also observed from the transistor channel, with spectra revealing the tetragonal to orthorhombic phase transition. This demonstration of CH3NH3PbI3 light-emitting field-effect transistors provides intrinsic transport parameters to guide materials and solar cell optimization, and will drive the development of new electro-optic device concepts, such as gated light-emitting diodes and lasers operating at room temperature.
Only a selected group of two-dimensional (2D) lead–halide perovskites shows a peculiar broad-band photoluminescence. Here we show that the structural distortions of the perovskite lattice can ...determine the defectivity of the material by modulating the defect formation energies. By selecting and comparing two archetype systems, namely, (NBT)2PbI4 and (EDBE)PbI4 perovskites (NBT = n-butylammonium and EDBE = 2,2-(ethylenedioxy)bis(ethylammonium)), we find that only the latter, subject to larger deformation of the Pb–X bond length and X–Pb–X bond angles, sees the formation of VF color centers whose radiative decay ultimately leads to broadened PL. These findings highlight the importance of structural engineering to control the optoelectronic properties of this class of soft materials.
Solar cells based on organic–inorganic lead iodide perovskite (CH3NH3PbI3) exhibit remarkably high power conversion efficiency (PCE). One of the key issues in solution-processed films is that often ...the polycrystalline domain orientation is not well-defined, which makes it difficult to predict energy alignment and charge transfer efficiency. Here we combine ab initio calculations and photoelectron spectroscopy to unravel the electronic structure and charge redistribution at the interface between different surfaces of CH3NH3PbI3 and typical organic hole acceptor Spiro-OMeTAD and electron acceptor PCBM. We find that both hole and electron interfacial transfer depend strongly on the CH3NH3PbI3 surface orientation: while the (001) and (110) surfaces tend to favor hole injection to Spiro-OMeTAD, the (100) surface facilitates electron transfer to PCBM due to surface delocalized charges and hole/electron accumulation at the CH3NH3PbI3/organic interfaces. Molecular dynamic simulations indicate that this is due to strong orbital interactions under thermal fluctuations at room temperature, suggesting the possibility to further improve charge separation and extraction in perovskite-based solar cells by controlling perovskite film crystallization and surface orientation.
Two-dimensional (2D) perovskites with the general formula APbX 4 are attracting increasing interest as solution processable, white-light emissive materials. Recent studies have shown that their ...broadband emission is related to the formation of intra-gap colour centres. Here, we provide an in-depth description of the charge localization sites underlying the generation of such radiative centres and their corresponding decay dynamics, highlighting the formation of small polarons trapped within their lattice distortion field. Using a combination of spectroscopic techniques and first-principles calculations to study the white-light emitting 2D perovskites (EDBE)PbCl 4 and (EDBE)PbBr 4 , we infer the formation of Pb 2 3+ , Pb 3+ , and X 2 − (where X = Cl or Br) species confined within the inorganic perovskite framework. Due to strong Coulombic interactions, these species retain their original excitonic character and form self-trapped polaron–excitons acting as radiative colour centres. These findings are expected to be relevant for a broad class of white-light emitting perovskites with large polaron relaxation energy.
Published studies of layered (2D) (100)‐oriented hybrid lead‐bromide perovskites evidence a correlation between increased inter‐octahedral (Pb‐Br‐Pb) distortions and the appearance of broadband white ...light emission. However, the impact of distortions within their constituent PbBr64− octahedra has yet to be assessed. Herein, we report two new (100)‐oriented 2D Pb‐Br perovskites, whose structures display unusually high intra‐octahedral distortions, whilst retaining minimal inter‐octahedral distortions. Using a combination of temperature‐dependent, power‐dependent and time‐resolved photoluminescence spectroscopic measurements, we show that increased intra‐octahedral distortion induces exciton localization processes and leads to formation of multiple photoinduced emissive colour centres. Ultimately, this leads to highly Stokes‐shifted, ultrabroad white light emission at room temperature.
2D perovskite white‐light emitters: Asymmetric pyridinium‐ and piperidinium‐based cations were used to template 2D hybrid lead‐bromide perovskites with unusually high intra‐octahedral distortions. This synthetic procedure leads to the formation of multiple photoinduced emissive colour centres, which ultimately results in highly Stokes‐shifted, ultrabroad white light emission.
We report on the excitonic luminescence of polycrystalline films composed of two-dimensional (2D) tin iodide perovskites. By combining steady-state and time-resolved spectroscopies between room ...temperature and 5 K, we identify an optically inactive, dark excitonic state within the spectral fine structure at the exciton energy, ubiquitous in the optical spectra of 2D perovskites. Lying at about 10 meV below the bright exciton that is responsible for luminescence, the dark state mediates non-radiative decay of the photo-excited population at temperatures below 100 K. However, at about 100-120 K, we observe a thermally activated transfer of population from the dark state back to the bright state which results in an increase in luminescence efficiency. Based on quantitative analysis of the observed exciton dynamics, we argue that the dark state acts as a reservoir of photo-excitations at ambient temperature. By feeding the bright state at a rate that is slower than the radiative rate, the dark state mitigates the loss of photo-excited population to other non-radiative pathways. Our work provides insights into the dynamics of the inevitable dark states in 2D perovskites and their relevance in enhancing the emissivity of the technologically relevant lead-free material architectures.
We report on the excitonic luminescence of polycrystalline films composed of two-dimensional (2D) tin iodide perovskites.
A new broadband‐emitting 2 D hybrid organic–inorganic perovskite (CyBMA)PbBr4 based on highly flexible cis‐1,3‐bis(methylaminohydrobromide)cyclohexane (CyBMABr) core has been designed, synthesized, ...and investigated, highlighting the effects of stereoisomerism of the templating cation on the formation and properties of the resulting perovskite. The new 2 D material has high exciton binding energy of 340 meV and a broad emission spanning from 380 to 750 nm, incorporating a prominent excitonic band and a less intense broad peak at room temperature. Significant changes in the photoluminescence (PL) spectrum were observed at lower temperatures, showing remarkable enhancement in the intensity of the broadband at the cost of excitonic emission. Temperature‐dependent PL mapping indicates the effective role of only a narrow band of excitonic absorption in the generation of the active channel for emission. Based on the evidences obtained from the photophysical investigations, we attributed the evolution of the broad B‐band of (CyBMA)PbBr4 to excitonic self‐trapped states.
Cis or trans? A new 2 D hybrid perovskite ((CyBMA)PbBr4) based on highly flexible cis‐1,3‐bis(methylaminohydrobromide)cyclohexane (CyBMABr) core presenting a broad emission spanning from 380 to 750 nm is presented here. Its design, synthesis, and photophysical properties are investigated, highlighting the effects of stereoisomerism of the templating cation on the formation and properties of the resulting perovskite.
Despite their excellent power conversion efficiency, MAPbI3 solar cells exhibit strong hysteresis that hinders reliable device operation. Herein it is shown that ionic motion is the dominant ...mechanism underlying hysteresis of MAPbI3 solar cells by studying the effects of electrical poling in different temperature ranges. Complete suppression of the hysteresis below 170 K is consistent with temperature activated diffusion of I− anions and/or the motion of the MA+ cations. Ionic motion has important effect on the overall efficiency of the MAPbI3 solar cells: the initial decrease of the power conversion efficiency while lowering the operating temperature is recovered and even enhanced up to 20% of its original value by applying an electrical poling. The open circuit voltage significantly increases and the current density fully recovers due to the reduction of the electron extraction barrier at the TiO2/MAPbI3 interface driven by the charge accumulation at the interface. Moreover, beside TiO2/MAPbI3 interfacial charge transfer, charge transport in TiO2 strongly affects the photovoltaic performance, as revealed by MAPbI3/ms‐TiO2 field effect transistors. These results establish the basis to develop effective strategies to mitigate operational instability of perovskites solar cells.
It is proven that ionic motion is the dominant mechanism behind MAPbI3 solar cell hysteresis by investigating electrical poling effects in a wide temperature range. The power conversion efficiency reduces at low temperature, but then recovers and improves up to 20% of its original value under electrical bias. This effect is attributed to the electron extraction barrier reduction at the TiO2/MAPbI3 interface.
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