Exciton and Carrier Dynamics in Two-Dimensional Perovskites (original) (raw)
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Journal of Physical Chemistry C, 2020
Two-dimensional (2D) Ruddlesden-Popper (RP) layered lead halide perovskites have recently emerged as a more stable alternative to their 3D counterparts while also exhibiting intriguing photophysical properties. Although time-resolved photoluminescence (TRPL) is an excellent diagnostic tool for the photophysics of these luminescent semiconductors, the most common approaches to analyzing TRPL transients do not discriminate between the mechanisms responsible for charge carrier dynamics; namely, the behavior of excitons, which is more relevant for optoelectronic applications, and the behavior of electrons and holes, which is most relevant for solar conversion. Here, we develop a kinetic approach to systematically resolve exciton and free carrier dynamics across a series of (PEA) 2 (CH 3 NH 3) n-1 Pb n I 3n+1 RP perovskite single crystals (PEA = phenethylammonium, n=1, 2, 3, 4, and ∞). Our approach uses repetitive excitation that builds up a steady state of free carriers, even when the exciton binding energies are large. Within the timescale of a TRPL experiment, the rapid changes in the total photoexcited carrier density cause the carrier dynamics to change from exciton-dominated to freecarrier-dominated, as expected from the Saha equilibrium. Thus, despite only measuring
Probe of the excitonic transitions and lifetimes in quasi-2D organic–inorganic halide perovskites
AIP Advances, 2022
Traditional organic-inorganic halide perovskites (OIHPs), in which perovskites layers are separated by an organic spacer material, have been mainly explored for photovoltaics devices, but they also offer promises for nonlinear optics and quantum light applications. These attributes include (a) high quantum efficiency, (b) large binding energy of excitons in low-dimensional structures, (c) polarons of long coherence times at room temperature, and (d) a large spin-orbit coupling. OIHP systems can be engineered to have photoluminescence (PL) emissions from UV to IR regions, in addition to power conversion efficiencies, in excess of 24%. This class of materials offers broad tunability of its properties, through controlling the number of atomic layers in the quantum well, tuning the organic spacer thickness, or even engineering the composition with exotic dopants. In this work, we present PL and time-resolved PL measurements of quasi-2D BA 2 PbI 4 and provide new insights on the temperature dependence of their excitonic dynamics and fine structures of their PL emissions. We observed long lifetimes, which can result from the formation of large polarons, screening the Coulomb interactions of the charge carriers and reducing the scattering of the carriers with charge defects.
Phonon-Assisted Trapping and Re-excitation of Free Carriers and Excitons in Lead Halide Perovskites
Journal of Physical Chemistry C, 2019
Despite the advances in solar cells based on lead halide perovskites, the nature of photogenerated charges and trap states within these materials remains unclear. A model describing recombination in CH 3 NH 3 PbI 3-x Cl x has been developed that accounts for phonon-assisted free-exciton and freecarrier trapping. We utilize optical spectroscopies and observe significant coexistence of the tetragonal and orthorhombic structural phases at low temperatures. From these measurements, we evaluate the longitudinal-optical phonon energy, exciton binding energy and temperaturedependent electronic bandgap. We use these parameters to model the temperature-and fluencedependent time-resolved photoluminescence decays, enabling us to demonstrate how shallow traps from which carriers can be re-excited can account for the delayed recombination in lead halide perovskites. The trap-state density reaches a maximum at the tetragonal to orthorhombic phase transition at ~140 K, suggesting the formation of disorder-induced trap states, which are shown to dominate the recombination dynamics in CH 3 NH 3 PbI 3-x Cl x .
Physical review letters, 2015
We studied the ultrafast transient response of photoexcitations in two hybrid organic-inorganic perovskite films used for high efficiency photovoltaic cells, namely, CH_{3}NH_{3}PbI_{3} and CH_{3}NH_{3}PbI_{1.1}Br_{1.9} using polarized broadband pump-probe spectroscopy in the spectral range of 0.3-2.7 eV with 300 fs time resolution. For CH_{3}NH_{3}PbI_{3} with above-gap excitation we found both photogenerated carriers and excitons, but only carriers are photogenerated with below-gap excitation. In contrast, mainly excitons are photogenerated in CH_{3}NH_{3}PbI_{1.1}Br_{1.9}. Surprisingly, we also discovered in CH_{3}NH_{3}PbI_{3}, but not in CH_{3}NH_{3}PbI_{1.1}Br_{1.9}, transient photoinduced polarization memory for both excitons and photocarriers, which is also reflected in the steady state photoluminescence. From the polarization memory dynamics we obtained the excitons diffusion constant in CH_{3}NH_{3}PbI_{3}, D≈0.01 cm^{2} s^{-1}.
Recombination Kinetics in Organic-Inorganic Perovskites: Excitons, Free Charge, and Subgap States
Physical Review Applied, 2014
Organic-inorganic perovskites are attracting increasing attention for their use in high-performance solar cells. Nevertheless, a detailed understanding of charge generation, interplay of excitons and free charge carriers, and recombination pathways, crucial for further device improvement, remains incomplete. Here, we present an analytical model describing both equilibrium properties of free charge carriers and excitons in the presence of electronic subgap trap states and their time evolution after photoexcitation in CH 3 NH 3 PbI 3−x Cl x . At low fluences the charge-trapping pathways limit the photoluminescence quantum efficiency, whereas at high fluences the traps are predominantly filled and recombination of the photogenerated species is dominated by efficient radiative processes. We show experimentally that the photoluminescence quantum efficiency approaches 100% at low temperatures and at high fluences, as predicted by our model. Our approach provides a theoretical framework to understand the fundamental physics of perovskite semiconductors and to help in designing and enhancing the material for improved optoelectronic device operation.
Two Origins of Broadband Emission in Multilayered 2D Lead Iodide Perovskites
Journal of Physical Chemistry Letters, 2020
Broadband emission in lead iodide 2D perovskites has been alternately attributed to self-trapped excitons (STEs) or permanent structural defects and/or impurities. Here, we investigate six different multilayered (n > 1) 2D lead iodide perovskites as a function of sample temperature from 5 to 300 K. We distinguish shallow defect-associated emission from a broad near-infrared (NIR) spectral feature, which we assign to an STE through subgap photoexcitation experiments. When we varied the thickness (n = 2, 3, 4), A-site cation (methylammonium vs formamidinium), and organic spacer (butylammonium vs hexylammonium vs phenylethylammonium), we found that the temperature dependence of broad NIR emission was strongly correlated with both the strength of electron−phonon coupling and the extent of structural deformation of the ground-state lattice, strongly supporting the assignment of this spectral feature to an STE. However, the extent to which formation of these STEs is intrinsic versus defect-assisted remains open to debate.
Stable biexcitons in two-dimensional metal-halide perovskites with strong dynamic lattice disorder
Physical Review Materials
With strongly bound and stable excitons at room temperature, single-layer, two-dimensional organic-inorganic hybrid perovskites are viable semiconductors for light-emitting quantum optoelectronics applications. In such a technological context, it is imperative to comprehensively explore all the factors-chemical, electronic, and structural-that govern strong multiexciton correlations. Here, by means of two-dimensional coherent spectroscopy, we examine excitonic many-body effects in pure, single-layer (PEA) 2 PbI 4 (PEA = phenylethylammonium). We determine the binding energy of biexcitons-correlated two-electron, two-hole quasiparticles-to be 44 ± 5 meV at room temperature. The extraordinarily high values are similar to those reported in other strongly excitonic two-dimensional materials such as transition-metal dichalcogenides. Importantly, we show that this binding energy increases by ∼25% upon cooling to 5 K. Our work highlights the importance of multiexciton correlations in this class of technologically promising, solution-processable materials, in spite of the strong effects of lattice fluctuations and dynamic disorder.
arXiv (Cornell University), 2023
The efficiency of two-dimensional Dion-Jacobson-type materials relies on the complex interplay between electronic and lattice dynamics; however, questions remain about the functional role of exciton-phonon interactions. This study establishes the robust polaronic nature of the excitons in these materials at room temperature by combining ultrafast spectroscopy and electronic structural calculations. We show that polaronic distortion is associated with low-frequency (30-60 cm −1) lead iodide octahedral lattice motions. More importantly, we discover how targeted ligand modification of this two-dimensional perovskite structure controls exciton-phonon coupling, excitonpolaron population, and carrier cooling. At high excitation density, stronger excitonphonon coupling increases the hot carrier lifetime, forming a hot-phonon bottleneck. Our study provides detailed insight into the exciton-phonon coupling and its role in carrier cooling in two-dimensional perovskites relevant for developing emerging hybrid semiconductor materials with tailored properties.
Long-range exciton transport and slow annihilation in two-dimensional hybrid perovskites
Nature Communications, 2020
Two-dimensional hybrid organic-inorganic perovskites with strongly bound excitons and tunable structures are desirable for optoelectronic applications. Exciton transport and annihilation are two key processes in determining device efficiencies; however, a thorough understanding of these processes is hindered by that annihilation rates are often convoluted with exciton diffusion constants. Here we employ transient absorption microscopy to disentangle quantum-well-thickness-dependent exciton diffusion and annihilation in two-dimensional perovskites, unraveling the key role of electron-hole interactions and dielectric screening. The exciton diffusion constant is found to increase with quantum-well thickness, ranging from 0.06 ± 0.03 to 0.34 ± 0.03 cm2 s−1, which leads to long-range exciton diffusion over hundreds of nanometers. The exciton annihilation rates are more than one order of magnitude lower than those found in the monolayers of transition metal dichalcogenides. The combinatio...