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Register a new userMulti-Pulse Terahertz Spectroscopy Unveils Hot Polaron Photoconductivity Dynamics in Metal-Halide Perovskites
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The behavior of hot carriers in metal-halide perovskites (MHPs) present a valuable foundation for understanding the details of carrier-phonon coupling in the materials as well as the prospective development of highly efficient hot carrier and carrier multiplication solar cells. Whilst the carrier population dynamics during cooling have been intensely studied, the evolution of the hot carrier properties, namely the hot carrier mobility, remain largely unexplored. To address this, we introduce a novel ultrafast visible pump - infrared push - terahertz probe spectroscopy (PPP-THz) to monitor the real-time conductivity dynamics of cooling carriers in methylammonium lead iodide. We find a decrease in mobility upon optically depositing energy into the carriers, which is typical of band-transport. Surprisingly, the conductivity recovery dynamics are incommensurate with the intraband relaxation measured by an analogous experiment with an infrared probe (PPP- IR), and exhibit a negligible dependence on the density of hot carriers. These results and the kinetic modelling reveal the importance of highly-localized lattice heating on the mobility of the hot electronic states. This collective polaron-lattice phenomenon may contribute to the unusual photophysics observed in MHPs and should be accounted for in devices that utilize hot carriers.
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Lead halide perovskite semiconductors are soft, polar, materials. The strong driving force for polaron formation (the dielectric electron-phonon coupling) is balanced by the light band effective-masses, leading to a strongly-interacting large-polaron. A first-principles prediction of mobility would help understand the fundamental mobility limits. Theories of mobility need to consider the polaron (rather than free-carrier) state due to the strong interactions. In this material we expect that at room temperature polar-optical phonon mode scattering will dominate, and so limit mobility. We calculate the temperature-dependent polaron mobility of hybrid halide perovskites by variationally solving the Feynman polaron model with the finite-temperature free-energies of =Osaka. This model considers a simplified effective-mass band-structure interacting with a continuum dielectric of characteristic response frequency. We parametrise the model fully from electronic-structure calculations. In methylammonium lead iodide at 300 K we predict electron and hole mobilities of 133 and 94 cm^2/V/s respectively. These are in acceptable agreement with single-crystal measurements, suggesting that the intrinsic limit of the polaron charge carrier state has been reached. Repercussions for hot-electron photo-excited states are discussed. As well as mobility, the model also exposes the dynamic structure of the polaron. This can be used to interpret impedance measurements of the charge-carrier state. We provide the phonon-drag mass-renormalisation, and scattering time constants. These could be used as parameters for larger-scale device models and band-structure dependent mobility simulations.
While polarons --- charges bound to a lattice deformation induced by electron-phonon coupling --- are primary photoexcitations at room temperature in bulk metal-halide hybrid organic-inorganic perovskites (HOIP), excitons --- Coulomb-bound el-ectron-hole pairs --- are the stable quasi-particles in their two-dimensional (2D) analogues. Here we address the fundamental question: are polaronic effects consequential for excitons in 2D-HIOPs? Based on our recent work, we argue that polaronic effects are manifested intrinsically in the exciton spectral structure, which is comprised of multiple non-degenerate resonances with constant inter-peak energy spacing. We highlight our own measurements of population and dephasing dynamics that point to the apparently deterministic role of polaronic effects in excitonic properties. We contend that an interplay of long-range and short-range exciton-lattice couplings give rise to exciton polarons, a character that fundamentally establishes their effective mass and radius, and consequently, their quantum dynamics. Finally, we highlight opportunities for the community to develop the rigorous description of exciton polarons in 2D-HIOPs to advance their fundamental understanding as model systems for condensed-phase materials in which lattice-mediated correlations are fundamental to their physical properties.
In recent years, metal halide perovskites have generated tremendous interest for optoelectronic applications and their underlying fundamental properties. Due to the large electron-phonon coupling characteristic of soft lattices, self-trapping phenomena are expected to dominate hybrid perovskite photoexcitation dynamics. Yet, while the photogeneration of small polarons was proven in low dimensional perovskites, the nature of polaron excitations in technologically relevant 3D perovskites, and their influence on charge carrier transport, remain elusive. In this study, we used a combination of first principle calculations and advanced spectroscopy techniques spanning the entire optical frequency range to pin down polaron features in 3D metal halide perovskites. Mid-infrared photoinduced absorption shows the photogeneration of states associated to low energy intragap electronic transitions with lifetime up to the ms time scale, and vibrational mode renormalization in both frequency and amplitude. Density functional theory supports the assignment of the spectroscopic features to large polarons leading to new intra gap transitions, hardening of phonon mode frequency, and renormalization of the oscillator strength. Theory provides quantitative estimates of the charge carrier masses and mobilities increase upon polaron formation, confirming experimental results. Overall, this work contributes to complete the scenario of elementary photoexcitations in metal halide perovskites and highlights the importance of polaronic transport in perovskite-based optoelectronic devices.
For the intrinsic carriers of MAPbBr$_{3}$, the temperature $T$ dependent mobility $mu(T)$ of behaves like $mupropto T^{-1/2}$ in piezoelectric tetragonal phase, $mupropto T^{-1.4}$ in non-piezoelectric cubic phase. But for the photo-generated carriers in other halide perovskites ABX$_{3}$, $mupropto T^{-3/2}$ behavior is typical. Due to the strong interaction of carrier with longitudinal optical phonon, in ABX$_{3}$ the carriers mainly exist as optical polarons. The softness of ABX$_{3}$ renders it without inversion center in tetragonal phase, which allows piezoelectric effect at low carrier concentration. The variations of $mu(T)$ behavior results from (1) the wave vector dependence of the piezoelectric interaction of polarons with acoustic phonons is different from that of ordinary polaron-acoustic phonon interaction; (2) the residual interaction of polaron with 2 longitudinal optical phonons can be ignored at low temperature, but is important at higher temperature; and (3) the concentration of intrinsic carriers is determined by temperature, while the concentration of photo-generated carriers is determined by the incident flux of photons.
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 multi-exciton 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 $44 pm 5$,meV at room temperature. The extraordinarily high values are similar to those reported in other strongly excitonic two-dimensional materials such as transition-metal dichalchogenides. Importantly, we show that this binding energy increases by $sim25$% upon cooling to 5,K. Our work highlights the importance of multi-exciton correlations in this class of technologically promising, solution-processable materials, in spite of the strong effects of lattice fluctuations and dynamic disorder.
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