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Register a new userQuantifying Charge Carrier Mobilities and Recombination Rates in Metal Halide Perovskites from Time-Resolved Microwave Photo-conductivity Measurements
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The unprecedented rise in power conversion efficiency of solar cells based on metal halide perovskites (MHPs) has led to enormous research effort to understand their photo-physical properties. In this paper, we review the progress in understanding the mobility and recombination of photo-generated charge carriers from nanosecond to microsecond time scales, monitored using electrodeless transient photoconductivity techniques. In addition, we present a kinetic model to obtain rate constants from transient data recorded using a wide range of laser intensities. For various MHPs the temperature dependence of the mobilities and recombination rates are evaluated. Furthermore, we show how these rate constants can be used to predict the upper limit for the open-circuit voltage Voc of the corresponding device. Finally, we discuss photo-physical properties of MHPs that are not yet fully understood, and make recommendations for future research directions.
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The formation of polarons due to the interaction between charge carriers and the crystal lattice has been proposed to have wide-ranging effects on charge carrier dynamics in lead--halide perovskites (LHPs). The hypothesis underlying many of those proposals is that charge carriers are protected from scattering by their incorporation into polarons. We test that hypothesis by deriving expressions for the rates of scattering of polarons by polar-optical and acoustic phonons, and ionised impurities, which we compute for electrons in the LHPs MAPbI$_{3}$ , MAPbBr$_{3}$ and CsPbI$_{3}$. We then use the ensemble Monte Carlo method to compute electron-polaron distribution functions which satisfy a Boltzmann equation incorporating the same three scattering mechanisms. By carrying out analogous calculations for band electrons and comparing their results to those for polarons, we conclude that polaron formation impacts charge-carrier scattering rates and mobilities to a limited degree in LHPs, contrary to claims in the recent literature.
Time-resolved, pulsed excitation methods are widely used to deduce optoelectronic properties of semiconductors, including now also Halide Perovskites (HaPs), especially transport properties. Howev-er, as yet no evaluation of their amenability and justification for the use of the results for the above-noted purposes has been reported. To check if we can learn from pulsed measurement results about steady-state phototransport properties, we show here that, although pulsed measurements can be useful to extract information on the recombination kinetics of HaPs, great care should be taken. One issue is that no changes in the material are induced during or as a result of the excitation, and another one concerns in how far pulsed excitation-derived data can be used to find relevant steady-state pa-rameters. To answer the latter question, we revisited pulsed excitation, and propose a novel way to compare between pulsed and steady state measurements at different excitation intensities. We per-formed steady-state photoconductivity and ambipolar diffusion length measurements, as well as pulsed TR-MC and TR-PL measurements as function of excitation intensity on the same samples of dif-ferent MAPbI3 thin films, and find good quasi-quantitative agreement between the results, explaining them with a generalized single level recombination model that describes the basic physics of photo-transport of HaP absorbers. Moreover, we find the first experimental manifestation of the boundaries between several effective recombination regimes that exist in HaPs, by analyzing their phototransport behavior as a function of excitation intensity.
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.
Lead halide perovskites are a remarkable class of materials that have emerged over the past decade as being suitable for application in a broad range of devices, such as solar cells, light-emitting diodes, lasers, transistors, and memory devices, among others. While they are often solution-processed semiconductors deposited at low temperatures, perovskites exhibit properties one would only expect from highly pure inorganic crystals that are grown at high temperatures. This unique phenomenon has resulted in fast-paced progress toward record device performance; unfortunately, the basic science behind the remarkable nature of these materials is still not well understood. This review assesses the current understanding of the photoluminescence (PL) properties of metal halide perovskite materials and highlights key areas that require further research. Furthermore, the need to standardize the methods for characterization of PL in order to improve comparability, reliability and reproducibility of results is emphasized.
Metal halide perovskites (MHPs) have become a promising candidate in a myriad of applications, such as light-emitting diodes, solar cells, lasing, photodetectors, photocatalysis, transistors, etc. This is related to the synergy of their excellent features, including high photoluminescence quantum yields, narrow and tunable emission, long charge carrier lifetimes, broad absorption spectrum along with high extinction absorptions coefficients, among others. However, the main bottleneck is the poor stability of the MHPs under ambient conditions. This is imposing severe restrictions with respect to their industrialized applications and commercialization. In this context, metal oxide (MOx) coatings have recently emerged as an efficient strategy towards overcoming the stabilities issues as well as retain the excellent properties of the MHPs, and therefore facilitate the development of the related devices stabilities and performances.This review provides a summary of the recent progress on synthetic methods, enhanced features, the techniques to assess the MHPs-MOxcomposites, and applications of the [email protected], novel approaches to fabricate the composites and new applications of the composites are also reported in this review for the first time. This is rounded by a critical outlook about the current MHPs stability issues and the further direction to ensure a bright future of MHPs@MOx
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