In this work, the spatially dependent recombination kinetics of mixed-halide hybrid perovskite CHNHPb(BrCl ) (0 ≤ x ≤ 0.19) single crystals are investigated using time-resolved photoluminescence spectroscopy with one- and two-photon femtosecond laser excitation. The introduction of chloride by substituting a fraction of the bromide leads to a decreased lattice constant compared to pure bromide perovskite ( x = 0) and a higher concentration of surface defects. The measured kinetics under one-photon excitation (1PE) shows that increasing the chloride addition quenches the photoluminescence (PL) lifetimes, due to substitution-induced surface defects. In stark contrast, upon 2PE, the PL lifetimes measured deeper in the bulk become longer with increasing chloride addition, until the halide substitution reaches the critical concentration of ∼19%. At x = 19% Cl concentration, a significant reversal of this behavior is observed indicating a change in crystal structure beyond the continuous trends observed at lower percentages of halide substitution ( x ≤ 11%). The observed opposing trends, based on 1PE versus 2PE, highlight a dichotomy between extrinsic (surface) and intrinsic (bulk) effects of chloride substitution on the carrier dynamics in lead bromide perovskites. We discuss the physical relation between halide exchange and bulk carrier lifetimes in CHNHPbBr in terms of the Rashba effect. We propose that the latter is suppressed at the surface due to disorder in the alignment of the MA and that it increases in the bulk with Cl concentration because of the reduction in lattice parameters, which compresses the space available for the MA orientational degrees of freedom.
The past decade has witnessed a growing interest in metal halide perovskite (MHP) materials, driven by their promising applications in photovoltaics and optoelectronics. The further pursuit of improved performance and stability will rely on a clear understanding of the fundamental properties of these materials. In this Feature Article, we outline a representative set of studies detailing how ultrafast laser spectroscopy can be used to access the optoelectronic properties and photophysical mechanisms of halide perovskites. Beginning with a concise synopsis of MHP history and structural properties, the dynamic processes in MHPs related to the photovoltaic performance are discussed. A brief overview of the representative time-resolved optical spectroscopic techniques in MHP research is then provided. Afterward, the recent advances in exploring the carrier, lattice, and spatially resolved dynamic processes in halide perovskite are summarized. The last section is devoted to a range of applications using halide perovskites. Finally, our conclusions and outlook for the field with some predictions for future opportunities round off this Feature Article.
Metal halide perovskites are currently among the most promising materials to reshape our renewable energy future through photovoltaics. Nevertheless, they are also among the more complicated materials to understand and to engineer functional photovoltaics devices from. Their current performance efficiencies have not reached the highest predicted value of 30.06%. Many efforts have been dedicated to developing MHP materials, while fewer efforts were directed to understand and engineer the interfaces and interfacial properties in MHPs. Recently, the understanding and engineering of interfacial properties in MHPs have become a hot topic due to the vital role of interfaces, especially carrier dynamics, on device stability and efficiency. This perspective highlights the importance of focusing research on interfaces and interfacial carrier dynamics in metal halide perovskites. After introducing current challenges in MHPs interfaces and interfacial engineering, we provide a perspective on the contribution that different time-resolved laser spectroscopies add to the growing field of perovskite photovoltaics.
CuI is one of the promising hole transport materials for perovskite solar cells. However, its tendency to form defects is currently limiting its use for device applications. Here, we report the successful improvement of CuI through Sn doping and the direct measurement of the carrier relaxation and interfacial charge-transfer processes in Sn-doped CuI films and their heterostructures. Femtosecond-transient absorption (fs-TA) measurements reveal that Sn doping effectively passivates the trap states within the bandgap of CuI. The I–V characteristics of heterostructures demonstrate drastic improvement in transport characteristics upon Sn doping. Fs-TA measurements further confirm that the CuSnI/ZnO heterojunction has a type-II configuration with ultrafast charge transfer (<280 fs). The charge transfer time of a CuI/ZnO heterostructure is ∼2.8 times slower than that of the CuSnI/ZnO heterostructure, indicating that Sn doping suppresses the interfacial states that retard the charge transfer. These results elucidate the effect of Sn doping on the performance of CuI-based heterostructures.
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