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and n = 1, 2, 3 or 4, and T z denotes surface terminations such as OH, O, and/ or F. MXenes are so-called for being principally derived by selectively etching the A-group (mostly Al) layers from the MAX-phases. They are distinct among 2D materials in that they are intrinsically metallic, easily processable in water, and have, in their short twelve-year history, displayed an array of unique properties such as record volumetric capacitances and conductivities, optical nonlinearities, and high laser damage thresholds. [1][2][3][4][5][6] While their potential to revolutionize energy storage and act as flexible electrodes rightfully attracted much attention, MXenes also hold promise as candidates in photonic and optoelectronic devices such as electromagnetic interference, EMI, shields, optical and terahertz (THz) switches, modulators, wavelength converters, and detectors. [7][8][9][10][11][12][13][14][15] The optical properties of MXenes are diverse [16] and can be tailored by chemical composition, structure, and/or morphology. [17,18] Their optical propertie scarise from a complicated interplay between free carriers, interband transitions as well as plasmon modes that occur in the visible and near-IR spectral ranges. [16,18,19] MXene plasmonic properties in particular are intriguing; while there is still much to be learned about the nature and properties of plasmons in different MXenes, localized surface plasmon resonances (LSPR) have already been hypothesized to underly efficient conversion of light to heat, saturable absorption in the visible and at telecommunication wavelengths, and high-performance photodetectors. [13,[20][21][22][23] In this work, we investigate the nature and dynamics of photoexcitations in three different members of the MXene family, viz. Ti 3 C 2 , Mo 2 Ti 2 C 3 , and Nb 2 C. We use two complementary pump-probe techniques to uncover the effects of optical excitations. The first is transient optical absorption, TA, that characterizes the effects on the optical properties in the visible and near-IR range, where LSPR features are dominant. The second, time-resolved THz spectroscopy (TRTS) measurements provide information about the dynamics of free charge carriers. The MXenes chosen here represent a broad range of electronic and optical properties. [16] As recent studies have uncovered, the most well-studied MXene, Ti 3 C 2 , is metallic with a high free carrier density (≈10 22 cm −3 ) and exhibits a pronounced LSPR extinction peak in the near-IR, in 750-800 nm range. [16,18,24] On 2D MXenes have diverse and chemically tunable optical properties that arise from an interplay between free carriers, interband transitions, and plasmon resonances. The nature of photoexcitations and their dynamics in three different members of the MXene family, Ti 3 C 2 , Mo 2 Ti 2 C 3 , and Nb 2 C, are investigated using two complementary pump-probe techniques, transient optical absorption, and time-resolved terahertz (THz) spectroscopy. Measurements reveal pronounced plasmonic effects in the visible and near-IR in...

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Annealing is one of the processing methods that are used for the fabrication of defect-free, photoactive perovskite films with compact grains in highly efficient and stable perovskite solar cells (PSCs). Thus, the annealing temperature is a key parameter for the control of the interdiffusion (of constituent elements) in photoactive films. In this paper, we present the results of a systematic study of the effects of annealing on the interdiffusion of constituent elements in efficient formamidinium-based PSCs. We also explore the effects of annealing-induced interdiffusion on layer microstructures, local strains, and the optoelectronic properties of perovskite films. We observe a dramatic upward diffusion of tin (Sn) and titanium (Ti) from fluorine-doped tin oxide and titanium dioxide (TiO2) to the perovskite films. We also observe a downward diffusion of lead (Pb) and iodine (I) from the perovskite films to the mesoporous layer of the electron transporting layer (ETL), after annealing at temperatures between 100 and 150 °C. The diffused I substitutes for Ti in the ETL, which improves the optoelectronic properties of the films, for annealing temperatures between 100 and 130 °C. The annealing-induced interdiffusion that occurs at higher temperatures (between 140 and 150 °C) results in higher levels of interdiffusion, along with increased local strains that lead to the nucleation of pores and cracks. Finally, the implications of the results are discussed for the design of PSCs with improved photoconversion efficiencies and stability.

In this paper, we use Polyethylene Oxide (PEO) particles to control the morphology of Formamidinium (FA)-rich perovskite films and achieve large grains with improved optoelectronic properties. Consequently, a planar perovskite solar cell (PSC) is fabricated with additions of 5 wt% of PEO, and the highest PCE of 18.03% was obtained. This solar cell is also shown to retain up to 80% of its initial PCE after about 140 h of storage under the ambient conditions (average relative humidity of 62.5 ± 3.25%) in an unencapsulated state. Furthermore, the steady-state PCE of the PEO-modified PSC device remained stable for long (over 2500 s) under continuous illumination. This addition of PEO particles is shown to enable the tuning of the optoelectronic properties of perovskite films, improvements in the overall photophysical properties of PSCs, and an increase in resistance to the degradation of PSCs.