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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.

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 all three. Optical excitation, with either 400 or 800 nm pulses, results in a rapid increase in lattice temperature, evidenced by a pronounced broadening of the plasmon mode that presents as a plasmon bleach in transient absorption measurements. Observed kinetics of plasmon bleach recovery provide a means to monitor lattice cooling. Remarkably slow cooling, proceeding over hundreds of picoseconds to nanoseconds time scales, implies MXenes have low thermal conductivities. The slowest recovery kinetics are observed in the MXene with the highest free carrier density, viz. Ti 3 C 2 , that supports phonon scattering by free carriers as a possible mechanism limiting thermal conductivity. These new insights into photoexcitation dynamics can facilitate their applications in photothermal solar energy conversion, plasmonic devices, and even photothermal therapy and drug delivery., (© 2022 Wiley-VCH GmbH. This article has been contributed to by U.S. Government employees and their work is in the public domain in the USA.)

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., (© 2022. The Author(s).)