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Van der Waals heterojunctions of two-dimensional transition-metal dichalcogenides are intensely investigated for multiple optoelectronics applications. Strong and adjustable interactions between layers can influence the charge and energy flow that govern material performance. We report ab initio quantum molecular dynamics investigation of the influence of the bilayer twist angle on charge transfer and recombination in MoS 2 /WS 2 heterojunctions, including high-symmetry 0° and 60° configurations, and low symmetry 9.43° and 50.57° structures with Moiré patterns. The twist angle modulates interlayer coupling, as evidenced by changes in the interlayer distance, electron-vibrational interactions, and spectral shifts in the out-of-plane vibrational frequencies. Occurring on a femtosecond timescale, the hole transfer depends weakly on the twist angle and is ultrafast due to high density of acceptor states and large nonadiabatic coupling. In contrast, the electron–hole recombination takes nanoseconds and varies by an order of magnitude depending on the twist angle. The recombination is slow because it occurs across a large energy gap. It depends on the twist angle because the nonadiabatic coupling is sensitive to the interlayer distance and overlap of electron and hole wavefunctions. The Moiré pattern systems exhibit weaker interlayer interaction, generating longer-lived charges. Both charge separation and recombination are driven by out-of-plane vibrational motions. The simulations rationalize the experimental results on the influence of the bilayer twist angle on the charge separation and recombination. The atomistic insights provide theoretical guidance for design of high-performance optoelectronic devices based on 2D van der Waals heterostructures. 

Defects, such as halide interstitials, act as charge recombination centers, induce degradation of halide perovskites, and create major obstacles to applications of these materials. Alkali metal dopants greatly improve perovskite performance. Using ab initio nonadiabatic molecular dynamics, it is demonstrated that alkalis bring favorable effects. The formation energy of halide interstitials increases by up to a factor of four in the presence of alkali dopants, and therefore, defect concentration decreases. When defects are present, alkali metals strongly bind to them. Halide interstitials introduce mid‐gap states that rapidly trap charge carriers. Alkalis eliminate the trap states, helping to maintain high current density. Further to charge trapping, the interstitials accelerate charge recombination. By passivating the interstitials, alkalis make carrier lifetimes up to seven times longer than in defect‐free perovskites and up to thirty times longer than in defective perovskites.

 

Defects, such as halide interstitials, act as charge recombination centers, induce degradation of halide perovskites, and create major obstacles to applications of these materials. Alkali metal dopants greatly improve perovskite performance. Using ab initio nonadiabatic molecular dynamics, it is demonstrated that alkalis bring favorable effects. The formation energy of halide interstitials increases by up to a factor of four in the presence of alkali dopants, and therefore, defect concentration decreases. When defects are present, alkali metals strongly bind to them. Halide interstitials introduce mid‐gap states that rapidly trap charge carriers. Alkalis eliminate the trap states, helping to maintain high current density. Further to charge trapping, the interstitials accelerate charge recombination. By passivating the interstitials, alkalis make carrier lifetimes up to seven times longer than in defect‐free perovskites and up to thirty times longer than in defective perovskites.