Quadruple cation mixed halide perovskite, GA0.015Cs0.046MA0.152FA0.787Pb­(I0.815Br0.185)3, single crystals were grown for the first time using an inverse temperature crystallization process. Solar cell devices in n-i-p stack configuration using thin films of the same materials showed power conversion efficiency above 20%. Complementary time-resolved spectroscopy confirmed that polycrystalline thin films and single crystals identically composed exhibit similar carrier dynamics in the picosecond range. Cooling of excited carriers and bandgap renormalization occur on the same time scale of 200–300 fs. The radiative recombination coefficient (1.2 × 10–9 cm3/s) is comparable to values reported for a GaAs semiconductor. At low excitation density, a long carrier lifetime of 3.2 μs was recorded possibly due to the passivation of recombination centers. This study clarifies discrepancies about the lifetime of hot carriers, the impact of radiative recombination, and the role of recombination centers on solar cell performance. The quadruple cation perovskites displayed short time dynamics with slow recombination of charge carriers.

Abstractα-RuCl3 is a promising candidate material to realize the so far elusive quantum spin liquid ground state. However, at low temperatures, the coexistence of different exchange interactions couple the effective pseudospins into an antiferromagnetically zigzag (ZZ) ordered state. The low-field evolution of spin structure is still a matter of debate and the magnetic anisotropy within the honeycomb planes is an open and challenging question. Here, we investigate the evolution of the ZZ order parameter by second-order magneto-optical effects, the magnetic linear dichroism and magnetic linear birefringence. Our results clarify the presence and nature of metamagnetic transitions in the ZZ phase of α-RuCl3. The experimental observations show the presence of initial magnetic domain repopulation followed by a spin-flop transition for small in-plane applied magnetic fields (≈1.6 T) along specific crystallographic directions. In addition, using a magneto-optical approach, we detected the recently reported emergence of a field-induced intermediate phase before suppressing the ZZ order. The results disclose the details of various angle-dependent in-plane metamagnetic transitions quantifying the bond-anisotropic interactions present in α-RuCl3.

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Exciton intervalley scattering, annihilation, relaxation dynamics, and diffusive transport in monolayer transition metal dichalcogenides are central to the functionality of devices based on them. Here, these properties in a large-size exfoliated high-quality monolayer MoSe 2 are addressed directly using heterodyned transient grating spectroscopy at room temperature. While the free exciton population is found to be long-lived (≈230 ps), an extremely fast intervalley scattering (≤170 fs) is observed, leading to a negligible valley polarization, consistent with steady state photoluminescence measurements and theoretical calculation. The exciton population decay shows an appreciable contribution from the exciton-exciton annihilation, with an annihilation rate of ≈0.01 cm 2 s −1. The annihilation process also leads to a significant distortion of the transient grating evolution. Taking this distortion into account, consistent exciton diffusion constants D ≈ 1.4 cm 2 s −1 are found by a model simulation in the excitation density range of 10 11-10 12 cm −2. The presented results highlight the importance of correctly considering the many-body annihilation processes to obtain a pronounced understanding of the excitonic properties of monolayer transition metal dichalcogenides.

Monolayer transition metal dichalcogenides (TMDCs) hold the best promise for next generation optoelectronic and valleytronic devices. However, their actual performance is usually largely affected by the presence of inevitable defects. Therefore, a detailed understanding of the influence of defects on the dynamic properties is crucial for optimizing near future implementations. Here, the exciton population and valley scattering dynamics in a chemical vapor deposition grown large size monolayer WSe2 with naturally abundant vacancy and boundary defects were systematically investigated using polarization controlled heterodyned transient grating spectroscopy at different excitation wavelengths and temperatures. Slow and multi-exponential decay dynamics of the exciton population were observed while no sign of any micron scale diffusive transport was identified, consistent with the effect of exciton trapping by defects. In general, two different kinds of exciton species were identified: one with short population lifetime (∼10 ps) and extremely fast intervalley scattering dynamics (<200 fs) and in contrast another one with a long population lifetime (>1 ns) and very slow intervalley scattering dynamics exceeding 100 ps. We assign the former to non-trapped excitons in the nanometer scale and the latter to defect-bound excitons. Temperature dependent intervalley scattering dynamics of the trapped excitons can be understood in terms of a two optical phonon dominated process at the K point in momentum space. Our findings highlight the importance of the intrinsic defects in monolayer TMDCs for manipulating exciton valley polarization and population lifetimes, which is key for future device applications.

We have studied backfolded longitudinal acoustic (LA) phonons in InAs/GaSb superlattices (SLs) intentionally grown with either InSb-like or GaAs-like interfaces (IFs). Raman scattering by folded LA phonons was found to be resonantly enhanced for incident photon energies slightly below the 2.0 eV electronic SL interband transition, observed previously by spectroscopic ellipsometry and by resonant Raman scattering from longitudinal optical SL phonons. Under resonant excitation, InAs/GaSb SLs with InSb-like IFs showed scattering by folded LA phonons up to the 7th order. For SLs with GaAs-like IFs, the folded LA phonon modes occurred at systematically higher frequencies than for SLs with the same nominal period and InSb-like IFs. The difference in frequency increased with decreasing SL period.

Exciton intervalley scattering, annihilation, relaxation dynamics, and diffusive transport in monolayer transition metal dichalcogenides (TMDCs) are central to the functionality of devices based on them. In article number 2000029 by Jingyi Zhu, Paul H. M. van Loosdrecht, and co‐workers investigate exciton‐exciton annihilation dynamics and its effect in the distortion of the diffusion grating in an exfoliated monolayer TMDC, MoSe2.

We present time-resolved magneto-optical spectroscopy on the magnetic Mott–Hubbard-insulating Kitaev spin liquid candidate α-RuCl3 to investigate the nonequilibrium dynamics of its antiferromagnetically ordered zigzag groundstate after photoexcitation.