Hybrid organic−inorganic lead halide perovskites have recorded unprecedented improvement in efficiency as fourth-generation photovoltaic materials. Recently, they have attracted enormous interest in nonlinear optics stemming basically from their excellent optoelectronic properties. Here, we investigate multiphoton absorption (MPA) in high-quality MAPbX 3 (MA = CH 3 NH 3 and X = Cl, Br, I) bulk single crystals synthesized by an inversetemperature crystallization (ITC) method. The two-photon absorption (2PA) coefficients under picosecond pulse excitation are determined to be β = 23 ± 2 cm/ GW and 9 ± 1 cm/GW for MAPbI 3 and MAPbBr 3 at λ = 1064 nm, and 13 ± 2 cm/ GW for MAPbCl 3 at λ = 532 nm. The 2PA coefficients are comparable to those of conventional semiconductors having similar bandgaps and can be explained by a two-band model. Furthermore, we characterize the three-photon absorption behavior of MAPbCl 3 at λ = 1064 nm, yielding γ = 0.05 ± 0.01 cm 3 /GW 2 . The polarization dependence of MPA is also probed to experimentally estimate the degree of anisotropy. The hybrid perovskites are promising materials for nonlinear optical applications due to polarizationdependent MPA response and subsequent strong photoluminescence emission, especially for the Br-and I-containing compounds.

We demonstrated an effective poly(p-chloro-xylylene) (Parylene-C) encapsulation method for MAPbI3 solar cells. By structural and optical analysis, we confirmed that Parylene-C efficiently slowed the decomposition reaction in MAPbI3. From a water permeability test with different encapsulating materials, we found that Parylene-C-coated MAPbI3 perovskite was successfully passivated from reaction with water, owing to the hydrophobic behavior of Parylene-C. As a result, the Parylene-C-coated MAPbI3 solar cells showed better device stability than uncoated cells, virtually maintaining the initial power conversion efficiency value (15.5 ± 0.3%) for 196 h.

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Defect investigation in two-dimensional transition-metal dichalcogenides (TMDs) is required because structural defects significantly affect the optical and electrical properties of TMD. Raman scattering can be an essential tool to study the defects in TMD, but defect-related Raman modes have been rarely studied. Here, we investigated the influence of sulfur vacancies and oxygen substitution on the optical properties of WS2 using the laser irradiation technique. The defect-induced photoluminescence (PL) exhibits distinct features depending on the type of defects, which shows different changes in the intensities and peak positions of the excitons, biexcitons, and defect-bound excitons. Defect-activated Raman modes revealed information about the defects and demonstrated the origin of the alteration in PL. The defect analysis of TMDs based on the correlation between PL and Raman scattering provides a clear understanding of the variations in their optical properties.