Two-dimensional (2D) layered materials, transition metal dichalcogenides and black phosphorus, have attracted much interest from the viewpoints of fundamental physics and device applications. The establishment of new functionalities in anisotropic layered 2D materials is a challenging but rewarding frontier, owing to their remarkable optical properties and prospects for new devices. Here, we report the anisotropic optical properties of layered 2D monochalcogenide of germanium sulfide (GeS). Three Raman scattering peaks corresponding to the B3g, A 1 g , and A 2 g modes with strong polarization dependence are demonstrated in the GeS flakes, which validates polarized Raman spectroscopy as an effective method for identifying the crystal orientation of anisotropic layered GeS. Photoluminescence (PL) is observed with a peak at around 1.66 eV that originates from the direct optical transition in GeS at room temperature. Moreover, determination of the polarization dependent characteristics of the PL and absorption reveals an anisotropic optical transition near the band edge of GeS, which is also supported by the density functional theory calculations. This anisotropic layered GeS presents the opportunities for the discovery of new physical phenomena and will find applications that exploit its anisotropic properties.

3.8% when first reported [1] to 22.1% [2] within a few years only. The striking rapid advances in the photovoltaic performance of PSCs are mainly attributable to the development of perovskite photoactive layer materials with superior properties, including strong light absorption from the visible to the infrared region, [3] a small exciton binding energy, [4][5][6] micrometerscale charge carrier diffusion lengths, [7,8] and high charge collection ability. [9] The PCE of PSCs based on methylammonium lead halide (CH 3 NH 3 PbX 3 ) as a model PSC device has reached 20.1% and 19.5% in inverted (p-i-n) and normal (n-i-p) architectures, [10,11] respectively. Some endeavors have been invested to improve quality of perovskite layer, such as N,Ndimethylformamide (DMF) used as a fumed source of perovskite, [12] CH 3 NH 3 I vapor and CH 3 NH 3 I employed in mixed solvents, [13][14][15] however, some urgent challenges are still to be solved for the photovoltaic performance of CH 3 NH 3 PbX 3 to be further improved. Specifically, (i) compact, void-free, and fully covered perovskite layers should be developed to reduce the leakage of photocurrent and increase the voltage in PSCs, (ii) a method of depositing perovskite layers with a flat surface is needed to improve adhesiveness to the hole transport layer, and (iii) the resistance of the perovskite phases to humidity, oxygen, and ambient Perovskite solar cells (PSCs) have attracted intensive attention as the most promising next-generation photovoltaic technology because they both enable accelerated development of photovoltaic performance and are compatible with low-cost fabrication methods. The strategy of interface engineering of the perovskite layer in PSCs is expected to result in further enhancement of the power conversion efficiency (PCE) of PSCs via minimizing the charge recombination loss. Here, a high current-voltage (stabilized power output) PCE of 20.4% (19.9%) in CH 3 NH 3 PbI 3 PSCs under reverse scanning conditions is demonstrated by incorporating a solution-processed polymer layerof poly(methyl methacrylate) (PMMA) between the perovskite photoactive layer and the hole transport layer. Moreover, steady-state and time-resolved photoluminescence spectroscopy and impedance spectroscopy are used to reveal the mechanism of the enhancement of the photovoltaic performance and its stability by the PMMA layer in a CH 3 NH 3 PbI 3 PSC device. The morphology modification, surface passivation, and protection of the perovskite layer by the insulating PMMA layer substantially contribute to the enhancement of photovoltaic performance and its stability, despite a slight reduction of the charge extraction efficiency.

Atomically thin-layered ReS 2 with a distorted 1T structure has attracted attention because of its intriguing optical and electronic properties. Here, we investigated the direct and indirect exciton dynamics of a three-layered ReS 2 by polarization-resolved transient photoluminescence (PL) and ultrafast pump-probe spectroscopy. The various time scales of the decay signals of the time-resolved PL (<10 ps), with monitoring of the populations of electron-hole pairs (exciton), and the transient differential reflectance (1 and 100 ps), with monitoring of the populations of electrons and/or holes in the excited states, were observed. These results reveal the characteristic exciton dynamics: rapid relaxation of direct excitons (electron-hole pairs) and slow relaxation of the momentum-mismatched indirect excitons accompanied by a one-phonon emission process. Our findings provide important information regarding the indirect band gap nature of few-layered ReS 2 and its characteristic exciton dynamics, boosting our understanding of the novel electronic and optical properties of atomically thin-layered ReS 2 .

Rhenium dichalcogenides have attracted considerable attention as new members of group VII layered semiconductor transition-metal dichalcogenides (TMDs) with respect to fundamental physics and potential applications. In this study, room-temperature photoluminescence (PL) spectra, as well as PL quantum yields (QYs) of thin-layer rhenium disulfide (ReS2), were evaluated. Low PL QYs of ∼10–4 were determined from a monolayer thickness to seven layers (1–7L) of ReS2 regardless of the layer number. These low PL QYs strongly suggest that the ReS2 is an indirect-bandgap semiconductor from a monolayer limit to the bulk, which is in contrast to those observed for group VI TMDs (MX2: M = Mo and W; X = S and Se). Our experimental findings will provide valuable information for the electronic and optical device applications in atomically thin-layered ReS2.