Defect Passivation of 2D Semiconductors by Fixating Chemisorbed Oxygen Molecules via h-BN Encapsulations.

Hexagonal boron nitride (h-BN) is a key ingredient for various 2D van der Waals heterostructure devices, but the exact role of h-BN encapsulation in relation to the internal defects of 2D semiconductors remains unclear. Here, it is reported that h-BN encapsulation greatly removes the defect-related gap states by stabilizing the chemisorbed oxygen molecules onto the defects of monolayer WS2 crystals. Electron energy loss spectroscopy (EELS) combined with theoretical analysis clearly confirms that the oxygen molecules are chemisorbed onto the defects of WS2 crystals and are fixated by h-BN encapsulation, with excluding a possibility of oxygen molecules trapped in bubbles or wrinkles formed at the interface between WS2 and h-BN. Optical spectroscopic studies show that h-BN encapsulation prevents the desorption of oxygen molecules over various excitation and ambient conditions, resulting in a greatly lowered and stabilized free electron density in monolayer WS2 crystals. This suppresses the exciton annihilation processes by two orders of magnitude compared to that of bare WS2. Furthermore, the valley polarization becomes robust against the various excitation and ambient conditions in the h-BN encapsulated WS2 crystals.

Oriented Crystal Growth during Perovskite Surface Reconstruction.

Surface passivation has become a key strategy for an improvement in power conversion efficiency (PCE) of perovskite solar cells (PSCs) since PSCs experienced a steep increase in PCE and reached a comparably matured point. Recently, surface passivation using a mixed salt of fluorinated alkyl ammonium iodide and formamidinium bromide demonstrated a remarkable improvement in both performance and stability, which can be tuned by the length of the alkyl chain. Nevertheless, the role of the alkyl chain in manipulating surface-limited crystal growth was not fully understood, preventing a further progress in interface control. In this study, we found that the length of the fluorine-substituted alkyl chain governed the crystal formation dynamics by manipulating surface tensions of different crystal orientations. The overall enhancement of the (001) plane, being the most favored, commonly resulted from the surface reformation of the perovskite film regardless of the chain length, while the highly oriented (001) over (111) was monitored with a particular chain length. The enhanced crystal orientation during surface recrystallization was responsible for the low trap density and thus effectively suppressed charge recombination at the interface, resulting in a considerable increase in open-circuit voltage and fill factor.

Polarization-insensitive broadband omni-directional anti-reflection in ZnO nanoneedle array for efficient solar energy harvesting.

Broadband omni-directional anti-reflection characteristics have been an important issue because they can maximize the optical absorption in photovoltaic devices. Here, we investigate the optical properties of ZnO nanoneedle arrays to demonstrate broadband anti-reflection, omni-directionality, and polarization insensitivity using optical simulations and experimental approaches. The results of this work clarify that the ZnO nanoneedle array plays an important role as a broadband anti-reflection layer due to its spatially graded refractive index, omni-directionality and polarization insensitivity. To take advantage of these structures, we prepared a ZnO nanoneedle array on the surface of conventional SiN x /planar Si solar cells to prove the broadband omni-directional anti-reflection for solar energy harvesting. Current density-voltage results show that SiN x /planar Si solar cells with ZnO nanoneedle arrays lead to a nearly 20% increase in power conversion efficiency compared to SiN x /planar Si solar cells, and a 9.3% enhancement in external quantum efficiency is obtained under identical conditions. Moreover, the photocurrent results of SiN x /planar Si solar cells with ZnO nanoneedle arrays clearly demonstrate the incident angle- and polarization-insensitive characteristics compared to those of typical SiN x /planar Si solar cells. Our results demonstrate the optical multi-functionality of ZnO nanoneedle arrays and pave the way for high-performance optoelectronic devices that require broadband omni-directional anti-reflection and polarization insensitivity.

3D/2D Bilayerd Perovskite Solar Cells with an Enhanced Stability and Performance.

The formation of a thin 2D perovskite layer on the surface of 3D perovskite films has become a popular strategy for obtaining a high-efficiency perovskite solar cell (PSC) with an ensured device stability. In this review paper, various experimental methods used for growth of the 2D layer are introduced with the resulting film properties. Furthermore, a variety of organic cation sources for the 2D layer, ranging from alkyl to phenyl ammonium, are explored to investigate their impact on the device stability and photovoltaic performance.