Atomically thin molybdenum disulphide (MoS₂) is a direct band gap semiconductor with negatively charged trions and stable excitons in striking contrast to the wonder material graphene. While large-area growth of...

Due to the lowest formation energies, sulfur vacancies are inevitable in the vapor-phase chemical vapor deposition (CVD) of MoS2, which act as deep donors and induce midgap defect states, making the material intrinsically n-type. The postgrowth oxygen passivation of such defects has been the subject of a large number of recent studies because passivation of defects augments the photoluminescence quantum yield by several orders. In this study, by introducing an SiO2/Si wafer in close proximity to the growth substrate, we were able to supply trace oxygen in situ during the growth while simultaneously enabling chemisorption of oxygen at defect sites on the basal plane of large-area MoS2 monolayers. Low-temperature photoluminescence spectroscopy allowed us to distinguish clearly the nature of oxygen bonding in defective MoS2 grown with and without the trace oxygen. Chemisorption of oxygen enabled elimination of defect-related bound exciton emission at the near band edge transition of MoS2, leading to about 300% enhancement in the photoluminescence. We observed unusual splitting of the first-order A1g Raman mode in monolayer MoS2 films when the sulfur vacancies are not compensated by oxygen. The present study provides new experimental evidence to better distinguish between chemisorption and physisorption of oxygen and may serve as an effective way to tune the optical properties of van der Waals crystals during the large-area CVD process.

Heterointegration of graphene with silicon has attracted growing interest, because the resulting unique heterojunction allows for efficient collection of light generated electron–hole pairs. This study aims to understand the role of device fabrication conditions and electronic properties of chemically doped graphene on the efficiency variations and stability of graphene/silicon solar cells. We observed significant variations in the efficiency between the devices doped with metal chloride and organic molecules. A strong degradation in the hole carrier mobility by the formation of metal particles/clusters, which could act like charge puddles, accounts for the limited efficiency in the former. Molecular doping, on the other hand, offered good doping homogeneity and no mobility degradation, leading to solar cells with efficiency as high as 9.2%. Our results demonstrate that the droop in efficiency over time observed in the chemically doped devices is due to oxidation limited charge carrier separation rather than doping reversal. The prospects of molybdenum oxide (MoOx) as a multifunctional layer in improving the device stability and efficiency are also discussed based on preliminary experimental findings.

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