Liquid phase exfoliation of MoS2 and WS2 in aqueous ammonia and their application in highly efficient organic solar cells

Abstract

Simple, scalable and cost-effective synthesis of quality two-dimensional (2D) transition metal dichalcogenides (TMDs) is critical for fundamental investigations but also for the widespread adoption of these low-dimensional materials in an expanding range of device applications. Here, we report on the liquid-phase exfoliation (LPE) of molybdenum disulfide (MoS₂) and tungsten disulfide (WS₂) in aqueous ammonia (NH₃(aq.)) as a greener alternative to commonly used but less environmentally friendly solvents. The synthesized nanosheets can be prepared in high concentrations (0.5–1 mg mL⁻¹) and exhibit excellent stoichiometric and structural quality with a semiconducting character. These characteristics make them ideal for application in organic optoelectronics, where optical transparency and suitable energetics are two important prerequisites. When MoS₂ and WS₂ are used as the hole transport layer materials in organic photovoltaics, cells with a power conversion efficiency of 14.9 and 15.6%, respectively, are obtained, highlighting the potential of the aqueous ammonia-based LPE method for the preparation of high quality TMDs. The method could potentially be extended to other TMDs.