Aqueous ammonia-based exfoliation of two dimensional MoS2 and WS2 and their application in non-fullerene organic solar cells

Recent advances in atomically thin two dimensional (2D) transition metal dichalcogenides (TMDs) have shown remarkable impacts in optoelectronic technologies which span from photodetectors, field-effect transistors, sensors, and solar cells. Several methods have been reported to synthesize atomically thin TMDs which include mechanical cleavage, chemical vapour deposition and liquid phase exfoliation (LPE). Amongst them, LPE is the most promising one since it serves as a comparably low-cost, fast and simple method, which results in high yield suspension of TMD nanosheets. LPE also enables film deposition by a solution-phase process for instance by printing or coating methods. Here we demonstrate the synthesis of MX2 nanosheets (WS2 and MoS2) using LPE in aqueous ammonia (NH3(aq)) and their application as hole transporting layer (HTL) in organic solar cells. By employing this technique, we are able to produce stable dispersion containing monolayer and multilayer MX2 nanosheets with lateral sizes ranging between 10s-100s nanometers, for both MoS2 and WS2. The low boiling point of the NH3 and water makes them more favourable than the commonly used high boiling-point solvent such as N-Methyl-2-pyrrolidone (NMP) or N-Cyclohexyl-2-pyrrolidone (CHP). Hence no high-temperature post-deposition annealing is required. The successful exfoliation of TMDs is shown via atomic force microscopy (AFM), transmission electron microscopy (TEM) and Raman spectroscopy. The semiconducting nature of the MX2 nanosheets is revealed by X-ray photoelectron (XPS), UV-Vis and photoluminescence (PL) spectroscopies. Finally, we integrated the synthesized nanosheets in non-fullerene organic solar cells (NF-OSCs) as an HTL replacing the hygroscopic and acidic poly(3,4-ethylene dioxythiophene): poly(styrenesulfonate) (PEDOT: PSS). Ultraviolet photoelectron spectroscopy (UPS) shows that the work function of ITO increases upon deposition of TMDs. Consequently, the thin layer of MX2 can improve contact with the active layer and hole extraction. The photovoltaic measurements show that integration of the MX2-based HTLs in NF-OSCs can enhance the performance of the devices. PCE of the NF-OSC with WS2- and MoS2-based HTLs can be increased to 11.8% and 11.4%, respectively. These results are comparable to conventional PCE of PEDOT:PSS-based NF-OSCs. This study carries great importance of 2D MX2 for the performance enhancement of organic solar cells. Furthermore, the simplicity of the synthesis of MX2 by LPE in NH3(aq) and their deposition make the approach feasible to implement in broader applications.