Photoluminescence properties of CuPc/MoS<sub>2</sub> van der Waals heterostructure

Abstract

Among two-dimensional (2D) materials, transition metal chalcogenides (TMDs) have attracted much attention due to their unique photoelectric properties. On the other hand, organic molecules have the characteristics of flexibility, wide source, easy fabrication and low cost. The van der Waals heterostructure constructed by the combination of 2D TMDs and organic semiconductors has attracted enormous attention in recent years. When organic semiconductors combine with TMDs to form van der Waals heterostructure, the hybridization of organic molecules could improve the photoelectric properties and other properties by taking the advantages of these two materials, Therefore, the combination of organic semiconductor molecules and TMDs can provide a research platform for designing many basic physics and functional devices and interesting optoelectronic applications. In this work, CuPc/MoS<sub>2</sub> van der Waals heterostructure is built, and its photoluminescence (PL) properties are investigated. It is observed that after introducing CuPc, a significant PL quenching phenomenon occurs in the heterostructure compared with the single layer MoS<sub>2</sub> and pure CuPc only. After fitting the PL of CuPc/MoS<sub>2</sub> heterostructure system and monolayer MoS<sub>2</sub> only, the ratio of trion to neutral exciton is clearly increased in the heterostructure. Furthermore, it is found that two mid-gap states D1 and D2 related to the CuPc are introduced into the band gap of MoS<sub>2</sub> by first principle calculation. Through the charge density analysis, we find that the D1 state originates from the sp<sup>2</sup> bonding state of the C-C bond while the D2 state comes from the anti-bonding state of the N-Cu bond. Meanwhile, the valence band maximum (VBM) and conduction band minimum (CBM) of CuPc/MoS<sub>2</sub> heterostructure are derived from the bonding and anti-bonding states of MoS<sub>2</sub>, respectively. The charge transfer occurs between the mid-gap states of CuPc and MoS<sub>2</sub>. However, owing to different positions of charge density distribution of CBM, D2, D1 and VBM, the charge pathway is dominated by non-radiation recombination, which cannot give new PL peak in heterostructure. However, this process reduces the number of carriers involved in the direct recombination of MoS<sub>2</sub>, which leads PL to quench in the heterostructure. This work would be applied to the manipulation of photoelectric characteristics and the design of TMD/organic-based photovoltaic applications.