Optoelectronic Properties of MoS₂ in Proximity to Carrier Selective Metal Oxides

Abstract

stacking nature, [5] strain, [6] and applied voltages. [7] Properties such as natural surface passivation, [8] high carrier mobility, [9] semiconducting band gaps, [1a] valley polarization, [10] strong light-mater interactions, [3,11] and the transitions from an indirect band gap in bulk form to a direct band gap in monolayer form [3,12] make them noteworthy materials to study. TMDCs such as molybdenum disulfide (MoS 2 ) have proven to be photoactive [13] and have been investigated as potential absorber layers in solar cells. [14] Many proof-of-concept devices have been constructed in the past few years, the majority of them based on exfoliated TMDC flakes. [13b,15] Exfoliation approaches are limited to the preparation of micrometer-scale devices and are challenging to scale to larger length scales. Even though there have been a few instances of larger scale TMDC solar cells. [16] In most of these cases, bulk MoS 2 or TMDC materials were used with layer thickness exceeding 100 nm. Therefore, these approaches could not benefit from the intrinsic properties of mono-and few-layer MoS 2 , such as an increased band gap [12] and high absorption coefficient in thin layers, [13b] and do not exploit the potential for transparent and semi-transparent photovoltaic applications. [17] In order to take advantage of the unique mono-and few layer properties of MoS 2 as absorption layers in solar cells, the TMDC films need to be integrated in a device stack that can separate and extract charge carriers from the TMDC light absorbing layer. Charge separation can be realized with carrierselective contact materials, such as titanium oxide (TiO x ) acting as an electron selective contact [18] and molybdenum oxide (MoO x ) acting as a hole selective contact. [19] Where these selective contacts help separate the generated charge carriers due to different work functions of the contact material. [20] TiO x and MoO x have already been reported to form type II heterostructures with MoS 2 allowing for charge separation. [21] One aim of this paper is to investigate the influence of the adjacent TiO x and MoO x contacts on the optoelectronic properties of monoand few layer exfoliated MoS 2 flakes, different from a similar study by Xu et al., [22] that also explores the relationship between MoS 2 and carrier selective contacts. The MoS 2 absorber layers, in our study are transferred onto the contact materials in order to avoid possible damage of the ultrathin absorber during contact deposition, instead of being sandwiched between silicon dioxide (SiO 2 ) and metal oxides. As 2D materials are sensitive Transition metal dichalcogenides are an exciting class of new absorber materials for photovoltaic applications due to their unique optoelectronic properties in the single to few-layer regime. In recent years, these materials have been intensively studied, often utilizing conventional substrates such as sapphire and silicon dioxide on silicon. This study investigates the optical properties of molybdenum disulfide (MoS 2 ) mono-, bi-, and mul...

Layer-Thickness-Dependent Work Function of MoS2 on Metal and Metal Oxide Substrates

“…This shift is an indication of a change of strain (E 2g 1 ) and doping (A 1g ) 58,59 where the monolayer on Ti−TiO x appears to be more strained or less compressed and more n-doped than the MoS 2 monolayer on TiO x . These findings match what we have already reported for MoS 2 bilayers on similar substrates, 17 where the change in the A 1g peak was related to screening, doping effects, or change in band alignment. Interestingly, the work function does not reflect an intrinsic change in doping because then we would expect the work function of the monolayer flake on Ti−TiO x to have a lower work function and not a higher one.…”

“…As reported previously 17 the substrate materials, (Ti, TiO x , Au, and MoO x ) were deposited via electron beam evaporation (DREVA LAB 450 VTD Vakummtechnick) on to p-doped silicon wafers with 100 nm of SiO 2 (Supplier: Seigert Wafer), with thicknesses of 30 ± 2.3 nm for the TiO x, and MoO x, , unless reported otherwise, and 100 ± 5.2 nm for the Ti and Au. MoS 2 mono-and multilayer flakes were then exfoliated from a bulk crystal (supplier: 2D Semiconductors) via a deterministic all-dry transfer method 46 and stamped directly on to their target substrates.…”

“…MoS 2 mono-and multilayer flakes were then exfoliated from a bulk crystal (supplier: 2D Semiconductors) via a deterministic all-dry transfer method 46 and stamped directly on to their target substrates. 17 Thick hexagonal boron nitride (hBN) flakes (∼95 nm) were stamped in a similar manner. A schematic and optical images of example heterostructures are represented in Figure 1.…”

“…Raman spectroscopy measurements were acquired using a Bruker Senterra system, with a 488 nm laser with a power of 2 mW at the sample with a spot size of 1 μm, as previously reported. 17 Atomic force microscopy (AFM) and Kelvin probe force microscopy (KPFM) measurements were acquired simultaneously with a conducting cantilever with a Cr/Pt coating (ElectriMulti75-G) in noncontact KPFM mode using a ParkSystems NX 10. For the KPFM measurement, the experimental output of the device is V exit , V exit = ±CPD, depending directly on the nullifying force, where CPD is the contact potential difference.…”

“…Molybdenum disulfide (MoS 2 ) and other 2D semiconductors have been progressively studied for use in electronic and optoelectronic applications. − Understanding the optoelectronic properties of these novel materials is paramount to engineering efficient and high-quality devices. In particular contact engineering − has been of great interest, with the goal of understanding the interface physics of 2D materials, such as MoS 2 , in contact with other 2D layers, − metals, , insulators, , and bulk semiconductors, − and creating optimized contacts for the given situation. The work function, in particular, is a very useful electronic property concerning contact engineering, device design, and device simulation.…”