**2. Two-dimensional transition metals ETL**

Recently, materials of two-dimensional (2D) dichalcogenide transition metals (TMDs), such as MoS2, WS2, TiS2, CdTe, and others, have been used as carrier layers in perovskite solar cells due to their high charge carrier mobility, unique optoelectrical properties, large exciton binding energy, very fast interface charge transfer properties as well as excellent physical and chemical stability properties [7]. Their optoelectronic properties were also found to correspond to the number of layers, dopants, and strains (straining). The phenomenon of the massive charge transfer process in these van der Waals crystals driven by the collective motion of excitonic surfaces enables a high interfacial charge extraction and reduces charge recombination for an effective photovoltaic process [8]. One of the uniqueness of the TMDs layer is that it has an atomic-scale thickness (very thin) and has high crystallinity. With its planar (2D) structure, it will produce a strong coupling when grown on the electrode surface. Therefore, it has great potential for a carrier layer in perovskite solar cells.

Transition metal dichalcogenide (TMD) has the chemical formula of MX2 where M is the transition metal from groups 4 to 10 in the periodic table system, and X is a chalcogen atom such as sulfur (S), selenium (Se), or tellurium (Te). **Figure 2** shows the typical structure of TMD. The structure has two layers of chalcogen that clamp a transition metal layer making this material have its uniqueness in electronic, optoelectronic properties, and chemical stability [10]. The electronic and optical properties of TMDs materials change significantly depending on the number of layers. For example, the MoS2 band gap increases from 1.29 eV (multilayered MoS2) to 1.59 eV (monolayer MoS2), and also this bandgap changes from an indirect bandgap to a direct bandgap as the number of layers decreases [11].

As is well known, most of these 2D TMD materials have ambipolar properties that enable the materials to transport both electrons and holes [12]. In other words, this allows 2D TMDs material to be used as ETL or HTL in n-i-p or p-i-n perovskite solar cells. However, most perovskite solar cell applications use these 2D TMD materials as HTL. Only MoS2 and TiS2 have been used as ETLs and have successfully produced efficiencies as high as 13.14% and 18.79% [7, 13]. **Table 1** shows several PSC device structures utilizing TMD as ETL. Recently, there was a first simulation study on the

#### **Figure 2.**

*Typical structure of transition metal dichalcogenide materials. (A) Typical layer stacking structure in bulk transition metal dichalcogenide structure. T and X represent the transition metal and chalcogen elements, respectively. (B) Top and side view of single-layer of TMD with 2H-phase. (C) Side view of single-layer TMD with 1T-phase. (Reprinted from [9]. © 2020, The Author(s)).*


#### **Table 1.**

*Photovoltaic parameters of perovskite solar cell devices using dichalcogenide transition metals (TMDs) as ETLs.*

*Two-Dimensional Transition Metal Dichalcogenide as Electron Transport Layer of Perovskite… DOI: http://dx.doi.org/10.5772/intechopen.103854*

photoelectric properties of WS2 as an ETL in perovskite solar cells reported with efficiencies as high as 25.70% [23]. By having high electron mobility as well as energy levels appropriate to the perovskite layer, the WS2 atomic layer is expected to function as an ETL capable of producing high-performance perovskite solar cell devices.
