**2.1 TiS2 electron transport layer**

TiS2 is one of the TMDC family that has been intensively studied recently due to its semi-metallic properties with low-bandgap value, i.e., 0.2 eV. With high electrical conductivity, i.e., 1 x 104 S m−1, this material is potential as an electrode in many applications including lithium-ion batteries and solar cells. Despite its excellent electrical properties, the use of TiS2 as independent electrode material in the application is limitedly demonstrated. It is mainly stacked with other materials such as MoS2 [24] or TiO2 to improve the properties in applications. For the case of MoS2 stacked with TiS2, the TiS2 can form Schottky contact with MoS2 with barrier height [24] between these two atomic layers can be varied by the doping type and concentration either in the MoS2 or TiS2 side (**Figure 3**). This certainly provides a wider opportunity to modify the electrical properties of the system for desired

#### **Figure 3.**

*PLDOS of TiS2–MoS2 (ML) FET-like junctions doped with different doping concentrations and the variation of band structure at interface B. a–d The doping concentrations are: N = 5 × 1019 cm−3, N = 1 × 1019 cm−3, N = 5 × 1018 cm−3, and P = 5 × 1018 cm−3. The thickness of TiS2 is four layers. On the right side, the plot shows the variation of band structure under different doping concentrations. The scale bar is from 0.0 to 90.0 (1/eV). Interface A is the interface between TiS2–MoS2. (Reprinted from [24]. © 2020, The Author(s)).*

performance in application. In the typical process, n-type-doped TiS2–MoS2 (ML) contacts exhibit a barrier height relatively larger, i.e., 1.0 eV below doping level degeneracy. Nevertheless, these n-type-doped contacts still have the potential as the switch in high-power as well as tunnel Schottky barrier MOSFETs. In contrary to the n-type doped system, the p-type-doped TiS2–MoS2 (ML) exhibits a zero barrier height at a particular doping concentration, i.e., 5 × 1018 cm−3. Under this condition, the depletion region width is zero and the band becomes flat, revealing that the contact is ohmic and the barrier height is small. These results reveal the unique unusual interfacial properties arising from this ultimate thin contact that promise a special function in the application. This phenomenon could be the driving factor for an efficient photocarrier extraction in the perovskite solar cells using ETL modified with MoS2 or TiS2 atomic layer.

For example, in the perovskite solar cells system with SnO2 ETL (**Figure 4**), there is an increase in the energy band alignment between the ETL and perovskite layer when the 2D TiS2 is attached to the surface of SnO2 [18]. The conduction band level

#### **Figure 4.**

*(A) Cross-sectional SEM image of the PSC. (B) The energy level diagram. (C) Representative J-V curves of the PSCs with SnO2 or SnO2 /2D TiS2 as ETLs. (D) EQE curve and integrated current density of the PSC with SnO2 /2D TiS2 as the ETL. (E) Histogram of the PCE of PSCs with SnO2 and SnO2 /2D TiS2 as ETLs analyzed from 25 cells. (F) Steady-state efficiency of the PSCs with SnO2 and SnO2/2D TiS2 as ETLs measured under constant voltages of 0.86 V and 0.92 V, respectively. (Reprinted from [18]. © 2019 Royal Society of Chemistry).*

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

of ETL (SnO2) reduced from 4.68 to 4.63 eV in the presence of 2D TiS2. This has narrowed the offset energy between the ETL and perovskite (conduction band level at 4.36 eV). As the result, the photogenerated carrier extraction becomes enhanced, improving the photocurrent and the power conversion efficiency. As shown in **Figure 4C**–**4F**, the power conversion efficiency increases from 19.65% to 21.73% when the SnO2 ETL is modified with the 2D TiS2 atomic layer. The nature of interfacial photocarrier dynamic improvement in the presence of the 2D TiS2 atomic layer can be seen from the increase of the *V*oc, FF, and the IPCE of the device. This process is also reflected by the decrease in the device hysteresis and the improvement of the stability properties.

**Figure 5** explains in detail how the photocarrier dynamic in the device was impressively modified in the presence of a 2D TiS2 atomic layer on the surface of SnO2 ETL. As presented, the photocurrent is enhanced impressively. This is the result of

#### **Figure 5.**

*Comparison of SnO2 and SnO2/2D TiS2 as ETLs in PSCs: (A) Jph-V eff curves; (B) J-V curves in the dark (the dash-dot lines represent the fitting lines); (C) Nyquist plots; (D) steady-state PL spectra; (E) transient PL spectra; (F) C-V characteristics. (Reprinted from [18]. © 2019 Royal Society of Chemistry).*

enhanced interfacial charge transfer as indicated by the transient and steady-state photoluminescence analysis result, which is also supported by the electrochemical impedance spectroscopy result, showing decrease in the interfacial charge transfer resistance in the device.

We also in our recent result have coupled the TiS2 atomic layer on top of the TiO2 surface to compensate for surface defect due to the oxygen vacancy, enhancing the interfacial charge transfer and transport dynamic when applied as ETL in perovskite solar cells [25]. The perovskite solar cells' performance improves from 18.02 to 18.73% (**Figure 6**). Electrochemical impedance analysis revealed that there is an improvement as high as 13% in interfacial charge transfer in the ETL with 2D TiS2 and 43% improvement in the charge recombination resistance (**Figure 7A**). The latter is verified by the increase in the photocurrent (**Figure 7B**) and the decrease in the leakage

#### **Figure 6.**

*Photovoltaic performance of the 2D TiS2-TiO2 NG and TiO2 NG-based PSC. (A) Schematic structure of 2D TiS2-TiO2 NG-based PSC. (B) J-V curves of the champion device. (C-F) The comparison of the photovoltaic parameters, i.e., PCE, Voc, Jsc, and FF, for the two devices. (Reprinted from [25]. © 2021 The American Chemical Society).*

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

#### **Figure 7.**

*Photoelectrical properties of the PSC device. (A) Electrochemical impedance spectra and equivalent circuit of the device. (B) Photogenerated current of the PSC device (Jph-Veff curve). (C) Semilog J-V curve of the PSC in the dark. The green lines represent the fitting line. (D) Double log J-V curve in the dark for photoelectrical dynamic in the device. Three distinct regimes of (i) the ohmic response, (ii) filled trap transition, and (iii) SCLC are shown by different colored regions. (Reprinted from [25]. © 2021 The American Chemical Society).*

current of the device when 2D TiS2 passivates the TiO2 surface (**Figure 7C**). We can relate this process to the reduction in the trap density in the device as shown by the value of *V*TFL of the double-log *J*-*V* curve as depicted in **Figure 7D** where the *V*TFL value depends linearly with the trap density in the device.

## **2.2 MoS2 electron transport layer**

MoS2 atomic layer is the most studied TMD system because of its excellent optical and electrical properties [26–28] and has been used widely in perovskite solar cells as a hole-transport layer (HTL) and an electron-transport layer (ETL) [11, 15, 26] in the form of colloidal or flakes thin film [15, 28–30]. **Table 1** lists down several perovskite solar cells using MoS2 as ETL with a particular device configuration. For example, Singh, Giri, et al. [13] have obtained power conversion efficiency as high as 13.2% from PSC devices using MoS2 material as ETL. In this study, they synthesized the MoS2 film directly on FTO substrate using microwave irradiation-assisted reduction method. It is found that the efficiency obtained by MoS2 material is close to the efficiency value obtained from TiO2 and SnO2 material making MoS2 material comparable to other ETL materials. Abd Malek et al. [16] have also developed different structures of MoS2 ETL on the ITO substrate. Instead of colloidal or flake structured film, an ultrathin layer of MoS2 prepared from ultrasonic spray pyrolysis was fabricated to

obtain its functionalities as ultrathin ETL in the PSC device. The result showed that the PCE device performance depended on the condition during the preparation of the MoS2 atomic layer, particularly the substrate temperature. It is demonstrated that substrate temperature of 200°C is suitable for growing high-quality MoS2 atomic layer on ITO surface, thus, optimizing the power conversion efficiency of the PSC (**Figure 8**). This MoS2 thin-film-based device as ETL has shown high-stability properties where its efficiency can be maintained as much as 90.24% of the original efficiency after 80 s exposure continuously under simulated solar light illumination (AM1.5).

In addition to being used singly in the ETL, TMD materials can also be combined with other organic or inorganic electron transport materials to form electron transport materials. For example, Ahmed et al. [31] have added a MoS2 layer on top of the TiO2 layer to be used as ETL in perovskite solar cells. The use of MoS2/TiO2 as ETL has successfully increased the efficiency of the device by 16% higher than the device

#### **Figure 8.**

*The photovoltaic parameter for MoS2 as ETL in PSC. (A) Schematic structure of the PSC device. (B) The J-V curves for the champion device, (C-F) PCE, Voc, Jsc, and FF of the MoS2 based PSC devices with different substrate temperatures, namely MoS180 (a), MoS200 (b), MoS220 (c), and MoS250 (d). (Reprinted from [16]. © 2020 Elsevier).*

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

that only uses TiO2 as ETL. Similarly, Huang et al. [18] have successfully produced an n-i-p type plane device using SnO2 and 2D TiS2 as ETL. High efficiency was recorded by this group, which was as high as 21.73% with a relatively small hysteresis value. The increase in efficiency in this device is due to the matching of the ETL energy level and the appropriate perovskite layer as well as the lack of electron trap density in the ETL.
