1. Introduction

Perovskite solar cells (PSCs) have received a great deal of attention in the past few decades due to their impressively high power conversion efficiency (PCE) [1]. To date, PCE as high as 25.6% has been successfully recorded. This performance has already been compared with the single-crystalline silicon solar cells system. With the advancement in the perovskite properties control, including the crystallinity properties, grain size, and stability properties, further improvement in the PCE is expected to be achieved soon. The continuous growth in the preparation of the high-performance charge selective layer in the perovskite solar cells further contributes to the rapid progress in the PCE improvement of the PSC [2].

Along with the transparent conducting electrode (TCE) and the top metal contact, a PSC device is composed of an electron-transport layer (ETL), an organometal-halide perovskite active layer, and a hole-transport layer (HTL). In these solar cells, the perovskite and its photoelectrical properties are the keys to the overall photovoltaic process. Its unique high-optical absorption constant drives massive photon absorption and exciton generation in the device. Despite this key fact, the carrier transport and interfacial charge transfer dynamics play another crucial factor for the generation of the overall PSC performance. These two parameters depend on the nature of the surface and the crystallinity properties of the charge-selective layers [3].

One of the serious problems in perovskite solar cell devices is the loss of charge carriers during the transport process in the carrier layer. This is because, the carrier layer has low crystallization, high grain boundary resistance as well as experiences loss of carrier charge during extraction to the outer electrode. The main factor of carrier charge lost during extraction to the outer electrode is due to the high interface resistance between the electrode and the carrier layer. Therefore, it is expected that when a carrier layer that has high crystallinity, very low thickness, and good coupling conditions with external electrodes is used, then the performance of the device will increase.

The electron transport layer (ETL), for example, TiO₂, and other semiconducting oxides, such as SnO₂, ZnO, have been widely applied in the perovskite solar cells fabrication. Despite the excellent performance demonstrated by them, this ETL suffers from large-density surface defects related to oxygen vacancy, particularly in the TiO₂ system. The defect from such vacancy causes immense trap-limited (Shockley-Read-Hull) transport in the extraction of the photogenerated carrier to the external electrode. This in many cases degrades the photovoltaic performance of the PSC up to a certain degree, reducing the power conversion efficiency of the device. Even though there exist several methods in the passivation of such defects, such as acid passivation, etc., the improvement is minute. In addition, this method may add additional resistance to the photocarrier transport reducing the power conversion efficiency. Along with these crucial factors, the crystallinity properties of the ETL add an additional issue to the photocarrier transport dynamic in the device. As normal in the high-performance PSC fabrication, mesoporous TiO₂ or SnO₂ was used as ETL along with a compact layer of TiO₂ or SnO₂ (See Figure 1), [4]. As the figure reveals, the mesoporous layer is composed of a large number of interconnected small grain particles that produce grain boundary resistance due to lattice mismatch among the connected particles. This resistance should be massive due to their large-scale existence on the layer. This certainly complicates the transport of photogenerated electrons to the electrode layer, such as high internal resistance or radiationless recombination [5, 6]. Therefore, the selection of the right material for the carrier layer is important in determining the performance of a device. Such resistance boundary further augments the presence of mesoporous-compact layer interface resistance in the ETL system of the PSC. From this picture, we can estimate the loss would be suffered by the device during the photovoltaic process. This means that if such ETL is replaced with the single-crystalline ETL system, the performance of the perovskite solar cells can be improved.

Figure 1.
Mesoporous TiO₂ ETL. (A and C) Top and side view of mesoporous TiO₂ layer on compact layer TiO₂. (B and D) Top and side of mesoporous TiO₂ layer. (Reprinted from [4]. © 2017 American Chemical Society).

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].

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)).

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 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.

Material	Device structure	J_sc (mA cm⁻²)	V_oc (V)	FF	PCE (%)	Ref.
TiS₂	FTO/TiS₂/MAPbI₃/spiro-OMeTAD/Au	23.38	1.05	0.71	17.37	[14]
TiS₂	ITO/TiS₂/ FA_xMA_1-xBr_xCl_yI_1-x-y/spiro-OMeTAD/Ag	24.68	1.00	0.75	18.79	[7]
MoS₂	FTO/MoS₂/MAPbI₃/spiro-OMeTAD/Au	21.70	0.89	0.63	13.14	[15]
MoS₂	ITO/MoS₂/Cs_x(MA_yFA_1-y)_1-xPb(I_zBr_1-z)₃/spiro-OMeTAD/Au	16.24	0.56	0.37	3.36	[16]
MoS₂/TiO₂	ITO/TiO₂/MoS₂/MAPbI₃/spiro-OMeTAD/Au	13.36	0.65	0.51	4.43	[17]
MoS₂/SnO₂	ITO/SnO₂/MoS₂/FA_xMA_1-xBr_xCl_yI_1-x-y/spiro-OMeTAD/Ag	24.57	1.11	0.79	21.73	[18]
MoS₂	Graphene/MoS₂/MAPbI₃/PTAA/Au	20.92	0.91	0.76	14.42	[19]
MoS₂	ITO/MoS₂/MAPbI₃/PCBM/Al	12.50	0.85	0.57	6.01	[20]
SnS₂	ITO/SnS₂/MAPbI₃/Spiro-OMeTAD/Au	23.70	0.95	0.61	13.63	[21]
SnS₂	ITO/SnS₂/MAPbI₃/Spiro-OMeTAD/Au	21.70	1.011	0.60	13.20	[22]

Table 1.

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

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 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.

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)).

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 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 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).

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 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.

Figure 5.
Comparison of SnO₂ and SnO₂/2D TiS₂ as ETLs in PSCs: (A) J_ph-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).

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 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.

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, V_oc, J_sc, and FF, for the two devices. (Reprinted from [25]. © 2021 The American Chemical Society).

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 (J_ph-V_eff 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).

2.2 MoS₂ electron transport layer

MoS₂ atomic layer is the most studied TMD system because of its excellent optical and electrical properties [26, 27, 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, 29, 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).

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, V_oc, J_sc, 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).

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 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.

2.3 WS₂ electron transport layer

Tungsten disulfide (WS₂) share common basic properties of TMD with other systems, such as high-mobility properties, unique optoelectronic properties, large exciton-binding energy, and good physical and chemical stability as well as ambipolar properties [11]. In addition, WS₂ has an energy level that is suitable for the perovskite layer of three types of cations (Figure 9) and can be easily synthesized by the ultrasonic spray pyrolysis method. WS₂ also has high stability as well as having fast interface charge transfer properties [32]. Among the available 2D TMD, the energy band structure of WS₂ is a much better match with the common perovskite of MAPbI₃ (Figure 10). Furthermore, it also has a relatively larger bandgap if compared with the other system in this class of materials, promising facile excitonic separation during the photovoltaic process and producing better power conversion efficiency.

Figure 9.
Energy levels of dichalcogenide transition metal materials (TMDs) as ETLs and MAPbI3 as perovskite layers in perovskite solar cells.

Figure 10.
Energy level diagram for n-i-p perovskite solar cells using WS₂ ETL.

Recently, we have realized the PSC device utilizing the WS₂ layer as ETL and evaluated how the number of layers of WS₂ influences the carrier dynamic in the device [5]. We prepared the WS₂ atomic layer via ultrasonic spray pyrolysis. Figure 11 shows a schematic diagram of the 2D atomic layer preparation. A modified commercially available ultrasonic spray system (Daiso, Japan) was used. A homemade solution container was placed on the top of the ultrasonic membrane of the system (Figure 11). Ultrasmall solution precursor mist can be produced from the process and fall on the ITO substrate surface that is positioned approximately 5 cm below the membrane. The temperature of the substrate was set at 350°C.

Figure 11.
Schematic diagram of ultrasonic spray pyrolysis for the preparation of TMD ETL.

The typical morphology of the WS₂ atomic layer on the ITO substrate is shown in Figure 12A. The WS₂ nanosheet’s morphology resembles a circular structure that is produced from the precursors’ mist that emerged from the ultrasonic spray membrane. Confocal Raman imaging further indicated the existence of a very thin layer of structure from the circular structure as shown in Figure 12B. Raman analysis then confirmed the phase crystallinity of the WS₂ (Figure 12C). As the figure reveals, there are two sharp peaks obtained from the Raman spectrum that is centered at 348.9 cm⁻¹and 412.3 cm⁻¹, which are associated with the in-plane (E_2g) and the out-of-plane (A_1g) vibration modes of the lattice (see inset in Figure 12C) [33, 34, 35, 36, 37, 38, 39]. According to the value of the separation between these two peaks, the thickness of the atomic layer is estimated to be in the range of 10 L. The X-ray diffraction analysis further confirmed the phase crystallinity of the WS₂ layer (Figure 12D) [40, 41, 42]. The high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) analysis results (Figure 12F and G) show that the sample is single crystalline. However, the presence of SAED composed of a triple spot is related to the stacking of the WS₂ atomic layer during the transfer to the lacey grid for HRTEM analysis. The XPS analysis then further confirmed the Raman and XRD analysis results on the phase crystallinity of the sample of which it belongs to WS₂ (Figure 12H–I).

Figure 12.
The morphology, phase crystallinity, chemical state properties of WS₂ nanosheet. (A) FESEM image of WS₂ nanosheet on the ITO substrate. (B-C) Raman imaging and spectrum of WS₂ were obtained using 532 nm laser excitation. The inset in (C) shows the corresponding main vibration mode of Raman. (D) XRD spectrum for WS₂ nanosheet showing 2H phase. (E-F) Low and high-resolution TEM image of WS₂ nanosheet. (G) SAED pattern of WS₂ nanosheet showing at least three stacking WS₂ nanosheets. (H-I) High-resolution scan of XPS at W and S binding energy of WS₂ nanosheet. (Reprinted from [5]. © 2020 Wiley-VCH GmbH).

PSCs device was fabricated using the WS₂ atomic layer as ETL and investigated how the thickness of the WS₂ ETL influenced the photovoltaic process. The structure of the PSC device is ITO/WS₂ nanosheets/Perovskite/Spiro-OMeTAD/Au. Perovskite used was triple cations system of Cs_0.05[MA_0.13FA_0.87]_0.95Pb (I_0.87Br_0.13)₃ [43].

It was found that the thickness, represented by the number of layers, of the WS₂ atomic layer ETL, strongly influences the power conversion efficiency of the PSC device (Figure 13). The results show that the PCE performance improves with the increase of thickness from 4 L to the optimum thickness of 7 L (WS30 sample in the figure). The optimized WS₂ ETL thickness can produce a PSC device with PCE as high as 18.21% with J_sc, V_oc, and FF as high as 22.24 mA cm⁻², 1.12 V, and 0.731, respectively. The average performance was 17.84%, 22.33 mA cm⁻², 1.10 V, 0.731 for PCE, J_sc, V_oc, and FF, respectively. However, due to an increase in the energetic disorder when using the WS₂ ETL, the device performance then declined when the thickness of the ETL increased above 7 L. From Figure 13, we can also see that the values of V_oc and fill factor (FF) are impressively high, which is higher than 1.1 V for V_oc and approximately 74% for FF. This reflects that the photogenerated carrier dynamic in the device is high and the photogenerated carrier is effectively extracted to the external circuit to produce photocurrent [44]. This is verified by the high-external quantum efficiency (EQE) of the device as shown in Figure 13F.

Figure 13.
The photovoltaic performance of PSC using different thicknesses of WS₂ ETL. (A) J-V curves for the champion PSC device. (B–E) The statistic plot for PCE, V_oc, J_sc, and FF, respectively. (F) EQE and integrated current density (J) of the corresponding device. (Reprinted from [5]. © 2020 Wiley-VCH GmbH).

To understand the extent effect of the WS₂ atomic layer as ETL in the PSC device, the device performance was compared with the reference PSC utilizing well-known SnO₂ ETL. In the typical process, the performance of SnO₂-based PSC shows lower performance than the WS₂ atomic layer–based device (Figure 14). Steady-state and transient photoluminescence analysis revealed that the interfacial charge transfer from the perovskite to ETL is high in the WS₂ atomic layer [45], the result of optimized coupling due to ultra-flat surface morphology offered by the WS₂ atomic layer. This phenomenon is further confirmed by the electrochemical impedance spectroscopy analysis result where it is obtained that the interface charge transfer resistance is lower in the WS₂-based PSC device than the SnO₂-based device. Thus, it can be remarked that the WS₂ atomic layer enables highly active interfacial charge transfer for a high-performance PSC device.

Figure 14.
The comparison of the photovoltaic parameter between WS₂ (7 L, WS30 sample) and SnO₂-based PSC device. (A) J-V curves for the champion device. (B–E) The comparison of PCE, V_oc, J_sc, and FF for the two devices, respectively. (F–H) Steady-state PL, time-resolved PL spectra (TRPL), and electrochemical impedance spectra for WS₂ and SnO₂-based PSC devices, respectively. (Reprinted from [5]. © 2020 Wiley-VCH GmbH).

3. Conclusions

2D atom thick TMD promises facile charge extraction and transport in the perovskite solar cells due to its ultimate thin and single-crystalline nature. The optimization of the 2D TMD layer to obtain a large dimension on the substrate surface is necessary to further promote a highly dynamic photogenerated carrier in the perovskite solar cells device. These materials may become a potential platform for high-performance perovskite solar cells.

Acknowledgments

We acknowledged the financial support from the Universiti Kebangsaan Malaysia for supporting this project under GUP-2019-071 and DIP-2021-025.”

Conflict of interest

The authors declare no conflict of interest.

Xu Guo

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