3.1. Investigating how the HTL layer’s properties are impacted by changes in thickness and doping density on PV solar cell
The proper transportation and collection of photo-generated holes can be obtained by the suitable HTL layer and it can also restrict the flow of electrons (minority carrier). As a result, it reduces carrier recombination. Hence, it’s highly essential to demonstrate the consequences of the HTL layer on SC performance parameters. The effect of thickness and doping density of Sb2S3 on the performance parameters of the proposed SC has been represented in Fig. 2.
The values of thickness and have been changed within 10 to 120 nm and the value of doping concentration within \({10}^{11 }\)to \({10}^{22}\)cm− 3, respectively, while other parameters remain constant. The values of Voc and Jsc act a little variation with raising the thickness of the HTL layer. However, the FF and η demonstrate an almost insignificant amount of reduction. The parameters values of Voc, FF, and η initially show a constant tendency with increasing the doping concentration of HTL layer up to 1021 cm− 3. After that, it rises slightly when the doping density is more than 1021 cm− 3 and maintains a constant value. The increase in doping concentration may contribute to reduce the recombination of carriers that facilitate proper transportation and collection of carriers and as a result the rise in the performances of Voc, FF, and η. The decrease of FF with reducing doping concentration in Sb2S3 HTL indicates the rise of series resistance in the introduced solar cell [26][27]. The optimal thickness may be considered as 100 nm and optimal doping density may be considered to be and\({ 10}^{18}\) cm− 3 for obtaining the maximum η of 40.33%.
3.2. Examining the effects of varying thickness and doping density on MoTe2 absorber layer
The simultaneous influence of MoTe2 layer on the performances with the variation of thickness and carrier concentration has been observed and presented in Fig. 3. The percentage of incident photons entered into the MoTe2 contributes for enhancing Jsc and Voc changes because of absorber concentration. Thus, it’s highly needed to optimize the thickness as well as carrier density. These values have been switched within 100 to 1500 nm and 1013 to 1022 cm-3, respectively. It has been demonstrated that Voc enhances due to the rise of the doping concentration while Jsc reduces. On the contrary, the increase in thickness reduces Voc below the doping concentration of 5×1017 cm-3 and then it rises marginally. The FF and η rise because of the enhancement of the two varied parameters (thickness and doping concentration). The higher thickness of the MoTe2 contributes to enhance the absorption of the incident photons which displays higher values of η, FF and Jsc. The rise of doping density may also attribute to enhance the carrier recombination rate and hence, the value of Jsc and Voc reduces [28][29][30]. At thickness value of 500 nm and doping concentration of 5×1016 cm-3, the numerically simulated value of η, FF, Voc, Jsc, is 1.13 V, 40.78 mA/cm-2, 87.63%, and 40.33%, respectively. For achieving a better performance of Al/FTO/CdS/MoTe2/Sb2S3/Pt SC, we consider the optimum points of doping density and thickness are 5×1016 cm-3 and 500 nm, respectively.
3.3. The role of MoTe2 thickness and defect density in shaping the properties of PV Structure
The presence of defects in the SC system has an opposing impact on the PV performances. For that, it’s highly beneficial to supervise the impact of defect density on the MoTe2 layer. The simultaneous influence of defect density and thickness of MoTe2 on device performances FF and has defined in Fig. 4. It is noticeable that the parameters values of Voc, Jsc, FF and η have been decreased from 1.24 to 1.00 V, 40.77 to 17.32 mA/cm-2, 83.19 to 44.18%, and 42.05 to 5.62% respectively for changing the defect density within 1.0×1012 to 1.0×1021 cm-3 at a constant thickness of 500 nm. The PV parameters enhance with the rising the thickness up to the defect density 1018 cm-3 and reduces after that limit. The value of Voc reduces noticeably with increasing the thickness.
The simulated results demonstrate a clear correlation between defect density and the reduction in parameters values for the MoTe2 SC. Shockley Red Hall recombination losses are influenced by defects located within the band gap of the absorber layer. A consequences of the SRH mechanism is the reduction in photo-generated carriers, leading to diminished SC performances [31][32, 33]. At the defect density of 1014 cm− 3 and thickness of 500 nm, the numerically simulated determined performance parameters of Voc, Jsc, FF and η are 1.33 V, 40.78 mA/cm− 2, 87.68% and 40.33%, respectively. We have chosen the value of defect density and thickness 1014 cm− 2 and 500 nm, respectively for determining the optimum performance of the displayed solar cell. By setting the defect density at 1014 cm− 2 and thickness at 500 nm, we aimed to determine the best performance for the SC presented.
3.4. Analysing the effects of changing the temperature and rare surface recombination velocity on PV Structure
The simulation work, displayed in Fig. 5(a), investigated the influence of working temperature (ranging from 274 K to 525 K) on the performance of the MoTe2 SC. Increasing the working temperature shows a consistent reduction in the PV performances except Jsc. It has been noticed that the value of all the PV parameters (Voc, FF, and η) decreases monotonically except Jsc with increasing the operating temperature. The numerically simulated η for the SC without the HTL layer is 29.63% and 22.28% at a temperature of 275K and 525K, respectively. While the maximum η of the newly designed SC with Sb2S3 HTL is 40.76% and 32.97% at the temperature of 275K and 525K, respectively. Increased working temperature promote greater collision rates between photogenerated electron-hole pairs and vibratoinal atoms, resulting in Jsc fluctuations in proposed SC. The diminishing energy bandgap in the semiconductors, caused by the increasing the temperature, is responsible for the drop in Voc [34, 35]. These outcomes are consistent with the findings of multiple research groups investigating temperature-related effects on PV performances [35, 36].
The examination on the back surface recombination velocity (BSRV) on the PV performances has been carried out for the SC, with and without HTL and is visually represented in Fig. 5(b). It can demonstrates a reduction in the values of Voc, Jsc, FF, and η has been reduced from 1.13 to 0.91V, 40.70 to 36.00 mA/cm-2, 87.4 to 77.8% and 40.33 to 25.20%, respectively for changing BSRV from 1×101 to 1×105 cm/s for the baseline Solar structure. No significant changes are observed in the PV performances of as BSRV varies, indicating a consistent behavior. The insertion of the HTL layer creates a high electric field which contributes to decreasing the recombination loss. Therefore, the PV parameters remain constant. On the contrary, the absence of HTL accelerates the recombination and the PV parameters reduce [37][38].
3.5. Exploring how the PV parameters of the solar cell is affected by its interface defect density
We have investigated the effect of defect density at MoTe2/Sb2S3 and MoTe2/CdS interfaces of the MoTe2 heterojunction SC under the illumination of AM1.5G and 300 K temperature as represented in Fig. 6 (a) and (b). The variation of defect density within \({10}^{10}\) to \({10}^{20}\) cm− 2 is applied at the interface of MoTe2/Sb2S3 keeping others parameters fixed. The result demonstrates a constant behavior in all PV parameters up to 1014 cm− 2, followed by a decrease in response to rising defect density.
At the CdS/MoTe2 interface, higher defect density results in a significant drop in the values of FF, η and Voc. The value of Jsc remains unaltered at 40.87 mA/cm2 until the defect density reaches 1016 cm− 2. The degradation of η and Voc values from 40.33 to 27.30% and 1.1 to 0.3 V for changing the defect density between \({10}^{10}\) cm− 2 to \({10}^{20}\) cm− 2 respectively.
The defect density at CdS/MoTe2 interface has a dominant impact over that of the MoTe2/Sb2S3 interface of the demonstrated structure. The interface defects contribute of increasing the rate of photo-generated carrier recombination which degrade the PV performances [29, 36, 39, 40]. The optimum performance was achieved at the position of 1014 cm− 2 of the defect density for both the interfaces. The trap state exists at the interfaces behave like a recombination center and contributes for degradation to the photo-generated carriers. In this way, the interface defects demand a reduction in the performance of the SC [41].
3.6. Understanding the effects of series and shunt resistance on the PV performances
Several factors are liable for the non-ideality behaviour of a solar structure. One of them is parasitic resistance (Rs & Rsh). Both these resistances diminish the output performances of the solar structure. The Rs and Rsh resistance significantly affects SC performance parameters. The Rs originated from the connection among various terminals between front and back contact in the heterojunction SC structure. In contrast, Rsh is generated from defects in the SC that causes to increase in reverse saturation current. Therefore, these resistances have a significant role in SC PV performance.
Figure 7 (a) and (b) present a comparative analysis of how Rs and Rsh affect the PV parameters of the MoTe2 structure, both in the absence and presence of an Sb2S3 HTL. The values of FF, and η reduces with the variation of Rs from 0 to 6 Ω-cm2 at a fixed Rsh of 103 Ω-cm2. Otherwise, values of Voc and Jsc remain fixed over the entire range of Rs. The improper connection at the front contact, back contact and combination among various terminals causes power loss which in turn outcomes in a fall down in the conversion efficiency [28][42]. With the enhancement of Rs, the behavior of FF decreases dramatically because of solder bond degradation, but Jsc drop for the optical transmission loss resulted from the encapsulate discoloration [43].
Furthermore, we have investigated the effect of Rsh on PV parameters by altering values within \({10}^{1}\) to \({10}^{11}\) Ω-cm2 at constant Rs of 1 Ω-cm2 and displayed in Fig. 7(b). From the simulated results, it highly noticeable that the performance parameters enhancing with increasing the Rsh except Jsc which is constant. The parameters values increased up to \({10}^{3}\) Ω-cm2, and then got saturated above the \({10}^{3}\) Ω-cm2. The calculated η has been determined 4.16% and 40.33% at the Rsh value of \({10}^{3 }\)to \({10}^{6}\) Ω-cm2, respectively. The presences of bulk and interface defects in the proposed Al/FTO/CdS/MoTe2/Sb2S3/Pt SC are liable for decreases in the values of Rsh which in turn reduces the PV performance parameters [28][44].
3.7. The C-V characteristics of the optimized device
To characterize the semiconductor devices like solar cells, the C-V profile is an integral part and is used to enhance the device's performance. This study reveals additional basic features of a device structure, where doping density and build-in potential can be derived with the help of two following relations [24]:
\(\frac{1}{{\text{C}}^{2}}\) = \(\frac{2}{ {\text{q}\text{N}}_{\text{a}}{{\epsilon }}_{0}{{\epsilon }}_{\text{s}}{\text{A}}^{2}}\)(Vbi-V) … … … … … … … … … … … … … (4)
Na=\(\frac{2}{\text{q}{{\epsilon }}_{0}{{\epsilon }}_{\text{s}}{\text{A}}^{2}\left[\frac{\text{d}}{\text{d}\text{v}}\left(\frac{1}{{\text{c}}^{2}}\right)\right]}\) … … … … … … … … … … … … …… (5)
Where, the mentioned C, Na, \({\epsilon }_{0}\), \({\epsilon }_{s}\), A, V, q are the values of measured capacitance, doping density (1/cm3), the permittivity of the free space (8.85×10− 14 F/cm, dielectric constant as per Table 1, area of the cell (cm2), applied potential, the charge of an electron (1.6×10− 19 C), respectively. From these two relations, we can find the build-in potential (Vbi) and doping density (Na) from the intercept and the slope of the line in C− 2 versus V plot.
To evaluate the consistency of the studies, the C-V measurements may examine at diverse frequencies ranging from 0.1 kHz to 1 MHz. The total p-n junction may be the summation of the diffusion and depletion capacitance. At forward bias, the depletion capacitance is smaller than diffusion capacitance, while the relationship is opposite for reverse biased condition. At zero bias condition, the capacitance is found 48 nF/cm2 as shown in Fig. 8 (a). The mathematical expression for the depletion capacitance is as follows [45]:
C(V) = A\(\sqrt{\frac{\text{N}\text{q}{{\epsilon }}_{0}{{\epsilon }}_{\text{s}}}{{2(\text{V}}_{\text{b}\text{i}}-\text{V})}}\) … … … … … … … … … … … … … … (6)
Their physical meanings are stated as above. At a constant frequency, the capacitance exhibits a substantial and rapid rise with the increasing polarization potential. The Mott-Schottky plot of the proposed structure is shown in Fig. 8 (b). The intersection of the 1/C2 with the voltage axis originates the flat band potential. Here, the negative slope reveals that holes are the majority carrier and electrons are the minority carrier and the space charge area broadly occupies the p-type MoTe2 absorber layer. Localized deep states in the MoTe2 layer lead to a noticeable marginal deviation in the 1/C2 curve. The C-V characteristic with varying absorber layer thickness from 0.10 to 0.80 µm is shown in Fig. 8 (c), by keeping frequency constant at 1 MHz. The capacitance value is almost constant up to a certain level and changes dramatically after that level. The same kind of relation has been discussed in the earlier report [46].
The C-V characteristics with varying absorber doping density at a constant frequency of 1 MHz has studied in Fig. 8 (d). With the increases in bias voltage, the capacitance value also changes proportionally. The charge development at the interface increases with the increases in absorber doping density which further contributes to enhancing the capacitance value [41].
The depletion width (\({W}_{d}\)) and the length of carrier diffusion can be calculated by these two obtained values (Vbi, V), by the following two relations [24]:
$${W}_{d}={\left(\frac{2{\epsilon }_{s}{\epsilon }_{0}{V}_{bi}}{qNa}\right)}^{0.5}$$
7
$$\text{L}\text{n} \sim \sqrt{{\text{D}}_{\text{n}}{\tau }}=\sqrt{{{\mu }}_{\text{n}}\frac{kT}{q}{\tau }}$$
8
Where Dn is the diffusivity of the electron, \({\tau }\) is the minority career lifetime, µn is the electron mobility, \(\frac{kT}{q}\) is the thermal voltage at room temperature. Before recombination, electrons may travel by this average distance towards the boundary of the space charge region. The active thickness required for the photocurrent in a MoTe2 Solar structure can now be easily determined through the equation Lp=Ln+Wd [24].
3.8. The final output performance MoTe2 solar structure
There are two MoTe2-based heterostructure solar cells. One is the baseline (without HTL) MoTe2 SC structure and the other is the proposed (with HTL) heterojunction SC design of Al/FTO/CdS/MoTe2/Sb2S3/Pt, where Sb2S3 has been used as an HTL layer and placed within left contact and MoTe2 absorber layer. Figure 9(a) represents a comparison of the J-V curve of baseline and proposed SC structures. The thickness of FTO, Sb2S3, MoTe2, CdS and has been chosen as 0.05 µm, 0.1 µm, 0.5 µm, 0.05 µm, respectively. It has been demonstrated that the PV performance of this proposed SC design are greater than the reference MoTe2 solar structure. The PV parameters of the baseline MoTe2 SC is 0.95 V, 38.15 mA/cm2, 81.09%, and 29.35%, respectively. While the enhanced η of the presented Al/FTO/CdS/MoTe2/Sb2S3/Pt SC is 40.33% containing Voc of 1.13 V, Jsc of 40.78 mA/cm2, and FF of 87.63%. The absorber layer with Sb2S3 HTL creates a strong electric field via the creation at p+-p junction which contributes a degradation at the carrier recombination rate at the MoTe2/Sb2S3 interface [21, 26, 27, 46, 47]. This reduction of carrier recombination contributes to enhancing the performance of our presented solar structure.
The wavelength-dependent external quantum efficiency (EQE) curves of MoTe2 SC for both reference and presented structures have displayed in the Fig. 9(b). It has also been cleared that the EQE of MoTe2 SC with the HTL layer of solar cells is higher than without HTL and the wavelength has altered within (300 ~ 1200 nm) for detecting this effect. The enhancement of the EQE in the longer wavelength may be because of the insertion of the Sb2S3 HTL layer which diminishes carrier recombination by creating an electric field at the interface of MoTe2/Sb2S3 [21, 40].
Table 4 reveals the comparison of the performance features of MoTe2 SC studied by various research groups and proposed structure. This study shows that the η varied from 22.43–29.13% in the previously investigated MoTe2 SC structures. The value of PV performance parameters is lower because of the mismatch of band arrangement at the Sb2S3/ MoTe2 interface that inhibits the photo-generated carrier’s conduction. Moreover, the presence of shallow and deep-level defects facilitates SRH recombination [48]. Thus the photovoltaic performances of MoTe2 solar cell structure need to be enhanced. Our proposed structure has achieved better values by restricting the carrier recombination at Sb2S3/MoTe2 and MoTe2/CdS layers [48]. The present study of Al/FTO/CdS/MoTe2/Sb2S3/Pt SC demonstrates insight into the development of high-efficiency MoTe2 SC by suppressing carrier recombination.
Table 4
Comparison values of PV parameters with various research works.
No
|
Structure
|
Depth (µm)
|
\({\varvec{V}}_{\varvec{o}\varvec{c}}\) (V)
|
Jsc (mA/\({\varvec{c}\varvec{m}}^{2})\)
|
FF (%)
|
PCE (%)
|
References
|
1
|
FTO/ZnO/MoTe2/Cu2Te
|
0.5
|
0.88
|
38.14
|
85.26
|
29.13
|
[1]
|
2
|
ZnO/Zn2SO4/CdS/CdTe/MoTe2
|
0.08
|
0.7
|
28
|
---
|
28.00
|
[8]
|
3
|
CdS/MoTe2/As2Te3
|
0.1–10
|
1.07
|
27.79
|
84.20
|
25.06
|
[10]
|
4
|
CdS/MoTe2/In2Te3
|
0.1-5.0
|
1.07
|
27.90
|
84.20
|
25.17
|
[14]
|
5
|
FTO/CdS/CdTe/Sb2S3
|
1.0
|
1.15
|
28.74
|
86.03
|
28.41
|
[19]
|
6
|
SnO2/Zn2SnO4/CdS/CdTe/ZnTe
|
0.1–2.9
|
1.05
|
24.34
|
87.20
|
22.43
|
[49]
|
7
|
SnO2/Zn2SnO4/CdS/CdTe/SnTe
|
0.1-3.0
|
1.05
|
24.40
|
87.20
|
22.48
|
[50]
|
8
|
Zn2SnO4/CdS/MoTe2/CZT
|
0.1–10
|
1.07
|
27.85
|
84.10
|
25.11
|
[13]
|
9
|
Al/FTO/CdS/MoTe2/Pt
(without HTL)
|
0.5
|
0.95
|
38.15
|
81.09
|
29.35
|
Proposed SC
|
10
|
Al/FTO/CdS/MoTe2/Sb2S3/Pt
(with HTL)
|
0.5
|
1.13
|
40.78
|
87.63
|
40.33
|
Proposed SC
|