Simulation and Optimization Characteristic of Novel MoS 2 / cSi HIT Solar Cell

Monolayer Mos2 has excellent optoelectronic properties, which is a potential material for solar cell. Though MoS2/c-Si heterojunction solar cell has been researched by many groups, little study of MoS2/c-Si solar cell physics is reported. In this paper, MoS2/c-Si heterojunction solar cells have been designed and optimized by AFORS-HET simulation program. The various factors affecting the performance of the cells were studied in details using TCO/n-type MoS2/i-layer/p-type c-Si/BSF/Al structure. Due to the important role of intrinsic layer in HIT solar cell, the effect of different intrinsic layers including a-Si:H, nc-Si:H, a-SiGe:H, on the performance of TCO/n-type MoS2/i-layer/p-type c-Si/Al cell, was studied in this paper. The results show that the TCO/n-type MoS2/i-layer/p-type c-Si/Al cell has the highest efficiency with a-SiGe:H as intrinsic layer, efficiency up to 21.85%. The back surface field effects on the properties of solar cells were studied with p + μc-Si and Al as BSF layers. And the effect of various factors such as thickness and band gap of intrinsic layer, thickness of MoS2, density of defect state and the energy band offset of MoS2/c-Si interface of TCO/n-type MoS2/i-layer nc-Si:H/p-type c-Si/Al cells, on the characteristics of solar cells, have been discussed for this kind of MoS2 heterojunction cells. The optimal solar cell with structure of TCO/n-type MoS2/i-type nc-Si:H/p-type c-Si/BSF/Al, has the best efficiency of 27.22%.


Introduction
Since physicists Andre Anaheim and Konstantin Novoselov successfully isolated graphene from graphite in 2004 [1], two dimensional layered materials have been widely concerned due to its excellent physical and chemical properties.However, graphene is a zero-gap material, which limits its applications in some fields.Recently, researchers have been refocusing on other grapheme, like 2D materials to overcome the shortage of graphene and broaden its range of applications.In contrast to the zero-gap graphene, transition metal sulfides are tuneable band structure and applicable in wide fields due to its excellent optional and electrical properties [2] [3].Monolayer MoS 2 has an ultra-thin lamellar structure with thickness about 0.65 nm [4], a direct band gap of 1.9 eV [5], and high electron mobility of 200 cm 2 •V −1 •s −1 [6].So it is a potential candidate material for TFTs, FETs, photodetectors, sensors and solar cells.
It has been reported that MoS 2 exhibits one order of magnitude higher light absorption than Si and GaAs [7].MoS 2 /c-Si heterojunction solar cell has been researched by many groups lately.L. Hao et al. achieved a PCE of 1.3% in MoS 2 /Si junction by the method of magnetron sputtering [8].Tsai et al. realized the increase of the PCE of MoS 2 /Si from 4.64% to 5.23% in Al/Si solar cells [9].Rimjhim Chandhary achieved the efficiency up to 12.44% in MoS 2 /Si heterojunction solar cell through simulation [10].Comparatively speaking, the performances of solar cell in experimental results are poorer than the simulation results.This is because solar cell in experiment is affected by some uncontrollable factors and not optimized, which make the solar cell not in optimal conditions.While the previous works are focused on the fabrication of MoS 2 /c-Si heterojunction, little understanding of device physics is obtained.In order to improve the efficiency of solar cell, especially effect and physics mechanism of device structure parameters has been concerned such as the intrinsic layer, the defect in MoS 2 /Si interface, the BSF, the thickness of MoS 2 etc.
As is well known, the heterojunction with intrinsic thin layer (HIT) solar cell is the best module in Si-based cells with the highest efficiency up to now.It can be expected that the MoS 2 /Si heterojunction, combined with HIT, would become one of good ways to develop high-performance solar cells.In this paper, detailed studies of the property of MoS 2 /c-Si have been carried out with AFORS-HET.In order to deeply understand the physics of this device, we analyzed the influence of intrinsic layer on performance of TCO/n-type MoS 2 /i-layer/p-type c-Si/Al cells, and studied the relationships between the cell parameters, such as thickness and band gap of intrinsic layer, thickness of MoS 2 , density of defect states (DOS) and the energy band offset of MoS 2 /c-Si interface, and characteristics of heterojunction cells, to improve the performance of solar cell.By optimization of the various cell parameters, we obtained the optimal solar cell structure of TCO/n-type MoS 2 /i-type nc-Si:H/p-type c-Si/BSF/Al solar cell with efficiency of 27.22%.

Physical Model
where E pka and E pkd are the Gaussian peak positions of the acceptor and donor states; σ A and σ D are the full width at half maximum (FWHM) of the acceptor and donor states, respectively; G AG and G DG are the density of the acceptor state and the density of the donor state.
Density of location state in band tail is described by an exponential function, and its distribution in the forbidden band are shown in equations ( 3) and ( 4) respectively.
( ) ( ) ( ) ( ) where g A (E) is conduction band tail defect density of states; g D (E) is valance band tail defect density of states.E C is conduction band edge; E V is the valance band edge.G A0 and G D0 are prefactor; E A and E D indicate tail characteristics of the energy transfer.These complex states take the role of traps and composite centers.The composite model mainly considers SRH and Auger recombination, which have a decisive influence on the electrical and optical properties of thin film silicon materials.

Device Structure
Sanyo Ltd. has developed a silicon heterojubction solar cell named heterojunction with intrinsic thin layer with an efficiency up to 20%, which makes the HIT structure popular.However continuing to improve the efficiency of HIT solar cell is a big challenge.Owing to the unique electronic characteristics and stronger photoresponsivity in visible light spectrum from 400 nm to 680 nm [11] than Siand GaAs, the monolayer MoS 2 was used as window layer to make up a novel HIT cell.The intrinsic layer and back surface field (BSF) layer adopted to improve the efficiency of solar cell.Transparent conductive film (TCO) and Al back contact are also considered.The structure of solar cell is shown in Figure 1.
The main parameters we draft are shown in  flectance of the solar cell is 0.1, the backside emissivity is 1.The surface recombination rate of the electrons and holes at the front and rear contact surfaces is 1 × 10 7 cm/s, these coefficients are given in the AFORS-HET software.2.

Results and Discussions
According to Table 2, it is clear that the cell with a-SiGe:H as intrinsic layer has the best performances with efficiency 21.85%.However, using the a-Si:H as intrinsic layer of cell has the poor performances, which efficiency just is 13.4%.
This result is mainly ascribed to the recombination rate in p/i interface.
In order to explain the results, we investigated the energy band and recombination rate of solar cells.
( ) It can be noted that when ΔE V increases, J will be reduced; this will lead to the decreasing of cell performance.From Figure 2(a), when a-Si: H serving as the intrinsic layer, ΔE V is larger, according to Equation (5) that will result in J smaller and makes the cell efficiency worse.In addition, From Figure 2(b), it shows that the recombination rate of solar cell with a-Si:H as the intrinsic layer is higher, consequently, the recombination rate of the i/p interface is increased, and resulting in the open circuit voltage has a low value, so using a-Si:H as the intrinsic layer of heterojunction solar cell, the efficiency is lower than the other materials.For a-SiGe:H, ΔE V is small so that the efficiency of the TCO/n-type MoS 2 /i-layer/p-type c-Si/Al solar cells with a-SiGe:H as intrinsic layer is higher, reaching 21.85%.For the solar cell with nc-Si:H as the intrinsic layer, it can be seen from Figure 2(a) that the conduction band offset is large that equals to 0.25 eV.Conduction band offset suppresses the surface recombination and makes the cell efficiency up to 21.83%.MoS 2 /c-Si interface leads to the decreasing of V oc , J sc , FF and Eff.Initially V oc and J sc slightly decrease with the increasing density of defect state of MoS 2 /c-Si interface.However, V oc and J sc obviously decrease when value larger than 1 × 10 11 cm −2 •eV −1 .FF and E ff almost keep at constant initially with increasing density of defect state of MoS 2 /c-Si interface, but greater than 1 × 10 12 cm −2 •eV −1 , they visibly reduce.The results indicate that the density of defect states of interface is less than 1 × 10 11 cm −2 •eV −1 , the performances of solar cell decrease slowly, however, the performances of solar cell reduce rapidly when the defect states of MoS 2 /c-Si interface higher than 1 × 10 11 cm −2 •eV −1 .Therefore, the density of defect states of interface should be under1 × 10 11 cm −2 •eV −1 in order to obtain good performance, and the better performances can be obtained due to the larger J sc when the density of interface defect states is lower than 1 × 10 11 cm −2 •eV −1 .It is because that defect states of interface and monolayer MoS 2 works as charge carriers traps that provide the channel for carriers recombination.The photo-generated carriers come mainly from p-type c-Si layer in n-type MoS 2 /p-type c-Si heterojunction solar cells.And there is a potential barrier resulting from the valence band offset at n-type MoS 2 /p-type c-Si interface, which hinders the photo-generated minority carrier holes from being collected by front electrode.As a result, the valence band offset strongly affects the interface transport properties of photogenerated holes.As well known, the influence of ΔE V on the interface transport properties and performances of solar cells can be got by changing the electron affinity of n type MoS 2 layer and p-type c-Si layer [10].The performances of TCO/n-type MoS 2 /i-layer nc-Si:H/p-type c-Si/Al solar cell as a function of ΔE V are given in Figure 4.As we can see from Figure 4(a), initially the V oc keeps at constant with the increasing of ΔE V from 0.55 eV to 0.7 eV, then decreases when ΔE V larger than 0.7 eV, however, when ΔE V greater than 0.76 eV the value of V oc keeps at constant again.From Figure 4(b), the J sc almost remains at a constant with increasing of ΔE V .In case of the fill factor, it slightly reduces from 83.64% to 83.58% with increasing of ΔE V from 0.56 eV to 0.76 eV.However, the value keeps at constant when ΔE V greater than 0.86 eV.E ff decreases continuously with the increasing of ΔE V .It is found that when ΔE V is under 0.762 eV, ΔE V has little impact on J sc and FF, with the increasing of ΔE V , J sc stands at fixed value, but FF and V oc decrease.It indicates that an appropriate high minority carrier band offset can lead to an effective suppression of interface recombination at MoS 2 /c-Si hetero-interface, but too high band offset may enhance the interface recombination.So it is evident that ΔE V should be kept lower than 0.762 eV in order to obtain good performance of solar cell.For this result, we studied the energy band structure with different ΔE V , the results display in Figure 5.

Effect of Defect States and the Energy
As we can see form Figure 5, the degree of band bending at interface between MoS 2 and c-Si increases when ΔE V is larger, which will lead to the higher valence band barrier for holes and enhancing the build-in potential.From Equation ( 5), it is easy to note that J will decrease with the increasing of ΔE V , and then the performance of the solar cell will go bad.The offsets between the band edges, ΔE C and ΔE V of the MoS 2 /c-Si junction influence strongly the carrier transporting across the hetero-interface, so setting the suitable carrier band offset is necessary.Above all, the defect state of MoS 2 /c-Si interface should be under 1 × 10 11 cm −2 •eV −1 , and the ΔE V should be under 0.762 eV so that we can get the good performance for solar cell.It implies that the monolayer MoS 2 is a potential material for solar cell if we deal well the MoS 2 /c-Si interface.

Optimizing the Performance of TCO/n-Type MoS2/i-Layer nc-Si:H/p-Type c-Si/Al Heterojunction Solar Cells with BSF
It is known that BSF working as passivation plays an important role in improving the performance of solar cell [18].In order to improve the performances of solar cell, the effect of BSF on the performance of the cell was studied by TCO/n-type MoS 2 /i-typenc-Si:H/p-type c-Si/BSF/Al cell structure in this paper.
We choose p + -μc-Si:H as back surface layer.On one hand, the band gap of p + -a-Si:H is about 1.74 eV, and it has a large ΔE c because of its large band gap which has a good effect on the reflection of electrons.On the other hand, nevertheless, ΔE V also increases resulting in the holes difficultly reach to the back electrode.Compared with p + -a-Si:H, the bandgap of p + -μc-Si:H is about 1.45 eV, that is to say, the ΔE c is relatively low which can lessen the reflection of the effective electrons.Furthermore, the electrons of the back surface field are collected, therefore, using p + -μc-Si:H as the back surface layer can transport enough of the majority of carriers [18].The optimizing result is shown in Figure 6.

The Optimization of Band Gap of nc-Si:H Intrinsic Layer
The intrinsic layer plays an important role in interfacial modification and band offset of solar cell, consequently, optimizing the intrinsic band gap to improve the efficiency of solar cells is necessary.Therefore, in this section, the band gap of intrinsic layer is variable from 1.6 eV to 1.8 eV, other parameters keep at constant.
The performances curve of the solar cell with different band gap of nc-Si: H is presented in Figure 7.According to Figure 7(a), the open-circuit voltage V oc increases obviously from 766.6 mV to 771.6 mV with the increase of intrinsic band gap from 1.60 eV to 1.80 eV.From Figure 7(b), increasing with the energy gap, the J sc almost holds at constant, it means that the band gap of the intrinsic layer has little effect on the short-circuit current density.( ) ( ) { } ( ) S.  Since E ff varies in proportion with V oc and J sc , therefore, efficiency of solar cell will reduce with the photogenerated current and the open circuit voltage decreasing.

Optimization the Best Performance of MoS2/c-Si Solar Cell
From the above work we did, we finally obtained the best performance of solar cell which efficiency is up to 27.22% with the density of defect states and band offset of MoS 2 /c-Si interface lower than 1 × 10 11 cm −2 •eV −1 and 0.762 eV respectively, MoS 2 thickness of 0.65 nm, intrinsic layer thickness of 2 nm and the band gap is 1.7 eV.The J-V curve of solar cell is shown in Figure 11.

Conclusions
The TCO/n-type MoS 2 /i-layer nc-Si:H/p-type c-Si/Al solar cells were investigated by AFORS-HET.The main studies are the intrinsic layer selection and optimization, MoS 2 /c-Si interface optimization and the effect of BSF layer on the cell performance.The effect of different intrinsic layers including a-Si:H, nc-Si:H and a-SiGe:H on solar cells was studied.For intrinsic layer a-SiGe:H, the solar cell has the best performance with the efficiency 21.85%.The results show that when the density of defect states is lower than 1 × 10 11 cm −2 •eV −1 and the band offset is lower than 0.762 eV, the solar cell has better performances.The parameters of optimal cell structure with TCO/n-type MoS AFORS-HET is used to analyse and simulate the properties of heterojunction solar cells by solving the one-dimensional semiconductor equation based on S. Xu et al.DOI: 10.4236/jmmce.2017.55027325 J. Minerals and Materials Characterization and Engineering Shockley-Read-Hall recombination statistics.In the simulation mode, the energy band electron distributions of solar cells include the valence band, the conduction band extension state, the localized states of valence band tail and the localized states of interval domain.The localized states in the band gap are mainly caused by the dangling bond.The tail domain is mainly caused by strain bond angle.The localized states in the band gap have a double Gaussian function distribution, which were positively correlated.Its distribution equations as follows Capture cross-section for donor states, e, h (cm 2 ) 1.0 × 10 −14 1.0 × 10 −15 1.0 × 10 −14 1.0 × 10 −15 1.0 × 10 −14 1.0 × 10 −15 1.0 × 10 −14 1.0 × 10 −15 1.0 × 10 −14 1.0 × 10 −15 1.0 × 10 −14 1.0 × 10 −15 Capture cross-section for acceptor states, e, h (cm 2 ) 1.0 × 10 −15 1.0 × 10 −14 1.0 × 10 −15 1.0 × 10 −14 1.0 × 10 −15 1.0 × 10 −14 1.0 × 10 −15 1.0 × 10 −14 1.0 × 10 −15 1.0 × 10 −14 1.0 × 10 −15 1.0 × 10 −14 Figure 2. (a)energy band of MoS 2 /c-Si cell; (b)recombination rate of MoS 2 /c-Si cell.
Band Offset of MoS2/c-Si Interface of the TCO/n-Type MoS2/i-Layer nc-Si:H/p-Type c-Si/Al HIT Solar CellFor MoS 2 /c-Si heterojunction solar cells, the density of defect states of MoS 2 /c-Si interface is the important influencing factor that determines the transport properties of the cell.Here, in this paper, the TCO/n-type MoS 2 /i-type nc-Si:H/p-type c-Si solar cell is studied.Because the bandgap adjusted of a-Si: H is less convenient than nc-Si:H.The bandgap of a-SiGe:H is small, though its bandgap is adjustable.Hence, we select the nc-Si:H as intrinsic layer in TCO/n-type MoS 2 /i-type/p-type c-Si solar cell.The performances of the solar cell with TCO/n-type MoS 2 /i-layer nc-Si:H/p-type c-Si/Al structure as a function of MoS 2 /c-Si interface defect states show in Figure3.From Figures 3(a)-(d), we can see that the larger density of defect state of

Figure 3 .
Figure 3. Performances of TCO/n-type MoS 2 /i-layer nc-Si:H/p-type c-Si/Al MoS 2 /i-layer nc-Si:H/p-type c-Si/Al as a function of defect states.as a function of different ΔE V .

Figure 4 .
Figure 4. Performances of TCO/n-type TCO/n-type MoS 2 /i-layer nc-Si:H/p-type c-Si/Al MoS 2 /i-layer nc-Si:H/p-type c-Si/Al as a function of defect states.as a function of different ΔE V .

Figure 5 .
Figure 5.The energy band structure of the cell with different ΔE V .

Figure 6
Figure 6 is the J-V curve of TCO/n-typeMoS 2 /i-layer nc-Si:H/p-type c-Si/BSF/Al solar cells.According to Figure 6, the result reveals that the open circuit voltage of solar cell increases from 652.7 mV to 771.1 mV, in case of short-circuit current density, it increases from 39.97 mA/cm 2 to 42.46 mA/cm 2 and cell efficiency increases from 21.83% to 26.99%.It is obvious that the

Figure 7 (
c) indicates that the FF decrease obviously from 82.82% to 82.41%.In case of E ff , it is found that the E ff keeps at constant nearly with the increases of intrinsic band gap.The observably improvement of the open voltage is attributed to the change of barrier in the MoS 2 /c-Si solar cell, causing the smaller probability of carrier at the interface in the heterojunction smaller, thus the reverse saturation current

Figure 8 .
Figure 8. Performances of TCO/n-type MoS 2 /i-type nc-Si:H/p-type c-Si/BSF/Al with different thickness of intrinsic layer.

Figure 9
Figure 9 shows the quantum efficiency characteristic of the cells with various values of thickness of intrinsic layer.It is obvious that the short-wave response of the cell is deteriorated with the increase of the thickness of nc-Si:H.According to the Equation (6) and Equation (7), this will lead to the photogenerated current and the open circuit voltage decrease.It indicates that the corresponding photogenerated carriers are not collected effectively, which leads to a decrease in Jsc of the cell.

Figure 9 .
Figure 9.The QE curve of solar cell with different thickness.

Figures 10 (
Figures 10(a)-(d) show the cell performance of solar cells with the different MoS2 thickness.From Figure 10(a), it is evident that Voc indicates that the thickness of MoS 2 has no effect on Voc.However, Figure 10(b) shows that the short-circuit current density decreases gradually from 42.49 mA/cm 2 to 42.16 mA/cm 2 with the increasing MoS 2 thickness from 0.65 nm to 9.75 nm.For Figure 10(c), we can see that the value of fill factor follows oscillating behaviour with increasing the thickness of MoS 2 but it continuously reduces from 82.49%to 82.43% in the thickness range 1.95 to 7.15 nm, and it reaches a maximum of 82.49% for the thickness at 8.45 nm.From Figure10(d), we can note that the efficiency of solar cell decreases from 27.22% to 26.8% with increasing the thickness of MoS 2 .The reduction of Jsc may be ascribed to the increase in absorption losses at the surface layer and the larger series resistance in solar cell.The monolayer MoS 2 is a direct gap which is beneficial to electron jumping.Hence, we can achieve the best performance of the cell up to 27.22% at the thickness of MoS 2 is 0.65 nm.But the efficiency reduces with increasing the thickness of MoS 2 due to it is indirect gap for multilayer MoS 2 .

Figure 10 .
Figure 10.Performances of TCO/n-type MoS 2 /i-type nc-Si:H/p-type c-Si/BSF/Al with different thickness of MoS 2 .

Figure 11 .
Figure 11.J-V curve of solar cell with best performance.

Table 1 .
Parameters used in present simulation.
Capture cross-section for donor states, e, h (cm2

3.1. The Effect of Different Intrinsic Layer on the Performance of TCO/n-Type MoS2/i-Layer/p-Type c-Si/Al HIT Solar Cell MoS
[16]s layered structure, while crystal Si is a diamond-like structure, hence, when MoS 2 film was deposited straight on the Si surface, this would results in large quantities of lattice defects at the interface[16].So it is necessary to conduct interface modification before deposited MoS 2 on Si[16].The buffer layer can balance carrier injection and reduce the leakage current.When intrinsic layer is inserted into MoS 2 /c-Si interface, the intrinsic instead of the Si surface forms a contact with MoS 2 film.It is good way for MoS 2 /Si solar cell to obtain high performance by introducing the intrinsic layer for solar cell.Therefore, in this paper, we compared the performance of TCO/n-type MoS 2 /i-layer/p-type c-Si/Al solar cells by using different intrinsic layer, including a-Si: H, nc-Si:H and a-SiGe:H.The results are listed in Table

Table 2 .
The simulated performance of the solar cells with different intrinsic layer.
2 /i-layer nc-Si:H/p-type c-Si/BSF/Al are, thickness of MoS 2 0.65 nm, intrinsic layer thickness of 2 nm and the band gap of 1.7 eV, with the open circuit voltage V oc 771.6 mV, J sc 42.81 mA/cm 2 , fill factor FF 82.42%, conversion efficiency of cell up to 27.22%.