ABSTRACT

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%.

Keywords:

Intrinsic Material, Cell Efficiency, Molybdenum Disulfide

1. 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 MoS₂ 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 cm²∙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 MoS₂ exhibits one order of magnitude higher light absorption than Si and GaAs [7] . MoS₂/c-Si heterojunction solar cell has been researched by many groups lately. L. Hao et al. achieved a PCE of 1.3% in MoS₂/Si junction by the method of magnetron sputtering [8] . Tsai et al. realized the increase of the PCE of MoS₂/Si from 4.64% to 5.23% in Al/Si solar cells [9] . Rimjhim Chandhary achieved the efficiency up to 12.44% in MoS₂/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 MoS₂/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 MoS₂/Si interface, the BSF, the thickness of MoS₂ 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 MoS₂/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 MoS₂/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 MoS₂/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 MoS₂, density of defect states (DOS) and the energy band offset of MoS₂/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 MoS₂/i-type nc-Si:H/p-type c-Si/BSF/Al solar cell with efficiency of 27.22%.

2. Physical Model and Device Structure

2.1. Physical Model

AFORS-HET is used to analyse and simulate the properties of heterojunction solar cells by solving the one-dimensional semiconductor equation based on 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

$g_A\left(E\right)=G_{AG}\exp \left\{-1/2\left[{\left(E-E_{pka}\right)}^2/{\sigma }_A^2\right]\right\}$ (1)

$g_D\left(E\right)=G_{DG}\exp \left\{-1/2\left[{\left(E-E_{pkd}\right)}^2/{\sigma }_D^2\right]\right\}$ (2)

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.

$g_A\left(E\right)=G_{A0}\exp \left[\left(E-E_c\right)/E_A\right]$ (3)

$g_D\left(E\right)=G_{D0}\exp \left[\left(E_V-E\right)/E_D\right]$ (4)

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.

2.2. 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 MoS₂ 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 Table 1, which are taken from various references [10] [12] [13] [14] [15] . For the c-Si, defect density is chosen as oxygen defect at 0.55 eV with a concentration of 1 × 10¹¹ cm⁻³. The surface re-

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 × 10⁷ cm/s, these coefficients are given in the AFORS-HET software.

3. Results and Discussions

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

MoS₂ has layered structure, while crystal Si is a diamond-like structure, hence, when MoS₂ 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 MoS₂ on Si [16] . The buffer layer can balance carrier injection and reduce the leakage current. When intrinsic layer is inserted into MoS₂/c-Si interface, the intrinsic instead of the Si surface forms a contact with MoS₂ film. It is good way for MoS₂/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 MoS₂/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 2.

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. Figure 2(a) and Figure 2(b) are the energy band and the recombination rate of the heterojunction solar cells with different intrinsic layer respectively. As well known, band offset can be observed at interface when two semiconductors with different band gap contact. Therefore, there is different band offset for p region of solar cell with different intrinsic layer. Band offset has

an important impact on the performance of solar cell. According to the Equation (5) [17]

$\begin{array}{c}J=J_{sc}-[\left(q{D}_n{N}_d/L_n\right)\exp \left(-\left(q{V}_D+\Delta E_c\right)/k_{0}T\right)\\ +\left(q{D}_p{N}_A/L_P\right)\exp \left(-\left(q{V}_D-\Delta E_V\right)/k_{0}T\right)]\times \left(\exp \left(qv/k_{0}T\right)-1\right)\end{array}$ (5)

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 MoS₂/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%.

3.2. Effect of Defect States and the Energy 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 Cell

For MoS₂/c-Si heterojunction solar cells, the density of defect states of MoS₂/c-Si interface is the important influencing factor that determines the transport properties of the cell. Here, in this paper, the TCO/n-type MoS₂/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 MoS₂/i-type/p-type c-Si solar cell. The performances of the solar cell with TCO/n-type MoS₂/i-layer nc-Si:H/p-type c-Si/Al structure as a function of MoS₂/c-Si interface defect states show in Figure 3.

From Figures 3(a)-(d), we can see that the larger density of defect state of MoS₂/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 MoS₂/c-Si interface. However, V_oc and J_sc obviously decrease when value larger than 1 × 10¹¹ cm⁻²∙eV⁻¹. FF and E_ff almost keep at constant initially with increasing density of

defect state of MoS₂/c-Si interface, but greater than 1 × 10¹² cm⁻²∙eV⁻¹, they visibly reduce. The results indicate that the density of defect states of interface is less than 1 × 10¹¹ cm⁻²∙eV⁻¹, the performances of solar cell decrease slowly, however, the performances of solar cell reduce rapidly when the defect states of MoS₂/c-Si interface higher than 1 × 10¹¹ cm⁻²∙eV⁻¹. Therefore, the density of defect states of interface should be under1 × 10¹¹ 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 × 10¹¹ cm⁻²∙eV⁻¹. It is because that defect states of interface and monolayer MoS₂ 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 MoS₂/p-type c-Si heterojunction solar cells. And there is a potential barrier resulting from the valence band offset at n-type MoS₂/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 MoS₂layer and p-type c-Si layer [10] . The performances of TCO/n-type MoS₂/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 MoS₂/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.

As we can see form Figure 5, the degree of band bending at interface between MoS₂ 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 MoS₂/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 MoS₂/c-Si interface should be under 1 × 10¹¹ 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 MoS₂ is a potential material for solar cell if we deal well the MoS₂/c-Si interface.

3.3. 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 MoS₂/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.

Figure 6 is the J-V curve of TCO/n-typeMoS₂/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/cm² to 42.46 mA/cm² and cell efficiency increases from 21.83% to 26.99%. It is obvious that the

performances of the solar cell with BSF are obviously improved. It means that BSF is a good way to improve the performance of solar cell. BSF can collect the photogenic minority carrier in back surface, improving the internal quantum efficiency of solar cell. In addition, BSF layer can introduce barrier for minority carriers, which can reduce the recombination of the photons on the surface.

3.4. Effects of nc-Si:H Intrinsic Layer on the Performance of Solar Cell

3.4.1. 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. 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 MoS₂/c-Si solar cell, causing the smaller probability of carrier at the interface in the heterojunction smaller, thus the reverse saturation current

decreases in p-n junction, but the open circuit voltage increases. The FF decreases with the increase of band gap due to the increase in the open-circuit voltage V_oc. The conversion efficiency of the cell slowly enhances with the increase of the band gap. The efficiency is basically kept constant when the band gap become more than 1.7 eV. Hence, the intrinsic band gap of 1.7 eV was optimized for achieving high performance of cell which efficiency up to 27.16%.

3.4.2. Optimization of the Thickness of nc-Si: H Intrinsic Layer

Figures 8(a)-(d) show the performances curves of TCO/n-type MoS₂/i-layer nc-Si:H/p-type c-Si/BSF/Al solar cells with different thickness. From Figure 8(a), it can be noted that the open circuit voltage Voc decreases gradually from 771.6 mV to 762.8 mV with the thickness increases from 2 nm to 10 nm, Figure 8(b) displays that the short current density continuously reduces from 42.81 mA/cm² to 42.16 mA/cm² with increases of thickness. Figure 8(c) indicates that the FF initially increases slightly with the increase of the intrinsic layer thickness from 2 nm to 8 nm. However, when thickness of nc-Si:H greater than 8 nm, the FF gradually decreases. It is depicted in Figure 8(d), with the thickness of the

intrinsic layer is variable from 2 nm to 14 nm, the efficiency of the cell is reduced rapidly from 27.16% to 25.91%. The decrease of the open voltage can be attributed to an decrease of the built-in electric field. The quantum efficiency characteristic of the cells with different thicknesses of intrinsic layer is used to explain these results, as shown in 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.

$J_{sc}=q{{\displaystyle \int }}^{\text{}}\phi \left(\lambda \right)\left\{1-R\left(\lambda \right)\right\}QE\left(\lambda \right)\text{d}\lambda$ (6)

$V_{oc}=\left(nkT/q\right)\ln \left(1+J_{ph}/J_0\right)$ (7)

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.

3.5. Optimization of Thickness of n-Type MoS₂ Emitter Layer

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 MoS₂ has no effect on Voc. However, Figure 10(b) shows that the short-circuit current density decreases gradually from 42.49 mA/cm² to 42.16 mA/cm² with the increasing MoS₂ 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 MoS₂ 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 Figure 10(d), we can note that the efficiency of solar cell decreases from 27.22% to 26.8% with increasing the thickness of MoS₂. 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 MoS₂ 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 MoS₂ is 0.65 nm. But the efficiency reduces with increasing the thickness of MoS₂ due to it is indirect gap for multilayer MoS₂.

3.6. 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 MoS₂/c-Si interface lower than 1 × 10¹¹ cm⁻²∙eV⁻¹ and 0.762 eV respectively, MoS₂ 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.

4. Conclusions

The TCO/n-type MoS₂/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, MoS₂/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 × 10¹¹ 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 MoS₂/i-layer nc-Si:H/p-type c-Si/BSF/Al are, thickness of MoS₂ 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/cm², fill factor FF 82.42%, conversion efficiency of cell up to 27.22%.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (Grant No. 51472096), the R & D Program of Ministry of Education of China (No. 62501040202). The authors would like to acknowledge Helmholts-Zentrum Berlin for providing AFORS-HET simulation software.

Cite this paper

Xu, S., Zeng, X.B., Wang, W.Z., Zhou, G.T., Hu, Y.S., Wu, S.X. and Zeng, Y. (2017) Simulation and Optimization Characteristic of Novel MoS₂/c-Si HIT Solar Cell. Journal of Minerals and Materials Characterization and Engineering, 5, 323-338. https://doi.org/10.4236/jmmce.2017.55027

References