Monolayer MoS 2 has excellent optoelectronic properties, which is a potential material for solar cell. Though MoS 2/c-Si heterojunction solar cell has been researched by many groups, little study of MoS 2/c-Si solar cell physics is reported. In this paper, MoS 2/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 MoS 2/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 MoS 2/i-layer/p-type c-Si/Al cell, was studied in this paper. The results show that the TCO/n-type MoS 2/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 MoS 2, density of defect state and the energy band offset of MoS 2/c-Si interface of TCO/n-type MoS 2/i-layer nc-Si:H/p-type c-Si/Al cells, on the characteristics of solar cells, have been discussed for this kind of MoS 2 heterojunction cells. The optimal solar cell with structure of TCO/n-type MoS 2/i-type nc-Si:H/p-type c-Si/BSF/Al, has the best efficiency of 27.22%.
Since physicists Andre Anaheim and Konstantin Novoselov successfully isolated graphene from graphite in 2004 [
It has been reported that MoS2 exhibits one order of magnitude higher light absorption than Si and GaAs [
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%.
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 ( E ) = G A G exp { − 1 / 2 [ ( E − E p k a ) 2 / σ A 2 ] } (1)
g D ( E ) = G D G exp { − 1 / 2 [ ( E − E p k d ) 2 / σ D 2 ] } (2)
where Epka and Epkd are the Gaussian peak positions of the acceptor and donor states; σA and σD are the full width at half maximum (FWHM) of the acceptor and donor states, respectively; GAG and GDG 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 ( E ) = G A 0 exp [ ( E − E c ) / E A ] (3)
g D ( E ) = G D 0 exp [ ( E V − E ) / E D ] (4)
where gA (E) is conduction band tail defect density of states; gD (E) is valance band tail defect density of states. EC is conduction band edge; EV is the valance band edge. GA0 and GD0 are prefactor; EA and ED 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.
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 [
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.
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 [
According to
In order to explain the results, we investigated the energy band and recombination rate of solar cells.
an important impact on the performance of solar cell. According to the Equation (5) [
J = J s c − [ ( q D n N d / L n ) exp ( − ( q V D + Δ E c ) / k 0 T ) + ( q D p N A / L P ) exp ( − ( q V D − Δ E V ) / k 0 T ) ] × ( exp ( q v / k 0 T ) − 1 ) (5)
It can be noted that when ΔEV increases, J will be reduced; this will lead to the decreasing of cell performance. From
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
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 Voc, Jsc, FF and Eff. Initially Voc and Jsc slightly decrease with the increasing density of defect state of MoS2/c-Si interface. However, Voc and Jsc obviously decrease when value larger than 1 × 1011 cm−2∙eV−1. FF and Eff 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 Jsc 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 ΔEV 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 [
As we can see from
As we can see form
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 ΔEV, and then the performance of the solar cell will go bad. The offsets between the band edges, ΔEC and ΔEV 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 ΔEV 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.
It is known that BSF working as passivation plays an important role in improving the performance of solar cell [
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.
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
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 Voc. 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%.
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
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
J s c = q ∫ φ ( λ ) { 1 − R ( λ ) } Q E ( λ ) d λ (6)
V o c = ( n k T / q ) ln ( 1 + J p h / J 0 ) (7)
Since Eff varies in proportion with Voc and Jsc, therefore, efficiency of solar cell will reduce with the photogenerated current and the open circuit voltage decreasing.
Figures 10(a)-(d) show the cell performance of solar cells with the different MoS2 thickness. From
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
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 Voc 771.6 mV, Jsc 42.81 mA/cm2, fill factor FF 82.42%, conversion efficiency of cell up to 27.22%.
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.
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