Raytracing Modelling of Infrared Light Management Using Molybdenum Disulfide (MoS2) as a Back-Reflector Layer in a Silicon Heterojunction Solar Cell (SHJ)

Abstract

The silicon heterojunction solar cell (SHJ) is considered the dominant state-of-the-art silicon solar cell technology due to its excellent passivation quality and high efficiency. However, SHJ's light management performance is limited by its narrow optical absorption in long-wave near-infrared (NIR) due to the front, and back tin-doped indium oxide (ITO) layer's free carrier absorption and reflection losses. Despite the light-trapping efficiency (LTE) schemes adopted by SHJ in terms of back surface texturing, the previous investigations highlighted the ITO layer as a reason for an essential long-wavelength light loss mechanism in SHJ solar cells. In this study, we propose the use of Molybdenum disulfide (MoS₂) as a way of improving back-reflection in SHJ. The text presents simulations of the optical response in the backside of the SHJ applying the Monte-Carlo raytracing method with a web-based Sunsolve high-precision raytracing tool. The solar cells' electrical parameters were also resolved using the standard electrical equivalent circuit model provided by Sunsolve. The proposed structure geometry slightly improved the SHJ cell optical current density by ~0.37% (rel.), and hence efficiency (η) by about 0.4% (rel.). The SHJ cell efficiency improved by 21.68% after applying thinner back ITO of about 30 nm overlayed on ~1 nm MoS₂. The efficiency improvement following the application of MoS₂ is tentatively attributed to the increased NIR absorption in the silicon bulk due to the light constructive interface with the backside components, namely silver (Ag) and ITO. Study outcomes showed that improved SHJ efficiency could be further optimized by addressing front cell components, mainly front ITO and MoS₂ contact engineering.

Keywords: computer simulations; dimensionality reduction; light trapping; photovoltaic cells; raytracing; thin films.

Figure 1

The optical characteristic of MoS2 n and k-value was reprinted with permission from Ref [34]. https://refractiveindex.info/about (accessed on 14 July 2021).

Figure 2

(a) Sunsolve electrical equivalent circuit model, (b) silicon wafer along with MoS₂ few layers at the backside, (c) SSP solar cell and (d) SHJ solar cell. (All figures not to scale).

Figure 3

Photo-current density loss was calculated for a silicon wafer and a silicon wafer/MoS₂ structure.

Figure 4

Photo-current density loss was calculated for the SSP solar cell and the SSP solar cell/MoS₂ structure with various MoS₂ thicknesses.

Figure 5

Pie charts represent optical current density in SHJ cell using MoS₂ as back-reflector layer for various structure: (a) SHJ; (b) SHJ/MoS₂ (1 nm)/ITO (70 nm)/Ag; (c) SHJ/MoS₂ (1nm)/ITO (30 nm)/Ag; and (d) SHJ/ITO (30 nm)/MoS₂ (1 nm)/Ag.

Figure 6

A NIR light rays components constructive interference has conceptually drawn in the pink circle.

Figure 7

J_sc, V_oc and efficiency trend in All SHJ device configurations in Table 2.

Figure 8

Graph of EQE vs wavelength and various losses in D1, D2, D3 and D4: (a) EQE and Reflectance for various SHJ structures (D1, D2, D3 and D4); (b) EQE in NIR; (c) Front reflection; (d) Escape front reflection; and (e) escape rear reflection.