Optical absorption of MoS2 based ultrathin solar cells

Transition metal dichalcogenides (TMDCs) are promising candidates for ultrathin solar cells because of their strong light absorption and chemical stability. Among the different TMDC structures proposed for solar energy harvesting applications, MoS2 homojunctions exhibit the largest open-circuit voltages so far. However, these devices typically lose approximately half of the incident light due to reflection. Here, we calculate the absorbance of MoS2-based structures including h-BN, SiO2 or MgF2 as antireflection layers and an Ag back mirror, using a model that considers multiple reflections and interference. We find a considerable reduction of reflectance losses with the addition of an h-BN layer. h-BN is a layered material like MoS2, and therefore, it can be transferred onto the solar cell with the same techniques as for constructing the MoS2 junction. We find that combining an h-BN antireflection layer and an Ag back mirror, a MoS2 slab of 97 nm increases its absorbance to reach 89% in the photon energy range between 300 and 700 nm. This result can be further improved using a cavity structure consisting of h-BN/MoS2/h-BN/Ag. Inside the cavity, a MoS2 absorber that is 57 nm thick absorbs 91% of the photons. Finally, we find that the absorber thickness inside the cavity structure can be reduced drastically with only a small loss to the absorbance, namely 87% absorbance at a thickness of only 7 nm. This comprehensive study of absorbance contributes to the design of a new generation of high-efficiency ultrathin solar cells based on TMDC materials.