High-Performance Optoelectronic Devices: Exploring the Potential of Janus SbTeBr/SbSI Van der Waals Heterostructures for better Solar Cell Efficiency

Two-dimensional (2D) materials are known for their atomic-scale thickness and unique electronic, optical, and mechanical properties which made them potential candidates to enhance the performance of optoelectronic devices such as solar cells. However, the efficient separation of photogenerated carriers (electrons and holes) in 2D materials is still a limitation and in traditional solar cell materials, the recombination of these carriers significantly reduces energy conversion efficiency and leads to energy loss. This recombination process, if not properly managed will reduce the effectiveness of solar energy conversion. Although many 2D materials have excellent optical and mechanical properties, still their individual performance in light conversion remains inadequate. To address these challenges, new study published in Journal Physical Chemistry Chemical Physics and conducted by PhD candidate Hong-yao Liu, Associate Professor Huan Yang and led by Professor Yujun Zheng from the Shandong University developed novel Janus monolayer-based 2D/2D vdWH called SbTeBr/SbSI which is suitable for high-performance optoelectronic applications.

The researchers began their investigation into the SbTeBr/SbSI heterostructure and performed advanced theoretical and computational analysis including the Vienna Ab initio Simulation Package to model the interactions within the heterostructure which allowed them to predict the stability and properties of the heterostructure without the need for immediate experimental synthesis. The team also examined four different stacking arrangements in order to determine the most stable configuration of the SbTeBr/SbSI heterostructure. They generated these configurations by rotating and flipping the SbTeBr and SbSI monolayers and the binding energy of each configuration was calculated to identify the most energetically favorable structure. According to the authors, among the four configurations, the one with a binding energy of -226 meV per unit and an interlayer distance of 3.65 Å is the most stable. They also studied the electronic properties of the SbTeBr and SbSI monolayers, as well as the SbTeBr/SbSI heterostructure using the HSE06 hybrid functional to obtain accurate band gap values and found that the SbTeBr and SbSI monolayers to have indirect band gaps of 2.12 eV and 1.90 eV, respectively. When these monolayers were combined to form the heterostructure, the resulting SbTeBr/SbSI vdWH displayed a narrower indirect band gap of 1.28 eV. This band structure indicated a type II alignment with the valence band maximum originated from the SbSI monolayer and the conduction band minimum from the SbTeBr monolayer. The researchers also analyzed the charge density distribution to investigate charge transfer between the layers and found that charge transfer occurred from the SbSI layer to the SbTeBr layer supported by the plane-averaged charge density difference. Additionally, they observed an electrostatic potential difference of 0.9 eV along the z-direction which indicates the presence of a dipole-induced intrinsic electric field. Furthermore, the optical absorption properties of the SbTeBr/SbSI heterostructure were studied to evaluate its potential for solar energy applications and the absorption coefficient of the heterostructure was found to be high which ensures that the heterostructure can effectively capture solar energy. Moreover, the heterostructure exhibited a red shift in the light absorption edge compared to the individual monolayers, which indicated improved absorption characteristics in the visible spectrum. Professor Yujun Zheng and team studied as well the mechanical properties of the SbTeBr/SbSI heterostructure and showed that heterostructure has good mechanical stability and that the heterostructure is flexible and can withstand external stress. Indeed, this flexibility makes it suitable for applications in flexible optoelectronic devices. They also evaluated the photovoltaic performance of the SbTeBr/SbSI heterostructure calculated the power conversion efficiency (PCE) using the Scharber model. The heterostructure achieved a PCE of 8.3%, which is higher than previously reported Janus heterojunctions in the literature. The authors believe this high efficiency is attributed to the effective charge separation facilitated by the type II band alignment and the long carrier recombination time.

In conclusion, Professor Yujun Zheng and colleagues demonstrated the SbTeBr/SbSI heterostructure exhibit type II band alignment with an indirect bandgap of 1.28 eV which is ideal for solar cell applications and a power conversion efficiency of 8.3% which surpassed many existing 2D material-based heterostructures.  The mechanical properties of the SbTeBr/SbSI heterostructure indicated that it is flexible and can respond well to external stress. This flexibility is critical for the development of next-generation flexible optoelectronic devices which can be used to develop new wearable technology and foldable solar panels. Beyond solar cells, the unique properties of the SbTeBr/SbSI heterostructure make it suitable for light-emitting diodes, photodetectors, and transistors. Indeed, the ability to tailor the electronic and optical properties through stacking and combining different 2D materials opens up new avenues for designing custom devices with specific functionalities.