Photovoltaic effect in an electrically tunable van der Waals heterojunction

Abstract: Semiconductor heterostructures form the cornerstone of many electronic and optoelectronic devices and are traditionally fabricated using epitaxial growth techniques. More recently, heterostructures have also been obtained by vertical stacking of two-dimensional crystals, such as graphene and related two- dimensional materials. These layered designer materials are held together by van der Waals forces and contain atomically sharp interfaces. Here, we report on a type- II van der Waals heterojunction made of molybdenum disulfide and tungsten diselenide monolayers. The junction is electrically tunable and under appropriate gate bias, an atomically thin diode is realized. Upon optical illumination, charge transfer occurs across the planar interface and the device exhibits a photovoltaic effect. Advances in large-scale production of two-dimensional crystals could thus lead to a new photovoltaic solar technology.

• The paper demonstrates that the electrically tunable MoS2/WSe2 vdW heterojunction exhibits a photovoltaic effect with a 0.2% power conversion efficiency.
• It utilizes mechanical exfoliation and precise stacking techniques to achieve atomically sharp interfaces with type-II band alignment for effective charge separation.
• The study highlights the potential for flexible, low-cost solar cells and calls for further optimization of device architectures to enhance efficiency.

Photovoltaic Effect in Electrically Tunable van der Waals Heterojunctions

This paper reports on the development and experimental validation of an electrically tunable van der Waals (vdW) heterojunction composed of molybdenum disulfide (MoS$_2$) and tungsten diselenide (WSe$_2$) monolayers. These two-dimensional (2D) materials are stacked to form a type-II heterostructure, showcasing significant promise for photovoltaic applications. Unlike conventional semiconductor heterostructures that are typically fabricated through epitaxial growth methods, the use of 2D atomic crystals allows for new device configurations with atomically sharp interfaces held together by vdW forces.

Experimental Findings

The authors have meticulously fabricated vdW heterostructures through mechanical exfoliation and precise stacking techniques. The diode-like current-voltage (J-V) characteristics of the heterojunction confirm type-II band alignment, which is essential for the separation of photo-generated electron-hole pairs. The application of an external gate voltage allows the tuning of electrical characteristics, transforming the heterojunction into an atomically thin diode.

Optical illumination of the fabricated vdW heterostructures results in charge transfer across the junction and the device exhibits a photovoltaic effect. The paper reports a power conversion efficiency of approximately 0.2%, with an external quantum efficiency of around 1.5%. Although these efficiencies are modest compared to current photovoltaic materials, the capability to fabricate such devices from 2D materials presents novel opportunities for engineering highly flexible and potentially low-cost solar cells.

Technical Implications

The findings underscore the potential of vdW heterostructures in photovoltaic applications, leveraging the unique properties of 2D transition-metal dichalcogenides (TMDCs). The optoelectronic behavior of MoS$_2$ and WSe$_2$ is notable for their direct bandgaps in monolayer form, which enhances their suitability as optical absorbers. Moreover, the observed need for vertical geometry and series resistance issues with configurations like graphene/TMDC/graphene stacks prompt further exploration into stacked heterojunctions or alternative configurations like planar junctions for optimization.

Implications and Future Directions

The successful implementation of a vdW heterojunction with photovoltaic capabilities invites theoretical and practical evaluations on scalability and material engineering of 2D layers. The simplicity in fabricating these heterostructures and tuning them electrically suggests a significant cost advantage and mechanical flexibility over traditional solar materials, offering a competitive edge in markets where planar flexibility is valued.

Potential barriers include the need for enhancing optical absorbance and understanding the trap-assisted recombination mechanisms observed in the current experiments, indicated by the dominance of Shockley-Read-Hall processes. Continued advances in material synthesis, particularly in stacking technologies and interface engineering, could yield multilayer heterostructures with improved absorption and carrier extraction efficiencies.

Conclusion

This paper effectively contributes to the growing body of research on 2D materials and their application in novel photovoltaic devices. By demonstrating the feasibility and tunability of vdW heterostructures and presenting strong evidence of photovoltaic effect, it sets the stage for diverse applications in flexible and cost-effective solar technology. Subsequent research should focus on optimizing device architecture and expanding material choices for achieving higher efficiencies, potentially leading to the next generation of photovoltaic technologies.