Electroluminescence and photocurrent generation from atomically sharp WSe2/MoS2 heterojunction p-n diodes

Abstract: The p-n diodes represent the most fundamental device building block for diverse optoelectronic functions, but are difficult to achieve in atomically thin transition metal dichalcogenides (TMDs) due to the inability to selectively dope them into p- or n-type semiconductors. Here we report the first demonstration of an atomically thin and atomically sharp heterojunction p-n diode by vertically stacking p-type monolayer tungsten diselenide (WSe2) and n-type few-layer molybdenum disulfide (MoS2). Electrical measurement demonstrates excellent diode characteristics with well-defined current rectification behaviour and an ideality factor of 1.2. Photocurrent mapping shows fast photoresponse over the entire overlapping region with a highest external quantum efficiency up to 12 %. Electroluminescence studies show prominent band edge excitonic emission and strikingly enhanced hot electron luminescence. A systematic investigation shows distinct layer-number dependent emission characteristics and reveals important insight about the origin of hot-electron luminescence and the nature of electron-orbital interaction in TMDs. We believe that these atomically thin heterojunction p-n diodes represent an interesting system for probing the fundamental electro-optical properties in TMDs, and can open up a new pathway to novel optoelectronic devices such as atomically thin photodetectors, photovoltaics, as well as spin-/valley-polarized light emitting diodes and on-chip lasers.

• The paper demonstrates that an atomically sharp WSe2/MoS2 heterojunction p–n diode exhibits excellent current rectification with an ideality factor of 1.2, indicating minimal recombination loss.
• It achieves a photocurrent external quantum efficiency of approximately 12%, significantly surpassing lateral homojunctions by optimizing charge separation.
• The device produces pronounced electroluminescence with enhanced hot electron luminescence, offering new insights into electron–orbital interactions in transition metal dichalcogenides.

Electroluminescence and Photocurrent in Atomically Sharp WSe2/MoS2 Heterojunctions

The investigation presented in this paper focuses on the creation and analysis of a vertically stacked heterojunction p-n diode formed by synthetic p-type monolayer tungsten diselenide (WSe2) and exfoliated n-type molybdenum disulfide (MoS2). This study addresses the challenges related to selective doping in monolayer transition metal dichalcogenides (TMDs), offering new insights into electro-optical properties and potential applications in next-generation optoelectronic devices.

Key Findings

• Current Rectification and Ideality Factor: The WSe2/MoS2 heterojunction demonstrates a well-defined current rectification behavior with an ideality factor of 1.2. This low ideality factor suggests excellent diode behavior, attributable to the atomically sharp interface that minimizes recombination loss.
• Photocurrent Generation: The maximum external quantum efficiency (EQE) observed is approximately 12%. This high EQE is significantly higher than the 0.1–3% typically seen in lateral electrostatically doped WSe2 homojunctions. The atomically sharp vertical p-n junction optimizes charge separation, contributing to enhanced photoconversion efficiency.
• Electroluminescence: Under forward bias, electroluminescence (EL) is prominently observed, enabling efficient excitonic emissions. Notably, enhanced hot electron luminescence (HEL) are observed, offering substantial insights into electron-orbital interactions in TMDs. The HEL enhancement by two orders of magnitude suggests electric field induced carrier redistribution.

Implications

The findings from this study have strong implications for the development of highly efficient, atomically thin optoelectronic devices. The observed high EQE and distinct electroluminescence characteristics suggest the potential for utilizing WSe2/MoS2 heterostructure devices in a variety of applications such as photovoltaic devices, spin- or valley-polarized LEDs, and on-chip lasers. Moreover, the enhanced hot electron luminescence provides a novel approach to probe electron-orbital interactions in TMDs, which is critical for understanding fundamental material properties.

Future Directions and Speculations

Advancements in fabrication techniques for these heterostructural diodes could lead to improved interface quality and device performance. Temperature-dependent EL studies indicate the role of interlayer perturbation on electron-orbital interaction, a finding that suggests further exploration into tuning optical properties through thermal management could yield more efficient devices. Additionally, understanding the mechanisms behind HEL could inform the design of devices that exploit these interactions for enhanced functionality. Future studies can focus on integrating these diodes into more complex circuit architectures to evaluate their performance in real-world electronic systems.

Overall, the work on atomically sharp WSe2/MoS2 heterojunctions represents a compelling step towards the realization of high-performance, two-dimensional optoelectronic devices, offering foundational insights that drive both theoretical and applied research in the field of advanced materials.