Hybrid, Gate-Tunable, van der Waals p-n Heterojunctions from Pentacene and MoS2

Abstract: The recent emergence of a wide variety of two-dimensional (2D) materials has created new opportunities for device concepts and applications. In particular, the availability of semiconducting transition metal dichalcogenides, in addition to semi-metallic graphene and insulating boron nitride, has enabled the fabrication of all 2D van der Waals heterostructure devices. Furthermore, the concept of van der Waals heterostructures has the potential to be significantly broadened beyond layered solids. For example, molecular and polymeric organic solids, whose surface atoms possess saturated bonds, are also known to interact via van der Waals forces and thus offer an alternative for scalable integration with 2D materials. Here, we demonstrate the integration of an organic small molecule p-type semiconductor, pentacene, with a 2D n-type semiconductor, MoS2. The resulting p-n heterojunction is gate-tunable and shows asymmetric control over the anti-ambipolar transfer characteristic. In addition, the pentacene-MoS2 heterojunction exhibits a photovoltaic effect attributable to type II band alignment, which suggests that MoS2 can function as an acceptor in hybrid solar cells.

• The paper demonstrates the fabrication of hybrid, gate-tunable van der Waals p–n heterojunctions using pentacene and MoS₂.
• The paper employs charge transport measurements, scanning photocurrent microscopy, and finite element simulations to reveal type II band alignment and asymmetric transfer characteristics.
• The paper reports a photovoltaic response with a 0.3 V open-circuit voltage and 3 nA short-circuit current, underscoring its potential for advanced optoelectronic applications.

Overview of Hybrid Gate-Tunable van der Waals p-n Heterojunctions from Pentacene and MoS₂

The paper presents a study on the integration of organic semiconductor pentacene with two-dimensional (2D) transition metal dichalcogenide (TMDC) MoS₂, forming a van der Waals (vdW) heterostructure. This investigation explores the potential of 2D materials for innovative device architectures, specifically focusing on the creation and characterization of hybrid, gate-tunable, p-n heterojunctions with notable photovoltaic applications.

Key Findings and Methodologies

The researchers demonstrate the fabrication of heterojunctions using mechanically exfoliated MoS₂ flakes, which were integrated with thermally evaporated pentacene films. The devices showed asymmetric anti-ambipolar transfer characteristics when operated as a three-terminal, gate-tunable diode. The heterojunctions exhibited a photovoltaic response, which confirmed the potential utility of MoS₂ as an electron acceptor in organic solar cell configurations.

Quantitative analysis was conducted using various techniques such as direct charge transport measurements, scanning photocurrent microscopy, electrostatic force microscopy, and finite element modeling. The study reported a type II band alignment at the pentacene/MoS₂ interface, a critical characteristic leading to the observed anti-ambipolar behavior and gate-tunability.

Numerical Results and Device Performance

Experimental results from over ten devices revealed consistent qualitative behavior with asymmetry in anti-ambipolar transfer characteristics, attributable to differences in transconductance on either side of the gate voltage peaks. The photovoltaic effect was evidenced by an open-circuit voltage (VOC) of approximately 0.3 V and a short-circuit current (ISC) of about 3 nA under illumination. However, the power conversion efficiency remained low (~0.004%), suggesting room for optimization.

Finite element simulations provided insights into the observed asymmetry in transfer characteristics, identifying the series resistances of the semiconductor channels as the controlling factor. This was further substantiated by electrostatic force microscopy measurements, highlighting the influence of channel resistances on current flow and transfer characteristics.

Implications and Future Prospects

The results hold significant implications for the advancement of 2D material-based electronic and optoelectronic devices. The application of MoS₂ as a non-fullerene acceptor positions it as a potential candidate for hybrid solar cells, while the observed gate-tunable characteristics may influence future designs of vdW heterostructure devices in telecommunications and integrated circuits.

A promising future direction involves leveraging the scalability of organic semiconductors and the solution-processability of 2D TMDCs to enhance the performance of these hybrid structures. Additionally, pursuing vertical bulk heterojunction configurations may boost carrier collection efficiency and improve photovoltaic metrics.

In conclusion, this research delineates foundational progress in creating hybrid organic/2D semiconductor devices, opening avenues for high-performance applications in next-generation optoelectronic technologies. The study emphasizes the versatility and potential of vdW heterostructures, encouraging further exploration within the material science and semiconductor fields.