Ultrafast Exciton Dissociation and Long-Lived Charge Separation in a Photovoltaic Pentacene MoS2 van der Waals Heterojunction

Abstract: Van der Waals heterojunctions between two-dimensional (2D) layered materials and nanomaterials of different dimensions present unique opportunities for gate-tunable optoelectronic devices. Mixed dimensional p-n heterojunction diodes, such as p-type pentacene (0D) and n-type monolayer MoS2 (2D), are especially interesting for photovoltaic applications where the absorption cross-section and charge transfer processes can be tailored by rational selection from the vast library of organic molecules and 2D materials. Here, we study the kinetics of excited carriers in pentacene-MoS2 p-n type-II heterojunctions by transient absorption spectroscopy. These measurements show that the dissociation of MoS2 excitons occurs by hole transfer to pentacene on the time scale of 6.7 ps. In addition, the charge-separated state lives for 5.1 ns, up to an order of magnitude longer than the recombination lifetimes from previously reported 2D material heterojunctions. By studying the fractional amplitudes of the MoS2 decay processes, the hole transfer yield from MoS2 to pentacene is found to be approximately 50 percent, with the remaining holes undergoing trapping due to surface defects. Overall, the ultrafast charge transfer and long-lived charge-separated state in pentacene-MoS2 p-n heterojunctions suggest significant promise for mixed-dimensional van der Waals heterostructures in photovoltaics, photodetectors, and related optoelectronic technologies.

Insights into Ultrafast Exciton Dynamics and Charge Separation in Pentacene-MoS(_2) Heterojunctions

The study of van der Waals heterostructures, particularly those combining two-dimensional (2D) materials such as transition metal dichalcogenides (TMDs) with organic molecules, presents a promising avenue for the development of advanced optoelectronic devices. The paper under review focuses on a mixed-dimensional type-II heterojunction between pentacene and monolayer molybdenum disulfide (MoS(_2)), providing an in-depth exploration of exciton dynamics critical for photovoltaic applications.

The researchers employ transient absorption spectroscopy to elucidate the kinetics of charge carriers within this heterojunction. The pentacene-MoS(_2) p-n junction is distinguished by an ultrafast exciton dissociation in MoS(_2) facilitated by a hole transfer to pentacene occurring on a timescale of approximately 6.7 picoseconds. Notably, the study reports a long-lived charge-separated state with a lifetime of 5.1 nanoseconds, significantly surpassing recombination lifetimes typically observed in n-n 2D TMD heterojunctions.

The experimental setup involves the growth of large-area monolayer MoS(_2) films via chemical vapor deposition (CVD), followed by thermal evaporation of pentacene. The interface properties and electronic interactions are characterized using several instruments, including atomic force microscopy (AFM) and Raman spectroscopy. The introduction of pentacene to MoS(_2) induces a bathochromic shift in exciton peaks, corroborating the formation of a depletion region and charge transfer.

Numerically, the research quantifies a 50% yield for hole transfer from MoS(_2) to pentacene. The reported hole transfer rate outpaces most relaxation processes except ultrafast carrier trapping, which occurs sub-picosecond. This suggests a competitive process advantageous for photovoltaic efficiency, especially when compared to 2D-2D heterojunctions that exhibit more rapid charge recombination.

This work addresses the challenge of achieving high open-circuit voltages in photovoltaic cells, as the pentacene-MoS(_2) heterojunction boasts a theoretical maximum close to 1.1 volts due to favorable band offsets. However, the efficacious dissociation of excitons in pentacene is partially undermined by the sub-picosecond singlet fission process, which may necessitate the substitution of pentacene with alternative organic molecules to improve photocurrent.

In terms of theoretical and practical implications, the results provide a platform for further exploration and optimization of mixed-dimensional heterojunctions. The pentacene-MoS(_2) system exemplifies the potential and challenges in leveraging organic and inorganic interfaces to engineer superior optoelectronic devices. Future research may focus on identifying materials with optimal exciton dynamics, energy level alignments, and charge transport properties to enhance the photovoltaic performance further.

In summary, this detailed investigation into the photoinduced charge transfer and recombination dynamics in van der Waals heterojunctions delineates the intricate balance between charge dissociation and recombination. It opens avenues for the rational design of heterostructures in which charge transfer can be finely tuned to maximize device efficiency without compromising the longevity of charge-separated states.