Hot carrier diffusion-assisted ideal carrier multiplication in monolayer MoSe2

Abstract: Carrier multiplication (CM), the process of generating multiple charge carriers from a single photon, offers an opportunity to exceed the Shockley-Queisser limit in photovoltaic applications. Despite extensive research, no material has yet achieved ideal CM efficiency, primarily owing to significant energy losses from carrier-lattice scattering. In this study, we demonstrate that monolayer MoSe2 can attain the theoretical maximum CM efficiency permitted by energy-momentum conservation principle, using ultrafast transient absorption spectroscopy. By resolving the scatter-free ballistic transport of hot carriers and validating our findings with first-principles calculations, we identify the cornerstone of optimal CM in monolayer MoSe2: superior hot-carrier dynamics characterized by suppressed energy dissipation via minimized carrier-lattice scattering, and the availability of abundant CM pathways facilitated by 2Eg band nesting. Comparative analysis with bulk MoSe2 further emphasizes the enhanced CM efficiency in the monolayer, attributed by superior hot-carrier diffusion and access to additional CM pathways. These results position monolayer MoSe2 as a promising candidate for high-performance optoelectronic applications, providing a robust platform for next-generation energy conversion technologies.

• The paper demonstrates ideal, quantum-limited carrier multiplication in monolayer MoSe₂ using ultrafast transient absorption spectroscopy.
• It employs first-principles DFT and ballistic hot-carrier transport, revealing a sharp quantum yield jump from 1 to 2 at photon energies above 2Eg.
• The findings imply that 2D TMDs can overcome the Shockley-Queisser limit, paving the way for advanced optoelectronic and photovoltaic devices.

Hot Carrier Diffusion-Assisted Ideal Carrier Multiplication in Monolayer MoSe₂

Overview

This work provides a comprehensive investigation into carrier multiplication (CM) in monolayer MoSe₂, demonstrating for the first time the achievement of the theoretical ideal CM efficiency limit as imposed by energy-momentum conservation. Using ultrafast transient absorption spectroscopy supported by first-principles calculations, the study elucidates the critical electronic and nonequilibrium transport mechanisms underpinning this unprecedented efficiency. The findings position monolayer MoSe₂—and by extension, other monolayer transition metal dichalcogenides (TMDs)—as leading candidates for next-generation optoelectronic devices aiming to significantly surpass the Shockley-Queisser efficiency limit in photovoltaics.

Experimental Demonstration of Ideal Carrier Multiplication

Ultrafast transient absorption (TA) measurements reveal a stepwise quantum yield (QY) in monolayer MoSe₂: QY jumps from 1 to 2 at photon energies exceeding $2E_g$ (band gap), directly corresponding to the generation of two free carriers per absorbed photon. This step-like, quantized response contrasts sharply with both gradual QY increases in bulk MoSe₂ and sub-ideal performance in all previously studied materials. The result indicates carrier multiplication operating at the quantum limit, where all excess photon energy above $2E_g$ efficiently produces multiple electron-hole pairs with no observable energy loss beyond what is strictly required by energy and momentum conservation.

Strong numerical results include:

• Quantum yield (QY) of 2.0 at the $2E_g$ threshold, corresponding to 100% CM efficiency (see Figure 1d of the paper, with exact simulation-experiment agreement).
• Hot-carrier diffusion coefficient of $$D_\text{mono} \approx 1\times10^4\,\mathrm{cm}^2/\mathrm{s}$$, exceeding competing materials and even gold by 1–2 orders of magnitude.
• Ultrafast, ballistic carrier propagation with velocities up to $7.2\times10^5\,\mathrm{m}/\mathrm{s}$, confirmed via spatiotemporal TA microscopy.

These measurements are carried out at low photon fluence to minimize nonlinear many-body effects, thus isolating the intrinsic material response.

Microscopic Mechanisms: Electronic Structure and Transport

First-principles DFT calculations uncover that the electronic band structure of monolayer MoSe₂ possesses 2$E_g$ band nesting near the $K$ and $K'$ points. This nesting creates abundant energy- and momentum-conserving CM channels for photons with energy just above $2E_g$, a feature absent in bulk MoSe₂ due to stricter conservation constraints and the lack of such nesting. The valley symmetry of the monolayer ensures both intra- and inter-valley CM channels, further maximizing the number of available CM processes.

The observed hot-carrier diffusion is initially ballistic (exponent $\beta=2$) for sub-picosecond timescales, rapidly transitioning to diffusive behavior. Ballistic expansion facilitates immediate spatial separation of high-energy carriers, suppressing Auger recombination and exciton-exciton annihilation during and following CM. This dynamic is crucial for preserving multiplied carrier populations, in stark contrast to bulk MoSe₂ where higher carrier-lattice scattering rates (resulting in $2E_g$0) drive rapid thermalization and limit CM efficiency.

Comparison to Other Dimensionalities and Materials

The performance of monolayer MoSe₂ is compared across material classes (0D, 1D, 2D, 3D), demonstrating:

• 2D systems, especially monolayer TMDs, exhibit superior CM efficiency relative to both quantum dots and bulk analogues.
• Specifically, the unique combination of uninterrupted in-plane transport, strong out-of-plane quantum confinement, and band nesting in TMDs enables the suppression of loss channels that heavily limit 0D/1D/3D systems.

Van der Waals 2D materials with continuous crystalline stacking (unlike polycrystalline quantum dot structures) avoid carrier trapping at interfaces, thus supporting ultrafast and unhindered carrier propagation.

Practical and Theoretical Implications

From a device engineering perspective, the realization of ideal CM in monolayer MoSe₂ implies:

• Photovoltaic devices could exceed the Shockley-Queisser efficiency limit via direct exploitation of ideal CM and minimized thermalization losses.
• Photodetectors and light emitters based on monolayer TMDs can achieve higher quantum efficiencies, sensitivity, and speed due to efficient hot-carrier extraction and reduced recombination.

Theoretically, the study upends conventional understanding that ideal CM is unachievable in real solids due to rapid carrier-lattice thermalization. The data here show that 2D confinement, band nesting, and symmetry properties can robustly circumvent these limitations. It also raises the prospect that similar behavior might be engineered in other TMDs or van der Waals heterostructures, opening a promising direction for both fundamental exploration and material design.

Future Directions

Unresolved issues remain concerning the ultrafast nature of the CM process—occurring faster than the experimental temporal resolution—which could be directly probed with advanced femtosecond tr-ARPES. Additionally, distinguishing unambiguously between free-carrier CM and multiple exciton generation (MEG) may require manipulating the dielectric environment or applying external fields to disentangle competing channels.

Research into integrating monolayer MoSe₂ with scalable growth processes, stability under device operation, and coupling with various electrodes/heterostructures is warranted to translate laboratory performance to practical technologies.

Conclusion

This paper establishes that monolayer MoSe₂ achieves ideal, quantum-limited carrier multiplication efficiency, a feat attributed to 2$2E_g$1 band nesting, valley symmetry-enabled CM channels, and ultrafast ballistic carrier transport that collectively suppress recombination and energy loss channels. The outcomes redefine performance boundaries for optoelectronic materials, challenging previous assumptions about CM limits and positioning 2D TMDs as essential for future high-efficiency energy conversion platforms.