In traditional semiconductor photovoltaic devices, "hot carriers" generated after the absorption of high-energy photons undergo rapid relaxation through phonon scattering, resulting in severe energy loss and limiting the photovoltaic conversion efficiency. Carrier Multiplication (CM), a process that generates two or more electron-hole pairs upon absorbing a single high-energy photon, is regarded as an effective mechanism to break the Shockley–Queisser limit. However, the energy threshold for initiating CM is typically above 2Eg, meaning a large number of sub-2Eg high-energy photons cannot be effectively utilized. Two-dimensional transition metal dichalcogenides (TMDs) are ideal platforms for CM research due to their strong Coulomb interactions, high density of states, and dangling bond-free surfaces. Although previous studies have observed a CM threshold of approximately 2Eg in materials such as MoTe₂and MoS₂, further reducing this threshold remains a challenge. Theoretically, researchers have proposed that band structure renormalization induced by electron-phonon coupling (EPC) may break the CM threshold limit, but experimental verification is still lacking and the theory needs further improvement.

To address this issue, Prof. LIU Xinfeng from the National Center for Nanoscience and Technology, China (NCNST), in collaboration with Prof. XU Haiyang's team from Northeast Normal University, published a research paper titled "Defect-Engineered Reduction of the Carrier Multiplication Threshold in Monolayer WS₂ to 1.6Eg" in ACS Nano. The study breaks the carrier multiplication limit of TMDs to 1.6Eg through defect engineering. By introducing in-gap states (IGS) to enhance electron-phonon coupling (EPC), the second electron can transition into the conduction band through IGS-assisted upconversion after absorbing energy, thereby realizing a low-threshold energy CM (LTE-CM) process at a photon energy of 1.6Eg. The research utilized chemical vapor deposition (CVD)-grown monolayer WS₂ and introduced high-density symmetric disulfur vacancies (V_2S) through thermal annealing. The types and concentrations of defects were confirmed by techniques such as scanning transmission electron microscopy (STEM) and X-ray photoelectron spectroscopy (XPS), revealing that symmetric V_2S is the most effective in promoting LTE-CM.

This study demonstrates an ultra-low threshold carrier multiplication process in monolayer WS2 enabled by artificially engineered in-gap states and strong electron-phonon coupling. Optoelectronic measurements show that when the photon energy exceeds 1.6Eg, the photoresponsivity (R) and internal quantum efficiency (IQE) of the device increase sharply, and the power-dependent exponent of the photocurrent exceeds 1, indicating obvious CM characteristics. Transient absorption (TA) spectroscopy further verifies the occurrence of CM, showing that when the pump photon energy exceeds 1.6Eg, the carrier quantum yield (QY) increases from 1 to nearly 2, indicating that each photon generates more than one electron-hole pair. This is the first experimental discovery of a carrier multiplication process with a threshold energy below 2Eg. Temperature-dependent experiments and Raman spectra confirm that V_2S significantly enhances EPC and promotes the phonon-assisted upconversion process. Density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations reveal the key role of in-gap states in the CM process: the introduction of V_2S forms intermediate energy levels in the bandgap. Valence band electrons excited by impact ionization first transition to the intermediate states, and then jump to the conduction band through the degeneracy and lifting of degeneracy processes of the intermediate energy levels and the multiphonon-assisted upconversion process, completing the LTE-CM process. Theoretical calculations provide the energy required for each transition process, revealing that lattice softening caused by symmetric V_2S is most conducive to the aforementioned processes, which is consistent with the experimental results.

Figure 1. Design of LTE-CM in monolayer WS₂. (a) Schematic diagram of the LTE-CM process. (b) Schematic of annealing for vacancy-doped WS₂, indicating three types of vacancies: monosulfur vacancy (VS), symmetric disulfur vacancy (V_2S), and tungsten vacancy (VW). (c) Atomically resolved STEM image of WS₂ annealed for 2 hours, with three defect types marked by dashed circles of corresponding colors. (d) XPS spectra of W 4f/5p and S 2p core levels of untreated WS2 and samples annealed for 1 hour and 2 hours. Red arrows indicate the redshift of the spectral peaks with increasing annealing time.

Figure 2. Experimental evidence of LTE-CM. (a) Schematic diagram of a monolayer WS₂ device on a Si/SiO₂ substrate. (b) Responsivity (R) and internal quantum efficiency (IQE) under different excitation photon energies. (c) Summary of the α parameter under different photon energies. (d) Schematic diagram of the TA setup. (e) Dependence of QY on pump photon energy. (f) Comparison of CM behavior between monolayer WS₂ and other materials.

Figure 3. Mechanistic explanation of LTE-CM. (a) Temperature dependence of the full width at half maximum (FWHM) of the A exciton in V_2S-WS₂ excited by a 320 nm laser. Solid lines are fitting curves using the Debye-Einstein approximation. (b) Variations of the peak position and FWHM of the out-of-plane phonon mode of V_2S-WS₂ with annealing time, excited by a 488 nm laser. (c) Schematic diagram of the dynamics of the second electron in LTE-CM. (d) Statistics of the energies required for the three processes in four different WS₂ configurations.

Contact:

LIU Xinfeng

National Center for Nanoscience and Technology (NCNST)

E-mail: liuxf@nanoctr.cn