Vacancy-engineered electronic reconstruction for spin-polarized photocurrent with signal enhancement in Ti₂CO₂ MXene

Polarization-sensitive and spin-polarized photocurrent generation paves a critical path for constructing multifunctional integrated optoelectronic devices. However, two-dimensional material systems that simultaneously possess intrinsic tunable electronic structures and strong polarization-dependent photoresponse remain extremely scarce. Herein, we systematically investigate the modulation mechanisms of vacancy engineering on the electronic structures, optical properties, and device photoelectric transport performance of Ti₂CO₂ MXene with different layer thicknesses using first-principles calculations combined with the nonequilibrium Green's function method. At the material level, our investigations reveal that oxygen vacancies primarily induce bandgap narrowing and significantly enhance the optical absorption of the material in the low-energy region. In contrast, titanium vacancies in monolayer Ti₂CO₂ and carbon vacancies in the upper layer of bilayer Ti₂CO₂ strongly break the parity-time (PT) symmetry of the crystal lattice, leading to bandgap closure, the introduction of defect states near the Fermi level, and pronounced spin-splitting effects. Device-level simulations further confirm that vacancy engineering can effectively modulate polarization-dependent photocurrents and achieve a substantial enhancement of the extinction ratio in specific photon energy ranges. Among all the studied configurations, bilayer Ti₂CO₂ with an upper-layer carbon vacancy exhibits the optimal optoelectronic performance: its maximum photocurrent approaches 90a₀² per photon near 0.7 eV, and the extinction ratio reaches as high as 1.5 × 10³ near 1.0 eV. Notably, both the titanium-vacancy and upper-layer carbon-vacancy configurations can generate highly spin-polarized photocurrents with a maximum spin polarization approaching 100%. These findings highlight that vacancy-engineered Ti₂CO₂ is a highly promising candidate for high-performance polarization-sensitive photodetectors and spin-polarized optoelectronic devices.