Understanding the microscopic tribo-piezo-flexoelectric coupling mechanism at ferroelectric polymer-metal interfaces from molecular and electronic structure

Despite the simultaneous occurrence of triboelectric, piezoelectric, and flexoelectric effects in bending ferroelectric polymers, their synergistic potential to exceed the collective output achievable by individual mechanisms has not been systematically explored, and the underlying microscopic coupling mechanism remains poorly understood. Herein, this study employs first-principles calculations to investigate the contact electrification mechanism at the ferroelectric polymer-metal interface under bending, using polyvinylidene fluoride (PVDF) and copper (Cu) as a model system. The results indicate that the piezoelectric d₃₁ and the flexoelectric effect facilitate charge transfer by lowering the lowest unoccupied molecular orbital (LUMO) energy level and providing additional polarization. Furthermore, the alignment of the polarization induced by the piezoelectric d₃₁/d₃₃ and flexoelectric effects with the spontaneous polarization of PVDF is identified as the key intrinsic factor for the tribo-piezo-flexoelectric coupling. Specifically, the combined influence of the piezoelectric d₃₁ and flexoelectric effects lowers the electron transfer barrier, thereby boosting the triboelectric effect. The enhanced triboelectric effect induces additional dipoles within PVDF, further increasing its polarity and strengthening the piezoelectric effect. When the induced polarization aligns with the spontaneous polarization of PVDF, the intensified dipole polarity provides additional electrostatic potential energy for charge transfer, ultimately synergistically enhancing both triboelectric and piezoelectric responses. This endows the devices with significantly superior performance that surpasses the linear superposition of single-effect or dual-effect contributions, offering a viable pathway to address the inherent performance limitations.