Metal halide perovskite light-emitting diodes (PeLEDs) have emerged as promising emitters for visible light communication (VLC) applications owing to their excellent optoelectronic properties and cost-effective fabrication. While various strategies have been proposed to enhance PeLEDs’ modulation performance, their practical application in VLC systems remains at an early, demonstrative stage. A key challenge lies in imbalanced charge injection and interfacial defects, which severely limit the modulation bandwidth and VLC performance. Specifically, the potential of interfacial engineering in solving these issues remains underexplored. Herein, we propose a multifunctional interfacial modification strategy by employing zwitterionic betaine citrate (BC) to simultaneously passivate defects and achieve balanced charge injection. The BC molecules effectively coordinate with undercoordinated Pb²⁺ ions, reducing the trap state density, while their ionic interaction with PEDOT:PSS decreases the hole injection barrier. Additionally, the employed methanol solvent selectively removes insulating PSS, improving interfacial conductivity and morphology. The synergistic effects of improved charge injection, defect passivation and optimized interface morphology lead to reduced non-radiative losses and a significantly lower resistance–capacitance (RC) time constant, both of which are critical for enhancing VLC performance. Consequently, optimized devices achieve a modulation bandwidth of 4.9 MHz and a data rate of 55.56 Mbps, far exceeding those of control devices and representing the best performance reported to date for devices with comparable active areas. This work systematically investigates the role of interfacial engineering in VLC performance and offers a viable pathway toward high-speed PeLED-based VLC systems.