Efficient spin-to-charge is crucial for developing high-performance spintronic devices, such as terahertz (THz) emitters and spin batteries. Here, we demonstrate that precise engineering of the Pt layer in $\mathrm{FeNi}/\mathrm{Pt}/\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$, using scalable magnetron sputtering, significantly enhances conversion efficiency over a broad frequency range (from GHz to THz). Terahertz emission spectroscopy and ferromagnetic resonance measurements identify an optimal Pt thickness of $\approx 2\mathrm{nm}$, where the THz emission intensity increases by a factor of $\approx 8$ compared with a sample without the Pt layer. In the 2 nm Pt samples, both the THz emission amplitude and effective spin Hall angle exhibit a monotonic increase with temperature. First-principles calculations indicate that this enhancement arises mainly from Rashba-split surface states at the Pt/$\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ interface, rather than from preserved topological surface states. These findings highlight the crucial role of Pt interface engineering in boosting spin-to-charge conversion in topological insulators, with promising implications for broadband communication and information processing.

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• Spin dynamics
• Spin-orbit torque
• Spintronics
• Topological insulators
• Ultrafast magnetic effects